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  #1  
Old 04-02-2008, 12:30 PM
tims
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Very fast or short f/ratio mirrors - better for astro-photography?

I asked this question on another forum but didn't get many replies and still am not sure what, if any, are benefits of using large fast mirrors. So...

I'm interested in astro-photography and the benefits (if any) of using large Dobsonian telescopes with fast mirrors.
In conventional photography, if I understand correctly a very fast or short f/ratio camera lens will enable shorter exposure times so does the same apply to very "fast" or short f/ratio mirrors?
Will a large fast mirror (say > 25 inch/ F4) in a Dobsonian allow shorter exposure times as opposed to a smaller mirror with a longer f/ratio?

cheers


Tim
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  #2  
Old 04-02-2008, 12:55 PM
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Yep
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Old 04-02-2008, 02:13 PM
tornado33
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If you halve the F number you can use 1/4th the exposure time
eg an F10 tha reaches the sky background limit in 1 hour, an F5 will do so in 15 minutes.

However, go faster then about F6 and coma starts to appear as elongated stars at the edges. faster f ratio = more coma. Thankfully Baader Planatarium boght out an excellent coma corrector that unlike the even more expensive televue parracoor, virtually eliminates all coma in systems down to F4
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Old 04-02-2008, 02:48 PM
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Don't forget that the critical focus zone (for imaging) gets much, much smaller, so it can be a bit of a bugger to get perfect focus on faster systems.

If you get the CCDCalc program, you can see what the critical focus zone is for a particular scope combo will come out like.

http://www.newastro.com/newastro/boo...camera_app.asp

Using a dob for long exposure photography can be problematic, especially truss tube designs as they are not as rigid. Not only that, you've got to get them tracking, which on the massive aperture you're talking about, means some pretty tricky gear.

Turbo
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Old 04-02-2008, 08:53 PM
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Peter Ward
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This is part myth. Aperture alone defines what stellar limit you will reach.

For extended objets, eg Nebulae the F-ratio does matter.

I'd personally be looking at the mount first. Even the Hubble would produce crappy images if it wobbled all over the sky
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Old 05-02-2008, 12:34 AM
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Hi All
IMHO the obsession with lower exposure times is to settle for less than decent images.
The more photons you collect the better your signal to noise ratio and the better your image quality.
If you dont have the expensive and quality mount with guiding then track and stack. The photons dont care if they are collected in one exposure or one thousand exposures. The stamina of the observer will determine the length of the total exposure.
Some astrophotographers go for exposures in excess of 12 hours. It just depends on your expectation.
It is also my understanding that larger focal ratios mean smaller FOV's and smaller focal ratios mean larger FOV's. Large aperture allows for fainter objects to be seen and that would be the advantage of the larger aperture.
Kind regards
Steve
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Old 05-02-2008, 07:28 PM
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Quote:
Originally Posted by turbo_pascale View Post

....

If you get the CCDCalc program, you can see what the critical focus zone is for a particular scope combo will come out like.

http://www.newastro.com/newastro/boo...camera_app.asp

....

Turbo
A HUGE thankyou!
I used a simple program like this years ago, then had a harddisk die on me but couldn't remember the programs name or where I got it.
Maybe this is it and it's gone through a few() changes since I last had it?!
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  #8  
Old 06-02-2008, 06:29 AM
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Quote:
Originally Posted by skwinty View Post
Hi All
If you dont have the expensive and quality mount with guiding then track and stack. The photons dont care if they are collected in one exposure or one thousand exposures. The stamina of the observer will determine the length of the total exposure.
Steve
The photons don't care, but the camera does. You get read-out noise for every sub, so with 1000 tracked and stacked you get about 32 [sqrt(1000)] times as much read-out noise as with a single exposure. Also, if the initial exposure is too short to trigger a signal in the camera, then adding a thousand zeros still gives you zero.
Geoff
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Old 06-02-2008, 01:02 PM
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Had a better play around with CCDCalc, very similar to the one I used to use. The old one also had film and afocal tho.
Still, will come in very handy, thanks again.
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Old 06-02-2008, 04:18 PM
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Quote:
Originally Posted by ghsmith45 View Post
The photons don't care, but the camera does. You get read-out noise for every sub, so with 1000 tracked and stacked you get about 32 [sqrt(1000)] times as much read-out noise as with a single exposure. Also, if the initial exposure is too short to trigger a signal in the camera, then adding a thousand zeros still gives you zero.
Geoff
Hi Geoff
Yes,point taken but remember that the Poisson noise will become the dominant noise source and will exceed the track and stack read out noise.
Regards
Steve
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  #11  
Old 07-02-2008, 04:31 PM
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Hi Geoff
Attached is a synopsis of signal to noise ratio and the impact of read out noise when tracking and stacking.
Regards
Steve
Attached Files
File Type: pdf Signals and noise.pdf (105.0 KB, 16 views)
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  #12  
Old 08-02-2008, 09:42 PM
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g__day (Matthew)
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So help me out here guys - place a parabolic dish - say a metre in diameter down a 40 metre tube, cover said dish in a CCD array, cool it to - 75 and do you have a quality 1 metre diameter telescope with beautiful readout (low Q/E) signature with an F ratio of effectively 0?

I can see how longer shots improve your signal, well up to the point your either saturate your wells in each pixel, and I can see how there is diminishing signal gain with time and I get you have readout noise and residual sky glow noise to contend with - and that this is why a series of shorter stacked shots - (so long as they can get sufficient signal) may be better in particular circumstances then longer duration single shots. But in essence is lower F ratio just better signal to noise due to less magnification?

How or rather why does lowering F-ratio work from a lay person's perspective please?
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  #13  
Old 08-02-2008, 10:00 PM
Zuts
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Hi,

This is probably totally wrong but....

For a given aperture a shorter F ratio means that you have a wider field of view. This means that assuming the CCD chip is getting completely covered you would be getting more light onto the same area because bigger FOV.

Paul
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  #14  
Old 09-02-2008, 12:03 PM
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Correct me if I'm wrong....
For a given aperture, lets say a perfectly square subject on an f4 scope covered a 5x5 grid of 25 pixels.

At f8 the same object would be twice the size and have 4 times the area, covering 100 pixels, with the same amount of photons coming in(due to same aperture), four-fold increase in exposure length... but 4 times the detail(pixels).
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  #15  
Old 09-02-2008, 01:54 PM
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That is how I figure it as well MrB - for extended objects like nebula you increase S/N ratio (or decrease exposure times) with shorter focal ratio as the same amount of photons are collected onto fewer CCD detectors.

For objects like stars which are pinpoint, focal ratio is completely irrelivent because it doesn't matter if you are shooting at F2 or F10 the stars are still a pinpoint of light, so aperture is the only determination with regards to how deep you can image.
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Old 09-02-2008, 04:50 PM
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Quote:
Originally Posted by Kal View Post
T
For objects like stars which are pinpoint, focal ratio is completely irrelivent because it doesn't matter if you are shooting at F2 or F10 the stars are still a pinpoint of light, so aperture is the only determination with regards to how deep you can image.
In a sense....
Limiting stellar magnitude is a function of sky fog limit. The longer the f ratio the longer you can expose and the fainter your limiting magnitude ( kind of the opposite of imaging extended objects).
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  #17  
Old 09-02-2008, 10:48 PM
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Hi all
herewith a synopsis of focal ratio, focal length and aperture
Regards
steve
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File Type: pdf Focal Length.pdf (27.8 KB, 25 views)
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