Real problems with flats

[Stub Mandrel]

Last night, I took proper bias and dark frames and when I came in I pointed the scope at a white-painted wall about 12 feet away and took eight flats, turning the scope/camera ninety degrees after each pair of shots in case the light wasn't as even as it looked.

Now I discovered that my sensor was watermarked from the cleaning wipe I had used on it :-( I also suffered from dew and I got little rings from a bright star just out of shot...

  The flats got rid of a lot of the watermark, but I also had really strong vignetting combined with a 'doughnut' effect. One of the RAW flats seems to show the same problems as the raw images, so I would have expected it to work, but instead I get this awful result.

I have read lots of advice on making flats, but clearly my flats don't match the pictures well at all. They were taken at the same ISO (800), but auto exposure so more like 1/13 of a second instead of two minutes!

Any ideas what I am doing wrong? Would flocking the inside of the plastic dew shield (this is the cheap Lidl/Bresser scope) reduce those odd reflections?

[sharkmelley]

How was the wall illuminated?  It sounds a bit of a hit and miss method in any case.

Get yourself a sheet of 3mm White Acrylic Perspex Plastic Sheet from eBay to put over the mouth of the scope (or dewshield) and take your flats in daylight (but not direct sunlight).   Doing this my flats are the best they have ever been.

Mark

[brrttpaul]

I struggled with flats myself, but I was under the impression that you shouldnt change the orientation of the camera?  otherwise I would try what you did to have a go at them. Im presuming that if you have moved the mount indoors then they would be no good or am i wrong again lol

[Stub Mandrel]

I took the scope off the mount, but I think i have found the cause of two of the problems - my home-made camera adaptor 

In a flash of inspiration this morning I realised two things:

1 - the inner diameter might be too small - yep it's 1" (25.4mm and y sensor is 27mm across the diagonals, so I need to bore it out , I can open it up to 29mm.

2 - I hadn't got round to anodising it or painting the inside black

So I think I'm the architect of my own misfortune here.

[gooselk]

[...] turning the scope/camera ninety degrees after each pair of shots in case the light wasn't as even as it looked.

Did you turn the scope with the camera attached or did you turn the camera in the focuser? If the latter, that may have been your problem.

I'm not an expert on this as you can tell but having a different light distribution across your set of flats (that you may have introduced by turning the scope) that may also give you problems. But that's just a guess.

Clear skies ... Lars

[Stub Mandrel]

I turned the scope and the camera together.

I've done some quick experiments and my adaptor tube definitely makes a doughnut if I shine a light through it.

[Stub Mandrel]

Lawks! I feel a right wassock.

I bored out my home made 1 1/4" to M42 adaptor (which fits a canon adaptor) to about 29mm, which is about 2mm greater than the diagonal measurement of my camera's sensor. Then gave the inside two coats of matt black, and cleaned the sensor with a cotton bud and cleaning solution off of a wipe.

Some trial flats show no dirt or doughnut effect and just much reduced vignetting of the very corners.

The imaging time wasn't wasted, because I have learned a lot. Strange thing is I couldn't figure out what i had done wrong, and could only imagine it was reflection on the inside of the dew shield. Slept on it and I woke up and instantly realised I was taking pictures down nearly two inches of shiny silver tube.