After months of trying I cannot calibrate my RGB images!
- Thread starter tomb18
- Start date
cloudbait
Well-known member
That right there points to something wrong with either the flat frames or the calibration process. The histogram for the flat just looks odd to me. Maybe that's because of the extreme vignetting (that and the distortion show that this sensor is too large for the telescope). My flats are much more symmetrical (and better exposed). I think the suggestion to compare sky and panel flats is good. I could never make a panel flat work on my SCT. If the two don't look the same, it means your whole procedure for taking flats needs to be examined.
What is certain beyond reasonable doubt is that the shadows cast by the dust on the filters is changing. Either because the filter wheel position isn't repeatable, or because the mirror is moving and shifting the light path, or some other flexure. That doesn't fully explain the failure of the flats, but it's the only explanation for that embossed appearance. I suspect the mirror, since the effect is greater on shadows cast from a greater distance from the sensor.
Hi
I took my light panel flats yesterday and they make no difference. In fact, they introduce more reflections than before.
How exactly would I do this and what would I expect?
just calibrate the panel flat subs with a master sky flat, or the other way around. when you integrate the flat-calibrated flats, you should get an almost perfectly flat image. if the illumination differs between the panel flat and the sky flat, you'll see circular artficats in the integrated image.
I did this and there is a gradient and the dust motes show up. The flats with the light panel were at the park position whereas the sky flats were in a different location. Clearly something is moving.
If I grab the camera and move it up and down, there is a motion. It does not seem rigid like on my FSQ with a NiteCrawler. It appears that it is the focuser drawtube that is allowing the motion.
I talked with Moonlite about this and they told me a way to tighten this up. Two setscrews on the bpottpom. It required a fair bit of turning to tighten it up. Now there appears to be no motion.
Unfortunately, I took down the scope for the summer so won't be able to test until the fall / winter.
So, can this be the source of issues? When I start imaging I start around 45 degrees above the horizon up until zenith. At 45 degrees there must be sag, and at 90 there is none. Is this the source?
Tom
cloudbait
Well-known member
Neither a loose camera nor a wiggling mirror should change the overall appearance of the flatfield. Both can shift the dust shadows a few pixels, giving that embossed look to them in the calibrated images. You still have some problem (maybe serious) if your issues are mitigated by using an underexposed flat.
just be careful with this as these are crayford focusers and rely on friction to move the drawtube. if you tighten the screws too much the motor will have a hard time moving the drawtube. i like and use moonlite focusers but i hate this aspect of them. rack and pinion is a lot more accurate for imaging.
as for the flats, i would have expected to see large rings in the flat-calibrated flats if the difference between sky flats and panel flats were changing the sensor illumination. so it sounds like you must have some source of reflections in the light path still, if both flats are similar.
can you post a screenshot of the flat-calibrated master flat so we can see what the gradient you are referring to looks like?
rob