78 replies to this topic

[davidgmd]

Can one use an AT72EDII with say a 31mm Nagler or a 35mm Panoptic? Or will vignetting occur? 

 

I asked TS Optics if a TSFLAT2 field corrector could be used with a 72 APO and they replied this:

 

"thank you for your interest. The TSFLAT2 will work with the 72mm f6 refractor. Working distance is 130mm but we suggest the corrector only for sensors up to 25mm diameter and smaller. Than you will have good stars also in the edges."

 

I replied to them asking:

 

Oh thank you very much, I will use it for visual widefield only. You think it will be good with eyepieces 40mm field stop?

 

and they replied:

 

"yes it will work. The Flattener should be adapted into the 2" Filterthread of the diagonal."

     
The recommended optical path length between the TSFLAT2’s M48 thread and the eyepiece focal plane for a 430 mm focal length scope is 128 mm. See https://www.teleskop...onnection-10307.

   
If you attach the TSFLAT2 to the filter threads on the end of the diagonal’s nosepiece, the resulting optical path length will be the OPL of the diagonal plus the length of the nosepiece. The OPLs of 2” diagonals are often in the 100-110 mm range, and nosepieces tend to run 25-35 mm. That would give you a total OPL of anywhere from 125-145 mm. It’s in the right ballpark which may well be good enough, and might be right on the money depending which diagonal you’re using.

  
For longer focal length refractors, the ideal OPL is shorter. That can require a diagonal with a removable nosepiece so the TSFLAT2 can be attached directly to the diagonal body, essentially replacing the nosepiece as described here: https://www.cloudyni...ual/?p=14016318

[GGK]

One eyepiece, one scope and a gadget like a field flattener or focal reducer, for a dark site trip for rich field views, which are you taking and why?

 

What is a better wide field view for you? 3º, 4º, 5º, 6º, 7º?

 

No cameras..

Wide field / rich field observing takes up over half of my scope time these days, and I've optimized my scope selection and eyepieces accordingly.

 

If I could only take one scope, it would be the 100mm, f/4 Pentax flat field Petzval refractor for its 400mm focal length wide field capability.

 

I use most of my scopes for rich field observing - the rich field views in my 9.25" SCT with a focal reducer and 31mm Nagler show an impressive view at ~1.9^o - but I spend a lot more time observing between 4^o and just under 8^o, with the ~6^o field being the sweet spot for me, which is why I need the short focal length. 

 

The eyepiece is a harder choice because nothing is perfect.  The 31mm Nagler with Dioptrx will provide the most detailed view, but at a loss of aperture from the oversized exit pupil.  The 25mm ES100 will show the largest field - about 5.8^o - using the full 100mm aperture with my eye pupil size.  The 26mm Nagler gives a slightly better image quality than the 25mm ES100, but with a drop in field size to about 5^o, which is a little smaller than I'd like if having only one option.  This is why I never use only one eyepiece.

 

I'd likely take the 25mm ES100 and sacrifice some edge of field sharpness / contrast, and loss of some of the faintest stars in the outer field.  The image will be outstanding no matter which of these eyepieces I take.

 

Depending on the time of year / right ascension range, I might select the Tak FSQ-85 Petzval instead with the A-P focal reducer to get a 6.9^o field with the resulting 338mm focal length.  I'd likely still take the 25mm ES100 to keep the full 85mm effective aperture at f/4.

 

Gary

[Procyon]

Wide field / rich field observing takes up over half of my scope time these days, and I've optimized my scope selection and eyepieces accordingly.

 

If I could only take one scope, it would be the 100mm, f/4 Pentax flat field Petzval refractor for its 400mm focal length wide field capability.

 

I use most of my scopes for rich field observing - the rich field views in my 9.25" SCT with a focal reducer and 31mm Nagler show an impressive view at ~1.9^o - but I spend a lot more time observing between 4^o and just under 8^o, with the ~6^o field being the sweet spot for me, which is why I need the short focal length. 

 

The eyepiece is a harder choice because nothing is perfect.  The 31mm Nagler with Dioptrx will provide the most detailed view, but at a loss of aperture from the oversized exit pupil.  The 25mm ES100 will show the largest field - about 5.8^o - using the full 100mm aperture with my eye pupil size.  The 26mm Nagler gives a slightly better image quality than the 25mm ES100, but with a drop in field size to about 5^o, which is a little smaller than I'd like if having only one option.  This is why I never use only one eyepiece.

 

I'd likely take the 25mm ES100 and sacrifice some edge of field sharpness / contrast, and loss of some of the faintest stars in the outer field.  The image will be outstanding no matter which of these eyepieces I take.

 

Depending on the time of year / right ascension range, I might select the Tak FSQ-85 Petzval instead with the A-P focal reducer to get a 6.9^o field with the resulting 338mm focal length.  I'd likely still take the 25mm ES100 to keep the full 85mm effective aperture at f/4.

 

Gary

That is very impressive Gary. And about what I'm trying to achieve. I want to use my 25mm ES 100 with an AT72EDII and get about 5.8º? Unless vignetting happens.... I want to keep my 100mm f/6 achro for 3-4º views and get a 72 Apo for 5-6º fields. But I see Petzvals with faster focal ratios and wider rich fields is where the action is huh?. 

Edited by Procyon, 01 May 2025 - 05:19 PM.

[GGK]

That is very impressive Gary. And about what I'm trying to achieve. I want to use my 25mm ES 100 with an AT72EDII and get about 5.8º? Unless vignetting happens.... I want to keep my 100mm f/6 achro for 3-4º views and get a 72 Apo for 5-6º fields. But I see Petzvals with faster focal ratios and wider rich fields is where the action is huh?. 

A Petzval design certainly provides benefit to my wide field observing capability, and I've moved in that direction nearly 100% for wide fields.  The only negative is price. 

 

I like to see as many threshold stars and DSOs as possible from image center to the field stop, and I like very wide fields at 5^o and larger.   In this range, my eye pupil diameter is my limiting factor for aperture size and is also what drives down my preferred focal ratios into astrograph territory.  Since my eyes prevent me from increasing aperture at my preferred wide field size range, I've focused on improving my scopes and eyepieces to see more image detail.

 

The Petzval refractors that I use tend to do everything right.  The field is very flat, which removes the defocus blur in the outer field.  They're APOs with no CA blur and no loss of contrast that would occur if light was transferred from the spurious disk into a background halo.  The baffling and light scatter control is excellent, keeping contrast up.  The lens polish and coatings are superb which also helps contrast, focus sharpness and light transmission.  The focusers are larger diameter for a large fully illuminated light circle to minimize vignetting.  And the focusers handle the large heavy eyepieces without problems.  I also prefer to direct connect the diagonal body to the focuser tube to minimize vignetting from the diagonal at the fast focal ratios.

 

The cumulative impact of improving all small details makes a difference, but it's difficult to know which attributes matter the most because they're hard to isolate for comparisons. 

 

I found that eyepiece performance can improve in Petzval refractors as long as the eyepiece has close to a flat field itself.  Eyepiece aberrations are visibly smaller when the whole field is in sharp focus with no blur or light scatter from the scope.  I find that there is often a noticeable increase in faint visible image detail when using a great well-corrected eyepiece vs. a generally very good eyepiece, especially in the outer field.  The difference often shows up as being able to detect faint DSOs vs. not knowing they're present in the image, or seeing 4 stars in the Trap vs. a single glowing area with 0 to 2 stars defined. To reemphasize - I'm discussing low power wide fields, which is usually in the 10 to 20X range of magnification.  I also observe wide fields under all sky conditions from dark to full moon, and performance of all equipment and combinations will vary depending on sky brightness.

 

All of this above doesn't mean that I don't like using fast achromats or EDs.  I just decided to really push my wide field scopes and eyepieces to the max image quality since I spend so much time using them.  I also have a great time observing asterisms, which keeps me pushing to larger true fields.

 

Gary