Weird flats

[DiscoDuck]

Just testing a new camera (ZWO ASI1600MM-C) on my GSO RC8 scope. I am getting flats like the attached.

This is heavily stretched, and the differences are quite small, i.e. 27000 ADU in the centre and 29000 at the edge. The shadow at the top is from the OAG and is about 24000 ADU. But my question is why is it darker in the middle rather than lighter than the edge?!

Setup is an ASI1600MM-C + filter wheel (1.25" Luminance filter) + OAG + CCDT67 reducer.

Not tried it on stars yet as of course a week of rain has greeted my purchase!

Anyone who can advise if there's an issue or what might cause this centre darkening please?? There's nothing there in the bias frames so seems to be a property of the optics and not the camera. But my previous setup with a QHY8 and AstroTech flattener showed a brighter image centre as expected.

Thanks,
Paul

[Atmos]

Are you using the ZWO filter wheel?

[DiscoDuck]

Yes. The 8 x 1.25" one. Got it attached right on the camera so that I think the filters are only about 16mm from the sensor

[kens]

Whats the light source for your flats? Coud it be an artefact of that?

[DiscoDuck]

A light box. Same one I've been using with my QHY8 the last few years so I don't think it's that. Nor the scope as that hasn't changed. All that's new is the camera, filter wheel and reducer.

Also tried a shot without any filter (my 8th filter slot is empty) and got a similar flat. So it's not something to do with the luminance filter.

[billdan]

Were you focused at infinity?, because it looks like the shadow of the secondary is showing.

[DiscoDuck]

Thanks Bill. That sounds an embarrassing but likely explanation. Until the skies clear I can't test focus though - no easy distant targets from the window here!

That said, I measured it all out in comparison to the QHY8 it replaced, and it should have been within a couple of mm of focus. But I could've stuffed that up. Not sure how far out it'd have to be to give such an effect.

[DiscoDuck]

Finally got a clear night in Adelaide last night ... at least for a little while!

Was a couple of mm out of focus only, and flats taken when in focus are exactly the same as that previous one, i.e. about 10% darker in the middle. Seems even to happen without the reducer in place.

No big deal, as the lights seem to have the same profile and so the flats calibrate them ok. Just seems curious.

[Shiraz]

http://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/

might be useful.

[DiscoDuck]

Thanks Ray. That's some good idea for me to investigate.

[DiscoDuck]

There certainly are some bright surfaces showing up looking up the tube. i.e. at the telescope end of the focuser and another one at the ring where it goes through the primary mirror (it's an RC scope).

I'll get some flocking material and see what I can do.

Thanks again all.

[DiscoDuck]

Just thought I'd update the thread with the results of my flocking - in case anyone else has similar issues in the future.

Simply flocking two surfaces seems to make a big difference (before and after shots attached). I applied some adhesive backed velvet material to the telescope end of the focuser and the ring at the entrance to the hole through the primary mirror.

A shot with my phone looking up the focuser to a flat box changed from the before to the after shot. Most of that light left round the edge is from the inside of the tube that goes up through the primary mirror, but it is very minor and would be hard to flock without dismantling the scope I think.

A test flat (single frame so apologies for the coarseness) is attached. Apart from the fact it's a smidge off centre (I need to tweak my focuser's collimation and/or my collimation ring) it's a lot better I think. (The shadow of the OAG is only 10% off the centre - so I might leave that where it is for now).

Thanks again for the advice.