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Eyepiece projection with c-mount lens

NV Optics
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18 replies to this topic

#1 stnagy

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Posted 21 March 2021 - 07:58 AM

Eyepiece projection with the ENVIS objective lens is obviously a very popular way to do night vision astronomy. 

 

I have a Mod-3C and am wondering whether using a c-mount lens having a different focal length would be worth trying. The way I understand it, you can calculate the effective focal reduction of the ENVIS / eyepiece combination by dividing the ENVIS focal length (27mm) by the eyepiece focal length. For the Tele Vue 55mm plossl projection system it would be about 27/55 = 0.49x, basically reducing the f-ratio of the OTA by 2. With the 67mm kit the effective f-ratio is reduced even further. 

 

But what would happen if you used a shorter focal length c-mount lens instead of the ENVIS objective? Something like this 12mm Tamron lens:

 

https://www.bhphotov...mount_12mm.html

 

Would you end up reducing the effective f-ratio by 12/55 = 0.21x? And what’s the impact of the 27mm Mod 3 eyepiece on all of this? 

 

Thank you!



#2 a__l

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Posted 21 March 2021 - 08:40 PM

I think there will be problems with vignetting and distance between NV lens and eyepiece. Maybe Eye Relief for TV55 (38mm) will be enough, maybe not. You need to check.
I am currently using a Zeiss Tevidon 24mm f/1.4 with the TV67 and has slight vignetting. With TV 55 it is better, but more aberrations. I just bought a Nikkor 24mm (F bayonet) f/2 and will experiment with it.



#3 a__l

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Posted 21 March 2021 - 08:53 PM

Zeiss is a good lens, maybe even better than Envis, but need to check side by side.
I also like Nikkor, but so far I have more focal lengths.


Edited by a__l, 21 March 2021 - 09:02 PM.


#4 ButterFly

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Posted 23 March 2021 - 04:55 PM

Eyepiece projection with the ENVIS objective lens is obviously a very popular way to do night vision astronomy. 

No.  It may seem like a distinction without consequence mathematically, but there is a huge difference in the real world.  Here is a snippet from Televue's imaging methods page under the afocal section:

 

An alternative way of determining effective focal length considers the camera lens and eyepiece combination acting as a relay system. Using the example above, the 50mm camera lens with the 10mm eyepiece gives 5x relay magnification. 5x600mm telescope objective focal length yields 3000mm focal length.

 

The practical difference is that an eyepiece has many elements working together to make a well corrected field, and using it afocally is right on spec.  Same with the objective of the NV device.  A random pairing of an eyepiece and a camera lens forming an image at some random place usually leads to very small usuable fields with garbage at the edges, largely because the eyepiece is being used waaaay off spec.  Garbage in garbage out, especially when it's intensified.

 

Even when it was still commonly used, eyepiece projection was reserved largely for planets because they are small.

 

 


Would you end up reducing the effective f-ratio by 12/55 = 0.21x? And what’s the impact of the 27mm Mod 3 eyepiece on all of this? 

Think in terms of exit pupil to make your life much easier.  Any lens you put on your NV device has an opening, called the entrance pupil.  That opening constrains how much exit pupil it can handle.  When the exit pupil matches that entrance pupil, the effective f/ratio of the whole system is the f/ratio of the lens you put on the NV device.  When the exit pupil is smaller than the entrance pupil, the effective f/ratio goes UP in the same ratio as the areas diameters of the exit pupil to the entrance pupil.

 

So, if I can handle a 22mm exit pupil, but I only deliver an 11mm exit pupil, I use one-fourth the available area.  The effective f/ratio of the system is four two times that of the lens you put on the NV device.

 

This is all relative so far.  To get the actual effective f/ratio, you need to know the actual f/ratio of the lens you put on the NV device.  If we assume the same entrance pupil, then a 12mm focal length lens is 2.25x times faster than a 27 mm lens.

 

The f/ratio of a 22mm entrance pupil with a 12mm focal length is f/0.54.

 

If that sounds absurd, it's becuase it is.  Looking at the specs of the lens you linked to, it has f/ratios between f/1.4 and f/16, because of the zoom.  The fastest actual effective f/ratio of a system with this lens can NEVER go below f/1.4.  If you try to go faster, the exit pupil just won't fit into the entrance pupil, and you reduce the effective aperture used by just enough to reach the lens' f/ratio.

 

Just like with any ordinary eyepiece, the magnification presented to the NV device is still: scope focal length / eyepiece focal length.  Just like with any ordinary eyepiece, the image scale goes up by a factor of 2.25 (27 / 12) while decreasing total light by 2.25^2 ~ 5x.


Edited by ButterFly, 23 March 2021 - 05:03 PM.


#5 a__l

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Posted 23 March 2021 - 06:55 PM

If you started referring to TV, then look at the diagram: 

https://www.televue....id=36&Tab=_work

Exit pupil location of telescope matches monocular entrance pupil location.

Monocular entrance pupil location depends on the lens used and is often not listed in the specification. Therefore, verification is only by experiment.

 

Example for using the entrance pupil for my Fujinon

https://en.fujifilma...a-Pixel 1.0.pdf


Edited by a__l, 24 March 2021 - 01:36 AM.


#6 stnagy

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Posted 24 March 2021 - 12:00 PM

Think in terms of exit pupil to make your life much easier.  Any lens you put on your NV device has an opening, called the entrance pupil.  That opening constrains how much exit pupil it can handle.  When the exit pupil matches that entrance pupil, the effective f/ratio of the whole system is the f/ratio of the lens you put on the NV device.  When the exit pupil is smaller than the entrance pupil, the effective f/ratio goes UP in the same ratio as the areas diameters of the exit pupil to the entrance pupil.

 

So, if I can handle a 22mm exit pupil, but I only deliver an 11mm exit pupil, I use one-fourth the available area.  The effective f/ratio of the system is four two times that of the lens you put on the NV device.

Is the entrance pupil size on the lens the same as the diameter of the objective? 



#7 ButterFly

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Posted 24 March 2021 - 12:34 PM

Is the entrance pupil size on the lens the same as the diameter of the objective? 

Yes.  It's just the hole that lets in the light.



#8 stnagy

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Posted 24 March 2021 - 12:35 PM

Yes.  It's just the hole that lets in the light.

Thanks, that explanation made a ton of sense. 



#9 ButterFly

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Posted 24 March 2021 - 12:37 PM

Sensor illumination may be a problem, or not - I don't know the size of the chip.  Someone needs to measure that.

 

I use a similar lens on my ASI120MM.  It does fine once the spacing is set correctly.  On the ASI183, there is lots of vignetting.



#10 a__l

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Posted 24 March 2021 - 02:16 PM

Is the entrance pupil size on the lens the same as the diameter of the objective? 

In general, NO.

It has a "virtual" size. The entrance pupil is the diaphragm you install (1.4 ... 16) and how it is seen by the set of lenses that are installed in front of it.


Edited by a__l, 24 March 2021 - 02:16 PM.


#11 chemisted

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Posted 24 March 2021 - 07:39 PM

Is the entrance pupil size on the lens the same as the diameter of the objective? 

The entrance pupil is equal to the focal length of the eyepiece divided by its focal ratio.  For an ENVIS lens that is 26.5/1.2 = 22mm.  See the last post in the following thread by Glenn LeDrew.

 

https://www.cloudyni...-fast-nv-scope/



#12 stnagy

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Posted 24 March 2021 - 07:52 PM

The entrance pupil is equal to the focal length of the eyepiece divided by its focal ratio.  For an ENVIS lens that is 26.5/1.2 = 22mm.  See the last post in the following thread by Glenn LeDrew.

 

https://www.cloudyni...-fast-nv-scope/

Yes, that makes sense. For a telescope that would essentially be the diameter of the objective, which is why I asked the question in that way, but I am sensing that’s not necessarily the case with camera lenses. 



#13 a__l

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Posted 24 March 2021 - 08:36 PM

https://en.wikipedia.../Entrance_pupil

 

Determine by experiment for specific objective. As I wrote earlier.


Edited by a__l, 24 March 2021 - 08:57 PM.


#14 chemisted

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Posted 25 March 2021 - 06:54 AM

https://en.wikipedia.../Entrance_pupil

 

Determine by experiment for specific objective. As I wrote earlier.

Here is a quote from your reference: "The f-number ("relative aperture"), N, is defined by N = f/EN, where f is the focal length and EN is the diameter of the entrance pupil."  Rearranging it we have: EN = f/N as I stated above.



#15 a__l

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Posted 25 March 2021 - 08:48 PM

https://en.wikipedia.../Entrance_pupil

 

"Depending on the lens design, the entrance pupil location on the optical axis may be behind, within or in front of the lens system; and even at infinite distance from the lens in the case of telecentric systems."

 

 

I know from experiment that my Zeiss Tevidon 25mm f/1.4 only works with the TV55 and does not work with the N31. No focus.
What formulas will you confirm this? I think it will be difficult to do.



#16 a__l

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Posted 25 March 2021 - 09:50 PM

I will continue further.

chemisted, Formula EN = f / N intended for photographic objective use.

Do you have confidence in its correctness when used afocal with an eyepiece? When the location of the entrance pupil is undefined (may be behind, within or in front of the lens system).



#17 chemisted

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Posted 26 March 2021 - 06:46 AM

The equation is accurate for providing the diameter of the entrance pupil which was the question that I was answering.  That is all I have to say on the matter.


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#18 a__l

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Posted 26 March 2021 - 04:38 PM

The entrance pupil is equal to the focal length of the eyepiece divided by its focal ratio.  For an ENVIS lens that is 26.5/1.2 = 22mm.  See the last post in the following thread by Glenn LeDrew.

 

https://www.cloudyni...-fast-nv-scope/

https://www.televue....id=36&Tab=_work

 

Televue defines the size as "about 20mm".

 

For OP. Exit pupil with TV55 for NV+Tak TSA-120 - 7.3 mm, SW 130PDS - 11 mm


Edited by a__l, 26 March 2021 - 05:01 PM.


#19 a__l

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Posted 13 April 2021 - 01:23 PM

My new kit. Final conclusions when the weather will be. Afocal Nikkor 24 f/2

Attached Thumbnails

  • Nikkor_24 (1).jpg

Edited by a__l, 13 April 2021 - 01:29 PM.



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