35mm Panoptic vs. 55mm Plossl for Pronto
haw...@my-dejanews.com
Hello, I have a Pronto with a 27mm Panoptic, but would like to have more
field, should I go with the 35mm Panoptic or the 55mm Plossl? I like the
27mm so much that I lean toward the 35mm Panoptic, but I want the true field
of the 55 Plossl. Does anyone have experience with either of these on the
Pronto?
How about the 40mm Televue Widefield, what would it have for true field?
Thanks for helping
Gary
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Jimrossi94
I have both the 35 Panoptic and the 55 Plossl for my 4" Renaissance. The
differences:
The 55 gives a smaller apparent field of view
The 55 has a lot of eye relief (maybe too much)
The 35 gives a wider field of view
The 35 is huge (maybe a problem for the Pronto?)
The 35 is EXPENSIVE
I started with the 55, and then got the 35 Panoptic, in my scope I find that
the actual field of view is amost indentical, with the 55 noticibly brighter.
The color of individual stars is stunning in the 55.
Given the choice I would choose the 35 Panoptic, the wider apparent field makes
it a pleasure.
Clear Skies,
Jim
Kenneth L. Schwarz
Jimrossi94 (jimro...@aol.com) wrote:
: I have both the 35 Panoptic and the 55 Plossl for my 4" Renaissance. The
: differences:
: The 55 gives a smaller apparent field of view
: The 55 has a lot of eye relief (maybe too much)
: The 35 gives a wider field of view
: The 35 is huge (maybe a problem for the Pronto?)
No problem. I use it with the Panoramic alt-az mount and it
balances if the OTA is pushed forward.
: The 35 is EXPENSIVE
Bevan & Leigh Harris
haw...@my-dejanews.com wrote in article
<6k5vr7$e6i$1...@nnrp1.dejanews.com>...
> Hello, I have a Pronto with a 27mm Panoptic, but would like to have more
> field, should I go with the 35mm Panoptic or the 55mm Plossl? I like the
> 27mm so much that I lean toward the 35mm Panoptic, but I want the true
field
> of the 55 Plossl. Does anyone have experience with either of these on
the
> Pronto?
>
> How about the 40mm Televue Widefield, what would it have for true field?
>
No experience, but here are the relevant figures for you:
FOV Mag Exit Pupil
55mm Plossl 5.8º 9x 8.1mm
40mm Wide-Field 5.5º 12x 5.9mm
35mm Panoptic 5.0º 14x 5.1mm
You could scratch (Oops... bad choice of words!) the Plossl because it will
result in some vignetting in the Pronto and also produces an exit pupil
that is too large to use effectively. Either the Wide-Field or the 35mm
Panoptic would be good choices, but I understand the Wide-Field is
discontinued.
For the sake of comparison, your 27mm Panoptic gives a 3.9º true field, 18x
magnification and a 4.0mm exit pupil.
A question for those who are wiser than I - would the minor vignetting and
the overly large exit pupil of the 55mm Plossl cancel each other out in any
way?
Clear skies,
--
Bevan Harris (Lat 31º52'23.20"S Long 116º02'21.36"E Alt 30m)
NOTE: To obtain true e-mail address, substitute "spam" with "BMH" and
"blackhole" with "bigpond".
Alson Wong
I moderated a panel discussion on eyepiece selection at RTMC on Saturday.
Al Nagler was one of the panelists, and during a discussion of exit pupils,
he noted that with a refractor, there is no problem with using an eyepiece
that gives an exit pupil greater than 7mm: with a refractor, the exit
pupil cannot be overly large. With an 8.1mm exit pupil you are not
"wasting light" because your pupil is already fully illuminated. The
advantage with using exit pupils larger than 7mm is that the true field of
view is larger. With reflecting telescopes, however, large exit pupils
will cause image of the central obstruction to increase, which will result
in "wasting light" because your eye is no longer fully illuminated but is
now seeing the shadow of the central obstruction. A complete discussion of
this topic is at Tele Vue's web page
<http://www.rahul.net/resource/regular/products/tele-vue-optics/pg4.htm>.
Personally, I prefer the 35mm Panoptic to the 55mm Plossl. The 55mm does
have a slightly wider true field of view, but the view in the 35mm is more
impressive because of the greater magnification and larger apparent field
of view. I use the 35mm in my 4" f/9 refractor for a 2.6 degree FOV, which
was nearly enough to see all of the North American Nebula last night.
OTOH, the view in the 35mm vs. the 27mm Panoptic is that much different.
--
Alson Wong
Riverside Astronomical Society
http://www.pe.net/~wpl/ras.html
30th Annual Riverside Telescope Makers Conference, May 22-25
http://home.sprynet.com/sprynet/hrmeyer/rtmchome.htm
Bevan & Leigh Harris <sp...@blackhole.com> wrote in article
<01bd8714$92a933e0$LocalHost@spiff>...
Alson Wong
Alson Wong <aw...@gte.net> wrote in article <6kd9aa$e4v$1...@gte2.gte.net>...
> OTOH, the view in the 35mm vs. the 27mm Panoptic is that much different.
Correction: the view in the 35mm vs. the 27mm Panoptic is NOT that much
Peter Besenbruch
On 25 May 98 04:46:37 GMT, "Bevan & Leigh Harris" <sp...@blackhole.com>
wrote:
>No experience, but here are the relevant figures for you:
>
> FOV Mag Exit Pupil
>55mm Plossl 5.8º 9x 8.1mm
>40mm Wide-Field 5.5º 12x 5.9mm
>35mm Panoptic 5.0º 14x 5.1mm
I have a little experience putting the 40mm Pentax XL on a Pronto. It
is an ideal low power eyepiece for the Pronto.
___________________________________________________
Hawaiian Astronomical Society http://www.hawastsoc.org
HAS Deepsky Atlas http://www.hawastsoc.org/deepsky
Delete the "nobulk." for the true e-mail address.
Bevan & Leigh Harris
> Al Nagler was one of the panelists, and during a discussion of exit
pupils,
> he noted that with a refractor, there is no problem with using an
eyepiece
> that gives an exit pupil greater than 7mm: with a refractor, the exit
> pupil cannot be overly large.
<snipped discussion>
Thanks for the info Alson! I should have figured it myself, but I hadn't
quite realized the effect the secondary had in the equation. <g>
Which would be the better eyepiece then? (I'm assuming that the true field
of the 55mm Plossl will be limited by the baffling in the Pronto, so both
eyepieces would produce a 5.5º). What it comes down to in my mind is the
greater exit pupil of the Plossl vs the wider apparent field of the 40mm
Wide-Field. I would tend to go for then Wide-Field as I would assume the
image size is a little larger. As my own pupils max out at 5mm, would I
benefit any from the significantly greater exit pupil of the Plossl?
> Alson Wong
> Riverside Astronomical Society
> http://www.pe.net/~wpl/ras.html
> 30th Annual Riverside Telescope Makers Conference, May 22-25
> http://home.sprynet.com/sprynet/hrmeyer/rtmchome.htm
>
>
> Bevan & Leigh Harris <sp...@blackhole.com> wrote in article
> >
> > FOV Mag Exit Pupil
> > 55mm Plossl 5.8º 9x 8.1mm
> > 40mm Wide-Field 5.5º 12x 5.9mm
> > 35mm Panoptic 5.0º 14x 5.1mm
>
Ratboy99
Is one eyepiece sharper (altogether or to the edge of field) than the other?
I am trying to make the same decision for my 5" refractor - I would like the
55mm for the 2.56 degree field of view and 7mm exit pupil, but I was wondering
if the Panoptic is going to smoke it in terms of image quality.
rat
Alson Wong
The Pronto is an f/6.8 scope, so the only benefit to you in having an exit
pupil larger than 5mm would be a larger FOV. However, the true FOV with
the 40mm WF is 5.42 degrees vs. 5.73 degrees with the 55mm Plossl, a
difference of only 6% which is not very noticeable. The 40mm WF will
provide a more impressive image because of the higher magnification and
apparent FOV. The 40mm WF is no longer sold by Tele Vue so if you want one
you'll have to get one used. Three alternatives would be the 40mm Pentax
XL, 40mm Meade SWA, and 40mm University Optics Konig MK-70, although I have
not personally used any of them. I use a 35mm Panoptic on my f/9
refractor, which is also used as my low-power eyepiece on my f/4.5 Dob with
a Paracorr.
--
Alson Wong
Riverside Astronomical Society
http://www.pe.net/~wpl/ras.html
30th Annual Riverside Telescope Makers Conference, May 22-25
http://home.sprynet.com/sprynet/hrmeyer/rtmchome.htm
Bevan & Leigh Harris <sp...@blackhole.com> wrote in article
<01bd88a2$b090c8c0$LocalHost@spiff>...
Alson Wong
I have not noticed a dramatic difference in sharpness between the two. If
my calculations are correct (a 5" f/8.4 scope?), the 35mm Panoptic will
give you a 2.22 degree FOV and a 4.2mm exit pupil. The true FOV with the
35mm is slightly smaller but the view should be more impressive because of
the higher magnification and larger apparent FOV. The larger exit pupil
with the 55mm may be helpful on large object of low surface brightness such
as the North American Nebula, M33, and M101, but generally the views with
the 35mm Panoptic are superior.
--
Alson Wong
Riverside Astronomical Society
http://www.pe.net/~wpl/ras.html
30th Annual Riverside Telescope Makers Conference, May 22-25
http://home.sprynet.com/sprynet/hrmeyer/rtmchome.htm
Ratboy99 <ratb...@aol.com> wrote in article
<199805280158...@ladder01.news.aol.com>...
> Is one eyepiece sharper (altogether or to the edge of field) than the
other?
> I am trying to make the same decision for my 5" refractor - I would like
the
> 55mm for the 2.56 degree field of view and 7mm exit pupil, but I was
wondering
> if the Panoptic is going to smoke it in terms of image quality.
>
> rat
>
Mike McJimsey
The Eagle Optics web site (http://www.eagleoptics.com) still lists
prices for all of the widefields, including the 40mm (for $314).
Mike
Alson Wong (aw...@gte.net) wrote:
: The 40mm WF is no longer sold by Tele Vue so if you want one
: you'll have to get one used.
--
+--------------------------------------+
| Mike McJimsey |
| Danville, CA |
| 37.806N 121.936W |
| mike_mcjimsey@#nospam#hp.com |
| To reply, remove #nospam# |
| |
| Sight is a faculty; seeing is an art.|
+--------------------------------------+
Bevan & Leigh Harris
Alson Wong <aw...@gte.net> wrote in article <6kiomt$pqm$1...@gte1.gte.net>...
> The Pronto is an f/6.8 scope, so the only benefit to you in having an
exit
> pupil larger than 5mm would be a larger FOV. However, the true FOV with
> the 40mm WF is 5.42 degrees vs. 5.73 degrees with the 55mm Plossl, a
> difference of only 6% which is not very noticeable. The 40mm WF will
> apparent FOV. The 40mm WF is no longer sold by Tele Vue so if you want
one
> you'll have to get one used. Three alternatives would be the 40mm Pentax
> XL, 40mm Meade SWA, and 40mm University Optics Konig MK-70, although I
have
> not personally used any of them. I use a 35mm Panoptic on my f/9
> refractor, which is also used as my low-power eyepiece on my f/4.5 Dob
with
> a Paracorr.
>
Thanks again Alson. You didn't quite answer my question though, so perhaps
I worded it badly. Thinking it through though, I believe I've answered it
myself. Basically my question was asking what would the effect be if a
given eyepiece nominally delivers a true field larger than the telescope
can produce? In the case of the Pronto this field is 5.5 degrees, while
the 55mm will nominally produce a true field of 5.73 degrees. I would
suggest that the two eyepieces (55mm Plossl and 40mm Wide-Field) would
produce an almost identical true field, but the Plossl would exhibit
softness around the edge due to vignetting. As the nominal true field
increased (lets say the 55mm was a [fictional] Panoptic), this vignetting
effect would worsen. Close enough?
Alson Wong
Bevan & Leigh Harris <sp...@blackhole.com> wrote in article
<01bd8a57$d7c2d620$LocalHost@spiff>...
>
> Thanks again Alson. You didn't quite answer my question though, so
perhaps
> I worded it badly. Thinking it through though, I believe I've answered
it
> myself. Basically my question was asking what would the effect be if a
> given eyepiece nominally delivers a true field larger than the telescope
> can produce? In the case of the Pronto this field is 5.5 degrees, while
> the 55mm will nominally produce a true field of 5.73 degrees. I would
> suggest that the two eyepieces (55mm Plossl and 40mm Wide-Field) would
> produce an almost identical true field, but the Plossl would exhibit
> softness around the edge due to vignetting. As the nominal true field
> increased (lets say the 55mm was a [fictional] Panoptic), this vignetting
> effect would worsen. Close enough?
Yes, I understand your reasoning, but I don't know if you would actually
get any vignetting in the Pronto with the 55mm Plossl, as the Pronto may
have been designed with the 55mm Plossl in mind. I've never used this
combination so I can't answer your question. Your best bet would be to
contact Tele Vue directly.
Mike McJimsey
Bevan,
I think there has been some confusion about how and what determines the
true field of view in a given eyepiece. The actual true field is
determined by the size of the field stop in the eyepiece and the focal
length of the objective. The TV 55mm Plossl has a field stop diameter
of 46mm (which is the largest possible field stop for a 2" eyepiece
because the field stop is actually just the end of the eyepiece barrel
- there is no separate field stop) and the objective focal length is
480mm. To calculate the maximum true field, take half of the field
stop diameter (23mm), divide by the focal length (23/480=0.04792),
take the inverse tangent (arctan(0.0497)=2.743), and then double it
(2.743*2=5.486), which, rounded off to the nearest tenth of a degree,
is the 5.5 degrees that TV specifies as the largest true field of
view for the Pronto. By the way, the 35mm Panoptic has a stop diameter
of 38.7mm, which converts to a true field of 4.6 degrees. The TV website
does not have information for their discontinued Wide-Field series, so
I cannot say for sure what the true field of the 40mm is. You could
call or write Eagle Optics and ask if they see a separate field stop
in the 40mm Wide-Field and if they don't, then you can be sure that
it has the same true field as the 55mm Plossl. My guess is that the
true field fo the 40mm Wide-Field is so close to the that of the 55mm
Plossl that other factors like exit pupil size, magnification, weight
and size would be the determining factors. I believe that the Pronto
is designed so that there is essentially no vignetting over the entire
field of any 2" eyepiece. So you should feel comfortable dropping in any
2" eyepiece and know that you are getting the most performance you can
from that eyepiece and are not being limited by the telescope. I hope
this has been helpful.
Kindest regards and clear skies,
Mike McJimsey
Bevan & Leigh Harris (sp...@blackhole.com) wrote:
: Thanks again Alson. You didn't quite answer my question though, so perhaps
: I worded it badly. Thinking it through though, I believe I've answered it
: myself. Basically my question was asking what would the effect be if a
: given eyepiece nominally delivers a true field larger than the telescope
: can produce? In the case of the Pronto this field is 5.5 degrees, while
: the 55mm will nominally produce a true field of 5.73 degrees. I would
: suggest that the two eyepieces (55mm Plossl and 40mm Wide-Field) would
: produce an almost identical true field, but the Plossl would exhibit
: softness around the edge due to vignetting. As the nominal true field
: increased (lets say the 55mm was a [fictional] Panoptic), this vignetting
: effect would worsen. Close enough?
AndersonRM
I was "given" a 50mm refractor recently as a kind of joke.
It's a typical dept. store job, plastic 0.97 focuser, plastic parts
(except for the aluminum tube) and holes in the middle of the
tube for some long lost alt-az monstrosity.
However, when I tried it out, just for fun, it worked very well!
The objective is air spaced, with two surfaces mag-fluoride coated.
I used a 1-1/4" to 0.97 adapter and decent eyepieces.
The thing shows good contrast (500mm f-10), nice detail
on the moon and not much colour. The diffraction image is
slightly "off" but not by very much. This is proof of one of
the saddest things in astronomy; The dept. store scopes aren't
that bad optically. They are crippled by the awful Japanese sized
toy eyepieces that come with them and the shaky, useless mounts
they have. Put it on a decent camera tripod, use good eyepieces
and voila, a respectable, if not perfect instrument.
-Rich
JT Gill
I have a 60mm with a focal length of 800mm, and it was difficult to
use, but after reworking the mount,and buying a nice 17mm celestron
plossl (.965) and barlow, it performs very nicely, It would probably
be quite nice with a better diagonal. Of course, it wasn't worth its
original asking price, but it isn't a throwaway by any means.
JT
Bevan & Leigh Harris
Thanks Mike!! I've been using the formula TFOV = AFOV * FL(ep) /
FL(scope).
BTW, I don't actually have a Pronto, I'm just interested in how TFOV and
other properties are calculated, and the effect of other obstructions in
the optical path. For instance, in the thread " TV 55mm and Meade 8"SCT",
Brent Peterson <pete...@mail.ptdprolog.net> asks...
> I have a Meade 8" LX50 SCT and I'd like to purchase a TV 55mm eyepiece
for
> the expanded field of view. The question: The 55mm has a 46mm field
stop
> and the opening in the back of the SCT is only 36mm. Will I get the full
> 1.3 degrees FOV that a 46mm field stop should give me?
to which Michael Richmann <rich...@concentric.net> replied ...
> You'll get the FOV. You just won't get complete field illumination.
> A bigger question that hasn't been asked here is will the secondary
> shadow be objectionable and the answer is most likely except under
> good dark conditions. You'd probably be better off to consider a 35 mm
> Panoptic as has been discussed in many a thread...
Judging from what you've said, I would have thought the field would be
reduced to around 1.0 degrees because the rear aperture of the scope would
act as an artificial field stop. I'll concede that this wouldn't be a
sharp field stop, as it is some way forward of the focus point, but I doubt
if it would be far enough away that it is totally out of focus (causing an
overall loss of field illumination). Perhaps I'd be better to describe it
as severe vignetting.
I know that these issues have been discussed ad infinitum in this ng, but
could someone give me a pointer to where I could find this information?
Anyway, I'll go off and play with a few figures to compare the two AFOV
formulae.
Thanks!!!
Mike McJimsey wrote in article ...
> Bevan,
>
<snip>
> 480mm. To calculate the maximum true field, take half of the field
> stop diameter (23mm), divide by the focal length (23/480=0.04792),
> take the inverse tangent (arctan(0.0497)=2.743), and then double it
> (2.743*2=5.486), which, rounded off to the nearest tenth of a degree,
> is the 5.5 degrees that TV specifies as the largest true field of
> view for the Pronto. By the way, the 35mm Panoptic has a stop diameter
> of 38.7mm, which converts to a true field of 4.6 degrees. The TV website
<snip>
> and size would be the determining factors. I believe that the Pronto
> is designed so that there is essentially no vignetting over the entire
> field of any 2" eyepiece. So you should feel comfortable dropping in any
> 2" eyepiece and know that you are getting the most performance you can
> from that eyepiece and are not being limited by the telescope. I hope
> this has been helpful.
>
Clear skies,
Mike McJimsey
Bevan,
I would highly recommend purchasing the book "Telescope Optics, Evaluation
and Design" by Harrie Rutten and Martin van Venrooij (ISBN 0-943396-18-2)
published by Willmann-Bell. There is a whole chapter dedicated to stops
and vignetting. In the book, they analyze a 200mm Schmidt-Cassegrain
design (among others) for vignetting. It turns out that there are no
less than 15 different places that could potentially cause vignetting
in a Schmidt-Cassegrain. They include a graph which shows the relative
illumination in the focal plane for five different configurations (standard
cylindrical baffle tube with and without 5% oversized primary, widened
baffle tube with and without oversized primary, and a conical baffle tube
instead of a cylindrical one). In the first four cases, illumination
drops to zero at about a 28mm distance off axis and at about 38mm for
the fifth case. At 23mm off axis (which corresponds to the edge of
the 46mm stop diameter of the 55mm Plossl), the illumination has
dropped to 30-50%, depending on the which case is considered.
Therefore, Michael Richmann was correct in saying that the entire
field of the 55mm Plossl will be illuminated, but only to about 40%
of the on-axis illumination level. The reason that the telescope
can illuminate a 46mm field through the 38mm hole in the back of the
telescope is that the focal plane is located approximately 100mm from
the back of the telescope. Oblique rays that just reach 23mm off axis
are coming from the secondary mirror on the opposite side of the axis
at enough of an angle to reach 23mm off axis after just clearing the
38mm hole. I hope I haven't confused you too much; it's much easier
to understand when you have the book in front of you. So I would
concur with your description of the situation as severe vignetting.
Kindest regards and clear skies,
Mike McJimsey
Bevan & Leigh Harris (sp...@blackhole.com) wrote:
: Michael Richmann <rich...@concentric.net> replied ...
: > You'll get the FOV. You just won't get complete field illumination.
: > A bigger question that hasn't been asked here is will the secondary
: > shadow be objectionable and the answer is most likely except under
: > good dark conditions. You'd probably be better off to consider a 35 mm
: > Panoptic as has been discussed in many a thread...
: Judging from what you've said, I would have thought the field would be
: reduced to around 1.0 degrees because the rear aperture of the scope would
: act as an artificial field stop. I'll concede that this wouldn't be a
: sharp field stop, as it is some way forward of the focus point, but I doubt
: if it would be far enough away that it is totally out of focus (causing an
: overall loss of field illumination). Perhaps I'd be better to describe it
: as severe vignetting.
Bevan & Leigh Harris
Hi Mike,
Mike McJimsey wrote in article ...
> I would highly recommend purchasing the book "Telescope Optics,
Evaluation
> and Design" by Harrie Rutten and Martin van Venrooij (ISBN 0-943396-18-2)
> published by Willmann-Bell.
I've been eyeing this title for a while now. It's now firmly on my
acquisitions list, though probably not at the top of it.
<snip>
> I hope I haven't confused you too much; it's much easier
> to understand when you have the book in front of you. So I would
> concur with your description of the situation as severe vignetting.
>
I get the basic idea of what you're saying. I'll read over it again [and
again] - as well as do a few pencil scratchings - which should help me to
understand it more fully.
Thanks!!