Hi All
Very new SC user but very impressive. I have a QHY600 which I will use for, amongst other things, lucky imaging and photometry.
Accurate photometry relies on excellent linearity. The QHY600 delivers this, as measured by the SC sensor analysis, very well. However it is possible to correct for linearity. I have done this with the output of a single run of the sensor analysis using excel. I was able to reduce the small non linearity.
However I would like to make multiple linearity runs and be able to average these runs together to improve the reliability of the results.
Is there a way for sharpcap to automate the acquisition of multiple linearity measurements? Perhaps at each exposure level SC could take multiple images and take an average or similar statistic.
Can I also ask about the method and mathematics used to create the linearity measurements.
Many thanks for the help.
Paul
Linearity Measurements
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Re: Linearity Measurements
Hi Paul,
the full analysis procedure can only be kicked off from the UI, but if you're just interested in measuring sensor response then you could build something using the SharpCap scripting language (which is Python-based) if you have any experience with programming. There's a whole sub- forum which is a good place to go looking for people who've done something similar already to steal (sorry repurpose) their code - viewforum.php?f=14
The actual mechanism for measuring the linearity is fairly simple – SharpCap initially works out roughly the exposure needed to get you a histogram peak at about 65%, from which it can estimate what the saturation exposure should be. Then, starting from a low exposure value (1% of estimated saturation), SharpCap gradually increases the exposure. As each data point is added, SharpCap performs a linear regression of the ADU values against the exposure times. The linearity limit is measured as the last point before the one where the deviation of the measured ADU from the linear fit exceeds 0.5%.
Note that it's obviously very important to have constant illumination during this measurement. Some people are suggested using phone or tablet displays as a source of illumination for sensor analysis, but that's a really bad idea because the brightness tends to vary in response to ambient light levels. To get the very best results from this you should also make sure that the area that you are measuring is subject to even illumination – if some parts of the area are brighter than others then it will likely affect the are to measurement than any of the others.
Also, you might be interested to know that SharpCap takes three brightness measurements at each exposure when measuring the linearity and averages those three.
Cheers, Robin
the full analysis procedure can only be kicked off from the UI, but if you're just interested in measuring sensor response then you could build something using the SharpCap scripting language (which is Python-based) if you have any experience with programming. There's a whole sub- forum which is a good place to go looking for people who've done something similar already to steal (sorry repurpose) their code - viewforum.php?f=14
The actual mechanism for measuring the linearity is fairly simple – SharpCap initially works out roughly the exposure needed to get you a histogram peak at about 65%, from which it can estimate what the saturation exposure should be. Then, starting from a low exposure value (1% of estimated saturation), SharpCap gradually increases the exposure. As each data point is added, SharpCap performs a linear regression of the ADU values against the exposure times. The linearity limit is measured as the last point before the one where the deviation of the measured ADU from the linear fit exceeds 0.5%.
Note that it's obviously very important to have constant illumination during this measurement. Some people are suggested using phone or tablet displays as a source of illumination for sensor analysis, but that's a really bad idea because the brightness tends to vary in response to ambient light levels. To get the very best results from this you should also make sure that the area that you are measuring is subject to even illumination – if some parts of the area are brighter than others then it will likely affect the are to measurement than any of the others.
Also, you might be interested to know that SharpCap takes three brightness measurements at each exposure when measuring the linearity and averages those three.
Cheers, Robin