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Showing results for tags 'fwhm'.
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Hi all, last night i had an incredible seeing with a fwhm value of about 0.8 (800mm fl , qhy 8 w 6 megapixel). That is by far the best value i have ever reached. (yes , i am still learning. ..) Questions to the pros on sgl: do we have to have temperatures below 0 centigrade for great seeing? And how is it possible to have great seeing when there is still a lot of humidity in the air? (i do understand that temperatures below freezing are good because there is far less movement in the air and that humidity drops when it starts to freeze. .. but i think there might be more to this???)
I was wondering how my focusing and guiding is working and wanted to know if I´m within reasonable range of what I can acchieve. My camera is a Canon 550D, pixel size 4.3 micrometers Scope is a Skywatcher 150-PDS, 750 mm FL. When I analysed one of my subframes (300 second exposure) in Pixinsight using the FWHMEccentricity script I get the following values: Median FWHM: 2.892 px Median eccentricity: 0.4329 MAD FWHM: 0.3114 px MAD eccentricity: 0.1106 MAD residual: 1662 DN Star support: 10059 This was one of the better subs that night. If I check the stacked image (38 frames) I get Median FWHM of about 3.26. If I´m not mistaking my resolution is 1.18 arcsecs/pixel so that would mean a FWHM of 3.41 arcsecs. Is that OK considering my gear? What would be the best one could hope for? Attached is a 100% crop from one of the better subs that night. It was quite windy so I´m glad they turned out so well. Only had to discard 10 or so from the entire night. My wind shield seems to work
I have yet to be entirely happy with the focus on my pictures. When cropping them quite heavily (I use a DSLR, so smaller DSOs need a bit of cropping) the stars always feel a bit bloaty and look out of focus - much more than the image from the full frame. I have been using the focus aid in APT, checking out the FWHM tool. I don't know much about the science behind it, but my understanding is that the number needs to be as low as possible. Last night I was focussing and tweaked it down to around 1 to 1.5 - obviously it was fluctuating with the atmosphere/interference, but I thought that it was a pretty good score. I locked the focus very carefully, making sure that I didn't change the figure - I usually tighten half-way, check the value and then re-do it (if needed) before locking it down completely. I only twigged tonight that DSS shows a FWHM maximum on the light frames after it has checked them and, when I looked for the images I took last night, the FWHM figures were coming out at over 4.5. I am presuming that this is far from ideal focus, but is also a long way from what I thought it was. Is there any relation between the FWHM measurements in APT and DSS? Is there a reason for the discrepancy or am I just going to need to be even more careful with focussing? Are there vital settings in APT that I need to set? Will check for more scope related settings next time I get out. Might also need to get the Bhatinov mask out rather than relying on just APT Focus Aid. Thanks, as ever, for any thoughts