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Night Vision at f/long

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#1 Jeff Morgan

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Posted 02 March 2017 - 01:28 AM

So I have had this ETX 90 that I have been using for about 10 years as a spotting scope at the rifle range. It's poor mount and finderscope precluded use for astronomy. Well, finally gave the de-forked ETX a good workout on the FTX mount - I'm impressed!

 

Mainly I intend to use it for double star observations, so I can use my larger scope time for DSO's.

 

But just for kicks, I wanted to see how it would do with my L3 NV monocular. We all obsess over getting the fastest optics for our NV tubes, and I wanted to gain some experience with a slow scope. This is not an exhaustive report, because we only hit a few objects before switching out the ETX for the Comet Catcher.

 

Surprisingly, it did really well. With a focal ratio of f/13.8 I was not expecting much. Yet NGC 2362 and NGC 2354 were actually better in the NV eyepiece than in the 22 Panoptic or the 12 Morpheus. Many more faint cluster members revealed. In fact, given the moon was still up, the sparse 2354 was barely distinguishable without NV.

 

Just for kicks, we tried the HorseHead nebula in the ETX - and we both saw it with a 12nm H-alpha filter. Not a great view, the filter and long focal ratio caused photon starvation of the tube and scintillation. But it was definitely there.

 

Mainly, I see the ETX as a double star scope - I was able to push as high as 278x with a very good image on Gamma Leonis. But it may also be viable for NV work in limited roles - such as open clusters.



#2 bobhen

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Posted 02 March 2017 - 08:42 AM

I have come to the conclusion that NV (like a camera) can work with just about any scope. Some (as with a camera) work better than others but as you have found, just about any scope is improved with NV.

 

Bob



#3 Eddgie

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Posted 02 March 2017 - 08:54 AM

For stellar targets (Clusters and Globulars), I too find that I can use very high powers by barlowing and get good results.

 

For bright nebula like Orion, the results can be pretty staggering.

 

The dimmer the target, the more I enjoy a faster system.  

 

Your success though does point to an area where some have criticized NV and I am curious about this area.  For double stars obviously we would not see colored doubles with different colors, but I know from my own experience that I can split doubles that I could not even see were doubles without NV.   In fact, using the 12" dob, I saw that many of the brighter stars in M29 had tiny little close companion stars (one is a triple) that I had never even seen before.

 

The question though would be if you can see diffraction rings on any stars.  Very curious about this. I would think at high enough magnification in a small enough scope it might be possible, but I have never seen an Airy Disk with diffraction ring mostly because I have not ever used high enough magnfication.

 

Did you see any diffraction rings?   Again, just curious.   I would probably still use glass eyepeices for doubles but this question has always been in my mind.



#4 Jeff Morgan

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Posted 02 March 2017 - 11:18 AM

For stellar targets (Clusters and Globulars), I too find that I can use very high powers by barlowing and get good results.

 

For bright nebula like Orion, the results can be pretty staggering.

 

The dimmer the target, the more I enjoy a faster system.  

 

Your success though does point to an area where some have criticized NV and I am curious about this area.  For double stars obviously we would not see colored doubles with different colors, but I know from my own experience that I can split doubles that I could not even see were doubles without NV.   In fact, using the 12" dob, I saw that many of the brighter stars in M29 had tiny little close companion stars (one is a triple) that I had never even seen before.

 

The question though would be if you can see diffraction rings on any stars.  Very curious about this. I would think at high enough magnification in a small enough scope it might be possible, but I have never seen an Airy Disk with diffraction ring mostly because I have not ever used high enough magnfication.

 

Did you see any diffraction rings?   Again, just curious.   I would probably still use glass eyepeices for doubles but this question has always been in my mind.

 

No, I have not tried to barlow up to reveal a diffraction rings. I will try this, I suspect the halo will preclude it on brighter stars.

 

 

I would agree that conventional eyepieces will be my first choice in double stars. But NV could be an interesting "special case" tool. I will keep trying it with the little Mak just to see how it works on various targets.

 

Aside from curiosity, I wanted to get this out there for potential newcomers - they don't necessarily have to dump existing scopes to use NV. Faster is of course better, but moderate focal ratios can still benefit.



#5 pwang99

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Posted 02 March 2017 - 06:00 PM

Actually another thing to point out is that NV is also the only way I've seen the moons of Uranus and Neptune.



#6 Eddgie

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Posted 04 March 2017 - 09:48 AM

On nights of less than excellent seeing, it is hard to see the first diffraction ring of brighter stars even using a glass eyepiece!

 

I get it though..  I do think this would be the problem. The Halo might make it difficult but with sufficient magnification, I have to think it could be done!

 

Best scope I think to try it with would indeed be a smaller MCT because of the easy of reaching high powers and the smaller larger diffraction pattern of the smaller aperture. 

 

I am very curious to know though so if you are able to do so I would love to hear about it!



#7 Rickster

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Posted 04 March 2017 - 12:54 PM

We had excellent seeing last night and I had my 2120/LX3 out, so I thought I would see if I could get you a double star shot.  I picked Castor since it should be relatively easy:  mag +1.58 and +2.97 with 5.2" separation.  I was able to resolve the two stars within the NV halo.  The stars were two distinct very bright blobs. 

 

I took some pix using an old handheld Canon Powershot, but didn't have much luck.  Even though the upper atmosphere was unusually steady, surface winds were gusting to 30mph.  The scope was dancing so badly that it actually helped if I placed my hand on it.  Also, the camera image blew out the stars.  Given those caveats, here is the best image I got (cropped). 

Attached Thumbnails

  • IMG_0043 cropped.jpg


#8 Rickster

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Posted 04 March 2017 - 01:09 PM

Here is the same image with the gamma and contrast dialed back.  It is still grossly worse than the actual real time image, but at least the diffraction rings are more visible.

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  • IMG_0043 cropped mod.jpg


#9 pwang99

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Posted 04 March 2017 - 05:38 PM

Random, possibly very dumb question: in terms of enhancing the contrast on the diffraction rings as viewed through NV, does it change anything if you were to use a narrowband filter?



#10 Rickster

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Posted 04 March 2017 - 09:14 PM

Even better would be NV with adjustable gain.  If we only knew someone who had that...smile.gif 



#11 Eddgie

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Posted 05 March 2017 - 06:53 PM

The problem for me would be that I would probably need a 5x Barlow to get sufficient magnification to be able to get the image big enough.  

 

I am not sure that the above image shows diffraction rings.  I would think you would need maybe 250x for that.

 

My guess is that what is seen here may be some refraction artifact from the corrector, which has better correction than most Apos, but does not focus red perfectly.

 

People don't of course realize this, but Celestron shows it clearly in their ray traces for the EdgeHD and the Meade ACF (Page 8).  In both cases, you can see that there is a red blur that is about two Airy Disk diameters. 

 

I see this all the time in systems with one or more refractive elements!!!! If you focus green, there is almost always a red blur that is often twice the size of the point formed by the other wavelengths.

 

My guess then is that unless the view was at something like 200x, (whatever you would need to see the ring using a standard eyepeice) anything that looked like a ring around the Airy Disk was probably the defocused red for CA introduced by the corrector.  We don't see this visually of course because our eye is not sensitive to red, but of course we know the image intensifier shows red like carzy.

 

Just my guess though, but a look at the ray traces on page 8 of the EdgeHD white paper shows a red blur circle around the Airy Disk and unless the view was at 200x or so, my guess is that this was what was being seen. 




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