Kevin_A If I run at 1s at 500ms on dec and ra at 40% aggression I can change it to 80ms and absolutely no difference.
Hi Kevin, remember that the 80ms number is the max duration of a pulse. If the autoguider never asked for more than 40ms (for example), then setting it to 80ms or 2000ms will not change anything, since there is nothing to limit in that case.
I can run at 0.5s up to 3s and always the same 0.7 to 0.8 rms.
Yeah, this is the weird part.
With the rapid changes in the periodic error (i.e., large slopes caused by the higher harmonics), the error should improve from the 0.7" down to the 0.3" to 0.4" range that your mount should be capable of.
A 3 second guide exposure would allow your 0.16" slope to move by 0.16 x 3 arc seconds before any correction is issued, or around 0.5 arc seconds; and that is just for the RA axis.
If the declination axis has the same amount (but independent) of 0.5" RMS error, then the total would come to be 0.707" total RMS... (RMS is the square root of the sum of squares), or close to what you are seeing.
Lopping that exposure time down to 1 second should have limited the excursion from 0.5" down to 0.16", and make the error be limited by other things than the mount's PE itself.
The fact that you are not seeing an improvement points to something else being the "tallest tent pole." I.e., something else is causing a 0.5" type error (in the RA axis) so that shortening the exposure time did not improve anything.
Asiair always just picks 6-8 stars, unsaturated but not very sharp stars in the box.
And that may be the problem.
Remember that some time back, I had shown a number of bar graphs showing the "typical" SNR distribution of stars. The ASIAIR uses SNR as a weight to determine the average centroid. And, given 6 stars, it is probably just the equivalent of using the uniformly weighted centroid of only 2 stars. I.e., you are getting very little improvement from multiple stars, and that could be the "tallest tent pole."
On an ASIAIR with a 55mm aperture guide scope at 200mm focal length (f/3.6) and an ASI178MM at a gain of 250 (analog gain of 25 dB) and an exposure time of 0.5 seconds, I can consistently get 12 stars from it, even in the star starved region around M81/M82 where I was playing around last week when clouds were thinner.
Notice a couple of things in this choice. The guide scope is actually a Borg 55FL with its reducer, chosen to give me a large aperture (and flat enough to cover the camera). The ASI178MM was chosen to give me a large FOV (if all else is equal, the larger FOV means more stars to pick from). The camera gain is pushed enough so that singularly bright stars will saturate, and causes the one or two bright stars to not get included in the centroid average (otherwise the SNR weighting will reduce to just using that single one or two bright star for guiding).
So, I would recommend tuning your guide system so that you can get all 12 stars that ASIAIR will process (the most the ASIAIR will use; guide systems like DONUTS will use all stars in the FOV that it can see, and DONUTS also uses uniform weighting instead of SNR weighting).
If the centroid measurement is coarse, then you are limited by centroid measurements, not by the feedback loop.
Check too that your stars are not distorted towards the edge of the field. I don't think that is your primary problem, but it can be as you tune the guide system.
You may have been too timid with pushing up the gain to get the stars. Don't be. With the short exposure times (i.e., shorter than 10 seconds), you can pump the gain up quite a bit before the dark noise dominates over the read noise.
After you have the guide scope tuned to consistently get 12 stars (while ignoring saturated stars) the next thing to do is to reduce the guide rate to 0.25x sidereal on ASIAIR (the ASIAIR will not let you go below that). It was a serendipitous discovery when looking at the sawtooth graph, but so far has served me well since I started using it.
By the way Kevin, on a different topic, I took my own advice about drilling the EAF to keep it from falling below the declination plate for both my FSQ-85 (first image) and my FOA-60Q EAFs (second image).
So, I have at least 4 EAFs now that are mounted this "direct mounting" way.
Chen
