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Experiments in extreme focal reduction

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#1 Starman81

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Posted 21 May 2018 - 01:38 AM

Actually, my plan is to try the 55 Plossl (providing a 2x FL reduction) combined with a 2 inch .8x reducer ahead of it to yield a 360 FL in my TEC 140... at f:2.6.  In the ST 120 that combo would get it to f:2.  It's worth a try.  Gavin (Gavster) in London is doing the same thing; he's just a lot faster than I am at moving forward.  

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... I'll wait to report on how it reacts to the significant FL reduction I have in mind.  

 

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I guess I'm not the only one who has been thinking of extreme focal reduction grin.gif ... Maybe we should start a different thread on it...

 

After acquiring the TNVC/TeleVue adapter for afocal use, I got the idea to use a focal reducer first to reduce my TV 40PL and then use the Mod 3 afocally. What lead me to want to try this was that I wanted to match the afocal reduction potential of a TV 55PL without having to buy one and without having to buy expensive 2" filters (h-alpha, longpass, etc) as I already have several 1.25" filters. 

 

The  Mod 3 used with the TV 40PL afocally effectively reduces the system by 0.65x and coupling that with my 0.5x 2" focal reducer would theoretically get you to about 0.33x! 

 

I have only tried this in my EON 80.  It doesn't have sufficient back-focus for focal reducers when using the Mod 3 at prime focus, so it is a prime candidate for afocal NV. However, the spacing of the 0.5x reducer with the TV 40PL was tricky and I couldn't achieve more than 0.8x reduction. That, plus the 0.65x reduction of using the 40PL yielded a total reduction of 0.52x. These reduction numbers were verified by measuring the distances between field stars in SkySafari and comparing with the eyepiece view. 

 

So, the double reduction did work but there was some extreme field curvature present that really took away from the view. I had not experienced anywhere near this level of field curvature when using the 0.5x focal reducer in scopes with sufficient backfocus and the Mod 3 at prime focus, so it really did jump out at me.

 

So, I think more testing is necessary before I give up on it,  but due to my experience with the EON 80, I am kind of expecting to see an objectionable level of field curvature.

 

EDIT: This is the new thread, thanks Mod (star drop)


Edited by Starman81, 21 May 2018 - 03:34 PM.

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#2 GeezerGazer

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Posted 21 May 2018 - 03:06 AM

I guess I'm not the only one who has been thinking of extreme focal reduction grin.gif ... Maybe we should start a different thread on it...

 

After acquiring the TNVC/TeleVue adapter for afocal use, I got the idea to use a focal reducer first to reduce my TV 40PL and then use the Mod 3 afocally. What lead me to want to try this was that I wanted to match the afocal reduction potential of a TV 55PL without having to buy one and without having to buy expensive 2" filters (h-alpha, longpass, etc) as I already have several 1.25" filters. 

 

The  Mod 3 used with the TV 40PL afocally effectively reduces the system by 0.65x and coupling that with my 0.5x 2" focal reducer would theoretically get you to about 0.33x! 

 

I have only tried this in my EON 80.  It doesn't have sufficient back-focus for focal reducers when using the Mod 3 at prime focus, so it is a prime candidate for afocal NV. However, the spacing of the 0.5x reducer with the TV 40PL was tricky and I couldn't achieve more than 0.8x reduction. That, plus the 0.65x reduction of using the 40PL yielded a total reduction of 0.52x. These reduction numbers were verified by measuring the distances between field stars in SkySafari and comparing with the eyepiece view. 

 

So, the double reduction did work but there was some extreme field curvature present that really took away from the view. I had not experienced anywhere near this level of field curvature when using the 0.5x focal reducer in scopes with sufficient backfocus and the Mod 3 at prime focus, so it really did jump out at me.

 

So, I think more testing is necessary before I give up on it,  but due to my experience with the EON 80, I am kind of expecting to see an objectionable level of field curvature.

The problem is finding the right combination.  I really dislike vignetting, especially when it shows up in photos, and reducers often interfere with the light cone to cause it.  I've had pretty good luck with the 2" .8x reducer (Orion) in my TEC, but it might cause vignetting with the steeper light cone in the faster ST120.  The .5x reducers I have, both 1.25" and 2", cause vignetting in both scopes.  Visually they work OK with mild vignetting, but a photo will show the vignetting much more prominently.  I hadn't even considered the possibility of field curvature.  

 

If you find a working combination that doesn't vignette, please let me know!  waytogo.gif


Edited by GeezerGazer, 21 May 2018 - 03:10 AM.


#3 Gavster

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Posted 21 May 2018 - 03:15 AM

The problem is finding the right combination.  I really dislike vignetting, especially when it shows up in photos, and reducers often interfere with the light cone to cause it.  I've had pretty good luck with the 2" .8x reducer (Orion) in my TEC, but it might cause vignetting with the steeper light cone in the faster ST120.  The .5x reducers I have, both 1.25" and 2", cause vignetting in both scopes.  Visually they work OK with mild vignetting, but a photo will show the vignetting much more prominently.  

This probably would be a good topic for a new thread.

The vignetting does become much more obvious when you are taking phone photos of the views and like GeezerGazer I really dislike seeing the vignetting. I’ve tried a few different focal reducers with my 55mm afocal setup. The best results I have are with reducers with a large image circle - this is the key attribute for me so far.

The best reducer I’ve had is the Astro-physics 0.75 photo visual reducer. It works well on my C11 (much better than the standard 0.63 x reducer) and my Tec160fl and ap130gtx. With the 55mm plossl and 0.75x reducer I can get the f ratios of my scopes down to 3.7, 2.6 and 2.4. So pretty fast and also a decent fov for those larger nebulae!


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#4 Eddgie

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Posted 21 May 2018 - 08:51 AM

Ah, I think that it may be that people are starting to understand why I spent so much money on the Boren Simon 6" f/2.8.

 

I know when I posted my review in the Reflector forum, there were people that said that they would never own a telescope with such a big obstruction (40%) but as it turns out, to work at 6" and f/2.8, there really is no way around it.  To get a fully illuminated field, you just have to have a bigger than standard obstruction.

 

I love my Boren Simon.   I labored over the choice instruments to be used for wide field nebula exploration and by then, I had a very clear objective of having a fully illuminated field.

 

It is hard to get to that goal without using an astrograph, or a telescope that looks a lot like an astrograph.  Secondary mirrors have to be large, or refractors have to be very fast to start with and have short focuser tubes or oversize tobe to avoid vignetting at the front of the tube when heavy reduction is used.

 

I came to believe that starting with a fast imaging reflector and reducing it was the way to go, and that lead me to the Boren Simon, where someone had already done the design and construction for me.

 

I paid a lot for it, but it has been extremely satisfying to use.  I can see even faint nebula at the edge of the field, and it has far better limiting magnitude than the 80mm refractor I tried (which did not have as big of a true field though I did not run it with a reducer). 

 

I have been a big proponent of using imaging Newts for NV because you can compress them without too much illumination falloff (and of course in the case of the Boren Simon, that was designed to work with a reducer, no falloff).


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#5 Mazerski

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Posted 21 May 2018 - 12:01 PM

FYI -- I purchased the Boren-Simon 8" based on Eddgie's write up of his 6". I never would have known about spacing requirements and the use of a filter wheel, etc. I really appreciate his help in getting the scope all set up and ready to go.

 

What a great scope. I had no idea that many emission nebula are so big - I couldn't know due to an east coast sky and using glass eyepieces, even in a 15" scope with a h-beta filter, all I see at Alnitak (Orion's belt) is a bright star. Period. With the Boren Simon 8", the horsehead and flame look great (with Night Vision and 7nm Ha filter). The star stuff in Cygnus looks great as do many objects. I use the scope at f/2.8 with included reducer or at f/4. 

 

I can only hope I have opportunity to use this scope in a dark sky. I was in Arizona in April with just PVS 7, Mod 3 and 1x, 3x, 7x... the views were fantastic. I can see a lot from my area but not like the dark sky in AZ.

 

Not the best photo of scope but its all I have at this time - this is on the AYO Digi II with the Intes Micro M715 Mak-Cass and Nexus DSC.

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#6 Eddgie

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Posted 21 May 2018 - 12:29 PM

Great to see you chime in, Mazerski.

 

That is a very beautiful setup. I like the carry handle and the mount is beautiful.

 

I know we had some trouble with the configuration, but I am happy to hear that it all worked out and that you are enjoying the Boren Simon.  I really do enjoy mine and for nebula, it is wonderful.

 

And under dark sky, it is going to blow your mind....



#7 GeezerGazer

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Posted 21 May 2018 - 02:21 PM

This probably would be a good topic for a new thread.

The vignetting does become much more obvious when you are taking phone photos of the views and like GeezerGazer I really dislike seeing the vignetting. I’ve tried a few different focal reducers with my 55mm afocal setup. The best results I have are with reducers with a large image circle - this is the key attribute for me so far.

The best reducer I’ve had is the Astro-physics 0.75 photo visual reducer. It works well on my C11 (much better than the standard 0.63 x reducer) and my Tec160fl and ap130gtx. With the 55mm plossl and 0.75x reducer I can get the f ratios of my scopes down to 3.7, 2.6 and 2.4. So pretty fast and also a decent fov for those larger nebulae!

This is probably the key for refractors.  I don't think a .5x reducer can be used without vignetting in most scopes.  I really like the ST120 results so far but I have not used it in afocal mode yet.  I should order a 55P and see what happens when I throw the .8x reducer in the mix.  

 

Eddgie, I almost ordered one of the Boren-Simon Newts on your high praise.  But I am a refractor guy at heart and wanted to see how much performance I could get out of this very inexpensive ST120.  I like it so far; it continues to amaze me that it performs as well as it does with NV.  The really big improvement for my observing came after you explained your experience with your iOptron mount.  I really like the compact size and light weight of the AZ Pro, and its goto and tracking have been perfect.  I was pleasantly surprised that it can satisfactorily handle my TEC 140, and I appreciate more than ever that it tracks.  It sits very cleanly on the portable pier I built a few years ago.

 

Bill P. was with me the first night I turned on the AZ Pro in NM, and he was used to the procedure for a recent model Cube... which is a different mount.  We tried it his way without success.  With the AZ Pro, I must level it then turn it on; it calibrates based on GPS automatically, then it finds a bright target.  All I do is center that target in the FoV and it is aligned/calibrated for goto.  I don't know how much easier it could get!  Tracking and goto were perfect all 3 nights.  So far... really glad to own and use it!  Thanks for your input.  



#8 GeezerGazer

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Posted 21 May 2018 - 02:27 PM

This probably would be a good topic for a new thread.

The vignetting does become much more obvious when you are taking phone photos of the views and like GeezerGazer I really dislike seeing the vignetting. I’ve tried a few different focal reducers with my 55mm afocal setup. The best results I have are with reducers with a large image circle - this is the key attribute for me so far.

The best reducer I’ve had is the Astro-physics 0.75 photo visual reducer. It works well on my C11 (much better than the standard 0.63 x reducer) and my Tec160fl and ap130gtx. With the 55mm plossl and 0.75x reducer I can get the f ratios of my scopes down to 3.7, 2.6 and 2.4. So pretty fast and also a decent fov for those larger nebulae!

It does deserve a separate thread, but I'm waiting until I can actually do afocal... which means I'll order a 55P today!  Dang, it's easy to spend money.  lol.gif



#9 Starman81

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Posted 21 May 2018 - 03:13 PM

For NV observing with a telescope, there are two options for focal reduction.:

 

1) Prime focus NV observing - use a readily available 2" focal reducer (0.7x and 0.5x are common) or if back-focus is limited...

 

2) Go the afocal route with the use of the TNVC/TeleVue adapter and any eyepiece that can accomodate the adapter (TeleVue works well) with a focal length > 26mm. The TV 55PL gets you the most reduction (~0.5x) and even the TV 40PL can you get 0.65x reduction, but in a 1.25" format (which may be important if your filters are all 1.25" or you don't have a 2" diagonal or you need to keep the weight of the setup minimized. 

 

The third option is one of extreme focal reduction, which is to use the afocal approach with an eyepiece of f/l > 26mm for its inherent focal reduction and to couple that one of the off-the-shelf focal reducers as mentioned. In this thread we can discuss those experiments in extreme focal reduction and the results.



#10 Eddgie

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Posted 21 May 2018 - 07:34 PM

 

The really big improvement for my observing came after you explained your experience with your iOptron mount.  I really like the compact size and light weight of the AZ Pro, and its goto and tracking have been perfect.  I was pleasantly surprised that it can satisfactorily handle my TEC 140, and I appreciate more than ever that it tracks.  It sits very cleanly on the portable pier I built a few years ago.

 

 

I know that the iOptron mounts have a checkered history, and mine took a little tweaking to hold the 120ED, but it has been a fantastic little mount.  Compact, light, very stable, excellent Go2 and tracking with the 6". 

 

I am happy to hear that the iOptrion is working well for you.



#11 GlennLeDrew

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Posted 21 May 2018 - 10:06 PM

I'm a tad confused by these claimed extreme resultant f/ratios approaching f/0.3.

 

If an eyepiece is delivering the image through an objective lens on the NV device, the f/ratio limit is set by that of the little objective lens itself. If the NV objective is, say, f/1.4, that's the fastest you're gonna get. Further focal reduction, with the enlarging exit pupil getting bigger than the little objective's entrance pupil, results in aperture reduction. The main gain, if anything, is just a larger FoV. For obstructed systems, the larger secondary shadow also reduces light grasp.

 

Is there something I've missed?


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#12 Eddgie

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Posted 21 May 2018 - 10:17 PM

Is there something I've missed?

I may be confused about your confusion, but I don't think they are talking about "f/" .3.  I think they are talking about an effective focal reduction of .33x  

 

Unless I missed where someone said f/0.3.  I did see that someone mentioned .33x reduction.

People doing EAA are using stacked reducers to get from .66 in their SCTs to .33x, an I think this may be what was mentioned in the thread above.  If someone said f/.03, my apologies because I missed that claim.


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#13 Starman81

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Posted 23 May 2018 - 12:00 PM

I'm a tad confused by these claimed extreme resultant f/ratios approaching f/0.3.

 

If an eyepiece is delivering the image through an objective lens on the NV device, the f/ratio limit is set by that of the little objective lens itself. If the NV objective is, say, f/1.4, that's the fastest you're gonna get. Further focal reduction, with the enlarging exit pupil getting bigger than the little objective's entrance pupil, results in aperture reduction. The main gain, if anything, is just a larger FoV. For obstructed systems, the larger secondary shadow also reduces light grasp.

 

Is there something I've missed?

 

Hi Glen. I think you misread 0.33x focal reduction as focally reducing all they way down to f/0.3. That's not what is being said. By double focal reducing, we can theoretically get the focal ratios down to 0.33x of their original native values (an f/5 can be reduced to ~f/1.7, for example). At what cost will this focal reduction be achieved? That's what I'm interested in finding out. Basically how to balance the desire for extreme focal reduction with the tolerance of any aberrations that may be induced. 

 

Thanks though for highlighting the f/ratio limit. The objective in the Mod3 I believe is f/1.2, so that would be the fastest we could get before aperture reduction. I understand the max limit now. 



#14 Starman81

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Posted 05 June 2018 - 07:07 PM

Okay, now THIS is what I'm talking about... Probably looks crazy to many and the question is: why on earth would you want to do this? If you want to get more and more TFOV you need to get shorter focal length telescopes (since the focal length and apparent FOV of the Mod 3 is a fixed constraint at ~26mm and 40°)...

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#15 Starman81

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Posted 05 June 2018 - 07:16 PM

But when you get down to the really low power/low mag/max TFOV end, you are getting more into the handheld configuration using various afocal lenses (2x, 3x, 5x or 6x with a 50mm finder used at prime focus). But what if you want to use the Mod 3 with a GIGANTIC TFOV in a telescopic configuration?

 

That's where this combo comes into play. That huge stack to Mod 3 + TNVC/TV adapter + TV 55PL + 2" filter + TV 2" Barrel extender + 2" 0.5x focal reducer gets you the largest TFOV when plugged into the shortest focal length scopes available. Pictured below it is my AT60ED, with a native f/6 focal ratio. My indoor tests have shown this combination gets somewhere in the neighborhood of ~9° TFOV, maybe even 10°! The focally reduced system equates to a super-fast f/2.2 (by my estimates). 

 

Yes, there is a huge lever arm to contend with and I would definitely want to keep my hand on the guide handle of the Unistar Light Deluxe mount at all times, but it may be less taxing to observe this way than perhaps straight-through with the Mod 3 at prime focus in a 50mm finder scope... Maybe. And there may be untold numbers of aberrations that went largely unnoticed in my daytime tests. I guess I'll have to wait and see. 

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#16 Jeff Morgan

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Posted 05 June 2018 - 10:18 PM

Wow!

 

If the FOV quality proves acceptable to you, it might be worthwhile for convenience (and safety) to put a tube ring just under the sliding dew cap and hang a weight from it to counterbalance some of that stack.


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#17 moshen

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Posted 06 June 2018 - 01:42 AM

Syed, good experimenting. I'll be curious how much vignetting / light drop-off there is. The fully illuminated image plane is only so large. Maybe join in on some of the astro-phonetography and put some images up! wink.gif


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#18 Starman81

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Posted 08 June 2018 - 11:54 AM

I was able to the get the TV 55PL afocal configuration in the AT60ED out a few nights ago for testing. The hour was very late and the Summer MW was well-positioned. Conditions were quite transparent (I could see all the stars of the Teapot asterism) and NELM was 4.5+. 

 

I tested both with the use of the 0.5x focal reducer and without. The only filter that was used was the 2" Lumicon Night Sky. Both combos (with and without the focal reducer) turned the suburban night sky into that of a very dark site. Using NV @ f/2.2 and also @ f/2.9 through the diminutive 60mm was equal to using a 4" refractor in SQM 21.5+ skies--no joke! Except the 4" refractor can't get you these massive TFOVs.

 

So it's clear that I found the AT60ED afocal NV combo with the TV 55PL very enjoyable and highly recommend it. This telescopic configuration, as opposed to a Mod 3C used at prime focus in a 50mm finder scope straight through, was relaxing and much easier to use. 

 

Mod 3 C + TV 55PL + Lumicon Night Sky filter +  0.5x focal reducer

Estimated ~f/2.2, 4x, and ~10° TFOV (vignetted), ~8.5° unvignetted (though not all ‘usable’; 5-6° sweet spot)

 

This combination had its compromises but it had some great benefits as well. When I first turned on the Mod 3, it was dark and blurry and thought, ‘I guess it was too good to be true!’ Then I remembered I had set the gain to near the lowest possible setting for my daytime/indoor testing and turned the gain knob up and voila! I was greeted by hundreds of stars in Cygnus in my FOV. The TFOV measured in the field was ~8.5° (Saturn to Kaus Media, with room to spare) with accounting for vignetting. The total TFOV with vignetting was probably closer to 10°, so we can estimate the magnification to be about 4x. Only the central 5-6° was usable, but the speed of the system was so fast that even with the gain at the highest setting there was no scintillation! The field curvature outside of this central region was definitely severe. So with this setup, there was an obvious sweet spot, though it was reasonably large.

 

Mod 3 C + TV 55PL* + Lumicon Night Sky filter

Expected reduction: 0.48x; f/6 --> f/2.9; 6.5x and 6.0° TFOV.
Actual reduction: ~6.0° as estimated in the field, so I would agree with the expected reduction numbers.

 

The combo without the reducer had much more of its usable field (by %) actually usable. Field curvature was much less as well. The increase in magnification was quite noticeable as M22 went from being a bright fuzzy ball to partially resolved and other mag 7 globs went from somewhat noticeable as globs to obvious. I could still see the dark nebulae in the heart of the MW star cloud at the spout of the Teapot with this combo, so it is not too much behind compared to to focally reduced setup and the extra magnification helps.

 

Lumicon Night Sky filter- A great filter. Stars were not attenuated as I saw tons and tons of them in the Summer MW and the nebulae such as the the Lagoon, Trifid, Swan, Eagle all came through decently. A great combo filter when you don’t want to lose the stars and still see the brighter nebulae.
 

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#19 moshen

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Posted 08 June 2018 - 02:49 PM

Nice reporting! The Lumicon Night Sky 645nm long pass is my most used filter. I have two of them, one for the filter wheel and one for quick 1x viewing.



#20 GeezerGazer

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Posted 16 June 2018 - 01:04 AM

Syed, nice report. Have you or Moshen used a Lumicon Deep Sky filter? I was using it with very nice results too. Very little attenuation with nice H-a enhancement.

The addition of a reducer does introduce or perturb field aberrations... but a gentler reducer might work OK without loosing so much true field to them. Gavin had pretty good luck with his 55P and AP .75x. And I’ve had good luck with my .8x reducer, but have not used it afocally. I’ll remedy that as soon as I get home and will include some photos to show results. I do have a 2” - 7nm H-a. I also have a 2” - .5x reducer, so will try to do a comparison photo to show differences in useable field. AND, I should have the adapter to use my 1.25” 40mm MaxView. So with some adapter fiddling, I should be able to show some differences. This will take a couple of days after I get home tomorrow.

Edited by GeezerGazer, 16 June 2018 - 01:12 AM.


#21 Starman81

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Posted 16 June 2018 - 03:41 PM

Syed, nice report. Have you or Moshen used a Lumicon Deep Sky filter? I was using it with very nice results too. Very little attenuation with nice H-a enhancement.

The addition of a reducer does introduce or perturb field aberrations... but a gentler reducer might work OK without loosing so much true field to them. Gavin had pretty good luck with his 55P and AP .75x. And I’ve had good luck with my .8x reducer, but have not used it afocally. I’ll remedy that as soon as I get home and will include some photos to show results. I do have a 2” - 7nm H-a. I also have a 2” - .5x reducer, so will try to do a comparison photo to show differences in useable field. AND, I should have the adapter to use my 1.25” 40mm MaxView. So with some adapter fiddling, I should be able to show some differences. This will take a couple of days after I get home tomorrow.

 

Thanks Ray. I have not yet used a Lumicon Deep Sky filter. Looking at the transmission graph of Deep Sky vs Night Sky filters (labeled h-alpha filter), it looks like it lets in quite a bit more light than the Night Sky filter. If you can, please post a pic. I am curious to see the color of the filter. 

 

As for the focal reducing... I will try my 0.7x 2" reducer on the TV 55 Plossl as well next time and see how that works. I'm sure it will be less aberrated, but there is something to be said of the TV 55 Plossl with the 0.5x reducers (aberrations and all). The central part of the FOV is not compromised and in that region, with this setup, you are going to get the brightest NV views in your scope. It is akin to a monocentric eyepiece for planetary observing but instead of supposed maximum sharpness in a monocentric, we're talking of maximum brightness in the NV doubly focally reduced system. 



#22 GeezerGazer

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Posted 19 June 2018 - 04:50 AM

I spent 3 hrs looking at Arcturus tonight!  Well, sort of.  Mainly I was using just about everything I have to test reducers and afocal together in two scopes, an ST 120 f:5 and a ScopeStuff VersaScope 60 f:3.8.  I took photos to help demonstrate what I found when using afocal alone or afocal with a reducer attached.  Forgive that some of these are not precisely focused; my goal was to test the presence of vignetting and field aberrations.  So here goes...

 

The first set of photos was through the ST 120, using 2" nose attached to a T2 prism diagonal.  To make this work in my scope which has a replacement GSO two speed focuser which is shorter than the stock focuser, I employ a 35mm extension tube in front of the diagonal and I add a 15mm T2 extension tube under the 1.25" ClickLock eyepiece holder.  This scope has lots of back focus available.  These photos were taken using an Optolong 685nm IR filter to block LP at home... I rarely observe from home.  

 

At prime, I found a reflection at 6 o'clock just below Arcturus, probably from the filter at the bottom of the nose; I never use IR filters at my green zone observing site, so this was new to me.  At my home, I'm surrounded by sodium vapor and LED street lights with no place to hide from the direct glare.  

 

With afocal, I found one or two reflections at 12 o'clock, just above Arcturus.  I have no doubt that this was caused by the afocal arrangement.  When I moved Arcturus from the center, the reflection moved with the star.  Look for them in every afocal image, whether using the 40P or the 55P.  

 

1.  This photo is at native/prime, using a 1.25" nose on the Mod 3C, filter attached to the nose. This photo is for perspective, to show the native FoV:  ISO 32, 1/3s, 8s stack

IMG_1877.JPG

 

2.  Removed the 35mm extension tube and added a 1.25"/ .5x reducer between the 685nm filter and the 1.25" nose on the Mod 3C:  ISO 100, 1/12s, stacked 8s.  This arrangement shows significant vignetting; not as significant visually at the Mod 3C eyepiece.  Photos tend to highlight the effects of vignetting.

IMG_1879.JPG

 

3.  Replaced the 35mm extension tube and put the 685nm filter on the bottom of a Scopetronix, MaxView 40mm Plossl.  This older eyepiece has a sliding outer barrel with TT threads on the top; I had an adapter made by RAF Camera to screw onto the TT threads and into the Envis lens for a strong connection.  This arrangement provides very little falloff in field illumination:  ISO 29, 1/6s, 8s stack

IMG_1882.JPG

 

4.  Same arrangement as #3, but added a 1.25"/ .5x reducer between the filter and the eyepiece barrel; resulted in significant vignetting, enough so that any gain in the FoV was offset by the vignetting:  ISO 29, 1/10s, 8s stack   

IMG_1883.JPG

 

5.  Changed to 2" ClickLock eyepiece holder and used a TV 55 Plossl, TNVC adapter to Envis lens; no filter:  ISO 29, 1/25s, 8s stack

IMG_1884.JPG

 

6.  Same as #5, but removed the 2" diagonal nose and replaced it with a 2" to TT Orion .8x reducer, screwed the reducer directly to the diagonal, providing a slightly larger FoV:  ISO 29, 1/35s, 8s stack

IMG_1885.JPG

 

7.  Same as #5, but added a 2"/ .5x reducer to the bottom of the 55P (the .8x reducer used for photo 6 was removed from the diagonal and the 2" nose applied); resulted in significant vignetting:  ISO 29, 1/50s, 9s stack

IMG_1886.JPG



#23 GeezerGazer

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Posted 19 June 2018 - 05:52 AM

The next scope I used was a 60mm achro f:3.8 VersaScope from ScopeStuff.  I like this little scope for really wide FoVs.  

 

For these photos, I screwed the T2 diagonal directly to the back of the scope and used a 1.25" Clicklock eyepiece holder.  A 1.25" nose with the 685nm filter was applied to the Mod 3C; this is native focal length:  ISO 29, 1/2s, 8s stack

IMG_1889.JPG

 

This second photo is the same arrangement but the 1.25" nose was replaced with a C to TT adapter and filter capture ring holding the 1.25"/ .5x reducer with 685nm filter.  At this close proximity to the focal plane, there is visible reduction, but placing the reducer farther from the focal plane to work at .5x results in very severe vignetting.  Vignetting was bad enough with this arrangement:  ISO 29, 1/7s, 8s stack

IMG_1888.JPG

 

As you can see, Arcturus was setting behind a tree so I didn't have time to do any further testing with the 40P in afocal.  Maybe tomorrow.  

 

Here's some of my thoughts about the test results.  First, vignetting does not bother some observers and it is less obvious/annoying at the eyepiece than it is in photographs.  For me, the level of vignetting caused by the addition of both the 1.25" and 2"/ .5x reducers in these photos is beyond my threshold of tolerance.  Afocal with both the 55P and 40P does reduce the focal ratio without causing significant vignetting.  But adding the .5x reducers or the .8x reducer did present more vignetting than I want to see, even visually, and in photos, it caused so much vignetting that the increased FoV was nearly cancelled by the vignetting.  For me, there is no gain in adding the reducer to the afocal arrangement because I like taking photos.  Visually, I saw the gain as being quite insignificant.  I MUCH preferred the native FL images from my smaller scope over the highly vignetted image with similar FoV from the ST 120.  

 

Having said this, and having taken photos to show what I saw, I will mitigate the results by adding that if I used a higher ISO or longer exposure, resulting in a brighter image, the vignetting would not look nearly as bad.  Alas, I prefer a darker sky background, so even illumination of the sensor is key for me.  If I'm able tomorrow, I'll take 3-4 more photos to show what this would look like, and I'll try to keep it on Arcturus for a fair comparison.  I'll also test to see if the filter and afocal reflections which showed up at my heavily light polluted home, reappear at a darker site.  

 

I'm hoping these images will help those who are wondering whether they should go with prime vs afocal.  These photos demonstrate that problems (reflections, distortions, etc) can arise with either method, especially when dealing with bright, point light sources... like Arcturus.  

 

Finally, different optical systems will react differently to the addition of reducers and different reducers, made for a system may have a dramatically different effect.  These test images represent but two refractor telescope optical systems.  So YMMV.  

 

Here are 3 more images I took tonight, all of the Gamma Cygni H-a complex.  All three were with the 60mm VersaScope achro at f:3.8 (about 8.4x).  These were taken with 12, 7 & 5nm H-a filters.  Settings were adjusted to keep the image at about the same brightness.  Note how the star field shows so well with the 12nm, but fades with progressively narrower band pass filters, while the narrower filters show the H-a subject better. I think these photos clearly demonstrate why I do not like vignetting.  Cropping the edges of these images with a dark halo just isn't appealing to me.  

 

12nm H-a, ISO 3712, 1/8s, 8s stack

IMG_1895.JPG

 

7nm H-a, ISO 1600, 1/4s, 8s stack

IMG_1892.JPG

 

5nm H-a, ISO 2500, 1/5s, 8s stack

IMG_1894.JPG


Edited by GeezerGazer, 19 June 2018 - 09:17 AM.

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#24 Eddgie

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Posted 19 June 2018 - 12:23 PM

Lots of effort and I appreciate your efforts.

 

Here are some thoughts, though they come with the very strong admittance that the changes in exposure and the way the camera behaves because of the different image scale/pixel ratio is hard to fully assimilate into my own observations.

First, and perhaps most noticeable to me is that my first pass was not looking at the vignetting so much as it was looking at limiting magnitude and this is what I see.  It looked to me like the prime focus view showed much fainter stars than the other views.  Now, this is again possibly as much to do with the scale on the chip as anything else, but in the first image, I could see fainter magnitude stars around the target star than I could in the other views. 

Second, and something I have been saying for quite some time, is that where illumination falloff takes the real toll is when the nebula is larger than the field of view, and this is quite common when viewing very large nebular regions. This is really well shown in the Gamma Gygni pictures where the field is probably fully illuminated over most of the photocatode.  The illumination falloff of a compressed image is going to mean that one trades some brightness for the area at the center of the field for seeing less detail and structure at the edge of the field.  These last pictures show that very well in that the nebula, particularly near the edge of the field in between 9 and 12 is quite detailed, and getting it by using any kind of aggressive reducer is likely to suppress this structure. 

 

So, it is a two bladed knife.  Achieving a wide field with reducers for the purpose of viewing nebula means that you get a wider field, but the loss of illumiation works against showing you the complete range of structure in the true field.  You might get more brightness at the center but here, the reduced angular size may prevent one from seeking out the finest detail.

 

I am not saying one way is better than the other, but with everything in optics and telescopes, there is always a compromise.

 

This is why I recommend starting with scopes that have by design, a fast focal ratio and a very large, fully illuminated field.

With reflectors, the secondary mirror is almost always the limit and pushing past the design (which is of course based on a 7mm exit pupil and the low light behavior of the human eye that can't easily see the illumination falloff anyway).

 

With SCTs, the situation is worse, with edge of field illumination falling to miserable levels once reduced past about f/5 but even at .5x of reduction, the field illumination loss at the edge of the field of an SCT (when using an image intensifer) is hard to see.

 

With refractors, generally the front of the focuser tube imposes the same limit as the central baffle of the SCT.  You can only go so wide before the field illumination falls off, but unless the reduction is quite aggressive (and I have to say that in some of these cases, that is likely what is happening) this may not be easily seen.   Shortening a refractor to be able to reach focus though does induce the possibility of aperture reduction occurring due to the objective side of the focuser tube intruding into the light path.

 

This is nice work though. It cleanly shows the vignetting, but it may not really show the true effects of illumination falloff on extended targets.

 

My request/recommendation would be this because (in my opinion) the most powerful driver of focal ratio is nebula. Try finding a small, dim extended target that fits easily into the size true field you estimate would be fully illuminated.  Take a picture of the target there, and then move the brightest center of that target to the edge of the field and image it again.  This would be something I would enjoy seeing and might be useful to others in trying to decide whether aggressive reduction would be useful to them when exploring larger nebula. 

 

I think afocal is probably the most effective way to go (if the scope is not an astrograph) because any good designer would have allowed for about a minimum of a 10mm fully illuminated field and at least 75% illumination at the edge of the field when using the largest field stop that would work with a 7mm exit pupil.   

 

Again, this is why I recommend astrographs.  They are designed to provide near full illumination over the full frame of a given size chip.  My Boren Simon delivers very high illumination over the size of an APS-C sensor, and this is close to the size of the photocathode.  I am very pleased with the way nebula remains as bright at the edge of the field as it is at the center and this is why I request/recommend this kind of comparison.  II don't think is really not all about true field for nebula, but rather finding a balance that shows you the most nebula possible in any given field size.

 

Wonderful work though and it very dramatically illustrates the vignetting, but I am perplexed as to why the prime focus image shows the best limiting magnitude on stars.  I could see several stars (going by position) in the prime focus view that were difficult to see in the other views.  Maybe the compression used caused this???


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#25 Eddgie

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Posted 19 June 2018 - 12:40 PM

With respect to my last, and I hope you don't mind me using a copy of the image, here is a picture that I would use to kind of explain my concern. This picture shows some of the nebula (in the circle) that I can see at the edge of the field.
point of interest.jpg
 
For this picture you used a small finder type objective and with the speed, this frame was probably fully illuminated, but as can be seen in the picture, this nebula at the edge is pretty dim.

Suppose though, you started with a larger but slower scope and reduced your way there.  Let's say that you started at f/6 and were able to reduce (though whatever method) to f/3.  Accounting for the scopes starting fully illuminated field and accounting for the increased falloff due to the expanded true field size, it could be possible that while you could match the resulting brightness at the center of the field, by the time you got to the outside, you could actually be working at the equivalent brightness of a slower focal ratio.  You might see the nebula at the center of the field appearing just as bright as at the center of the smaller scope (or perhaps a tiny bit brighter) but you might have so much illumination falloff that even though the field could be the same angular size, you could miss seeing this edge of field nebula.

If you like experimenting and taking pictures, this might be a fun thing for people to see and I know I would sure like to see it. 
 
Great work in showing the effects of vignetting with aggressive filters though. I very much enjoyed it and thank you for the effort and eagerness to share it with us!


Edited by Eddgie, 19 June 2018 - 12:48 PM.

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