[g__day (Matthew)]

Playing round with this tonight - taking 10 minute shots of M16 and looking at how well the faints double stars are resolved - I confirmed something the Jim McMillian's notes on guiding posited; chasing seeing errors swamps all other sorts of guide errors (especially at longer focal lengths).

PHD defaults to about 0.15 pixel movement before a guide pulse is sent. Now 2*2 binned at full focal length on a C9.25 works out to be each pixel on my guide camera sees 2.2 arc seconds of sky. So I have to set pixel movement before I guide pulse is sent to 1.15 pixels.


1. Drift is likely < 0.10 arc seconds / second
2. PE is likely < 0.067 arc seconds / second
3. Seeing < 2.0 arc seconds / second

So Jim walked through most guide errors are likely to be chasing the seeing guide errors – typically indicated by periodic oscillations on your guiding graphs. Assuming your guide star is bright enough, and that you take shots between 2-3 seconds to optimise Signal / Noise in guiding shots, Jim had the following recommendations:

1. Set RA Aggressiveness relatively low to significantly reduce errors
2. Set RA Maximum move as low as possible
3. Set minimum move high enough to ignore corrections smaller than seeing corrections

So this implies many corrections are chasing the seeing. I typically set minimum drift before a pulse is issued to 0.2 pixels (but at my imaging 2.2 pixels an arc second – Jim would be advising my setting to be more like 0.8 – 0.9 pixels).

So I tired tonight varing minimum pixels from 0.6 up to 1.2 pixels - the absolutle sweet spot was 1.15 pixels - stars were beautifully round and resolved.

Too I set RA Aggressiveness to 50%, RA Hystersis to 25%, Max Dec duration to 80 msecs, and took 3 second guide shots (on a Meade DSI II Pro mono). The OSC index crept down as the min pixels before a guide is issued raised, but the RMS (v1.10.8) moves randomly between 0.80 - 0.90.

A give way should have been when you first select a star - prior to calibration - if it jumps alot - that's seeing - and you shouldn't try and guide that out!


Matthew

PS

Watching a star for 30 secs - without guiding - leet me see this much stare wobble (atomspherics)

My data from just watching a star (not guiding)

Star wobble - not guiding

Time X Y
3 309.3 287.5
6 309.4 286.8
9 311.8 286.3
12 309.2 286.4
15 310.8 285.9
18 309.7 285.7
21 309.3 285.5
24 311.2 285.8
27 309 286.3
30 308.4 285.7

Mean 309.8 286.2
Std Dev 1.03 0.58



That Std Dev should give me a hint to my pixel size before guided pulses are sent. The default of 0.15 pixels on my rig - chasing that seeing - would kill good guiding!