New Feature : Improved Star detection in Multi-star FWHM Focus
Posted: Sat Apr 10, 2021 4:42 pm
Hi folks,
this week's update has been delayed as I have spent a considerable amount of time working on an improved star detection routine. So far this is active in Multi-star FWHM focus only, but if it is successful then I will expand the use to other areas such as live stacking and polar alignment.
The goal was to both increase the sensitivity of star detection (to allow stars to be detected in situations where they previously could not) and to simplify the various options associated with star detection - to make it more likely to 'just work'.
What are the changes in the new version?
1) There is no need to specify a 'black level' value any more. In the old star detection this told the code to ignore the dark background of the image (below the 'black level'). The new code works out the background automatically (and also takes account of variations in the background across the image).
2) There is no 'minimum star width' setting any more - this was used to try to eliminate hot pixels, but now there is a simple check box to suppress hot pixels which can be used with noisy cameras.
3) There is no 'digital gain' any more - setting this wrong could cause more problems than it solved and you had no way of seeing the effects.
4) Instead of digital gain there is an option to 'Use Display Stretch' this means that if you have made stars show up by using the stretch functionality of the mini-histogram, the star detection code can be set to use the stretched image instead of the unstretched image.
5) The code now calculates something closer to the HFD (half flux diameter) than the FHWM. The HFD can be used for all the same calculations as FWHM, but is usually considered to be a more stable and robust measurement of star size. I haven't changed the name of the measurement though - that would just confuse people.
6) Because HFD is better than FWHM, there is no Minimum brightness any more - HFD allows a reasonable estimate of the size of a star no matter how dim it is. There is still a maximum brightness because taking measurements from saturated stars is inaccurate.
7) There is a Sensitivity control - turn this up to detect more stars, turn it down to detect less. Like any sensitivity, if you turn it too high then you could start detecting things that are not stars (hot pixels, warm pixels, noise, etc). Under certain circumstances, SharpCap can detect that this has been set too high (basically if the number of detected stars is very large). When this happens, SharpCap will automatically turn down the sensitivity 1 point per frame until a reasonable value is reached.
8) There is still a noise reduction control. I really wanted to get rid of this, but sometimes when the image is very noisy you need to reduce the noise to get star detection to work. Adding noise reduction all the time tends to cause problems in less noisy images with small stars. In the end there still needs to be a noise reduction control, but it should only be used as a last resort.
9) The new code can detect much bigger (further out of focus) stars than the old code - I have seen out-of-focus samples work out to 20-25 pixel FWHM. Because of this the 'No stars found' value for the focus graph has been changed from 20 to 50 - it needs to be a value higher than any reasonable real measurement.
10) The number of stars needed for a measurement has been reduced from 10 to 5.
Some guidelines for using the settings:
Start with the defaults :
Suppress Hot Pixels : On
Use Display Stretch : On
Sensitivity : 75
Max Brightness : 95%
Noise Reduction : 0.0
In most situations, only adjust the sensitivity
Turn it up if not enough stars are detected. If too many stars are detected, or noise/hot pixels are detected as stars, or star detection reacts very slowly then turn the sensitivity down.
If your stars are mostly very faint, use Display Stretch
Applying a display stretch to brighten the stars helps the star detection code find them. This can be very helpful for finding faint stars (particularly out of focus ones). Note that the measurements of FWHM/HFD are based on the true pixel values, not the stretched ones.
Other tips:
If your frames are very noisy, use the noise reduction option. A little extra noise reduction can also improve the speed of star detection if it is slow
If you are using a large pixel camera that is undersampling, you may get very small stars - you might need to turn off hot pixel suppression
Very keen to hear any positive or negative reports on this.
cheers,
Robin
this week's update has been delayed as I have spent a considerable amount of time working on an improved star detection routine. So far this is active in Multi-star FWHM focus only, but if it is successful then I will expand the use to other areas such as live stacking and polar alignment.
The goal was to both increase the sensitivity of star detection (to allow stars to be detected in situations where they previously could not) and to simplify the various options associated with star detection - to make it more likely to 'just work'.
What are the changes in the new version?
1) There is no need to specify a 'black level' value any more. In the old star detection this told the code to ignore the dark background of the image (below the 'black level'). The new code works out the background automatically (and also takes account of variations in the background across the image).
2) There is no 'minimum star width' setting any more - this was used to try to eliminate hot pixels, but now there is a simple check box to suppress hot pixels which can be used with noisy cameras.
3) There is no 'digital gain' any more - setting this wrong could cause more problems than it solved and you had no way of seeing the effects.
4) Instead of digital gain there is an option to 'Use Display Stretch' this means that if you have made stars show up by using the stretch functionality of the mini-histogram, the star detection code can be set to use the stretched image instead of the unstretched image.
5) The code now calculates something closer to the HFD (half flux diameter) than the FHWM. The HFD can be used for all the same calculations as FWHM, but is usually considered to be a more stable and robust measurement of star size. I haven't changed the name of the measurement though - that would just confuse people.
6) Because HFD is better than FWHM, there is no Minimum brightness any more - HFD allows a reasonable estimate of the size of a star no matter how dim it is. There is still a maximum brightness because taking measurements from saturated stars is inaccurate.
7) There is a Sensitivity control - turn this up to detect more stars, turn it down to detect less. Like any sensitivity, if you turn it too high then you could start detecting things that are not stars (hot pixels, warm pixels, noise, etc). Under certain circumstances, SharpCap can detect that this has been set too high (basically if the number of detected stars is very large). When this happens, SharpCap will automatically turn down the sensitivity 1 point per frame until a reasonable value is reached.
8) There is still a noise reduction control. I really wanted to get rid of this, but sometimes when the image is very noisy you need to reduce the noise to get star detection to work. Adding noise reduction all the time tends to cause problems in less noisy images with small stars. In the end there still needs to be a noise reduction control, but it should only be used as a last resort.
9) The new code can detect much bigger (further out of focus) stars than the old code - I have seen out-of-focus samples work out to 20-25 pixel FWHM. Because of this the 'No stars found' value for the focus graph has been changed from 20 to 50 - it needs to be a value higher than any reasonable real measurement.
10) The number of stars needed for a measurement has been reduced from 10 to 5.
Some guidelines for using the settings:
Start with the defaults :
Suppress Hot Pixels : On
Use Display Stretch : On
Sensitivity : 75
Max Brightness : 95%
Noise Reduction : 0.0
In most situations, only adjust the sensitivity
Turn it up if not enough stars are detected. If too many stars are detected, or noise/hot pixels are detected as stars, or star detection reacts very slowly then turn the sensitivity down.
If your stars are mostly very faint, use Display Stretch
Applying a display stretch to brighten the stars helps the star detection code find them. This can be very helpful for finding faint stars (particularly out of focus ones). Note that the measurements of FWHM/HFD are based on the true pixel values, not the stretched ones.
Other tips:
If your frames are very noisy, use the noise reduction option. A little extra noise reduction can also improve the speed of star detection if it is slow
If you are using a large pixel camera that is undersampling, you may get very small stars - you might need to turn off hot pixel suppression
Very keen to hear any positive or negative reports on this.
cheers,
Robin