matt_d Is there a maximum RMS error/pixel scale that you find to be acceptable to get the most out of your glass?
I find that how well you need to guide depends most on two things. One is the spot diagram of the glass, and the other is the "seeing."
If all the random processes are independent, you can basically sum all the variances. I.e., the resultant RMS (which is basically the standard deviation) is simply sqrt( rms12 + rms22 + rms32 + ...) where rmsx is the RMS (standard deviation) of each thing that contributes to the error, and since standard deviation squared is the variance, the different variances simply add if they are independent events.
That is why "total rms" is just sqrt( rmsRA2 + rmsDeclination2).
Centroid estimation has its own RMS error, etc etc. And so is atmosperic turbulence.
Now, we look at the spot diagram of OTAs (I do not buy an OTA that has no published spot diagram). You will find something from SharpStar/Askar OTAs to have chromatic spot diagrams of the order of 30 arc seconds even at the optical axis. This is the spot diagram of an Askar FRA600 at the optical axis (notice the blue bloat):
Obviously, if the seeing contributes 3 arcsec and the autoguiding contibutes 1 arc sec, their total contribution is tiny (since we square the RMS values) compared to the already bloated 30 arc seconds. I.e., you will only bloat the total RMS to sqrt( 900 + 9 + 1 ) = 30.2 arc sec. But this is because the spot diagram is so large to start with.
But if you take Japanese optics with 10 arc second type spot diagrams, then the total RMS becoms sqrt( 100+9+1) = 10.5 arc sec. I.e., you have bloated the star by a larger 5%. This is the FSQ-106 (similar aperture as the Askar above) (note that the side of the square is 100µm instead of 200µm for the Askar spot diagram):
(Notice lack of blue bloat.)
This is the spot diagrams for the FOA-60Q (has the best Strehl number for Takahashi OTAs):
Notice that the on-axis spot diagram is even better than the FSQ-106. It has a tiny aperture (60mm) though, and a large f-number(f/15), so I only use one for solar work (which ironically, don't need autoguiding)
So, how well you need/want to guide depends on the rest of your equipment (not even accounting for how well they are collimated by technicians before shipping it to you).
Similarly, if the seeing is 3 arc seconds, then trying to autoguide to 0.3 arc second is wasted.
Chen
