I have been chasing a problem in my guiding for several hours yesterday night and tonight. I use an OAG with a SCT (C8); PHD2 (managed by NINA) is the brain.
The experienced problem was that, for a specific target, guiding would work well “in most cases”; but in some other cases - always after some NINA operation that stopped and restarted guiding - I would get a rapidly accelerating drift, as if PHD2 was guiding in the opposite direction (mostly RA).
I investigated the rotator; calibration; settings; nothing. I did notice that exposure time seemed to be correlated - shorter exposure times seemed less likely to trigger this behavior.
Finally, I saw it - and it explained everything. During a re-calibration attempt, I noticed all the stars in the field moving the opposite direction of the “star” locked for calibration; and it dawned on me: PHD2 was locking on a ghost, a reflection. You can see it here - a bit exaggerated with a longer exposure to see it well:
📷 image.png
Knowing it’s a ghost, you can see hints that it is somewhat peculiar. But to PHD2 (and me, honestly) it was a reasonable star - for an OAG field in a SCT… HFD of 3.5, nice curve… but when the field moves down, that one moves up; when the field moves up, that one moves down; when the field goes right or left, that does go right or left as well… I don’t think this is caused by the moon - I guess I’ll know for sure in a couple of weeks; the moon is 45 degrees away tonight, and yesterday it was 60 away or so; more likely is a relatively bright star nearby.
I couldn’t come up with any way to fix that in PHD2; HFD of 3.5-4 is good for normal stars; it’s not a hot pixel - so, darks or BPM won’t help. My only workaround was to change my chosen framing rotation; I found one which is not my favorite, but the ghost is gone there.
It’s the first time I have seen something like this with this OAG (a Pegasus Indigo); has anyone else experienced similar problems? What did you do to address them? I was literally bumping my head against the wall, at some point. Should I even venture “chasing ghosts” by cleaning the OAG prism or what else?
