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How to determine: mean central region ADU

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john downing
How to determine: mean central region ADU

I am studying the CCD Photometry Guide and have a questIon:
Page 16, Camera Linearity: "Point the telescope at the screen and adjust the brightness until a 10 second exposure will result in a mean central region ADU count of 10,000".

Can someone please direct me to a resource that will provide me with a way to measure mean central region ADU counts? I am using an SBIG ST-8XE, operating with CCDOps and APT for image acuisition.

I am assuming (was taught not to do this!) that you are stating that the "stable" light source must have a variable brightness? 

Thanks,

John Downing

DJCA

TRE
TRE's picture
Central region with 100 pixels on a side?

Could be the mean of ADU counts is 10,000 ADUs in each of the central 10000 pixels of the image. 100 x 100 pixels. Start with enough illumination to make that happen for some short exposure time. Then do a run of increasing exposure times, recording the mean value of those 10,000 pixels. At some long exposure time it will not be linear, as shown on the plot on the same page (16).

The plot happens to depict a very linear CCD with a 16 bit analog to digital converter.  It quits at 65536 ADUs or 2^^16. A typical CCD may begin to falter at 40,000 or 50,000 ADUs.  If your camera has a 14 bit A/D then you only get a maximum of 16384 ADUs, and you might go nonlinear at 13,500 analog-to-digital Units (ADUs). A 12 bit ADC yields 4096 steps. So this chart only ilustrates the general idea. Each user needs to tayler their measurements to their camera's specification.

 

Ray TRE

spp
spp's picture
Mean central region ADU's

"I am assuming (was taught not to do this!) that you are stating that the "stable" light source must have a variable brightness? "

If you use 3 lamps, each holding a bulb with a different wattage, you can have a variable, stable light source.

"Can someone please direct me to a resource that will provide me with a way to measure mean central region ADU counts?"

When you're setting up the linearity test, experimenting to get the right initial illumination and the right exposures, the mean central ADU counts you use can be approximate.  Usually, imaging software will show the x,y, positions and the ADU value of the pixel under the cursor.  Just move your cursor around over the central region of the image and watch the ADU's.  This is good enough to get an idea of the approximate mean ADU count that you can use for the setup.

When plotting the linearity limit you need to be more precise.  For this I use the Measure>Pixel Tool in AIP4W (Astronomical Image Processing for Windows).

Phil 

 

john downing
How to determine: mean central region ADU

Thank you for the answers Phil & Ray. I have AIP4Win but have not used it very much. Will need to get more acquainted. I now have the information required to perform a linearity test on my vintage ST-8XE. I have applied for a mentor and am studying the guide but occasionally have questions like this. 

John 

TRE
TRE's picture
many ways

Hi John

I also used AIP4WIN to get the ADU count and all the basic camera parameters. Been awhile, but I think it may have a way to get a mean count of the central region. It displays a single number. Check the book that comes with it. You may even be able to set the size of the region. My ST8 rolled over at < 50,000 ADU, deviating by 1% at 47500 ADU. When observing, I limit exposure duration so that 47500 ADU is not exceeded. That usually can happen for a red star in the I-band.

If you will worry about 60 Hz flicker during a 1 to 10 second exposure, my method could really yield nightmares;  AIP4WIN V 2.0 Barry and Burnell use an LM317 voltage regulator to drop battery to 1.25V that feeds 120 ohms and ~10 milliamps to an LED. Schematic on page 235. Study section 8.3 . Page 230 mentions 100 pixel squares.

My light source was 3 white LEDs in a plastic white salad bowel. That provided a fairly even illumination. The LEDs were driven in parallel by a pulse generator having lots of space between pulses relatve to the pulsewidth so that the PRF could be increased without running the pulses together.  Pulse height was made sensible for the LED ( a volt or two? don't remember ) I think I added ~100 ohms series R and adjusted pulse height to make it happy.  If you don't want all the LEDs going off at the same time, add a little different inductance to each diode.  That would be hard to do for a 50 or 60 Hz light bulb. But that really isn't needed because the pixels make a pretty good LPF at time scales less than 17 msec.

 I used a constant 5 second exposure to avoid shutter accuracy problems. I didn't need 80 seconds to saturate like the setup in the book. Also, since the exposures were all 5 seconds, I needed just one bias and one flat and one dark. I tested at -20C, but could have readily tested at many temperatures because the exposures were all a short 5 seconds. You will have different darks for each temperature.

I set PRF to something like 400 Hz, then reduced the PW to get a low ADU count for a 5 second exposure, then cranking the PRF up to get more photons in 5 seconds. An oscilloscope was used to set it up and showed the pulse train to make sure the duty factor was <50% ( my choice, to limit power to the LEDs). Took about 30 minutes to do the quick power calc, slobber the diodes and R together and scope it out. Easy, low-light, no heat, and I was able to return the salad bowl to the kitchen before anybody was the wiser.

I got the same answers after running the tests 5 times. Easy since the exposure was just 5 seconds. If you a electronics savy you'll like this crazy method.

Ray

john downing
Your ST-8

"My ST8 rolled over at < 50,000 ADU, deviating by 1% at 47500 ADU. When observing, I limit exposure duration so that 47500 ADU is not exceeded. That usually can happen for a red star in the I-band."

Thanks for the information Ray. I do have electronics savvy and may well give this a try. Quick question: Is your ST-8 ABG or NABG? Mine is ABG per the serial number (purchased second hand), I'm looking for a frame of reference re: your <45,500. 

I continue ot be impressed with the amount of time and energy people are willing to put into answering these questions. Thanks again, John 

TRE
TRE's picture
I should know this, I did

I should know this, I did know it, but now have forgotten if my ST8XME is ABG. I always have in mind that I have forgotten so I never really push it over about 35000 ADUs. I have had it so long it has well over a million exposures. Now it seems to have developed an intermitant hot pixel in th LLQ. I think I worked it to death.

AIP4WIN does  a good readable treatment of calibration. Good luck with the LEDs.

Ray

john downing
Thanks Ray, that is a big

Thanks Ray, that is a big help. If <35,000 adu works for you it will certainly work for me as I get started. I know I need to conduct a true lineariy test eventually. But for right now I have so much to learn that I will just keep it down to below that level and I shouldn't get in trouble. Perhaps this will help: "The XEA at the end of the serial number indicates that the ccd is ABG though. X (USB interface), E (Enhanced CCD) A (ABG)." This is from Bill Lynch, SBIG Repair. 

Appreciate the assistance,

John 

 

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