Taking flats - How to figure out saturation level?

[vincentnm]

Hi All,

In order to take a good flat, it is recommended that you reach 1/3 to 2/3 saturation level for your camera. The literature talks about Well capacity and ADUs (Analog to Digital Units).

But, nowhere can I find how this maps to actual exposure time.

How on earth do I know whether I have reached half well capacity of half max ADU on my flats?

Thanks,

Vincent.

[SteveL]

you check the mean/average value of the image, and if that is running somewhere between 1/3 to 1/2 of full value (65535 usually for 16 bit CCDs), then that is good enough.

Exposure times are something you need to experiment with... its not the time thats important, its the levels in the image that you are looking to get right. If the average is too bright, decrease the exposure times, and vice versa

[brianb]

How on earth do I know whether I have reached half well capacity of half max ADU on my flats?

If your capture software has a "live histogram" this will do niceley. If not, increase the exposure, keep increasing by 50% until the image is obviously completely saturated (doesn't get any whiter with further increase in exposure) then back off two steps.

[vincentnm]

Thanks guys. I've got an easy answer from another forum. In Maxim DL, The ADU's displayed on the status bar when I move my mouse around the image!

Thanks,

Vincent.