Recovery from lost guide star?

[Mark Mayer]

When PHD2 loses its guide star (with multi star guiding turned on) will it keep searching search for a new guide star, for example when guiding is blocked by clouds, or if the mount moves unexpectedly, or is the observer required to manually intervene?

How this is handled has a big impact on automated astrophotography sessions, and I need to learn more.

[Bruce Waddington]

Hi Mark.  Here’s the explanation from the manual:

 

 

You should be using an automation app that can take sensible actions for lost-star events.  If the star is lost for an extended period of time, your mount is likely to drift significantly far off-target.  In that case, resumption of guiding by PHD2 when any guide star enters the search region may not get you the results you want.  The automation app needs to do things like plate-solving and re-centering of the target before resuming the imaging session.

 

Regards,

Bruce

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[steve]

On 23/5/22 6:15, Bruce Waddington wrote:

How this is handled has a big impact on automated astrophotography sessions, and I need to learn more.

Hi

For both local and remote sessions, we use EKOS. It will recover from lost guide-stars, re-centre, re-focus and restart capture.

Perfect for nights with drifting cloud. Reliable meridian flips complete the scenario. All you to get some sleep:)

Cheers and HTH

[Mark Mayer]

Hi Bruce,

Thanks for the tips. One more question:

Does the size of the tracking rectangle impact the accuracy of tracking? e.g. will the star wander inside a large rectangle, and increase rmsd?

I do use a dark library and plate solve during sessions. Need to find out from Robin (SharpCap) how he handles lost star events

because as you note, if it goes on too long and the mount drifts, PHD2 will not be able to locate its guide star.

Currently I intervene manually, but that means having to babysit the session.

Mark

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[Bruce Waddington]

Hi Mark.  The tracking region size has no effect on the guiding accuracy.  The size has two direct effects and some less-obvious implications.  The direct effects are these:

1. After a full-frame auto-find is completed, each guide star in the list will be centered in its own tracking region.  With each subsequent guide camera exposure, PHD2 will look only in those tracking regions to re-locate each star and compute how far it has moved since the previous exposure.  Obviously, as the region sizes increase, the processing time needed to do this will increase.

2. If sub-frames are being used for single-star guiding, only the data within the tracking region is downloaded from the camera.

Larger tracking regions have some other, subtle effects.  As the region gets larger, there is a greater likelihood that multiple stars will fall within the boundaries.  Auto-find is generally able to sort these out and tries to avoid stars that are too close together, but there could still be some potential for "hopping" between guide stars.  There is also a higher likelihood that un-calibrated sensor noise including alpha-particle hits will fall inside the region and interfere with guiding.  On the flip side, there are probably only two benefits to increasing the region size.  If the mount is subject to wild excursions in guiding, for example because of serious cable snags, there is less likelihood that the guide star will be lost.  If the guide star is lost, there will usually be intervention needed to restore guiding either by the automation application or by the user.  Second, a larger tracking can be of temporary benefit for doing long-interval Guiding Assistant measurements or sometimes for measuring the declination backlash.   After the GA session is complete. , the region size should be restored to the lower value.

Bottom line, there isn't usually any need to change the region size from its default value and the guiding performance isn't affected by the region size.

Hope this helps,

Bruce

[Pict...@earthlink.net]

Is it the case that tracking region size is measured in pixels and not arc-seconds so that if one changes the guide camera to a different camera with different size pixels this may lead to a different effect in guide star acquisition and persistence?  Generally, I believe the observer would think in terms of arc-seconds and thus may have to adjust the tracking region size when changing guide cameras to achieve expected results.

Manning B