Sunday, June 23, 2013

Working on calibration frames

I received some feedback on my image of M51 that I should improve my calibration frames.

First, I took some new darks and bias frames. I removed the camera from the scope, put the 1.25" adapter on it and put a tight cap on it to make sure that no light leaks in. Then I put the whole setup into the freezer to make sure that it stays at -10C. I took 50 dark frames and 100 bias frames.

Here are the results:

Dark Frames:
600 min, 1x1 binned:

450 min, 2x2 binned:

But when I used these new frames on my recent image of M51, I noticed some strange black spots:

Here is the middle part:

I posted on the ccd-newastro mailing list. One explanation was that it could be frost that formed on the camera when it was in the freezer. I should try it again - next time outside when it's dark (and cold enough that I can get the temperature constantly down to -10C).

Bias Frames:
1x1 binned:

2x2 binned:

First observation is how much noisier the 2x2 binned frames are - which is expected. But I wasn't prepared how much it would be.
Second, there is a very bright vertical stripe on the left hand side. I checked with Terry on the starlightxpress Yahoo! mailing list - he said that there is a firmware update that will reduce this. But these are the areas that I usually crop out anyway, so I'm not too concerned. 
Third, there is a vertical gradient in the bias frames. But the difference top to bottom is only less then 20 ADU.

I was first surprised about the maximum readout values of the darks, they were 50000! But these were just hot pixels. In areas without hot pixels, the maximum was 2000-3000. The same as they were for the bias frames - these are expected.

I then also took new flats. This time at night to make sure that no other light is leaking in. On visual inspection they looked very similar to the ones that I previously took.

For better inspection, I subtracted one from the other:

It's hard to see anything - here I stretched the image more:

So, apart from the slightly different level of both images, the only difference I can see is in the upper left corner. This could be caused by stray light (which wasn't there when I took the flats at night).

With regards to flats, I'm not sure what I could do differently with my flat panels. I could try to take sky flats again - but don't want to get up so early (and when I come home, I usually play with the kids).

Summary:

  • My bias frames are OK, but I should add a delay between them to make sure that the camera does not heat up.
  • Don't take dark frames in the freezer - rather outside when it's colder.
  • Doesn't make a difference between taking flats during the day or at night. The only way to improve them is to take sky flats.