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spaceydee
Postmaster
Reged: 04/16/04
Posts: 15124
Loc: Where the Kittens Are
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I recently purchased a TMB92SS, which is an f5.5. Was wondering what eyepiece people would recommend to perform the startest??
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Dee
space-scientist
student violinist
Nexstar8i,SV80S,80/9D,FC100,94 Brandon,TMB92SS,GM8
8" f/7 Discovery,12.5" Portaball, PST
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Rick
Carpal Tunnel
Reged: 04/12/05
Posts: 2511
Loc: Tokyo, Japan
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You will need a focal length that will give you around 185x, or ~2.5mm, so that will limit your choices. Are you unhappy with the images?
clear skies,
Rick
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www.japanastro.com
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NeoDinian
Experienced Postmaster
Reged: 10/05/05
Posts: 11783
Loc: Rockford Illinois
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You can always Barlow another EP. This will help with the Exit-Pupil and Eye-Relief...
Basically, you want as much magnification as your skies will allow. The more the better.
--------------------
Neo... (Jeff)
-=-=-=-=-=-=-
10" LX200-GPS/SMT UHTC "Draco"
 Rockford, Il.
NeoDinian's Eye on the Sky!
Coming soon:
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spaceydee
Postmaster
Reged: 04/16/04
Posts: 15124
Loc: Where the Kittens Are
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no, I'm not unhappy with the images, just curious what people would suggest.
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Dee
space-scientist
student violinist
Nexstar8i,SV80S,80/9D,FC100,94 Brandon,TMB92SS,GM8
8" f/7 Discovery,12.5" Portaball, PST
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ausastronomer
scholastic sledgehammer
Reged: 06/30/03
Posts: 756
Loc: NSW Central Coast (Australia)
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Hi Dee,
The philosophy on this for many decades was that you needed "at least 50X per inch of aperture", to properly star test a telescope. Some of the earlier posts in this thread allude to that.
That theory has somewhat gone out the window in recent years. Suiter indicates in his book "Star Testing Astronomical Telescopes" that lower powers can be used.
In my experience 25X to 30X per inch of aperture is more than adequate, which in your case is about 110X, which equates to about a 5mm eyepiece. Keep in mind however that the higher you can go under the prevailing conditions, the better.
Consider this. In recent years "amateur" telescope apertures have increased dramatically. If the 50X per inch criteria were to be followed, these telescopes would need to be star tested at ridiculously high powers, rarely attainable under normally encountered seeing conditions.
Lets take that a step further. Steve Kennedy who is rapidly earning a well deserved reputation as the worlds' premium manufacturer of large fast optics, Star Tests every single optic he produces to aid in final figuring and to ascertain it's quality. If he stuck to the 50X per inch of aperture criteria, he would get about 1 mirror out the door every 3 years or so. Simply because he would need to reach 1100X to star test his smallest mirror, which is 22" at 50X per inch of aperture. Seeing conditions might allow him to do that once every 3 years or so, if he was lucky.
Cheers
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John Bambury
AS of NSW
AS of Hunter Valley
18"/F4.5 Obsession with Argo Navis and Servocat, 10"/F5 Newtonian with Argo Navis
Lots of Pentax XW's. Plus 12mm & 17mm Nagler T4's, 31mm Nagler T5 & 13mm ETHOS. TV Paracorr and 2.5X Powermate
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bratislav
super member
Reged: 09/07/06
Posts: 127
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Quote:
I recently purchased a TMB92SS, which is an f5.5. Was wondering what eyepiece people would recommend to perform the startest??
I would suggest buying a cheap webcam and learning to use Roddier test.
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spaceydee
Postmaster
Reged: 04/16/04
Posts: 15124
Loc: Where the Kittens Are
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I was wondering if any of the eyepieces that I have are "good" enough for a fast scope (to star test) or if I have an excuse to get a new one. Seriously though, I've got a 3-6mm Nagler Zoom , 4-6mm TMB supermonos and 4,6mm Celestron plossls (e.p. kit). I'm sure the NZ is the best of the bunch for doing a star test but am not sure if I shouldn't be considering a regular Nagler?
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Dee
space-scientist
student violinist
Nexstar8i,SV80S,80/9D,FC100,94 Brandon,TMB92SS,GM8
8" f/7 Discovery,12.5" Portaball, PST
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ausastronomer
scholastic sledgehammer
Reged: 06/30/03
Posts: 756
Loc: NSW Central Coast (Australia)
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Hi Dee,
The TMB Supermonos are easily the best choice. Use the shortest focal length of the 3 that seeing will allow. The idea is to use the eyepiece with the best on axis performance and introducing the least on axis aberrations. In this regard the TMB supermonos are a better choice than the Nagler Zoom, or a fixed focal length Nagler for that matter, if you have a driven mount. I assume you have it on your GM8? Keep in mind that those eyepieces would also do a fine job.
Cheers,
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John Bambury
AS of NSW
AS of Hunter Valley
18"/F4.5 Obsession with Argo Navis and Servocat, 10"/F5 Newtonian with Argo Navis
Lots of Pentax XW's. Plus 12mm & 17mm Nagler T4's, 31mm Nagler T5 & 13mm ETHOS. TV Paracorr and 2.5X Powermate
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Jan Owen
super member
Reged: 02/12/06
Posts: 197
Loc: Sun City West, Arizona
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With that scope, I'd go with something VERY close to a 7mm *something*...
My choice would be a 7mm ortho... Your choice of manufacturer or model number will probably do (but a UO volcano top 7mm will do nicely, or an old Meade 7mm Ortho, like mine, which is AT LEAST as good, will do as well, or even better)...
A 7mm eyepiece will give you close to a 1mm exit pupil, and that's about where you want to be in star testing...
Have fun!!!
Jan
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Keep on looking up...
Meade 60mm f/15 achro refractor from WAY back...
Vernonscope 94mm f/7 Brandon triplet APO
Orion EON ED 120mm f/7.5
Celestron 5.5" f/3.6 Comet Catcher Schmidt-Newt
8" f/6 Newtonian w/Spooner optics on Atlas G mount
Meade 10" f/10 SCT
12" f/5 Meade LightBridge Dob/Newt
13.1" f/4.5 Coulter Odyssey early Dob/Newt
Many eyepieces and accessories
Always lusting after large MCT, but never buy one
Edited by Jan Owen (08/01/08 06:37 PM)
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hoof
Pooh-Bah
Reged: 04/07/05
Posts: 1403
Loc: San Rafael, CA
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I typically use my 4mm TMB SuperMono. You want the maximum power, with the best on-axis performance.
One of the complications is the fact that eyepieces themselves often have error as well. You could have a perfect mirror/lens cell, throw in an eyepiece that introduces 1/3rd wave spherical error, and come to the false conclusion that the telescope itself has a problem. Of course, it's usually much harder to spot problems in eyepieces, usually you have to do a "one of these eyepieces is not like the other" approach, where the star-test yields good results with all but the suspect eyepiece.
TMB Supermonos should do well in the star test because they're designed and built for maximum on-axis contrast, which requires minimizing any inherant eyepiece-introduced abberation. UO Orthos, or any other really-good-contrast eyepieces should work as well.
I don't recommend barlows, because the barlow itself might introduce error. Keep it simple, to make it easier to isolate the telescope itself.
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Jonathan Hoof
15" F/4.14 Discovery Truss
8" F/5.9 Orion XT8i
6" F/6 Intes-Micro MN66
4.5" F/4 Orion Starblast
80mm F/7.5 Orion 80ED
18x50 IS Canon binoculars
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Jan Owen
super member
Reged: 02/12/06
Posts: 197
Loc: Sun City West, Arizona
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OOPS!!!
I sailed right by a solution you already own...
The 92mm F/5.5 and your TV 3-6mm Zoom, set at 5.5mm... That's a very good eyepiece, which you already have, AND it delivers a 1mm exit pupil, when set at 5.5mm. A 1mm exit pupil is about the ideal for star testing...
The zoom has more glass than an ortho or monocentric, but it can still be VERY good...
In a nutshell, go for a 1mm exit pupil. Don't use a diagonal or a Barlow (use extensions if necessary to reach focus). If you choose not to try the 3-6 zoom, use a single focal length eyepiece that is both of high quality, few lens elements, few air to glass surfaces, which delivers as close to a 1mm exit pupil as you can get (which, by definition, is 5.5mm eyepiece, or get as close on either side as you can get)... So, if you've got the 5 and 6mm supermonos AND the 3-6 zoom, you can try all THREE, and see if one works better than the others... In that group, they should all work GREAT...
Jan
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Keep on looking up...
Meade 60mm f/15 achro refractor from WAY back...
Vernonscope 94mm f/7 Brandon triplet APO
Orion EON ED 120mm f/7.5
Celestron 5.5" f/3.6 Comet Catcher Schmidt-Newt
8" f/6 Newtonian w/Spooner optics on Atlas G mount
Meade 10" f/10 SCT
12" f/5 Meade LightBridge Dob/Newt
13.1" f/4.5 Coulter Odyssey early Dob/Newt
Many eyepieces and accessories
Always lusting after large MCT, but never buy one
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Granger
member
Reged: 03/25/07
Posts: 19
Loc: SE Missouri
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The Eyepiece you use for star-testing isn't that critical. The main thing is to use very high power - 0.5mm exit pupil or less (50+ per inch). Preferably more. At this high power the telescopes limits should overwhelm any limits of the eyepiece.
You can generally see the limits of the telescope (diffraction rings and airy disk) at around 1-2mm depending on your vision. By making them much larger they become very obvious and detailed.
Personally I use a 2.5mm BO-planetary. But most modern eyepieces are pretty good quality so just use a barlow on whatever you have.
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C8 F/10 on Super Polaris
80mm ED F/6.8 on Porta
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suburbanskies
super member
Reged: 12/18/04
Posts: 196
Loc: New Jersey, USA
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With a fast f/5.5 scope, you do not want to use an ortho if you really want to do a serious startest. Orthos will introduce some on-axis spherical aberration at such a fast f-ratio. A Nagler would be a safe choice.
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ausastronomer
scholastic sledgehammer
Reged: 06/30/03
Posts: 756
Loc: NSW Central Coast (Australia)
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Quote:
OOPS!!!
I sailed right by a solution you already own...
The 92mm F/5.5 and your TV 3-6mm Zoom, set at 5.5mm... That's a very good eyepiece, which you already have, AND it delivers a 1mm exit pupil, when set at 5.5mm. A 1mm exit pupil is about the ideal for star testing...
Jan
It might be a good eyepiece and she might already own it but its clearly not as good a choice as the 4mm,5mm or 6mm TMB Supermonos which she also already owns and I recommended to her a couple of posts ago.
Cheers,
--------------------
John Bambury
AS of NSW
AS of Hunter Valley
18"/F4.5 Obsession with Argo Navis and Servocat, 10"/F5 Newtonian with Argo Navis
Lots of Pentax XW's. Plus 12mm & 17mm Nagler T4's, 31mm Nagler T5 & 13mm ETHOS. TV Paracorr and 2.5X Powermate
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Jan Owen
super member
Reged: 02/12/06
Posts: 197
Loc: Sun City West, Arizona
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No need to be upset... I don't disagree with you...
Frankly, as it happens, I didn't see your previous message(s), because I was responding directly to Dee...
Her original message indicated her scope's data, and I responded about using something like a 7mm eyepiece, for the exit pupil it would deliver... The human eye *likes* exit pupils around .8 to 1.0mm (though what I recommended delivers an exit pupil of 1.27)... It's true that telescope objectives, if pushed to exit pupils down in the range of about .5 or so, can push past aberrations of the eyepiece, and bring you to where you're looking at just objective aberrations, if you can SEE them... In my case, I have a lot of floaters as the result of a detached retina and the following surgery, so, while I do use exit pupils as low as .3mm (pretty much the extreme limit) in some circumstances, I'll be able to more easily interpret the images at closer to 1mm exit pupil, so I mostly use something in that (+/-) 1mm range most of the time...
That's also what Suiter suggests, and numerous others suggest the range between .8 and 1, so I'm not alone.
As for the supermonos versus, the zoom, I own a 2-4 zoom, but haven't used it yet, because we're in the annual Arizona summer monsoon right now, and that'll go on until around the middle of September or so... I didn't mention the supermonos or the zoom in my earlier message because I didn't realize she HAS all that nice hardware...
In my later response, if you'll re-read it, I suggested that I like the supermonos and abbe orthos just fine, and she likes her zoom, so I suggested she use BOTH the monocentrics, AND the zoom, and see which works best for HER... I'm a firm believer in whatever works best for YOU is the best answer...
So, in neither of my messages was I trying to intentionally ignore your recommendations, or recommend against them; I wss just responding directly to Dee, and didn't SEE your messages until later...
No intent to offend, ignore, or cast aside your recommendations.
For the most part, I concur...
As for the mono's superiority to the TV Zooms, that may be, but that remains to be seen for me, even though it may be true. And it sounds like Dee likes hers at LEAST as well as her monos... So, I think we need a little more time, in this case
Jan
Edited by Jan Owen (08/03/08 06:10 PM)
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KaStern
professor emeritus
Reged: 04/18/06
Posts: 526
Loc: Dortmund
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Hello Dee,
I like to use d x 2 magnification when startesting an optic.
That would be a 2,75mm eyepiece for your 92mm apo.
But the 3-6mm Nagler Zoom will be sufficiant and even your 4mm eyepieces
can be used for startesting your f/5.5 apo.
However, I suggest to use a baader solar continuum filter. That will
eliminate all colour issues from the telescope and the eyepieces.
www.baader-planetarium.de/zubehoer/okularseitiges_zubeh/filterkurve_kontinuum.gif
With the narrow filter you cann find out exactly how well your scope ist
corrected for spherical aberration at around 540nm.
I assume that you will discover some slight differences between the NZ,
the Plössl and the Monozentric eyepieces, at the same magnification.
The is, because at f/5.5 many eyepieces begin to suffer from spherical aberration.
Clear skies, Karsten
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200/1200mm ATM Dobson
150/1800mm Yolo
80/1470mm Achromat
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 6632
Loc: Beatrice, Nebraska
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Quote:
With a fast f/5.5 scope, you do not want to use an ortho if you really want to do a serious startest. Orthos will introduce some on-axis spherical aberration at such a fast f-ratio. A Nagler would be a safe choice.
Well, you might be confusing "spherical aberration of the exit pupil" (the so-called "kidney Bean" effect) with true spherical aberration, as the first term does not significantly impact the on-axis image quality. I have never seen a decent orthoscopic (Abbe) eyepiece introduce *any* significant spherical aberration at f/5 or above. Certainly, my 1970's vintage 6mm Brandon Orthoscopic shows no spherical aberration in my 10 inch f/5.6 Newtonian (custom mirror). An ortho would work well as a telescope testing eyepiece. Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
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suburbanskies
super member
Reged: 12/18/04
Posts: 196
Loc: New Jersey, USA
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Quote:
Quote:
With a fast f/5.5 scope, you do not want to use an ortho if you really want to do a serious startest. Orthos will introduce some on-axis spherical aberration at such a fast f-ratio. A Nagler would be a safe choice.
Well, you might be confusing "spherical aberration of the exit pupil" (the so-called "kidney Bean" effect) with true spherical aberration, as the first term does not significantly impact the on-axis image quality. I have never seen a decent orthoscopic (Abbe) eyepiece introduce *any* significant spherical aberration at f/5 or above. Certainly, my 1970's vintage 6mm Brandon Orthoscopic shows no spherical aberration in my 10 inch f/5.6 Newtonian (custom mirror). An ortho would work well as a telescope testing eyepiece. Clear skies to you.
Hi David,
I first heard about this from a well-known telescope manufacturer. I think his response to any doubt about spherical aberration in orthos on-axis in fast scopes would be "buy a copy of ATMOS (optical design software) from me, model it and see for yourself" 
Mark
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SkyscraperJim
sage
Reged: 07/29/07
Posts: 386
Loc: Providence, RI
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Quote:
I recently purchased a TMB92SS, which is an f5.5. Was wondering what eyepiece people would recommend to perform the startest??
While at Stellafane this past weekend the optical judges (I didn't enter a scope, but I was on the hill during the Friday night judging) recommended an eyepiece of approximately the same focal ratio of your scope, so something in the 5-6mm range would be appropriate.
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Jim
TMB/APM 130/780 (#185) on G11
Televue Pronto (#3533) on Half Hitch
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Mike K
sage
Reged: 04/01/07
Posts: 418
Loc: Central Texas
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So, Dee... Did you get a chance to do a star test? Any conclusions?
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Clear skies,
Mike K.
30°31" N 97°44" W, LP: Red
Observe: Once or twice a week back yard, once a month under dark skies
Favorites: Globulars, planets, face-on spirals
Equipment: CPC925/XT10i/TMB-92SS/PST
Eyepieces: Naglers, Ethoi, UO HDs, Hyperion Zoom
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