11" Petzval Refractor you can build
Chris1011
The design uses an 11" F15 doublet up front with equal curves on the crown. It
can be used by itself as a long focus planetary scope (it would look similar to
the car mounted 11" that shows up at Riverside). If you build the 5.5" doublet,
also with equal curves, you will have an 11" F7.5 Petzval system.
Specs are as follows:
Crown glass - index 1.522, V = 59.5 K5 or similar glasses.
R1= 61 inches convex, R2 = 61 inches convex, thickness approx 1"
Flint glass - index 1.620, V= 36.4, F2 or similar flint glass
R3 = 61 inches concave, R4 = slightly concave, approx 2500 inches, adjusted for
best color correction. Center thickness approx. 3/4 to 1 inch.
The crown and flint are in contact with a thin spacer, .002" or so, use a
postage stamp cut in 6 pieces.
To make the Petzval system, you need a second lens half way down the focus
(approx. 77 inches from the front curve). The lens needs to be at least 5.5
inches in diameter, 6 inches would be better. Same glasses as above are used,
but radii are now approx. half that of the above design. To get critically flat
field performance, the radii are 34.5", 34.5" biconvex on the crown, 34.5"
concave on the flint, with the rear surface a weak concave of approx. 440
inches. Crown thickness is anywhere from 3/4" to 1". Flint center thickness is
about 1/2 inch, give or take. Spacing between crown and flint is 0.35 inches.
Don't worry if the radii don't come out exact. The design is extremely
forgiving. It will work even if the radii are as much as 1 inch longer or
shorter. You can vary the spacing to null the spherical correction. The color
error won't be affected very much.
Field curvature in this design is around 120 inches, and if you play with the
position of the second lens and increase the rear radius, you can actually
attain a backward field curvature, or null it out completely somewhere in
between. Field coverage will be quite sharp over a 3" to 4" circle. Of course,
the field will not be fully illuminated, as is the case with all Petzval
designs. To get full illumination of a 3" field will require a 9" diameter
second lens. This would be totally impractical for this type of system.
Color correction for this combination is .037% from C to F, or 1 part in 2700.
This is approximately 35% better than an equivalent achromat. The color error
will be noticeable on very bright objects, but will be pretty much absent on
any and all deep sky objects. Based on the views I got at Riverside through the
11" car mounted refractor, the deep sky views would be superb. Based on the
color photographs that a friend of mine took with a smaller fast achromat, deep
sky astrophotos of wide field objects should be quite impressive.
This design can be scaled down quite easily. If you want to build a 9" version,
just scale everything by 9/11, etc. Do we have any gutsy ATMers out there?
PS. Plese don't ask me how to grind, polish or in any way construct these
optics.There are plenty of articles in ATM magazines to help in that regard.
Roland Christen
ASTRO-PHYSICS
Excelsior Optics
How would the omission of the smaller doublet affect the color correction,
if at all? Also, is K7 a suitable candidate for the crown?
Maurizio
Jay Reynolds Freeman
second doublet to the focal point?
--
Jay Reynolds Freeman -- freeman at netcom dot com -- I speak only for myself.
Chris1011
How would the omission of the smaller doublet affect the color correction,
if at all? Also, is K7 a suitable candidate for the crown?
>>
K7 is suitable. You will have to tweak R4 a bit to bring in the color and
perhaps adjust the spacing for null spherical. K7 is very hard to find. Easier
to find is water white plate or opthalmic crown, which have similar specs to
K5. There is tons of that stuff in China. They are using it for almost
everything, 4 to 6" doublets, as well as cheap Newtonian mirrors.
The color correction of the doublet alone is that of an achromat, pure and
simple , i.e. 1 part in 2000 from C to F. At F15, this is actually slightly
less OPD than the Petzval configuration at F7.5
Roland Christen
ASTRO-PHYSICS
TMBack
>With the second doublet in place, what is
>the distance from the second doublet to
>the focal point?
About 38.24".
Thomas Back
TMB Optical
Tim Parker
Excelsior Optics wrote:
> Hi Roland,
>
> How would the omission of the smaller doublet affect the color correction,
> if at all? Also, is K7 a suitable candidate for the crown?
>
> Maurizio
Roland's optical design sounds interesting. But, with all due respect, haven't
we gone way beyond the cost of a long-focus Newtonian of similar aperture with
the need to purchase all that glass? It'd be interesting to try out, though.
Even the achromat version, though I wouldn't even think about transporting a
scope that big around (the Petzval would be nicely portable, though). But then I
wouldn't want to cart an 11" f/8 Newtonian around, either.
I once drew up a design for an 18" f/4.5 Springfield Newt with an eyepiece height
of 6' and that I could offload from the back of my pickup without TOO much
trouble (but probably more than an 11" f/8 Newt!) - too expensive (for me),
though, so I've never built one. Also, the Springfield mount in large apertures
is more suited to other optical configurations, like Cassegrains and their
derivatives (aka Coudes), refractors, and more exotic designs. The Cass
configurations are particularly interesting to me because there's not the
aperture limit like with Newtonians, beyond which the eyepiece is so high off the
ground that it defeats the purpose of the mount - to observe comfortably from a
sitting position (many old large springfields had a seat mounted to the pier, but
it doesn't sound any more fun to me being several feet up on a stool versus
several feet up on a ladder in the dark). The biggest drawback to Springfield
designs is the additional reflections needed to bring the light cone through the
dec and polar axes (on the other hand, most people with SCTs and refractors use a
star diagonal, so Springfields only add one reflection for fixed-eyepiece
comfort). With my 8" Newt, I simply don't use the star diagonal while imaging,
so I'm working at the Newtonian focus (with a slightly larger secondary than
would otherwise be required for an f/6). For the 12.5" Cass I'm building, I will
leave the Cassegrain focus option for imaging (or discriminating planetary
observations) by having a focuser at the back of the tube, and an interchangeable
baffle tube/tertiary mirror to deflect the light cone through the dec and RA axes
for the Springfield.
planetarily,
-Tim.
--
***************************************************************
Dr. Tim Parker
JPL Mail Stop 183-501
4800 Oak Grove Drive
Pasadena, CA 91109-8099
***************************************************************
All opinions herein stated are mine, not those of Caltech, JPL,
or NASA. Trust me.
Bill McHale
: Well, here it is, a simple design for an 11" refractor using inexpensive glass.
: The design uses an 11" F15 doublet up front with equal curves on the crown. It
: can be used by itself as a long focus planetary scope (it would look similar to
: the car mounted 11" that shows up at Riverside). If you build the 5.5" doublet,
: also with equal curves, you will have an 11" F7.5 Petzval system.
: Specs are as follows: <snipped for brevity's sake>
: Color correction for this combination is .037% from C to F, or 1 part in 2700.
: This is approximately 35% better than an equivalent achromat. The color error
: will be noticeable on very bright objects, but will be pretty much absent on
: any and all deep sky objects. Based on the views I got at Riverside through the
: 11" car mounted refractor, the deep sky views would be superb. Based on the
: color photographs that a friend of mine took with a smaller fast achromat, deep
: sky astrophotos of wide field objects should be quite impressive.
: This design can be scaled down quite easily. If you want to build a 9" version,
: just scale everything by 9/11, etc. Do we have any gutsy ATMers out there?
: PS. Plese don't ask me how to grind, polish or in any way construct these
: optics.There are plenty of articles in ATM magazines to help in that regard.
Roland,
Thanks, this is a really cool design. Just a quick question; when you say
that the color error is 35% better than an equivalent achromat, do you
mean an f/7.5 achromat or an f/15 achromat?
Thanks again for participating in this news group : )
--
Bill
***************************************************************************
Artificial Intelligence Is No Match For Natural Stupidity.
---------------------------------------------------------------------------
Home page - http://www.gl.umbc.edu/~wmchal1
***************************************************************************
Bill McHale
: Jay,
:>With the second doublet in place, what is
:>the distance from the second doublet to
:>the focal point?
: About 38.24".
Please Thomas, lets try to be a little more precise with our figures : ).
So we would be talking about an optical tube in the 120" range? Not to
bad for such a big scope. Lets see, if we scale it we would have a 6" in
a 5' tube and a 4" in a 40" tube... not to shabby really.
TMBack
>>120" range? Not to bad for such a big scope. Lets
>>see, if we scale it we would have a 6" in a 5' tube and
>>a 4" in a 40" tube... not to shabby really.
> Yep, you're just about right. I get a total track from R1
>to focus of 166.8402".
Make that 116.8402".
Thomas Back
TMB Optical
Jay Reynolds Freeman
glass-pushing. As a once and sometime mirror-maker, I am intrigued -- I
always wanted to try a refractor -- but, what has kept me from doing so
is the near certainty that in small apertures, anything I might turn out
would be seriously embarrassed by the high-end small refractors I own
(Vixen fluorites, Christen triplets), while in large apertures I would
have to buy a flat-bed truck -- or invest in a set of fins and a
nose-cone -- to transport The Thing to somewhere worth setting it up. I
had not realized that a Petzval design was so simple and tolerant, or I
might have pushed this project beyond the wistful thinking stage.
I do have a couple of questions, however, either for Roland or for
other experts on the group.
(1) What do you recommend for coating, if anything? Specifically,
are there any suppliers willing to low-reflection-coat amateur optics in
onesy-twosy quantity, and is low-reflection coating riskier than
aluminization? I don't know how to think about this one. What are the
trade-offs?
(2) How do you propose we test large refractor optics? Zygos aren't
cheap. Big flats aren't cheap. Waiting for seeing good enough to
star-test, in the 25 cm aperture range, might stretch out the project
time more than many of us would put up with.
I have often wondered if there is some simple correcting lens
arrangement that one could cobble up, that would allow null-testing
refractor optics with a source pinhole at relatively close distance on
one side of the lens, and the knife-edge at the matching image point on
the other side of the lens. If there were a simple rig that one could
make from small, readily available, high-quality lenses, I bet there
would be lots more refractors built.
(3) One of the common reasons for wanting a refractor is planetary
performance, and one of the fixes for the chromatic aberration of a
simple doublet is a moderately narrow-band color filter -- the 36-inch
at Lick has a yellow filter, Wratten something-or-other, in place for
public-night planetary viewing. Is there some simple lens design, other
than the ones Roland has discussed here, that would be particularly
suitable if it were stipulated in advance that it were to be used with
some particular filter, or perhaps with one of several filters?
(I suspect that the answer for one filter is, build the design just
given and figure spherical aberration to zero with the filter in place.
What I am wondering is, whether there is a trade-off between
spherochromatism and longitudinal color, for an instrument to be used
with many different filters in turn, such that you might accept a bit
more color, since you were going to filter it out anyway, in return for
a bit less spherical aberration in each of your proposed filter bands.
Roland and Tom, am I making sense?)
RAnder3127
However, the constructor used two
achromats, one with an F15 focal
ratio (larger) and one with an F8 focal
ratio. The small lens was about 2/3
of the way to the focuser. The whole
scope suffered from severe spherical
aberration.
Is your design similar to that of the
"Neo-achros" from Vixen?
-Rich
"Death warrants have been issued for N'Sync, S-Club 7, The Backstreet Boys,
Britney Spears, Aqua and Christina Aguilera. Anyone caught listening to this
music will be summarily executed."
-The Coalition Against Bad Music.
Chris1011
With the second doublet in place, what is the distance from the
second doublet to the focal point?
>>
Almost exactly half the remainder. It's a giant .5x telecompressor.
Roland Christen
Chris1011
So we would be talking about an optical tube in the 120" range? Not to
bad for such a big scope.>>
Thats assuming you want to stick your eyeball and ocular inside the tube. The
tube distance from front element to the focus point is approximately 116.8233
inches for D light. You gotta leave at least 6.8233 inches of back focus for
2" diagonal etc. Also the focuser takes up about 7". So the tube length is only
103 inches.
Roland Christen
ASTRO-PHYSICS
Chris1011
Roland's optical design sounds interesting. But, with all due respect, haven't
we gone way beyond the cost of a long-focus Newtonian of similar aperture with
Yes we have, however, the glass is not that expensive. Pyrex isn't cheap
either, at least the precision annealed stuff I buy for my test plate material.
And don't you need a secondary mirror for your Newt along with all the spider
katzenjammer to hold it up in the air? Nevertheless, with the secondary lens
you have a flat 3" or greater field with no coma or astigmatism. To get that in
a mirror system requires a Cassegrain with a pretty fancy Hyperbolic primary
mirror with a field flattener at the film plane. This lens can be built
practically without figuring. You can use the polished surfaces of R1, and R2
to check the sphere on R3 by interference and vice versa. R4 can be checked
with a pinhole and only needs to be nulled. No parabolic surface required.
Roland Christen
ASTRO-PHYSICS
Chris1011
Thanks, this is a really cool design. Just a quick question; when you say
that the color error is 35% better than an equivalent achromat, do you
mean an f/7.5 achromat or an f/15 achromat?
Pretty much all 2 element achromats have a color error of 1 part in 2000
(.05%). It is independent of F ratio or diameter. Of course, the effect of that
color error increases with shorter F ratios. This 4 element design has a color
error of 1 part in 2700, or .037%. So do your geometry to see what effect that
has vs an F7.5 and an F15 achromat.
Roland Christen
ASTRO-PHYSICS
Chris1011
"Neo-achros" from Vixen?
-Rich>>
I have no idea what Vixen is doing. This 11" design would be similar (probably)
to the original Televue MPT, however, that scope was F5, I believe.
Roland Christen
ASTRO-PHYSICS
Tim Parker
Chris1011 wrote:
> >>
> Roland's optical design sounds interesting. But, with all due respect, haven't
> we gone way beyond the cost of a long-focus Newtonian of similar aperture with
> the need to purchase all that glass?>.
>
> Yes we have, however, the glass is not that expensive. Pyrex isn't cheap
> either, at least the precision annealed stuff I buy for my test plate material.
> And don't you need a secondary mirror for your Newt along with all the spider
> katzenjammer to hold it up in the air?
Yep.
> Nevertheless, with the secondary lens
> you have a flat 3" or greater field with no coma or astigmatism. To get that in
> a mirror system requires a Cassegrain with a pretty fancy Hyperbolic primary
> mirror with a field flattener at the film plane.
That's the single most important aspect of modern refractors, so far as I'm
concerned. Very important for wide field photography, and maybe CCD imaging, if
you have a truly large chip.
> This lens can be built
> practically without figuring. You can use the polished surfaces of R1, and R2
> to check the sphere on R3 by interference and vice versa. R4 can be checked
> with a pinhole and only needs to be nulled. No parabolic surface required.
I actually might just try that one of these days. ...so long as you'll allow me
to put it on a springfield mount 8^)
planetarily,
-Tim.
***************************************************************
Dr. Tim Parker Phone: (818) 354-2451
JPL Mail Stop 183-501 Fax: (818) 354-0966
Chris1011
(1) What do you recommend for coating, if anything? >>
Don't bother, make it 10% larger and you gain in light what a coating would
give you. The only problem is internal reflection between R2 and R3 if left
uncoated.
>> (2) How do you propose we test large refractor optics?>>
Big flats are not cheap, but a distant reflection off an insulator will work.
Null the curves that are concave by simple pinhole reflection. Then test the
convex curves against the concave by using a fluorescent light and looking at
the interference fringes.
>> I have often wondered if there is some simple correcting lens>>
No, there isn't.
>> (3) One of the common reasons for wanting a refractor is planetary
performance, and one of the fixes for the chromatic aberration of a
simple doublet is a moderately narrow-band color filter >>
This does not fix the chromatic aberration, it just removes the light
wavelengths that are out of focus. With this 11" design, the sphero-chromatism
is very small, not enough for you to see any difference with any color filter.
There is no single lens design that would be practical for use with filters. We
tried that with an 8" F18 singlet and an interference filter. The bandwidth was
still great enough to cause significant image deterioration due to primary
color.
Roland Christen
ASTRO-PHYSICS
TMBack
>Thats assuming you want to stick your eyeball and
>ocular inside the tube. The tube distance from front
>element to the focus point is approximately 116.8233
>inches for D light. You gotta leave at least 6.8233
>inches of back focus for 2" diagonal etc. Also the
>focuser takes up about 7". So the tube length is
>only 103 inches.
That's true, but a 11" refractor needs a decently
long dew cap, so you get fairly close to 116-inches
again (12" dew cap + ~103" = ~115").
Nice design, BTW.
Thomas Back
TMB Optical
Dan Chaffee
>
>There is no single lens design that would be practical for use with filters. We
>tried that with an 8" F18 singlet and an interference filter. The bandwidth was
>still great enough to cause significant image deterioration due to primary
>color.
Forget practical for a minute; I wonder how that filter would do on
my 3" f/80 singlet?
D Chaffee
Dan Chaffee
>
>The crown and flint are in contact with a thin spacer, .002" or so, use a
>postage stamp cut in 6 pieces.
So close and yet so far--too bad you can't oil them. Although
I gather from friends that oiling something that big isn't so easy.
>
>This design can be scaled down quite easily. If you want to build a 9" version,
>just scale everything by 9/11, etc. Do we have any gutsy ATMers out there?
May I assume that making a version of this design that retains
all radii of curvature/spacing, but with a reduced aperture, say
9" would give slightly less residual color than simply scaling down?
Seems to work when other non-apo designs are stopped down.
Anyhow, I'm interested. Unfortunately, it aint gonna be
this year. It would be my best excuse yet to make a sphereometer<g>
Dan Chaffee
Bill McHale
: Roland,
Thats what I had been thinking as well. Still with a removable ore
sliding due shield one could get the transportable length down to under
9'.
: Nice design, BTW.
I agree.
--
Bill
***************************************************************************
It's Your Right To Be Stupid, But It Doesn't Mean You Should Be.
Bill McHale
: I've seen this type of scope in practice.
: However, the constructor used two
: achromats, one with an F15 focal
: ratio (larger) and one with an F8 focal
: ratio. The small lens was about 2/3
: of the way to the focuser. The whole
: scope suffered from severe spherical
: aberration.
: "Neo-achros" from Vixen?
Well I think Roland's scope has different proportions. My 120mm Neo-Achro
had the second doublet about 3/4ths of the way to the focal point so I
would guess the answer is sort of : ). BTW, my scope had decent
correction for spherical abberation.
Bill McHale
:>>Roland,
Ack... don't you hate it when a teacher leaves something as an exercise
for the student? LOL.
Chris1011
Nice design, BTW.
Thomas Back
TMB Optical
>>
Thanks. It is not perfect due to the limitation of having exactly equal curves
on the crown elements. i'm sure that a bit of optimization with Zemax would
result in even better wide field performance at the expense of unequal curves.
Roland Christen
ASTRO-PHYSICS
Chris1011
Anyhow, I'm interested. Unfortunately, it aint gonna be
this year. It would be my best excuse yet to make a sphereometer<g>
This design does not need a spherometer. all you need is a pinhole and
measuring tape. Remember that the convex surfaces are equal to the concave. You
can measure the concave with the pinhole source. Make the convex equal by using
a fluorescent light to see the interference fringes against the concave
surface. The radii are not critical. if you miss by 1 inch, the design will
still work well.
Roland Christen
ASTRO-PHYSICS
Foolmoon
How much would it cost to construct a refractor of this size if you
are doing most of the work yourself? Just a ball park idea would be
fine.
bratis...@my-deja.com
fre...@netcom.com (Jay Reynolds Freeman) wrote:
> (3) One of the common reasons for wanting a refractor is planetary
> performance, and one of the fixes for the chromatic aberration of a
> simple doublet is a moderately narrow-band color filter -- the 36-inch
> at Lick has a yellow filter, Wratten something-or-other, in place for
> public-night planetary viewing. Is there some simple lens design,
other
> than the ones Roland has discussed here, that would be particularly
> suitable if it were stipulated in advance that it were to be used with
> some particular filter, or perhaps with one of several filters?
Check ATMJ #12 where Hamid Mahdy outlines a doublet intended to be used
with yellow filter. By cutting blue and UV and shifting the color
correction curve one can get excellent performance from an ordinary
doublet. Same principle can be applied to this Petzval design (or
any other achromat for that matter).
Bratislav
Sent via Deja.com http://www.deja.com/
Before you buy.
Sue and Alan
Some wise soul once recommended I leave off the dew shield for my 8" f/11.5
<g>. The reasons were that the dew shield slows down the cooling of the
lens and its presence can also give you some dew shield thermal problems
that are slightly detrimental to seeing. A Kendrick heater takes care of
the dew problem.
Leaving off the dew shield has some other benefits. The resulting ota
weighs a bit less and balances a bit lower down (this is really appreciated
with a heavy three element lens). It also reduces the wind loading - a
rather important consideration with a long tube. And it is one less thing
to make.
Clear skies, Alan
Bill McHale wrote in message <8ci0b2$kt17$1...@news.umbc.edu>...
>TMBack <tmb...@aol.com> wrote:
>: Roland,
>
>:>Thats assuming you want to stick your eyeball and
>:>ocular inside the tube. The tube distance from front
>:>element to the focus point is approximately 116.8233
>:>inches for D light. You gotta leave at least 6.8233
>:>inches of back focus for 2" diagonal etc. Also the
>:>focuser takes up about 7". So the tube length is
>:>only 103 inches.
>
>: That's true, but a 11" refractor needs a decently
>: long dew cap, so you get fairly close to 116-inches
>: again (12" dew cap + ~103" = ~115").
>
>Thats what I had been thinking as well. Still with a removable ore
>sliding due shield one could get the transportable length down to under
>9'.
>
>: Nice design, BTW.
>
>I agree.
>
Bob Schmall
Why don't you have the glass done in China to your specs, and market
them under a different name than A-P at considerably lower price? with
that correction and aperture, you'd sell a ton of them in the 6-8 inch
range. And be proud of them as well.
Bob
Make it sound easy, don't I?
On 05 Apr 2000 13:56:29 GMT, chri...@aol.com (Chris1011) wrote:
>Well, here it is, a simple design for an 11" refractor using inexpensive glass.
>
>The design uses an 11" F15 doublet up front with equal curves on the crown. It
>can be used by itself as a long focus planetary scope (it would look similar to
>the car mounted 11" that shows up at Riverside). If you build the 5.5" doublet,
>also with equal curves, you will have an 11" F7.5 Petzval system.
>
>Specs are as follows:
>
>Crown glass - index 1.522, V = 59.5 K5 or similar glasses.
>R1= 61 inches convex, R2 = 61 inches convex, thickness approx 1"
>
>Flint glass - index 1.620, V= 36.4, F2 or similar flint glass
>R3 = 61 inches concave, R4 = slightly concave, approx 2500 inches, adjusted for
>best color correction. Center thickness approx. 3/4 to 1 inch.
>
>The crown and flint are in contact with a thin spacer, .002" or so, use a
>postage stamp cut in 6 pieces.
>
>To make the Petzval system, you need a second lens half way down the focus
>(approx. 77 inches from the front curve). The lens needs to be at least 5.5
>inches in diameter, 6 inches would be better. Same glasses as above are used,
>but radii are now approx. half that of the above design. To get critically flat
>field performance, the radii are 34.5", 34.5" biconvex on the crown, 34.5"
>concave on the flint, with the rear surface a weak concave of approx. 440
>inches. Crown thickness is anywhere from 3/4" to 1". Flint center thickness is
>about 1/2 inch, give or take. Spacing between crown and flint is 0.35 inches.
>Don't worry if the radii don't come out exact. The design is extremely
>forgiving. It will work even if the radii are as much as 1 inch longer or
>shorter. You can vary the spacing to null the spherical correction. The color
>error won't be affected very much.
>
>Field curvature in this design is around 120 inches, and if you play with the
>position of the second lens and increase the rear radius, you can actually
>attain a backward field curvature, or null it out completely somewhere in
>between. Field coverage will be quite sharp over a 3" to 4" circle. Of course,
>the field will not be fully illuminated, as is the case with all Petzval
>designs. To get full illumination of a 3" field will require a 9" diameter
>second lens. This would be totally impractical for this type of system.
>
>Color correction for this combination is .037% from C to F, or 1 part in 2700.
>This is approximately 35% better than an equivalent achromat. The color error
>will be noticeable on very bright objects, but will be pretty much absent on
>any and all deep sky objects. Based on the views I got at Riverside through the
>11" car mounted refractor, the deep sky views would be superb. Based on the
>color photographs that a friend of mine took with a smaller fast achromat, deep
>sky astrophotos of wide field objects should be quite impressive.
>
>This design can be scaled down quite easily. If you want to build a 9" version,
>just scale everything by 9/11, etc. Do we have any gutsy ATMers out there?
>
>PS. Plese don't ask me how to grind, polish or in any way construct these
>optics.There are plenty of articles in ATM magazines to help in that regard.
>
>Roland Christen
>ASTRO-PHYSICS
ZodiacMan
fre...@netcom.com (Jay Reynolds Freeman) wrote:
>
> (3) One of the common reasons for wanting a refractor is planetary
> performance, and one of the fixes for the chromatic aberration of a
> simple doublet is a moderately narrow-band color filter -- the 36-inch
> at Lick has a yellow filter, Wratten something-or-other, in place for
> public-night planetary viewing. Is there some simple lens design,
other
> than the ones Roland has discussed here, that would be particularly
> suitable if it were stipulated in advance that it were to be used with
> some particular filter, or perhaps with one of several filters?
>
> Jay Reynolds Freeman -- freeman at netcom dot com -- I speak only
for myself.
>
Rodger Gordon in TPO, "The Practical Observer", Volume 8 Issue 1
indicated that he got some correction for chromatic aberration and
increased contrast by using GOLD coatings on the star diagonal that he
uses in his achromatic refractors.
I've never tried it as nobody is commercially making gold coated star
diagonals that I know of, so I don't know if it will work. But, I have
been curious about the potential benefits ever since I read the article.
Rodger also indicated that other planetary and lunar observing benefits
could be had by coating the secondary of a reflector with gold.
--
ZodiacMan
ZodiacMan
Yes, it's started now I see from the responses -- the million dollar
question.
When is Roland Christen or Thomas Back going to start pumping these out
or taking orders, and what's going to be the cost to buy one?
Though if you can afford to buy one, you don't need to ask how much it
will cost, right? <grin>
Chris1011
Why don't you have the glass done in China to your specs, >>
Well, I can ask the same thing, why don't you? Maybe you can make a ton of
money with that design. How about it?
Roland Christen
TMBack
>Some wise soul once recommended I leave off the dew
>shield for my 8" f/11.5 <g>
The guy must be an anti-dew shield fanatic. :-)
Still remember the great views of Jupiter at Astro-
fest last year through your 8" AP.
Thanks,
Thomas Back
TMBack
>Check ATMJ #12 where Hamid Mahdy outlines a
>doublet intended to be used with yellow filter.
Actually, it was a OG530 filter (orange) that Hamid
used in the design. One, that I might add, I helped
him devise (among about ten other apo and achromatic
designs for his own use), but for reasons unknown to me,
he never gave me any credit, and stopped our 3 year
correspondence.
Thomas Back
TMB Optical
Sue and Alan
could make themselves. If you want one, I suspect you had better get to
work.
Clear skies, Alan
ZodiacMan wrote in message <8ck67o$h2$1...@nnrp1.deja.com>...
> [SNIP]
>When is Roland Christen or Thomas Back going to start pumping these out
>or taking orders, and what's going to be the cost to buy one?
> [SNIP]
Bill McHale
: Bill and Thomas,
: Some wise soul once recommended I leave off the dew shield for my 8" f/11.5
: <g>. The reasons were that the dew shield slows down the cooling of the
: lens and its presence can also give you some dew shield thermal problems
: that are slightly detrimental to seeing. A Kendrick heater takes care of
: the dew problem.
Doesn't a dew shield also help with baffeling?
: Leaving off the dew shield has some other benefits. The resulting ota
: weighs a bit less and balances a bit lower down (this is really appreciated
: with a heavy three element lens). It also reduces the wind loading - a
: rather important consideration with a long tube. And it is one less thing
: to make.
Well with a Petzval, one doesn't have the same sorts of balance problems,
indeed one might actually want a little extra weight on the front end to
allow a balance point in front of the second doublet.
--
Bill
***************************************************************************
Body By Nautilus. Brain By Mattel
Excelsior Optics
One quick question; will "N/A" cut it for the glass, or will we need "P"?
Maurizio
Frez
> Rodger Gordon in TPO, "The Practical Observer", Volume 8 Issue 1
> indicated that he got some correction for chromatic aberration and
> increased contrast by using GOLD coatings on the star diagonal that he
> uses in his achromatic refractors.
>
> I've never tried it as nobody is commercially making gold coated star
> diagonals that I know of, so I don't know if it will work. But, I have
> been curious about the potential benefits ever since I read the article.
>
> Rodger also indicated that other planetary and lunar observing benefits
> could be had by coating the secondary of a reflector with gold.
>
> --
> ZodiacMan
Hi
Vernonscope seems to still make one. See:
http://www.astroptx.com/BRANDON.htm
Who knows if the Astroptx web site has been
updated in this decade, but it is listed. A bit
pricey too.
Frez
Chris1011
One quick question; will "N/A" cut it for the glass, or will we need "P"?
>>
I'm not sure what you mean by N/A. The glass should be fine annealed and free
of stria. Large plates usually have excellent internal characteristics. Small
chunks that are remolded are the worst.
Roland Christen
Bob Schmall
I've got plenty on my plate now. I already teach three nights a week
besides the day job, the astronomy, the woodworking and this computer
thingy. Besides, I have more money than I could ever dream of
spending...
That was a serious suggestion, BTW. You could do it right and the
hard-core among us would know that it was your stuff.
But then you probably have plenty on your plate also.
Bob
RAnder3127
>used in the design. One, that I might add, I helped
>him devise (among about ten other apo and achromatic
>designs for his own use),
Unless your primary goal was observing
planets, and not in their true colours,
this sounds like a waste of time.
Way too much light loss for deepsky
work.
-Rich
"Death warrants have been issued for N'Sync, S-Club 7, The Backstreet Boys,
Britney Spears, Aqua and Christina Aguilera. Anyone caught listening to this
music will be summarily executed."
-The Coalition Against Bad Music.
ap...@my-deja.com
I get f/13.5 for the 11" doublet.
Can these elements tolerate .001" wedge?
How does modern plate glass compare in homogenity to old glass
used in the great refractors of last century?
Al
Chris1011
Can these elements tolerate .001" wedge?>>
Don't know. I would keep wedge down to half that.
>>How does modern plate glass compare in homogenity to old glass
used in the great refractors of last century?
>>
Tody's plate is quite a bit better.
Roland Christen
jbr...@my-deja.com
The URL is
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=308045066
and the description is
This is a ball sphereometer used for measuring the curvature of a
telescope mirror or lens. Consists of the temperature-stabilized
aluminum sphereometer body 6 1/2" in diameter and precision dial
indicator (NOT a Chinese import !) for direct reading of the curve.
Sells new for over $400.00. Great for "morale building" during long
mirror-grinding sessions as you can easily see the progress you are
making without going to the Focault tester. Radius of sphereometer is 3
inches; can be used on mirrors or lenses 6" or larger in diameter. The
dial indicator itself has one inch of travel, and will measure
sagetta of at least 1/2 inch. The dial indicator is in excellent
condition; the sphereometer body has a couple of small nicks and scrapes
from production-line use, but it is in otherwise very good condition.
Has not been dropped or otherwise abused.
Closes 4/23/00.
Thank you.
Chris1011
The word is spelled spherometer.
Roland Christen
Herm
Herm
Excelsior Optics
Maurizio
Bob May
is $74 for the .001" and $86 for the .0005", a Peacock is $35 and
those are all 1" reading instruments w/secondary dial brand new from
Enco which isn't the cheapest source for those products. I personally
won't spend over $50 for that spherometer because of the prices above.
The ring (it is a ring spherometer rather than a 3 point one) is
easily done on any lathe with the proper stock. It would be better to
find a .25" range dial indicator with .00001" divisions and build the
spherometer from that as you will get better readings.
--
Bob May
Don't subscribe to ACCESS1 for your webserver for the low prices. The
service has
been lousy and has been poor for the last year. Bob May
Bob May
let me tell you that refractor design is something that I am trying to
understand but it's an interesting one to say the least.
I have several pieces of glass (SK10 and F15 in particular) in the 12"
size. Do I need to keep to the same glasses for the second doublet or
can I use something else which will produce a similar secondary
spectrum?
I know that this one is probably going to take some thinking to some
degree but I do have the glass and am interested in the doing of a
nice large refractor to match my 200" refractor so people can see the
difference that modern optics has done for the astronomer.