Question about the "BASIC CCD Parameters" Script

[jeffweiss9]

To add to this old thread,
 I've been using the BasicCCDParameters v0.3.0 script (the latest I find here) and it worked fine, agreeing with AIP4WIN measurements on the same input frames for my ST2000XM camera EXCEPT the script no longer calculates the dark current.   I don't give it a second, long exposure dark as I am not using a DLSR with internal dark current subtraction, but it looks like with the recent mods for DLSR's it no longer works for CCD's.  It looks like Ruediger is the author of that version.  Can you take a look at that?
Thanks,
Clear skies,
-Jeff

[ruediger]

Hello Jeff,

is it possible that you upload the five files (2 bias, 2 flats, 1 dark) to endor file server?
A crop of the center region with about 1000x1000px should be sufficient.

I have an idea what might happen, but have to look at the files first.

Regards,
 Rüdiger

[jeffweiss9]

Hi, Ruediger-
  Will do as soon as I hear back on my registration request for Endor.
-Jeff

[jeffweiss9]

Ruediger-
Ok, the five files from one of the cases at -20 degC, no binning, 8 min dark are uploaded to the jweiss account.
-Jeff

[ruediger]

Ok, the five files from one of the cases at -20 degC, no binning, 8 min dark are uploaded to the jweiss account.

Can't find them... did you put them to the forum shared folder?

Rüdiger

[jeffweiss9]

Ok. Moved them into Forum Shared Folders / jweiss subdirectory.
-Jeff

[ruediger]

Hi Jeff,

I made the following observations after using the CCD Parameters 0.3.0 script with your files:

BIAS1 has a mean value of about 1029.8 ADU
DARK  has a mean value of about 1031.5 ADU, exposure time is 480 sec.

When only one dark file is supplied, the dark current (e-/sec) is computed as follows (see also 8.2.2.6 of www.willbell.com/aip4win/Errata%20to%202nd%20%20Printing%202nd%20Edition%20of%20HAIP.pdf)

dark current = gain * ( mean(DARK) - mean(BIAS1) ) / dark exposure time.

This would lead to a value of 0.001 e-/sec. I don't know, if this value really makes sense or if there's some onchip circuitry like in Canon DSLR that measures dark current outside the chip area and subtracts it before readout takes place.

The line 411 of the script prevents computation of dark current, if the mean values of the dark file is not substantially larger than the bias mean value. Too enforce computation of a result, I patched line 411 as follows:

Code: [Select]

if ( darkMinusBiasMean[c] > 0 /*dark1StdDev[c]*/ ) {

Regards,
 Rüdiger

[georg.viehoever]

Hi Ruediger,

in my experiments with my EOS40D I see that dark current does not follow the usual linear law for exposure times<30 seconds. To get useful values, you need to use darks considerably larger that 30 seconds (I usually do 120 secs).

Georg

[jeffweiss9]

Hi, Ruediger-
  Thanks. I made your mod to the script and it gives quite consistent results with AIP4WIN that implements the HAIP method.  AIP4WIN gets 0.001857 e/pix/sec@-20degC mean dark current and your script gives 0.002 e/sec.  I'm quite sure there is no dark subtraction from the flat or the bias frames.  The SBIG spec for the camera (ST2000XM) is <0.1 e/pixel/sec at 0 degC, so I'm not sure that it is an unreasonable with the temperature difference (although I don't have these frames at 0 degC). 
  I ran the (original) script with a second dark (16min) in addition to the original dark (8min), all -20 degC, and got exactly the same measurements, except .003 e/sec dark current, instead of .002 e/sec.
-Jeff

[ruediger]

Hello Jeff,

thanks for your feedback! I was not aware, that CCDs might have such a low dark noise (0.002 e/sec at -20°C). This results in only one electron dark noise in 500 seconds exposure time. Incredible. Wish my Canon DSLR comes close to this :-)

Rüdiger

[Alejandro Tombolini]

Hi,

Ruediger, excellent script, thank you.

I was experimenting with Canon 450 and I do not understand too much about numbers..., perhaps someone can help.

Ex. Differents darks gives differentes dark current, is it ok?

The fullwell seems too low... 

What does negative numbers mean?

Regards,
Alejandro.

[ruediger]

Ex. Differents darks gives differentes dark current, is it ok?

For a given ISO and sensor temperature, the dark current should be the same. However, a Canon RAW is getting processed inside the camera, so to fully understand what's going on in your DSLR, you have to read the following article from Craig Stark: http://www.cloudynights.com/item.php?item_id=2786. To quote from the article: "Canon is re-scaling your data before it hits the CR2 file.  Based on the thermal signal [...] it is both shifting your histogram left (i.e., subtracting a constant from the whole image) and scaling the intensity (changing the contrast or gain). This makes the camera appear to have very low dark current[...].

Also, from the timestamps of the darks you supplied, I see that you mixed darks from different months, so probably the temperatures when the darks where taken differed considerable. As a rule of thumb (I don't know if there is some law of physics behind) dark current doubles about every 7°C.

The fullwell seems too low... 

All of your files where shot at ISO 800, that's the reason. Every lower ISO step will double your fullwell capacity. Most Canon astrophotographer use ISO 800, because that's the "magic" unity gain ISO, meaning that one electron relates to one ADU value (normalized to a 12 bit A/D converter resolution) and readout noise is low. But for bright objects (planetary nebula, globular clusters etc.) there is nothing wrong with using lower ISO to increase fullwell capacity and dynamic range.

What does negative numbers mean?

An unprocessed dark file must have a mean value that is greater than that of a bias file, because the dark current is added to every pixel. Canon DSLR measures and subtract dark currents, therefore negative values might occur.

Regards,
 Rüdiger

[Alejandro Tombolini]

Hi Rüdiger,
Thank you for taking the time for such detailed answers.

Yes, Darks are from different months. The temperature, when known, is represented in the name by two digit before the number of the image (in °C). If it is not known says XX

The article is very interesting, and is made with a camera identical to mine, which is even more usefull, but...

It is not so clear yet the desition of which ISO is conveninte to use reading the parameters and the article. Should I aim to a gain=1?, mean to shot maybe to ISO 200 with Canon XSi..., or ISO 400 with Canon 5DII... (I am not understanding this!)

Regarding exposure duration tested for duration until 2 minutes, the article says:

So, the gain (slope of the line) is increasing and the offset (y-intercept) is decreasing in both cases as we increase the exposure.  This would imply that for something very dim (dimmer than the flats I recorded here as the first points you see on the line), the output image intensity would read darker if recorded with a 2 minute exposure than if recorded with a short exposure.

I don't know if it make sense, as for dim details usually the exposures are longer than 2 minutes.

too many doubts...

Regards, Alejandro.

[ruediger]

It is not so clear yet the desition of which ISO is conveninte to use reading the parameters and the article. Should I aim to a gain=1?, mean to shot maybe to ISO 200 with Canon XSi..., or ISO 400 with Canon 5DII... (I am not understanding this!)

The more I read about (Canon) DSLR and their best use in astrophotography, the more confused I get. I try to give an answer as far as I understand it:

The gain for different ISO values is measurable with the Basic CCD Parameters script and e.g. leads to 0.283 e/ADU at ISO 800 for a typical Canon DSLR with APS-C chip size.
But ISO 800 is also often cited as "unity gain ISO" despite the "real" unity gain (1 e ~ 1 ADU) is already reached at ISO 200. This is because the A/D converter resolution of 14 bits is higher than can be utilized because of read noise, meaning the lowest 2 bits contain mostly noise. Look also at fullwell capacity, read noise and dynamic range as a quotient of these values.
So if some authors (including C. Stark) sometime speak of "unity gain ISO", it is the ISO value where "1 e ~ 1 ADU" holds if ADU values are normalized to a 12 bit A/D converter resolution.

So you can continue using ISO 800 to benefit from the low read noise (in electrons) despite the lower fullwell capacity. If necessary you can combine long- and short exposed frames with HDR techniques to extend the dynamic range.

Rüdiger

[Josef]

Hi

I have debayered RAW from Canon 40D to monochrome CFA, but script cannot recognised exposure and ISO from some file and a script not worked.

Could you help me?

Josef