Quote:
Originally Posted by DiscoDuck
BTW, you mention the fact that stars are elongated when it's not guiding well. Could it be argued that it's the FWHM that matters at the end of the day, e.g. you could get round stars when RA is a bit loose in guiding by deliberately making your dec guiding equally as bad! Ideally we want small FWHM in both axes, and that, I'd suggest, should be the goal rather than just trying to make them the same value and have round stars? OK, I'm being pedantic, and both approaches are trying to get to the same end, but I'd suggest we shouldn't be assessing "are the stars round" to see if our guiding is good, but "are the stars small"? In my case, it makes a difference, as I tend to have slightly rubbish guiding in BOTH dec and RA, so get round stars - though a tad wider than they should be! Just an idle thought on a late Friday arvo!  |
totally agree. You can for example get nice round stars by defocusing them a bit, but that really isn't the point. Similarly, one often sees tracking performance assessed on the basis of how flat the phd2 graph is. You can get the graph as flat as you like if you make the exposures long enough (eg to cover a full worm cycle), but all that will mean is that the guiding system is not making any corrections - not that there is nothing to correct. I am quite happy to see phd2 jumping about all over the place if it ends up with better stars (ie, if in the process of following the seeing, it also corrects some mount errors).
Agree that minimising and equalising FWHM in both axes is the goal and this should be the basis for assessing guiding solutions.