Reflector flats, plot thickens...

[ollypenrice]

We can't get decent flats in Yves' 14 inch/Starlight H36 and Yves can't get them in his 8 inch RC either.

I'm using my standard system, EL panel, calibration using a master bias as a dark, exposed to between 20K and 30K ADU (I've tried both. Stacking and calibrating is done in AstroArt, which works perfectly for my refractors and Atik cameras.)

The flats are too aggressive and over correct by about twice what they they should, so we get inverse vignetting.

I tried flats at around a 1 sec exposure first then tried them again at around 5 seconds with a diffuser. Still the same.

I contacted Astro Art and they said their algorithm was right and it would be incorrect calibration of the flats causing the problem. Seemed reasonable, so I shot temperature and exposure matched darks for the flats and it made no difference whatever.

I have never seen this in my refractor images so it makes me wonder about things like light bouncing around between the mirrors and scattering off the sides etc.

Anyone had this?

My very effective (but not nice) bodge is to do a stack without flats and a stack with, then average them in AstroArt with a real time preview which allows me to get a flat field. Pixinsight does the rest in DBE.

Olly

[NickK]

Hmm is that RC affected by the same issue that the GSO has with light coming in at the side of the FoV when using a flat panel?

[Jessun]

We had the same problem with the Orion RC/Geoptik combo (green problem aside), and I still haven't quite nailed the flats. I reckon shortening the exposure won't chane a thing, the difference between the edges and the centre will be the same, just with lower ADU counts so after the division things are still off.

I have looked again at M Dovington's book, where he recommends calibrating the flats with flat darks allowing for a play with contrast afterwards to adjust the flat for these effects.

I haven't done any tests on this yet, but I can see how that could push the Master flat in a direction we want.

Your method is I would guess is halving the problem? (Big problem plus no problem divided by two)

ATB

[ollypenrice]

Good to know that we are not alone. What's odd is that I have never had this before. Another reason to love refractors!!

Olly

[Catanonia]

I have had the same problem with the SuperNewt which is a GSO Newt.

To fix it I put a black teashirt with elastic band over the rear of the scope to stop stray light and weird patterns on my flats.

Also try a few sky flats to eliminate the panel and setup. I have started to do this now in combination with the black teashirt.

[FraserClarke]

Have you tried using sky flats taken at twilight? Dome/internal flats are notorious at big telescopes for producing large scale structure which isn't really there.

[rfdesigner]

oh yes!

Good reminder Earl.

My F5 newt has had some odd problems as the camera can see past the primary mirror, if the tube isn't covered at the mirror end then it throws everything.

I'm assuming the 14" is a newt and not a cass. (I can't remember).. if a cass then the tube in the centre of the primary mirror would need extending.

Reflectors are good, but as you're looking in both directions in the tube you see problems at both ends.

Derek

(lived with newts since the 90s)

[ollypenrice]

Thanks folks, I think we might be onto something here. The ODK is a Cassegrain, an ODK, with a closed bottom end. I have never had much luck with sky flats but maybe it is time to try again.

Olly

[RobH]

I'm having similar issues with my open truss RC....I downloaded the skyflats plugin for maxim yesterday, but for some reason it stays greyed out

Also, I don't like opening my obs in anything other than proper dark due to the trail behind it that has its fair share of scumbags using it.

Any other way apart from sky flats would be good for me.

Rob

[rfdesigner]

Other thoughts that occurred:

If your flats are essentially too bright in the middle and too dim at the edge, then you'd get what you're seeing.

I think (but not sure) this could happen with an undersized EL pannel, so dumb question: is the pannel really large enough? (granny eggs etc. I know)

The other thought is what happens if the EL pannel is held further away from the scope?.. by doing this the stray light should at least be a lower intensity so although it won't solve the problem, it might well reduce it and so show if it's a stray light problem.

Derek

[r3i]

Glad it's not just me - having similar problems with my AA8RC.

I don't get to image as often as I'd like and am still a bit of a novice at this game. I've only recently started having a go with flats (using a DIY lightbox), but so far it's been hit and miss.

I took some flats on an imaging session a month or so ago and they seemed to work well. But the flats from a session at the weekend were hopeless and I got that inverted vignetting effect. Need to work out what if anything I did differently. Will just have to keep trying I guess.