Quote:
Originally Posted by nebulosity.
Thanks a lot for your reply Greg, extremely helpful 
I'll give a more detailed description of what I did, so I got the camera going around 6 o'clock (my cooled, modded 350d) and after about half an hour (to let it cool down) I started capturing darks. Then around 9 o'clock framed and focused and started imaging. I imaged all night and in the morning when the sky had brightened and I couldn't see any more stars I took the flats. I didn't touch the camera (left it pointing at the sky) beyond changing the exposure to 1/200 and the iso to four hundred. The histogram was about 2/3 of the way up so that may have been a problem?
The camera would have sat on around -8 most of the night I think.
All the darks and bias frames were stacked with a pure average and the flats with a 2x2 median (what ever that is, it's what Nebulosity recommended)
I removed the bias from the flats, and the darks and flats from the lights.
I'm not sure if Nebulosity has a clip max/min combine but it can do a standard deviation stack which I think chucks out any pixel thats over a certain value, would that be similar you reckon?
The dust is mainly on my sensor thats why it's so big and dark.
Cheers and thanks again
Jo |
Hi Jo,
Firstly I am not 100% sure but one thing that stands out there is you changed the ISO. DSLRs have different responses at different ISOs. For one thing dynamic range falls as you raise ISO. ISO is simply amplification of the signal, its amplifying the image you took much like stretching an astroimage. So if the lights are at ISO800 and your darks are at ISO400 or your flats are at ISo400 that may make a difference.
If you are taking flats using the sky there is an area called the null area I only found out about recently. Its on the opposite side of the sky to the sun and is considered to be the most neutral part of the sky. I can't say I have applied this myself but try pointing the scope to the null area of the sky.
Did you put something over the end of your scope for doing flats like a cotton sheet or a Tshirt or something? You may need that as well or you may pick up some fading stars.
Also when I use bias I don't subtract it from the flat when making the master flat. I subtract when applying it. Not sure if there is any difference there but just in case.
2/3rds of a histogram on a DSLR may be too bright. Flats are divided into the image. If the flat is too bright I have also seen it through an image into a wobbly.
Do you have any light leaks if you are taking darks at 6pm, its still light then. Have you visually checked the darks to make sure they were in fact dark with no light leaks? Some DSLRs can leak light. There was an issue with the first batch of Canon 5D3's for this. It was a simple fix involving black electrical tape!
Another point on flats. It may not be an issue with DSLRs because of their curtain shutter but with my CCD with their aperture blade style shutter, I have to take flats at least 3 seconds long otherwise the shutter leaves an artifact in the image. You can see the shutter blades. At 3 seconds they disappear. It may not be an issue with DSLRs but I thought I would mention it in case you can see shutter artifacts.
Greg.