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observations on an adjustable-magnification night vision setup

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#26 shams42

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Posted 04 August 2016 - 09:10 AM

The confirmation screen should look like this.

 

Screen Shot 2016-08-04 at 10.01.47 AM.png



#27 shams42

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Posted 04 August 2016 - 09:12 AM

And here are some pics of the adapter itself and the adapter installed on M43 eyepiece threads of the Baader Zoom.

Attached Thumbnails

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#28 shams42

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Posted 04 August 2016 - 09:18 AM

A couple more things of note about this setup:

 

1) The image will be flipped vertically compared to directly inserting the NV device.

 

2) Edge aberrations become much worse. 

 

Again, using this setup with a Paracorr on my f/4.5 Newt is fine, yielding nice sharp stars to the edge. Sans coma corrector, though, the view is a mess. This is strange since I can barely see any coma at all when I use the night vision device directly in the focuser. I don't know how this setup would behave in a refractor.

 

Still, being able to zoom in on globs and planetary nebulas is great fun, and (to me) completely justifies the effort in this setup. 



#29 Dark Night

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Posted 08 August 2016 - 04:31 PM

I'm the first to admit that it looks kind of silly. If we were into appearances we'd be spending all this money on sports cars, not telescopes and night vision. 

 

It's a really, really long stack.  

 

It's clearly not for everyone, but the benefits are pretty amazing. Being able to zoom in on a globular, galaxy, or small planetary nebula will add a whole new dimension to the night vision experience. 

BEAUTIFUL scope!



#30 Jeff Morgan

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Posted 09 August 2016 - 07:37 AM

Yesterday I scored a Baader Zoom on the 'Mart, and just now ordered my Precise Parts adapter. Yippee!

 

I also noticed they have adapters for camera lenses (Canon FD for example). In another thread someone was showing their telephoto -> NV set-up. It looked great excepting the filter was basically friction-fit inside the bayonet adapter. This might be a lot better set-up, though it costs more ($212 to be exact).



#31 shams42

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Posted 09 August 2016 - 12:17 PM

Great, Jeff! I hope you enjoy it. Please post your impressions.

 

For my money, the views of globs and p/n alone justify the cost.

 

My initial impression is that it works best with minimal filtration - either unfiltered (for p/n) or with 610nm red filter. More aggressive and (IMO) you are just giving up too much light for high mags to work well. Though I haven't yet tried it with the IR pass filter.



#32 shams42

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Posted 10 August 2016 - 11:07 AM

Here's why I think that this setup is not great with Ha filters.

My scope is native f/4.5. With the Paracorr, it becomes ~f/5.2. The lowest zoom setting is 24mm. The equivalent focal length of my NVD Micro is 27.8mm. The magnification factor is 27.8/24 = 1.16.

Therefore, the equivalent focal ratio at the minimum zoom setting is f/6.

That's starting to get slow for H alpha observing. I definitely notice more noise. And at the highest zoom setting, it's about equal to f/18.

You can watch nebular objects disappear into the noise floor as you zoom in and reemerge as you zoom out with the H alpha filter in place.

Of course, there is much more light available when running unfiltered (or with minimal filtration). Even at the highest zoom settings, globs and p/n are quite usefully displayed.

And you don't have to use the whole zoom range, of course, to derive benefits from adjustable magnification.

I think I will look for a coma corrector that does not impart additional magnification to use with this setup.

#33 shams42

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Posted 10 August 2016 - 11:12 AM

And this is another reason why I think that big, fast dobs are the ideal telescope for observing small objects with night vision. Imagine a 24" f/3 scope. With Paracorr II, the equivalent f-ratios at various zoom settings would range from f/4 to f/12. If I've done my math correctly, the magnification range in such a scope would be 87x to 263x

(Of course, I don't know how well the Baader Zoom would cope with such a steep light cone. Probably not too well).

#34 Jeff Morgan

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Posted 10 August 2016 - 11:23 AM

Here's why I think that this setup is not great with Ha filters.

My scope is native f/4.5. With the Paracorr, it becomes ~f/5.2. The lowest zoom setting is 24mm. The equivalent focal length of my NVD Micro is 27.8mm. The magnification factor is 27.8/24 = 1.16.

Therefore, the equivalent focal ratio at the minimum zoom setting is f/6.
 

 

I don't doubt what you are seeing - but I am not following how magnification change focal ratio.



#35 shams42

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Posted 10 August 2016 - 12:28 PM

Eyepiece magnification doesn't change the focal ratio. The focal ratio is dictated by the optics, and the influence of any reducers or barlows in the light path. 

 

Focal ratio is just the steepness of the light cone. The steeper it is, the more light is being squeezed into each unit of the detector (pixels for cameras, microchannels for intensifiers). This makes the image brighter, because each unit of detector area is being illuminated by a larger area of sky. 

 

Increasing the focal length while leaving the aperture fixed increases the f/ratio and produces a more 'zoomed in' image. The photon flux from the object (which is fixed) is being spread over a larger area. This reduces the number of photons hitting each particular part of the detector, reducing the brightness of the image. And of course, this brightness is really the signal. The noise is fixed. So longer focal length (with aperture fixed) means more magnification at a reduced signal to noise ratio (SNR). 

 

This principle is very well known to photographers and astrophotograhers. 

 

Eyepiece magnification works in just the same way. As human beings, we don't place our retina in prime focus. We have to use an eyepiece to focus the image at infinity, and then the light passes through the eye's own optic before striking the retina. The degree of 'spread-outness' of the image for ordinary visual observing is dictated by the combination of the telescope's f/ratio and the focal length of the eyepiece. 

 

The key parameter for measuring image brightness in visual observing is the size of the exit pupil. The exit pupil can be calculated by dividing the eyepiece focal length by the f/ratio of the optic. A 12mm eyepiece generates a 2mm exit pupil in any f/6 scope, regardless of aperture. The image produced by a 8" f/6 or a 24" f/6 will be equally bright when a 12mm eyepiece is used, as both will have a 2mm exit pupil. Of course, the image presented by the 24" will be of a much smaller area (more magnified) than the 8". And indeed, if you tried to match the magnification produced by the 24" in the 8", you'd have to use a much different eyepiece in the 8" scope (in fact, a 4mm eyepiece), which would dramatically reduce the brightness of the image it produces. The 8" at equal magnification would have a .67mm exit pupil -- very dim, and not useful for DSO, while the 24" at equal magnification is sitting right at the 2mm exit pupil sweet spot for DSO observing. 

 

The 'brightness' (or SNR) of any telescope-eyepiece combination is dictated by two things. The f/ratio and the eyepiece focal length. Changing either one has the exact same consequence for determining the parameter that drives the SNR: the image flux per unit area of the detector. To wit: zooming from 24mm to 12mm is exactly the same, mathematically, as doubling the f/ratio. Regardless of which parameter you change, the light ends up being more spread out across the detector (by a factor of 4, in this example), and the SNR of the resulting image is reduced accordingly.

'

That's why I was describing the zoom factor as being equivalent to adjusting the effective focal ratio of the optical system, which has well known consequences for the performance of intensifiers. I was simply trying to express the effect of various zoom settings in a metric that most of us are quite familiar with: the f/ratio.


Edited by shams42, 10 August 2016 - 01:15 PM.

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#36 pwang99

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Posted 10 August 2016 - 01:46 PM

I was not expecting a little optics lesson right after lunch, but that was very nicely and clearly articulated.  Thanks, shams42!



#37 Vondragonnoggin

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Posted 10 August 2016 - 02:42 PM

Eyepiece magnification doesn't change the focal ratio. The focal ratio is dictated by the optics, and the influence of any reducers or barlows in the light path. 

 

Focal ratio is just the steepness of the light cone. The steeper it is, the more light is being squeezed into each unit of the detector (pixels for cameras, microchannels for intensifiers). This makes the image brighter, because each unit of detector area is being illuminated by a larger area of sky. 

 

Increasing the focal length while leaving the aperture fixed increases the f/ratio and produces a more 'zoomed in' image. The photon flux from the object (which is fixed) is being spread over a larger area. This reduces the number of photons hitting each particular part of the detector, reducing the brightness of the image. And of course, this brightness is really the signal. The noise is fixed. So longer focal length (with aperture fixed) means more magnification at a reduced signal to noise ratio (SNR). 

 

This principle is very well known to photographers and astrophotograhers. 

 

Eyepiece magnification works in just the same way. As human beings, we don't place our retina in prime focus. We have to use an eyepiece to focus the image at infinity, and then the light passes through the eye's own optic before striking the retina. The degree of 'spread-outness' of the image for ordinary visual observing is dictated by the combination of the telescope's f/ratio and the focal length of the eyepiece. 

 

The key parameter for measuring image brightness in visual observing is the size of the exit pupil. The exit pupil can be calculated by dividing the eyepiece focal length by the f/ratio of the optic. A 12mm eyepiece generates a 2mm exit pupil in any f/6 scope, regardless of aperture. The image produced by a 8" f/6 or a 24" f/6 will be equally bright when a 12mm eyepiece is used, as both will have a 2mm exit pupil. Of course, the image presented by the 24" will be of a much smaller area (more magnified) than the 8". And indeed, if you tried to match the magnification produced by the 24" in the 8", you'd have to use a much different eyepiece in the 8" scope (in fact, a 4mm eyepiece), which would dramatically reduce the brightness of the image it produces. The 8" at equal magnification would have a .67mm exit pupil -- very dim, and not useful for DSO, while the 24" at equal magnification is sitting right at the 2mm exit pupil sweet spot for DSO observing. 

 

The 'brightness' (or SNR) of any telescope-eyepiece combination is dictated by two things. The f/ratio and the eyepiece focal length. Changing either one has the exact same consequence for determining the parameter that drives the SNR: the image flux per unit area of the detector. To wit: zooming from 24mm to 12mm is exactly the same, mathematically, as doubling the f/ratio. Regardless of which parameter you change, the light ends up being more spread out across the detector (by a factor of 4, in this example), and the SNR of the resulting image is reduced accordingly.

'

That's why I was describing the zoom factor as being equivalent to adjusting the effective focal ratio of the optical system, which has well known consequences for the performance of intensifiers. I was simply trying to express the effect of various zoom settings in a metric that most of us are quite familiar with: the f/ratio.

Except digiscoping with an eyepiece does not work in the same way as general eyepiece magnification or employ of barlow. I highlighted the differences in my original thread. While i agree in theory about general barlow use or focal length increases changing the f ratio with eyepieces, digiscoping does not do the same thing working with collimated light bundle.

 

It also becomes visibly apparent when using a barlow with your NVD vs digiscoping with NVD. That's why i compared various methods. native FL vs Barlow use vs digiscoping with adjustable (zoom) mag eyepiece.

 

Otherwise i would use any of the 1.5x, 2x, 3x, 4x, 5x barlows I own vs a large adapter and digiscoping methods

 

If it was solely for the convenience of zooming mags but still getting the same dimming as focal length increases, i would not use digiscoping methods


Edited by Vondragonnoggin, 10 August 2016 - 02:47 PM.


#38 shams42

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Posted 10 August 2016 - 02:48 PM

I don't understand how any optical device can magnify an image without spreading a finite number of photons over a larger geometric area.

Do you have any references on this?

#39 Vondragonnoggin

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Posted 10 August 2016 - 02:54 PM

I referenced this along with many observations in original thread:

 

https://en.wikipedia...cal_photography



#40 Vondragonnoggin

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Posted 10 August 2016 - 02:59 PM

Particularly this section describes the effect I am getting:

 

https://en.wikipedia...econdary_lenses



#41 Vondragonnoggin

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Posted 10 August 2016 - 03:01 PM

While my optical theory may not be up to snuff on some things, perhaps I should ask someone like Glenn LeDrew to chime in and correct me on what may be incorrect.

 

I'll PM him a link and see if he has something to add here that I would definitely fall short on explaining.



#42 Vondragonnoggin

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Posted 10 August 2016 - 03:07 PM

I sent him a PM to correct my ramblings if they are off.



#43 GlennLeDrew

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Posted 10 August 2016 - 07:52 PM

The effective focal length and f/ratio of an afocal imaging system.

 

Consider the eyepiece and camera or NVD lens (I'll use the general term "camera") as a relay system. The magnifying power of this unit is determined by the ratio of the camera lens focal length to eyepiece focal length. Example:

 

The camera's lens has a focal length of 50mm. The eyepiece f.l. is 25mm. The resulting relay system has a magnification of 50 / 25 = 2X.

 

The effective f.l. of the full system is the telescope f.l. multiplied by the relay system magnification. Example:

 

The telescope has a f.l. of 1,000mm. Our earlier eyepiece/camera lens combo of relay magnification 2X is attached. The effective f.l. of the full system is 1,000 * 2 = 2,000mm.

 

The f/ratio as presented to the sensor equals the effective f.l. of the full system divided by the objective aperture. Example:

 

Our 1,000mm f.l. telescope has an aperture of 200mm, for a 'native' f/5. The effective f.l. with the 2X relay system is 2,000mm. The working f/ratio is 2,000 / 200 = f/10. Similarly, one can calculate the working f/ratio as the 'native' f/ratio multiplied by the relay magnification, or f/5 * 2X = f/10.

 

Camera lenses of diminutive size can have apertures smaller than the exit pupil. In such case the system becomes stopped down in the same manner as for the eye when a larger-than-iris exit pupil is employed.

 

As can be seen, in order to obtain an image of high surface brightness the relay magnification cannot be large. This means using a shorter f.l. camera lens and/or an eyepiece of longer f.l. If the relay magnification is <1X, as when the camera lens f.l. is shorter than the eyepiece f.l., the working f/ratio is faster than 'native' for the objective alone. This is just like installing a reducer in the prime focus configuration.

 

The ultimate in system speed and efficiency occurs when the eyepiece exit pupil just fully fills the camera lens iris aperture. Then the working f/ratio is the same as that for the camera lens by itself--and the full objective aperture is utilized. For example, a 50mm f/4 camera lens has an entrance pupil of 50 / 4 = 12.5mm. To first order, a 12.5mm exit pupil will just fully fill the iris of this lens, with the fastest possible working f/ratio for the system being f/4. 


Edited by GlennLeDrew, 10 August 2016 - 07:55 PM.

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#44 Jeff Morgan

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Posted 10 August 2016 - 08:44 PM

Now I get it. Thanks Glenn!

 

In the context of thread, both the barlow and the "relay camera" slow down the system. The parallel light bundle (not converging) doesn't enter into play.

 

The advantage then comes from the adjustability of the zoom (vs. a barlow of fixed amplification). The ability to optimize scale and detail.

 

But no free lunch on the image brightness.


Edited by Jeff Morgan, 10 August 2016 - 08:45 PM.


#45 Vondragonnoggin

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Posted 10 August 2016 - 09:09 PM

Interesting. Not sure why the 16mm setting digiscoping is brighter than a 2x barlow native then.

 

If there is no brightness benefit vs barlow, I'll ditch the digiscoping method and just use one of my various barlows, but that isn't what I experienced using ENVIS lens mated to the zoom afocally.

 

Being able to zoom isn't that big a deal to me by itself.


Edited by Vondragonnoggin, 10 August 2016 - 09:17 PM.


#46 shams42

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Posted 10 August 2016 - 09:20 PM

The effective focal length of these night vision devices is 27.8mm, right? Then at the 16mm zoom setting, the magnification factor is 27.8/16 = 1.74. It looks brighter because it's a little less magnified.

 

I don't have a barlow so I can't try this experiment -- but I believe that a zoom setting of about 14mm should yield a 2x magnification factor and should be equally bright compared to using the night vision device at prime focus with a 2x barlow.



#47 shams42

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Posted 10 August 2016 - 09:22 PM

Oh - also, the photocathode of my night vision device is set deeper into the body than is the typical depth of the field stop of my eyepieces. I know that the NVD Micro requires a fair amount of in-focus compared to my eyepieces. Since the magnification factor of a barlow lens is related to the spacing between the lens elements and the field stop (or, in our case, the photocathode), it could be that a typical 2x barlow produces more than 2x when used with a night vision device due to the extra spacing. 


Edited by shams42, 10 August 2016 - 11:20 PM.


#48 Vondragonnoggin

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Posted 10 August 2016 - 10:09 PM

Yes, I realize the 16mm reference was off after I posted but was just leaving work and no time to correct that.

 

I'm not going to repeat my previous posts, but if you want to read my observations (because I am recalling it was still brighter at 10mm and 12mm settings), they are in that thread that was linked off the first post. That was months ago I did that so trying to remember is less advantageous to the actual posts that were made right after experimentation. In essence, the image scale using digiscoping adapter, and zooming in with afocal placement of ENVIS yielded a larger image scale than 2x yet still appeared brighter. I saw no benefit back then of digiscoping if this wasn't the result.

 

Maybe it was averted imagination, but I usually don't try and embellish experimentation. Again though, if theoretically no gain over barlow use, then no benefit to me.



#49 Vondragonnoggin

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Posted 10 August 2016 - 10:17 PM

I still appreciate Glenn stopping in to straighten my theories out. 

 

Lots of knowledge on these forums!



#50 shams42

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Posted 10 August 2016 - 11:22 PM

Yes, I realize the 16mm reference was off after I posted but was just leaving work and no time to correct that.

 

I'm not going to repeat my previous posts, but if you want to read my observations (because I am recalling it was still brighter at 10mm and 12mm settings), they are in that thread that was linked off the first post. 

 

Yes, I read that report with great interest. You are the person that inspired me to try this!

 

I guess I should stop calling it 'digiscoping.' I think it is better to call it afocal night vision? Not sure what the proper term would be.




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