Use Display Stretch - Autofocus
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kaymann
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Use Display Stretch - Autofocus
Recently I have had autofocus tell me there were no stars to be found when I could see several dozen stars that would normally be accepted. I could not figure what was going wrong so I unchecked "Use Display Stretch" and bam plenty of stars and focus went to work. So what is that option and when would it be appropriate to use? BTW I looked in the manual and could not find anything...
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admin
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Re: Use Display Stretch - Autofocus
Hi,
when that option is checked, the star detection first stage (finding the bright points in the image) uses the stretched image rather than the raw image from the camera. This *usually* helps when you want to find faint stars, but obviously how much it helps (or not) depends on how much stretch is in use. Later stages of star detection (like measuring the FWHM) do use the unstretched version of the image, as using the stretched one would give incorrect results.
With those options around star detection, there are no hard and fast rules - the options are there to adjust them to get a situation that works, which may vary depending on equipment, conditions, etc. One other thing to note is the 'sensitivity' of the star detection... I *think* that auto adjusts to some extent - increasing if too few stars are found and decreasing if there are too many. So, turning off the display stretch option could have then triggered an auto-adjustment of the sensitivity - maybe.
cheers,
Robin
when that option is checked, the star detection first stage (finding the bright points in the image) uses the stretched image rather than the raw image from the camera. This *usually* helps when you want to find faint stars, but obviously how much it helps (or not) depends on how much stretch is in use. Later stages of star detection (like measuring the FWHM) do use the unstretched version of the image, as using the stretched one would give incorrect results.
With those options around star detection, there are no hard and fast rules - the options are there to adjust them to get a situation that works, which may vary depending on equipment, conditions, etc. One other thing to note is the 'sensitivity' of the star detection... I *think* that auto adjusts to some extent - increasing if too few stars are found and decreasing if there are too many. So, turning off the display stretch option could have then triggered an auto-adjustment of the sensitivity - maybe.
cheers,
Robin