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  1. #1
    Join Date
    Sep 2006
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    I live in Massachusettes; Observatory in New Mexico
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    Default Dusk and Dawn Flats

    I did a silly thing the other day - I started thinking about flats. In doing so I completely confused myself. Every night i take flats (Thank-you, ACP Scheduler!) A set at dusk and another at dawn. Now my C14 sits on a German Equatorial mount so when it passes from East to West or West to East it does the wonderful equatorial mount dance. So does that mean the flats taken at dawn are "upside" down compared to the dusk flats and hence the dawns and dusks can't be combined into masters? For the life of me I cant visualize the orientation of the camera after the flip! So, do folks keep two sets of masters - one for East images and one for West - as i've been doing? Or is that not necessary ?

    Thanks,

    George

  2. #2
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    Mar 2012
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    Currently I live in Houston but will be retiring soon to Indiana
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    Default

    George,

    key is is whether you are using a Rotator or not. If you are, then you need two different sets of flats for East and West. If you don't and your East and West frames are upside down from each other, then the orientation of your system never changed and you can use one set of flats for both and combine the dusk and dawn flats.

    hope that helps,

    Best,

    Jim Morse

  3. #3
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    Default

    Thanks Jim, but Arrrgh! I can't grasp it! If the east and west flats are upside down to each other, how can they be combined? The dust spots in the upper left on an east flat are in the lower right on a west flats as are the vignetting values. I trust you are correct, I just can't grasp it I don't use a rotator.

  4. #4
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    Default

    Do this. Take a sheet of paper, mark a rectangle on it and in the rectangle, draw something that is asymmetrical such as a box shaded on one side. Also draw some spots on it like dust donuts. This is an image pointing East. Now turn it 180 degrees and that's the image pointing west. All you did was turn the image upside down but it's in the exact same objective orientation no matter how you turn it.

    now to to your point, when you calibrate your lights, the ones from the west are upside down compared to the east ones and that applies to both the lights and the flats. You now have two choices. You can either use the east flats with the east lights and vice versa or you can flip both the west flats and lights so everything is in the same orientation. But just like the lights, so long as you orient the flats the same way, they can all be combined into one flat master.

    That said, the first way is the much better choice, otherwise you start messing with your darks and your head will really be spinning.

  5. #5
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    Sep 2006
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    Default

    Thanks again, Jim! I think the light has come on (pun insinuated on purpose) The software (in my case MaximDl) matches the pixels regardless of where they are displayed on the screen . So if pixel 1,1 is in the upper left corner as displayed for Dawn Flats and in the right hand corner as displayed for Dusk flat, that is irrelevant when the master flat is created - the pixel calculations are done strictly on the basis of their position on the chip as are the flat, dark, and bias adjustments on the light frames.

  6. #6
    Join Date
    Nov 2005
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    Virgil, NY
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    Default

    Hi George, (Jim,)

    You only have to take one set of flats, and they can be from either side of the pier. I think what I describe here will confirm what you have just concluded.

    Assuming your filter wheel and camera are bolted together behind the rotator, you can take flats at any rotation angle and apply them to the target images taken at any other rotation angle. Flipping over the pier and rotating the camera giving you an “upside-down” image doesn’t change the relationship between the filter wheel and the camera.

    Consider this: Close your eyes and in your imagination take the camera and filter wheel off the telescope. The relative position of the filter wheel (and filters) and the camera does not change regardless of which way you roll the camera around in your hands. If you take a picture standing upright, any donuts caused by dust on the filters or the CCD window will fall where they may. If you stand on your head, the dust donuts will be in the same place as before with respect to the camera X,Y coordinates even though the image itself is upside down.

    The only fly in this ointment is if there is significant vignetting AND ... AND the rotational axis of the camera/filter wheel is not colinear with the optical axis of the OTA. In that case vignetting will have a rotational dependence. This would also be true if there is some asymmetric obstruction in the optical train somewhere.
    Dick
    www.VirgilObservatory.us
    Pier-mounted Meade 12-inch SCT "classic"
    w. focal reducer to f/5.3 ~ FL 1630mm
    Optec TCF-S focuser
    SBIG CFW-8A and ST7-XME
    FOV ~ 15' x 10'
    H-alpha, BVRI, RGB & Clear filters
    MaxIm and, of course, ACP!
    AAVSO Code: BRIC

  7. #7
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    Default

    Thank you to Jim and Dick. Flat creation is so easy with ACP Sched that I shoot them every dawn and dusk. At any one time I have 30 flats for each filter. Each morning I add any flats that were created the night before, drop the oldest, and create a new master for each filter. The whole process takes about five minutes. I just put a sticky note on my monitor it says "flats, don't think about them!". If you've ever attended a meeting of citizen astronomers the most talked about topic, the most disagreement is about flats. How many, how to shoot them, how to combine them. My advice is choose a method and then "...don't think about them!"

    ..george

  8. #8
    Join Date
    Nov 2005
    Location
    Virgil, NY
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    Default

    Good advice. If your recipe works for you, stick with it.
    Dick
    www.VirgilObservatory.us
    Pier-mounted Meade 12-inch SCT "classic"
    w. focal reducer to f/5.3 ~ FL 1630mm
    Optec TCF-S focuser
    SBIG CFW-8A and ST7-XME
    FOV ~ 15' x 10'
    H-alpha, BVRI, RGB & Clear filters
    MaxIm and, of course, ACP!
    AAVSO Code: BRIC

  9. #9
    Join Date
    Mar 2012
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    Currently I live in Houston but will be retiring soon to Indiana
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    Default

    Of course, those poor sots with a rotator don't quite have that luxury, but it has other benefits

 

 

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