SFn8CieR Not sure what he means by using timelapse?? but it is able to be autoguided using 2 to 3 sec exposures with less pulse which is what you are saying.
He also needs to learn the difference between "less" and "fewer" :-). In math, we knew it as countable vs uncountable sets :-).
Like you, I have no idea what he means by timelapse. My guess is he wants you to use 2 to 3 second exposures, but do not talk to the mount that often. Perhaps talk to the mount every two exposures.
Yeah, he is claiming that the mount is good enough that you only need to now and then give it a kick. Good, large legacy mounts are like that. You only need to occasionally steer it just a little in case a polar misalignment is causing the mount to drift away over time. Encoders don't track the sky, it tracks a little glass plate :-).
I have been telling people exactly this when they ask about encoders. Both cases uses some feedback from a known source. In the case of an encoded mount, it is coming from something local in the mount, and in the autoguiding case, the feedback is coming from the stars. The latter is actually far superior (much higher resolution than any encoder man can build) if only there is no atmospheric turbulence. Multistar is an effort to reduce the effect of atmospheric turbulence by averaging the centroid of many stars to reduce the variance of the error (with equal weights, the average of two stars (since the are acctected by different pockets of turbulence) reduces the variance by two, etc. So, given enough stars, there is no affect from turbulence.
The problem with something like ASIAIR multi-star is they use Signal-to-Noise ratio weighting -- so the autmospheric turbulence on brighter stars have a bigger influence on the error than dimmer stars (lower SNR). This is why I recommend lots of gain for the guide camera. That would clip the brightest stars and they get dropped from being used. The remaining stars have a more uniform SNR distribution.
If you don't do this (adding an extra 10 dB of gain), what appears as 12 stars is only equivalent to using 1 to 3 stars -- you might as well use single star guiding. Once the brightest two or three stars are removed, 12 star multiguiding is probably equivalent to something like using 6 stars on ASIAIR. Much better, but it would be even better if they had not use SNR-weighted centroid averages.
Chen
