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Different size stars

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#1 hungerford


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Posted 05 March 2013 - 02:09 AM

I am having a problem with the size of star when imaging in LRGB and when I stack all the frames to produce the colour image the stars looks wrong.
I refocus the scope after changing filter and I am satisfied with the focus but the star size is different still.
What is creating the different star size between filter.
Thank you for your help.

#2 blueman


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Posted 05 March 2013 - 02:38 AM

Well, sometimes it is atmospheric conditions that cause the colors to bend differently. Such as moiture and dust or just the amount of atmosphere itself.
But also the area that you shoot may have interstellar gas and dust that can change the way the light appears in the wavelengths.
However it is a pretty common thing, usually red is the one that is bigger and that is why you see so many red raccoon eyes on stars in images.
Sometimes I think that focusing is just tighter in one color than another on a night due to conditions.
You can shrink the offending stars before they are combined with the other colors. This will make them all the same size more or less and eliminate the raccoon eyes. Ths can be done with deconvolution or using a star bloat removal action in Photoshop.

#3 Hilmi


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Posted 05 March 2013 - 02:43 AM

Are you using a low end refractor? That could result in not being able to full focus certain colors. I'm not an expert since I never faced this issue myself as I have a reflector, but I think you might get slightly better results with an IR/UV blocking filter. I figure a barlow would also help by slowing down the scope and giving a larger critical focus area.

DISCLAIMER: I dont have much confidence in my interpretation of the situation, but it should be easy to test my hypothesis on your equipment

#4 Konihlav



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Posted 05 March 2013 - 03:35 AM


- if you are using an achromatic refractor (or anything other, just very low-end type (cheap) telescope) than that's pretty normal

- even an ED refractor is not perfect in UV, i.e. namely in Blue channel the UV leaks in and makes really bigger B-channel stars then it should (on one side, the B-stars are normal to be bigger as stars are either yellowish or bluish and that color must be shown somehow, on the other side, using e.g. IDAS LPS-P2 that cuts out UV or Baader UV/IR Block that blocks from 420nm to 6xx something really helps! - I am using it with my ED).

- CCD filters - are you using dedicated LRGB CCD filters for CCD imaging? they are supposed to cut UV/IR. But that's not completely true. The Astronomik or Baader or OrionUSA filters are far from perfect, they all leak. On the other side Astrodons are "perfect" but have other problem - in the Blue color they pass 100% (have 100% transmittance) already before 400nm wavelength (like from 380nm up) and this is problem for all refracting optics (lenses...) while this is perfect for mirror systems (reflecting optics). I am using Astrodons with IDAS LPS-P2 or with Baader UV/IR when I do LRGB imaging with my only ED-like telescope. For Newtonian I do not need to.

what I describe are common issues that took me 2-3 years to understand and discover and resolve :)

good luck!

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