Problem with Bias frames

[Pomerol]

I've been getting used to my Atik 383+mono over the last few months and haven't done much because of the lousy UK weather. I had noticed that Bias had introduced noise on a previous target but didn't persue it as I wasn't happy with the quality of the subs.

Took 12 x 10 minute Ha subs of ngc7822 a couple of night ago followed by 20 bias and 20 flats (using light panel) and one dark.I then do a Bpm to remove any hot pixels.

I am using Maxim for camera control and took the Bias using the Bias option in the Autosave box.

When I ran DSS, the resulting image came out very noisy.

I then re-ran DSS without the flats...no change, then re-ran without the Bias and was surprised to see a really good quality image.It looked as if the noise was being introduced by the Bias.I am not using Darks so no double Bias.

I next took another set of Bias, this time using the Dark option in Autosave with 0.001 second exposure and re-ran DSS.........noisy again.

The easy option is to ignore Bias but that is not in my DNA.

Any help to understand what is happening would be much appreciated.

Rgds

Ian S.

[steppenwolf]

You may find that just subtracting Darks will do all you need as these calibration frames also contain the Bias data (as you have already ascertained).

[Bloom]

Why don't you use Maxim for preprocessing as well?

[Pomerol]

Bloom, have used DSS for over 5 years, know it well and have never had a problem that I could attribute to DSS.

Steve, found a Master Dark from a couple of months ago which had the right temperature (-34) but was 900 seconds not 600.

Ran this in DSS with flats(no bias) and it is better than with the Bias but still not as noise free as without the Bias.

[rfdesigner]

I have the 383L+ I measured the readout noise and some other stuff when I got it, so took LOADs of bias frames under different conditions. The camera really ought to be cooled for the bias frames, was it?.. the on chip amplifier can get noisy if you don't cool it.

Also I found I got better results with darks and bias by using median rather than mean summing to get the master bis/dark... cuts out quite a lot of rubbish.

Derek

[Pomerol]

Derek, that was the answer.

Just ran a set of Bias cooled to -30, put them into DSS and...................no noise as I had before.

I guess it makes good sense to run the Bias at the same temperature as the Lights but can't recall seeing that mentioned in any of the multitude of calibration information that is available.

Many thanks for your input and have noted your comment on using "Median".

Rgds

Ian S.

[gkec]

Derek, that was the answer.

Just ran a set of Bias cooled to -30, put them into DSS and...................no noise as I had before.

I guess it makes good sense to run the Bias at the same temperature as the Lights but can't recall seeing that mentioned in any of the multitude of calibration information that is available.

Many thanks for your input and have noted your comment on using "Median".

Rgds

Ian S.

I am surprised at the fact that temperature is not mentioned. CCD performance does vary with temperature which is why the astronomy specific cameras are always cooled. The temperature should be the same for the darks,lights,bias and image but that can be hard to achieve. Each pixel will have a unique response to light level, exposure time and temperature. What the dark/bias/flats do is model this for each pixel so it make sense to have constant temperature and exposure time (except for darks which can be viewed as the constant term).

[gkec]

I am surprised at the fact that temperature is not mentioned. CCD performance does vary with temperature which is why the astronomy specific cameras are always cooled. The temperature should be the same for the darks,lights,bias and image but that can be hard to achieve. Each pixel will have a unique response to light level, exposure time and temperature. What the dark/bias/flats do is model this for each pixel so it make sense to have constant temperature and exposure time (except for darks which can be viewed as the constant term).

Also to add -- if the readout time is selectable noise will vary with readout time. Fast switching electronic signals induce noise so a slow readout is usually preferable although not always practical (e.g. autoguiding).

[Pomerol]

Just copied the following from the DSS FAQ page...........says Temperature is "not important" for Bias. It also says temperature is not important for flats?

Also checked 3 other web pages which didn't mention temperature as important for Bias.

I now know that for Bias with the Atik 383 temperature is very important and will use it also in future for Flats.

Bias Frames (aka Offset Frames)

The Bias/Offset Frames are used to remove the CCD or CMOS chip readout signal from the light frames.

Each CCD or CMOS chip is generating a readout signal which is a signal created by the electronic just by reading the content of the chip.

It's very easy to create bias/offset frames: just take the shortest possible exposure (it may be 1/4000s or 1/8000s depending on your camera) in the dark by covering the lens.

The bias frames must be create with the ISO speed of the light frames. The temperature is not important.

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master bias/offset frame

Rgds

Ian S.

[rfdesigner]

Excellent.. nice to see another problem dealt with.