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Old 29-06-2015, 03:52 PM
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Atmos (Colin)
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Join Date: Aug 2011
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Okay, few things to cover now that the daily grind is out of the way!

Quote:
Originally Posted by rmuhlack
What I am trying to get at is what is the effect (with respect to both Ray's original formula and to the resulting resolution of the final image) of performing:
1) a 2x drizzle integration on dithered sub-frames that were captured with 2x2 binning, compared with
2) integration (without drizzle) of dithered sub-frames that were captured with 1x1 binning
The reason I said that you cannot have your cake and eat it too comes down to physics. Going back to "Astronomy 101", your aperture is what determines the number of photons entering the optical system. Let's say for instance that you have a FLI 16803 (because it is sheer awesome) and a telescope that allows you to image the entire M42 complex in one shot.

Let's say that when imaging M42 there are 100 million photons/second entering the telescope. This means that on average we are getting 6 photons/second/pixel. If we do a 2x2 bin this increases to about 24 photons/second/pixel, this is why imaging times can be shorter. What you do need to remember though is that in both images, the same amount of photons were present (if both images have the same overall imaging time).

The next step is if we have a 2x2 bin and that is drizzled. Without going into too much detail, drizzling works by basically creating a pseudo unbinning (just changing the image scale) and then when combining the images runs a mathematical calculation on a pixel to pixel basis of the statistical weighted average of what each pixels value should be. Sorry if that sounds like a messy explanation!
The point though is that there ends up potentially being a considerable amount of lost data through this process. Because every image has a lot of information that isn't of use due to being over sampled, it is just lost forever.

Of course, without doing some testing when I get the time and weather permits, I would say that imaging at 2x2 bin and then drizzling would be a waste of time. I doubt you would get fully back to unbinned resolution and from what I can tell, you would lose SNR on the inbinned images anyway.

Quote:
Originally Posted by rally
Well I think I get it !

So a larger Image circle is simply the scopes greater ability to see more of the sky at the same focal length ?

The image scale will of course remain the same !

So a given pixel receives much the same light as another one on a different system (with a different image circle), just that it can illuminate a larger imaging chip/plate/film.
So its capturing more light overall and making it useable, but not at the expense of a given pixel
So one scope must still be more efficient in terms of light gathering power even though

So it begs the question (at least for me) - Is F Ratio actually the FL divided by scope Aperture ? - Or is it focal length divided by some sort of "Effective" scope Aperture"
Or have I confused your F No. with F ratio ? - Maybe thats what I have done !
I understand where you are coming from with that but luckily telescopes don't work that way. What a telescope does is collect light from a large portion of sky and then focus it towards a point. As was previously mentioned, the "better" telescopes have a larger corrected field BUT there isn't a finite quality of light that is able to enter the optical train.

To give an example. Let's say you put a KAF-8300 on two 8" F/10 telescopes, one with a corrected field of 40mm and one with 100mm. Neither will suffer too much of vignetting although realistically the 100mm one should fair better simply because it has a larger corrected field. As both telescopes have the same focal length, image scale is exactly the same but with an ironic twist, the 40mm corrected telescope should be ever so slightly faster

The reason for this is that to have larger corrected field you must by definition have a larger secondary mirror. Granted, it won't make much of a difference. Now to get back to what you asked, the reason it doesn't make a big difference between a 40mm & 100mm corrected field is that the photons aren't being scattered across the entire corrected field. The 100mm one can have a larger chip and get a larger FOV but it doesn't have any effect on what is in the centre of view, because, by definition, all of the photons are concentrated into the centre of view

Hope that helps.
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