Flat Frame Creation Question

[tdgm]

Hello

In the past have been taking flats in morning after an imaging session. I have been doing "TShirt". My question is that when I finish my session it is a a certain ambient temperature requiring a certain focus but when I take the "TShirt" flat the ambient temperature is different so the focus would be different if I was imaging an object. Since the dust donuts are on the mirror which changes shape with temperature and thus the image captured should the flats been at the same amnbient temperature as the image or not?


Steve

[Geoff]

The dust donuts are not caused by dust on the mirror, they are caused by dust on the filters and/or the cover glass of the ccd chip. I take t-shirt flats with the sensor at the usual temperature I use for imaging, although I am not sure this is necessary. Change of focus due to the mirror having a different temperature to the night before will have a negligible effect.