what is the difference between a sub frame and a dark frame. I know you have to take dark frames with the lens cap on at the same temperature and time as light frames. But how is a sub frame different then the other two.

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interstellaryeller: Celestron C8/ Meade DS 2000: Lon 81º 31'W Lat 40º 04' N: If it uses lens or lipstick its bound to give you trouble.

When you take multiple light frames and combine them into a single image each of the original light frames is a sub frame. Dark frames are used to remove the heat-related noise from the original light frame.

Light Frame: Exposure capturing light data of your object
Dark Frame: Exposure with lens covered or camera close to capture heat-related noise. Generally subtracted from light frames
Flat Frame: Exposure to flat light (e.g., dawn sky) to capture abberations in the imaging train or dust. Generally light frames are divided by the flat to correct the light frame.
Sub Frame: One of many light frames combined (summed, averaged, median, etc.) to create a longer-exposure equivalent light frame.

e.g., I take 10 5-minute luminance shots, apply dark frames to remove noise, flat frames to remove dust motes, and then align and average the frames to create a master image. Each of the 10 initial shots is a sub frame.

Not to complicate things, but this is all assuming you don't mean sub frame in the sense that it is a cropped portion of an original image or imaging chip.

HTH,

--Andy

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Observatorio de la Ballona