Strange guiding problem

[Davey-T]

I have a 10" Meade SCT which I have been guiding with an PHD,  OAG and Loadstar for a while with no problems but tonight when I tried to calibrate it did east and west OK in about a dozen steps but then wobbled about trying to calibrate north and going east at the same time and taking 20 odd steps but not appearing to move, it then wobbled off crookedly in a south calibration direction again not appearing to move then claimed it was guiding but kept jumping between north and south where usually it does all south corrections.

Permanently mounted in obs'y and nothing altered since last time I used it a few nights ago.

I swopped the Loadstar to the piggy backed refractor and it calibrated and started guiding properly so no idea what's going on ?

Dave

[Ibbo!]

can only think there is something that has worked loose or a dragging cable

[Davey-T]

Cables all tied up as before, maybe there's a spider in the OAG  will take it apart today.

Dave

[opticalpath]

If calibration is working on one scope but not on the other, and it's on the same mount and using the same calibration procedure, pointing at the same area of sky ..... I think what's left is something mechanical causing unpredictable movement of the SCT optics or OAG optical path.  If you don't spot something obvious, there are some things you could try to narrow down the source of the problem:

1. First double-check for any obvious looseness in the mechanical coupling of Lodestar to OAG, OAG to camera, OAG to focuser and focuser to OTA.  Check the SCT mirror-lock is secure.

2. Next, try to reproduce the behaviour 'manually', i.e. not using PHD calibration, in order to remove that from the equation. Set the OAG/ Lodestar to take rapid continuous images of a star within a small sub-frame. Choose a bright star and short exposure so you get several (sub-) frames per second appearing on the laptop/ PC screen. Now initiate moves in each direction at guide speed using the hand-box and observe how the star moves. (Magnify the sub-frame so the movement is easy to see.)  Do you get smooth movements of the star or the same erratic behaviour you saw when PHD initiated the moves during calibration? That will eliminate (or not) PHD calibration as the problem area.

3. Assuming that the manual moves show the same behaviour, repeat step 2 but this time imaging small sub-frames via the main imaging camera. If you don't see the same behaviour then it's likely to be the Lodestar to OAG connection that's the problem.  If you still see the same behaviour ..... remove the OAG from the imaging train and repeat step 3. That should tell you whether it was the OAG/ OAG couplings or not. If not, and you still see the erratic moves with no OAG in place, look to the imaging camera coupling and focuser, and to the SCT mirror.

Adrian