Takahashi FC design vs. FS Design
Chuck Gulker
<< I would like to hear from people who have compared the older Takahashi
FC (as in the FC-100) to the newer FS designs >>
I've had my new FS-78 for several months now, and have been extremely
pleased with the scope. I can't comment on the FC having never looked
through one.
I can say that my FS passes the star test with flying colors (make that lack
of color!). I'm anxious to observe the planets. Very happy. Very happy.
Chuck
Fred & Julie Burger
I would like to hear from people who have compared the older
FS-102). I have heard that at about the same time they switched to the
newer design, they also went from hand polishing to all machine
polishing of the optical surfaces and I was wondering if this has
affected the optical quality (good, bad, or not at all) of their
refractor lenses.
I know that the older FC type was superb as I have owned several,
but I would like to know how this new design made with machine
polishing techniques compares.
Many thanks in advance!
Best Wishes,
Julie :-)
f...@sos.net
Wavefront Web Pages - A web site for Maksutov users
http://www.sos.net/~fjb/Q1.htm
Starvick
The new design is excellent as is expected from Takahashi. Jeff
Herm Perez
Machine polishing of spherical optics is perfectly ok, as long as its
done slowly so that you get a smooth surface.
Herm
On Sun, 17 May 1998 00:57:44 GMT, f...@sos.net (Fred & Julie Burger)
wrote:
Todd Gross
> I would like to hear from people who have compared the older
>Takahashi FC (as in the FC-100) to the newer FS designs (as in the
>FS-102). I have heard that at about the same time they switched to the
>newer design, they also went from hand polishing to all machine
>polishing of the optical surfaces and I was wondering if this has
>affected the optical quality (good, bad, or not at all) of their
>refractor lenses.
> I know that the older FC type was superb as I have owned several,
>but I would like to know how this new design made with machine
>polishing techniques compares.
> Many thanks in advance!
> Best Wishes,
> Julie :-)
Julie.. my personal experiences are mixed, and so is what I hear from various
users. Mike Harvey swears the old line is sharper. I found an FC100 with so-so
optics, and an FC100 with superb optics. I have found most FC60,50, and FS128s
I've tried near perfect, yet an FS78 not at all, not even close.
It seems it still varies scope to scope unfortunately to some degree. These
scopes from what I ahve heard used to be more consistent than A/P. Now I think
that has switched. A/P is very consistent scope to scope. However, (and this
is very controversial) from the dozens of ppl I've talked to about it, the
best 5" and over A/P scopes apparently are not matching the best 5" and over
Takahashi scopes in actual field use. I have heard of an important exception
to this (Rich N.) with an FS128 up against a 5" EDT. The 6" FCT is so raved
about by it's users in fact, they are almost delirious. Whether that is
because they paid 10,000 or not I am not sure.
Rich N.
-snip
> I would like to hear from people who have compared the older
> Takahashi FC (as in the FC-100) to the newer FS designs (as in the
> FS-102). I have heard that at about the same time they switched to the
> newer design, they also went from hand polishing to all machine
> polishing of the optical surfaces and I was wondering if this has
> affected the optical quality (good, bad, or not at all) of their
> refractor lenses.
> I know that the older FC type was superb as I have owned several,
> but I would like to know how this new design made with machine
> polishing techniques compares.
> Many thanks in advance!
> Best Wishes,
> Julie :-)
>
Hi Julie,
I have a very nice FC-100. But so far I haven't
had it side by side with the FS-102. The FS-102s
I've looked through gave very sharp high contrast
images. Maybe if the weather gives us a break
I can do a side by side between the old and new
Tak 4" inchers.
Rich
Rich N.
-snip
> that has switched. A/P is very consistent scope to scope. However, (and
this
> is very controversial) from the dozens of ppl I've talked to about it,
the
> best 5" and over A/P scopes apparently are not matching the best 5" and
over
> Takahashi scopes in actual field use. I have heard of an important
exception
> to this (Rich N.) with an FS128 up against a 5" EDT. The 6" FCT is so
raved
> about by it's users in fact, they are almost delirious. Whether that is
> because they paid 10,000 or not I am not sure.
Hi Todd and Julie,
The FS-128 I was looking through was giving excellent images
of the moon and Mars. It wasn't as if my AP 130EDT was blowing
away the FS-128. I just thought I could see some features on Mars
a little better with my EDT. Even though I did try to make the switch
from one scope the other fairly quickly the seeing could still have made
the difference in the image.
Rich
Todd Gross
rtant
>exception
>> to this (Rich N.) with an FS128 up against a 5" EDT. The 6" FCT is so
>raved
>> about by it's users in fact, they are almost delirious. Whether that is
>> because they paid 10,000 or not I am not sure.
>Hi Todd and Julie,
>The FS-128 I was looking through was giving excellent images
>of the moon and Mars. It wasn't as if my AP 130EDT was blowing
>away the FS-128. I just thought I could see some features on Mars
>a little better with my EDT. Even though I did try to make the switch
>from one scope the other fairly quickly the seeing could still have made
>the difference in the image.
thanks for the clarification. I also seem to be getting the impression from
you and others that the f/8 EDT may outperform the f/6 EDF on planetary,
but nobody has sweared to that
MHarveyWW1
Hi Julie....
As Todd indicated in his earlier response, my experience
has been that the FC scopes I've owned and used (about a dozen, including my
own FC-125) are superior to any of the FS's that I've been able to use (four
102's and one 128).
I agree that it could be an issue of quality control, BUT...I cannot remember
a 'bad' or even 'sub-par' FC - and I have YET to see an FS that I'd rate 'very
good', let alone 'excellent'. (I'm sure that there MUST be SOME 'bad' FC's...I
just never encountered one).
The five FS's in question were inferior in contrast and image
brightness...exhibited more spurious color...and suffered noticeable image
breakdown at very high magnifications.
As regards AP scopes....none of the FS's have matched the performance of the
EDF's or EDT's I've had occasion to use.
The FC's proved to be equal and, in some cases, superior.
Mike Harvey
Mike Harvey
TMBack
>The five FS's in question were inferior in contrast
>and image brightness...exhibited more spurious color...
>and suffered noticeable image breakdown at very high >magnifications. As
regards AP scopes....none of the FS's
>have matched the performance of the EDF's or EDT's
>I've had occasion to use.
>Mike Harvey
All of the above statements are true. A Japanese
amateur that I know explained that Takahashi, because
of the current economic conditions in Japan, has gone
to machine figured lenses (to save time and money), and
they are much poorer in quality control. There is always
a chance of receiving a good TAK FS APO, however the
older TAK FC series almost always had a great optical
figure. I tested a recent FS-102, and the image started to
break down over 160x on Jupiter (seeing excellent). The wavefront was on the
order of 1/3 to 1/4 wave over-
corrected, with noticable color aberration on bright stars.
The older FC series and Astro-Physics EDF/EDT scopes
can be used at much higher magnifications without image breakdown.
Thomas Back
Rich N.
Todd Gross <to...@weatherman.com> wrote in article
<toddg.1177...@weatherman.com>...
I've not seen the 130EDT outperform the 130EDF.
I don't know that it will.
The only time I've looked through a 130EDF was at Saturn
using quite low power something under 60x.
In general I perfer the little longer focal ratio (f/8) for planetary
viewing.
Rich
Dave Pisak
TMBack wrote in message
<199805181613...@ladder03.news.aol.com>...
About three weeks ago I bought a new FS102, and the optical quality
is beyond reproach, the star test, near as I can tell, is perfect. As
a
matter of fact, what supprised me the most was how well the image
held up under very high power, it seems to be limited only by light
gathering ability.
I have not had the opportunity to use an older model (FC), so I can't
say how they compare, but a lot more data points would be needed
before one could draw any conclusions one way or the other.
Any FS owners out there wish to comment?
Dave
Paul S. Walsh
My brief experience with a new FS102 reflects Mr. Pisak's opinion. Frankly
I've been knocked out by it's performance and what star testing I've done
does yield what I construe to be text-book results. I am following these
threads with great interest and, weather willing, I should be able to add
more if this thread is still warm. I CAN convey local word-of-mouth from
last year's Table Mt. Star Party to the effect that when pitted against a
5" AP, the Tak 102(FS) had absolutely nothing to apologize for. The
matched EM-10/USD mount is wonderful, as well.
-Paul S. Walsh
AndersonRM
In article <199805181613...@ladder03.news.aol.com>, tmb...@aol.com
(TMBack) writes:
> All of the above statements are true. A Japanese
>amateur that I know explained that Takahashi, because
>of the current economic conditions in Japan, has gone
>to machine figured lenses (to save time and money), and
>they are much poorer in quality control.
This would also back up Terrence Dickinson's observation that
Vixen fluorites are now not as well made as they used to be-if
Vixen is following the same track in mfg. as Takahashi.
-Rich
Rich N.
AndersonRM <ander...@aol.com> wrote in article
<199805182024...@ladder01.news.aol.com>...
It looks as if Vixen has the fluorite element
behind the front element. Not the same placement
as the FS series. But that still doesn't address the
question of lens quality.
Rich
Fred & Julie Burger
I haven't looked through the 130mm f/6 EDF yet, but I have heard that
it is every bit as good as the f/8 EDT. I do own the older f/8 EDT and
it is a fantastic portable lunar and planetary telescope. Isn't the
latest f/8 EDT really a EDF as far as having the same glass and
design, just extended to f/8? If so, it should be quite a scope!
Best Wishes,
Julie :-)
P.S. Thank you for the Takahashi accessories Todd!
On Mon, 18 May 1998 10:47:26 GMT, to...@weatherman.com (Todd Gross)
wrote:
MHarveyWW1
>I've been knocked out by it's performance and what star testing I've done
>does yield what I construe to be text-book results.
Hi Paul...
Perhaps you and Mr. Pisak received exellent examples of the FS scopes. I hope
that you continue to be very happy with them.
My comments are simply the product of my experiences. It may very well be that
someone who has not had the opportunity to use the old FC scopes, and therefore
can't make a direct comparison, would be satisfied with the FS series,
regardless.
Let me simply repeat my hands-on experience -
* the FS scopes, in side by side comparisons with the FC's, showed spurious
color (there was 'zero' in the FC's).
* FS's exhibited image breakdown at high powers (none provided acceptably
crisp and contrasty images at anything over 60-70X to the inch).
* High power images were dimmer in the FS's.
*The images in the FS's were markedly less contrasty at ALL magnifications.
Delicate tonal shadings (Mars and Jupiter) that were easy in the FC's were
totally invisible with the FS's.
In the FC we're talking about simply astounding performance. I once saw an
FC-100 (in excellent seeing) provide images of Jupiter that looked like
etchings , at 150x PER INCH!!! (actual magnification =X600).
Remember that this was not my scope...so I'm not inflating the claim just for
bragging rights<g>.
As for my FC-125, I regularly used well over X600 during Mars oppositions
(seeing permitting, of course) with no observable image breakdown. I would not
expect to achieve this with the FS glass, as it takes near-perfection of the
surfaces to achieve that kind of performance...something machine-figuring is
not likely to produce.
Please don't take my comments for "Takahashi-bashing"...anyone who's been
around this forum long knows that I have been one of Tak's leading proponents.
I'll still champion the FC's but, until I can find some that're up to
traditional Takahashi standards, I must remain less than enthusiastic about the
FS series.
Mike Harvey
Mike Harvey
Rich N.
MHarveyWW1 <mharv...@aol.com> wrote in article
<199805190238...@ladder03.news.aol.com>...
Hi Mike,
It is interesting how experience will differ. My experience is
with my FC-100 and two FS-102s, one FS-128 and one FS-152.
One FS-102 was being run at high power, 100x per inch
of aperture and it did very well. We were checking out
the FS-102 owners new Pentax 5.2mm SMC Xl eyepiece
in his new FS-102 and my old (1993) AP 155mm F/9 EDT.
The second FS-102 while not used at high power was showing
fine contrast when looking at the lunar surface.
If the weather permits maybe I can do a side by side
with my FC-100 and a FS-102.
The FS-128 was giving excellent lunar and planetary detail
and virtually as good as my AP 130EDT. And yes 130EDT
will take 100x per inch with good seeing.
The FS-152 was splitting doubles with a new AP 155EDF.
Both were performing very well. Maybe later this year if
we have his FS-152 and my 155EDF together we can see
how well they perform on planetary detail.
Even my little FS-78 gives very nice images.
From your posting one would think the Tak FS series
telescopes are dogs and that is just not my experience.
I hope the new Tele Vue telescopes are as good as the
Tak. FS series OTA. I would gladly buy another TV
OTA if it will perform as well as its comptitions in a
similar aperture.
Rich
Paul S. Walsh
> Please don't take my comments for "Takahashi-bashing"...anyone who's been
> around this forum long knows that I have been one of Tak's leading
proponents.
> I'll still champion the FC's but, until I can find some that're up to
> traditional Takahashi standards, I must remain less than enthusiastic
about the
> FS series.
>
> Mike Harvey
I take your comments exactly as you present them - an honest expression of
your personal experience. I've had past experiences with scopes of
supposed "Excellent Quality" that caused me to wonder how on EARTH they had
ever passed the Q.C. Department and had to be returned immediately. Even if
all of our discussions to could be reduced to the "A-List of Perfect
Telescope Choices" we'd all still have to be extremely diligent regarding
routine manufacturing processes and changes in Corporate ownership, etc. I
appreciate your input to this discussion. Without canaries, the miners die
a quiet death.
-Paul S. Walsh
Todd Gross
>The five FS's in question were inferior in contrast and image
>brightness...exhibited more spurious color...and suffered noticeable image
>breakdown at very high magnifications.
>As regards AP scopes....none of the FS's have matched the performance of the
>EDF's or EDT's I've had occasion to use.
>The FC's proved to be equal and, in some cases, superior.
>Mike Harvey
>Mike Harvey
Mike.. how were the actual (in/out of focus) star tests on those FSs, and the
FCs. The latest two FS128s have been superb on the out of focus test.
Not the FS78. Also.. the TV85 was not perfect, and I understand the TV101,and
TV140 also were not made with perfect out of focus star tests in mind. Can
you tell me about your out-of-focus star test is on the TV140 considering you
can bring up the scope to 700x with no image breakdown? Would like your
opinion on this please, you never really mentioned it.
Todd Gross
> All of the above statements are true. A Japanese
>amateur that I know explained that Takahashi, because
>of the current economic conditions in Japan, has gone
>to machine figured lenses (to save time and money), and
>a chance of receiving a good TAK FS APO, however the
>older TAK FC series almost always had a great optical
>figure. I tested a recent FS-102, and the image started to
>break down over 160x on Jupiter (seeing excellent). The wavefront was on the
>order of 1/3 to 1/4 wave over-
>corrected, with noticable color aberration on bright stars.
> The older FC series and Astro-Physics EDF/EDT scopes
>can be used at much higher magnifications without image breakdown.
Thomas.. you know I respect you and Mike Harvey very much and I need your
opinion. Putting your comments together with his over teh past year has led to
a slight point of confusion for me.
The TV85 from Al nagler did offer wonderful views, but I never got to test it
on Jupiter. The star test was good, and the outer ring in and out of focus was
matched. However, it was not perfect..I forget the actual flaw, but it was
not a question of under or overcorrection per se. I have heard that the same
is happening on the TV101s and TV140s.. and Al indicates that he goes for
the sharpest image in - focus, and does not dwell on the in/out of focus
pattern. Coincidentally, the outer ring in the star test will then match.
What do you think of this? Mike is very happy, ecstatic with the 140. Isnt'
this contrary though to what you have indicated to me? The FS's that you
viewed through.. did they have imperfect star tests? You didn't mention it.
the two FS128s I've owned, have featured perfect star tests, although there is
some color (slight) on brightest stars inside of focus on this FS128 and the
previous FC100 I have owned (no false color on the moon,or stars in focus)
Can you comment?
Todd Gross
>your personal experience. I've had past experiences with scopes of
>supposed "Excellent Quality" that caused me to wonder how on EARTH they had
>ever passed the Q.C. Department and had to be returned immediately. Even if
>all of our discussions to could be reduced to the "A-List of Perfect
>Telescope Choices" we'd all still have to be extremely diligent regarding
>routine manufacturing processes and changes in Corporate ownership, etc. I
>appreciate your input to this discussion. Without canaries, the miners die
>a quiet death.
>-Paul S. Walsh
well put!
I still have to wonder if there isn't some optical variation scope to scope.
As I mentioned, the latest A/P scopes don't seem to vary much, in terms of
star test, they are all excellent (the traveler I just tested for example)
HOWEVER, even an FC100 I tested was considerably undercorrected (not the
current one) I have to wonder if perhaps being a newbie star tester at the
time, I hadn't allowed it too cool down or something, because from what you
both are saying (tom and mike) you haven't run into a bad FC.
Does anyone know if the current FCT models are hand figured?
Todd Gross
>* the FS scopes, in side by side comparisons with the FC's, showed spurious
>color (there was 'zero' in the FC's).
The latest FC100 that I have is showing color out of focus on brightest stars,
along with the FS128.
> * FS's exhibited image breakdown at high powers (none provided acceptably
>crisp and contrasty images at anything over 60-70X to the inch).
>
> As for my FC-125, I regularly used well over X600 during Mars oppositions
>(seeing permitting, of course) with no observable image breakdown. I would not
>expect to achieve this with the FS glass, as it takes near-perfection of the
>surfaces to achieve that kind of performance...something machine-figuring is
>not likely to produce.
>Please don't take my comments for "Takahashi-bashing"...anyone who's been
>around this forum long knows that I have been one of Tak's leading proponents.
>I'll still champion the FC's but, until I can find some that're up to
>traditional Takahashi standards, I must remain less than enthusiastic about the
>FS series.
Mike, you haven't told us how the out of focus star test was on the FSs. I
have run into a wide variation lately, with a pretty poor FS78 star test
(maybe 1/4 wave) to a near perfect FC50, FC60 (2), and FS128 (2).
In focus, my current FS128 was put up against a new 6" A/P EDF for many weeks,
and the buyer of the 6" A/P was frustrated by the poorer lunar images on the
6" compared to the Tak, however, it could have been cool-down or other issues.
He did not star test the scopes.
I guess what I am asking is whether or not the star tests on those poorer
Taks, supported your in-focus findings. You rarely refer to that kind of star
test, and am curious as to what you think ?
thanks!
MHarveyWW1
>Mike, you haven't told us how the out of focus star test was on the FSs
Todd...
I haven't really done a true in/out focus test on the FS's I've used.
Unfortunately the scopes' owners were always near at hand and my purpose was
not to 'trash' their toys. As I didn't really know these guys, I didn't think
they would appreciate a brutal analysis.
I simply played the role of the owner's observing partner...used the scopes for
as long as possible and traipsed back and forth between the FS and an FC.
Next time I come across one I promise I'll find a way to do the test!
Mike
Mike Harvey
kar...@pacific.net.sg
Hi everybody,
I'm rather interested in what has been discussed here, as I'm a
refractor owner myself.
I'm just curious: when one says one's scope does not suffer from any
image breakdown even at high magnifications, does one mean that more and
more detail is seen as magnification is increased to that given power,
or does it just mean that the image still has acceptable sharpness?
Thanks for the clarification!
Regards,
Qu
Dave Pisak
kar...@pacific.net.sg wrote in message
<356266...@pacific.net.sg>...
Given equal optical quality detail or resolution is
directly proportional to aperture. beyond 40 to 50X
per inch, no more detail can be seen. If the optics
are of high quality, the image beyond 50X / inch
will remain sharp even though the information you
are getting is the same.
Using higher power can make the available detail
easier to see provided it is of high contrast, most low
contrast detail is completely lost at high power (100X/inch).
Dave
Todd Gross
>Todd...
>I haven't really done a true in/out focus test on the FS's I've used.
>Unfortunately the scopes' owners were always near at hand and my purpose was
>not to 'trash' their toys. As I didn't really know these guys, I didn't think
>they would appreciate a brutal analysis.
>I simply played the role of the owner's observing partner...used the scopes for
>as long as possible and traipsed back and forth between the FS and an FC.
>Next time I come across one I promise I'll find a way to do the test!
>Mike
>Mike Harvey
thanks! Because I want to know if you think it would have run hand in hand. My
latest FS128s have been exquisite in star test, but I haven't tested my
current one on Jupiter yet, and you got me a bit nervous!! My star test of an
FS78 did completely support your findings though, as it was 1/4 wave at best.
It was still nice, but I thought it could have done better in focus as well
for it's aperture.
Now, the bonus question.. how does your TV140 test out?? I heard the TV101,
and TV140s were not perfect in their star test, despite great in-focus
performance. My TV85 was not perfect. I have a followup question, but let me
ask you if you have star tested your 140 first. (I assume you have..and if so,
at 5 wavelengths??)
Michael J Edelman
Dave Pisak wrote:
> Given equal optical quality detail or resolution is
> directly proportional to aperture. beyond 40 to 50X
> per inch, no more detail can be seen. If the optics
> are of high quality, the image beyond 50X / inch
> will remain sharp even though the information you
> are getting is the same.
>
> Using higher power can make the available detail
> easier to see provided it is of high contrast, most low
> contrast detail is completely lost at high power (100X/inch).
Although 100x per inch can be useful for splitting doubles.
Keep in mind that these are *not* limitations of optics, but of the eye,
and can vary for different users. At 100x/inch you have an exit pupil of
only 0.5mm, and that's where the difficulty lies.
--
Michael Edelman http://www.mich.com/~mje
Telescope guide: http://www.mich.com/~mje/scope.html
Folding Kayaks: http://www.mich.com/~mje/kayak.html
Airguns: http://www.mich.com/~mje/airguns.html
Phil Harrington
MHarveyWW1 wrote:
> Unfortunately the scopes' owners were always near at hand and my purpose was
> not to 'trash' their toys. As I didn't really know these guys, I didn't think
> they would appreciate a brutal analysis.
> I simply played the role of the owner's observing partner...used the scopes for
> as long as possible and traipsed back and forth between the FS and an FC.
Come on, Mike! Don't get soft on us. :-)
Phil
--
***********************
Phil Harrington
Go ahead...make my day! Visit the Star Ware Home Page
http://ourworld.compuserve.com/homepages/pharrington
TMBack
Hi Todd,
TMBack wrote:
>>All of the above statements are true. A Japanese
>>amateur that I know explained that Takahashi, because
>>of the current economic conditions in Japan, has gone
>>to machine figured lenses (to save time and money), and
>>they are much poorer in quality control. There is always
>>a chance of receiving a good TAK FS APO, however the
>>older TAK FC series almost always had a great optical
>>figure. I tested a recent FS-102, and the image started
>>to break down over 160x on Jupiter (seeing excellent).
>>The wavefront was on the order of 1/3 to 1/4 wave over-
>>corrected, with noticeable color aberration on bright
>>stars.
>> The older FC series and Astro-Physics EDF/EDT scopes
>>can be used at much higher magnifications without image
>>breakdown.
Todd wrote:
>Thomas.. you know I respect you and Mike Harvey very
> much and I need your opinion. Putting your comments
>together with his over the past year has led to a slight point
>of confusion for me.
>The TV85 from Al nagler did offer wonderful views, but I
>never got to test it on Jupiter. The star test was good, and the >outer ring
in and out of focus was matched. However, it was
>not perfect.. I forget the actual flaw, but it was not a question
>of under or overcorrection per se. I have heard that the
>same is happening on the TV101s and TV140s.. and Al
>indicates that he goes for the sharpest image in - focus,
>and does not dwell on the in/out of focus pattern.
>Coincidentally, the outer ring in the star test will then match.
I would be very interested to hear just what is considered
the "best in-focus" image. I can tell you what I think it is. In
a design like Al Nagler's air-spaced Petzval TV-101 and
TV-140, adjusting the air-spaces for the best visual null is
the way to optimize these designs. At focus, I would look
for an Airy disk, imaged at infinity, that had the greatest concentration of
energy in the center, with the faintest first diffraction ring possible. By
doing this, you are increasing
the Strehl ratio, thus increasing the contrast or MTF of the
lens. I wouldn't be surprised if that is just what Al is doing.
If after doing this, the star test is not perfect, then there
are higher order aberrations that are slightly affecting
the inside and outside patterns. Every lens (especially
fast APOs) have some zonal spherical aberration and spherochromatism (the
variation of spherical aberration
with color). These higher order aberrations are not easy
to detect in the star test, but can conspire against a "perfect"
test, even if the null visual wavelength is full corrected.
In a triplet ED design like the Astro-Physics lenses, the
best image is obtained by hand figuring one or both of the
two air-to-glass surfaces, and testing by interferometer.
Roland is able to achieve better than 1/10 wave P-V, 1/50
wave RMS consistently with this method. It is hard and time consuming work.
>What do you think of this? Mike is very happy, ecstatic
>with the 140. Isnt' this contrary though to what you have
>indicated to me? The FS's that you viewed through.. did
>they have imperfect star tests? You didn't mention it.
I think Mike's lens must be outstanding, and would be
very surprised if it didn't star test well. The FS-102 had a
very poor star test for an Apochromat. The outer rings
were quite different: overcorrection. Outside focus the
outer ring was overly hard and sharp, inside of focus, the
outer ring was diffuse and fuzzy. And as I stated, the lens
started to lose it over 160x, on Jupiter. I don't want to
downplay the quality of Takahashi lenses, either the FC
or FS series. I'm sure that the owners that have stated on
s.a.a., that their lenses are superb, are telling the truth.
I'm more concerned that new purchasers of the FS series
TAKs may find unit to unit variations that the FC series
didn't have.
>the two FS128s I've owned, have featured perfect star
>tests, although there is some color (slight) on brightest
>stars inside of focus on this FS128 and the previous FC100
>I have owned (no false color on the moon,or stars in focus)
Todd, I'm going to have to call you on the perfect star
tests that you say you see in so many instruments. I have
used and tested more than 1000 scopes in the last 25+ years -- mostly at star
parties, also at observatories, prototypes,
friends and my own scopes, and I have never seen a
perfect star test, ever! Now I'm not saying you are making
this up. I do believe what you say. But I think you are either
not using the proper technique, or not looking at all the
possible variations in the visible aberrations in the star test.
T. Welford did a study of the star test and found that its
sensitivity for 3rd order aberrations to be 1/20 wave on the wavefront, 1/60
wave for sharp, or 5th order aberrations.
I have never seen an optic with a true quality this high.
And I will add this: If you or I tested a long focus
Apochromat that had a better than 1/20 wave P-V smooth wavefront, it _would_
look perfect in the star test. If there
is an error in the star test, there is an error somewhere
in the optic, period. But just because the star test isn't
perfect (and it never is), doesn't mean the lens or mirror
is bad. It is the type and degree of aberration that is
important.
As for color, in an Apochromat, I can say that all current
available lenses show some slight color error, even the
best corrected ones, like the Astro-Physics, Zeiss and Aries triplets.Normally
no color is seen, however, at very high magnifications (over 70x per inch), on
a bright star like
Vega, a very small trace of violet can be seen in the first diffraction ring,
or in the out-of-focus pattern. Having said
that, this amount of color is really academic, as it doesn't
affect contrast or color fidelity. The Fluorite doublets are
much worse in this regard, and the FS series seems worse
than the FC series, at least in my tests. They both are still
very well color corrected, just not as good as the Fluorite/
Super ED triplets.
Thomas Back
Jeff Richards
Actually, .25mm
Jeff
Ian Turner
In article <199805192046...@ladder03.news.aol.com>,
tmb...@aol.com (TMBack) wrote:
>Hi Todd,
>
>
>TMBack wrote:
> I would be very interested to hear just what is considered
>the "best in-focus" image. I can tell you what I think it is. In
>a design like Al Nagler's air-spaced Petzval TV-101 and
>TV-140, adjusting the air-spaces for the best visual null is
>the way to optimize these designs. At focus, I would look
>for an Airy disk, imaged at infinity, that had the greatest concentration of
>energy in the center, with the faintest first diffraction ring possible. By
>doing this, you are increasing
>the Strehl ratio, thus increasing the contrast or MTF of the
>lens. I wouldn't be surprised if that is just what Al is doing.
>If after doing this, the star test is not perfect, then there
>are higher order aberrations that are slightly affecting
>the inside and outside patterns. Every lens (especially
>fast APOs) have some zonal spherical aberration and spherochromatism (the
>variation of spherical aberration
>with color). These higher order aberrations are not easy
>to detect in the star test, but can conspire against a "perfect"
>test, even if the null visual wavelength is full corrected.
++++++++++++++++++++++++++++++++
Thomas
I'm sure I understand what Todd is speaking of in the TV 85 & I have
personally expierenced it in the TV 101.
I even had Al himself look thru it and spoke with him at length one late
evening. I think you are correct that is exactly what he is doing with the
optics.
One other comment regarding the TV 140 the scope is advertised as a 5.5"
scope yets it's clear aperture is not why would they design it this way?
As always Thomas thanks for the ongoing education!
Ian
--
_____________________________________________________________________
Ian Turner
Replace the xxxxxxxxxx with mindspring
Astrophoto and CCD Imaging Tips
http://www.geocities.com/CapeCanaveral/Hangar/7247/
_____________________________________________________________________
dbledsoe
TMBack wrote
>> I have used and tested more than 1000 scopes in the last 25+ years --
mostly at star parties, also at observatories, prototypes, friends and my
own scopes, and I have never seen a perfect star test, ever! Now I'm not
saying you are making this up. I do believe what you say. But I think you
are either not using the proper technique, or not looking at all the
possible variations in the visible aberrations in the star test.<<
Hello Thomas,
Very well said. I read about so many "perfect" star tests here it leaves me
always wondering if I'm the unlucky person who seems to get lousy scopes. I
have had my fair share of what I consider nice performers but none have ever
had a perfect star test (such as you describe). I currently have an FS78
that I am quite pleased with. Has it star tested perfect? No. But it is
nice of contrast and shows very little color. Lunar observation has
revealed what I consider exceptional detail in an 80mm range scope.
You bring up an important point, that of being conservative in star testing.
Some seem to be hyping the star tests like certain manufacturers hype wave
front errors. No doubt they are sincere in their reports but perhaps they
don't have all the tools necessary to be as accurate as they think. One of
those tools would certainly have to be the experience to know how to
interpret what they see. And before anyone jumps on me I'll say right here
that I too am a long long way from having such experience. As you point
out, perfection is likely out there but it is far and few between.
I always enjoy your writings, very informative, as I have enjoyed for so
many years.
Best, Don
Todd Gross
> Todd, I'm going to have to call you on the perfect star
>tests that you say you see in so many instruments. I have
>used and tested more than 1000 scopes in the last 25+ years -- mostly at star
>parties, also at observatories, prototypes,
Tom, as I told you in email, I use the term perfect too loosely, I mean under
1/8th wave, and indiscernible to me, as a newbie star tester. I've only tested
about 40 or 50 scopes at most, I'm as I stated, nothing close to you in
expertise. If I can't see the error in the refractor, it means it is 1/8th to
1/10th wave or better. I verified that recently with an FC100 which Bob Luffel
tested too. He came up with the same. I've only seen about 7 or 8 in the 40-50
that have hit that category, btw. I currently have a few. These are not
perfect, just damn good.
Todd Gross
>I always enjoy your writings, very informative, as I have enjoyed for so
>many years.
>Best, Don
I agree, Tom has helped me too. I have used perfect too loosely. However, see
my web page for comparisons, and it will all fit into place, I always assume
ppl have read it. As I mentioned, I am referring to what I can discern as
better as 1/8th wave as perfect, I can't see beyond that, being impatient, and
a newbie tester. I have compared enough to have a general idea of what I am
doing, but that is it!!
David Whysong
Michael J Edelman (m...@mich.com) wrote:
: Keep in mind that these are *not* limitations of optics, but of the eye,
: and can vary for different users. At 100x/inch you have an exit pupil of
: only 0.5mm, and that's where the difficulty lies.
Why is this, anyway? What's the problem with small exit pupils (other
than floaters)?
Dave
David Whysong dwhy...@physics.ucsb.edu
Astrophysics graduate student, University of California, Santa Barbara
PGP Key fingerprint = 9D 51 6E CE 5F 38 F0 E7 56 3C 01 45 CF 03 9F 4A
See http://www.physics.ucsb.edu/~dwhysong for my PGP public key.
bratislav
TMBack wrote:
> Every lens (especially
> fast APOs) have some zonal spherical aberration and spherochromatism (the
> variation of spherical aberration
> with color). These higher order aberrations are not easy
> to detect in the star test, but can conspire against a "perfect"
> test, even if the null visual wavelength is full corrected.
A useful indication of how well an APO (or for that matter any
refractor) is corrected at visual wavelengths is star testing
through a filter. OIII is almost perfect line filter, and will
let enough light to allow for an isolated test of residual
sperical aberration. A Wratten #47 is probably good enough for
APOs, as they sport far less color.
If extrafocal test looks perfect with the filter, but
'muddy' without, your lens suffers from spherochromatism, but
it is figured as well as it could be.
But as Thomas said, I'm yet to see the 'perfect' one.
(except for some Newtonians stopped down with fairly small off
axis mask; similar trick works on a large well figured APO;
I guess that would make Thomas' AP one heck of a 4" :-)
Bratislav
JohnLX200
No, actually 0.254 mm if you must be exact. ;-)
There are even 3 significant figures in the 100x to support it, sort of.
John...@aol.com
MAPUG administrator / detail freak (sometimes)
>
>Actually, .25mm
>
>Jeff
TMBack
Bratislav wrote:
>A useful indication of how well an APO (or for that matter any
>refractor) is corrected at visual wavelengths is star testing >through a
filter. OIII is almost perfect line filter, and will let >enough light to allow
for an isolated test of residual sperical >aberration. A Wratten #47 is
probably good enough for APOs,
>as they sport far less color. If extrafocal test looks perfect with >the
filter, but 'muddy' without, your lens suffers from >spherochromatism, but it
is figured as well as it could be.
Good point. One way to eliminate the sphero-chromatic and
chromatic aberration contribution while star testing a refractor is
to use a filter, and the sharper the bandpass the better. A filter centered
around 550nm would be the best, as this is where the eye is most sensitive to
detail. A good compromise would be a Wratten
#58 deep-green or a #11 yellow-green filter.
>But as Thomas said, I'm yet to see the 'perfect' one.(except for >some
Newtonians stopped down with fairly small off axis mask; >similar trick works
on a large well figured APO; I guess that >would make Thomas' AP one heck of a
4" :-)
>Bratislav
Yes, I must correct my statement about a perfect star test. I have
stopped down my 7.1" AP to one inch, and the test was textbook
(we are taking f/64 here!) Also, various high quality Newtonians that I have
stopped down to one inch does the same trick. The
only problem is that is not quite enough aperture. :-)
Thomas Back
TMBack
Ian Turner wrote:
>I even had Al himself look thru it and spoke with
>him at length one late evening. I think you are
>correct that is exactly what he is doing with the
>optics.
Good to hear that I was on the right track.
>One other comment regarding the TV 140 the
>scope is advertised as a 5.5" scope yets it's clear
>aperture is not why would they design it this way?
You are not supposed to know this! :-) The front
elements have a clear aperture of 5.5 inches, but
the two element corrector slighly masks down the
aperture at the focal plane. I really can't say, but I
would ask Al.
>As always Thomas thanks for the ongoing
>education!
Thank you!
Thomas Back
TMBack
Don wrote:
>Very well said. I read about so many "perfect"
>star tests here it leaves me always wondering if
>I'm the unlucky person who seems to get lousy
>scopes.
Hi Don,
Good to hear from you again. I feel the same
as you, as I was the unlucky person with optics,
for more years then I care to remember, until I
bought my Astro-Physics telescope.
>I currently have an FS78 that I am quite pleased
>with. Has it star tested perfect? No. But it is nice
>of contrast and shows very little color. Lunar
>obervation has revealed what I consider ex-
>ceptional detail in an 80mm range scope.
I'm certain your FS-78 is an excellent scope. A
local amateur just purchased a FS-78 and it tests
very well and he is very pleased with it. As I said,
Takahashi still can make superb optics.
>You bring up an important point, that of being
>conservative in star testing. Some seem to be
>hyping the star tests like certain manufacturers
>hype wave front errors. No doubt they are
>sincere in their reports but perhaps they don't
>have all the tools necessary to be as accurate
>as they think. One of those tools would certainly
>have to be the experience to know how to
>interpret what they see.
I think the star test is like any skill. It takes time
and experience to learn all the subtle aspects. A
quick glance never shows all the errors in an
optic. As you say, the tools to help are there, but
even Richard Suiter's book on star testing can be
confusing to some amateurs.
>I always enjoy your writings, very informative,
>as I have enjoyed for so many years.
Thanks Don,
Thomas
TMBack
Todd wrote:
>Tom, as I told you in email, I use the term
>perfect too loosely, I mean under1/8th wave,
>and indiscernible to me, as a newbie star
>tester. I've only tested about 40 or 50 scopes
>at most, I'm as I stated, nothing close to you in
>expertise. If I can't see the error in the refractor,
>it means it is 1/8th to 1/10th wave or better. I
>verified that recently with an FC100 which Bob
>Luffel tested too. He came up with the same.
>I've only seen about 7 or 8 in the 40-50 that
>have hit that category, btw. I currently have a
>few. These are not perfect, just damn good.
Hi Todd,
I understand now, and you are right, seeing
errors under 1/10 wave is a difficult task. It is
something of a challenge for me to see the
smallest errors and make sense of them. It does
take a lot of practice, just like planetary observing.
At the star parties that I go to, I carry my Pentax
3.8mm XP eyepiece, and ask the owner if I could
star test his/her scope.
This way I get an idea of how good the optics are,
ATM and commerical. Then I enjoy the objects
in the night sky, least anyone thinks I just star test
all night (and I do know people who star test more
then they observe!).
7 or 8 out of 40 or so telescopes is quite a crop of
excellent scopes. It is my belief that any smooth optic
1/10 wave P-V or better is superb, and is worthy of
the highest quality observing, for its aperture.
Thomas Back
Todd Gross
Todd wrote:
>>Tom, as I told you in email, I use the term
>>perfect too loosely, I mean under1/8th wave,
>>and indiscernible to me, as a newbie star tester.
Tom wrote:
>Hi Todd,
> I understand now, and you are right, seeing
>errors under 1/10 wave is a difficult task. It is
>something of a challenge for me to see the
Ian, Tom, and all... I've heard from Mike Harvey, btw, and
he confirmed (before he left for a trip) that his star tests on the TV140 are
not identical on either side of focus, he said he'll explain
when he comes back, so it supports our determination that
Al Nagler is forgoing the perfect star test on the 85mm, 101mm, and 140
. Tom, you explained previously what your take was on why this might
be....
Secondly.. in terms of the various scopes that had matching patterns on
either side of focus.. note that 4 of them were tiny aperture, and in
retrospect I think the 2 FC60s were marginal, certainly close to 1/8th, as
it wasn't as bad as 1/4 wave by any means. You pointed out to me that you
weren't surprised the smallest aperture scopes could be finished off more
easily to high quality anyway. Well, anyway, I could detect some slight
error...undercorrection in each case. I also recall now that I had a slight
undercorrection on the first FS128. Tested both on stars and artificial
star. I've heard of overcorrected Takahashis too (including from you), I just
haven't seen any. That leaves only the following scopes..
FC50 (1)
Traveler (1)
FC100(1)
FS128 (1)
90mm Vixen f. (1)
55mm Vixen f. (1)
or 6 scopes that I can't determine the error because it is under 1/8th wave at
the eyepiece. The Pegasus 16" test isn't fair, because seeing was okay,
but not great, and I didn't give a lot of effort to an earlier A/P
Traveler, and a Meade 12.5" that also looked quite good. Sooooo... once again,
yup, you're right, I was throwing "perfect" around much too loosely.
Also..note, Many of these scopes I sought out because the previous owners had
star tested them.. or offered evidence of their quality. This included the
FC100, FS128, and 90mm Vixen fluorite, so this wasn't pure "chance" !!!!
Tom, I've noted that a lot of folks do the star testing at 10 wavelengths or
more..and as you have suggested, I close in on 5 wavelengths to bring out the
more subtle errors. The Brandon 80mm scopes that I test, even the better one,
showed an imperfect test clearly down near 5 wavelengths, it was much harder
to see at 10. When Don sent me the 2nd scope he claimed the star test was
excellent, and yet I noted the flaw right away down towards 5 wavelengths,
after corresponding with you. My point is, I think a lot of folks pull out the
pattern too far, supporting Don's point that ppl through around "perfect"
(myself being guilty too, yes) too frequently.
How this all relates to the original thread is that I have run into only one
relatively speaking not-so-hot FS series scope, one quite good, and one
superb, leaving me to believe that there is variation scope-scope.
btw.. of the APOs, The color correction was the worst on the Takahashis, best
on the Astrophysics.. in that outside of focus, all Takahashis I have tested
of late showed some violet...as you also pointed out.
Todd
ssqu...@hsresources.com
In article <6jt8jg$rqp$1...@news01.micron.net>,
"dbledsoe" <dble...@micron.net> wrote:
> Some seem to be hyping the star tests like certain manufacturers hype wave
> front errors. No doubt they are sincere in their reports but perhaps they
> don't have all the tools necessary to be as accurate as they think. One of
> those tools would certainly have to be the experience to know how to
> interpret what they see. And before anyone jumps on me I'll say right here
> that I too am a long long way from having such experience. As you point
> out, perfection is likely out there but it is far and few between.
One of the most interesting aspects of the whole topic of optical design and
its implementation for me has been the star test. My experience with it is
certainly less than many, but the more I have tested, the more I find I
understand about its intricacies. I recently had the opportunity to make use
of Suiter's software in the evaluation of an MN-61 Mak Newt. For this design
(20% obstruction) Suiter's book was lacking in its displays. The software
truly allows the reproduction of his images and many many more. I was able
to develop a whole book of various abberations to compare against for this
telescope, for various amounts of defocus. While seeing is still my biggest
problem in testing, I feel that this software can make a difference if your
experience is lacking.
To keep this on topic, does anyone know the differences in coatings between
the FC and the FS. The exterior fluorite element is supposed to have a very
hard coating. What is it?
Stew
-----== Posted via Deja News, The Leader in Internet Discussion ==-----
http://www.dejanews.com/ Now offering spam-free web-based newsreading
Michael J Edelman
David Whysong wrote:
> Michael J Edelman (m...@mich.com) wrote:
>
> : Keep in mind that these are *not* limitations of optics, but of the eye,
> : and can vary for different users. At 100x/inch you have an exit pupil of
> : only 0.5mm, and that's where the difficulty lies.
An error of 100%, as one poster noted ;-)
> Why is this, anyway? What's the problem with small exit pupils (other
> than floaters)?
Resolution of the retina.
Martin Tom Brown
> Dave Pisak wrote:
>
> > Given equal optical quality detail or resolution is
> > directly proportional to aperture. beyond 40 to 50X
> > per inch, no more detail can be seen. If the optics
> > are of high quality, the image beyond 50X / inch
> > will remain sharp even though the information you
> > are getting is the same.
This cannot be true. Using a shorter focal length eyepiece
magnifies what is there. The spatial frequencies in the image
are *fundamentally* limited by the diameter of the objective.
Perhaps we are using confusing terminology here...
I take sharp to mean limited only by my eye resolution.
> > Using higher power can make the available detail
> > easier to see provided it is of high contrast, most low
> > contrast detail is completely lost at high power (100X/inch).
>
> Although 100x per inch can be useful for splitting doubles.
Yes - nice high contrast targets.
> Keep in mind that these are *not* limitations of optics, but of the eye,
> and can vary for different users. At 100x/inch you have an exit pupil of
> only 0.5mm, and that's where the difficulty lies.
It is a hard limitation on the optics.
A 10" diameter lens allows 0.46" arc detail resolution
Human eye resolution is around 2' arc - varies with the person.
Once the magnification is such that 0.46" arc corresponds to more
than one cell on your retina the image is smoothly interpolated
between them on all scales shorter than 0.46" arc.
This does mean that an experienced observer can spot a closer double
as being present by a tell-tale non round stellar image, but when you
are at magnifications higher than 30x per inch of aperture the image
is noticeably less sharp. Maybe a bit too smooth is a better phrase.
Most of the time high magnifications are prevented by the atmosphere.
(at least in the UK)
Regards,
--
Martin Brown <mar...@nezumi.demon.co.uk> __ CIS: 71651,470
Scientific Software Consultancy /^,,)__/
Bill Greer
dwhy...@hugh.physics.ucsb.edu (David Whysong) wrote:
>Michael J Edelman (m...@mich.com) wrote:
>
>: Keep in mind that these are *not* limitations of optics, but of the eye,
>: and can vary for different users. At 100x/inch you have an exit pupil of
>: only 0.5mm, and that's where the difficulty lies.
(100x/inch --> 0.254mm exit pupil)
>Why is this, anyway? What's the problem with small exit pupils (other
>than floaters)?
Earlier this year I experimented with aperture stops while observing
Jupiter in the daytime. I used a 130mm APO refractor at 115x and
stopped the aperture down in 10mm increments (or decrements) until I
reached a 10mm clear aperture. The exit pupil ranged from 1.13mm to
0.087mm.
My notes concentrated on Jovian detail until I reached a 30mm CA
(0.261mm exit pupil). At this exit pupil I noted: "crud worse, eye
and eyepiece".
I had rotated the eyepiece to verify my suspicion that some of the
'crud' was not originating within my eye. It rotated with the
eyepiece. The eyepiece was a 7mm Nagler. Perhaps with other eyepiece
designs (or cleaner or dirtier optics) more or less 'crud' would be
visible from that source.
Eye floaters and eyepiece 'crud' was visible at somewhat larger exit
pupils also, but it had become bad enough to note at the 0.261mm exit
pupil. At the smaller exit pupils (0.174 and 0.087mm) this 'crud' was
quite objectionable -- expecially against the brighter daytime
background -- dominating the FOV.
At a 40mm aperture (0.348mm e.p. --> 73x per inch) Jupiter's two most
prominent belts could still be made out in the daytime sky and my
attention was focused on the planet, not on floaters or other 'crud'.
At 60x per inch (my maximum useful magnification base on other
planetary observations, at night, at full aperture, under excellent
seeing), floaters and eyepiece 'crud' are not a problem. Personally,
I see no gains (for planetary observing) in increasing magnification
beyond 60x per inch, but as Michael correctly pointed out; this can
vary for different users.
Bill Greer
Fred & Julie Burger
Wow, you guys are great! Thanks for all of this tremendous advice!
I was offered two trade deals, one involved an older FC-100 and the
other a FS-102. Unfortunately, the trade for the FC-100 didn't work
out and so I will be getting the FS-102. My next big question regards
the amount of color present in this design as mentioned by the
venerable Thomas Back. Is there more color visible in the FS than in
the older FC design? On paper, which design is technically better?
Please forgive me if this next question has already been addressed in
another thread. Has anyone experienced any problems with the fluorite
element being in front like in the newer FS design? Besides storing
the instrument with dessicant to minimize exposure to moisture, are
there any other precautions that I should be taking with such a lens?
Again, many thanks!
Best Wishes,
Julie :-)
On 19 May 1998 20:46:45 GMT, tmb...@aol.com (TMBack) wrote:
...snip...
>
> As for color, in an Apochromat, I can say that all current
>available lenses show some slight color error, even the
>best corrected ones, like the Astro-Physics, Zeiss and Aries triplets.Normally
>no color is seen, however, at very high magnifications (over 70x per inch), on
>a bright star like
>Vega, a very small trace of violet can be seen in the first diffraction ring,
>or in the out-of-focus pattern. Having said
>that, this amount of color is really academic, as it doesn't
>affect contrast or color fidelity. The Fluorite doublets are
> much worse in this regard, and the FS series seems worse
>than the FC series, at least in my tests. They both are still
>very well color corrected, just not as good as the Fluorite/
>Super ED triplets.
>
>
>Thomas Back
>
>
>
>
>
>
f...@sos.net
Wavefront Web Pages - A web site for Maksutov users
http://www.sos.net/~fjb/Q1.htm
Augustine Kim
I have an FC-100N f10 and it shows considerably less color than
my FS-78. At 400X , I can see barely a trace of violet around Vega.
Has anyone done a comparison between the f8 and f10 versions
of FC-100 ?
Augustine
Fred & Julie Burger wrote in message <35635934...@news.nas.com>...
Chuck Gulker
Hi Todd, it's quite clear after reading your last post that star testing is
still very much subject to a person's experience level. Going from
'perfect' to 'marginal' (FC60's) is a big jump in assessment. You also
changed your opinion on the FS128. One adjective seems too little to
accurately describe the performance of a lens. If you have 3 guys of less
than expert level star testing expertise, I would bet you'd get three
different assessments of a given lens. Luckily for the most part, the top
small APO and Flourite refractors all probably perform splendidly in the
field. The skill of the observer will more than make up for the small
differences in optical performance. This is not to say that we shouldn't
strive to purchase perfection. We should just keep in mind that we will
never achieve it and will only drive ourselves nuts in trying to get there
<g>! Now what was the price of the new AP90?
Take care, Chuck
kar...@pacific.net.sg
> > Why is this, anyway? What's the problem with small exit pupils (other
> > than floaters)?
>
What is then, the minimum exit pupil that would not be smaller than the
resolution of the retina?
Thanks!
Qu
Todd Gross
In article <35635934...@news.nas.com> f...@sos.net (Fred & Julie Burger) writes:
>From: f...@sos.net (Fred & Julie Burger)
>Subject: Re: Takahashi FC design vs. FS Design
>Date: Wed, 20 May 1998 22:39:42 GMT
>Wow, you guys are great! Thanks for all of this tremendous advice!
>I was offered two trade deals, one involved an older FC-100 and the
>other a FS-102. Unfortunately, the trade for the FC-100 didn't work
>out and so I will be getting the FS-102. My next big question regards
>the amount of color present in this design as mentioned by the
>venerable Thomas Back. Is there more color visible in the FS than in
>the older FC design? On paper, which design is technically better?
Julie!! Hi!! I specifically tested for color in an FC100 and an FS128
recently. Both showed color (violet) just outside of focus in the star test on
Vega/Deneb. It may have been a bit worse on the 128, but I think it is more
because of the brightness being enhanced. All these doublets are not as true
APO as the triplets, it seems, although I have seen no evidence of false color
on planets, and the moon (although I didn't evaluate it ultra-critically)
Todd Gross
In article <6jvpqm$n3v$1...@news2.i-2000.com> "Augustine Kim" <au...@i-2000.com> writes:
>From: "Augustine Kim" <au...@i-2000.com>
>Subject: Re: Takahashi FC design vs. FS Design
>I have an FC-100N f10 and it shows considerably less color than
>my FS-78. At 400X , I can see barely a trace of violet around Vega.
>Has anyone done a comparison between the f8 and f10 versions
>of FC-100 ?
>Augustine
I'd be interested in further comparisons between the N and the regular. Also
between the triplet FCT and the doublets
Todd Gross
I think it'll be around $2000.00. :)
I know this is off your point but.. I thought again I should explain..Whenever
I get a bit carried away, Tom Back puts me back in line, (like after I
erroneously evaluated a Brandon quickly at 10 wavelengths, and totally
missed an incredibly gross error at 5 wavelengths) so I thought I should
re-check my notes/memories on my star tests...since I was throwing around the
word "perfect" too loosely. I felt it deserved clarification.
That is.. I have become more experienced with time, so if I may consider
something near-perfect earlier , and then remember that I was comparing it to
another scope that was so imperfect that my judgement was skewed. Also..I felt
it important to point out that I am NOT an expert star tester, just one who
has studied Suitors book and put scope up against scope up against scope. . .
since there were those joining the thread that could evaluate scopes beyond
1/8th or 1/10th wave.
And as for the FC60s..I believe I saw a bit more softness on the outside of
focus some of the time, but not all of the time.. so "marginal" was the only
word that came to mind, but they were superb. The FS128 I had at the time was
great, but I think there indeed it was slightly softer on the outside of
focus, very slightly.. but it could have been due to cool-down. The current
FS128 seems identical on either side of focus once cooled, but OTOH I haven't
taken it to very high power yet, or viewed it under 5 wavelengths. ( 5 rings)
Both Vixens I was very careful with.
I really think it is important for people to star test their scopes. I
discovered some really important problems doing so. However, the various
aberrations "mix" and can be very confusing. Certain aberrations such as S.
Aberration and zonal defects can sometimes though be easily determined in just
a minute at the eyepiece, and it is gratifying to know that this can be done
in many cases. I find the Ronchi test very hard to determine anything, but the
extra-focal star test quite revealing, and easier to read on refractors.
You are right that different newbie star testers like myself will come up wtih
different impressions. Bad scopes however are sometimes fairly easy to pick
out. The variable that I have brought up in this thread that is important
though, is the Televue line whihc is not optimizing the star test itself.. as
a few ppl explained, but going solely for in-focus clarity apparently, and
this changes things...
Fred & Julie Burger
Hi Augustine,
You are so lucky, those f/10's are excellent! The one that I
owned was superb and that with the 4" f/12 Nikon ED and the 4" f/10
Zeiss APQ are probably some of the finest 4" refractors made.
The f/10 demonstrated noticably less color than the FC at f/8 (I
am hearing that the FS has a little more color than the FC) and star
tested out at around 1/10th wave (confirmed by the next owner).
Unfortunately, not that many were made and they are extremely
rare and hard to find. You see WTB ads for these from time to time,
but never any for sale. I have been seeing lots of "for sale" ads for
the FS's though.
Congratulations and Clear Skies for your new scope!
Best Wishes,
Julie :-)
On Wed, 20 May 1998 19:45:00 -0400, "Augustine Kim" <au...@i-2000.com>
wrote:
>I have an FC-100N f10 and it shows considerably less color than
>my FS-78. At 400X , I can see barely a trace of violet around Vega.
>Has anyone done a comparison between the f8 and f10 versions
>of FC-100 ?
>
>Augustine
>
Rich N.
Fred & Julie Burger <f...@sos.net> wrote in article
<3564350...@news.nas.com>...
> Hi Augustine,
> You are so lucky, those f/10's are excellent! The one that I
> owned was superb and that with the 4" f/12 Nikon ED and the 4" f/10
> Zeiss APQ are probably some of the finest 4" refractors made.
> The f/10 demonstrated noticably less color than the FC at f/8 (I
> am hearing that the FS has a little more color than the FC) and star
> tested out at around 1/10th wave (confirmed by the next owner).
> Unfortunately, not that many were made and they are extremely
> rare and hard to find. You see WTB ads for these from time to time,
> but never any for sale. I have been seeing lots of "for sale" ads for
> the FS's though.
> Congratulations and Clear Skies for your new scope!
> Best Wishes,
> Julie :-)
Hi Julie,
Didn't the FC-100 f/10 have a smaller, @ 2.4", focuser tube?
Thanks,
Rich
Fred & Julie Burger
Hi Chuck,
I don't know what the price is on the new AP 90mm EDL, but if you
find out would you please tell me?
It looks like it will be a terrific little scope and I would love
to buy one if they're not too expensive. I even have a picture of the
prototype (thanks to Allen Chan) posted on my web site (under
Stellafane).
Best Wishes,
Julie :-)
On Wed, 20 May 1998 19:45:10 -0400, "Chuck Gulker"
<cgu...@columbus.rr.com> wrote:
><g>! Now what was the price of the new AP90?
>
>Take care, Chuck
>
>
>
>
Fred & Julie Burger
Hi Rich,
I remember it being smaller, but I can't remember the actual
size. Maybe Augustine can help us with this information?
Best Wishes,
Julie :-)
On Thu, 21 May 1998 18:01:36 GMT, "Rich N."
<rich.neu...@tandem.com> wrote:
>Hi Julie,
>
>Didn't the FC-100 f/10 have a smaller, @ 2.4", focuser tube?
>
>Thanks,
>Rich
Jeff Medkeff
Probably Mar...@nezumi.demon.co.uk (Martin Tom Brown) said
this:
>Once the magnification is such that 0.46" arc
>corresponds to more than one cell on your retina
>the image is smoothly interpolated between them
>on all scales shorter than 0.46" arc.
I assume by "one cell on your retina" you are referring to
ganglion cells, and not rods or cones.
Right?
--
Jeff Medkeff | Check out the s.a.a. photos page at
Rockland Observatory | http://shutter.vet.ohio-state.edu/saa.htm
Sierra Vista, AZ |
Augustine Kim
Hi Rich,
Yes. My FC-100N f10 has a smaller focuser than does the f8 version.
In fact, it is the same size as the one that comes with FC-76 or FS-78.
Inside diameter of the drawtube is slightly under 2", which prevents
2" diagonal from being fully inserted when used with a 2" adapter.
I am considering having a 2" diagonal modified with a smaller barrel
so that it will slide inside the drawtube of the focuser.
Clear skys,
Augustine
Rich N. wrote in message
<01bd84e3$64893c40$5ee0fc82@Neuschaefer_Rich.mis.tandem.com>...
Rich N.
Augustine Kim <au...@i-2000.com> wrote in article
<6k331s$e0j$1...@news2.i-2000.com>...
> Hi Rich,
> Yes. My FC-100N f10 has a smaller focuser than does the f8 version.
> In fact, it is the same size as the one that comes with FC-76 or FS-78.
> Inside diameter of the drawtube is slightly under 2", which prevents
> 2" diagonal from being fully inserted when used with a 2" adapter.
> I am considering having a 2" diagonal modified with a smaller barrel
> so that it will slide inside the drawtube of the focuser.
>
>
> Clear skys,
>
> Augustine
Thanks very much Augustine!
I saw the f/10 FC-100N at a star party but
it was back around 1990 and I wasn't sure
if my memory was accurate.
Rich
AndersonRM
In article <toddg.1178...@weatherman.com>, to...@weatherman.com (Todd
Gross) writes:
>In focus, my current FS128 was put up against a new 6" A/P EDF for many
>weeks, and the buyer of the 6" A/P was frustrated by the poorer lunar images
on the 6" compared to the Tak, however, it could have been cool-down or other
>issues. He did not star test the scopes.
This is one positive thing about SCTs vrs. apos. If you get a good SCT,
even if a friend's performs somewhat better, you don't feel too bad. However,
if I purchased an apo at $1000 per inch and a friend's identical scope showed
more detail on a planet, i'd be irritated. It would eat at me until I dumped
the
scope and found a better one. I find some of the current discussion of
the performance of TAKs vrs. TAKs vrs. AP's vrs. other AP's to be "smoothed
over" with comments like, "they both performed very nicely." Diplomatic yes,
informative when dealing with multi-thousand dollar scopes, no.
-Rich
AndersonRM
In article <199805191336...@ladder01.news.aol.com>,
mharv...@aol.com (MHarveyWW1) writes:
>I haven't really done a true in/out focus test on the FS's I've used.
>Unfortunately the scopes' owners were always near at hand and my purpose was
>not to 'trash' their toys. As I didn't really know these guys, I didn't think
>they would appreciate a brutal analysis.
Please be candid. It's more interesting since you are discussing scopes
with (hopefully) 1/8th or better wavefronts. Comments like "nice" and
"wonderful" are for criticizing what the hotel on your last vacation was like.
Vagueness is no use.
-Rich
AndersonRM
In article <199805192046...@ladder03.news.aol.com>, tmb...@aol.com
(TMBack) writes:
> I can tell you what I think it is. In
>a design like Al Nagler's air-spaced Petzval TV-101 and
>TV-140, adjusting the air-spaces for the best visual null is
>the way to optimize these designs. At focus, I would look
>for an Airy disk, imaged at infinity, that had the greatest concentration of
>energy in the center, with the faintest first diffraction ring possible. By
>doing this, you are increasing
If the compressor was removed from the TV's, would the front
elements function like a good F12 apo?
-Rich
AndersonRM
In article <356217...@asac.ericsson.se>, bratislav
<epa...@asac.ericsson.se> writes:
>A useful indication of how well an APO (or for that matter any
>refractor) is corrected at visual wavelengths is star testing
>through a filter. OIII is almost perfect line filter, and will
>let enough light to allow for an isolated test of residual
>sperical aberration.
Not really. The optical quality of the glass "sandwich" of those
filters is not that great and can effect image quality. I've used
them as planetary filters out of curiosity and they fall apart to
a certain degree at high power. Just like current glass solar
filters.
-Rich
AndersonRM
In article <toddg.1180...@weatherman.com>, to...@weatherman.com (Todd
Gross) writes:
> I also recall now that I had a slight
>undercorrection on the first FS128. Tested both on stars and artificial
>star. I've heard of overcorrected Takahashis too (including from you), I just
>haven't seen any. That leaves only the following scopes..
According to Peter Ceravolo, telescopes are often undercorrected
intentionally to compensate for continuously falling nighttime
temperatures that cause the figure of the scope to remain
in a perpetual flux.
-Rich
AndersonRM
In article <6jvpu9$i4o$1...@m5.columbus.rr.com>, "Chuck Gulker"
<cgu...@columbus.rr.com> writes:
> If you have 3 guys of less
>than expert level star testing expertise, I would bet you'd get three
>different assessments of a given lens.
Bingo! That explains some contradictions i've seen in posted
test reports.
-Rich
AndersonRM
In article <3562F271...@mich.com>, Michael J Edelman <m...@mich.com>
writes:
>Resolution of the retina.
Can I trade mine in for an Owl or Eagle's? :)
I hear that they have a resolution something like
10x as fine as a human and can see 11th mag
stars.
-Rich
AndersonRM
In article <35632d9c.15907213@news>, verdn...@mcn.net (Bill Greer) writes:
>I see no gains (for planetary observing) in increasing magnification
>beyond 60x per inch, but as Michael correctly pointed out; this can
>vary for different users.
True, but preference depends on your own "comfort" ratio
of magnification to the "softeness" of the view. Some people
like Saturn at 250x in an 8 inch, others like 350x. Neither sees
more detail.
-Rich
David Whysong
: > Why is this, anyway? What's the problem with small exit pupils (other
: > than floaters)?
: Resolution of the retina.
Ok, let me try to figure this out and tell me where I go wrong....
The limit everyone seems to quote is about 50x per inch of aperture, which
corresponds roughly to 2x per millimeter. For my 235mm f/10 SCT, that
makes for a useful magnification limit of about 470x. At this power,
the exit pupil is 0.5 mm. That is, for any incoming plane waves from
a point source (i.e. a star, assuming diffraction from the 'scope is
negligable), the beam of light that enters my eye is still 0.5mm wide.
However, my eye focuses these plane waves essentially to a single
point on my retina. I don't see that diffraction becomes a problem,
since my pupil is large compared to the exit pupil.
It seems to me that resolution of the retina would discriminate between
the angle of incoming plane waves, right? So you would still be limited
to a few arc minutes of resolution in the true field of view of the
eyepiece. If that's true, then magnification should be limited only by
when the diffraction pattern of the telescope becomes intrusive (which
seems to happen at around 470x in my SCT).
So unless I'm confused (which is quite likely), the resolution of the
retina is not directly a factor. Rather, when the line spread function
is roughly equal to the angular resolution of the retina, more power
gains you nothing. But this is really a limit of the telescope, not
the eye.
What am I missing?
Thanks,
Dave
David Whysong dwhy...@physics.ucsb.edu
Astrophysics graduate student, University of California, Santa Barbara
PGP Key fingerprint = 9D 51 6E CE 5F 38 F0 E7 56 3C 01 45 CF 03 9F 4A
See http://www.physics.ucsb.edu/~dwhysong for my PGP public key.
Todd Gross
>According to Peter Ceravolo, telescopes are often undercorrected
>intentionally to compensate for continuously falling nighttime
>temperatures that cause the figure of the scope to remain
>in a perpetual flux.
>-Rich
I believe that would be overcorrected (to adjust for the nightime
undercorrection) ??
Dave & Frani Pisak
Todd Gross wrote in message ...
no, mirrors generally show signs of overcorrection as they cool
down, so perhaps some manufactures leave their mirrors slightly
undercorrected to compensate for this. Whether this applies to
refractor lenses as well, I don't know.
Dave
kar...@pacific.net.sg
Hi David,
I'm interested in what you're are discussing here. However, I realize I
don't even know how an exit pupil forms (i.e. why we see it as it is),
and what determines its size?
Would you mind explaining it to me? Also, what do you mean by
"resolution of the retina would discriminate between the angle of
incoming plane waves", and what is "line spread function"?
Please pardon my apparent ignorance.
Thanks!
Qu
> Ok, let me try to figure this out and tell me where I go wrong....
>
Nils Olof Carlin
David Whysong wrote:
> Ok, let me try to figure this out and tell me where I go wrong....
>
> The limit everyone seems to quote is about 50x per inch of aperture,
which
> corresponds roughly to 2x per millimeter. For my 235mm f/10 SCT, that
> makes for a useful magnification limit of about 470x. At this power,
> the exit pupil is 0.5 mm. That is, for any incoming plane waves from
> a point source (i.e. a star, assuming diffraction from the 'scope is
> negligable), the beam of light that enters my eye is still 0.5mm wide.
> However, my eye focuses these plane waves essentially to a single
> point on my retina. I don't see that diffraction becomes a problem,
> since my pupil is large compared to the exit pupil.
Well, here's how I think about it:
The eye is an imaging device that (optical errors corrected) gives an Airy
disk for each point source on the retina. It has a resolution that is high
in a small area of the field, and here it is about 2 arcminutes for double
stars of not too low or high brightness (for my eyes at least).
The Airy disk size is dependent on the aperture, whether it is the actual
pupil, the exit pupil of an optical magnifier (=telescope), or any small
aperture such as you can make for experiment with pinholes in a piece of
metal (beer can aluminium works fine for me).
The Dawes' limit for a 1 mm aperture is also 2 arcminutes, so with an exit
pupil of 1 mm, the eye and the aperture limit the resolution appr. equally.
With larger exit pupils the eye cannot see the diffraction phenomena (and
you need not have "diffraction limited optics", that's why binoculars give
apparently sharp images), but going to 0.5mm exit pupils or less, the
diffraction is what limits the resolution, and you can see clearly the Airy
disk with its rings (also known as the point spread function!).
With smaller exit pupils the diffraction unsharpness only gets more
obvious, and also the brightness of a surface object decreases. This is why
you have little use for smaller than about 0.7mm for planets, but bright,
high-contrast objects can benefit by 0.5 mm or so. (You may enjoy even
higher powers, but do you really see more of details??)
>
> It seems to me that resolution of the retina would discriminate between
> the angle of incoming plane waves, right? So you would still be limited
> to a few arc minutes of resolution in the true field of view of the
> eyepiece. If that's true, then magnification should be limited only by
> when the diffraction pattern of the telescope becomes intrusive (which
> seems to happen at around 470x in my SCT).
>
> So unless I'm confused (which is quite likely), the resolution of the
> retina is not directly a factor. Rather, when the line spread function
> is roughly equal to the angular resolution of the retina, more power
> gains you nothing. But this is really a limit of the telescope, not
> the eye.
As I have tried to explain, the angular resolution of the retina is the
factor that determines the point of cross-over between retina and telescope
limited viewing as the exit pupil is varied - and for best resolution, it
must be telescope diffraction-limited. If your retina has lower resolution
than average, the cross-over moves to smaller exit pupil, so you can still
enjoy diffraction limiting, but at smaller exit pupils (and less bright
images) than your more fortunate fellows.
Try making pinholes of various diameters, and look at point sources of
light (maybe a pinhole, or two to test Dawes'limit) to see the Airy disks.
If you use sewing needles of different sizes, you can measure the holes by
seeing how deep a needle goes and measure it here with a micrometer).
Peripheral parts of the retina used for averted vision has lower
resolution, so you may use even smaller exit pupils to pick up details of
bright deep-sky objects (small, pright planetary nebulae, globulars).
Nils Olof
Nils Olof Carlin
Qu wrote,
>
> I'm interested in what you're are discussing here. However, I realize I
> don't even know how an exit pupil forms (i.e. why we see it as it is),
> and what determines its size?
If you hold an eyepiece at some distance, you can see distant objects
inverted, and very small. If you have the e.p in a telescope, the distant
thing you see small is the bright aperture - this is the exit pupil.
Its size can be calculated as the aperture divided by the magnification, or
the eyepiece focal length divided by the f/ ratio of the objective.
Nils Olof
RA
As I usually laugh at Mathematics in regards to views thru they
eyepiece, the reality is this in regards to the limits of 10 inches of
aperature. Basing my information on a 10" Schmidt I previously owned.
Typically, 25x per inch of aperature was the average limits to
acceptable viewing during and average night of viewing in Mag 4 skies, as
in backyard city viewing. Planetary images remained as crisp and sharp as
one would hope to see in any telescope of reasonable size. Under better
nights of viewing in these Mag 4 skies, 30x could be obtainable, and
under ideal conditions limited to these mag 4 skies, 35x was obtainable
as in viewing Jupiter or Saturn. I use these two objects because they are
very critical in optical performance. If conditions wouldnt allow for it,
the views of these planets would break down and loose sharpness and
contrass. By saying 35x per inch or 350x, this is the power that the
telescope would obtain and still remain extremely sharp and pleasing
without any breakdown as compared to lower powers. Magnicifation above
this would start to degrade image quality.
Under dark skies of Mag 6.5 or better, which is like my dark viewing
site, magnification limits under average days could hit about 30x per
inch of aperature. As seeing improved, 40x to 45x per inch of aperature
was usable. This meant that again, a view of Saturn would keep getting
larger and remain just as pleasing and sharp as at lower magnifications,
hitting the wall at 45x per inch of 450x using an F10 schmidt.
Under unusually ideal conditons, ( this I have only witnessed 3 times
in the last two years,) 50x to 55x per inch were obtainable. This was
using a 4.8mm nagler with my F10 10" scope or about 550x. Views of
Saturn were magnificant showing definate ring seperations and sharp
shadows of the ring and moons of the planets surface. Jupiters would show
9 bands or more even at 500x. This was extremely rare, but was neverthe
less, possible.
On nights of excellent yet very rare viewing, you'll hear everyones
ooohs and ahhhs. On nights of excellent viewing, I've seen storms within
the great red spot of Jupiter at 1100x using a 7mm nagler with a 2x
barlow. On nights of excellent viewing like this, aperature of 10" or
more simply wallops smaller trendy APO's of any flavor.
Realistically, I'd say to not really hope to use more than 30 to 35x
power per inch of aperature with any reflector type of scope if sharpness
and contrass is what you are looking for in better than average nights of
viewing. Quality refractor can make it to 50 to 55x per inch of
aperature also. My former 6" AP F9 could take 300 to 350x maximum before
quality would degrade under absolute ideal conditions. Thats still about
50x to 55x per inch of aperature. If you want to split dots of light to
see a little space between two stars, then you can just keep pumping up
the power until you run out of barlows.
Ralph
RCK
Thomas,
Could you please elaborate on what you mean by your comment:
"Every lens (especially fast APOs) have some zonal spherical
aberration and spherochromatism (the variation of spherical aberration
with color)"
How are the aberrations detected? What should I see inside focus
and outside focus?
Bob Kuberek
TMBack wrote:
> Hi Todd,
>
>
> TMBack wrote:
>
> >>All of the above statements are true. A Japanese
> >>amateur that I know explained that Takahashi, because
> >>of the current economic conditions in Japan, has gone
> >>to machine figured lenses (to save time and money), and
> >>they are much poorer in quality control. There is always
> >>a chance of receiving a good TAK FS APO, however the
> >>older TAK FC series almost always had a great optical
> >>figure. I tested a recent FS-102, and the image started
> >>to break down over 160x on Jupiter (seeing excellent).
> >>The wavefront was on the order of 1/3 to 1/4 wave over-
> >>corrected, with noticeable color aberration on bright
> >>stars.
>
> >> The older FC series and Astro-Physics EDF/EDT scopes
> >>can be used at much higher magnifications without image
> >>breakdown.
>
> Todd wrote:
>
> >Thomas.. you know I respect you and Mike Harvey very
> > much and I need your opinion. Putting your comments
> >together with his over the past year has led to a slight point
> >of confusion for me.
>
> >The TV85 from Al nagler did offer wonderful views, but I
> >never got to test it on Jupiter. The star test was good, and the >outer ring
> in and out of focus was matched. However, it was
> >not perfect.. I forget the actual flaw, but it was not a question
> >of under or overcorrection per se. I have heard that the
> >same is happening on the TV101s and TV140s.. and Al
> >indicates that he goes for the sharpest image in - focus,
> >and does not dwell on the in/out of focus pattern.
> >Coincidentally, the outer ring in the star test will then match.
>
>
> I would be very interested to hear just what is considered
> the "best in-focus" image. I can tell you what I think it is. In
> a design like Al Nagler's air-spaced Petzval TV-101 and
> TV-140, adjusting the air-spaces for the best visual null is
> the way to optimize these designs. At focus, I would look
> for an Airy disk, imaged at infinity, that had the greatest concentration of
> energy in the center, with the faintest first diffraction ring possible. By
> doing this, you are increasing
> the Strehl ratio, thus increasing the contrast or MTF of the
> lens. I wouldn't be surprised if that is just what Al is doing.
> If after doing this, the star test is not perfect, then there
> are higher order aberrations that are slightly affecting
> the inside and outside patterns. Every lens (especially
> fast APOs) have some zonal spherical aberration and spherochromatism (the
> variation of spherical aberration
> with color). These higher order aberrations are not easy
> to detect in the star test, but can conspire against a "perfect"
> test, even if the null visual wavelength is full corrected.
>
> In a triplet ED design like the Astro-Physics lenses, the
> best image is obtained by hand figuring one or both of the
> two air-to-glass surfaces, and testing by interferometer.
> Roland is able to achieve better than 1/10 wave P-V, 1/50
> wave RMS consistently with this method. It is hard and time consuming work.
>
> >What do you think of this? Mike is very happy, ecstatic
> >with the 140. Isnt' this contrary though to what you have
> >indicated to me? The FS's that you viewed through.. did
> >they have imperfect star tests? You didn't mention it.
>
> I think Mike's lens must be outstanding, and would be
> very surprised if it didn't star test well. The FS-102 had a
> very poor star test for an Apochromat. The outer rings
> were quite different: overcorrection. Outside focus the
> outer ring was overly hard and sharp, inside of focus, the
> outer ring was diffuse and fuzzy. And as I stated, the lens
> started to lose it over 160x, on Jupiter. I don't want to
> downplay the quality of Takahashi lenses, either the FC
> or FS series. I'm sure that the owners that have stated on
> s.a.a., that their lenses are superb, are telling the truth.
> I'm more concerned that new purchasers of the FS series
> TAKs may find unit to unit variations that the FC series
> didn't have.
>
> >the two FS128s I've owned, have featured perfect star
> >tests, although there is some color (slight) on brightest
> >stars inside of focus on this FS128 and the previous FC100
> >I have owned (no false color on the moon,or stars in focus)
>
>
> Todd, I'm going to have to call you on the perfect star
> tests that you say you see in so many instruments. I have
> used and tested more than 1000 scopes in the last 25+ years -- mostly at star
> parties, also at observatories, prototypes,
> friends and my own scopes, and I have never seen a
> perfect star test, ever! Now I'm not saying you are making
> this up. I do believe what you say. But I think you are either
> not using the proper technique, or not looking at all the
> possible variations in the visible aberrations in the star test.
> T. Welford did a study of the star test and found that its
> sensitivity for 3rd order aberrations to be 1/20 wave on the wavefront, 1/60
> wave for sharp, or 5th order aberrations.
> I have never seen an optic with a true quality this high.
> And I will add this: If you or I tested a long focus
> Apochromat that had a better than 1/20 wave P-V smooth wavefront, it _would_
> look perfect in the star test. If there
> is an error in the star test, there is an error somewhere
> in the optic, period. But just because the star test isn't
> perfect (and it never is), doesn't mean the lens or mirror
> is bad. It is the type and degree of aberration that is
> important.
>
> As for color, in an Apochromat, I can say that all current
> available lenses show some slight color error, even the
> best corrected ones, like the Astro-Physics, Zeiss and Aries triplets.Normally
> no color is seen, however, at very high magnifications (over 70x per inch), on
> a bright star like
> Vega, a very small trace of violet can be seen in the first diffraction ring,
> or in the out-of-focus pattern. Having said
> that, this amount of color is really academic, as it doesn't
> affect contrast or color fidelity. The Fluorite doublets are
> much worse in this regard, and the FS series seems worse
> than the FC series, at least in my tests. They both are still
> very well color corrected, just not as good as the Fluorite/
> Super ED triplets.
>
> Thomas Back
TMBack
Bob wrote:
>Thomas,
>Could you please elaborate on what you mean
>by your comment "Every lens (especially fast
>APOs) have some zonal spherical aberration
>and spherochromatism (the variation of spherical
>aberration with color)"
>How are the aberrations detected? What should
>I see inside focus and outside focus?
>Bob Kuberek
All fast refractors have some residual aberrations.
Zonal spherical aberration is spherical aberration
that effects a small "zone" of the optic. A turned
edge is an example of a zonal error. Typically, in
a refractor, a center zone is due to bad polishing
or figuring. A zone at .7 of the aperture or at the
margin (edge) can also be from the design limitations.
These will show up as zones in the star test, just
like a Newtonian (see Suiter's or Taylor's book on
the star test).
Sphero-chromatism is harder to detect. Using sharp
cut-off filters (line filters) or deep color ones (Wratten
number 47, 15, 25), you can detect under or over-
correction in the star test. A well correction lens with spherochromatism will
look overcorrected in the blue
and violet wavelengths (#47 filter) and the lens will
look undercorrected in the red wavelengths (#25
filter).
Thomas Back
bratislav
RA wrote:
> As I usually laugh at Mathematics in regards to views thru they
> eyepiece,
Or you can simply say "I didn't understand".
> Typically, 25x per inch of aperature was the average limits to
> acceptable viewing during and average night of viewing in Mag 4 skies, as
> in backyard city viewing.
...
> Under better
> nights of viewing in these Mag 4 skies, 30x could be obtainable,
...
> Under dark skies of Mag 6.5 or better, which is like my dark viewing
> site, magnification limits under average days could hit about 30x per
> inch of aperature. As seeing improved, 40x to 45x per inch of aperature
> was usable.
Sorry, but even your 'practical notes' hold very little water, if any.
The nights of best seeing are ones with mag 2 or 3 skies, with slight
fog. Good seeing is nothing to do with transparency, in fact it usually
happens when transparency is quite bad. Reason being that fog will
only form if air is exceptionally calm (no convection/air currents),
that is there is no (or very little) temperature gradient. And our
little
friend math says that we need less than 0.1 degree per meter of gradient
to get that elusive wavefront distorted less than a fraction of
wavelength
before enters our eye.
> If you want to split dots of light to
> see a little space between two stars, then you can just keep pumping up
> the power until you run out of barlows.
Yeah, gimme ten stacked TV 5X barlows and a 2.5mm Vixen eyepiece. Then
I'll
split all those pesky spectroscopic binaries !
Bratislav
JosephB41
<<>>
"If the compressor was removed from the TV's, would the front
elements function like a good F12 apo?"
No, since the fluorite element is in that rear group. It probably wouldn't even
be a usable telescope, let alone an apochromat.
Joe Bergeron (JABer...@aol.com)
AndersonRM
In article <toddg.144...@weatherman.com>, to...@weatherman.com (Todd
Gross) writes:
>I believe that would be overcorrected (to adjust for the nightime
>undercorrection) ??
I never really thought about it, I just remember him telling us this
about 5 years ago.
-Rich
AndersonRM
In article <199805252111...@ladder03.news.aol.com>, tmb...@aol.com
(TMBack) writes:
> A well correction lens with spherochromatism will
>look overcorrected in the blue
>and violet wavelengths (#47 filter) and the lens will
>look undercorrected in the red wavelengths (#25
Exact description of my two lesser 94mm Brandons and to a
greater extent, my mid-1980's fast 80mm Vernonscope.
-Rich
AndersonRM
In article <01bd87fa$c5d84260$LocalHost@default>, "Nils Olof Carlin"
<nilsolo...@swipnet.se> writes:
> the
>diffraction is what limits the resolution, and you can see clearly the Airy
>disk with its rings (also known as the point spread function!).
And this manifests itself as edges on edges on lunar or planetary limbs.
-Rich
AndersonRM
In article <356A05...@asac.ericsson.se>, bratislav
<epa...@asac.ericsson.se> writes:
>Sorry, but even your 'practical notes' hold very little water, if any.
>The nights of best seeing are ones with mag 2 or 3 skies, with slight
>fog.
Not always. I've seen transparent nights with seeing as good as
foggy ones and the image contrast isn't degraded by the lack of
transparency.
-Rich
Nils Olof Carlin
Bratislav,
> > If you want to split dots of light to
> > see a little space between two stars, then you can just keep pumping up
> > the power until you run out of barlows.
>
> Yeah, gimme ten stacked TV 5X barlows and a 2.5mm Vixen eyepiece. Then
> I'll
> split all those pesky spectroscopic binaries !
Here's another neat trick - use a Meade ED Apochromatic refractor, that can
"resolve double stars to the theoretical limit with a clearly defined black
space in between", according to the manufacturer ;-)
The ad doesn't mention, but I suspect the theory that comes with the Meade
EDs does not necessarily fit other brands or models, though.
Nils Olof
DBogan3220
OK I just can't contain myself after looking at a few posts how do you
guy's know that that Takahashi is no longer Flourite and is using ED glass for
their FS design. I looked through a 6 inch F8 Tak at RTMC this past weekend at
M-13 and at least this object appeared quite nice, at least as good as in the
AP 155 F9 and F7 that were on the telescope field. Ii makes me wish that
Jupiter was a lot higher this past weekend.
Over this past weekend at RTMC I learned of a Japenese publication that
did an in depth review of the Takahishi FS 128 and the AP 130EDF and the
results were that the AP scope was favored, the Magazine I believe is called "
Skywatcher " and of course being in Japan is printed in Japenese so I was
wondering if anyone out there has access to this publication and can read
Japenese could give this newsgroup a brief rundown on the article.
I won't even comment on Meade refractor's they should not even be
mentioned in this thread. < grin >
Clear Skies
Dwight L Bogan
Paul S. Walsh
DBogan3220 wrote...
> OK I just can't contain myself after looking at a few posts how
do you
> guy's know that that Takahashi is no longer Flourite and is using ED
glass for
> Dwight L Bogan
As I understand it, the FS design merely switched the placement of the
fluorite to the front of the OTA and placed an ED behind it. I searched
for the posting you refer to regarding the elimination of the fluorite
altogether and could not find it. Would you be so kind as to point out
which posting mentions this? If it were true, I imagine a LOT of people
would be keenly interested - including many Attorney's General who would
have a major false Ad-claim case in the brewing.
-Paul S. Walsh
bro...@my-dejanews.com
In article <01bd8864$6529fe80$LocalHost@default>,
"Nils Olof Carlin" <nilsolo...@swipnet.se> wrote:
>
>
> Bratislav,
>
> > > If you want to split dots of light to
> > > see a little space between two stars, then you can just keep pumping up
>
> > > the power until you run out of barlows.
> >
> > I'll
> > split all those pesky spectroscopic binaries !
>
> Here's another neat trick - use a Meade ED Apochromatic refractor, that can
> "resolve double stars to the theoretical limit with a clearly defined black
> space in between", according to the manufacturer ;-)
>
> The ad doesn't mention, but I suspect the theory that comes with the Meade
> EDs does not necessarily fit other brands or models, though.
>
> Nils Olof
>
The other thing that the ad doesn't mention is that, in fact, the Meade
refractors are achromats and are NOT Apochromatic in the generally accepted
definition of the term.
Kevin
-----== Posted via Deja News, The Leader in Internet Discussion ==-----
http://www.dejanews.com/ Now offering spam-free web-based newsreading
Kenneth L. Schwarz
DBogan3220 (dboga...@aol.com) wrote:
: OK I just can't contain myself after looking at a few posts how do you
: guy's know that that Takahashi is no longer Flourite and is using ED glass for
: their FS design. I looked through a 6 inch F8 Tak at RTMC this past weekend at
: M-13 and at least this object appeared quite nice, at least as good as in the
: AP 155 F9 and F7 that were on the telescope field. Ii makes me wish that
: Jupiter was a lot higher this past weekend.
:
: Over this past weekend at RTMC I learned of a Japenese publication that
: did an in depth review of the Takahishi FS 128 and the AP 130EDF and the
: results were that the AP scope was favored, the Magazine I believe is called "
: Skywatcher " and of course being in Japan is printed in Japenese so I was
: wondering if anyone out there has access to this publication and can read
: Japenese could give this newsgroup a brief rundown on the article.
:
: I won't even comment on Meade refractor's they should not even be
: mentioned in this thread. < grin >
: Clear Skies
: Dwight L Bogan
:
AndersonRM
>The other thing that the ad doesn't mention is that, in fact, the Meade
>refractors are achromats and are NOT Apochromatic in the generally accepted
>definition of the term.
>
>
The Meades don't operate like F9 achromats. The colour correction
is superior. Maybe in the same way the Pronto is superior to
an ordinary achromat. Didn't Zeiss make "semi-apos" some time
ago?
-Rich
Kenneth L. Schwarz
DBogan3220 (dboga...@aol.com) wrote:
: Over this past weekend at RTMC I learned of a Japenese publication that
: did an in depth review of the Takahishi FS 128 and the AP 130EDF and the
: results were that the AP scope was favored, the Magazine I believe is called "
: Skywatcher " and of course being in Japan is printed in Japenese so I was
: wondering if anyone out there has access to this publication and can read
: Japenese could give this newsgroup a brief rundown on the article.
Sorry about the empty post a little while ago. Let me try again!
I assume you're referring to the April '98 issue which does review
the AP130EDF, but doesn't compare it to the Tak.
The article is extremely positive. They are thrilled with mechanical
execution and visual performance. The only problem they found was that
when they used the reducer for 6x7 photography, there were red
"ghosts" in the images. They didn't go into detail; the flaw was
mentioned in a caption of a so-so photo taken by the scope of M42.
This image didn't (to my eye) demonstrate the issue.
Has anyone tried the reducer for 6x7 with this OTA? I'm not planning
to buy a 130EDF myself, but I'd be particularly interested if someone
has had success in wide-field photography on 6x7 film using a Traveller
with the reducer.
Thanks,
- Ken