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Re: [ccd-newastro] Bad flats

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  • Ron Wodaski
    OK, had a look. Yep, them s bad flats. :-( A few observations: * When you removed the dew shield, you changed your optical system. That s going to affect your
    Message 1 of 32 , Apr 30, 2013
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      OK, had a look. Yep, them's bad flats. :-(

      A few observations:

      * When you removed the dew shield, you changed your optical system. That's going to affect your flats. Whatever method you use for flats, you have to use the same setup (including passive elements like a dew shield) for both. Granted, it wasn't working out, but moving forward you'll need to keep this in mind.

      * The flat panel may be too bright, but you can use a diffuser to help cut that down. for example, put the flat panel 8' from the telescope (if possible), and hang a smooth (ironed if necessary; wrinkles can be bad) sheet about midway between scope and flat panel. You can use multiple diffusers if necessary.

      * Sky flats with that type of optical system invites trouble - especially reflections, which could be a big part of your problem.

      * The detail in the blue and green images is a mystery to me, but should go away with multiple diffusers unless it's very close to the chip. The fact that your white flat sort of worked makes me think those are artifacts, should not be in a flat. They look like flats taken looking through a tree or something like that. <g>

      * You had uneven illumination overall, which contributed to the problems. You need more uniform illumination of your flats.

      Flats are very critical for images through fast optics. Working on your technique will take time, but try to focus on the fundamentals: make sure you get even illumination; keep your optical system the same throughout; avoid anything that will cause reflections (which is one kind of uneven illumination).


      On Apr 30, 2013, at 8:23 AM, Mark Striebeck <mark.striebeck@...> wrote:

      > Hi,
      >
      > I recently took some flats with my hyperstar lens:
      > (blue<https://plus.google.com/u/0/photos/103097764320602190090/albums/5776759269803293169/5872408081947503442>,
      > green<https://plus.google.com/u/0/photos/103097764320602190090/albums/5776759269803293169/5872408210366415954>,
      > red<https://plus.google.com/u/0/photos/103097764320602190090/albums/5776759269803293169/5872408377941323250>,
      > luminance<https://plus.google.com/u/0/photos/103097764320602190090/albums/5776759269803293169/5872408494287288850>).
      > The blue and green ones look very bad. I took these flats at dawn. What did
      > I do wrong with the blue and green flat?
      >
      > MarkS
      >
      >
      > [Non-text portions of this message have been removed]
      >
      >
      >
      > ------------------------------------
      >
      >
      >
      >
    • phils67
      My flat box construction was constrained by a number of factors including size, weight, transportability, and field use where no AC would be available. The
      Message 32 of 32 , May 9, 2013
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        My flat box construction was constrained by a number of factors including size, weight, transportability, and field use where no AC would be available. The (fattening) solution was a deep cookie tin that just fits over the tube of my 8" Newtonian. (Think Christmas cookies.) Light is provided by an array of around 80 LEDs mounted on a white board in a pattern that has them evenly spaced from each other. The two diffusers are mounted 1" and 2" from the LEDs. Each diffuser is two double sided vellum sheets glued to a 3/16" thick ring of plywood. This gives me eight diffusing surfaces with an inch gap between each set of four. The best feature is that the top of the tin completely seals and protects it when it's not in use.

        The flat box was tested by taking sets of flats with the box rotated between the sets then subtracting one from another. The flats work great for fixing vignetting and dust donuts and I'm still a long way from trying to do millimag astrometric measurements. Actually, I don't think I can do that type of work from a severely light polluted city that's under the jet stream.

        Phil

        --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
        >
        > OK, to those who have been using thin diffusers; let me explain why a thick diffuser is better (although a number of multiple thin diffusers is equivalent to a thick one - just harder to do. :-)
        >
        > .....
        >
        > In the end, of course, whatever you use only needs to be good enough for your purposes. But I think it's useful to at least know how far one can take these things, so you can decide what good enough really means for you and your situation. There simply happens to be a good distance between what works for one individual, and actually professional best practices; how far you want to go toward best practices depends on your interests, your objectives, your location, and how much time you can put into it. I'm told professionals actually get paid to pick nits; good for them, right?
        >
        > Ron Wodaski
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