Quote:
Originally Posted by netwolf
Is that right essentially you can use the Master Scalable Dark and Master Bias with any Lights with exposure time less than that of the Darks. So ideally you only need to keep Darks of the longest Exposure you are likley to do for a given Temp and Binning.
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Yes, that is correct. Though the Master Scalable Dark frame is in essence just master dark frame. Its not scaled until the calibration is being performed as the scale factor will depend on the light frame. As you note, for scaling to be successful the exposure time dark frame must be at or longer than any of your light frame exposures. In other words with a 1800s (30min) dark, you can scale to a light frame sub exposure of anything below this. 120s, 300s, 900, 1200s whatever. This includes really short exposures such as .30s for flats if needed. What you can't do is scale up a dark frame. i.e. a 300s master dark can't accurate calibrate a 600s light frame.
Quote:
Originally Posted by netwolf
Do Bias frames need to be taken in the optical train?
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No. Bias frames are taken with the camera shutter closed. It is the signature of camera, not the optical path. Bias frame measure the bias level, bias structure and finally read out noise.
Note a dark frame also contains the above elements, but one important addition - the measurement of dark current which changes with time.
Quote:
Originally Posted by netwolf
So then am I correct in assuming that Flat frames dont have to be the same Exposure lenght as the Lights?
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Yes. Flat frames only need to be at an exposure duration that illuminates the field to capture pixel to pixel sensitivity. You mentioned 1/3 to 2/3 saturation which is correct. So for a camera with a well depth of 100,000 ADU, you want the hit around 33,000 ADU count as the saturation value. Need not be exact. I usually work on a 3,000 ADU tolerance on either side of the goal value. Reaching this value will vary depending on the light source. A bright EL panel may get you there in .3s whereas a sky flat on a darkening dusk sky could take 120s or more.
If you are taking sky flats, dark scaling is really the only way to achieve the task. A constantly varying sky brightness at dawn and dusk coupled to different light bandpass through each filter will generate flat exposures anywhere between .5s through to 240s. Taking darks for each possible flat sub exposure is impractical.
FWIW. I typically take 8 x 30min darks and use this for lights and flats with a bias to scale. Taking bias frames is quick and easy so there's no reason not to use them. This provides the most versatile calibration library as there is absolutely nothing I can't calibrate. In addition, its easy to refresh regularly as opposed to shooting darks for each light frame sub exposure you may use.
Hope this helps.