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Filter size for night vision

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#26 rexowner

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Posted 08 January 2021 - 12:04 PM

 

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

 

For refractors or Catadioptrics:

- In the case of a focal reducer, a filter drawer like the modular Baader UFC has

   multiple threads available, and adds less than an inch in light path might be the

   best way.  11 different adapters on the telescope side, and 9 different ones on the

   camera side.  As close to the focus as other equipment allows.

- In the case of the Newtonian with the coma corrector, probably on the bottom of

  the eyepiece for visual if a filter slide can't be added outside the tube.  Otherwise probably

  a filter slide inside the tube if absolutely necessary.


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#27 George9

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Posted 08 January 2021 - 03:53 PM

Mounting aside, I would think by the angles that:

 

Focal reducer: Before the focal reducer (between objective and focal reducer) because the reducer increases the angles (makes it faster).

 

Coma corrector: Not much difference either way. The Paracorr is a mild amplifier but no big deal.

 

Amplifier: I am guessing for a Powermate or telecentric, after the amplifier would be best (i.e., between the amplifier and the eyepiece or NVD). For a simple Barlow, I am not so sure because even though the Barlow lengthens the effective focal ratio, it magnifies the angles. Have to draw it out. Certainly for solar astronomy in H-alpha, I would never put a 0.3A filter after a Barlow.

 

Of course if you have a 2" filter and it is working well on that scope, then I guess the answer can be to always put it as the first element after the objective.

 

George


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#28 a__l

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Posted 12 January 2021 - 02:37 PM

http://karmalimbo.co...tio_Oct2020.pdf

Interesting information about the use of narrow-band filters. These are not quite discussed h-alfa filters, but the trend from telescope "fast" is understandable. At f/2 there might be nothing left ...



#29 rexowner

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Posted 12 January 2021 - 05:17 PM

http://karmalimbo.co...tio_Oct2020.pdf

Interesting information about the use of narrow-band filters. These are not quite discussed h-alfa filters, but the trend from telescope "fast" is understandable. At f/2 there might be nothing left ...

Interesting article.  I could have this conceptualized wrong, but I thought about why some filters were

characterized as e.g. "f/2", i.e. they would work above that focal ratio, but not faster.

 

Why aren't NV filters characterized this way?  I figured since they work at 1x, i.e. 40 degrees, it's probably

a non-issue.  Not sure there's ever a steeper light cone that at final magnification anywhere a filter could

go.

 

Do I have this right?



#30 a__l

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Posted 12 January 2021 - 07:46 PM

There will be no problem for 1x. Most objects are small and can be viewed in the center.
Photo lenses with an input diameter greater than 2" filter and high speed. The outgoing beam can be a problem.



#31 Highburymark

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Posted 14 January 2021 - 10:00 AM

I deliberated over buying a 2” Chroma 3nm filter for ages. At £700 (U.K.) it was a substantial investment. I finally took the plunge a couple of months ago. Then something very unusual happened. I got lucky. A week after receiving the new filter, Chroma virtually doubled the price. So am wondering if Astrodon prices are about to rise too? If so, might be prudent for the OP to buy sooner rather than later.
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#32 George9

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Posted 24 January 2021 - 04:40 PM

Thanks for all the help.

 

Wow! Through Televue, I got the TNV-14, using it with a 67mm converted TV Plossl, 32mm TV Plossl, and 11mm Delite. I picked up a used 7nm 2" H-alpha filter online. I was viewing in 2nd to 3rd magnitude skies, out an open window. M42 was well placed. At 1x, the filter showed a little more than the unfiltered view, but no big deal. But then in a 92mm refractor, unfiltered was fine but filtered it was just amazing. I have seen M42 many times in 8 and 10" scopes in dark skies and sure, it is great, and certainly more detailed than looking out a window in the winter in a 92mm. But the visual IMPACT of such a bright image rivaled M42 in an 18" in very dark skies. All the faint outer wisps right there, and the core densely bright.

 

Don't get me wrong, M42 in an 18" in a dark sky has detail and even color that are out of this world. But that NV image was just so surprising. My wife, who is a good sport about astronomy, was just amazed.

 

I had the gain much of the way up with the filter. Some scintillation, but the brightness just looked so good.

 

George


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#33 Jeff Morgan

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Posted 24 January 2021 - 06:44 PM

Congrats!

 

I suppose it's human nature and we all do it ... turn our intensifiers towards nice bright targets we are already familiar with - like M42. And that's ok, and it looks great. But it is hardly a test of what the eyepiece can do - M42 is already bright.

 

When you start pointing it at the fainter stuff ... it's really going to blow your socks off.


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#34 George9

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Posted 28 January 2021 - 11:28 PM

Well, I'm gaining experience. Again out my window in mag 3 skies, the Flame Nebula is bright. IC 434 was right there but dim. The Horsehead was hard in the 92mm, but when I moved it to a 10" Newtonian, the Horsehead was easy.

 

I couldn't get Barnard's Loop. I am getting a very high transmission, very narrow H-alpha filter for solar that may be interesting on the TNV, although it is a little small.

 

George



#35 George9

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Posted 05 February 2021 - 10:19 PM

Just to follow up. I did try my solar filter. It's a prototype 10A triple-cavity 90%-transmission H-alpha filter from Solar Spectrum and 1" in diameter. The best was at 1x with just the TNV objective. Looking out my window in skies that only show seven stars in Orion plus a vague blob near M42. More or less full-Moon skies. Plus hazy out.

 

Barnard's Loop was right there. I thought it was a cloud until I realized where I was looking. Flame and IC434 show up at 1x. M42 looks bloated. Because of the tilt, I cannot see all of Barnard's Loop at once. About half at a time. While the filter is very narrow, I am told that being triple cavity makes it a little less sensitive to tilt. The end effect is that there is a tilt effect but not as extreme as you might think.

 

I cannot believe I can see Barnard's Loop out my window in these skies. I waited too long to get the TNV.

 

I could not get Barnard's Loop at all in the 7nm Baader H-alpha. Not much of the Flame or IC434 at 1x either.

 

In the 92mm f/7 afocal with a 32mm Plossl, I can see the Flame and IC434. The Horsehead was indistinct with the haze. If I hadn't seen it on a clear night, I would not have noticed it. The 10A was only a little better than the 7nm Baader H-alpha in the scope (maybe because the 10A vignettes the 28mm field stop of the Plossl to 24mm). On M42, I actually liked the 7nm Baader better. I am guessing for afocal telescope viewing, a 2" 3nm filter is likely better. But for the 1x viewing, wow.

 

I noticed that my 8nm OIII and 13nm H-beta barely benefit from the TNV. Yes somewhat better, but not worth the hassle. The DPM NPB filter, with its H-alpha band pass, is a little better. So guess it is really about H-alpha and I guess IR, which I have not tried yet.

 

I expected full gain on the unit to be brighter even with almost no light coming in, but I guess it is good news that it doesn't flare out when there is no light coming in. It scintillates as I expected at full gain. And at the lowest gain, I would say it is about the same brightness as naked eye. In a really dark location (nighttime -> dark room -> dark closet with door closed), I find after accommodating the darkness, I can detect light naked eye almost as well as the unit at full gain, but the big difference is that in the unit I can make out shapes and naked eye it is just a sense of light.

 

George


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#36 faackanders2

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Posted 09 February 2021 - 09:26 PM

I have a 2" astrocrumb filter slide (w/ denk adapter) for my dob with spider not directly above focuser.  I am able to use 2" filters with both 1.25" and 2" eyepieces as well as my Denk binoviewer.  I have 3 filters plus empty per slide.


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#37 George9

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Posted 19 June 2021 - 08:08 PM

Another follow up even though I know it is old news here. I just got a 2" Antlia 3nm H-alpha used. Wow. That is impressive in my super-light-polluted skies. 

 

2" 7nm Baader at 1x in the Televue-supplied TNV-14. North American Nebulae found but not that distended. Gamma Cygni Nebula detectable. No Veil.

 

2" 3nm Antlia at 1x. North American Nebula big and contrasty. Gamma Cygni Neg extensive. Veil visible. Same field of view as the 7nm (in terms of effect of tilt off axis).

 

1" 1nm Solar Spectrum at 1x. Even more detail and extension in each. But a notably narrower field of view.

 

I will try them all under dark skies, but the 3nm Antlia covers it all in light polluted skies. I am sure an Astrodon or Chroma is a little better, but you really get your money's worth in the $400 Antlia 3nm vs the $300 Baader and presumably the $1300 2" Chroma.

 

Night vision with the Antlia 3nm H-alpha plus the Baader 685nm IR-pass recommended here revolutionize light-polluted astronomy. 

 

George


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#38 joelin

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Posted 21 June 2021 - 01:52 PM

interesting to hear about the 1nm Ha filter... why hasn't anyone made them easily accessible?? it seems 3nm is the lowest 



#39 George9

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Posted 21 June 2021 - 04:15 PM

That's a prototype solar filter I got from Solar Spectrum really for solar H-alpha work. But since it is a triple-cavity, very-high-transmission filter (similar in profile to the Alluxa filters), it works here, too. I don't know what it would cost retail, but presumably more than the 3nm Alluxa and Chroma filters.

 

George




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