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Filter size for night vision

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#1 George9

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Posted 28 December 2020 - 07:38 PM

I have read many threads in this forum, as well as other material, but I remain confused on filter sizes. For context, I ordered the Televue afocal setup. Mainly because I did not feel comfortable figuring out all the intricacies of picking the right tube criteria, getting the right casing, etc. This way I can always swap cases in the future if I decide to go to prime focus. And I already own many TV eyepieces. I have an 18" f/4.3, a 10" f/5, and several refractors f/6 to f/7. I figure I will spend a lot of my time with a 55mm Plossl on the 18", which is basically serving as my reducer when paired with the NV objective lens.

 

My question is what filter size do I need for an afocal setup? It seems a 2", assuming I use a 2" eyepiece like the 55mm Plossl. That places the filter into the telescope's native focal ratio, which should be fine in my case. In theory, a 1.25" would fit after the eyepiece before the NV objective, but I guess the problem is that the effective focal ratio will be much shorter and therefore go off band at the edges?

 

And for 1x use in front of the NV objective, I see people using 2" filters and TV offers a 2" adapter. But it seems to me that a 1.25" would work as well. The edges in a 2" filter will be off-band anyway, no?

 

And for prime focus, there a 1.25" filter seems obvious. But if you are using a reducer, then you might be better off putting the filter before the reducer, which means you might need a 2". After the reducer, you will have a shorter f-ratio and go off band on the edges, I think.

 

I ask because the difference in cost between a 1.25" and 2" is large. It seems that many people like a 5nm H-alpha filter, with some preferring 3.5nm and some preferring 6 or 7nm. And some also have a 12nm for special cases.

 

(Does the high transmission and sharp shoulders of a filter like an Astrodon--especially a big one--make a big enough difference to warrant a very costly purchase?)

 

George

 



#2 a__l

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Posted 28 December 2020 - 11:02 PM

I only use two 2" filters. Baader H-alfa 7 nm and Baader IR Pass 685 nm.
This is enough for the afocal 67 TV and prime focus on my telescopes and for my four lenses (1x - 8x).


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#3 Jeff Morgan

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Posted 29 December 2020 - 02:05 AM

I have read many threads in this forum, as well as other material, but I remain confused on filter sizes. For context, I ordered the Televue afocal setup. Mainly because I did not feel comfortable figuring out all the intricacies of picking the right tube criteria, getting the right casing, etc. This way I can always swap cases in the future if I decide to go to prime focus. And I already own many TV eyepieces. I have an 18" f/4.3, a 10" f/5, and several refractors f/6 to f/7. I figure I will spend a lot of my time with a 55mm Plossl on the 18", which is basically serving as my reducer when paired with the NV objective lens.

 

My question is what filter size do I need for an afocal setup? It seems a 2", assuming I use a 2" eyepiece like the 55mm Plossl. That places the filter into the telescope's native focal ratio, which should be fine in my case. In theory, a 1.25" would fit after the eyepiece before the NV objective, but I guess the problem is that the effective focal ratio will be much shorter and therefore go off band at the edges?

 

And for 1x use in front of the NV objective, I see people using 2" filters and TV offers a 2" adapter. But it seems to me that a 1.25" would work as well. The edges in a 2" filter will be off-band anyway, no?

 

And for prime focus, there a 1.25" filter seems obvious. But if you are using a reducer, then you might be better off putting the filter before the reducer, which means you might need a 2". After the reducer, you will have a shorter f-ratio and go off band on the edges, I think.

 

I ask because the difference in cost between a 1.25" and 2" is large. It seems that many people like a 5nm H-alpha filter, with some preferring 3.5nm and some preferring 6 or 7nm. And some also have a 12nm for special cases.

 

(Does the high transmission and sharp shoulders of a filter like an Astrodon--especially a big one--make a big enough difference to warrant a very costly purchase?)

 

George

 

If you're going to use a 2" eyepiece, you need 2" filter(s).

 

If you are afocal only, you can probably skip 1.25" filters. For 1x work, step-up adapter(s) get you from the ENVIS thread to M48 (2" astro filter).

 

If you do prime, you will want both formats. For example, 1.25" to use with a 1.25x barlow or 2.5x Powermate. Having the 2" ScopeStuff C-mount adapter gives you both filter threads, but I find the larger 2" format a little easier to handle in the dark.

 

Another use for 1.25" is large telephotos (135mm faster than f/2.8, and everything larger). In these cases, the aperture is too large for 2" filters, so the only option is to epoxy a 1.25" filter ring inside the camera lens adapter.

 

Unfortunately, trying to save money and use only 1.25" puts serious constraints on the operation.


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#4 Joko

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Posted 29 December 2020 - 02:37 AM

Based on my own experience, i started with 1.25" then i moved to 2" filters. 

1.25" are good for prime use but in afocal it will depends on the eyepieces you have. And long focal eyepieces have 2" thread.

Also they are always in my 2" filters wheel so it helps a lot to swap filters.

Of course 2" are more expensive than 1.25".


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#5 bobhen

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Posted 29 December 2020 - 07:08 AM

I only use Prim Focus and I only use 1.25” filters.

 

I use a 2” reducer and use the filter after the reducer on the intensifier with little or no noticeable issues other than those minor issues caused by the reducer. This works for all of my telescopes.

 

I also use a 50 mm repurposed guide scope for handheld, ultra-wide filed observing and use 1.25” filters for that purpose as well.

 

I have no trouble with the filters in the dark anymore than any other 1.25” filter or small eyepiece.

 

The only time I need to change a filter is when I change from observing nebula with the Ha filter to observing non-nebula objects using a Pass filter.

 

Hope this is of some help.

 

Bob


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#6 George9

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Posted 29 December 2020 - 09:55 AM

Thanks, all. That is indeed very helpful. I think I get it now.

 

George


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#7 GOLGO13

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Posted 29 December 2020 - 12:49 PM

I'm 1.25 inch only right now. I mostly use prime focus though. For a-focal I'd probably go 2 inch since the 55mm plossl with the new adapter is probably the best solution for getting good reduction. 

 

The benefit for me is the cost savings...but eventually I'll probably get the 55mm plossl and adapter. So far I personally prefer to use prime focus, but a-focal is fine also...and sometimes the only option for many folks.


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#8 Kathi

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Posted 29 December 2020 - 02:13 PM

Most of the time I use my NV with the 55mm > 67mm TeleVue Plössl. So I always use 2'' filters.

Kathi


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#9 hoof

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Posted 31 December 2020 - 07:21 PM

I use 1.25” filters for 1x viewing, 2” filters (usually the 7nm h-alpha) for use in my scopes, and the 1.25” 7nm for my 80mm f/3.75 finder (with my TV 40mm and 2” erect prism diagonal). Works well :)
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#10 GeezerGazer

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Posted 01 January 2021 - 02:39 PM

Afocal use is most easily and productively accommodated by using 2" filters, unless you are using 1.25" eyepieces or are limited to a 1.25" focuser.  But even 1.25" eyepiece can be used with 2" filters if you use a 2" to 1.25" adapter that has filter threads (care must be taken to prevent contact between the end of the eyepiece barrel and the filter).  

 

Most use 1.25" filters in prime focus, BUT,  there is always a way to use 2" filters.  

 

I use prime focus >95% of the time.  And I use camera lenses.  The only reason I previously maintained a set of 1.25" filters was for use behind my camera lenses.  So I figured out a way to use 2" filters with the camera lenses, and sold all 5 of my 1.25" filters.  

 

So how do I use 2" filters with NV? 

  1.  Attached to TV 55/67 or other 2" eyepieces in afocal.  I never use shorter FL eyepieces in afocal for higher magnification; most NVers do.

  2.  Attached to 2" focal reducers or Barlows when used in afocal or prime.  

  3.  Attached as a first surface filter to my 1x Envis lens, Computar 50mm, or Fujinon 75mm, in mono or bino modes

  4.  Attached within my Nikon camera lens adapter for prime use (between the prime lens and the NVD).  

 

1.25" filters are less expensive; 2" filters are more versatile.  Using 2" filters for everything required some adapters, but adapters are readily available from sources like Ebay, ScopeStuff, RAF Camera, or even many telescope retailers like Astronomics, Agena, OPT, and others.    In 2017, I wanted to save money by purchasing 1.25" filters for prime use.  In 2020, I was able to sell $1100 worth of 1.25" filters.  Currently, I use four filters, 12, 7 & 3.5nm H-a band pass and a 640nm long pass, Lumicon Night Sky.  I have owned and experimented with Astronomik, Baader, Optolong, Astrodon, Chroma and Antlia H-a filters and found all to be satisfactory with my NVDs and optical systems.  I have considered reducing my filters to just 3, by selling the 7 & 12 and purchasing an 8nm H-a.  Your optical systems are fast enough, especially using the TV 55/67, to accommodate the narrowest H-a filters.  

 

You will find that the selection of H-a filters is somewhat subjective, based on your personal preferences, your sky conditions and your optical systems.  But basically, the narrower the filter band pass, the more contrast can be seen between the H-a subject and the background sky... which is a reason the narrowest filters are helpful with very dim H-a subjects.  

 

Yes, the 2" filter will also be off band at the edges when attached to the Envis lens.  But it will also show minor vignetting, from the Envis to 2" adapter impinging on the Envis FoV.  The 1.25" filter with the Envis to 1.25" adapter will show minor to moderate vignetting, depending on the length of the 1.25" filter cell.  Minor vignetting is not noticeable when used visually.  

 

My recommendation is to not be too concerned about nuances and just jump in.  You will be able to use 1.25" filters in a lot of situations, but you can't go wrong if you start with 2" filters which can be used in more ways.  

 

Happy New Year!


Edited by GeezerGazer, 01 January 2021 - 03:11 PM.

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#11 GeezerGazer

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Posted 01 January 2021 - 03:03 PM

I forgot to mention.  There is a third size filter that is being used experimentally, so far with good results.  Chroma and Astronomik both will cut a custom filter size.  CNer, Longbond, had Astronomik cut a 6nm H-a filter 24mm in diameter.  Using a 1" O-ring inside his C-mount Mod 3, he places the filter against it, then adds two more O-rings on top of it.  The filter remains in place and is not impacted by the length of the Envis lens.  This may or may not be the optimal placement for an H-a pass band filter, but it provides filtering for ALL optical systems without needing adapters.  This may become another option, exploiting the versatility of C-mount devices.  


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#12 George9

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Posted 01 January 2021 - 04:21 PM

Awesome, thank you! Great community. I am just waiting for my NV. Should be coming fairly soon.

 

George



#13 Jeff Morgan

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Posted 02 January 2021 - 01:02 AM

I forgot to mention.  There is a third size filter that is being used experimentally, so far with good results.  Chroma and Astronomik both will cut a custom filter size.  CNer, Longbond, had Astronomik cut a 6nm H-a filter 24mm in diameter.  Using a 1" O-ring inside his C-mount Mod 3, he places the filter against it, then adds two more O-rings on top of it.  The filter remains in place and is not impacted by the length of the Envis lens.  This may or may not be the optimal placement for an H-a pass band filter, but it provides filtering for ALL optical systems without needing adapters.  This may become another option, exploiting the versatility of C-mount devices.  

 

How interesting.

 

How does he get the filter out for non-nebula targets?


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#14 GeezerGazer

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Posted 02 January 2021 - 02:10 AM

How interesting.

 

How does he get the filter out for non-nebula targets?

 

Hi Jeff,

It is Longbond's story to tell, but I found it to be yet another way in which C-mount NVDs allow different ways to expand options.  As you know Mr. Wiley Coyote, some of us just like to experiment!  bow.gif   I'd probably use a plastic pick with curved end to pluck out the O-rings.  But the system makes sense to me for prime use with camera and C-mount lenses or in telescopes... any system in which a first surface filter is impractical.  This would not work for a PVS-14; but in a Mod 3C or Micro, it might even be useable in afocal mode.  The experiment is in progress, so we'll have to wait for his report.  wink.gif


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#15 Mike Lockwood

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Posted 02 January 2021 - 10:35 AM

Just keep in mind, the best place for a narrowband filter is where the light is most parallel because this causes the least "off-band" issues.

 

So, even with my fastest telescope (f/2.55), using a 2" filter on an eyepiece is best for this.  There are other interesting places where a filter could be inserted if you think creatively, such as inside an eyepiece.

 

Unfortunately there isn't much we can do about filters in front of the 1x lens.


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#16 ButterFly

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Posted 02 January 2021 - 07:35 PM

Two inch filters are more versatile, unless a very nice refractor gets in the way.

 

Anytime there is more than one filter that is a good option, a slide is very helpful.  These are also things that can travel from scope to scope.  Higher focal length eyepieces, especially the 67PP or 41 Pan, leave lots of room in a refractor for a slide (or at least a drawer) as well.  A different mount on each scope and the slide can travel between them.

 

There is enough room on my ES80 with a diagonal to fit a slide holder for my astrocrumb slide.  That's something I plan on working out soon enough.  The hope is that I can also focus regular eyepieces as well.


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#17 longbond

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Posted 03 January 2021 - 08:44 AM

I forgot to mention.  There is a third size filter that is being used experimentally, so far with good results.  Chroma and Astronomik both will cut a custom filter size.  CNer, Longbond, had Astronomik cut a 6nm H-a filter 24mm in diameter.  Using a 1" O-ring inside his C-mount Mod 3, he places the filter against it, then adds two more O-rings on top of it.  The filter remains in place and is not impacted by the length of the Envis lens.  This may or may not be the optimal placement for an H-a pass band filter, but it provides filtering for ALL optical systems without needing adapters.  This may become another option, exploiting the versatility of C-mount devices.  

Looks like I've been outed by our resident Kremlinologist, Ray (GeezerGazer) smile.gif. Indeed, I've been working on unmounted 24mm diameter filters for NV devices. I don't want to veer too far off-topic, but this might be a good filter solution and a consideration for NV device choices. However, this requires a removable objective lens.

 

For some time, I've been using Nikon camera lenses with a Fotodiox C-mount adapter for my Mod-3. Lately, I've been working with C-mount machine vision lenses because of their fast f-ratios at longer focal lengths. Although they're usually more compact than their full-frame camera lens counterparts, some of my bigger C-Mount lenses use 82mm filters, which is way too big for front-mounted 2" filters. I briefly considered large #29 red filters, but I need H-Alpha, so I went internal.

 

Every NV device that accepts a screw-in C-Mount Envis lens has a deep well for the extension of the Envis' rear lens elements. Luckily, most C-Mount lenses don't extend beyond the approx. 5mm of the threads. This well is about 13.5mm deep, which opens-up a whole lotta space.

 

I contacted Astronomik to make me an unmounted 24mm dia. 6nm H-Alpha filter, 1mm thickness. No problem. I made sure that their diameter tolerances were tight enough so that it would fit in the 24.3mm minimum inside diameter of my Mod-3's threading.

 

You might think that having an unmounted filter is dicey, but it works very well. The Mod-3 window is about 19mm dia., so that leaves an unused 2.5mm-wide aperture rim on the filter for handling. As Ray mentioned, I use three 24mm o.d. O-rings to hold the filter in place. They don't cut into the light cone.

 

This appears to work universally except (ironically) with the Envis lens. Fortunately, it's easy to mount a 1.25" or 2" filter in front of an Envis. For afocal and prime focus with reducers, there may be more optimal location choices for narrowband filters but for C-Mount lenses, there aren't many choices.

 

About removal...it's easy. No tools or extra dexterity required. Just unscrew, tip it over and the O-rings and filter slide out. The O-rings help keep the filter perpendicular to the optical axis during removal.


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#18 jay.i

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Posted 04 January 2021 - 11:12 AM

I posted a similar question a few months ago, asking if I need to buy each filter twice, or which ones make sense to get in 1.25" and which make sense to get in 2".

 

The short answer is, just get 2". The price stings but it's the way to go.

 

I occasionally use my ENVIS at 1x but it's rare. The need for a 1.25" filter specifically for ENVIS just doesn't make sense (even though I bought 2 cheaper filters for it initially, 7nm Ha and 685nm long pass). If you were just going to get 1, I'd make it a 7nm Ha filter.

 

For everything else (50/1.3 Computar lens, afocal with refractor+TV67+reducer, and prime focus with refractor), I use 2" filters. The filters get threaded into my diagonal. I'd like a filter slide to go in between the diagonal and reducer (downstream) but I haven't wanted it badly enough to actually spend the cash and time on it.

 

BTW, if you buy a filter ring adapter for the ENVIS lens, make sure not to thread your filter too tightly into it. My Baader 7nm Ha seems like it might be permanently mated to the ENVIS adapter :)



#19 rexowner

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Posted 05 January 2021 - 03:37 AM

Just keep in mind, the best place for a narrowband filter is where the light is most parallel because this causes the least "off-band" issues.

 

So, even with my fastest telescope (f/2.55), using a 2" filter on an eyepiece is best for this.  There are other interesting places where a filter could be inserted if you think creatively, such as inside an eyepiece.

 

Unfortunately there isn't much we can do about filters in front of the 1x lens.

I might be mistaken, but not sure I’m following your argument.

 

It seems like anywhere between the last element of the telescope, and before the first element of the

eyepiece has an identical angle, before or after the focal point doesn’t matter wrt angle.

 

I could have this wrong, but it seems like size of the light cone is as big of an issue e.g. even a 36mm

filter could work in some  2” setups depending on where the focus is. I agree, it’s simpler to just use a

2” filter, which will handle the light cone b4 the eyepiece, so probably the way to go.  I’ve used smaller

filters with 2” Eyepieces, e.g. in a diagonal, but this is probably too much of a pain for the general case.

 

I agree within the eyepiece could make sense, but that would very much depend on the eyepiece.   I’m

guessing that within the eyepiece would be hard to generalize .

 

Having said that, you’re right. The bottom of the eyepiece will pretty much always be practical and work well. 


Edited by rexowner, 05 January 2021 - 03:39 AM.


#20 George9

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Posted 05 January 2021 - 07:44 AM

(I thought Mike was contrasting putting the filter between the objective and the eyepiece versus other places say in an afocal system, such as after the eyepiece or after the 1x lens. With the latter two allowing a smaller filter but being higher angled. George)



#21 rexowner

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Posted 05 January 2021 - 10:49 PM

It appears to me that it depends.

 

I completely agree that you won't get in trouble using 2" filters, and the best
place to put them is at the end of the eyepiece. 

 

However, you may not need to use 2" filters depending on your set-up.  Why would
anyone do this?  In my case, cost, but mostly availability.

 

I happened on a 36mm unmounted Baader IR-Pass, and if I did the math right, I can
use it down to 15X in my 85mm f/7, and 21X in my 127mm f/5.2 with no vignetting.

 

I am doing afocal (PVS-14 and Tele Vue adapters), and am using a
Baader UFC filter drawer with a T-2 diagonal for 1-1/4" EPs.  The drawer attaches
to the front of the diagonal.

 

Because the PVS-14 only has an AFOV of 40 degrees, should be 
able to use smaller filters if they are between the telescope and eyepiece,
where the angle of the light cone is straightforward to figure out if one
knows the size and location of the field stop (focus).

 

Assumptions
- Path length from field stop to barrel top is 81mm for 1-1/4" EPs with the
T-2 diagonal, and 114mm for 2" EPs, per Baader specs
- Field stop locations and sizes were per Tele Vue specs.
- I just did this for Tele Vue's EPs they recommend them for NV, and they
supply the data.  Not sure the data are available for others.  This is
one of the reasons I like Tele Vue -- they seem to respect the customer
enough to let them figure out what's going on.
- I did this pretty quickly today, so I could be off.

 

Attached are the results I got for my particular setup.  The size of the
light cone at the filter is the gray columns.  If it's smaller than 26mm
(1.25" filter glass size in green) can even use a 1-1/4" filter down to
33x.  The 36mm unmounted filter should work down to 15x/21x with no
vignetting.  Above that, a larger filter becomes necessary, but until
I can scare up a 2"/M48, the filter drawer allows for 50mm square unmounted
filters which is at least an option.

 

YMMV.

Attached Files


Edited by rexowner, 06 January 2021 - 02:01 AM.


#22 Mike Lockwood

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Posted 06 January 2021 - 09:41 AM

(I thought Mike was contrasting putting the filter between the objective and the eyepiece versus other places say in an afocal system, such as after the eyepiece or after the 1x lens. With the latter two allowing a smaller filter but being higher angled. George)

Yes.  I didn't need to say 2", but I did because that's what I use with my mostly wide-field NV viewing with 2" eyepieces or filter slides.  1.25" works on a 1.25" eyepiece, of course.
 

For the least shifting of the passband of a narrowband filter, you want the filter in a convenient place in the optical path where the light is closest to parallel (slowest/least angled).  Generally the bottom of the eyepiece is the simplest place to put it, but there are other possibilities.  Putting it in front of the 1X objective is generally least desirable due to the wide field of view that it has, thus the widest angular spread of light coming in, and thus the most shifting of the passband.



#23 a__l

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Posted 06 January 2021 - 07:36 PM

Yes.  I didn't need to say 2", but I did because that's what I use with my mostly wide-field NV viewing with 2" eyepieces or filter slides.  1.25" works on a 1.25" eyepiece, of course.
 

For the least shifting of the passband of a narrowband filter, you want the filter in a convenient place in the optical path where the light is closest to parallel (slowest/least angled).  Generally the bottom of the eyepiece is the simplest place to put it, but there are other possibilities.  Putting it in front of the 1X objective is generally least desirable due to the wide field of view that it has, thus the widest angular spread of light coming in, and thus the most shifting of the passband.

It makes sense to use H-alfa starting from 3x. Use a good lens that matches the 2" filter (by diameter or filter size).

 

For 1x, objects are small. They are always in the center. Therefore, you can safely use such a filter in front of the lens.


Edited by a__l, 06 January 2021 - 07:38 PM.


#24 ButterFly

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Posted 08 January 2021 - 03:50 AM

As one tilts an interference filter with respect to the wavefront, the shape of the bandpass, as well the location of its peak, change.  Over the past year, a few members have been sharing their band shift data.  The data make clear that one wants a uniform parallel wavefront to hit the filter squarely in order for the filter to perform as well as intended.

 

Televue's diagram shows the issues fairly well:

 

TelescopeEPRayTrace_plain_NightVision3.g

 

There are three places where the blue lines are parallel (collimated light from a given field angle):  all the way to the left, all the way to the right, and in the middle between the eyepiece and the NV objective.  The difference between these three areas of parallel light is the angle presented to the filter for a given field angle.  All the way to the left, it is tiny.  At the right and in the middle, it is magnified.  Placing an interference filter all the way to the right does nothing, so that's the last of that.  If your apparent field of 40 degrees covers two degrees of true field, consider what happens.  An object that appears at a field angle of ten degrees is actually half a degree away in true field.  The angle presented to the filter is half a degree at the left side but ten degrees in the middle.  That's much more off-banding placing the filter in the midddle where the light is parallel as opposed to at the left where the light is parallel.  The angles magnify.

 

Placing an interference filter in front of an objective is crazy expensive.  Imagine the cost of a filter for a 20" aperture when a 2" filter is already hundreds.  It's also tied to a particular aperture.  Going from one scope to another is cumbersome, if at all possible.

 

As a compromise betweeen cost, effectiveness, and interoperability, one usually places the filter in the focal plane of the scope.  Any scope.  The filter thus easily goes from one scope to another.  The downside is that the once parallel light from a particular field angle now presents to the filter as a cone with a range of angles.  For a scope with an f-ratio f, the tangent of the cone angle is 1/2f before the focal plane.  On axis-light crosses the filter by as much as +/- an angle with a tangent of 1/2f.  Our on-axis light thus offbands slilghtly.  That's worth the trade-off for the convenience.  As a rough estimate, the maximum crossing angle is the maximum angle for the on axis light cone, plus the field angle of the field of view.  Here are a few cone angles for given f/ratios (the field angle is twice the cone angle):

 

f/ratio  Cone angle

6         4.76

5         5.7

4         7.1

3         9.5

2         14

 

So an f/2 scope with a true field of ten degrees would have light cross a filter a the focal plane by at most around (10/2 + 14), or 19 degrees.  For my f/4.78 dob with a max field of around 1.5 degrees, it's at most 6.7 degrees, and for my f/6 with a max field of around six degrees, it's about 7.8 degrees.  That offbanding is worth it to have the same filter work in both scopes, not have to pay thousands for a single filter, and still have it work quite well.

 

Having a filter that is physically larger in no way impacts that off banding inherent to the material the filter is made of.  A two inch filter merely allows one to have light from a given field angle cross the filter without being clipped at the edges where there would no be longer any filter.  You can imagine how rapidly even a two inch filter can clip a fast light cone when it gets closer to the objective.


  • rexowner and Jsquared like this

#25 Joko

Joko

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Posted 08 January 2021 - 04:29 AM

To resume, there are only 2 ways to use a filter in afocal or handheld (in prime, just screw the filter at the bottom of the Night Vision Device)

1/ In front of the objectifve (or the eyepiece).

2/ Between the objectifve (or the eyepiece) and the NVD

 

Filters can be expensive and i doubt anyone here bought filters larger than 2".

So it's easy to thread at the bottom of the eyepiece.

It can be added in front of the objective if not larger than 2".

 

If none of the previous solutions are possible, just thread the filter at the bottom of the NVD.

 

An other question might be, where to thread the filter when the user also add a focal reducer or a coma corrector.

My best way to answer is...experimentation. It may depends on the instruments, so always try and find what gives the best results.




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