AstroBen60 I mean having a general 0.3asec/" derivative and peaks like 0,8"/sec
Hi Ben,
You would have to set guide parameters (such as guide rate, max pulse duration, etc) based upon the worse of the two (actually worse possible, which is probably not shown in the piece of paper, since it emphasize the peak amplitude [mostly dependent on fundamental] and not the region of worst case slope [highly dependent on the higher harmonics] -- this is why I'd recommend doubling the number that you can see on the piece of paper, to be safe).
The 0.8 arcsec/sec does not bode well (read on further down this post, since you also mentioned a 0.21 arcsec/sec number). The reason is that if you were to use a 1 second guide exposure, a guide star (or stars) would have moved by 0.8 arc seconds within that exposure. You now have to measure the centroid of that small streak instead of the centroid of a fixed star.
Using a 0.5 second guide exposure would reduce that streak to a more acceptable 0.4 arc seconds. I would gently suggest that you try a shorter guide exposure, as long as your guide instrument is up to it. To use 0.5 second exposure, you will definitely need to use multiple-star centroiding (averaging the centroids of multiple stars reduces the variance of the "seeing" error by two for each doubling of stars, if they all have the same weight in centroid computation). You will need a sensor with a large FOV so that it maximizes your chance to see more stars. And you need a larger aperture since you need a larger light bucket.
As an aside, I had for a couple of years been using a Borg 55FL objective as my guide scope (200mm focal length or 250mm f.l. depending on whether the reducer is used; thus f/3.6 or f/4.5) and an ASI178MM as the guide camera that has a decent sized FOV compared to what others typically use as their guide camera. For the past month, I have been experimenting with using a 40mm aperture, 180mm focal length as a guide scope (with same ASI178MM) to see if I can use a camera with a smaller "light bucket," but with a larger FOV to try to find as many guide stars -- that FMA180 appears to work quite well -- ASIAIR could always find 12 stars even in star-starved regions near M81/M82 with a camera gain of 250 (analog gain of 25 dB, which appears excessive, but is fine). So, a smaller and lighter guide instrument that works just as well as my earlier setup. I had to drill some holes in the EAF to mount one on the FMA180(pro), but just a 30 minute exercise (helical focusers are impossible to get pinpoint guide stars without some kind of aid).
Just remember that guide exposures never produce pin-point stars. The star is moving (due to the slope of the PE curve) while exposure is occurring. The best you can do is to focus really well so at least you reduce other centroid measuring errors. This is that "sawtooth" that I keep referring to.
Now, your 0.21 arcsec/sec is much more encouraging, and lines up with what others are seeing with their 430 second periods.
With that kind of slope, you should be able to use short max pulses of around 150 ms even with a guide rate of 0.25x sidereal rate, and a 0.5 second exposure (2 FPS). After discovering that sawtooth graph, it occurred to me that reducing the guide rate will also reduce the sawtooth amplitude (and thus centroid estimation error), so I have been using 0.25x sidereal guiding since. It has been a rather major factor in guiding for me. Whether you can also use a slower guide rate will depend on the worse case slope of the PE curve, and it looks like your mount should be able to handle it (although I think Kevin has reported that ASIAIR cannot choose 0.25x sidereal guide rate with ZWO's own mount; so you may be out of luck there if you use ASIAIR -- try INDIGO Sky instead of ASIAIR if you want to stick with a small microprocessor as your controller).
Good luck.
Chen
