Long exposure FWHM varies dramatically from short exposure and Pixinsight SFS measurements

[Anthony Quintile]

I recently updated to the most recent version of Sharpcap.

Although there has always been some disparity between what Sharpcap calculates for subframe FWHMs versus what Subframe Selector measurements calculate in Pixinsight, the difference is much more dramatic now. Also, there is dramatic disparity between FWHM on short exposures for focusing in Sharpcap and those for longer exposure (180 seconds) subframes. Additionally, although this is somewhat subjective, my guiding results and third-party forecasts don't reflect abnormally bad seeing conditions, (nor does the final PI measurements as stated above).

The past few nights, I am getting FWHM values while focusing in Sharpcap (4 second exposures) of around 3.5 px/3.5 arcsec/px. (Image scale is 1.03"/px).

For subframes (180 seconds) the FWHM values calculated by Sharpcap are around 2px/2" greater, in the mid 5s.

Pixinsight SFS measurements for the same data show around 3.5" median value for 180 second subframes.

I am not sure how Sharpcap is calculating FWHM, but maybe it is influenced by the brightness of the stars or something? (longer exposure=brighter stars in the subframe).

I can get my head around it, but the high FWHM numbers are a bit unnerving and it makes it more difficult to determine whether or not the seeing is too rough to keep imaging or not.

Maybe I am doing something wrong or missing something?

Please let me know-

Anthony Quintile

[Anthony Quintile]

I realize that this may not be a "bug" per se. If I need to submit logs let me know, but I don't know that they're relevant in this case.

[admin]

Hi,

SharpCap does exclude stars that are fully or very nearly saturated from the FWHM measurement (if the star has saturated pixels then we don't know exactly what the 'M' maximum pixel value really is, so you can't properly measure FWHM). However, the biggest single cause of odd FWHM values is difficulty measuring the background brightness of the image near to the star - if the background is under estimated then the FWHM will measure higher than it should, over-estimation leads to the opposite. This effect is largest when faint stars are are being measured, since the background uncertainty is larger relative to the brightness of the star. Often you can get a more accurate measurement by reducing the star detection sensitivity so that fewer faint stars are being used as part of the measurement.

The main goal of measuring star FWHM in SharpCap is to provide a relative measure so that you can compare the quality of two frames either to check for seeing fluctuations, or during focusing, etc. For this purpose, the exact numerical value of the FWHM doesn't really matter so much, just that it moves in the right direction when focus or seeing change. That being said, if the value remains very different from other measurements once you have tweaked the settings to restrict to brighter stars, I would like to take a look at one or two frames just to see if I can understand why.

thanks,

Robin

[Anthony Quintile]

I'll go dig around, but where do I find the star detection sensitivity for Live Stacking FWHM Filter screen?

Is it the "Sensitivity" setting in the Multi star FWHM focus dialogue box? Or is there another place to set that for Live Stacking's FWHM Filter dialogue box?

I probably set that pretty high when previously using an OAG on an SCT. Currently I am using a 60mm f4.6 guide scope which shows a lot more stars than the OAG.

What is the default setting for "Sensitivity"?

Thanks for the info...

-Anthony

[Anthony Quintile]

Realizing now the OAG part is irrelevant, but the SCT part is. Was having a hard time finding enough stars to focus on with the SCT, set sensitivity higher...

[admin]

Hi,

you need to look in the 'Alignment' tab in live stacking (because the stars are primarily detected to align the frames, being able to measure the FWHM is a secondary thing). The default value is 75, or you can just use the 'Reset All' button on that tab to reset the star detection settings to default.

Another useful thing on that tab is the ability to highlight the stars that are being detected - this lets you check that you are detecting proper stars, not galaxies, hot pixels, etc.

cheers,

Robin