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Old 28-11-2016, 10:52 AM
glend (Glen)
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Originally Posted by Satchmo View Post
It needs to be pointed out that it depends what you are trying to image . If you are doing planetary imaging then there is benefit from a greater degree of oversampling or density of pixels as you are going to pick up better random moments of excellent seeing in very short exposures and weed them out for stacking. You would have far less chance of capturing the finest moments when you are doing 5 min plus subs.
Yes indeed, the so called "Lucky Imaging" approach gives you a set of high speed video type frames where optimal seeing ones can be stacked and bad ones discarded. Ideal for planetary imaging. Traditional long subs effectively 'dumb down' incorporating all the bad seeing periods as well. A problem with 'Lucky Imaging' is the massive processing and storage load it can produce. Adaptive Optics may help in long sub capture but it does not seem to get much market penetration, perhaps due to complexities and cost.
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  #22  
Old 28-11-2016, 01:00 PM
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Camelopardalis (Dunk)
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Originally Posted by Slawomir View Post
Great result Dunk Recently I was getting around 1.7 Half Flux Radius for the NGC1763 in Ha which I think is not too bad neither.
Thanks Suavi! I didn't really know what to expect, so it's good to have a frame of reference. From my DSLR it's typically a bit higher ~2.5 so I figured I was just lucky with the seeing, but no doubt the mono sensor plays into it too.
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  #23  
Old 28-11-2016, 04:54 PM
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Slawomir (Suavi)
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Originally Posted by Satchmo View Post
It needs to be pointed out that it depends what you are trying to image . If you are doing planetary imaging then there is benefit from a greater degree of oversampling or density of pixels as you are going to pick up better random moments of excellent seeing in very short exposures and weed them out for stacking. You would have far less chance of capturing the finest moments when you are doing 5 min plus subs.
A good point Mark. I am solely interested in DSOs

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Originally Posted by RickS View Post
Hi Suavi,

I have a copy of Telescopes, Eyepieces and Astrographs: Design, Analysis and Performance of Modern Astronomical Optics if you'd like to borrow it. It's a very interesting read. They are big on ray fan plots as well as spot diagrams.

http://www.willbell.com/TM/Telescope...trographs.html

Cheers,
Rick.
Sounds great, thank you Rick. Would it be okay to borrow your book for a few weeks after the NYE?

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Originally Posted by Camelopardalis View Post
Thanks Suavi! I didn't really know what to expect, so it's good to have a frame of reference. From my DSLR it's typically a bit higher ~2.5 so I figured I was just lucky with the seeing, but no doubt the mono sensor plays into it too.
I think 3nm filters make a significant difference in terms of star sizes, in particular when used with my doublet.
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  #24  
Old 28-11-2016, 04:58 PM
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Originally Posted by Slawomir View Post
Sounds great, thank you Rick. Would it be okay to borrow your book for a few weeks after the NYE?
Sure, Suavi. Just drop me an email and we can meet up somewhere. Are you likely to be at the next AAQ meeting?

Cheers,
Rick.
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  #25  
Old 28-11-2016, 06:13 PM
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Shiraz (Ray)
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Originally Posted by codemonkey View Post
There's some argument for sampling at 3x the possible resolution of the system if you don't want to leave any detail on the table, but like everything it's a trade-off.

Additionally, the diffraction limit isn't really a hard limit and changes depending on which formula you use. Dawes for instance gives you 1.14" for a 4" scope.

Jon Rista (posts on CloudyNights) uses the following formula to estimate FWHM:

FWHM = SQRT(Seeing^2 + Dawes^2 + ImageScale^2 + GuideRMS^2)

I'm going to make some (reasonable) assumptions and say we have typical seeing of 2", and a guiding RMS of 0.4", which means for your current (native FL) system, we get:

FWHM = SQRT(2^2 + 1.14^2 + 1.07^2 + 0.4^2)
FWHM = 2.56992218"

Keeping everything the same except switching to an (unobstructed) 6" aperture you would improve your resolution as follows:

FWHM = SQRT(2^2 + 0.76^2 + 1.07^2 + 0.4^2)
FWHM = 2.42538657"

And for the sake of illustration, were you to continue to use your 4" scope but go for pixels half the current size, you'd be looking at:

FWHM = SQRT(2^2 + 1.14^2 + 0.535^2 + 0.4^2)
FWHM = 2.39704506"

As you can see, the diffraction limit is not a hard wall, and you actually get (admittedly marginal) increase in resolution by going for even smaller pixels.

My 2c is that a 4" is fine for you, especially given that you primarily shoot nebulae.
maybe be a little wary of that formula Lee. It is a kludge that is good for getting an understanding of the underlying concepts, but the incorporation of a non-continuous sampling function (image scale), for which "FWHM" is almost meaningless and for which the addition of variances is not appropriate, means that it fails when you push too hard.

Last edited by Shiraz; 28-11-2016 at 09:58 PM.
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  #26  
Old 28-11-2016, 06:45 PM
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Slawomir (Suavi)
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I fixed the formula

FWHM = SQRT(Seeing^2 + Dawes^2 + ImageScale^2 + GuideRMS^2 + ( SubLength^2 / pixel well depth ))

Last edited by Slawomir; 28-11-2016 at 07:13 PM.
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