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# Night Vision Intensifying Eyepieces

NV
336 replies to this topic

### #326 Jeff Morgan

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Posted 01 November 2019 - 12:24 PM

Excuse me, I'm using a translator. If words are not clear or are funny ....    ...

I have almost finished reading and translating the 13 pages . Can someone answer me to one or more questions, please  ?  :

1)  Anyone know how to calculate the magnification with a Plossl 55mm with an NVD on it? Is there a general formula for any eyepiece ?

2)  And by directly placing the NVD in the focusing point ( and therefore without putting an eyepiece ) , what magnification compared to the focal length of the telescope, and the advantage of this assembly?

3) If we put a reducer x0,7 or x 0,5, on a dobson, is there a central black spot, because of the very very low magnification (plossl 55mm + reducer). Because having a very fast F / D is good for the arms of galaxies ...and also avoid buying a small telescope to have large fields , or increase the fully visible objects in the field

4) Finally a last opinion, also for connoisseurs / specialists, I read that it was necessary to put a ring Canon EBX 2120 (?) , Between the plossl and the paracorr 2, to have a better image, is this true?

Thank you very much for your knowledges.
alain .

1) As an approximation, divide the NV eyepiece focal length (27mm) by the afocal "host" eyepiece to get the reduction factor:

27/55 = .49x.

Magnification is then telescope focal length / NV eyepiece focal length * reduction.

Since that is an approximation, I just use 0.5x. Based on true field, it is observational very close, and very close to the result with an actual 0.5x reducer.

2) You are discussing prime focus in this question. Magnification is computed just like a conventional eyepiece. The advantage of the prime focus arrangement is physical handling compared to the afocal method. Less strain on the focusers, star diagonals, and focuser boards. Changing magnifications is generally somewhat easier by adding barlows or focal reducers as desired.

3) No practical exit pupil considerations with devices such as cameras or NV eyepieces. The NV eyepiece can "see" a very large exit pupil - 18mm or more depending upon model.

4) I'm not sure I understand this question. Perhaps you are referring to a field flattener? Certainly not a requirement observationally, I have been using my NV eyepiece on a variety of telescopes and telephoto lenses for three years. But it would be an interesting thing to try.

### #327 gliese

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Posted 01 November 2019 - 05:52 PM

Thank you very much for your quick  !  ... and complete answer on the 4 questions !  !

So for " 3)" :  ...yes ,  I read for example the tests of Mike Lockwood, which showed that the exit pupil did not count. Probably because, as I understand him , the 20mm front lens of the NVD could capture an exit pupil of 20mm maximum. And therefore no problem with an exit pupil of 6 ou  8 ou 10 mm or more  .  But my question, and my fear, is that , when the magnification is very weak , it appears a black spot ( = the secondary mirror) very visible in daytime observations. This appears below the minimum magnification . With 55mm and F / D at 4 or 3, we have very very low magnification, and therefore a black marks very very marked , visible .

And some people say , that they  add a focal reducer , so it's  reinforce still a lot , this central black spot, already very visible with a 55mm without reducer of focal .

This central black area must be very strong , with a 55mm + reducer , and the NVD manages to erase this central black area ? ! The NVD  is already an amazing technologie , but if it erases this blind black center spot , to put an image on it, it becomes miraculous, no ? (  I'm very surprise  and/or don't understand this "phenomenon" )

yes the " 4) "  is not written by me , very clearly.... I saw that a person said that it was necessary to put a small tube = small extension, between the NVD and the coma corrector Paracorr 2, to have the best correction , the best optical path/distance   ....( but it seems to me finally , to be a detail not very important, not very "disabling")

It's almost midnight here, good late afternoon to the Americans   ; With maybe a holiday with no work  for them too , if the 1st of November is the feast/party  of saints and dead people , as in europe

### #328 Eddgie

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Posted 01 November 2019 - 06:11 PM

Thank you very much for your quick  !  ... and complete answer on the 4 questions !  !

So for " 3)" :  ...yes ,  I read for example the tests of Mike Lockwood, which showed that the exit pupil did not count. Probably because, as I understand him , the 20mm front lens of the NVD could capture an exit pupil of 20mm maximum. And therefore no problem with an exit pupil of 6 ou  8 ou 10 mm or more  .  But my question, and my fear, is that , when the magnification is very weak , it appears a black spot ( = the secondary mirror) very visible in daytime observations. This appears below the minimum magnification . With 55mm and F / D at 4 or 3, we have very very low magnification, and therefore a black marks very very marked , visible .

And some people say , that they  add a focal reducer , so it's  reinforce still a lot , this central black spot, already very visible with a 55mm without reducer of focal .

This central black area must be very strong , with a 55mm + reducer , and the NVD manages to erase this central black area ? ! The NVD  is already an amazing technologie , but if it erases this blind black center spot , to put an image on it, it becomes miraculous, no ? (  I'm very surprise  and/or don't understand this "phenomenon" )

yes the " 4) "  is not written by me , very clearly.... I saw that a person said that it was necessary to put a small tube = small extension, between the NVD and the coma corrector Paracorr 2, to have the best correction , the best optical path/distance   ....( but it seems to me finally , to be a detail not very important, not very "disabling")

It's almost midnight here, good late afternoon to the Americans   ; With maybe a holiday with no work  for them too , if the 1st of November is the feast/party  of saints and dead people , as in europe

The operator is not looking at the view from the scope, they are looking at the view as displayed on a small phosphor screen.

I can assure you that there is no concern about seeing the secondary.  I use a 6" f/2.8 with a 40% obstruction, and it does not show any secondary shadow.

Night Vision surveillance telescopes have obstructions as large as 50%. No problems.

It is just like using a camera.  The camera does not record the secondary shadow.  Neither does the NV.  Since the user is looking at an image formed on a small screen, there is nothing between the screen and the retina to cast a shadow.

So, exit pupil is simply not a factor for NV. You can use super large secondary mirrors and ultra-fast systems, and you won't see even a tiny hint of secondary shadow.  The Photocathode does not record it just like a camera does not record it.

Edited by Eddgie, 01 November 2019 - 06:19 PM.

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### #329 gliese

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Posted 02 November 2019 - 05:32 AM

Thank you for your answer, which is distressing, and keeps away unpleasant surprises.

In this case, the Plossl 55mm is not the best , to have wide fields of vision , to observe Sh Sharpless objects. Or to have maximum brightness and maximum SNR . There is a 85mm eyepiece  .

And approach the F / D 1,2 NVD, which is the otptimum, the top, if I understood everything well. (and how to calculate this famous global F / D, resulting ...?)

And in addition, no need to systematically change the telescope  ( in order to have a big field , or not ) , if you buy a Plossl 85mm  =  http://russell-optic...m/two_inch.html     The 85mm eyepiece is at the bottom of the page of this website . His price = \$ 85 only  ... is the image quality there? , when we see the price of a "small" 55 mm  . And , has anyone tried this montage/formula , please?

Edited by gliese, 02 November 2019 - 05:36 AM.

### #330 Jeff Morgan

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Posted 02 November 2019 - 12:19 PM

Thank you for your answer, which is distressing, and keeps away unpleasant surprises.

In this case, the Plossl 55mm is not the best , to have wide fields of vision , to observe Sh Sharpless objects. Or to have maximum brightness and maximum SNR . There is a 85mm eyepiece  .

And approach the F / D 1,2 NVD, which is the otptimum, the top, if I understood everything well. (and how to calculate this famous global F / D, resulting ...?)

And in addition, no need to systematically change the telescope  ( in order to have a big field , or not ) , if you buy a Plossl 85mm  =  http://russell-optic...m/two_inch.html     The 85mm eyepiece is at the bottom of the page of this website . His price = \$ 85 only  ... is the image quality there? , when we see the price of a "small" 55 mm  . And , has anyone tried this montage/formula , please?

I have not tried the 85mm eyepiece but I am intrigued.

GeezerGazer has tried a larger Plossl - perhaps the 65mm? You can search it on the EAA forum. IIRC, his results were not entirely successful.

### #331 gliese

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Posted 03 November 2019 - 06:44 PM

Thanks for the information , and always with the speed of the Beep Beep

### #332 gliese

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Posted 05 December 2019 - 08:39 AM

I reread the answers on the calculation of magnification, because in practice, I just realized, that these answers gave a classical calculation of magnification , and therefore I have a doubt :

The question (1st november) = "1)  Anyone know how to calculate the magnification with a Plossl 55mm with an NVD on it? Is there a general formula for any eyepiece ? "

The answer (Viking 1 ) = 1) As an approximation, divide the NV eyepiece focal length (27mm) by the afocal "host" eyepiece to get the reduction factor:27/55 = .49x.

Magnification is then telescope focal length / NV eyepiece focal length * reduction.

.............. So if I don't make a  translation mistake , we have :  The magnification  =  telescope focal length / NV eyepiece focal length * reduction.   =  telescope focal length / NV eyepiece focal length(27mm)  X  ( NV eyepiece focal length (27mm) / the afocal "host" eyepiece(55mm) )  =  telescope focal / eyepiece focal  (because there is the same numerator and same denominator : 27 mm) .

And so it is the classic formula of the calculation of the magnification, which does not intervene at all the NVD?!  I think I misunderstood

### #333 Jeff Morgan

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Posted 05 December 2019 - 12:08 PM

Don't make things more complicated than they need to be. Six decimal accuracy is not required (from a practical perspective not even one decimal place is justified).

1) Divide the telescope focal length by the NVD focal length - the "classical formula". In almost all cases the NVD focal length is 27mm.

2) Apply any barlow or reduction factor. Barlows and reducers are straightforward.

3) For afocal use, the barlow or reduction factor is approximately NVD focal length divided by host eyepiece focal length.

For example, using my 16" (400mm) f/7 Newtonian:

Prime focus: 2800 / 27 = 104x. That is a rounded figure because it is visually indistinguishable from 103.7x.

Using a 1.5x barlow: 104 * 1.5 = 156x.

Using a 55 lossl afocal projection: 27/55 = .5 reduction, applied as 2800 / 27 * 0.5x = 52x

### #334 gliese

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Posted 05 December 2019 - 06:36 PM

First of all, thank you always for your faithful and express answers.

Yes, that's what I understood from your first calculation =   2800/27    x   27/55  =  2800/55  =  the classic formula of magnification  .....  So I confirm that the NVD and its focal are useless and does not count  . Which seems a bit strange for me

### #335 Jeff Morgan

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Posted 05 December 2019 - 10:56 PM

First of all, thank you always for your faithful and express answers.

Yes, that's what I understood from your first calculation =   2800/27    x   27/55  =  2800/55  =  the classic formula of magnification  .....  So I confirm that the NVD and its focal are useless and does not count  . Which seems a bit strange for me

Well yes, you could look at it that way. Just treat the system as if the NV eyepiece is giving you a 1x view of whatever the scope and host eyepiece produce. Perfectly valid for afocal use.

For users with the choice of afocal or prime configurations, there is a desire to know the equivalent reduction between the two methods.

For example, if you can determine that a 0.7x focal reducer that costs \$80 and weighs 6 ounces offers the same effective brightness (focal ratio) as a 41mm eyepiece that costs \$500 and weighs two pounds ... nice to know.

### #336 GOLGO13

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Posted 06 December 2019 - 09:48 AM

I personally find I prefer prime focus, but I also recognize the potential benefit of afocal. I'm glad I have both options, but I just prefer a shorter stack on the focuser. There are cases where having the afocal would be of benefit though...so having both options is a nice thing.

What I notice when doing Afocal (and I don't have a 2 inch capability...just a 40mm plossl), the edges are not as good as when doing prime focus. If afocal is all you have then it's not something to worry about.

### #337 gliese

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Posted 06 December 2019 - 11:07 AM

oh yes , I did not think about that, about the richness of the un-simplified calculation formula . Thank you Jeff  for this clarification , and that will no doubt serve other people  .

And interressante observation and remark from Aurora: There could be a worse image in afocal, and indeed more stacks of lenses

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