SharpCap Forums

Hello,

I have been live stacking with SharpCap for a couple of years now. I'm curious if I need to make any adjustments or even use the FWHM filter.

According to astronomy.tools calculator, the ideal pixel size for OK Seeing (2-4" FWHM) seeing is: 0.67 - 2" / pixel. My setup is 1.19". Perfect! The ideal pixel size for Good Seeing (1-2" FWHM) seeing is: 0.33 - 1" / pixel. I am at 1.19" so I'm slightly under-sampling.

Should I be making any adjustments in the FWHM filter settings? I've never done it and wonder if it's something that will improve the image.

Thank you!

Scott

Hi Scott,

the best thing to do is to watch the graph for a while - if there are frames that have a significantly worse FWHM than others then switch to the option to view the frames being added to the stack rather than the stack itself. If you can see that the higher FWHM frames are visibly worse than the lower FWHM ones (perhaps due to poorer seeing, tracking wobble, etc) then applying a filter may help. If you struggle to see any difference or the FWHM values are very close then there may not be anything to be gained.

cheers,

Robin

Just adding in a reply here because I do use the FWHM (and brightness) filters routinely as a prefilter and only save the raw frames that pass these two filters. I also use the SC FWHM tab to keep an eye on what is happening to FWHM during capture sessions. But the usefulness or otherwise of these tools may come down to weather? i.e.

For where I am in the UK even on a relatively clear night there are typically clouds passing over quite quickly. Quite often I start a capture and it can take say a couple of hours to get only 30 minutes of capture between bands of cloud. It saves a lot of storage space and processing time to pre-filter out the dross. Thin high cloud tends to blur things and raise FWHM - thicker cloud has more obvious effects! The other main one for me is dew where a gradual general increase in FWHM can signal that it is time to get out and clear the secondary mirror using a hair dryer. ...so it is usually well worth monitoring FWHM for this reason (as well as for any gradual temperature-driven need to refocus)

If however I lived in a dryer or more desert-like place then I dare say that that I would use the tools a lot less

Tim

Thank you Robin and Tim for the information! Makes total sense. I really appreciate your help!

Scott