Which hour angle is right

[obscura]

This was an idle check yesterday between Eqmod hour angle and the HEQ5 Synscan hour angle. The Eqmod HA was 15m behind the Synscan HA. Yep, lats/longs/altitude/time were the same in each so why is there this approx 15m diff? Could understand an hour diff and know the prob but 15m? Eqmod is off system time and the handset must have its own internal clock but ko time was off system time. Idle curiosity - any ideas? Did just take a look at polar finder and ..... seems to agree with the Synscan.

Eqmod? why the diff?

I am missing a trick somewhere or my marbles.

[chrisshillito]

I know the Synscan hour angle has been previously shown to be wrong due to the fact it doesn't take into account precession but rather just uses the J2000 coords of Polaris. Why not check with a planetarium - they will generally display the hour angle of objects.

Chris.

[obscura]

Did do, plus polar finder and Takahashi site. Polar finder doesn't give actual HA and difficult to get a reading from the clock there. Precession wouldn't have occurred to me. Planetarium, Eqmod, Takahashi appear to agree so, I'll keep to them. Thanks for the explanation Chris.

[chrisshillito]

Yes, its a rather slack implementation in the handcontroller I'm afraid. That said I suppose would be difficult to get 15 minutes precision anyway if lining up by eye so perhaps they thought it was good enough for purpose. If you use EQMOD's method then so long as you get the starting position set correctly (see documentation for method) it will automatically compensate for precession (and nutation), move the mount in RA to the right position and then track at sidereal to keep it correct whilst you fiddle with the adjustment bolts.

Chris.

[obscura]

The 15m was 15 minutes in time approx 4degrees - not angular. Apologies if I misled you. Haven't used Eqmod's method as yet but my curiosity will lead me there. I did take a look some months ago but it was so dark, I couldn't see the reticle. I didn't want to get the Synscan out just for a light. I'll try and catch Polaris at dusk.

Cheers.

[chrisshillito]

Don't worry I wasn't misled - 15 minutes /4 degrees is still quite a small error when it comes to manually positioning the mount/reticule particularly given the design of the reticule itself and that's the only excuse I can think of for Synta's inability to do things properly. Its a long time since I used my synscan but it may even be that they don't ever apply precession even to the objects in their own database and rely on the star alignment process to compensate for any resulting errors.

Chris.

[obscura]

That's good. Setting polar always seems hit and miss. Its a best guess on setting circle/dropping pole into a little circle etc - not exactly precise. I can understand why many do a star drift. One day, me too. The reason to look at Eqmod HA was to dump the Synscan totally. I think that is done.

Cheers