How do I take flat frames?

I'm wondering how I would take flat frames for use with DeepSkyStacker? It talked about using a t-shirt and lighting it somehow. Do I stretch it over the lens and light it?

Seeing that you are only interested in vignetting you may try this. I did after a few different attempts and this one seems to work well for me. Quickly back to the T-shirt idea. As I understand, this is done by stretching the shirt over the front and taking a shot through the white T-shirt. Due to the shutter speed you will over expose and only see vignetting. Here is what I did:

I have a white plastic/tupperware container. Seeing that I am using a DSLR with no scope, but just the lens. I hold the container over the lens. You do this the next morning or early before sunset. point to the blue sky and fire away. The result is a nice white image.

Take care to use the same focal length and shutter speeds you plan to use later for the actual star shots.

Please correct me if I don't have my facts straight.

by my understanding, since stars are a "point" light source, the f-stop the photograph is taken at doesn't affect the visibility of the star to the stacking software.

If this is correct, does this also mean that if I correct for vignetting, I can shoot at F2 with the same precision as shooting at F4. I mean only for vignetting, not for CA or any other artifacts that could be introduced at the wider stop.

Regarding the white t-shirt, the proper technique is to hold a double layer over the lens, smooth out any wrinkles, and point the camera at an evenly illuminated field. The evenly illuminated field part is the key - the sky on an overcast day works, and a clear blue sky could work too if your focal length is fairly long. Trying to illuminate the t-shirt evenly with a flashlight, etc. will probably not work. You may have luck using a white wall or ceiling, but it can be surprisingly difficult to find such a background that's really evenly illuminated.

You should use the same f-stop and focal length to acquire your flats as you use for your lights. It is important that the exposure for the flats be such that no part of the flat field image is saturated and no part is black. Setting the camera to AV mode and letting it determine the exposure time automatically works well. Exposure times for flats will typically be well under one second.

I have had success using the white t-shirt with clear blue skies, but using a 1000mm telescope, not a camera lens. My preferred method is to use the twilight sky, about 15 to 20 minutes after sunset, pointing the camera about 45 degrees above the horizon in the southeast, and no t-shirt is needed for this method. With this technique too a long lens helps achieve even illumination.

Regarding stars being a point source, thus making f/ratio irrelevant, this is a myth. The assumption here is that all the photons from a star that arrive at your camera's sensor travel along a single line. If this were true, you could only see a given star when standing in one exact position on the earth - move a foot left or right and you wouldn't see it. We all know this isn't the case, of course. Photons from a given star enter the lens or telescope at every point on it's aperture, so the larger the aperture, the more photons are captured, and the brighter the star image. With telescopes, this means larger telescopes yield brighter star images in the same exposure time, regardless of the focal ratio. With camera lenses, the aperture is determined by the f-stop setting, so it does matter.

Don

DonR wrote in post #10362462
Regarding the white t-shirt, the proper technique is to hold a double layer over the lens, smooth out any wrinkles, and point the camera at an evenly illuminated field. The evenly illuminated field part is the key - the sky on an overcast day works, and a clear blue sky could work too if your focal length is fairly long. Trying to illuminate the t-shirt evenly with a flashlight, etc. will probably not work. You may have luck using a white wall or ceiling, but it can be surprisingly difficult to find such a background that's really evenly illuminated.

You should use the same f-stop and focal length to acquire your flats as you use for your lights. It is important that the exposure for the flats be such that no part of the flat field image is saturated and no part is black. Setting the camera to AV mode and letting it determine the exposure time automatically works well. Exposure times for flats will typically be well under one second.

I have had success using the white t-shirt with clear blue skies, but using a 1000mm telescope, not a camera lens. My preferred method is to use the twilight sky, about 15 to 20 minutes after sunset, pointing the camera about 45 degrees above the horizon in the southeast, and no t-shirt is needed for this method. With this technique too a long lens helps achieve even illumination.

Regarding stars being a point source, thus making f/ratio irrelevant, this is a myth. The assumption here is that all the photons from a star that arrive at your camera's sensor travel along a single line. If this were true, you could only see a given star when standing in one exact position on the earth - move a foot left or right and you wouldn't see it. We all know this isn't the case, of course. Photons from a given star enter the lens or telescope at every point on it's aperture, so the larger the aperture, the more photons are captured, and the brighter the star image. With telescopes, this means larger telescopes yield brighter star images in the same exposure time, regardless of the focal ratio. With camera lenses, the aperture is determined by the f-stop setting, so it does matter.

Don

Wow, extremely informative. Thanks for the information. I finished building a scotch mount so I want to see if I can stretch my photos to 1 or 2 minute exposures. The weather should clear up friday so I'll give it another shot then. I actually did a little research too and I plan on not simply "pointing the camera at an interesting part of the sky" lol. I looked up what should be visible in my neck of the woods this time of year.