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Ok - it must be me.
I took a few test shots of an attractive group of galaxies in - Pavo I think - last night. I then fired up the light box (thanks Exfso) and took a series of flats using the Sky Flat Assistant plug in with Maxim. Took darks at the correct temperature and a bias or 2 for good measure so the darks would be scaled and applied to the flats.
All good.
I then used the Calibration Wizard to point to the relevant folder and made sure all the cal files were there. They were.
To the combine routine - open Stack, add folder and select Auto-Calibrate. Measure, align and combine files into R, G, B and L groups. Then colour combine.
Am I missing something?
I have an image with a very green hue in the background to the right of the target and about a third to the left. Down the middle there is a strong magenta tone.
So I conclude its my flats. I take a single image - through a green filter as its the most sensitive - looks like some vignetting to me as the centre of the image is noticeably brighter than the edges. I then apply the flat from the same filter - similar look in terms of brightness - wham! Maxim has overdone it. I have effectively an inversion of the light with too much brightness at the outside and not enough in the centre.
Now my weighting of colours is around 1.23-R 1-G and 1.89-B, so when I do a combine, I have too much green for all the image except the centre. Where there is insufficient green, its a delightful magenta.
Quite arty but a poor astrophoto. My missus thinks I'm like Michelangelo painting the ceiling in the Sistene Chapel - "Michael, when are you going to finish??". The cold nights never produce anything worth looking at let alone printing.
So.....how do I reconcile this?
1. Is my green filter sensitive to light at different intensities so it passes more light when the light is brighter than it should - so my flat is not really flat?
2. Or is there a problem with my calibration routine? I am no Maxim DL expert - just a novice.
3. Or is it not vignetting but rather something else rather odd? I had a SN f4 and had a similar issue - you can see that from the shots in the original post. I now use an RC8 with the SBIG 8300. Is this some other optical aberration eg do I need a flattener?
4. Or am I just lousy at flats? I reviewed each of the flats and found that they stayed well within the ADU range - coming in at around 20k - 22k ADU. The EL flats as well as the T-shirt flats were pretty good too.
5. Or maybe its just me. Eminently possible but in this case I suspect not.
If anyone can put me out of my misery, I'd be grateful.
My suspicion is that number 2 is the culprit. Perhaps I am double or triple calibrating so the images are being progressively lightened at the margins with each iteration. Luckily I have kept a back up of uncalibrated images from last night so I can try and manually calibrate them first and then stack, align and combine.
Incidentally, I read somewhere that the FITS header for images taken with Maxim DL include a field identifying whether calibration has been completed. For the life of me I can't find it. Am I making this up?
If I can get to the computer tonight, I'll post some images so all the above makes sense.
Cheers
Pete
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