Quote:
Originally Posted by g__day
Andrew - when you say 1/3 of the way along - you mean from the left (dark) end don't you? So you expect you shot to look dim, not dark nor too bright (i.e. like a picture of the sky towards late sunset).
Terry - when you say a perfect flat should be perfectly flat - what do you mean? Saying a perfect egg should be a perfect egg - doesn't help a newbie trying to understand that definition.
Also when you say the average histogram should be half the total possible - I bet you're right - but what does that look like - i.e.
If its done right does the Histogram mountain in Photoshop CS2L
1. 1/2 the height of the box, or
2. half the width of the box or
3. half way along the bow or
4. something else entirely!
I guess I am asking for a lay persons guide to knowing if I got it right!
|
No 3 is correct. Halfway along the box.
In an ideal world (with perfect optic) an image taken of a flat even scene would result in every pixel having the same levels. This is what you want to achieve. The role of a flat field is to simulate this.
When you take an image through an optical train there will be some variation in the illumination across the field caused mostly by dust and vignetting in the lens (telescope).
If you divide the image by a "flat" field this will hopefully correct for the variation in illuimination in your image. The problem with this is the introduction of noise into the image from the flat field. This need to be limited as much as possible. If the numbers that you are dividing by are big then there is less noise added to the image compared to dividing by small numbers. The size of the numbers depends on the histogram in the flat- hence this explanation. If your CCD has a maximum count of ~32000 then making most of the numbers about half this seems to give the best result. This is achieved by having the histogram about half way along the box. What your flat actually looks like doesn't matter as long as it works.
A test is to take a number of flat images and keep one separate. Make a master flat using whatever tool your software provides. Then divide the flat frame you kept separate bu your master flat. The result should be a perfectly image with as little noise as possible.
Hope this is helpfull.