Re: [APML] SFL and Flats

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

>On the other
>hand, the slide still retains full information about faint objects
>until Zone VI. This
>information can be pulled out by good processing.

Just to make a comparison shot... Let's see if I understood which is the Zone IV: I have a neutral gray card, and I set the diafragm to the "correct" exposure. Now, I have to close it to underexpose 1EV, right? I.E. Is it a gray a bit darker than the neutral one?

>> Is the information recorded there in the linear zone
>> of film?
>Yes. For slides, the "straight line" portion of the HD curve
>roughly corresponds to Zone IV to VI. For negatives, it's
>Zone III to VIII or even IX.

I guess that it is due the latitude... I have to review that "zone stuff" <vbg>

>
>> BTW, I have a question for you... about film
>> flatfielding: When you divide, Do you have any
>> problems with the star's intensities in the center? [...]

 

>I also have this problem. Two factors may account for this.
>First, film is not linear. Even for a perfect flat, it only brings the sky
>to the correct brightness level, but not other objects. Indeed, it's
>very likely that the brightness of objects after such a flat fielding will
>be wrong. The second reason is due to the optics. It's always not
>surprising if stars at the field center look smaller.

 

The problem I have isn't the second one you mentioned. I see a difference of about 20 (in 8bits) between the stars in the center and in the corners. I'm talking about the star's intensities, or whole flux. They are overcorrecter due the division. That's the reason I always use substraction. I haven't see that problem (or so heavy) substracting.

>I did try something (as you mentioned, luminance dependent division)
>to account for the first effect.

 

Does it works well, or we have to make further corrections to the sky background?

How about linearizating the film? Becouse we usually shot wide fields, and almost all of the stars there have known magnitudes, I think that it would be not so difficult to characterice the film's HD curve (we are not making science... we can allow us some "large" errors), apply a transformation to make it "linear" in the whole range, then make the same with the flat, divide, and finally undo the first transformation. What do you think?

 

>> BTW-BTW: How do you divide using your flats? Do you
>> make any previous threatment to them? Or you use them
>> "raw"?
>Just plain division, no special treatment to the flats.

Ok. Thanks.

I'll give it a try. I'll make them using a large white wall instead of the sky, and I'll try to shot it to be in the VI zone...

 

 

Regards,

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