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Sorted - I think!
I think I have this all sorted now. I spent a fair amount of time over the last few days playing with flats. Here is a summary of the results:
1. Error 1 - I found a few times that the FW moved to the wrong filter – usually to the Ha filter which is in the first position in the wheel rather than to the luminence filter as instructed. This doubtless happened enough to throw the results.
2. Error 2 - I was testing flats against lights taken a few days earlier. I couldn’t work out why all the dust donuts obvious on the flats were not taken out of the lights. The answer was that the lights didn’t have the dust on them at all. I suspect the dust accumulated on the filters between lights and flats. The most persistent of the donuts was in a dark section of the light. Had it been brighter in that section of the image, I might have realised that there was no dust there in the first place.
3. Error 3 – I was testing my flats against a single exposure taken through a green filter. I didn’t realise that the calibrated flat was displayed with a different stretch than the uncalibrated image. So in flicking between the calibrated and uncalibrated image using the undo button, I was actually seeing not only the effect of calibration, there was also a different stretch which gave greater prominence to the fainter sections of the image.
4. Error 4 – I don’t think I was properly applying darks to the flats. I now set up master frames so they are dark subtracted.
I suspect it was a combination of all of these things – as well as pilot error more generally – that had me in a tangle.
A couple of other points:
5. Shutter artefacts don’t seem to be an issue. Generally, the shape of the flat doesn’t change from a sub-1 second exposure to a 30 second exposure. I played with twilight flats over the weekend and so managed flats out to 30+ seconds. The pattern of illumination was the same.
6. I couldn’t work out why I had a strange colour gradient. The sky background at the left of the image always had a blue hue while to the right it was quite green. Looking at the blue filtered images, it was clear that there was a lot more blue in the background on the left side. However, when I upped the target ADU for the blue filter from 20,000 to 28,000 the difference was much less stark. To be honest I can’t work out why this might be the case – other than perhaps that the limited sensitivity of the ST8300 to blue light (and the cut-off of the blue filter) means that the response of the sensor is not linear at that wavelength (though that sounds v odd) or that Sky Flat Assistant responds too quickly to the peak intensity so it cuts short the exposure before there are enough photons registered by the sensor to give a truly flat field. Anyway, the result is that I take blues at 28,000 ADU and R, G and L at 20,000. I have yet to work out Ha – my Ha is not parfocal with the other filters and the cable for my electric focuser has yet to arrive so I’ll sort that out later.
I could be wrong about all this – however I managed to take about 1.5 hours of the Triffid last night and it looks great – without colour gradient or obvious uneven illumination. Maybe its not a drama as the target is quite bright. I’ll only know when I head back to galaxies.
Thanks to everyone who contributed to solving this. I suspect almost all of the errors were spotted by other contributors to this thread before I did. Goes to show there are some v smart (and experienced) imagers out there.
Cheers
Pete
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