SharpCap Forums

Hi,
I am using this focus aid and generally find it best for my imaging. However, I did encounter a problem recently when amongst the stars automatically selected was included the nucleus of my target galaxy. As a result the FWHM results were quite poor even though I could judge from the small star images on the screen that I was close to focus. Is there a way of manually deselecting offending "stars", or alternatively just selecting manually the ones I want the measurement to be based on?
Thanks for any help you can provide.

Mike Hooper

Hi Mike,

in most cases this shouldn't cause a problem - as long as there are a good number of stars being detected in the image, SharpCap will ignore the ones with unusually large or unusually small FWHM values when calculating the average. So, if you had maybe 20 to 30 stars and the one galaxy nucleus has a much higher FWHM than the others, behind the scenes SharpCap should be discarding the unusally high FWHM value before averaging. If you have a relatively small number of stars being detected, this statistical technique may not be effective, so in that case try to make SharpCap detect more stars!

To actually not detect the galaxy, there isn't really anything direct you can do - the adjustments you have are the sensitivity and max brightness level - possibly turning down the max brightness might do the job if the core of the galaxy is near to saturated, but no real options beyond that.

Out of interest, was the unusually high value for average FWHM still showing a dip when in best focus? If so then it's still doing the job that is needed in terms of finding best focus even if the number isn't really representative.

cheers,

Robin

Hi Robin,
thanks for your quick reply. When this happens I will probably increase the exposure time to detect more stars. I cannot recall if the bar graph was trending into the yellow or green with a minimum value when the stars were visually in focus. All I recall was that the FWHM values were up to 50% higher when the galaxy nucleus was being selected, but there were probably only half a dozen stars also being selected at the same time. It was worse when I switched from L to RGB filters where less light was getting through.
Regards,
Mike

Hi Mike,

more stars should definitely help - with less than 10, the statistical tricks used to try to throw out the outlying values don't work very well. You could try increasing the sensitivity setting for the star detection too, and maybe turn on the 'use display stretch' option which can also help find more stars if you have the display stretch active.

cheers,

Robin

Hi Robin,
thanks again for your help. Live stacking has revolutionised my imaging and allows me to use my 20^ f/4 Obsession for imaging with very pleasing results. Thank you for this amazing invention!
Regards,
Mike