Strange Dark Calibration - any ideas

[[1ponders] (Paul)]

Went for a change of pace last night and broke out the 8" sct with ST7 to try to learn how to use the self-guide. After a few phone calls (thanks Dennis and Gerald) and a bit of hair pulling and cable swapping I managed to get it to work after a fashion. Time to try some imaging. M83. Nice and high, pretty clear night, just lums no colour, no drama no sweat. Until the camera applied the autodarks.

Details: 3/5min shots at -15deg, autodark taken and applied using CCDSoft. Aligned, stacked and digital development in IP.

As you can see from the first animation, the NoDark has a bit of noise and the dark reduction has definitely removed the noise and smoothed the image, but what are the black chunks it's taken out? The dark bits are present in the same position in all the original FIT files. The Dark was taken at the same temperature as the lights.

The second animation is from the lower right corner so the difference can be seen more clearly.

[Dennis]

Hi Paul

Have you tried the cold pixel removal tool in CCDSoft to see if these respond to that treatment? It may be that you have some cold pixels?

Cheers

Dennis

[[1ponders] (Paul)]

I'll give it a try Dennis.

Any idea why they would not show up in the No_dark image and only in the calibrated image? The underlying image looked quite good but after the dark was applied it was if chunks had been removed.

[[1ponders] (Paul)]

Nope, didn't work.

[Dennis]

Hmm, weird? Are you using CCDSoft to reduce the images, or another application? The only time I’ve experienced anything remotely similar is when the temps of the light and dark were different, which in your case it seems not.

Cheers

Dennis

[Dennis]

Hi Paul,

Just a long shot – you didn’t happen to have “Auto Dark” checked and also apply Darks manually? Sort of doubling up.

Cheers

Dennis

[sjastro]

If you are using a single dark then that's the problem. It's possible during the acquisition of the dark there was a cosmic ray hit which would cause the loss of real data during the reduction process.

The way of handling darks is to take 10-15 darks and median combine them. The median combine will ensure that odd effects such as cosmic ray hits will not turn up in the combined dark.

Save the combined dark, but be aware that library darks have a limited shelf life. You should prepare a new library dark every 2-3 months.

Regards

Steven
http://users.westconnect.com.au/~sjastro/small

[[1ponders] (Paul)]

Dennis; Yes using CCDSoft to reduce the images, only used autodarks, no manual darks used.


Steven; I thought about cosmic ray strikes but for a single image there are plenty of them and they are very clumped together. I agree I should collect multiple darks and median combine them, but I wasn't out for serious imaging last night, it was more of an equipment shakedown so just used the easiest method available.

[sjastro]

Originally Posted by [1ponders]

Steven; I thought about cosmic ray strikes but for a single image there are plenty of them and they are very clumped together. I agree I should collect multiple darks and median combine them, but I wasn't out for serious imaging last night, it was more of an equipment shakedown so just used the easiest method available.

Why don't you post the dark image? I've had light and dark images rained upon by cosmic rays. If the cosmic ray hits the chip perpendicular the resultant charge can spill over into adjacent pixels resulting in the clumping effect.

The other possibility albeit faint is that your peltier cooler was running at 100% when taking the dark.

Regards

Steven

[[1ponders] (Paul)]

I will. Any idea where I can find the saved dark image?

The peltier was set for -15 and it was 10 deg ambient so it's possible but not likely.

[[1ponders] (Paul)]

Looks like the dark is the culprit. I've found one from earlier on in the night that was only for 3 min, but it was autodark subtracted. It has similar black spots but in different places and not as many of them.

Ok so multiple darks and create a master definitely looks the way to go then.