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Old 17-01-2010, 03:25 PM
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pvelez (Pete)
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Testing the ST8300

My ST8300 has arrived. I haven't had the chance to use it in anger yet.

So far, I have been learning how to drive it. I've also been running a few tests to see how it stacks up against the SBIG specs.

SBIG records readout noise at ~10e RMS. My tests get me to 10.6 so thats spot on.

I'm intrigued by the dark current level. The specs have it at < 0.5e at 0C. Running mine at -10C, I get it down to 3.7e per second. Has anyone else tried this? I'd like to know how mine compares.

To test it, I've taken a 60 second dark, subtracted a bias frame and then worked out the mean pixel value around the centre of the image. I've then divided the result by 60 to get the dark current per second.

Also, its a bit interesting taking some flats. I have an electronic panel which I sit on top of my dew shield. But the shortest exposure allowed by the camera is 0.1 seconds and that saturates the frame. So I've inserted a gey t-shirt between light source and dew shield and upped the expoure time to 2 seconds which seems to work ok. Its not perfect but it will do for the moment.

Pete
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Old 17-01-2010, 03:39 PM
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Just had another go - after leaving the camera about half an hour for the temp to stabilise. With a new dark and bias, I'm now at 2.7e/sec.

Pete
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Old 17-01-2010, 09:06 PM
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Quote:
Originally Posted by pvelez View Post
Just had another go - after leaving the camera about half an hour for the temp to stabilise. With a new dark and bias, I'm now at 2.7e/sec.

Pete
Pete,

The bias (a zero second dark) is not necessary.

Darks already contain this data.

If you also make a bias correction after dark subtraction you are actually adding to the noise.


Regards
Peter
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Old 17-01-2010, 09:23 PM
Mighty_oz (Marcus)
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Is this the theory now, no bias if u take darks or just for these newer cameras as i have an older ccd

Thanks
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Old 17-01-2010, 09:38 PM
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Originally Posted by Mighty_oz View Post
Is this the theory now, no bias if u take darks or just for these newer cameras as i have an older ccd

Thanks
While I've covered this elsewhere on ISS, if you take a non-trivial duration
dark frame...and by that I mean one where dark current is a factor, say longer than 1/10th of a second, then you don't need a bias frame.

Just so we are clear: a bias frame = zero second dark frame

So any dark frame longer than zero seconds (i.e. any dark) already has a bias pattern.

When you subtract a dark frame, you also subtract *any* bias.

To attempt to also take a bias frame and further subtract it after dark frame subtraction *increases* the noise.

In short, don't do it.

Hope that helps

Last edited by Peter Ward; 17-01-2010 at 10:52 PM.
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Old 17-01-2010, 09:44 PM
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What do you use to correct the flats?
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Old 17-01-2010, 10:15 PM
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Octane (Humayun)
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Peter is correct.

I never understood why you'd want to subtract bias when it's already present in the darks.

Robin, you just subtract flat darks from your flat lights to calibrate them. That should be it.

H
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Old 17-01-2010, 10:51 PM
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What do you use to correct the flats?
A flat is just a variation on a light frame. Use equivalent darks as well.

BTW (after much pain and suffering )....I've come to the conclusion you can't beat median combined sky flats.

Adjust your exposures to get around 1/3rd well capacity at twilight. Start with narrow band filters, then RBG then L
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Old 17-01-2010, 10:54 PM
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Median combined sky flats are perhaps the best way to go, especially of you point the scope at a bunch of different areas in the sky to alter any gradients present. the median combine removes the gradients leaving behind a very good vignetting profile and all your dust bunnies.. Having said that, I use a LED Flat box that Robin made for me, it works well, however I do rotate the flat box around between flat frames to make sure any non-uniformity is compensated for..
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Old 18-01-2010, 05:34 AM
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Thanks for the tips on the bias - I'll give it another go this evening. Pity the weather is not looking so good for tonight.

Pete
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Old 18-01-2010, 07:07 PM
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Pete,

Try this..ensure the CCD temperature is the same, then

1. take 2 "bias" frames
2. Take 2 dark frames of ideintical duration (say 10 minutes each).

Subtract the 2 bias frames and measure the noise.

Convert to actual noise, i.e N(bias) = G x measurement / sqrt (2).

Subtract the 2 dark frames, measure the noise and similiarly convert to real dark noise, i.e N(dark) = G x measurement / sqrt (2).

From SBIG website G (or Gain) = 0.37 e- / ADU

Then

Dark signal = N(dark) ^ 2 - N(bias) ^ 2


Dark Current = Dark signal / exposure

See what you get.

Terry
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Old 19-01-2010, 12:39 PM
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Update

Thanks Terry - I'll give it a shot.

At the weekend I was focussing on learning how to drive the beast with CCDOps. I only have experience with a DSLR and the DSI II using Nebulosity.

I had been reading a chapter in Berry and Burnell (AIP4Win 2.0) on benchtesting a CCD and thought it might be interesting to try out some of the routines adn see how them compare with the published specs. This was the basis of my Dark - Flat, mean of centre and then divide by 60 approach on dark current. While I don't have my workings with me at the office, it looks like your suggestion is a different approach to get to the same result as my tests. I'll certainly look over them this evening.

On another front, I gave it a test drive last night with my LXD-75. I was really trying out focus and testing for internal reflections rather than actually taking decent photos. While I have drop in filters, I have no filter wheel yet.

Initial observations:

1. No filter, no coma corrector - its a v v sensitive camera (compared to the humble 350D). I took a 10 second shot of Orion and found it quite overexposed. I might have tried it out with the screw-in Ha filter I have but time got away from me.

2. Screwed in the MPCC in place of the 2 inch nose piece - I had lots of fun trying to work out the focus and appropriate spacing. Managed that (after a bit). I found that there was some heavy-duty vignetting with my set up. Shots were only 30 seconds and it looks like I took the shots through a tunnel. Will this go away if I move the MPCC further down the spacing tubes? I expect the flats will come into their own now. Perhaps it will be less apparent with filters?

3. Internal reflection - took a shot of Sirius and had a huge internal reflection. I suspect its the scope rather than the CCD as I used to get blue seagulls in the same field with the Canon. Dunno what to do about that.

I have no images to post - they were certainly not up to scratch. perhaps this evening.

Looks like the learning curve is steep. I'd better get the ice pick out and start climbing.

Pete
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