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NV: prime focus vs afocal

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#1 nimitz69

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Posted 13 June 2018 - 08:32 PM

As I continue on my vertical learning curve wrt NV I'm trying to understand under what circumstances prime focus vs afocal is 'better' or in general why you would do one over the other if you had a choice.

 

as I understand it your only choice with a monocular to do prime focus is a Mod 3 or micro.  With PVS-14s your only choice is afocal.  Given that it appears PVS-14s are a lot more readily available then Mod 3s or Micros it would seem afocal with them is the way to go - particularily if you have TV lens and can now use their adapter.

 

isn't there more flexibility in being able to use different EPs afocally then just the NV at 27mm prime focus?

 

what am I missing?



#2 TOMDEY

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Posted 13 June 2018 - 09:43 PM

Hi, nimitz69! I've been using NV at Prime Focus for a few years.

 

Some of the advantages are:
>Far less weight hanging off your focuser
>Far shorter than afocal stack
>Far less glass involved
>more throughput to the GaAs photocathode array
>convenient swap twixt "regular" and NV

 

I recently bought the TeleVue Afocal adapter, but haven't used it yet. Advantages:

>variable mag by eyep selection
>NV 1x objective lens stays right on
>eyep stays right in

 

Well, I'm all self-satisfied for having Both... (all Three of them!) but haven't compared yet.

 

But it would seem, that if the Prime Focus direct to GaAs provides the mag and f# that you want, that would be preferred. Otherwise, the recommended eyepieces have at least four lens elements and the NV lens probably another many. One other possible detractor re afocal... Eyepieces are designed and optimized for direct use by the eye. When we go afocal to a camera lens (aka NV 1x objective)... that is NOT what the eyepiece was designed for. So it's going to be a case-by-case try and see what you get.

 

Tom

 

Here's an image of one of my NV showing the simplicity of the Prime Focus adapter configuration. >>>

 

Attached Thumbnails

  • NV adapter for prime focus onto GaAs photocathode.jpg

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#3 moshen

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Posted 13 June 2018 - 11:17 PM

The only real advantage to afocal is you don't need the backspace requirements for focal reducing like you need with a focal reducer. For example you might have a dob that might not have the backspace required for prime focus & focal reducer.

For those that have a device w/o removable lens such as PVS-14, afocal is the only option.

 

For prime focus to change mag you use a focal reducer or barlow instead of eyepieces as with afocal. In return you get a much lighter and shorter stack, better ergonomics & much less glass in the path.

 

The Mod3 C-mount is the most flexible - it can do afocal & prime but you must do the work to get a 1x c-mount lens for 1x viewing. In return you can use it in prime, afocal, and with the ecosystem of other c-mount & camera lenses.

 

The PVS-14 in afocal usage is the easier plug-n-play path to get started immediately with 1x viewing but you lose the flexibility that a removable lens and c-mount interface provides.

 

BTW the Mod3-C mount is readily available. It's the 1x c-mount lenses that take some effort to get. So if you get a Mod3-C and a C-mount 1.25" adapter you can use it immediately in your telescope but you won't be able to use the Mod3 alone for 1x viewing until you get an appropriate objective.


Edited by moshen, 13 June 2018 - 11:21 PM.

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#4 Jeff Morgan

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Posted 13 June 2018 - 11:34 PM

Firstly, if you buy the PVS-14 it's a moot issue. Afocal only, learn to like it.

 

If you get a C-mount device it gets more interesting because you have choices. Tom identifies many key issues. I use both methods and see the following:

 

  • C-mount is far more convenient for changes. Pulling off my 0.7x reducer for prime focus is as fast and easy as removing a filter. Eight seconds tops with due caution.
  • Barlows are also a snap.
  • Focal reducers and barlows are cheaper than Tele Vue eyepieces.
  • Focal redcuers require substantial in-focus (about 12mm @ 0.7x, 55mm @ 0.5x).
  • Afocal may work better in scopes with limited focuser travel (Newtonians primarily), no scope mods necessary.
  • Out focus may be an issue with afocal, but easily solved with an extension tube.
  • You may already own the Tele Vue eyepieces you need to do afocal.
  • Swapping the TNVC adapter between eyepieces is a chore, you will probably want one for each eyepiece.
  • Undercut snags are exacerbated by the huge afocal stack. Tape those undercuts!

 

TNVC adapters are inexpensive (around $60 each), as are Tele Vue Plossls. So even if you have a C-mount device you may want to have the TNVC around.

 

My kit currently has one TNVC adapter, one 55 Plossl (for reduction) and one 22 Panoptic (mild barlow). My new scope build will have the back focus for a 0.5x reducer thus replacing the 55 Plossl. But since I never know what the future holds for scopes, I will keep the TNVC adapter and likely the big Plossl too.


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#5 nimitz69

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Posted 14 June 2018 - 09:28 AM

Thx guys,  I have an Orion XX14g Dob and the following TV EPs:  7mm Delite, 10mm,14mm Delos, 19mm, 24mm Panoptic .....

 

let's assume the in focus issue is a problem in my Dob & I'm unwilling to mod the scope

let's further assume I'd like to use my 3 Nikkor SLR lens (50mm, 85mm & 18mm-200mm zoom) for some non-telescope NV viewing of the sky

 

Does that eliminate any choices?



#6 nimitz69

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Posted 14 June 2018 - 09:56 AM

moshen:  just reread your post a couple of times ....  Eddgie recently told me new Mod 3s are not coming with a 1x objective lens, just the c adapter ...

 

In this configuration 'out of the box' I can put it in my Dob like an EP and do prime focus viewing at the focal length of the NV (27mm) and that's it, correct?

You said in order to do 1x viewing I'd need to find a 1x c-mount lens ... why would I want to do this?

 

Maybe I need to just list what I'd like to do which would more easily lend itself to what material solution options there are ... so

 

1.  I want to be able to use the NV on my current 14" f/4.6 Dob

2.  I want to be able to change mags as I view different objects.  This can be via my current TV EPs or barlows/reducers assuming I get equivalent results.  Selling some TV EPs would help fund the NV device after all

3.  I want to be able to use my Nikkor SLR lens for 'off telescope' NV viewing

4.  I probably want to do 1X viewing but honestly not sure of the mechanics ... is this 'off-telescope' viewing or not?

 

Given the above, what are my options for an NV solution?



#7 pwang99

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Posted 14 June 2018 - 09:57 AM

I've got an OCS adapter on order from Siebert Optics, which is a no-magnification extender that should (knock on wood) allow me to reach prime focus in my dob.  I'll definitely report here on how that works out.


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#8 Eddgie

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Posted 14 June 2018 - 10:04 AM

A PVS-14 can only be used afocally or at 1x and 3x with a military type 3x a focal lens. You would not be able to do prime focus.

 

You do want to do 1x an 3x because there is a lot of stuff that is really big like Barnards Loop and the Milky Way.

 

To do 1x you can use a 25mm CCTV camera lens or a 28mm SLR lens (with the appropriate c mount adapter).  In this case you just mount the lens and hand hold like a binocular though it is much lighter and at 1x stability is not a problem.

 

Trust me on this...You will do far more low power observing than you think.


Edited by Eddgie, 14 June 2018 - 10:04 AM.


#9 Eddgie

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Posted 14 June 2018 - 10:18 AM

And this needs to be repeated... The TV adapter is not the only way to do a focal.  Peter and I did a great deal of afocal by simply holding the device with the 1x lens up against the eyepiece! 

 

There is also the option of using an afocal camera mounting bracket.  The devices have standard 1/4-20 thread sockets.

 

So the TV adapter is not the only way to do afocal and I personally still simply hold me device to the eyepiece though the vast majority of my telescope use is magical.

 

And don't get confined by what you think you will or won't do because if you do buy a night vision device you may change you mind about what you will or won't do,  It may pretty radically change your thoughts on observing.


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#10 Jeff Morgan

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Posted 14 June 2018 - 12:17 PM

moshen:  just reread your post a couple of times ....  Eddgie recently told me new Mod 3s are not coming with a 1x objective lens, just the c adapter ...

 

In this configuration 'out of the box' I can put it in my Dob like an EP and do prime focus viewing at the focal length of the NV (27mm) and that's it, correct?

You said in order to do 1x viewing I'd need to find a 1x c-mount lens ... why would I want to do this?

 

Maybe I need to just list what I'd like to do which would more easily lend itself to what material solution options there are ... so

 

1.  I want to be able to use the NV on my current 14" f/4.6 Dob

2.  I want to be able to change mags as I view different objects.  This can be via my current TV EPs or barlows/reducers assuming I get equivalent results.  Selling some TV EPs would help fund the NV device after all

3.  I want to be able to use my Nikkor SLR lens for 'off telescope' NV viewing

4.  I probably want to do 1X viewing but honestly not sure of the mechanics ... is this 'off-telescope' viewing or not?

 

Given the above, what are my options for an NV solution?

 

In your earlier post you said that you were unwilling to mod your Dob. This rules out focal reducers because of the extra inwards focus required. This alone points you to afocal NV. The afocal path allows you to achieve faster f/stops without a scope mod.

 

Option 3 requires a C-mount device. You can buy C-mount adapters for telephoto lenses for under $20 on Amazon or eBay. Very easy. And very cool. cool.gif cool.gif cool.gif

 

Note: While these appear conflicting on the surface, you can do afocal NV with a C-mount device!

 

One other possible point of confusion - some C-mount devices are sold without the objective lens used for "normal" 1x viewing. Much in the manner most telescopes are not bundled with eyepieces. Therefore, the purchaser needs to track down a 1x CCTV objective lens. That is an extra step, but not an insurmountable obstacle since millions of these things have been made. Be sure to ask your vendor and plan accordingly.


Edited by Jeff Morgan, 14 June 2018 - 12:19 PM.


#11 Jeff Morgan

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Posted 14 June 2018 - 12:22 PM

There is also the option of using an afocal camera mounting bracket.  The devices have standard 1/4-20 thread sockets.

 

And the option of doing afocal with the Baader Zoom via it's M43 thread, but that method is the least versatile (limited to the Baader Zoom). Barlow effect only, no focal reduction.

 

https://www.cloudyni...t-vision-setup/

 

If long focal length eyepieces were discovered to have the M43 thread focal reduction would also be possible. But I am not aware of any.


Edited by Jeff Morgan, 14 June 2018 - 12:23 PM.


#12 moshen

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Posted 14 June 2018 - 01:53 PM

 

In this configuration 'out of the box' I can put it in my Dob like an EP and do prime focus viewing at the focal length of the NV (27mm) and that's it, correct?

You said in order to do 1x viewing I'd need to find a 1x c-mount lens ... why would I want to do this?

 

Maybe I need to just list what I'd like to do which would more easily lend itself to what material solution options there are ... so

 

1.  I want to be able to use the NV on my current 14" f/4.6 Dob

2.  I want to be able to change mags as I view different objects.  This can be via my current TV EPs or barlows/reducers assuming I get equivalent results.  Selling some TV EPs would help fund the NV device after all

3.  I want to be able to use my Nikkor SLR lens for 'off telescope' NV viewing

4.  I probably want to do 1X viewing but honestly not sure of the mechanics ... is this 'off-telescope' viewing or not?

 

Given the above, what are my options for an NV solution?

You can buy a Mod3 C-mount and use it in prime focus as a 27mm eyepiece. You only need to add a c-mount 1.25" adapter like: (https://www.ebay.com...vAAAOSwAuZX5Jvd)

 

1x objective for 1x viewing is 'off-telescope' viewing and is useful for both sky and terrestrial. You would also use the 1x objective in afocal mode by attaching the entire device including the 1x objective on top of an eyepiece. The main advantage with afocal is you can put it on top of a 55mm Plossl and get a 0.5x reduction without any further in-focus travel needed. For my refractors that in-focus travel is not a problem but it may be for your dob.

 

For #3 using a Nikkor SLR len for off telescope viewing you essentially need a NVD with c-mount like Mod3-C. A PVS-14 won't let you do that.

 

A Mod3-Cmount is the most flexible device, but as I said it requires extra effort to score that C-Mount 1x objective as it doesn't come with one.



#13 BJS

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Posted 14 June 2018 - 02:40 PM

And this needs to be repeated... The TV adapter is not the only way to do a focal.  Peter and I did a great deal of afocal by simply holding the device with the 1x lens up against the eyepiece! 

 

There is also the option of using an afocal camera mounting bracket.  The devices have standard 1/4-20 thread sockets.

 

So the TV adapter is not the only way to do afocal and I personally still simply hold me device to the eyepiece though the vast majority of my telescope use is magical.

 

And don't get confined by what you think you will or won't do because if you do buy a night vision device you may change you mind about what you will or won't do,  It may pretty radically change your thoughts on observing.

What Eddgie say above is how i have done Afocal for the last year or so....Btw I need to get a laynard for my Micro...where do you get them?  I did not find anything on amazon?  I need one so that i feel better handing the micro to other people!


Edited by BJS, 14 June 2018 - 02:41 PM.


#14 pwang99

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Posted 14 June 2018 - 04:32 PM

I use these parachute cord lanyards:  http://a.co/5Xk6dc6

 

They have nice little clips that make them easy to detach.

 

(FWIW, I don't think the above link is an affiliate link; I just used the "Share Link" button on the Amazon page.)


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#15 nimitz69

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Posted 14 June 2018 - 05:54 PM

Pwang99 ... I'm definitely interested to hear about that orcs adapter ..

 

what determines whether you have enough inward focus travel on a Dob?  F ratio? Focal length?  Focuser mechanics? It doesn't sound like there is any way to know without trying it out on your specific scope?



#16 Starman81

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Posted 14 June 2018 - 06:06 PM

Pwang99 ... I'm definitely interested to hear about that orcs adapter ..

what determines whether you have enough inward focus travel on a Dob? F ratio? Focal length? Focuser mechanics? It doesn't sound like there is any way to know without trying it out on your specific scope?

It boils down to just hope the scopes were designed. For mass-produced commercial builds (like your dob) just by experience we learn which lines have more back focus and which ones have less. This metric is not posted anywhere on the manufacturer's website or in any sales literature.

SkyWatcher newts/dobs are designed to have a lot of back focus which for visual-only observers can be quite annoying as they have to use extension tubes to reach focus with normal eyepieces, but for prime focus NV it is a highly-desirable characteristic.

Orion newts/dobs, while made by the same manufacturer, typically do not have as much back focus. Eddgie used to observe with an XX12G so I hope he chimes in. I know with my solid tube XT8g there wasn't enough back focus for me to use either the 0.5x or 0.7x reducers. Even so, I could just use the afocal method for 0.5x reduction in that case.

And do remember that of course you can use your NV device at prime focus (with the proper adapter) at the native focal ratio in any scope.

#17 nimitz69

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Posted 14 June 2018 - 06:23 PM

Since I have a XX14g I hope he does too ...



#18 nimitz69

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Posted 14 June 2018 - 06:38 PM

Ok, having finally reached my typical state of 'parayalsis by analysis' in attempting to get smart about a topic I know nothing about I think you all have beat enough facts into me to make a basic decision ....

 

the Mod 3 w/c adapter looks like my choice

 

the Micro is out because I want manual gain capability ...

 

So what does that leave me?

 

I need to score an appropriate 1x objective lens for the NV

I need adapters so I can easily do afocal with my current EPs

I may need to get that ocs adapter and then a .5 FR or a 55mm Plossel

I need to get some filters

I might need to get one or two SLR lens ... My DSLR lenses have manual focus but  the aperature rings don't move ...

I also need to decide who to get the NV from

 

am I missing anything? 



#19 nimitz69

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Posted 14 June 2018 - 06:49 PM

And the option of doing afocal with the Baader Zoom via it's M43 thread, but that method is the least versatile (limited to the Baader Zoom). Barlow effect only, no focal reduction.

 

https://www.cloudyni...t-vision-setup/

 

If long focal length eyepieces were discovered to have the M43 thread focal reduction would also be possible. But I am not aware of any.

Well since I happen to own the Baader Mark IV zoom already that's a nice bonus ....



#20 Jeff Morgan

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Posted 14 June 2018 - 08:58 PM

Pwang99 ... I'm definitely interested to hear about that orcs adapter ..

 

what determines whether you have enough inward focus travel on a Dob?  F ratio? Focal length?  Focuser mechanics? It doesn't sound like there is any way to know without trying it out on your specific scope?

 

Physical spacing. Newtonian designers tend to emphasize minimizing diagonal size, intercept distance, tube diameter, and focuser travel which all adds up to a limited focus range.

 

If you can reach focus with some of the better known in-travel eyepieces like the 31 Nagler, 17.3 Delos, or Leica ASPH you will be fine. My Mod 3 with the 2" ScopeStuff adapter needs about 3/16" more than the aforementioned eyepieces.

 

Should you really decide you like NV, modifying a Newtonian to create some more back focus is not very difficult at all. And if you split time with conventional an inexpensive extension tube keeps those eyepieces in the game.



#21 nimitz69

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Posted 14 June 2018 - 09:43 PM

Ok, I'll bite, what exactly is involved in ' not very difficult at all' to modify my Dob to get more back focus ?



#22 Eddgie

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Posted 14 June 2018 - 10:42 PM

For a scope to work the focal plane needs to be about 16mm from the top of the eyepiece when the focused is fully racked in.  If you want to use a 1.25" nose, you need to include the light path length of the 2" to 1.25" adapter or use a 2" to C mount.  Scopestuff sells one of these that has a 1.25" filter thread on the scope side so you can still use this size filter.

 

My dob is the Orion 12 and I can reach focus even when using a 2" to 1.25" adapter (about 3-4mm left).



#23 BJS

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Posted 14 June 2018 - 11:45 PM

I use these parachute cord lanyards:  http://a.co/5Xk6dc6

 

They have nice little clips that make them easy to detach.

 

(FWIW, I don't think the above link is an affiliate link; I just used the "Share Link" button on the Amazon page.)

Thanks for the link.  



#24 nimitz69

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Posted 15 June 2018 - 08:38 AM

For a scope to work the focal plane needs to be about 16mm from the top of the eyepiece when the focused is fully racked in.  If you want to use a 1.25" nose, you need to include the light path length of the 2" to 1.25" adapter or use a 2" to C mount.  Scopestuff sells one of these that has a 1.25" filter thread on the scope side so you can still use this size filter.

 

My dob is the Orion 12 and I can reach focus even when using a 2" to 1.25" adapter (about 3-4mm left).

Well tht sounds promising ... Let's hope the Orion XX14g is like the XX12g ... My scope is arriving this afternoon so in a day or two I should have a much better idea after getting it assembled ...



#25 Kevdog

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Posted 15 June 2018 - 10:50 AM

To give a little more backfocus you can also move your mirror as far up in the mirror mount as it can go, then "back off" to collimate.   To achieve focus I did this in my dob.   I think I gained another 12mm that way.




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