View Full Version here: : Help please from users of Synscan 4

04-08-2017, 01:11 PM
The Synscan 4 with my new mount ( EQ8 ) has strange behaviour and I am trying to find out if it is bug, a fault in the unit or a strange "feature" of the firmware.

During tracking in RA the celestial declination reported by the Synscan is slowly changing even though the declination axis of the mount is apparently not moving.

Has anyone else noticed this? If you get a chance could you check?

In my case the problem is seen via the following steps:

- start aligned scope from park
- select and rotate to object using Synscan
- on hand controller display the rotation angle of the mount's declination/altitude axis ( Utilities/ShowPosition/AX1 ) - whether tracking in RA or not, I have found this figure to be constant as soon as the mount has finished moving to the target.
- on hand controller display the Celestial declination ( Utilities/Show Position/DEC ) - it is this figure that is slowing increasing ( ~ 5" / min ) ( note it does not change if tracking is set to "off" ).

Does anyone else have this "issue"?

Do you have any ideas what it could be?

Note that this has nothting to do with PHD or flex of the tube or imaging system - the above is just related to the declination information being displayed ( and exported ) by the Synscan unit.

Much appreciated,


04-08-2017, 01:23 PM
Compensation for atmospheric refraction?

04-08-2017, 01:38 PM
Thanks for the reply Steffen.

I don't think so...

I measured the rate of drift on the meridian at 0deg declination and as low down on the horizon as I could and the rate of drift was higher on the meridian.

I thought it might be compensation for the declinaiton drift caused by polar alignment error ( as stored in the Synscan following alignment ) but the rate is too high. That is, after alot of tweaking of the mount I managed to get the Synscan polar alignment error down to < 15" in either axis and the rate was still around 5"/min when it should be << 1"/min with such low PA error.

04-08-2017, 04:20 PM
Hm, is it 5" per minute on all targets? What is the tracking rate set to?

The moon's apparent motion is currently about 3-4" per minute in Dec, for example.

04-08-2017, 04:59 PM
The tracking rate is set to Siderial.

Last time I measured it ( and from memory ) the drift rate is very different depending on RA:

0 deg Dec, Meridian: ~ 5"/ min
0 deg Dec, horizon: ~ 0.2" / min

If it clears I'm going to run some more tests tonight. In particular, see what the Synscan reports while tracking with PHD and also look to see if the drift direction is the same either side of Zenith.