Asiair plus pier side mix up

Dspital

I’ve seen a number of Celestron users having problems with meridian flips. The mounts behave as if the Asiair is getting the pier side information wrong after the flip, which then sends the telescope toward the ground. Is Asiair issuing GoTo commands with bad coordinates? It looks that way. Has anyone resolved this problem?

Dspital

Has anyone figured out how to make the Celestron mount behave properly after a meridian flip? Second iteration of plateslove and centering sends the mount in the wrong direction.
This issue is a persistent pain.

Dspital

I took another look at this issue. I was able to see that the plate solved location taken at the meridian before the flip routine started was trailing the target’s coordinates by approximately 22 minutes. I then set up an auto meridian flip with a thirty minute wait time. It worked! This is the first time anything involving a meridian flip has worked.

w7ay

Dspital I was able to see that the plate solved location taken at the meridian before the flip routine started was trailing the target’s coordinates by approximately 22 minutes.

This is an indication that the mount and the sky (plate solve) are not in sync.

First, check to make sure the time on the mount and the time on ASIAIR agrees. Also, make sure that your mount's local Longitude agrees with the longitude of the ASIAIR.

Depending on your mount, you may need a couple of star syncs to calibrate the mount before connecting to ASIAIR. For some mounts, you can substitute plate solved mount syncs for visual star syncs after connecting to ASIAIR. You might try some 3 to 5 syncs to see if your mount comes into sync. (Some mounts won't even connect until you have star synced it to identify the correct hour angle.)

ASIAIR is also known to not be able to sync mounts properly. In that case, check to see if your mount is leveled in the east-west direction. Each degree that the mount is not level causes the zero hour angle of an uncalibrated mount to be off by 4 minutes in time. Your 22 minutes delay could simply have come from a mount that is off (east-west) level by some 5.5 degrees, and lower to the ground in the west.

ASIAIR times the meridian transit based on time alone, and not on the hour angle of the mount. I.e., it thinks that a star transits the prime Meridian when the RA is equal to the Local Sidereal Time. If the tripod of your mount is tiled towards the western horizon, and the mount has not been star sync'ed, the mount will not perform a meridian flip. ASIAIR does not perform a Meridian flip -- it simply issues a GOTO to the mount, and if the mount thinks that the star has transited, it is the mount that performs a meridian flip upon receiving the GOTO command.

You will find that a leveled mount also makes polar alignment easier (otherwise there is a coupling between altitude adjustments and azimuth adjustments that makes you go back and forth a few more times).

If you have a mount (like an RST135) that has a home position sensor. You will also need to calibrate the mount after leveling it (but only need to do it once in a couple of years).

Chen

Dspital

Thanks for the reply. I’m aware of the issues you’ve pointed out. I leveled the tripods mounting plate before putting the mount on it. I understand that might not be perfect leveling but I was hopeful that it would be close. With the mount attached and the scope in place it might be off level. It’s not obvious how to level it when everything is attached. As far as doing multiple star alignments, I’ve done that in the past and my results were the same. Failed flips. The flips succeeded once I set a 25 min delay in the Asiair meridian flip set up. I tried 15 minutes but that failed. I will continue to play with it. I agree that it looks like mount and sky are not in sync.
Thanks for your help.

Dspital

Would using the Asiair to sync to a number of stars bring the mount into sync with the sky?

w7ay

Dspital Would using the Asiair to sync to a number of stars bring the mount into sync with the sky?

I think it will depend on your mount. You can try doing 5 or more syncs at various places in the sky to see if the mount learns from them through a sync to mount commands, instead of through a hand controller.

5 to 6 star syncs is about what is needed for a mount to create a decent model of itself, but you may need fewer than that even -- depends on how many star syncs for the mount to discover the HA of zenith. In the good old days when we used to do everything manually, Takahashi mounts had a single sync-to-zenith command, but I have not seen that command in today's mounts. But even that only sync the mount's HA to agree with a star at the declination of zenith. It can still be off when a star is farther north or south of the zenith, if the mount is not also perfectly polar aligned.

On mounts that have at least a relative encoder, there is sometimes an offset that you can set relative to the built in home sensor. Example:

https://www.youtube.com/watch?v=7IC1j8i-1H4

One thing that you can do to help diagnose the problem is to start up the mount, home it, and then before doing any star syncs or plate solve syncs, do a "blind" GOTO to some coordinate near the zenith. Then do a plate solve, and see how much the RA value is different to where you have asked it to do the GOTO. Try a coordinate east of the Meridian, since that is where your "auto" meridian flip starts at.

Another thing you can experiment with is to position the mount (with tracking turned on) north of zenith, just a little east of the meridian. Do a plate solve to record the RA (do not sync the mount to that solved location). Then do a declination slew (not a GOTO) to the south and do a second plate solve to discover the second true RA. The two RA should be identical. If the two RA do not agree (for example if polar alignment is poor), the meridian transit time will also be different for stars with different declinations.

If you like this kind of thing, take a look at Taki's white paper:

http://takitoshimi.starfree.jp/matrix/matrix_method_rev_e.pdf

Section 5.4 is the one pertaining to this thread, where he shows how to obtain the mount position using 2 and 3 stars, assuming there are no other mount errors (like cone errors, etc).

Chen