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Experiments in extreme focal reduction

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#26 slavicek

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Posted 19 June 2018 - 01:40 PM

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

attachicon.gif 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

attachicon.gif 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

attachicon.gif IMG_1895.JPG

 

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

attachicon.gif IMG_1892.JPG

 

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

attachicon.gif IMG_1894.JPG

Thanks for sharing this with us. I will have to read your report couple of times to digest it but it will definitely help me with my "NV trial and errors" at the telescope. 



#27 moshen

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Posted 19 June 2018 - 01:46 PM

Ray, nice experimenting! The photos are really useful to illustrate your testing.

 

I've been experimenting quite a bit with focal reduction and vignetting and have found success with a 2" GSO 0.5x focal reducer. With focal reducers a 1.25" doesn't have enough clear aperture fully illuminated image plane for the photocathode. If the PC uses a 18mm diameter focal plane of the scope without reduction, then at 0.5x reduction it effectively uses a 36mm focal plane and the 1.25" 0.5x reducer only has a 22mm clear aperture.  It doesn't have enough clear aperture to take a 36mm focal plane and compress it to 18mm to give the effective reduction without light falloff. 

 

For the 2" 0.5x reducer I space the reducer to give a 0.6x effective reduction. In the photo below there is still very slight falloff at the edges but not noticeable visually & I believe this is the restriction from the 2" focuser of the Borg 90FL I'm using.

 

hFUxMkZ.jpg


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#28 chemisted

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Posted 19 June 2018 - 03:27 PM

Ray,

 

I have a quick question.  From you notes, it looks like you used the 55mm Plossl with your T-2 diagonal. If this is so, the diagonal only has ~33mm clear aperture while the field stop of the eyepiece is 46mm and my understanding is this will not give the full benefit of wide field viewing that a 2 inch diagonal would provide.  Am I missing something?

 

Ed



#29 GeezerGazer

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Posted 19 June 2018 - 03:50 PM

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. 
 

 

Ed, I'll give it a try tonight at prime native FL, with a reducer and with afocal.  I might try to do a subjective comparison between an image and the view at the eyepiece as well.  



#30 GeezerGazer

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Posted 19 June 2018 - 04:23 PM

Ray,

 

I have a quick question.  From you notes, it looks like you used the 55mm Plossl with your T-2 diagonal. If this is so, the diagonal only has ~33mm clear aperture while the field stop of the eyepiece is 46mm and my understanding is this will not give the full benefit of wide field viewing that a 2 inch diagonal would provide.  Am I missing something?

 

Ed

You have a sharp eye.  I only had the T2 diagonal last night; I planned to take a 2" diagonal out tonight and run the 55P photo again.  It should show virtually no vignetting, similar to my 40 Plossl.  Even with the T2 diagonal (mine has a 34mm clear aperture) the 55P shows mild vignetting compared to the .5x reducers.  My observing buddy is returning my 2" diagonal tonight. 

 

Ray, nice experimenting! The photos are really useful to illustrate your testing.

 

I've been experimenting quite a bit with focal reduction and vignetting and have found success with a 2" GSO 0.5x focal reducer. With focal reducers a 1.25" doesn't have enough clear aperture fully illuminated image plane for the photocathode. If the PC uses a 18mm diameter focal plane of the scope without reduction, then at 0.5x reduction it effectively uses a 36mm focal plane and the 1.25" 0.5x reducer only has a 22mm clear aperture.  It doesn't have enough clear aperture to take a 36mm focal plane and compress it to 18mm to give the effective reduction without light falloff. 

 

For the 2" 0.5x reducer I space the reducer to give a 0.6x effective reduction. In the photo below there is still very slight falloff at the edges but not noticeable visually & I believe this is the restriction from the 2" focuser of the Borg 90FL I'm using.

 

hFUxMkZ.jpg

Moshen,

Your N. American nebula is really beautiful... you are an excellent Phonetographer!  So you've got your 2" reducer spaced at about 35mm instead of the 54mm required for .5x?  But I still see light falloff at the perimeter... fainter stars in the inky black halo.  I think Ed's proposed test is worth the time to do, because it will show what the perimeter might be missing because of illumination falloff.  I'll give it a try tonight and see what happens.  The benefit of a really fast, native focal ratio is abundantly clear.  I certainly can see the difference between my f:6 TV-60 at 14x and my f:3.8 VersaScope 60 at 8.4x... both provide a well illuminated image but at very different scale.  I was hoping to actually observe tonight, but it looks like I'm testing stuff again.  I'll see what targets I can squeeze in!  

Ray


Edited by GeezerGazer, 19 June 2018 - 04:24 PM.


#31 Eddgie

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Posted 20 June 2018 - 10:10 AM

Ray,

 

I have a quick question.  From you notes, it looks like you used the 55mm Plossl with your T-2 diagonal. If this is so, the diagonal only has ~33mm clear aperture while the field stop of the eyepiece is 46mm and my understanding is this will not give the full benefit of wide field viewing that a 2 inch diagonal would provide.  Am I missing something?

 

Ed

He is not using the full field so this does not matter that much.  A 55mm plossl has a field stop of 46mm and an apparent field of 50mm, but the NV device only uses a portion of that field.   The vignetting from the diagonal wold be mostly at the outside of the field in the 55mm Plossl, but again, the NV device does not use the full width of the field the Plossl could deliver.  Most of the vignetting would be outside of the field that the NV device can actually see.  There is some vignetting but it should not be severe.


Edited by Eddgie, 20 June 2018 - 10:18 AM.


#32 Eddgie

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Posted 20 June 2018 - 10:30 AM

Ray, nice experimenting! The photos are really useful to illustrate your testing.

 

I've been experimenting quite a bit with focal reduction and vignetting and have found success with a 2" GSO 0.5x focal reducer. With focal reducers a 1.25" doesn't have enough clear aperture fully illuminated image plane for the photocathode. If the PC uses a 18mm diameter focal plane of the scope without reduction, then at 0.5x reduction it effectively uses a 36mm focal plane and the 1.25" 0.5x reducer only has a 22mm clear aperture.  It doesn't have enough clear aperture to take a 36mm focal plane and compress it to 18mm to give the effective reduction without light falloff. 

 

For the 2" 0.5x reducer I space the reducer to give a 0.6x effective reduction. In the photo below there is still very slight falloff at the edges but not noticeable visually & I believe this is the restriction from the 2" focuser of the Borg 90FL I'm using.

 

hFUxMkZ.jpg

Great shot!  Love it!  This is a titanically rich area of the sky and one that bears study over powers ranging from 1x to 100x.  At every power I use (different lenses and telescopes, all fast) it gives up a different kind of detail.  Fantastic pic though!  

 

This really illustrates the fact that this region is essentially completely covered with nebula!  You can go out in every direction several degrees and see that it is continuous but the key to knowing that there is nebula in the field is to have a wide enough fully illuminated field that you can see the slight variations in density as you sweep past.

 

Just a really wonderful picture and I hope that for non-NV users, illustrates the kinds of things we can actually see in real time.

 

I hate taking them, but I love to see other people's pictures.

 

My cell phone is a mid-line Android, but the camera is not great.  Maybe with my next phone, I will spend more to get a good camera.  As much as I don't like imaging, this actually looks like fun because it is so much simpler and less gear intensive than most imaging I have done in the past. 

 

Thanks for posting!



#33 moshen

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Posted 20 June 2018 - 03:24 PM

Moshen,

Your N. American nebula is really beautiful... you are an excellent Phonetographer!  So you've got your 2" reducer spaced at about 35mm instead of the 54mm required for .5x?  But I still see light falloff at the perimeter... fainter stars in the inky black halo.  I think Ed's proposed test is worth the time to do, because it will show what the perimeter might be missing because of illumination falloff.  I'll give it a try tonight and see what happens.  The benefit of a really fast, native focal ratio is abundantly clear.  I certainly can see the difference between my f:6 TV-60 at 14x and my f:3.8 VersaScope 60 at 8.4x... both provide a well illuminated image but at very different scale.  I was hoping to actually observe tonight, but it looks like I'm testing stuff again.  I'll see what targets I can squeeze in!  

Ray

 

 

Ray, I have the 2" GSO reducer about 12mm away from the Mod3 body to the top of the reducer. I had a custom C-Mount to M48 female thread machined with that spacing by a nice CN user who did it at a reasonable cost (if anyone wants the contact info reach out and I can share it).

There is still vignetting but it's much better than with the 1.25" 0.5x reducer I had. The remaining vignetting may be from the scope itself - in this case the focuser is only 2".



#34 GeezerGazer

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Posted 20 June 2018 - 08:56 PM

Moshen,

I think your image of the N. American Neb. is really quite stunning as is... so your custom adapter to get the spacing to an optimal distance was a good investment!  With my equipment, I'm not sure it would help much... would probably have to calculate the light cone diameter at that point in my ST 120 to get the spacing right.  With my MaxView 40 in afocal mode, I'm getting about a .67x reduction, and I can use it in the tiny VS 60, the ST 120 and my 140... see results below.  BUT, I have not decided yet if I'm actually going to bother with afocal.  I don't really like the long stack in the diagonal.  It might be an OK system for a Newt (no diagonal), but in a modest size refractor, it is quite unruly, and balance issues are a concern, especially when using my 140 on the AZ Pro.  There are always tradeoffs, but I'm coming to the conclusion that the scale differences offered by my 3 scopes and a barlow, is about all I need.  I might look into a fast 75-105mm camera lens for wider TFoV than is offered by the VS 60.  I can't stand the vignetting caused by the 3x afocal when attached to the Envis lens.  

 

Last night I completed some additional testing as Eddgie suggested and agree that it provides a better test of edge illumination falloff.  I did use a 2" diagonal with the 55 Plossl, so results are a better measure of its performance than the previous examples.  Ed, I know you suggested a fainter nebula, but the half moon last night was brightening the sky so much, I thought the Gamma Cygni complex would be an OK target.  In each set of photos, I first placed Sadr at the bottom of the Fov and then placed it in the middle of the FoV for the second image.  All of these photos are unprocessed except for cropping and compression to fit here, white balance was left on automatic mode, and focus was not perfect in some, but they are good enough for comparing illumination falloff.  So here goes...

 

First set of photos taken through the Short Tube 120, F:5 native FL, T-2 prism diagonal (except with the 55 Plossl, when I used a 2" TV aluminized diagonal).   I used a 1.25"/ 7nm H-a for all images except with the 55 Plossl which required my 2"/ 7nm H-a filter.  

 

1.  ST 120 at native focal length (23x) for comparison, no reducer, 7nm H-a, Sadr at 6 o'clock, ISO 4000, 1/2s, 8s stack

IMG_1896.jpg

 

2.  Same settings as #1, Sadr at center

IMG_1897.jpg

 

3.  Using a 40mm Scopetronics MaxView for afocal TT connection, Envis to TT adapter into the Mod 3C, ISO 2500, 1/2s, 8s stack

IMG_1899.jpg

 

4.  Same as #3 but Sadr centered

IMG_1898.jpg  


Edited by GeezerGazer, 20 June 2018 - 08:59 PM.

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#35 GeezerGazer

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Posted 20 June 2018 - 09:20 PM

5.  2" diagonal, 55 Plossl, TNVC adapter to Envis, ISO 1250, 1/7s, 8s stack

IMG_1900.jpg

 

6.  Same as #5, Sadr centered, same settings

IMG_1901.jpg

 

7. Same as #5, but added 2" .5x reducer to bottom of the 55 Plossl, ISO 500, 1/2s, 8s stack

IMG_1903.jpg

 

8.  Same as #7, Sadr centered, same settings

IMG_1902.jpg

 

9.  I took this image at a much brighter setting to show how vignetting is mitigated by additional light.  Although the halo appears to be "smaller" at the perimeter as it is much more defined; the brighter image is less appealing to me.  ISO 100, 1/2s, 8s stack

IMG_1905.jpg


Edited by GeezerGazer, 20 June 2018 - 11:19 PM.

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#36 GeezerGazer

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Posted 20 June 2018 - 11:11 PM

Last two comparisons... still using the ST 120, and 7nm H-a filters:

 

10.  These two are using an Orion .8x reducer attached to the front of the T2 diagonal and the NV used at prime focus.  Almost no vignetting is visible.  ISO 4000, 1/10s, 8s stack

IMG_1907.jpg

 

11.  Same arrangement as #10, Sadr centered, ISO 1250, 1/2s, 8s stack

IMG_1908.jpg

 

12.  This one used the .8x reducer attached to the diagonal, with the 40mm MaxView in afocal, ISO 640, 1/3s, 8s stack; the outer field does not look pretty!

IMG_1910.jpg

 

13.  Same as #12 with Sadr centered, same settings

IMG_1909.jpg

 

Looking at these images, you can see where vignetting occurs and where aberrations are introduced in the outer field.  Although the Orion .8x provides much less vignetting and a pretty good image quality when used in the prime system (images 10-11), when placed in front of the 40mm Plossl (afocal), outer field distortions are abundant (images 12-13).  For both the 40 and 50mm Plossls, without reducers (images 3-6), used in afocal, I see obvious aberrations in the outer field through the ST 120, so the 8x reducer may not be the cause of the aberrations seen in images 12-13.  Again, images 10-11 demonstrate that the .8x reducer does not exacerbate aberrations in the outer field when used in a prime system.  I found the use of .5x reducers combined with either the 40 or 50mm Plossls to be wholly unsatisfactory because they produced severe vignetting combined with outer field aberrations in this scope.  

 

Reducers are usually designed for the optics used in most apochromatic refractor astrographs; that is, manufacturers design the reducers to work optimally with their scopes, eg, Astro Physics, TeleVue, Takahashi, William Optics, and other companies offer reducers for specific scopes in their line.  And, although generic (and less costly) reducers are available for us to try, I think their performance is less than optimal simply because they are made for a wide variety of scopes.  I know that my $250 Orion Short Tube 120 does not perform like a Tak TSA 120 on many levels, and it too, may be part of the problem.  But the ST 120 does perform well with NV at prime focus... even with the .8x Orion reducer.  But when I added that reducer to either afocal system (40 & 50 Plossls), performance at the edge of field was obviously effected.  

 

This is the end of my testing for now.  I'm hoping someone else will test and show better results with reducers.  But  here is one last photo I took last night... it's at prime focus through the ST 120, with no vignetting and no distortion... and no reducer.  ISO 160, 1/2s, 8s stack

Ray

IMG_1935.jpg


Edited by GeezerGazer, 20 June 2018 - 11:37 PM.

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#37 The Ardent

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Posted 21 June 2018 - 01:49 AM

Question: What if a 12.5mm c-mount lens is used instead of a 26mm? 

 

https://www.ptgrey.c...125ha-1b-lens-3



#38 GeezerGazer

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Posted 21 June 2018 - 02:28 AM

I suspect that this lens would not fully illuminate the sensor.  But if you buy it and it does work, please let all of us know.  If it did perform with NV, it would be like the image through a wide field camera lens... less than 1x.  



#39 moshen

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Posted 21 June 2018 - 03:50 AM

A lot of good testing. The 55mm plossl afocal image now looks much better vignetting wise with a 2" diagonal.  There's very little vignetting visible there.

 

The images done with afocal reduction (with either 55mm or 40mm eyepiece)  look to have star aberrations at the edge that adding a focal reducer exacerbates. I'm guessing that aberration comes from the eyepiece/afocal stack as I've not seen that with prime and extreme focal reduction with only a reducer (I mainly see field curvature & a lot of vignetting instead).

 

Trying the 2" 0.5x reducer with ~11-12mm spacing from the Mod3-c body in prime it could be compared to using the 40mm Plossl in afocal for a ~0.6-0.65x focal reduction. It'd be interesting to see how the edge star field aberrations compare between both methods that give a similar FOV.

 

I wonder what that aberration is that gives those curved stars around the edge - maybe a combination of field curvature and astigmatism?

 

Some great images of the region - out of all of them the prime or prime + 0.8x reducer look the cleanest to me.


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#40 Eddgie

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Posted 21 June 2018 - 08:46 AM

Good images.

 

What I was more interested to see was not the aberration on stars, but to see how various modes affected the view of nebula in a system that was not well illuminated at the edge of the field.

 

Now these are relatively bright areas so it would be harder to see the difference than if the nebula were a threshold object, but it will do.

 

One of the best examples of what I was hoping to see is in #10 and #11, and these will help explain what my caution is regarding compression and off axis illumination falloff.

 

Note that in #11, the dark lane extending down from the butterfly shaped part of the nebula has almost disappeared by the time it reaches the edge of the field, but when Sadr is in the center of the field, this area shows it's true nature, with the dark rift bordered on both sides by very bright nebula.  Again, compare this in both pictures and it suggests to me that in this mode, the outer edge of the field is loosing extreme amounts of illumination.  

 

Now for smaller objects that are known to fit completely into the field, I think this kind of reduction is useful, but if the goal is to see how much nebula can be shown inside the field and see the full extension of nebula for a bit larger and diffuse nebula, this kind of compression greatly reduces the performance at the edge of the field. 

 

Of course you can do both, but this is why I am such a proponent of using fast astrographs.  These scopes generally have very good off axis illumination and if you compress them, you preserve a lot more off axis brightness.  A lot of compression is a double edge sword.  It gives you a brighter image at the center, but the cost is that you loose a huge amount of performance at the edge. 

 

I would suggest that people that want to use NV consider refractors with focuser tubes sized for imaging (2.5" to 3") or reflectors with secondary mirrors that can support a 25mm image circle when not compressed.  This way, when compressed, the nebula at the edges of the field will be just about as bright as thew would be if you moved them to the center of the field so you can see all of the nebula that there is in the field.

 

This has been the revelation with the Boren Simon scope.  Even at f/2.8, the field illumination is excellent across the entire field.  When I look at this area, the lane mentioned above is shown well all the way across the field of view so I see it all in balance with how bright it really is.  I loose no detail at the edge of the field.

 

That is me though.. I like to see every bit of detail in the field because as I re-frame the view, I know that I am always seeing all of the detail.

 

These are great experiments!   I know you spent a lot of time with them and I appreciate your willingness to do this and share.


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#41 Eddgie

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Posted 21 June 2018 - 08:46 AM

And great pic of the Lagoon!



#42 GeezerGazer

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Posted 22 June 2018 - 12:01 PM

One of the best examples of what I was hoping to see is in #10 and #11, and these will help explain what my caution is regarding compression and off axis illumination falloff.

 

Note that in #11, the dark lane extending down from the butterfly shaped part of the nebula has almost disappeared by the time it reaches the edge of the field, but when Sadr is in the center of the field, this area shows it's true nature, with the dark rift bordered on both sides by very bright nebula.  Again, compare this in both pictures and it suggests to me that in this mode, the outer edge of the field is loosing extreme amounts of illumination.  

 

Now for smaller objects that are known to fit completely into the field, I think this kind of reduction is useful, but if the goal is to see how much nebula can be shown inside the field and see the full extension of nebula for a bit larger and diffuse nebula, this kind of compression greatly reduces the performance at the edge of the field. 

 

A lot of compression is a double edge sword.  It gives you a brighter image at the center, but the cost is that you loose a huge amount of performance at the edge. 

 

I would suggest that people that want to use NV consider refractors with focuser tubes sized for imaging (2.5" to 3") or reflectors with secondary mirrors that can support a 25mm image circle when not compressed.  This way, when compressed, the nebula at the edges of the field will be just about as bright as thew would be if you moved them to the center of the field so you can see all of the nebula that there is in the field.

 

Ed, your sage advice is sound and I heartily agree.  But it is also true that each of us has a different tolerance level to aberrations, reflections, illumination falloff, vignetting, etc.  So while having the optimal scope for observing with NV may be the preferred route, it is something that involves choice and expense.  While the ST 120 may perform somewhat marginally compared to a 6"-8" Boren-Simon, it does so at a cost of $250, and in prime, I think the edge of field illumination is quite good... and it is 1/14 the cost of a Tak TSA 120 that I sold 3 years ago.  This testing has revealed to me, that I should just skip the use of reducers in preference to prime focus through alternate scopes already in my possession.  

 

I'm going to be traveling for the next 3 weeks but when I return, I plan to do some more comparison photos through optional optics that I have (eg, see two photos below).  If I had an unlimited budget, I'd order one of the 4x or 5x Litton prime lenses and have a 10nm H-a filter made like jdbastro.  But alas, I do not have an extra $2700 for such an endeavor.  My methodology must take a different route and I suspect each will have it's own set of issues.  But for now, I can do my H-a Phonetography in prime at 37x, 23x, 14x and 8.4x with my modest refractors and at 1x with the Envis.  I can do H-a at 2x with a 50mm Nikkor camera lens but it will slightly vignette because it requires a 52mm filter rather than the 2" (48mm) H-a filter already in my possession.  Tradeoffs!

 

The 55mm plossl afocal image now looks much better vignetting wise with a 2" diagonal.  There's very little vignetting visible there.

 

The images done with afocal reduction (with either 55mm or 40mm eyepiece)  look to have star aberrations at the edge that adding a focal reducer exacerbates. I'm guessing that aberration comes from the eyepiece/afocal stack as I've not seen that with prime and extreme focal reduction with only a reducer (I mainly see field curvature & a lot of vignetting instead).

 

I wonder what that aberration is that gives those curved stars around the edge - maybe a combination of field curvature and astigmatism?

 

Some great images of the region - out of all of them the prime or prime + 0.8x reducer look the cleanest to me.

Yes, afocal seems to introduce the outer field aberrations that are increased with reducers... at least, in this scope.  I think what we need though is a comparison through a Newt... which I don't have.  And, I'd be willing to bet that a longer focal ratio scope would provide fewer if any aberrations afocally with NV.  And this is probably why most of the really fast Newts use a Paracor/Sipps for field correction.  Maybe Ed, Peter or Mike L. could chime in here.  I'm thinking that before I sell all my afocal stuff, I should run a comparison photo through my TEC 140 which is f:7 to see if there is a difference at the edge of field.  One of the things I found most annoying about afocal (other than its cumbersome application) was the reflections I would see within the field from bright stars like Sadr.  Every afocal photo I took that included Sadr showed a reflection of Sadr nearby.  Glenn LeDrew warned about afocal reflections several months ago; I think it was in a thread started by Mike Lockwood but don't remember the topic.  

 

Of the images through the ST 120 scope, I agree, prime or prime with the .8x reducer appear best.

 

Here are two more interesting photos showing scale difference at prime focus... and edge of field illumination/falloff.  Please disregard the bright sky background in these photos; it is a result of the 50% moon while taking these images.  I usually try to avoid bright moon phase when taking images for my records.  

 

This Phonetography image is through the ST 120 at f:5, prime focus, no reducer, of the Heart Nebula.  Like all of my images, this is through my iPhone 6+.  Look specifically at the edge definition/illumination of the three arms (that make the lobes of the heart) as they approach the perimeter.  ISO 4000, 1/3s, 8s stack

IMG_1950.jpg

 

This next photo is through the VersaScope 60 at f:3.8, prime focus, no reducer.  I didn't notice until I got home that the upper part of this photo seems darker than it should be.  Visually at the NVD, it appeared that the image was fully illuminated right to the edge of field with NO falloff.  I suspect that my phone shifted in its bracket, causing the lens to be slightly offset from the hole through the bracket; I plan to relieve that hole by 1/16" giving a less critical tolerance to prevent that in the future.  BUT, this image clearly shows that there is no dimming in the three arms that create the heart lobes.  So the edge brightness illumination that Ed so perfectly describes is present with the ST 120 and probably with the VersaScope 60.  The issue becomes an aesthetic choice; is this amount of illumination falloff significant enough to warrant the expense of a specialized scope to overcome the falloff.  I'm interested to hear the opinions of others here looking at this difference.  ISO 1250, 1/2s, 8s stack

IMG_1956.jpg


Edited by GeezerGazer, 22 June 2018 - 12:18 PM.


#43 bobhen

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Posted 22 June 2018 - 12:40 PM

Nice report.

 

And yes, all things are a trade off.

 

I’ve been experimenting with some reduction as well. I’ve been really pleased with the 2” .7 reducer when used with my Mewlon 210 and Tak 120. Visually, when used with both scopes, there is little falloff at the edges.

 

I liked the filtered views through my 102mm F5 refractor at prime as well (as most also like the views through the 120mm F5 refractors) but I couldn’t use the .7 reducer until I got a shorter GSO focuser.

 

With the fast F5 refractor and the reducer, there is more falloff at the edges. I’m not sure if it’s actual vignetting or field curvature. However, whatever it is just isn’t that bad and not really an issue unless you actually move objects to the edge and start looking for some distortion. Most large objects like the complete North American Nebula or M8 and M20 (seen together) fit in the well-corrected part of the field and with room to spare.

 

I actually like the reduced view in my 102mm achromatic refractor at F3.5 with some very slight edge falloff better than the view in my 60mm ED at a native F6 with no edge dimming.

 

Bob


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#44 GeezerGazer

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Posted 22 June 2018 - 01:10 PM

I should add that there is no doubt in my mind that through my refractors, the center of the image is more brightly illuminated than the edge of field.  This can be demonstrated in post processing when altering brightness, contrast or black point... where changes are not even across the FoV, but are gradual from the perimeter toward the center of the image.  I do simple post processing before adding images to my observing files, so I see this phenomenon on photos if I use lighting adjustments in iPhoto on my Mac.  Photos not taken through my telescopes do no react this way during post processing because they are evenly illuminated by the phones camera lens.  Eddgie's description of even illumination vs a bright center illumination is clearly demonstrated below...

 

Here is the same photo from the ST 120 in the preceding post, but using only Exposure in post processing at .5 and 1x decrease, simply to demonstrate how the field illumination is gradually effected from the perimeter toward the center of the image.  Our eye might not see the stronger center field illumination in the unprocessed image above (a trained photographer will see it), but post process clearly shows how the brighter center remains brighter even as exposure is reduced.  See how the reduction in exposure (darkness) gradually creeps in from the perimeter?  This happens only because the original photo was brighter at the center than at the perimeter... as Eddgie explained.  What I'd REALLY like to see here is a couple of photos through Ed's Boren Simon or Jeff's Tak Epsilon to SEE the difference an astrograph makes in field illumination.  

 

This is with .5x reduction in exposure

IMG_1950 (3).jpg

 

This is with 1x reduction in exposure

IMG_1950 (4).jpg


Edited by GeezerGazer, 22 June 2018 - 01:22 PM.


#45 GeezerGazer

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Posted 22 June 2018 - 02:02 PM

Nice report.

 

And yes, all things are a trade off.

 

I’ve been experimenting with some reduction as well. I’ve been really pleased with the 2” .7 reducer when used with my Mewlon 210 and Tak 120. Visually, when used with both scopes, there is little falloff at the edges.

 

I liked the filtered views through my 102mm F5 refractor at prime as well (as most also like the views through the 120mm F5 refractors) but I couldn’t use the .7 reducer until I got a shorter GSO focuser.

 

With the fast F5 refractor and the reducer, there is more falloff at the edges. I’m not sure if it’s actual vignetting or field curvature. However, whatever it is just isn’t that bad and not really an issue unless you actually move objects to the edge and start looking for some distortion. Most large objects like the complete North American Nebula or M8 and M20 (seen together) fit in the well-corrected part of the field and with room to spare.

 

I actually like the reduced view in my 102mm achromatic refractor at F3.5 with some very slight edge falloff better than the view in my 60mm ED at a native F6 with no edge dimming.

 

Bob

Bob, there really IS a difference when using NV visually compared to attaching a camera to take photos.  Our eyes have a much greater dynamic range than the camera and our brain tends to even out some of the lighting and curvature issues that pop up in photos.  Using my .8x reducer visually in prime, I am completely satisfied with the image at the NV eyepiece.  But replacing the .8x with the .5x reducer introduces vignetting way beyond my tolerance level.  In afocal, the edge of field aberrations were less noticeable visually and were limited to the very outer edge, but adding any of my reducers exacerbated them and the visual image was impacted beyond my tolerance level.  In my test, using the reducers with afocal created more problems than I want to see visually, but especially in photos.  I know that some scopes are much more tolerant when adding reducers to the optical chain, so your good results may be part of that equation.  Gavster in the UK also has very nice results with his 2" AP .75x reducer when used afocally.  And my eyes have a variety of issues that might also be exacerbated by the optical system using reducers.  There are many variables, but overall, I will probably shy away from reducers and afocal when I use my NVD.  I suppose the next test needs to involve an astrograph to show how much more evenly it illuminates the FoV.  That will have to be someone else since I do not have an astrograph!  

Ray



#46 moshen

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Posted 22 June 2018 - 02:15 PM

I see the same edge star aberrations with some of Lockwood's 55mm afocal photos that use a SIPS Paracorr.

Here's a link to one of them off his blog post (14" operating f/2.93): 

 

IMG_6505_mini.JPG

 

They are less evident in the photos where he uses a slightly slower scope but still there. Here it his 20" operating at f/3.45

 

IMG_6504_mini.JPG

 

 

Perhaps Glenn or Mike could tell us what kind of aberration that is and if it's introduced by the eyepiece as it's not something I see with a focal reducer.

 

BTW, it's pretty amazing what can be done with a $249 120mm f/5 scope. bow.gif


Edited by moshen, 22 June 2018 - 02:28 PM.


#47 moshen

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Posted 22 June 2018 - 02:35 PM

 

This next photo is through the VersaScope 60 at f:3.8, prime focus, no reducer.  I didn't notice until I got home that the upper part of this photo seems darker than it should be.  Visually at the NVD, it appeared that the image was fully illuminated right to the edge of field with NO falloff.  I suspect that my phone shifted in its bracket, causing the lens to be slightly offset from the hole through the bracket; I plan to relieve that hole by 1/16" giving a less critical tolerance to prevent that in the future.  BUT, this image clearly shows that there is no dimming in the three arms that create the heart lobes.  So the edge brightness illumination that Ed so perfectly describes is present with the ST 120 and probably with the VersaScope 60.  The issue becomes an aesthetic choice; is this amount of illumination falloff significant enough to warrant the expense of a specialized scope to overcome the falloff.  I'm interested to hear the opinions of others here looking at this difference.  ISO 1250, 1/2s, 8s stack.

I see the difference you mentioned. It's less noticeable on stars since they are point sources. The ST120 has a 2" focuser which limits the fully illuminated FOV, much like my Borg 90mm with its 2" focuser and why refractors designed as astrographs have very large focusers.  It's the front opening of the 2" focuser near the lens that limits the illuminated field. Make it short and illuminated field is larger but there is less focus travel and vice versa.  You can get both large illumination and good focuser travel by making the diameter larger. My 100mm Tak has a 2.7" focuser for this reason. On a $249 scope I think it's a very acceptable compromise though. lol.gif


Edited by moshen, 22 June 2018 - 02:40 PM.

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#48 GlennLeDrew

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Posted 22 June 2018 - 03:08 PM

moshen,

About Mike Lockwood's image pair you posted above. The dominant aberration is clearly field curvature, which simply results in increasing defocus with field angle. Present also is some astigmatism, here evinced as tangential elongation.

 

In all cases where vignetting is present, it is useful to know the source or sources. One potential causative mechanism in the afocal configuration is incorrect placement of the NVD lens's own entrance pupil w.r.t. the exit pupil. This is suggested when the device's image exhibits *notably* more vignetting than can be explained by our visual system's higher tolerance to illumination variance when looking directly by eye.

 

Might I presume that folks are ensuring optimal interfacing of pupils? Ideally via a coupling mechanism which permits adjustment of spacing between eyepiece and NVD objective? If one is constrained by a fixed-thickness coupler, or quantized spacings via 'step rings' and the like, this could result in additional fall-off on top of that imparted by the telescope end of things.


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#49 moshen

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Posted 22 June 2018 - 03:20 PM

moshen,

About Mike Lockwood's image pair you posted above. The dominant aberration is clearly field curvature, which simply results in increasing defocus with field angle. Present also is some astigmatism, here evinced as tangential elongation.

 

In all cases where vignetting is present, it is useful to know the source or sources. One potential causative mechanism in the afocal configuration is incorrect placement of the NVD lens's own entrance pupil w.r.t. the exit pupil. This is suggested when the device's image exhibits *notably* more vignetting than can be explained by our visual system's higher tolerance to illumination variance when looking directly by eye.

 

Might I presume that folks are ensuring optimal interfacing of pupils? Ideally via a coupling mechanism which permits adjustment of spacing between eyepiece and NVD objective? If one is constrained by a fixed-thickness coupler, or quantized spacings via 'step rings' and the like, this could result in additional fall-off on top of that imparted by the telescope end of things.

Thanks Glenn for chiming in.

I guessed it was field curvature and astigmatism so looks like that's the case. On field curvature, I thought it was mostly a function of the focal length. A 14" f2.93 with 55mm afocal is running roughly 530mm focal length.

 

On my focal reduced 90mm refractor (with a simple 2" 0.5x reducer) at f/3.6 I am running 325mm focal length. But I don't see nearly the amount of field curvature or astigmatism present on the edge of the field:

 

iZkXnru.jpg

 

Given the very different setups what aspect of it do you think is causing the difference in amount of FC and astigmatism seen? My hunch is the Plossl is introducing it - given it may not be well corrected at that speed at the edge, but I don't have a 55mm Plossl to compare.


Edited by moshen, 22 June 2018 - 03:30 PM.

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#50 Eddgie

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Posted 22 June 2018 - 04:10 PM

That will have to be someone else since I do not have an astrograph!  

Ray

I don't have a good enough quality of phone that I can take pictures, but I think these discussions are kind of hinting  at what I have suggested for some time now which is that an imaging scope with a good coma corrector/reducer like the ASA/Keller  might be the best compromise between speed, field quality, and ease of use.  The filter wheel makes it dead easy to compare views, and if I need more power I can Barlow up, or I can run the scope at f/4 with the MPCC.

 

All approaches have their strengths and all have their compromises, and the fact is that there are many ways to get there with NV and no matter how we choose to do it, we are going to see amazing sights! I still remember my first view of the fantastic star fields in the southern Milky Way in the Comet Catcher on my way back from LA.   I hear people on the other forums talking about rich field observing and I think how they would likely respond to one look though the Comet Catcher with the image intensifier.  I have been able to see views like this for three years now, and every time I see them it excites me. 


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