Clarification needed for capturing Flat frames

[Ehud]

Hi Robin,
The User Manual say:
"Capture and subtract dark flat frames – If this option is selected, SharpCap will capture dark flat frames (at the same exposure length as the flat frames) after completing the capture of flat frames. This option can give the best flat correction results (particularly with long exposure flat frames of 1s or more). When using this option, you must either cover the telescope when prompted to allow dark flat frames to be captured or specify a filter to use that blocks light to the camera. "

However, I am using 5 seconds exposure' and when testing through Darks capture histogram, even with Gain at 0 , the image is overexposed.

Please advise,
Ehud.

[Ehud]

Hi Robin,
also, I am not sure how this goes with with the direction to capture Darks using sane gain and exposure as using for the picture stacking itself. I am using a gain of 300 (for Galaxies EAA from Bortel 9).
Thanks.

[admin]

Hi,

light frames and dark frames need to be captured with the same gain, exposure and offset values to work together properly. Flat frames and dark flats need the same settings as each other, but do not need to be matched to the light/dark frames. That means you can use shorter exposures and lower gains for the flats and dark flats than for the light/dark frames.

If the image is looking over-exposed when capturing dark frames, the first thing I would check is to see if you have left the display stretch turned on , which could be making an image that is actually quite dark look light. The other thing to check is the offset (black level/brightness) control - sometimes if that is turned up high it can push a frame towards being light even when no light is hitting the sensor.

cheers,

Robin

[Ehud]

Hi Robin,
I think I got it:
Dark-Flats at the Flats capture process are not instead of the Darks that I capture separately.
Am I correct?
While capturing the Flats & Dark Flats, Darks subtraction should be enabled or
not?

Thanks,
Ehud.

[admin]

Hi,

yes, that's right - 'dark flats' are the dark frames matched to the flats - they are used to create a flat with the dark level background removed and not used in place of normal darks.

Best to turn dark correction off when capturing flats and allow the 'dark flats' option to do its bit.

cheers,

Robin

[Ehud]

Robin,
Thanks for the clarifications.