Non-linear Dark current

[Stonius]

Hey all, I've been following Craig Stark's tutorial on defining your camera's noise characteristics.

According to him, Dark Current should show a linear regression.

I notice my dark current over time increases in a non-linear fashion, more inline with a power series (y=ax^b rather than y=ab^x).

I'm assuming that the dark current scaling algorithm in Pixinsight expects a linear regression (twice as much time = twice as much dark current), so for me the scaling won't work.

I guess I could just take dark flats, but I'd ultimately like to figure out why this is so.

Someone suggested amp-glow may account for this non-linearity.

It could also be an issue with my bias frames which are required for this calculation, but I can't think what. As far as I can tell, the bias and resultant integrated master are fine. Just to be sure, I took the bias series at the same settings as the darks (except for exposure length, of course!)

ASI1600MM Pro, Filterwheel plugged through cameras onboard hub.
Slowest download speed (40). Cable from 1600 going direct to computer
Set point -15 C
It seems a consistent curve shape at exposures ranging from 30 to 600 secs at all gain levels.

Many thanks for any help.

Markus

[pfile]

PI doesn't rely on any characteristics of the input image (gain, exposure time, temperature, etc.) when scaling darks. what it does is iteratively scale the dark while measuring the noise in the calibrated result (on a square region at the center of the frame). when the noise is minimized, PI uses that scaling factor to scale the master dark and calibrate the whole light.

so in theory it shouldn't matter how the dark current behaves as aa function of time or temperature.

but in regards to your actual problem... i dunno. maybe someone with experience with CMOS cameras can chime in.

rob

[Stonius]

Thanks Rob, that's good to know

Best,

Markus

[mcgillca]

Dear Markus,

Hi - I measured my dark current for this camera recently as well.

What is a little odd is that there appears to be amp glow when reading an image, but not in the bias frames (not sure why).

So my dark current (as measured by difference between bias frame and dark frame), scaled as a constant plus a linear term. The linear term was very close to that reported by ASI on their web site.

Not sure why your's is exponential. Could you post your results as a graph?

Colin

[RickS]

What is a little odd is that there appears to be amp glow when reading an image, but not in the bias frames (not sure why).

Hi Colin,

Amp glow is usually of thermal origin, so it builds up over time.  I notice a lot more amp glow in my longer darks.

Cheers,
Rick.

[mcgillca]

Thanks, Rick - as I understand it, with the Pro model, the image is downloaded into a DDR so the amp glow is significantly reduced (it seems to be only turned on during reading the image, not during the imaging itself). That was one reason ASI introduced the memory - people were reporting significant amp glow with USB 2, but much less with USB 3 since it was a much faster download.

I should plot up my data and confirm it actually behaves that way!

Colin

[RickS]

Here are the results of testing dark current build up in my ASI1600mm Pro.  For frames longer than a few seconds it is quite linear.  For short frames (which I believe are timed by the camera and not the capture software) the results show some sort of CMOS weirdness.

Cheers,
Rick.

[Stonius]

Dear Markus,

Hi - I measured my dark current for this camera recently as well.

What is a little odd is that there appears to be amp glow when reading an image, but not in the bias frames (not sure why).

So my dark current (as measured by difference between bias frame and dark frame), scaled as a constant plus a linear term. The linear term was very close to that reported by ASI on their web site.

Not sure why your's is exponential. Could you post your results as a graph?

Colin

Sure. Maybe I'm reading things wrong.

Here's the only linear graph I get - when I look at total mean dark current for the entire exposure.

But when I look at the noise in terms of e-/sec you see the non-linear aspect of things.

The first graph shows you that dark current increases over time.

The second tells you that the rate of increase is not linear.

That's what I'm confused about.

Oh, I should point out, all graphs in 12 bit numbers because it relates to the camera better.

Also my 'strange' gain settings are because I wanted the driver gain settings to reflect e-/ADU conversions.

My Unity Gain sits at 135 (not 139 as specified) which is exactly twice the gain of 195 (the first being 1e-/ADU, the second being 0.5 e-/ADU). That way I'm working with actual stops of light, which makes a lot more sense to me.

Cheers, Markus

[RickS]

Hi Markus,

If you're getting linear growth in total ADU (bias + dark current) then that's exactly what I'd expect.  What exactly are you measuring in your second graph?  Dark current noise should be the square root of the dark current (because it's a form of shot noise).  Dark current noise and read noise will be the two major sources of noise in your dark frames.

Cheers,
Rick.

[RickS]

Gain 72 added to the graphs and I have also added plots of dark e- as well as total ADU.

[Stonius]

Hey Rick

The second graph is what happens for me when I take a gain setting and run bias and darks of varying lengths, then subtract the bias from the result and divide by the number of seconds I exposed for. IOW, it's the number of electrons per second.

Maybe I'm confused, but I would have assumed that given the dark current is supposed to be linear in nature, the number of electrons *per second* should be the same no matter the length of the exposure (as long as the gain stays the same).

So, at say, 135, the number of electrons *per second* varies between 0.031 and 0.008.

The rate at which the dark current accumulates is almost 4 times greater for a 30 second shot, as opposed to a 10 minute shot.

Also, I think I've been using the terms 'dark current' and 'dark noise' interchangeably. Sounds to me like you're saying the dark current *causes the dark noise at a level which is the square root of the dark current. Just trying to get my terms correct :-)

Cheers,

Markus

[RickS]

Hi Markus,

Here's my graph of dark current in e-/sec vs duration.  Interestingly, the differences between gain 72 and gain 200 I see are very small which is not the case in your graph.  There does seem to be some other factor at play at the short end of the graph but still the overall result still looks pretty close to a straight line if you look at the graph of mean e- vs duration.  I have a couple of ideas to try to see if I can tease out some additional info.  Will post if I find out anything interesting.

Dark current and dark current noise are definitely not the same thing!  The signal to noise articles by Craig Stark are a good introduction to some of the things you should understand if you want to dig into this: http://www.stark-labs.com/craig/articles/articles.html

Cheers,
Rick.

[Stonius]

Ah, that's interesting, thanks for posting. At least your graph has the same shape as mine

Not sure where the offsets are coming from. I might look at the bias frame data for mine. I used an integration to get an average of all biases, but maybe a single bias may remove the offset better.

Cheers
M

[RickS]

Not sure where the offsets are coming from. I might look at the bias frame data for mine. I used an integration to get an average of all biases, but maybe a single bias may remove the offset better.

Hi Markus,

I load all the frames into Blink and use the "Series analysis report" button to dump the data to a text file.  Then I import it into a spreadsheet and crunch the numbers there.

Cheers,
Rick.

[Stonius]

Ah, I didn't realise you could do that! Muchas Gracias!