Plate solve prior to meridian flip causing problems


#1

Hello all,

since 2 days I got my meridian flip in SGP worked out. All is well except for one thing: just before the MF SGP takes a picture of the current FOV to use that as a reference after the scope has flipped. After the MF SGP takes another shot and matches this with the reference shot. After that I get this message:

I understand that SGP is taking this frame because I checked the box in the window of the automated MF wicht says “Auto center after meridian flip”. If I uncheck this option, will SGP do an auto center based on the original reference frame (like a “normal” auto center action) or will it just continue without a plate solve and rely on the fact that the MF is 100% good (wich isn’t) and it has the same FOV before and after the MF?


#2

Getting that message means that the plate solution that came back was not close to the hint you provided. This likely means that your solve is bad and your scope was just synced to a random location. This could be very bad for your equipment during the next movement (physically damaging).

Unless your pointing model is extremely accurate… you NEED to have this checked or your post meridian frames will not be stackable. If you don’t check this option, the mount will just flip (slew) with no verification.


#3

Thanks Ken, that clears things up for me. I now have EQMOD set to “dialog based” with no reference stars / points added, because SGP did a very good job in plate solving and my objects would be centered perfectly (my setup is fixed on a pier in my observatory and is polar aligned very well, only the pointing is a bit off), but I understand that after a meridian flip it doesn’t work that way because my mount is too far off. Would it help if I would set EQMOD to “append on sync” and spend a night with making a pointing model on a bunch of stars spread around the sky?

Thanks in advance!


#4

Part of the problem is, I think, that when SGP does the image acquisition and solve just before the meridian flip it also does a sync. This sync should improve pointing close to that position but there are no guarantees about what is does for a position that is a long way away from the sync position.

From the perspective of a GEM, a meridian flip is a long slew. it is 180 degrees along the hour angle axis and 2 *(90 - declination) on the declination axis. The Cone error and Declination axis index in particular will be affected by this because the sign of the error is reversed.

It would, I think, be better if SGP did not do a sync immediately before the meridian flip but just did the image acquisition and plate solve to get a reference position to be used after the flip. This could at least be worth trying.

Getting a better mount model, one that corrects for the cone error and dec index for the OTA that you are using, should help though because the sync will then do less harm to the position after the flip.

Chris


#5

I agree Chris. And maybe it would even be better if SGP doens’t do anything at all before the meridian flip, besides stopping PHD en rotating the mount. Only after the MF it should perform a regular plate solve and recenter the scope just like it does when a sequence is started and the option “center scope” is checked.

As I said before: my mount is perfectly polar aligned, but in terms of positioning a bit off. That isn’t any problem as SGP does a perfect job with centering an object. I’ll post this at the forum for future updates.


#6

AIUI the use case that needs a solve before the flip is if the scope was positioned manually at the start of the slew rather than using a target position.

Chris


www.mainsequencesoftware.com