Understanding “a-focal” observing with Night Vision (Revised)

[alanjgreen]

Time does not stand still...

It has been 17 months since I wrote the original version of the above named article and there have been a few changes in the intervening time period…

1.       I now have 17 months more a-focal experience of using a telescope with a night vision device attached directly to the eyepiece.

2.       A new forum has been created for the discussion of such “Electronic Assisted” observing equipment on this website.

So I decided to revise my article and post it in this new “most applicable” forum.

Let us start with the basics…

 

What is a-focal observing?

“a-focal observing” simply means that the night vision device is attached directly to an eyepiece (after the focal point of the telescope). You are placing the night vision device’s objective at the exit pupil point in the light path.

The easiest way to achieve this is the use the “TNV-14 Eyepiece Adapter” (available from Tele Vue). This adapter has threads on either side to connect (1) any Dioptrx accepting Tele Vue eyepiece to (2) a PVS-14 Night Vision device.

http://www.televue.com/engine/TV3b_page.asp?id=36

Here is a picture of a Tele Vue 55mm Plossl connected to a PVS-14 using the TNV-14 adapter.

To perform “a-focal” observing we need to simply insert this “stack” into any telescope focuser. If the attached eyepiece can achieve focus then there will be a focused image available to view in the PVS-14.

 

Here we see the stack attached to my 20” dobsonian and my 107mm Borg refractor…

 

What are the advantages of a-focal?

The biggest advantage is that you WILL be able to reach focus in any scope. Unlike other options you are simply placing the night vision at the point of the exit pupil. For Newtonians, this is a big point.

 

Fundamentals of a-focal observing.

Now we are past the basics, we have some slight more complicated “fundamentals” to get our heads around…

1. The PVS-14 night vision device is designed to work at a focal ratio of f1.2 (which is very fast). To get the most from the device then we need to aim to send light from the eyepiece as fast as possible to take maximum advantage of the night vision device. A faster focal ratio results in a brighter image, a slower focal ratio results in a dimmer image.

- Here we have been given a “lucky break”. Because the PVS-14 has an effective focal length of around 26mm, if we use any eyepiece with a focal length greater than 26mm then the “effective” focal ratio of our system gets “magically” increased.

[I will show how we calculate this effective focal ratio shortly but think of this on a par with adding a focal reducer into the light train].

Unfortunately, any eyepiece with a focal length less than 26mm will decrease this “effective” focal ratio of our system.

 

2. The PVS-14 has a fixed forty (40) degree field of view. It does not matter how wide field our eyepiece is, the night vision device will only ever show the centre forty degrees. This means that you don’t need 100 degree Ethos or 82 degree Nagler eyepieces, narrower field of view Plossl, Panoptics & DeLite’s will be fine.

- Again, don’t panic! There will be so much to see in the forty degrees that it will feel like 100 degrees. I have come from 100 degree eyepieces and I have never once wondered where my huge FOV went

 3. Eyepiece eye relief is important. You need eyepieces with enough eye relief to match the distance from the top lens surface of the eyepiece to the position of the night vision objective lens. Too much or too little eye relief will result in loss or distortion to the outer edges.

 

What is the minimum set of eyepieces that I need?

I use a total of four (4) Tele Vue eyepieces with my Night Vision device:

1.       Tele Vue 55mm Plossl. This is my main work horse eyepiece. I use this eyepiece for >90% of my observing time. The reason it is my most used eyepiece is that it gives my telescopes the fastest possible “effective focal ratio” (which results in the brightest possible image at the eyepiece). In simple terms think of this eyepiece as being able to double the speed of your telescope (like a 0.5x reducer). I use this eyepiece for nebulae, galaxies & open clusters.

2.       Tele Vue 35mm Panoptic. I use this eyepiece occasionally when I want more magnification but still want a bright accelerated image (it acts like a 0.7x reducer for the effective focal ratio). An alternative to this eyepiece would be the Panoptic 41mm - I use the 35mm because it’s half the weight of the 41mm! I use this eyepiece for nebulas, galaxies, comets, large open clusters.

3.       Tele Vue Panoptic 27mm. I use this eyepiece again for greater magnification, usually for supernovae, globulars, comets & open clusters. I do not use this for nebulas and galaxies as the effective focal ratio is now too low and details are becoming lost at the eyepiece.

4.       Tele Vue DeLite 18.2mm. This is my least used eyepiece (as its focal length is smaller than the 26mm of the night vision device). In use, it has the effect of slowing my effective focal ratio and producing a dimmer image. It does however produce about the maximum useable magnification with my night vision a-focal setup and I have been successful using it for faint tiny supernovae and bright globular clusters.

 

What about the huge exit pupils?

[Exit pupil is the width of the light beam being emitted from the top of the eyepiece and traditionally astronomers baulk at anything wider that the width of the astronomers own eye pupil as it is not possible for our eye to consume the whole of the light beam]

[Exit pupil is calculated as the eyepiece focal length divided by the telescope focal ratio so a 55mm Plossl in an f4 scope will produce a light beam 13.75mm wide]

As the night vision objective lens is 20mm wide then it can take all that light in and process it with room to spare! Whilst your eye pupil would be flooded and loads of light wasted, no light is wasted in this case.

But as the eyepiece focal lengths get shorter (and the exit pupils get smaller too), the night vision device soon starts to become starved of light.

 

How do I calculate this “eyepiece focal ratio” exactly?

Now seems the right time to show the maths to calculate the “effective” focal ratio of your telescope/night vision setup:

Effective focal ratio = NVD / (EPFL / TFR) where

        NVD = night vision device focal length = 26mm

        EPFL = eyepiece focal length

        TFR = telescope focal ratio

 

As an example, if we have a telescope with a focal ratio of f4, the 55mm Plossl will produce an “effective” focal ratio of f1.9.

[Effective focal ratio = 26/ (55/4)  =1.9] 

 

Does the focal ratio of my scope actually change?

The answer is NO. These changes in “effective” focal ratio that I mention only happen inside the night vision device. If your scope is f4 then it will remain f4.

 

Is a-focal observing, low magnification observing?

Simple answer = Yes it is. You need to get as much light as possible into the night vision device as fast as you can get it to go. All of the photons that you can get into the device will be amplified by the night vision device enabling you to see views containing previously unseen detail. In some cases, the amount of new detail on offer will be overwhelming!

At first, you will want to change eyepieces to achieve greater magnification but you soon discover that you actually see less detail (due to loss of effective focal ratio and exit pupil) so you soon return to the longer focal length eyepieces.

 

How do I calculate the magnification that each eyepiece will give me?

There is no change here. Take your telescope focal length and divide by eyepiece focal length.

If your scope has a focal length of 1800mm then you would get the following magnifications from my eyepiece set:

-          55mm Plossl (1800/55 = x33)

-          35mm Panoptic (1800/35 = x52)

-          27mm Panoptic (1800/27 = x67)

-          18.2mm DeLite (1800/18.2 = x99)

 

How do I calculate the TFOV?

I used Sky Safari for this. I setup my eyepieces in the “equipment” section using a setting of 40 degrees for the fov and it did the rest…

 

Can I use a coma corrector with night vision?

If your telescope has a fast focal ratio and you find that you need a coma corrector now then you will still need it for use with night vision. I used a Tele Vue Paracorr2 with my 20” dobsonian before I had night vision and I am still using it with Night Vision. In a big reflector, the best place for filters in the light path remains on the bottom of the Paracorr.

 

What about filters?

This brings us nicely onto every astronomers “favourite” topic – filters!

When combined with filters, night vision devices can allow us to not only see what was not visible before but also to steal back some darkness by blocking out our old enemy, the moon!

In the Cumbrian countryside, the night sky has an SQL of around 21.6, class 4 Bortle. Please take this into account when reading my experiences as your SQM may not be the same as mine.

1.     General observing – For general observing, I do not use any filters as the best results are achieved by letting all the light into the night vision device. The PVS-14 has manual GAIN which means there is a knob that can be turned to decrease the gain and darken the image at the eyepiece – this is the only filter that I use in general observing.

        Moon – If the moon is up then I add a Baader 610nm Red filter into the light path. This is a good filter for reducing the effects of the moon on the sky background. It can also be effective if viewing low to the horizon where light pollution can be an issue.

2.     Filters for observing Nebulae

For nebulae viewing, a narrowband Ha filter in mandatory. I have tried 12nm, 6nm and 5nm and my preferred choice of bandwidth is the 5nm. As this filter is the “key” to seeing nebula then please do not scrimp of a “cheapie”. If you want to get the maximum from your expensive night vision device then only consider top brands such as Chroma, Astrodon, Astronomik or Baader.  I am currently using a Chroma 5nm Ha narrowband filter.

Your choice of Ha narrowband filter will directly affect whether you see some of the fainter nebulae objects or you do not see them!

3.      Filters for observing Galaxies

For galaxy viewing, there is no filter that can improve the unfiltered view. 
However, if the moon is up then I use either the Baader 610nm red filter or an Astronomik UHC Visual filter. If you are viewing tiny smudges then either are okay, if you are viewing larger galaxies with spiral arms, then I find that the Astronomik UHC Visual filter gives slightly more spiral arms than the 610nm red. Both beat unfiltered viewing if the moon is up.

@GavStar is using a “Baader IR pass” (685nm) filter from his city location for all non-Nebula targets to cut out the light pollution.

 

Which night vision units can I connect to my telescope for “a-focal” observing?

As a UK based astronomer there are very few options for us to purchase a Night Vision Device with the latest military specifications. The Tele Vue adapter works with the PVS-14 night vision device so this led me in that direction. 

I purchased my PVS-14 from www.actinblack.com based in Luxembourg.

Please do read my article on “Understanding Night Vision Tube Specs a little better” and do be prepared to wait a month or two for actinblack to get a new batch of tubes into stock (from which you can then pick the best one for astronomy use). I had to wait two months for a new batch of Photonis tubes to come into stock before I was sent three tube specification sheets to choose from via email.

Having selected my tube then it was delivered to me in under a week from placing the order.

 

Which telescope do I need for Night Vision?

This is a good question and one that will be debated long into the future. My opinion is that the best telescopes for a-focal night vision use are telescopes with fast focal ratio.

I am using an f3.6 dobsonian and an f5.6 refractor.

Our goal is to achieve the brightest possible image at the eyepiece and focal ratio is the key to achieve that.

As we can see from above, there is a rather restricted set of eyepieces needed for night vision astronomy but if we pair these eyepieces with telescopes of varying focal lengths then we can get a wide range of actual field of views and magnifications. This drove my minimal set to two telescopes, one long focal length dobsonian with good aperture and largest possible magnifications (with long focal length eyepieces) and one short focal length refractor for wide field with decent apperture (> 4") and light enough for travel.

 

What can I see using Night Vision a-focally?

At this point, I want to point you to some of the many posts from @GavStar  available on this website. His images do reflect what I can see visually with my two setups.

Let me go on to summarize what I have been seeing in the last 17 months since the initial article.

Nebulae

I have now almost completed the full Sharpless catalog (303 of 313 objects). The only ones that I have not seen are the ones that are too low to my horizon!

Galaxies

I am working through the 200 brightest galaxies available in the skies above us. This project is more than half way complete and so far I have observed the spiral arms of 68 galaxies with direct vision.

Supernovae

Last year I viewed 17 supernovae, down to a magnitude of 16.8

Globular Clusters

I have so far failed to give sufficient time to Globulars, but their brightness means that I have been able to see some of the smallest and faintest on offer above us. I will get to these once my Galaxy project is completed.

Comets

Night Vision works well on comets, in a side-by-side test with traditional eyepieces, I saw better results with the night vision device.

Open Clusters

Night Vision gives great results with open clusters. The smallest ones just jump out at the eyepiece as you nudge around.

Planets

Failure – night vision is no good for planets. They are too bright.

Moon

Failure – night vision is no good for the moon. It is too bright.

Here are a few links to some of my reports (there are many more if you use the search facility)...

Do you still use eyepieces for observing?

My eyepiece case has been mostly sold off now. I have a set of short focal length DeLite eyepieces for planetary and I have some eyepiece pairs for solar observing with my Lunt LS60.

I use eyepieces to complete the 2-star alignments of my telescopes then it’s become automatic to just switch straight to the Tele Vue Plossl and my night vision to get into my nights observing. With the GAIN turned down it really is no different to using an eyepiece and you just see so much more…

 

Hope this helps somebody,

Alan

Edited October 13, 2019 by alanjgreen

[GavStar]

I would add that there is now a ‘prime’ night vision option for European astronomers, which wasn’t available when Alan and I got our night vision monoculars. 
Please see this thread: