Gain setting when using sutract bias option

[Steve Teel]

When capturing a master flat using the subtract bias option, should one be careful to set the gain low enough so that an "OK" exposure time is greater than 1ms? I just recently noticed that some of my flats for a very fast system were black. I also noticed that the "Capture Flat" procedure uses 1ms when capturing bias frames, and sometimes my exposure time setting for the flat was less than 1ms, even though the gain was quite low, because the optical system was very fast.

[BlackWikkett]

You'll want to adjust your gain / exposure to put the peak of the white(On Shot Color camera) histogram curve peak at 50% of the horizontal scale. Make sure you don't clip anything on the left side, this may mean less than 50% of the main curve. Your flats should never be black. The 1ms setting for bias in effect is like covering the sensor to create bias frames to subtract the dark current from the flat.

[Steve Teel]

That's good advice, but my point is that it is possible, with a very fast optical system like F/2.8 or lower, to follow that advice exactly and get a black master flat. I suspect it's because the gain setting is just high enough that the exposure time required to hit the 50% peak white level is less than 1ms.

I would appreciate advice on how much higher than 1ms should the flat exposure time be in order to get a good result. For example, should I set the gain low enough that at 1ms of exposure time the peak white level is at 20%? 10%? Lower? Is there a problem if the gain has to be set to 0 in order to get a sufficiently low peak white level at 1ms exposure?

[BlackWikkett]

If the histogram peak is at 50% I would think it impossible to get a black master flat. If in your case you are at 50% histogram and getting black flat then some how you've broken physics. All kidding aside I would still shoot for the 50% with mono if using a one shot color. You can also shoot a master bias and add it to the flat creating tool. Set the exposure to 1 ms and gain that you'll use to make the flat. Cover the scope and use the Dark creation tool to make your BIAS frame. I'd use 50 exposure at 1 ms to make this. Once this master Dark BIAS is created us this as "Subtract Dark" in the Preprocessing section. Then start the capture flat frame as normal. You will not be able to select the BIAS option in the tool since you're all ready subtracting a dark. See if this helps.

[admin]

Hi,

In general you would want to take your master flats with a low gain setting which would lead to minimal noise in the individual flat frames. If you use a high gain setting then the individual flat frames will be noisy and you will need to take a very large number for the averaging process to be able to reduce that noise successfully. You may also run into the problem you describe where the correct exposure setting for the flat frame is less than the 1 ms used for the bias frame.

Hope this helps, Robin