I wonder what these flats are telling me?

[JKeay]

Maybe more of a newbie telescope question rather than a PI question (so it is here in off-topic).
Both my images and my flats have some subtle defects that look like little copies
of my reflector's mirror around the outside (I count at least 6). The flat is mid-gray
but an extreme stretch in PI does show them up.

Any thoughts?

 

[pfile]

these are dust donuts caused by particles of dust somwhere in front of the sensor. indeed they are forming an image of the objective/secondary. the dust is usually on your filters, but you can calculate how far in front of the sensor they are if you know the f/ratio of the telescope and the pixel size:

Useful Formulae

www.wilmslowastro.com

having said all that, this is pretty normal and it's one of the reasons to use flats in the first place. so i wouldn't worry about this. just make sure your flats are properly exposed and calibrated and they should take care of the corresponding donuts in the lights.

rob

 

[JKeay]

Rob,
Many thanks for that useful input. Now that the disease is named it only feels half as bad,

Ok...
pixel size 4.63um, objective 200mm, length 1m (ratio 5 I assume), binning 1x1, doughnut diameter ~400px => 9.26mm.
No filters (yet) so the dust maybe on the glass cover protecting the cmos array (ASI294mc).

I love that we can calculate it. Thanks again.

 

[John_Gill]

mmmh, I had a bit of muck on the sensor as the defect showed up in all filters .... so got the cleaning stuff out and just could not get rid of the mark ... and then realized that I did not take off the filter wheel and was only cleaning the Lum filter.... ooops!

 

[rdryfoos]

I see a cherboard pattern in the flat--what is this?

 

[pfile]

the bayer matrix, it's an undebayered OSC flat.

rob