Re: Re: [Utah-astronomy] Flat Field Frames
Joe, I had to run to the store to get the turkey so I didn't get a chance to reply to your question. My book "A Practical Guide to CCD Imaging" that I recently acquired says "The flat field takes into account the gain irregularies of the array's different pixels and the optical transmission irregularities, in particular, vignetting and the presence of DUST in the light path. We must therefore produce a flat field for each optical assembly and, if possible, at each observation session, since dust can appear, disappear, or move from one day to another. Since it is imperative not to change the optical configuration between the flat field and the image acquisitions, we can work with a maximum of two optical configurations per night: one at the night's start, for which we produce flat fields in the evening, the other at the end of the night, for which we produce the morning flat fields before sunrise (which requires some self-sacrifice, ie sleep). The flat field must be produced after focusing, in order for the light path to be the same during the flat field and the nocturnal sky image. Thus, the focusing is done on the stars, and the flat field must be finished before the first stars appear! The solution to this paradox consists in keeping the camera mounted from one night to another, or to accurately find the position that corresponds to the focus. In all cases, the first night's flat field, produced on an approximate adjustment, will not necessarily be very satisfying." This is all fine and dandy if you have an observatory or set up at a remote site so you don't have to worry about equipment being stolen. I don't feel confortable leaving my camera or telescope set up all night. Is it possible to focus on Venus or Jupiter before it gets dark? Then take the flats and download them directly to the computer. I know SBIG recommends putting a white T-shirt over the optical system but will there be enough light available once you get your camera focused? I didn't get a chance to try this Sunday night because I filled up my memory card. Hope this helps to clarify flat fields. I'm just trying to figure out the best way to do it with my portable setup. Debbie
From: Joe Bauman <bau@desnews.com> Date: 2005/11/22 Tue AM 10:53:59 MST To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Subject: Re: [Utah-astronomy] Flat Field Frames
Many thanks!
-- Black skies! -- Joe
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Wow, it's much more complicated than I thought. Thanks, Deb. I guess you could take flat-field exposures at dawn after a night of observing and before taking down your gear. Or maybe a flat-field box is a good way to go, sticking it on the telescope during your observing session and making an exposure, then taking the box off and going on with your project. It's almost more work than I want to do, with the trouble I have making the thing guide right! Thanks, Joe
--- Joe Bauman <bau@desnews.com> wrote:
Wow, it's much more complicated than I thought. Thanks, Deb. I guess you could take flat-field exposures at dawn after a night of observing and before taking down your gear. Or maybe a flat-field box is a good way to go, sticking it on the telescope during your observing session and making an exposure, then taking the box off and going on with your project.
The white flat-field picks up grains of defocused dust on all the mirror surfaces. You can easily see an example of these by taking a white flat field against a white wall using your 35mm digital camera and a zoom lens focused to infinity. If you are using filters, which also have color variations and dust on them, then I understand that you have to make beginning session white frames for each session. The reason for this is that each combination of filters will have a slightly different orientation and set of dust particles on them during each session. Thus, I understand that you put all the filters you want to use into your filter holder at the start of the session, lock your camera in position, and then take a series of white flats for each filter. During the session, you do not rotate the camera in or remove your camera from the eyepiece holder. Rotation or removing the camera will change the rotation orientation of the dust particles and dye variations in your white flats. I understand that this situation differs for dark flats, which measure variations in the CCD chip. Those can be taken across sessions, categorized by temperature, and then reused across sessions. - Canopus56 __________________________________ Yahoo! FareChase: Search multiple travel sites in one click. http://farechase.yahoo.com
--- Canopus56 <canopus56@yahoo.com> wrote: <snip> P.S. - That is why infinity focus is important. If you do the test with your 35mm camera on a white wall, you will notice that the dust particles will change size with focus. Hence, Patrick's suggestion of focusing on the celestial sphere and then rotating the scope down to take a picture of white board without changing the focus. - C __________________________________ Start your day with Yahoo! - Make it your home page! http://www.yahoo.com/r/hs
That is a really nice explanation and I appreciate your supplying it -- thank you. When you have a nice workable flat-field box, can you pass along the design and instructions for use? I have little skill in carpentry but I know a place that will manufacture things like that. Best wishes, Joe
--- Joe Bauman <bau@desnews.com> wrote:
Wow, it's much more complicated than I thought. Thanks, Deb. I guess you could take flat-field exposures at dawn after a night of observing and before taking down your gear. Or maybe a flat-field box is a good way to go, sticking it on the telescope during your observing session and making an exposure, then taking the box off and going on with your project.
The white flat-field picks up grains of defocused dust on all the mirror surfaces. You can easily see an example of these by taking a white flat field against a white wall using your 35mm digital camera and a zoom lens focused to infinity.
If you are using filters, which also have color variations and dust on them, then I understand that you have to make beginning session white frames for each session. The reason for this is that each combination of filters will have a slightly different orientation and set of dust particles on them during each session. Thus, I understand that you put all the filters you want to use into your filter holder at the start of the session, lock your camera in position, and then take a series of white flats for each filter.
During the session, you do not rotate the camera in or remove your camera from the eyepiece holder. Rotation or removing the camera will change the rotation orientation of the dust particles and dye variations in your white flats.
I understand that this situation differs for dark flats, which measure variations in the CCD chip. Those can be taken across sessions, categorized by temperature, and then reused across sessions.
- Canopus56
__________________________________ Yahoo! FareChase: Search multiple travel sites in one click. http://farechase.yahoo.com
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astrodeb＠charter.net -
Canopus56 -
Joe Bauman