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Stacking reducers with a c11 edge for night vision

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#1 Gavster

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Posted 29 November 2019 - 07:56 PM

I’ve posted before about how much I like my c11 for night vision use. In conjunction with a reducer and 55mm plossl I can get an effective f3.5 speed for nice real time views with good image scale. However, at my last outing with the c11, it struggled with my 3nm ha filter and my 5nm filter delivered nicer results.

We’ve had weeks of clouds in the UK but this evening at last we had a clear sky. We arranged a local Astro club meet up at our usual local light polluted site (sqm 19)! 
I set up both my Tak fsq85 and c11 edge side by side on my panther mount, to give widefield nv views with the Tak and more detailed views with the c11.

I had a nice session on the usual suspects (horsehead, flame, rosette, pac-man, cocoon, wizard, bubble, California, veil, monkeyhead, butterfly etc), switching between the Tak and the c11 depending on the size of the object.

My c11 edge has the special edge reduced attached to it so bringing the f ratios down to 7. Using a 55mm plossl gives a effective f3.3, nice but I’ve always hankered for a bit more speed. 
I was using my 0.75x gso rc reducer with my Tak and decided to experiment by using this reducer in the c11 edge. I was t expecting it to work due to the need for a special reducer, but to my surprise it worked great, giving lovely sharp stars and nebulae detail. M42 was incredible. 
With an effective speed of f2.5 (getting close to my f2 target for visual observing), this stacked reducer setup may well be my most used nv setup going forward - it was brilliant!

 

 

 

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#2 Todd N

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Posted 29 November 2019 - 08:54 PM

How are you deriving an effective f/3.3 for your C11 setup. Eyepiece projection entails increasing f-ratio.


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#3 GeezerGazer

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Posted 29 November 2019 - 11:39 PM

How are you deriving an effective f/3.3 for your C11 setup. Eyepiece projection entails increasing f-ratio.

 

Todd, he uses the dedicated Edge reducer at .63x and the 55mm Plossl which acts like a .5x reducer because the NVD is used afocally... the NVD with its own 1x objective lens has a focal length of 27mm, thus, 55 divided by 27 = 2 or .5x reduction.  Adding the GSO reducer adds another .73x of reduction within the optical chain.

 

Gavin, good to hear you are still experimenting. What TFoV are you achieving with the C11 in this configuration?  And, of course, how's the EoF performance?  

Ray



#4 Gavster

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Posted 29 November 2019 - 11:57 PM

Todd, he uses the dedicated Edge reducer at .63x and the 55mm Plossl which acts like a .5x reducer because the NVD is used afocally... the NVD with its own 1x objective lens has a focal length of 27mm, thus, 55 divided by 27 = 2 or .5x reduction.  Adding the GSO reducer adds another .73x of reduction within the optical chain.

 

Gavin, good to hear you are still experimenting. What TFoV are you achieving with the C11 in this configuration?  And, of course, how's the EoF performance?  

Ray

Ray

Something around 1.5 degrees with a magnification of 27x.

I didn’t take any phone photos this evening but visually eof performance was very good with little or no astigmatism. That’s what surprised me, how good the quality of the views were when the additional reducer was added, I was expecting the quality of the view to be poor...


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#5 Todd N

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Posted 30 November 2019 - 12:39 AM

Todd, he uses the dedicated Edge reducer at .63x and the 55mm Plossl which acts like a .5x reducer because the NVD is used afocally... the NVD with its own 1x objective lens has a focal length of 27mm, thus, 55 divided by 27 = 2 or .5x reduction.  Adding the GSO reducer adds another .73x of reduction within the optical chain.

 

Thank you. On the face of it it just didn't seem right with eyepiece projection in mind. From the equation that 27mm needs to be the distance from reducer to image plane, not the NVD lens but as you state it's a 1x lens so, I more so understand now how Gavster arrives at his figures. With SCTs and spacing of such arrangements are a fickle thing.

 

Another idea that has come to mind based on using a 55mm eyepiece is the exit pupil may be too large and the light intensity hitting the NVD is diminished. Maybe it doesn't matter much with such a device. I don't know. It of coarse is working well for Gavster.



#6 Gavster

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Posted 30 November 2019 - 12:56 AM

Thank you. On the face of it it just didn't seem right with eyepiece projection in mind. From the equation that 27mm needs to be the distance from reducer to image plane, not the NVD lens but as you state it's a 1x lens so, I more so understand now how Gavster arrives at his figures. With SCTs and spacing of such arrangements are a fickle thing.

 

Another idea that has come to mind based on using a 55mm eyepiece is the exit pupil may be too large and the light intensity hitting the NVD is diminished. Maybe it doesn't matter much with such a device. I don't know. It of coarse is working well for Gavster.

Nvds can handle exit pupils up to 20mm fine so no problems there.

More details here:

http://www.televue.c...id=36&Tab=_work


Edited by Gavster, 30 November 2019 - 12:58 AM.

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#7 Eddgie

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Posted 30 November 2019 - 10:31 AM

I’ve posted before about how much I like my c11 for night vision use. In conjunction with a reducer and 55mm plossl I can get an effective f3.5 speed for nice real time views with good image scale. However, at my last outing with the c11, it struggled with my 3nm ha filter and my 5nm filter delivered nicer results.

 

The numbers are the numbers, but in reality, the C11 is probably working at an effective brightness of maybe f/4.8. Now that is still pretty fast, but this is why narrow band filters don't give the brightness one would think when used in SCTs.

 

Here is how it breaks out.  First, you loose about 5% of your light collection due to the shading of the secondary obstruction, but that is not so much as compared to the transmission of the system, but it is the starting point for the math. 

 

When the shading is factored in, the system transmission at 650nm (which is at best 85% for 650nm and probably less than this) then the overall C11 transmission is really only about 80%.  Then we have the transmission losses of the reducer, the diagonal, the eyepiece, and the objective on the NV device, and of course the loss for the filter itself (most filters only pass about 97% in H-a.  When you factor all of the individual points for loss into the equation, the final effective brightness is probably more like f/4.8 or so (maybe a little higher).

This is probably why the C11 did not do well with the 3nm.  

 

 

I became far more aware of the issue with compound scopes when I started comparing my Comet Catcher to an 80mm f/6 Apo.  I was surprised to see that the image was almost as bright in the f/6 scope as in the f/3.5 Comet Catcher and when I factored in the secondary shading and transmission losses, I came to feel like the Comet Catcher was more like f/5 for non H-a and f/4.5 for H-a.

We tend to treat the transmission figures for NV the same a visual, but in most cases, there is a lot more transmission loss in red and near infra-red as there would be for visual, so system speeds should be corrected for both shading and transmission losses and in SCTs, this probably works out to something like a 30% to 35% reduction in the effective brightness of the entire system vs the paper numbers. 

 

The reason for the difference in H-a and non H-a is that since the transmission of XLT falls off pretty heavily as you get near the 900mm end of the spectrum, where NV is the most sensitive, then you are really only working at maybe 60% efficiency.  

 

Now f/4.5 is still pretty fast, but for a 3nm filter, it is not really fast enough to give a good result. 


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#8 Gavster

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Posted 30 November 2019 - 11:20 AM

The numbers are the numbers, but in reality, the C11 is probably working at an effective brightness of maybe f/4.8. Now that is still pretty fast, but this is why narrow band filters don't give the brightness one would think when used in SCTs.

 

Here is how it breaks out.  First, you loose about 5% of your light collection due to the shading of the secondary obstruction, but that is not so much as compared to the transmission of the system, but it is the starting point for the math. 

 

When the shading is factored in, the system transmission at 650nm (which is at best 85% for 650nm and probably less than this) then the overall C11 transmission is really only about 80%.  Then we have the transmission losses of the reducer, the diagonal, the eyepiece, and the objective on the NV device, and of course the loss for the filter itself (most filters only pass about 97% in H-a.  When you factor all of the individual points for loss into the equation, the final effective brightness is probably more like f/4.8 or so (maybe a little higher).

This is probably why the C11 did not do well with the 3nm.  

 

 

I became far more aware of the issue with compound scopes when I started comparing my Comet Catcher to an 80mm f/6 Apo.  I was surprised to see that the image was almost as bright in the f/6 scope as in the f/3.5 Comet Catcher and when I factored in the secondary shading and transmission losses, I came to feel like the Comet Catcher was more like f/5 for non H-a and f/4.5 for H-a.

We tend to treat the transmission figures for NV the same a visual, but in most cases, there is a lot more transmission loss in red and near infra-red as there would be for visual, so system speeds should be corrected for both shading and transmission losses and in SCTs, this probably works out to something like a 30% to 35% reduction in the effective brightness of the entire system vs the paper numbers. 

 

The reason for the difference in H-a and non H-a is that since the transmission of XLT falls off pretty heavily as you get near the 900mm end of the spectrum, where NV is the most sensitive, then you are really only working at maybe 60% efficiency.  

 

Now f/4.5 is still pretty fast, but for a 3nm filter, it is not really fast enough to give a good result. 

My actual experience shows a very different result to this analysis.

 

I have used lots of scopes with nv including fast refractors and dobs to do actual comparisons and have shown many images which show how well the c11 is working with nv such as this:

 

https://www.cloudyni...d-night-vision/

 

The c11 with the reducers and 55mm plossl is giving much better results than f4.8. I got great views with my c11 and 3nm filter last night, noticeably better than my 5nm filter.


Edited by Gavster, 30 November 2019 - 01:58 PM.

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#9 GeezerGazer

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Posted 30 November 2019 - 02:24 PM

Eddgie, wouldn't the light gathering ability of an 11" aperture offset the effects of light loss and shading, compared to a smaller aperture, where percentage points of light loss play a more significant roll.  For instance, the light loss of the same 2" diagonal used in the 80mm scope has greater impact (percentage wise) than in the 11" scope because the increased aperture offsets the percentage of loss.  Wouldn't the same be true for the reducers and eyepiece?  Certainly there is loss in a catadioptric/NV system, but it seems to me that increased aperture would play a significant roll in offsetting that difference when compared to a smaller aperture.  Would this help to explain Gavin's better results with his C11?  Or, is this rationale faulty? 

Ray


Edited by GeezerGazer, 30 November 2019 - 02:24 PM.



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