Understanding “afocal” observing with Night Vision...

[alanjgreen]

This is my second article on the subject of using military Night Vision technology with your telescope.

The first covered the Night Vision Devices themselves...

 

This article will focus on one method of connecting a Night Vision Device to your telescope... The "afocal" way!

 

What is afocal 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.

A modern night vision device then uses “electron multiplication” to increase the effective brightness of the image on a phosphor screen which is viewed directly at the telescope.

For more info:   https://en.wikipedia.org/wiki/Image_intensifier

See    http://www.televue.com/engine/TV3b_page.asp?id=36&Tab=_work   (How Afocal NVD Works)

  (Inside my garage IN THE DARK)

 

What are the advantages of afocal?

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.

 

Which night vision units can I connect to my telescope for “afocal” observing?

For a UK based astronomer (without budget constraints), the simplest option is to get a PVS-14 night vision device. These can be purchased ITAR FREE from actinblack (Luxembourg)

See   https://www.actinblack.com/product/act-pvs-14/

 

The advantage of the PVS-14 is that TeleVue make an adapter that allows it to be securely connected to any “Dioptrx” compatible TeleVue eyepiece.

 

Where can I buy the TeleVue TNVC afocal astronomy adapter?

Unfortunately, they are only available direct from TNVC (in the USA). TNVC have the sole distributor rights to sell the adapter. You will also have to email them to setup an order as their website only accepts orders from US addresses.

They are easy to deal with and will create an invoice that you have to pay for via bank transfer. You will get hit for UK customs charges too!

See    https://stargazerslounge.com/topic/310679-first-look-tele-vue-tnvc-night-vision-a-focal-astronomy-adapter/

Also    https://tnvc.com/shop/tele-vue-tnvc-night-vision-afocal-astronomy-adapter/

 

Which eyepieces do I need?

To use the TeleVue TNVC adapter then you need TeleVue eyepieces that accept “Dioptrx” (which luckily is most of them). Before you rush into buying more eyepieces then there are some “basics of afocal observing” to digest…

 

Basics of afocal observing

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.

- Here we have been given a “lucky break”. Because the PVS-14 has an effective focal length of 26-27mm, if you use any eyepiece greater than 27mm focal ratio then the effective focal ratio of your system gets increased. Unfortunately, any eyepiece shorter than 27mm will decrease the effective focal ratio of your system. An increased focal ratio results in a brighter image, a decreased focal ratio results in a dimmer image.

 

2. The PVS-14 has a fixed forty degree field of view. It does not matter how widefield your 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 and Panoptics will be fine.

- Again, dont 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. 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 of outer edges or distortion to the outer edges.

- In my case, this is why in the main that I opted for longer focal length Panoptics. They have a 68 degree field of view and any outer edge issues will be outside the centre 40 degrees that the night vision cares about.

 

What is the minimum set of eyepieces that I need?

Everyone using Night vision needs a 55mm TeleVue Plossl. This will be your most used eyepiece on all targets because it acts as a x0.5 reducer and guarantees the fastest focal ratio for any telescope system (for brighter image etc).

You probably need an eyepiece in the 30-40mm range which will act as a x0.7 (ish) reducer giving increased image brightness but a larger image scale due to more magnification (than the 55mm).

Beyond these two eyepieces then you can use whatever you have for testing before spending money.

You probably won’t want too many eyepieces under 27mm focal length due to the ever dimmer image.

 

Is afocal 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.

You are going to “see so much more” in the low power 40 degree field of view that magnification concerns will be lost in the past. I can see Hicksons in a 55mm Plossl (x38 magnification) in the big dob (I was very concerned about magnification before I got my setup – I am not concerned anymore!)

 

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.

Big Dob = 2086/55 = x38 (55mm Plossl gives x38 in my 2086mm Dob)

 

Is the focal ratio of my scope going to change?

Answer is NO. These changes in focal ratio that I mention only happen inside the night vision device. If your scope is f4 then it will remain f4. If you use a 55mm Plossl then the effective focal ratio “to the NVD" becomes f4/2 = f2)

 

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

Time for some maths. Eyepiece focal ratio = NVD / (EPFL / TFR)

    NVD = night vision device focal length = 26mm

    EPFL = eyepiece focal length = 55mm

    TFR = telescope focal ratio = f4.1

    Eyepiece focal ratio = 26 (55 / 4.1) = f1.94 (In big dob, the 55mm Plossl creates an effective focal ratio of f1.94)

 

How do I calculate the TFOV?

I used Sky Safari for this. I setup some “eyepieces” with 40 degree fov my “equipment” and it did the rest…

 

Hang on a minute if I use a 55mm Plossl in an f3.6 scope, what about the huge exit pupil?

Yes that’s right. When I use the 55mm Plossl in my f3.6 it is generating a 15mm exit pupil ! As the night vision objective is 20mm then it can take all that light in and process it! Whilst your eye pupil would be flooded and loads of light wasted, no light is wasted in this case.

 

Can I use a ParaCorr2 with night vision?

If you need a paracorr for coma now then you still will need it for NV too. It works no problems. In a big reflector, the best place for filters in the lightpath remains on the bottom of the Paracorr.

 

What about filters?

Now we are getting to the crux of the matter. Another major difference between the night vision device and your eye. It can see red. You can see green & blue.

If you add a “Ha CCD” filter into your light path then the latest night vision can produce an image from this narrow wavelength of light. This brings Nebula to your eye that you can only see in images. This is the area where night vision is a REVOLUTION.

If you will be using two inch eyepieces or a Paracorr then you should just go straight for two inch filters. The nebula filter of choice should be either (1) Astronomik 6nm Ha CCD filter or (2) Baader 7nm Ha filter.

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

For me, in the countryside, I am finding that “no filter” is proving the best solution for non-Nebula targets.

- But I am testing with (1) Baader 610nm Red filter (2) Astronomik CLS visual & (3) Astronomik UHC visual so the jury will be out for a few months.

- Basically, you may need a nice broadband filter (which passes infra red) to get rid of light pollution (if you have some) because the image intensifier will just intensify the light pollution too!

Right, I have run out of steam, please post questions below...

Alan

 

[Highburymark]

Great stuff Alan. Your NV equipment and observing reports - together with Gavin's - provide a great resource for many of us who are interested in this amazing technology.

[Stu]

Gavin has taught you well young Alan  

Good to see that you now get the need for a fast focal ratio and that mag is not so important. The results I’ve seen from light polluted sites have been amazing, and Gavin’s IOW results were spectacular.

I suspect this will remain a spectator sport for most of us though.

[John]

Very interesting stuff again Alan

I do think the cost needs to be mentioned though - currently it's going to preclude all but those with very healthy budgets for the hobby I fear ?

(hopefully not in the future though ?)

 

[Stu]

1 hour ago, John said:

hopefully not in the future though ?

Sadly I doubt that will be the case John, it is niche military stuff, and I can’t see the demand getting big enough to bring the cost down appreciably. Hope I’m wrong of course.

I was discussing this with Gavin, and I suspect that over time, the camera sensors will get ever more sensitive and allow real-time live views of these objects. The Sony A7 is already pretty good, but in a few years who knows where it might go, and being driven by phones and cameras, the volumes will be there to bring the price down.

I think it is great that the work done on CN and picked up and put together by Gavin is helping others to understand how to use the technology.

[GavStar]

Here’s a recent article on CN by a US Night Vision vendor, TNVC.

https://www.cloudynights.com/topic/617491-some-important-questions-when-shopping-for-night-vision-and-what-sets-tnvc-apart/

They make the point that night vision tubes are ‘grown’ rather than built. I think this will limit any price decreases. The main European supplier to astronomers, actinblack In Luxembourg, sometimes isn’t able to supply quality tubes off the shelf and means that you may have to wait a bit for them.

The guys in the US have many more options than us here, but that’s made us the afocal pioneers to some extent. I think it’s a very flexible method and am not worried about the long eyepiece plus monocular setup. 

I’ve been really enjoying the NV adventure discovering new stuff like the 1x sky scanning I was doing at the weekend. 

But I appreciate it’s not for everyone.

[PeterW]

The cost won’t likely shift, export restrictions will likely remain. There isn’t a huge number of these things made and the technology has only very slowly improved. Camera sensor quantum efficiency has been almost max for a number of years, just a case of pixel size and count and low noise detection circuitry. Better, but probably not amazingly so. Thermal imagers will be an area that we’ll see a lot of improvements (more spec, less cash) in the comming years, though they’re not much use for Astro.

It’s good to see another convert to the “green side” (though both of the young Jedi/night ninjas are “colourblind”).

preclude? Limit access maybe, but we’re happy to share the views.

PEterW

 

[scarp15]

Very interesting to read and presented with educational clarity in terms of this technology, it certainly is spicing up the observing reports. For many, traditional methods and getting to a dark, transparent sky from time to time, to extract what is humanly possible will continue to be the aspiration. Maybe that in time, as a few people become more able to invest in this technology, perhaps associations and star parties, may enable wider participation.  

[PeterW]

Gen3 has existed for 25yrs or so, very slowly improving. The filmless is a step up, but the amount of magic processing steps to end up with a robust product that will meet the specs is nuts. Can’t see the production ever rising hugely, the industry seems to mainly kept alive by defence contracts. Defence likes thermal and other infra red wavelengths using semiconductor  imaging sensors, so they might migrate away from “old analogue tech” which would be a loss for all of us. 

PeterW

[Stu]

3 minutes ago, PeterW said:

filmless is a step up

What is filmless NV Peter?

[DRT]

{please forgive and do not comment on the political angle of this post} But if Donald Trump withdraws from enough peace treaties won't the cost of military grade equipment come down?  

[PeterW]

Gen3 typically has a very thin barrier layer to protect the light sensitive layer from accumulated damage, but it reduces the petformance... some clever types managed to reduce the effects and so remove the layer and regain the performance.

http://aunv.blackice.com.au/forum?index=gen4

Cj7hawk explains it much better, one of the NV gurus. The filmless tubes have more sensitivity and also typically lower background noise... better for Astro... Gav has the “best the EU can make”, which gets pretty close.

PEterW

[DirkSteele]

Great stuff! Really enjoying (and learning) reading about this.

 

Gavin was kind to let me have a little play with his NV set up recently and I was very impressed.

[GavStar]

7 hours ago, DirkSteele said:

Great stuff! Really enjoying (and learning) reading about this.

 

Gavin was kind to let me have a little play with his NV set up recently and I was very impressed.

Apologies Matthew, as demos go it was pretty poor - not helped by me forgetting the power lead for my mount ?

Last weekend would have been a much better demo ?

 

[alanjgreen]

Some more photos for completeness

 

[DRT]

On 09/05/2018 at 15:47, alanjgreen said:

For a UK based astronomer (without budget constraints), the simplest option is to get a PVS-14 night vision device. These can be purchased ITAR FREE from actinblack (Luxembourg)

The old adage "If you have to ask the price, you can't afford it" is ringing in my ear, but...

What price range are we talking about for these devices?

[PeterW]

Plenty for the shiny new tech and a reasonable fraction for that “pot luck, get what you can find” type, which should still produce  decent benefit. For urban bound observers it’s a revelation.

PEter

[GavStar]

1 hour ago, DRT said:

The old adage "If you have to ask the price, you can't afford it" is ringing in my ear, but...

What price range are we talking about for these devices?

About £6k from actinblack in Luxembourg 

[DRT]

Just now, GavStar said:

About £6k from actinblack in Luxembourg 

{fall-off-my-chair-emoticon}

[PeterW]

.,,, or a bit over 2 for the lucky dip option... only a couple of Ethos, which can’t show you  any of this good stuff.....

Peter

[DRT]

5 minutes ago, PeterW said:

.,,, or a bit over 2 for the lucky dip option... only a couple of Ethos, which can’t show you  any of this good stuff.....

Peter

Given their intended customer base and use, do you need to wear a balaclava to get your hands on a cheap one?  

[Highburymark]

30 minutes ago, DRT said:

{fall-off-my-chair-emoticon}

I think of it as a second hand Ford Fiesta or the glory of the Horsehead Nebula from the middle of a city. Know which I'd rather have. But as I write this I'm thinking - how many SGLers are going to suggest buying the Ford Fiesta and driving to a dark site......

[DRT]

44 minutes ago, Highburymark said:

I think of it as a second hand Ford Fiesta or the glory of the Horsehead Nebula from the middle of a city. Know which I'd rather have.

Actually, what would be interesting to know is how one of these at £6k plus a "normal" refractor would compare to a £10k-£15k scope.

[alanjgreen]

On most targets the NV would blow the traditional scope out of the water.

The purists doting on star shapes and double stars should keep their kit, this is a world of low power observing. The stars lose their colours too, they have shades of greyness instead, so maybe open cluster purists would disapprove too.

- if our go afocal then you only need pop a traditional eyepiece instead so no great sweat (phew).

But If you want to see nebula then a 15k scope won't help you compared to viewing in H-alpha. NV will resolve even faint NGC globulars to the core.

You just need a reasonably fast scope, speed is important - focal ratio. The NV has a 20mm pupil that you need to fill with light to maximise what it can give you. I just ordered an f5.6 107FL Borg scope to be my travel companion. NV won't be at its best in one of those long anti-aircraft refractors I am afraid to say.

BUT you don't even need a scope, I saw the spiral arms in M101 at x1 magnification, these devices deliver even without a scope. The Milky Way and its many large nebula where wonderful at x1 the other night (unfiltered and with Ha filter). It's incredible to be out at night and be able to see around the garden, fields etc as if it were daytime!

The only thing a refractor combined with NV won't give you is image scale. You must use long focal length eyepieces to keep the exit pupil as big as possible. That's where a big fast dobsonian takes over = decent scale with a 55mm or 35mm eyepiece and huge exit pupils  

My main love are Galaxies, I have seen spiral arms in many new galaxies since May including the fireworks galaxy last week, I am really really looking forward to the next galaxy season... and the dark black winter skies.

- you still get more if the sky is darker

- you still get more with averted vision and time at the eyepiece for your brain to engage etc etc. It's not all free!

If I had 15k to spend on a scope then I would hope someone told me about NV before I spent my money and was left wondering what all these objects are that people are talking about.  That's all I try to do.

[DRT]

This sounds fantastic.

How much could I get for a 53 year old kidney? ?