Flats and a derotator

[Howardk]

Am I right in thinking that if i am using a derotator then the flats that i capture will be misaligned to the lights?

My derotator is attached to a focal reducer on telescope side which is fixed and on the camera side there is a filter drawer and the camera both of which rotate

Sometimes the rotation is 20-30 degrees

[admin]

Hi,

if I am following you correctly then the filter is on the camera side of the de-rotator. That is good because it means that any dust shadows from the filter will remain in the same place in the image, so the flat will work (or largely work) from that point of view. However, if your flat is also correcting for any optical issues in the telescope (vignetting, other uneven illumination, etc), then that part of the flat correction will get messed up by the rotation.

cheers,

Robin

[Howardk]

Yes

Its the vigneting that is messed up

I am going to try capturing flats at the midpoint of the rotation…see if that helps

[admin]

Hi,

the effect will be worse if the vignetting is at all uneven (ie one or two corners are darker/more extended vignetting than the others). That usually happens when the center point of the camera sensor isn't in the center of the optimum light cone of the telescope. It *might* be possible to improve matters by tweaking the collimation to try to even things out, but I'm not sure how much it will help in practice.

cheers,

Robin

[Howardk]

Seems that my flats are much better if i take the flats at the midway point in the rotation

So for example if the camera has rotated 50degrees then i would wind it back 25 degrees and take the flats there

Seemed to work last night anyway