Robert Q. Kimball
Well-known member
I have just started taking LRGB images. I assume that on the RGB subs the stars shouldn't be saturated. How can I use PixInsight to tell whether stars are saturated?
Identifying saturated stars
- Thread starter Robert Q. Kimball
- Start date
ngc1535
PixInsight Ambassador
Hmmm..
A couple of points:
1. Saturated stars- really white clipped stars, will happen when brightening an image. Typically processing to protect the stars affects the object (nebula or galaxy). Many people handle things independently with masks and extracted images.My philosophy is to cater the processing to object- which might not be stars.
2. To your question- if you look at the image without the auto STF applied (just linear)- anything in the image that white or nearly white is saturated or nearly saturated (it depends on if the gain forces the ADU counts to fill the entire bit space, 16-bits generally).
3. Another way to map saturated values (or any value) is to create an image with just those values showing up. A pixel math expression can do this: iif($T>0.8, 1, 0) If you create a new image with Pixel math, this will make a black image with white pixels everywhere the value is above 0.8.
4. There are so many ways to do this... You could use range mask to do exactly the same.
But #2 is really the best answer... it can be done by inspection.
-adam
A couple of points:
1. Saturated stars- really white clipped stars, will happen when brightening an image. Typically processing to protect the stars affects the object (nebula or galaxy). Many people handle things independently with masks and extracted images.My philosophy is to cater the processing to object- which might not be stars.
2. To your question- if you look at the image without the auto STF applied (just linear)- anything in the image that white or nearly white is saturated or nearly saturated (it depends on if the gain forces the ADU counts to fill the entire bit space, 16-bits generally).
3. Another way to map saturated values (or any value) is to create an image with just those values showing up. A pixel math expression can do this: iif($T>0.8, 1, 0) If you create a new image with Pixel math, this will make a black image with white pixels everywhere the value is above 0.8.
4. There are so many ways to do this... You could use range mask to do exactly the same.
But #2 is really the best answer... it can be done by inspection.
-adam
Note that this is equivalent to just:
$T > 0.8
Robert Q. Kimball
Well-known member
Thank Adam for the clear answer.
wagonwheel
Member
In a similar vein to the OP's question, is it possible to identify what an individual star's ADU actually is?
In regards to observing exoplanet transits, I need to ensure that my target star remains within the linear range of my camera.
In regards to observing exoplanet transits, I need to ensure that my target star remains within the linear range of my camera.
ngc1535
PixInsight Ambassador
There are probably better photometric tools...but Dynamic PSF will certainly give you some good information when you click on a star.
Presumably you would be doing this when observing? Most acquisition software have tools for this as well.
You asked for a star's ADU instead of pixel. There is a difference. A star's "brightness" or magnitude is photometric measurement based on a defined region of the pixel array. The ADU of a single pixel can be seen just by putting the cursor on the pixel... or using the inspection readout mode (this is the best way for realtime feedback visually).
-adam
Presumably you would be doing this when observing? Most acquisition software have tools for this as well.
You asked for a star's ADU instead of pixel. There is a difference. A star's "brightness" or magnitude is photometric measurement based on a defined region of the pixel array. The ADU of a single pixel can be seen just by putting the cursor on the pixel... or using the inspection readout mode (this is the best way for realtime feedback visually).
-adam
wagonwheel
Member
That's very helpful, thank you!