Flat frames

[aarong]

Hi guys,

Do you know what causes a flat frame to look like this? This was taken using a white t-shirt over the scope and pointed at a morning sky.

Is it just that the sky is too bright?

Scope is an lx90 8" ACF.

Thanks,
Aaron

[multiweb]

Dew? Out of focus secondary shadow?

[billdan]

Hi Aaron,

I used to get that occasionally with my QHY12 if I have the temp set below -20C and it is dew on the front of the CCD glass window, when I put a dew heater on the spacer in front of the QHY12 it hasn't appeared again.

Regards
Bill

[aarong]

Thanks guys.

It does look like an out of focus secondary shadow, but I didn't think of dew on the the front of the CCD glass window. I have a QSI683.

I have left the camera on the scope to try and get flats this evening, so I'll test both ideas.

[aarong]

I took some flat frames again this evening. It clouded over so couldn't check focus on a star, but didn't notice anything strange. I cooled the ccd down to temp over 10 mins (longer than I usually do) and all went well. Looks like it was dew on the ccd glass window.

Thanks again,
Aaron

[RobF]

10mins probably the min you'd want to be cooling down to below zero.
Sounds like a desiccant recharge due too....