Re: [APML] Southern Orion reprocessed

["Carlos Milovic F." <cmilovic@yahoo.es>]

Hi Stuart

 

>Carlos, I think you are right, that it is the interpolation that makes the >smaller pic look noisy. I didn't mean to imply the actual image was >noisy.

 

Also there are a lot of structures that were enchanced in that scale, so maybe it is just a problem of resolution, and not an artifact of the interpolation.

 

>As for the flat fielding technique, your image is seems very flat so I >would vote it works well. I would expect subtracting (using masks) risks >removing real signal whereas flat fielding only adjusts for vignetting or >light fall off. Is that the case?

 

When you substract what you are doing is to "remove" the sky from the image. If you build the mask in a proper way, you are not going to loose any information (also you need to prevent negative values, ant thus not having a "zero" background).

With substraction you are not correcting the information that lies in the image. You just setted the background point to be the same. With division you are rescaling the all information, including the sky, stars, nebulae, etc. Just think on a normalized flat (i.e. his median is 1). In the center, where lies the values above 1, the flat makes things to look fainter than in the corners, where you amplify the signal.

The division works so well in digital becouse the amount of light that is recorded is proportional to the exposure time and the position where it hits. For example, if in one second you record 2 photons in the center versus 1 in the corners, in ten seconds there will be 20 in the center and 10 in the corners. The flat will show the same proportion, so when you divide, all is rescaled in the proper way. What happens with film? We have a lot of problems, specially the non-lineality, but using some procedures to "prevent" that, you can reach much better results than with substraction.

Of course it is not a exact method (using film), and the results are not photometrically correct. We need to make a much better characterization of the film response, but anyway, becouse we are doing just aesthetically pleasant images, it is good enought.

 

Regards,

Carlos Milovic F.
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Astro & Photo - CMF
http://www.astrophoto.vze.com
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Visita "AstroFoto", el foro de astrofotografía en español
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