Quote:
Originally Posted by Merlin66
Ken,
Thanks for that.
Yes, I agree the PHD2 guys are very helpful. They implemented the spectroscopic slit guiding following my request.
The reason behind my questioning the exposure control was that in spectroscopy we generally don't have an option of guiding on fainter stars if the target star is too bright. It would sometimes be easier to reduce the exposure to give "better" results. I then worry about chasing seeing at 0.5sec etc. exposures.
(I have to say I have resorted to guiding on the secondary reflection from the slit plate to over come this.)
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What you could try then is to lower your gain (if possible) and if still too bright then reduce your exposure time. The use the Z-filter algorithm and apply a generous exposure factor. I designed this filter with fast sampling rates in mind (my mount is an Avalon and it works best with low latency/frequent corrections). As a "proper" frequency domain filter the z-filter lets you choose the cutoff frequency via the exposure factor to suppress high frequency noise from seeing. It is calibrated (roughly) so that the response is equivalent to the hysteresis algo with default settings operating with an exposure time of exposure factor x exposure time (or more correctly the sampling rate). You can tune the exposure factor to optimise removal of PE on the RA axis vs chasing the seeing. On the dec axis you can be quite aggressive with the exposure factor since there is no periodic error. This has the beneficial side effects of reducing the number of correction reversals and, in some situations, causing the dec guide graph to be offset from the axis when the corrections just balance out the PA drift. This results in almost no dec reversals and related backlash issues.
The MinMo setting also operates differently. In other algos the MinMo applies to the input deviations and acts as a crude filter. In the Z-filter is applies to the corrections so that miniscule corrections are not sent to the mount.
I'm currently developing the "image guiding" algorithm which may be helpful when the guide star is saturated. Still early days though and initial testing is not encouraging. The algorithm is fine but performance with sparse, noisy images is not good as it guides on the fixed pattern noise rather than the star.