Hello,
Just a simple question (I hjope), How can I measure the actual ADU of a star to determine if I am saturated? I have a 14 bit camera and need to stay under 16,384 ADU. I see a lot of jargon, and it may be displayed somewhere, but as a newbie I can't seem to get a definitive answer.
Thanks ... Sam
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Move the mouse pointer to the star and read the pixel values at the statusbar. Move a little around the center of the star to see the maximum of the values.
For a 14 bit camera you will likely see saturation at around 16000. It will not reach 16384. But for a bright star if several center pixels reach the same value, this is the limit of your analag to digital convertor.
There are also cameras which multiply the values with x 4 resulting in a range of 0..65535 and a practical limit of maybe around 65200. A quick test by measurung a bright star in an image will tell you how your camera & driver behaves.
If you move the mouse pointer to the statusbar, you will see the description of the indications. See bottom of screenshot.
Hi Han,
Thank you so much for answering me. In my copy of ASTAP, when I hold my cursor over a star, there are three numbers in parenthesees following the equal sign. Can I assume these relate to the ADU in RGB? I am using a Nikon D500 camera body.
Also, how do I interpret the star profile? I assume when the graph has a flat top the star is saturated, but if it has a peak, or several, it is not.
Thanks once again!!!
Sam
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The three values in the screen shot 122/122/122 are the displayed values, so the red.green and blue display in range 0..255. They will vary with the slider settings, so the displayed brightness. Not interesting for your purpose
The real pixel value of the star in my screenshot is 1222. This value will never change by any setting. This will reach around 16000 or 65000 in your frames.
Yes when the top is flat, the star is saturated. Like in my screenshot.
cs, Han
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Hi Han,
Here is my screen shot. In the bottom bar, after the pixel position equal sign, you can see the three values in parenthesee I am refering to.
Sorry to be a pain, perhaps the ADU values should be labeledas such?
Sam
Your image is calibrated with a dark and debayered to colour. That will shift the values and give you the pixel values of red,green and blue channel. So for your image the values 244.8, 239.28 and 246.54
But these valueswill be shifted/ polluted from the original sensor values. To check accurately if a star is saturated, you should load the original .CR2 or other raw you have. Then the status bar will display only ONE value because the image is still original grey.
A raw will likely show a checker pattern because the red, green or blue sensitive pixels will be different sensitive to light. Just find the brightest pixel by moving the cursor at the star center.
Han
Last edit: han.k 5 days ago
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Hi Han,
My camera is a Nikon DSLR D500 with crop sensor. As such it is is a one shot color CMOS sensor. So, my original NEF Raw image is in color.
Should I extract the green channel from the raw image before I take any measurements? Will that affect the photometry if I do this before any calibrations are applied?
Sorry to take up so much of your time on what I thought was a simple question!
Sam
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No problem. Feedback and questions help the development.
Yes if you use the green channel only extracting to green first will help. Any saturation in the red and blue sensitive pixel is not relevant. Extracting green will not modify the levels.
Note you can set a custom warning level at the top right of the photometry tab. Once you have found a safe level, set this at the photometry tab and it will warn you when it exceeds this level. It will work on the calibrated images so it better to set this value on the saturation level minus the dark level. So if your darks have level 500 and saturation is at 16000, set it at 15500 or better 15000 to be save. Or measure the maximum value of a calibrate image directly and set this value minus an extra safety margin. Once a star is identified as saturated, its value will be made invalid in tab photometry.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
Great, I have set the saturation level as you suggested. I have a better understanding now of how to read the ADU values. I will modify my routines and break out the grreen channel before I do any calibrating. Even though after separating the green channel, the display continues to show three values I will no longer worry about the Red & Blue numbers.
Next clear night I will give it another shot ... Thanks again !!!
Sam
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
Hello,
Just a simple question (I hjope), How can I measure the actual ADU of a star to determine if I am saturated? I have a 14 bit camera and need to stay under 16,384 ADU. I see a lot of jargon, and it may be displayed somewhere, but as a newbie I can't seem to get a definitive answer.
Thanks ... Sam
See attached screenshot.
Move the mouse pointer to the star and read the pixel values at the statusbar. Move a little around the center of the star to see the maximum of the values.
For a 14 bit camera you will likely see saturation at around 16000. It will not reach 16384. But for a bright star if several center pixels reach the same value, this is the limit of your analag to digital convertor.
There are also cameras which multiply the values with x 4 resulting in a range of 0..65535 and a practical limit of maybe around 65200. A quick test by measurung a bright star in an image will tell you how your camera & driver behaves.
If you move the mouse pointer to the statusbar, you will see the description of the indications. See bottom of screenshot.
cs, Han
The star profile is also very helpful to see if a star is saturated. See screenshot.
Last edit: han.k 5 days ago
Hi Han,
Thank you so much for answering me. In my copy of ASTAP, when I hold my cursor over a star, there are three numbers in parenthesees following the equal sign. Can I assume these relate to the ADU in RGB? I am using a Nikon D500 camera body.
Also, how do I interpret the star profile? I assume when the graph has a flat top the star is saturated, but if it has a peak, or several, it is not.
Thanks once again!!!
Sam
The three values in the screen shot 122/122/122 are the displayed values, so the red.green and blue display in range 0..255. They will vary with the slider settings, so the displayed brightness. Not interesting for your purpose
The real pixel value of the star in my screenshot is 1222. This value will never change by any setting. This will reach around 16000 or 65000 in your frames.
Yes when the top is flat, the star is saturated. Like in my screenshot.
cs, Han
Hi Han,
Here is my screen shot. In the bottom bar, after the pixel position equal sign, you can see the three values in parenthesee I am refering to.
Sorry to be a pain, perhaps the ADU values should be labeledas such?
Sam
Your image is calibrated with a dark and debayered to colour. That will shift the values and give you the pixel values of red,green and blue channel. So for your image the values 244.8, 239.28 and 246.54
But these valueswill be shifted/ polluted from the original sensor values. To check accurately if a star is saturated, you should load the original .CR2 or other raw you have. Then the status bar will display only ONE value because the image is still original grey.
A raw will likely show a checker pattern because the red, green or blue sensitive pixels will be different sensitive to light. Just find the brightest pixel by moving the cursor at the star center.
Han
Last edit: han.k 5 days ago
Correction: Then the status bar will display only ONE value because the image is still original grey.
Hi Han,
My camera is a Nikon DSLR D500 with crop sensor. As such it is is a one shot color CMOS sensor. So, my original NEF Raw image is in color.
Should I extract the green channel from the raw image before I take any measurements? Will that affect the photometry if I do this before any calibrations are applied?
Sorry to take up so much of your time on what I thought was a simple question!
Sam
No problem. Feedback and questions help the development.
Yes if you use the green channel only extracting to green first will help. Any saturation in the red and blue sensitive pixel is not relevant. Extracting green will not modify the levels.
Note you can set a custom warning level at the top right of the photometry tab. Once you have found a safe level, set this at the photometry tab and it will warn you when it exceeds this level. It will work on the calibrated images so it better to set this value on the saturation level minus the dark level. So if your darks have level 500 and saturation is at 16000, set it at 15500 or better 15000 to be save. Or measure the maximum value of a calibrate image directly and set this value minus an extra safety margin. Once a star is identified as saturated, its value will be made invalid in tab photometry.
Great, I have set the saturation level as you suggested. I have a better understanding now of how to read the ADU values. I will modify my routines and break out the grreen channel before I do any calibrating. Even though after separating the green channel, the display continues to show three values I will no longer worry about the Red & Blue numbers.
Next clear night I will give it another shot ... Thanks again !!!
Sam