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  1. #1
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    Default rotators and flats

    A few questions for those who use rotators (pyxis rotator specifically) Do you maintain a library of flats with a set for every 5 deg or so ( 0, 5, 10, etc) and then make sure ur PA's for images are a multiple of 5; or just add to the library for exact PA's as you need them? I have been trying to do the latter, but am finding the task of keeping them organized a bit daunting. Also, do you find the repeatablitiy (is there such a word?) of the PA by the pyxis good enough that the flats made weeks earlier are useable? It seems to me that "pretty close" would not be good enough

    ..george

  2. #2
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    Default

    I've wondered about that in the past - does Maxim's calibration tool know about rotators?
    Does it look for the closest PA when it's looking for a matching flat?
    If so, is there a max deviation it will allow when selecting a matching PA?

    If the answer is "no" to either of those questions, I'd suggest sending a feature request to the Maxim guys, maybe they can get it in the new v5.

  3. #3
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    Hi guys,

    George, I don't do a library with flats (although I do with darks/bias). I take flats for each target.

    John, I'm 99% sure MaxIm doesn't recognize PA as a criteria to determine which flat to use. I don't think there's a "standard" for the .fts header with respect to PA for MaxIm to use. As it is now, the PA captured by AutoFlat is the physical rotator PA since it doesn't know for sure what the actual sky PA is. (If the offset in RotatorInfo.txt is right, it could be calculated, but there's no guarantee it's right when taking flats.) The sky PA is what's captured for light images.

    Regarding the "aging" of flats, here's my experience. Nothing in my setup changes from night to night, so there's not much opportunity for new dust to get on the various surfaces. With respect to rotator accuracy, while the Pyxis is probably only accurate to about 1 degree, I don't find that to be a problem at all. The real issue is the CFW8 accuracy. Normally, it's fine, but once in a while, apparently the CFW8 isn't in the exact same place for the flats and images, so the flats don't "work" as well as they should (meaning dust donuts aren't completely eliminated). But, I don't think the accuracy of the CFW8 is affected by the time between when flats are taken and when the images are taken. The issue is far more random - and doesn't occur all that often.

    With my setup, I've worked pretty hard to eliminate as much reflected light as possible. I always image with a lined dew shield, I've flocked my scope, and I've painted all the exposed surfaces throughout the imaging train. Having said that, reflected light still causes problems with flat effectivness. The brighter the sky (like when imaging when the moon's out), the more likely that optical problems aren't completely eliminated by my flats.

    Yes, it's kinda ugly dealing with flats when using a rotator. While you might figure out a better way, here's what I do:

    1) Once I select a target, I'll do the flats for it as part of my imaging session. Often I'll do dusk flats for one target, and dawn flats for another one. I don't see any difference in "performance" between dusk and dawn flats. My "standard" is 12 exposures for each filter, 6 at the specified PA and 6 at PA + 180 (done automatically by AutoFlat.vbs).

    2) I save these flats along with the flat plan to document for sure what my sky PA intent is on my image processing computer.

    3) My normal imaging mode is to image Luminance over 1 (or more nights); the implication is I need to use both east and west flats. Then, I image the RGB, in that order; the implication is the R requires east flats, G requires both east and west flats, and B requires west flats.

    6) In MaxIm, My darks/biases are already setup as masters. Then I'll add the clear flats manually - "Clear1" are the east flats; "Clear2" are the west flats. Check "Clear1" flats (uncheck "Clear2" flats)
    and calibrate the east luminance images. (You have to review the ACP logfile to determine when the flip occurred.) Check "Clear2" (uncheck "Clear1") and calibrate the west luminace images.

    7) Do the same for the RGB images. If you do what I do in step 5, you only have to add east R flats and west B flats. You need to deal with the G flats the same as with the Clear flats.

    8) The closer you do the calibration to the actual imaging, the better - if only because you deal with the flats once, then forget them.

    None of this is particularly hard, but I'll admit it does add extra work and requires some extra organization. However, I know I'm happier with my results when using an internal guider than when using an external one. So, it's worth it for me.

    FWIW.

    Jim

  4. #4
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    Default

    Hi y'all

    Thanks for all the information. It will go along way to helping me stay organized. (By the way point 6,7and 8 seem like they should be 4,5and 6) What complicates the issue for me is that I'm trying to do a lot of photometry on some rather dim stars. So on any night I may be doing eight or ten of those and scheduling in on "pretty picture" image. I'd like to be guiding on most of the variables. But that means many many flats - too many to be taken in dusk and dawn. That's why I was asking about a library.

    Every once in awhile the pyxis will slip just a bit giving a misalignment value different from the usual 10.3 deg. If im aware of it I can just re-home the pyxis and everything is fine. Is there anyway to have a plan re-home the pyxis?

    ..george
    Last edited by George Sjoberg; Dec 13, 2007 at 22:15.

  5. #5
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    Hi George,

    (By the way point 6,7and 8 seem like they should be 4,5and 6)...
    Oops, guess I can't count!

    Every once in awhile the pyxis will slip just a bit giving a misalignment value different from the usual 10.3 deg. If im aware of it I can just re-home the pyxis and everything is fine. Is there anyway to have a plan re-home the pyxis?
    I don't think so. I'm sure it wouldn't be hard to implement, though. In the meantime, there might be a way to tighten up the Pyxis clutch. Perhaps that's worth pursuing with Optec? I guess the good thing is ACP does deal with it automatically with any plate-solves.

    Regards,

    Jim

  6. #6
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    Default

    Two last questions about flats: Do clouds adversely affect dawn or dusk flats? How about an overcast sky?

  7. #7
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    Hi George,

    The problem with any clouds (or being overcast) is they won't be evenly illuminated. So, flats end up introducing some sort of gradient, which you don't want. I don't even bother taking flats unless the sky is clear.

    Regards,

    Jim

  8. #8

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    I read an article somewhere by John Smith which indicated that a library of flats at 90 degree intervals gave good results.

  9. #9
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    Default

    Hmm ... I can see that as MIGHT be true in some fashion for vignetting, but dont see how that would work for dust donuts. If you run across the article again I'd appreciate reading it.

    ..george

  10. #10

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    Hi George, I've looked around John's Hidden Loft website without luck. I don't disagree with your assessment.

    I have a PIR, and while I couldn't live without it, the flat making overhead is a huge burden.
    John K
    St. Augustine, FL
    Weather: http://www.cloudappeal.com
    Images: http://www.pbase.com/jaxdilation

 

 

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