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NV afocal observing with TeleVue/TNVC adapter and a 20" f/3.0

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#51 GeezerGazer

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Posted 25 October 2017 - 04:17 PM

Masuyama also makes long FL Plossl type (50 degree AFoV) eyepieces at 45, 50 & 60mm.  But of course, these probably will not work mechanically with the TV adaptor.  

 

So the next question is... (assuming my scope has sufficient back focus) if my scope is f:7 and I screw a 2" .5x reducer to the bottom of a 55mm TV Plossl with sufficient spacing for it to actually work at .5x, then screw on a 5nm H-a filter, drop that in my diagonal and attach the Mod 3... what focal ratio is the Mod 3 actually seeing/working at?  Would this theoretically halve the scope's focal ratio twice?... not withstanding any visual aberrations introduced by the optical smokestack.  shocked.gif



#52 Eddgie

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Posted 25 October 2017 - 04:23 PM

Yeah, I can't either, but for someone using an SCT or MCT, the edge correction at these focal ratios is probably decent enough, though with the SCT, there is still going to be field curvature and coma but as the magnification gets lower, the aberratted blur gets smaller, so it could be that one of these would work.



#53 GlennLeDrew

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Posted 26 October 2017 - 08:43 PM

About the placement of interference filters behind the eyepiece... Mike is correct that the issue revolves around the AFoV. If the NVD lens was of a shorter f.l. and hence supplied a wider FoV, then in conjunction with a wide AFoV eyepiece bandpass de-tuning in the outer field would become worse.

 

If the NVD lens is of some limited FoV, then it will be the limiter, providing the same angular field limit irrespective of the eyepiece's AFoV (unless the latter is smaller than the former.)

 

Back in my film days I tried a UHC filter on a 50mm f.l. camera lens, which has a FoV roughly like that of a typical eyepiece AFoV. The outer field color shift was ugly, and response to nebulosity was notably impaired.

 

And so an interference filter of similar or narrower bandpass tasked with covering a similar field angle via placement between eyepiece and NVD lens will suffer de-tuning in the outer field.

 

About the use of a focal reducer with an eyepiece...

 

If the reducer results in an exit pupil larger than the NVD lens's entrance pupil, the image surface brightness does not increase from that obtained when the pupils match. You only get a reduced image scale (with concomitantly reduced resolution) and fainter stars captured.

 

About image scale and brightness...

 

Even though a smaller image scale resulting from a longer f.l. eyepiece might provide higher image surface brightness, the poorer resolving power often effectively fails to realize the impression of an improvement because finer, higher brightness structures are blended with dimmer bits so as to yield an averaged, blended object. That is, a dimmer but larger image can be more impressive simply due to the revealing of the smaller details.

 

The better way to quantify image brightness gains is to use a field-filling uniform-brightness source, such as a dimly lit wall indoors. No need to worry about focus whatsoever, for a uniform flux by definition is unfocused. Such a testing protocol will not be prone to such effects as point source depth of penetration and resolution of object details, both of which profoundly modify impressions of performance.



#54 Mike Lockwood

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Posted 26 October 2017 - 10:20 PM

I confirmed the de-tuning effect last night.

 

I hand-held multiple narrowband filters (mounted in a short threaded tube) in front of multiple NV units, all operating at 1X.  Objects in the center were seen with the filter not tilted, but when they were moved off-axis, they disappeared.  Tilting the filters brought them back.  So that's it, it's not the ENVIS, it's the filter.

 

My conclusion is that the best wide-field views (in terms of detail seen in objects that show up with narrowband filters) are not with 1X, but some higher power that allows the whole field to be narrow-band filtered without the dramatic de-tuning effect that makes objects at the edge of the field disappear.

 

Based on what I see with my 20" f/3.0, this should be achieved with a lens around f/3 or so, and ideally it is small enough that one can put a 2" narrowband filter in front of it.

 

So, just for a small side-topic here, but what lens is the best that the experienced NV users have found for this?

 

I'm just looking for suggestions, I don't want to go completely off-topic!


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#55 PEterW

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Posted 27 October 2017 - 04:06 AM

Nice check, as you point out these filters are designed for light going straight they, not at an angle.Or put the filter in front of the eyepiece running at a slower focal ratio for the filter benefit and then the eyepiece projection speeds up the view. A 2” filter nicely fits infeony of the 3xl ND for very wide views.

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#56 Jeff Morgan

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Posted 29 October 2017 - 10:26 AM

Doing some math.   The scope was working at f/3.45 in prime focus with Paracorr so effective focal length of about 1750mm

 

Effective focal ratios:

 

27mm Pan = ~ f/3.4 (Nominal f/stop increase)

 

41mm Pan = ~ f/2.4 (1.04 stops)

 

55mm Pan = ~ f/1.7 (1.41 stops)

 

I did not do the shorter focal length lenses because there is no gain here and like Borlow, the image just gets dimmer an dimmer.

 

So this begs the question of what aberrations one gets with "extreme afocal"?

 

For example, coma in the Newtonian example you start with. The final f/stop is f/1.7, which I would think far outstrips the ability of the Paracorr.

 

OTOH the Paracorr is upstream, so the correction has already happened by the time the signal gets to the afocal lens.

 

Has anyone gotten this fast yet, and what were the results?


Edited by Jeff Morgan, 29 October 2017 - 10:27 AM.


#57 Mike Lockwood

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Posted 29 October 2017 - 01:33 PM

I got that fast, and the photos are on my web page (which I assume you saw a link to earlier in the thread, but it sounds like you might not have).

 

My guess, based on what I see, is primarily off-axis astigmatism and field curvature seen in the eyepiece.



#58 GlennLeDrew

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Posted 29 October 2017 - 03:01 PM

If one has a more highly corrected eyepiece, the intensified view is concomitantly more highly corrected in its outer field. The NVD operating behind the eyepiece is fundamentally just like your eye operating behind the eyepiece; what the telescope in totality as a *system* delivers to whatever follows the eyepiece is what matters re. image quality. And as we all now know, the final f/ratio depends ultimately on the sub-system behind the eyepiece. For instance, our eye (f.l. ~22mm) when iris and exit pupil are about 7mm operates at around f/3.



#59 Jeff Morgan

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Posted 29 October 2017 - 05:58 PM

I got that fast, and the photos are on my web page (which I assume you saw a link to earlier in the thread, but it sounds like you might not have).

 

My guess, based on what I see, is primarily off-axis astigmatism and field curvature seen in the eyepiece.

 

I saw the photos, missed the focal ratio - I'll go back for a closer look.



#60 KJL

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Posted 02 November 2017 - 12:47 PM

Doing some math.   The scope was working at f/3.45 in prime focus with Paracorr so effective focal length of about 1750mm

 

Effective focal ratios:

 

27mm Pan = ~ f/3.4 (Nominal f/stop increase)

 

41mm Pan = ~ f/2.4 (1.04 stops)

 

55mm Pan = ~ f/1.7 (1.41 stops)

 

I did not do the shorter focal length lenses because there is no gain here and like Borlow, the image just gets dimmer an dimmer.

 

For those wanting to compute the effective focal ratio, it is simple to do:  

 

Find the exit pupil for the lens with the telescope you are going to use it in

Divide that number into the focal length of the lens on the camera (or NVD in our case)

 

The resulting number is the effective f/stop presented to the photocatode. 

This would be useful I think for people that have a scope that they are interested in doing afocal with. 

Thanks for the formula. So for my NVD Micro + C-mounted 25mm f/1.3 lens, an afocal setup with a 25mm Plossl will equate to the telescope's native f-ratio, right? It sounds like the Mod3's ENVIS lens is a 27mm.

 

It doesn't matter what the AFOV of the EP is (no difference between a 25mm Plossl and an ES100 25mm)?

 

How about the f-ratio of the NV's lens itself?



#61 PEterW

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Posted 02 November 2017 - 02:04 PM

Yes, just the focal length of the eyepiece, complex
Eyepieces may be better suited to faster scopes, but the NV lens will only observe the middle of the field of view so Nagler/100geg Fov will be wasted. All NV 1x lenses have about the same focal length, you could change those it’s a mount lens, but to get more benefit you’d need to go shorter.

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#62 KJL

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Posted 03 November 2017 - 07:23 AM

For lenses like my Computar — where the focal ratio can be adjusted from f/1.3 to f/16 — how does one calculate the net f-ratio from the point of view of the intensifier?

EDIT: I know this seems like asking the same question again, but I cannot believe the f-ratio of the intensifier’s lens doesn’t figure into the net f-ratio of telescope + eyepiece + lens train. After all, in extremis if I stop down the Computar lens eventually the image goes completely dark ....

Edited by KJL, 03 November 2017 - 07:26 AM.


#63 PEterW

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Posted 03 November 2017 - 08:42 AM

... keep all lenses wide open for the brightest view. If the fratio is slower than the other parts is will limit the image brightness, but these lenses are generally the fastest component.

Peter

#64 Mike Lockwood

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Posted 03 November 2017 - 11:29 AM

For lenses like my Computar — where the focal ratio can be adjusted from f/1.3 to f/16 — how does one calculate the net f-ratio from the point of view of the intensifier?

EDIT: I know this seems like asking the same question again, but I cannot believe the f-ratio of the intensifier’s lens doesn’t figure into the net f-ratio of telescope + eyepiece + lens train. After all, in extremis if I stop down the Computar lens eventually the image goes completely dark ....

As was said above, just leave the lens wide open.

 

Basically the longer the focal length of the eyepiece, the bigger the exit pupil it produces.  The exit pupil is what then goes into the lens on the front of the NV unit.  So, the bigger the exit pupil, and the more of the nightvision lens that is used, and the lower the effective f/#.

 

What it boils down to is if you're going to use a SIPS like I do, just get a 55mm TeleVue Plossl and that will give you the brightest NV image because the magnification is lowest.  A 41mm Panoptic also works quite well.



#65 SeymoreStars

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Posted 03 November 2017 - 11:54 AM

I will be testing it with my 35mm panoptic this weekend and will report back here. Hope to have the attachments for a video imager soon.



#66 KJL

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Posted 03 November 2017 - 07:48 PM

 

For lenses like my Computar — where the focal ratio can be adjusted from f/1.3 to f/16 — how does one calculate the net f-ratio from the point of view of the intensifier?

EDIT: I know this seems like asking the same question again, but I cannot believe the f-ratio of the intensifier’s lens doesn’t figure into the net f-ratio of telescope + eyepiece + lens train. After all, in extremis if I stop down the Computar lens eventually the image goes completely dark ....

As was said above, just leave the lens wide open.

 

Basically the longer the focal length of the eyepiece, the bigger the exit pupil it produces.  The exit pupil is what then goes into the lens on the front of the NV unit.  So, the bigger the exit pupil, and the more of the nightvision lens that is used, and the lower the effective f/#.

 

What it boils down to is if you're going to use a SIPS like I do, just get a 55mm TeleVue Plossl and that will give you the brightest NV image because the magnification is lowest.  A 41mm Panoptic also works quite well.

 

OIC, it's a direct match: if the iris of the NVD's lens cuts off the exit pupil then it's too small? Got it.

 

Now it makes sense why when used afocally I don't see any detuning effect (using your method above) from using a 2" filter between EP and the Computar lens: the exit pupil is much smaller than the full aperture of the Computar lens, or put another way the f/ratio of the telescope + EP is lower than the native f/1.3 of the Computar.

 

Thanks all for your help, and especially Mike for the article. As you know, John Pratte's own calculations preclude me from using my NVD at prime focus with my upcoming Sweet 16, but now I think I might be better off using it afocally anyway. It's more flexible, I don't have to refocus, etc. I've already purchased and tested the requisite (discontinued) Televue "Digital Camera Adapter Ring" (DEC-0037 in my case) and step-down thread converter to mate my NV Micro to Televue eyepieces with Dioptrix. It also fits over the threads of my XW eyepieces as well.

 

All set!



#67 KJL

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Posted 03 November 2017 - 08:03 PM

Can the TNVC/Televue adapter be used with other lenses? In particular, is the thread for the ENVIS 1x lens a standardized one for which step-down/up thread adapters could be purchased?



#68 Eddgie

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Posted 03 November 2017 - 08:37 PM

The ENVIS, PVS-7, and PVS-14 all share the same thread interface, which is 1.2" x 32 tread per inch. 

 

This is not a standard photo thread. I doubt that you would find almost anything off the shelf to adapt the Televue adatper to other objective lenses. 



#69 SeymoreStars

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Posted 03 November 2017 - 08:40 PM

Question how can the ENVIS to 1.25" adapter with Ha filter be introduced to this configuration?

 

Or simply how can an Ha filter be introduced to this configuration?

 

20171103 211956

Edited by sink45ny, 03 November 2017 - 08:44 PM.


#70 KJL

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Posted 03 November 2017 - 08:47 PM

The ENVIS, PVS-7, and PVS-14 all share the same thread interface, which is 1.2" x 32 tread per inch.

This is not a standard photo thread. I doubt that you would find almost anything off the shelf to adapt the Televue adatper to other objective lenses.


Thanks for the info.

#71 KJL

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Posted 03 November 2017 - 08:51 PM

Question how can the ENVIS to 1.25" adapter with Ha filter be introduced to this configuration?

Or simply how can an Ha filter be introduced to this configuration?


Ah, true nothing easily fits either the ENVIS lens or the TVNC adapter.

However for my much more standardized CCTV lenses with common filter thread specs, I can easily convert up to 48mm threads, screw on the filter, screw a 37mm step-down adapter to the other side of the filter, and finally attach the TV digital camera adapter to the other end.

This allows me to keep, say, an IR-pass or some other sky glow filter permanently attached to my Micro’s lens — useful for casual scanning of my light-polluted skies — and then quickly attach it to a TV eyepiece, all without removing the filter.

#72 Eddgie

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Posted 03 November 2017 - 10:54 PM

Question how can the ENVIS to 1.25" adapter with Ha filter be introduced to this configuration?

 

Or simply how can an Ha filter be introduced to this configuration?

 

Well, there are more ways to do afocal than just using the Televue system.

 

For example, you could get one of these and attach it to any eyepiece you like, mount the filter on the front of the NVD and put it on this bracket and point it at the eyepiece:

 

http://agenaastro.co...ra-adapter.html

 

Or, you could get a 40mm Plossl and put it into one of these.  Then you can use a T2 to 1.25" filter thread adapter and either screw it directly to the filter ring in the ENVIS, or mount a filter on the ENVIS and then screw the adapter into the filter.

 

https://www.ebay.com...pUAAOSwjDZYa2M8

 

So, there are other ways to do it rather than using the Televue adapter.

 

I kidn of like the first option better because it would make swapping filters much easier.


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#73 SeymoreStars

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Posted 03 November 2017 - 11:50 PM

I have a televue eyepiece projection adapter from many moons ago. I will give uncle Al a call to make sure his 40mm plossl will fit into it. I tested it with a 17mm plossl and it works by just resting the NVD in the eyepiece projection adapter, with the Ha filter screwed into the eyepiece. 


Edited by sink45ny, 03 November 2017 - 11:50 PM.


#74 chemisted

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Posted 04 November 2017 - 06:56 AM

I have asked Vic at TNVC (Clutch5150) to ask Uncle Al if an alternative face plate could be offered with a standard filter thread instead of the ENVIS thread.  This would provide a shorter path-length from TV eyepiece to NV device than a PreciseParts adapter that I will probably do if nothing better shows up.


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#75 SeymoreStars

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Posted 04 November 2017 - 11:05 AM

I confirmed the de-tuning effect last night.

 

I hand-held multiple narrowband filters (mounted in a short threaded tube) in front of multiple NV units, all operating at 1X.  Objects in the center were seen with the filter not tilted, but when they were moved off-axis, they disappeared.  Tilting the filters brought them back.  So that's it, it's not the ENVIS, it's the filter.

 

My conclusion is that the best wide-field views (in terms of detail seen in objects that show up with narrowband filters) are not with 1X, but some higher power that allows the whole field to be narrow-band filtered without the dramatic de-tuning effect that makes objects at the edge of the field disappear.

 

Based on what I see with my 20" f/3.0, this should be achieved with a lens around f/3 or so, and ideally it is small enough that one can put a 2" narrowband filter in front of it.

 

So, just for a small side-topic here, but what lens is the best that the experienced NV users have found for this?

 

I'm just looking for suggestions, I don't want to go completely off-topic!

We need an ENVIS focal reducer field flattener. lol.gif




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