ditto for me. I was thinking yesterday driving around town that the twilight method wouldn't be very practical even if you get focused by twilight. I'd still have to take the camera off to do the polar alignment because this is done with an eyepiece. So I'm thinking about constructing a lightbox myself. Most of the people on Images Plus yahoo group went this route. Or I could get a doberman to guard my equipment till morning. The lightbox route would be cheaper and more practical. Debbie

From: Joe Bauman <bau@desnews.com> Date: 2005/11/23 Wed AM 07:46:28 MST To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Subject: Re: Re: [Utah-astronomy] Flat Field Frames

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

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