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Use of flat field refractors and reducers with afocal nv

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#1 Gavster

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Posted 26 March 2020 - 11:42 AM

I was observing last night in my Sqm 18.3 back garden. Given the milky way is hiding at the moment, I decided to focus on a galaxy/cluster session with my 685 long pass and my pvs-14 (afocal only).

 

It was an interesting evening since my observations challenged some currently held beliefs of mine whilst strengthening some others.

 

I went for my preferred grab and go nv scope, the Tak FSQ-85. I had recently done some nv afocal observing with my 95mm f5.9 baader and any astigmatism in the eyepiece (eg the 55mm televue plossl) is, in my experience, exaggerated by afocal nv as a result of field curvature from the scope itself. The defocused edge stars in the Baader (when the stars were focussed in the centre) became curved lines as a result of the eyepiece astigmatism.

 

The Tak FSQ-85 has a lovely flat field so the edge stars are in focus and thus the astigmatism of the 55mm plossl is much less apparent. With my 41mm and 27mm panoptic the edge stars are great as these eyepieces have much better edge performance in fast scopes than the 55mm. 

 

So far, completely in line with my expectations and confirmation that the Tak is my best grab and go nv scope. I did once try a fast achro refractor afocally with nv and it was horrible across a lot of the fov even with heavy ha filtering. I guess I'm quite fussy about edge star performance when using nv (maybe my forty something eyes don't help).

 

Then I got my 0.75x RC GSO reducer out and this is when things did not go as I anticipated. Firstly the edge stars with the reducer and 55mm plossl were much worse than the 55mm plossl on its own (different to how I recalled the view looked when I used this mix before), so much so I don't think I will use this combo again despite the increase in effective speed and brightness. Secondly, the RC reducer and 41mm panoptic which previously I thought worked well just didn't give the same quality of views as the 55mm plossl on its own (similar effective speed for both at f2.5). The edge stars were decent (if a little blocky) but the views just didn't seem as crisp across the whole fov. I think in future I won't use the reducer anymore with this scope and will just stick to the eyepieces on their own. 

 

I then moved from the galaxies I had been observing (M81/82, leo triplet, markarian's chain, needle, black eye) to m13. This was rather small in the 5 degree fov that the 55mm plossl gave so I switched in my 27mm panoptic. The extra image scale gave an improved view despite the reduced effective speed (f5.3). I then went further and used an 18.2 delite, however although the image scale was bigger the extra scintillation gave a worse view for me even with the gain turned down. Certainly globulars can take slower speeds compared to nebulae (and the extra image scale helped with the brighter galaxies as well). But I think f5 is my personal limit.

 

 


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#2 Jeff Morgan

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Posted 26 March 2020 - 12:29 PM

Interesting reading Gavin!

 

The edge-of-field correction when doing afocal with the 55 Plossl is a situation I'm sure most of us would like to see improved. Your results in a flat-field telescope suggest a field flattener (for other refractors) could be worthwhile.

 

I do have refractors that could benefit from the Teleskop Service TSFLAT2 product.

 

However, Tele Vue has a reducer/corrector tailored to the 55 Plossl in development:

 

http://www.loptics.c...ightvision.html

 

From that snippet they at least have a working prototype, no idea on how close it is to commercial release. I will wait on that before I make any decisions on the refractor flattener.

 

Also, no idea on how that might work with other NV-compatible Tele Vue eyepieces.



#3 Gavster

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Posted 26 March 2020 - 01:10 PM

However, Tele Vue has a reducer/corrector tailored to the 55 Plossl in development:

 

http://www.loptics.c...ightvision.html

 

From that snippet they at least have a working prototype, no idea on how close it is to commercial release. I will wait on that before I make any decisions on the refractor flattener.

 

Also, no idea on how that might work with other NV-compatible Tele Vue eyepieces.

David Nagler told me that it will only work on the 55mm plossl unfortunately 



#4 GeezerGazer

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Posted 27 March 2020 - 05:06 PM

So far, completely in line with my expectations and confirmation that the Tak is my best grab and go nv scope. I did once try a fast achro refractor afocally with nv and it was horrible across a lot of the fov even with heavy ha filtering. I guess I'm quite fussy about edge star performance when using nv (maybe my forty something eyes don't help).

 

Then I got my 0.75x RC GSO reducer out and this is when things did not go as I anticipated. Firstly the edge stars with the reducer and 55mm plossl were much worse than the 55mm plossl on its own (different to how I recalled the view looked when I used this mix before), so much so I don't think I will use this combo again despite the increase in effective speed and brightness. Secondly, the RC reducer and 41mm panoptic which previously I thought worked well just didn't give the same quality of views as the 55mm plossl on its own (similar effective speed for both at f2.5). The edge stars were decent (if a little blocky) but the views just didn't seem as crisp across the whole fov. I think in future I won't use the reducer anymore with this scope and will just stick to the eyepieces on their own. 

 

I then moved from the galaxies I had been observing (M81/82, leo triplet, markarian's chain, needle, black eye) to m13. This was rather small in the 5 degree fov that the 55mm plossl gave so I switched in my 27mm panoptic. The extra image scale gave an improved view despite the reduced effective speed (f5.3). I then went further and used an 18.2 delite, however although the image scale was bigger the extra scintillation gave a worse view for me even with the gain turned down. Certainly globulars can take slower speeds compared to nebulae (and the extra image scale helped with the brighter galaxies as well). But I think f5 is my personal limit.

 

Gavin, your post is truly interesting; especially about the GSO-RC reducer, which sounded so promising.  I don't think you are any more fussy about EoF performance than I am!  lol.gif   I never quite found the afocal combination that worked in any of my scopes with an added reducer... which made me concluded that if I were to add a reducer, that it should be one of the much more expensive reducers that are specifically made (dedicated) for individual telescopes, like AP, TEC, SV or WO make for their scopes.  Although, I know your AP reducer with your TEC performs beautifully in afocal... so specific combinations do exist for very high EoF performance.

 

Like you, I have also found upper limits for effective focal ratio results.  In prime with my 8" native f:4 Newt, I can barlow 1.5x at f:6 for bright nebulae, and f:8 for point light sources.  But I find f:8 a little too slow for dim nebulae, most galaxies and planetary nebulae.  We all have preferences and tolerance levels that have to be considered along with the limitations of our equipment.  With my limited 8" of Newt aperture, I have concluded that except for the brightest PNs and galaxies, I would need more aperture and focal length for better visual or imaging results.  This is not an especially big drawback for me, but does seem to be the reality.  Phonetography does allow me to push those boundaries a little, but every NV target has it's own set of limitations, be it angular size or surface brightness.   

Ray




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