AstroDude42 On another scope i use the ASI 290MM Mini (2.9 μm pixel size, 1936 x 1096 pixels). So according to your findings, would it be better to use the 290MM Mini and Bin2 for guiding with the EdgeHD/OAG?
Hi Olli,
With a lighter load (FSQ-85) and a 55mm/250mm guide scope, I had been able to get around RMS error of 0.45 arc second when I use the ASI290MM (not the mini, but the USB 3 version of that camera) with ASIAIR multiple star guiding. Your load will push an RST-135 past the limits (it will slew fine, but may not guide that well).
When I use a 685nm IR pass filter on the guide scope, I can get a bit better than 0.4" RMS guiding, sometimes even 0.25" RMS. But the sensitivity with ASI290 is lower with the IR cut and it does not pick up enough stars for some regions of the sky. (It is not much better with my ASI462 due to the Bayer filters on the color camera.)
If you are using an instrument with 1500+ mm focal length, you will probably also want a longer focal length guide scope than what I use. Or use an OAG. This is where you want larger pixels than 2.9 µm to get better SNR (and for multi-star guding, better SNR).
I do not have my ASI485 yet (I expect it within a couple of days since my dealer just received stock yesterday), but that camera (assumuing ASIAIR uses mono-binning for guiding -- it would be stupid not to), with 2x2 binning, might be good for the long focal length case since the 485 has a much larger sensor area than the tiny 290/462.
Given the large dRA/dt of the periodic error of the RST-135 (PE of about 70 arc sec peak-to-peak, literally worse than the periodic error of a cheaper AZ-GTi), you definitely want to sample a guide frame with very short exposures. Basically, there are two regions on the sinusoidal (PE of harmonic drives are quite sinusoidal) where dRA/dt is so large that a guide star will appear as a short streak when you use 1 second or 2 second exposures. It is not easily visible, but when you are trying to guide to sub-pixel values, it can ruin guiding.
In the past (before binning), I had been using 0.5 second exposures on ASIAIR even though I could not get better than 1 FPS. That is to get rid of the dRA/dt problem (slope of the periodic error -- recall that the derivative of a sine is a cosine, and the period of the RST-135 is about 430.82 seconds).
For my short focal length guide scope, a 2x2 bin at 2 FPS will be a compromise between faster feedback versus a rougher plate scale caused by the binning. We'll see what happens (although I am perfectly happy with a 0.4" type guide error) when I have a chance to try binning. If you are using an OAG, I think it is almost a no-brainer that binning will be better -- as long as you have a large enough sensor size.
Chen