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Doug
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Posts: 28

12/12/2011 - I've never gone back to an unguided mount since I got my SBIG SG-4 standalone guider.  However, that is not to say that it has been without its problems.  The most notable has been star trails when using it with my Sky-Watcher Mak-Newt 190 (FL=1000mm).  At first I thought the problem was with the guider, but I have now concluded that it is associated with differential flexure.  The seriousness of the problem varies from object to object I have noted, but is always a constant drift in one direction, which invariably does not align to either DEC or RA.  Doing some very simple maths, assuming a tube ring spacing of 300mm, you end up with a value in the order of 0.001mm per pixel (EOS 60D) of lateral movement of the tube in its rings.

 

I note that the tube rings are felt lined so would be subject to compression depending on the attitude of the tube, and a 1/1000mm is not much.  Can be as large as 6 pixels over 2 minutes, which is noticable to approx 1 pixel per minute which is not really noticable on a short 2min exposure.  I have not seen this problem with my other scopes to the same extent, although the ED120 (FL=900mm) has been a bit problematical at times and I have noticed that the rings need tightening once in a while.  My MCT127 (FL=1500mm) which has a more rigidly mounted dovetail has not been a problem even on 5minute exposures.


The problem has not been evident on the smaller focal length tubes for obvious reasons, rigidly mounted, less resolution.  I shall have to monitor the guide parameters from the camera.  The guidescope (Sky-Travel 80mm) is a bit flimsy but the evidence would point to that not being the issue.

December 12, 2011 at 5:01 PM Flag Quote & Reply

Doug
Moderator
Posts: 28

05/03/2012 - I was interested to test the performance of the EQ5 at slightly above its recommended max payload limit, so I attached my Equinox 120ED Pro (7Kg), EOS 60D (1Kg), Tandem mounting bar (1Kg), ST80+SG4 (2Kg), Timer, Red-dot finder, ext tube (0.3Kg) = 11.3Kg.  The counterbalance weights that come with the mount are 2x5Kg.


I balanced the system with a 5Kg plus 3.2Kg quite easily so the total load on the bearings was just 19.5Kg.  Total specificied load on the bearings = Payload max (9Kg) + supplied counterweights (10Kg), so near enough OK.


Used M42 and M106 as two targets taking a total of 41 images across both at 121sec per frame.  Only one frame was rejected with evidence of stiction/trailing, which I thought was pretty good.  The mount did not complain at all during slewing.  I had spent some time to get the balance right in both Dec and RA though as I had adjustment over the weights, pivot point of the tandem bar and the longitudinal position of the scope relative to the mount.  Wind was light with the occassional moderate gust.  I did not find it particular difficult to focus on 10x liveview and oscillations on the mount damped out within a few seconds.


Pretty pleased with the results as this now means I don't have to take the additional time to get the NEQ6 out to run this optical configuration.

March 5, 2012 at 6:05 PM Flag Quote & Reply

Doug
Moderator
Posts: 28

11/03/2012 - Performed more assessment on the suitability of the EQ5 Pro Synscan to carry a heavier than recommended load and guide satisfactorily.  The total all up payload was 11Kg within 5% which is 2Kg above the recommended maximum.  The configuration balanced very well so the only strain is due to the moment around the centre of rotation, and the actual static load on the bearings.


Results were very promising when compared with data taken from the NEQ6.  You can reckon on approx 1 pixel drift per minute for both configurations which is not really apparent over a 2 minute frame.  I had convinced myself that this was differential flexure, and on the balance of probabilities I would still say that is the case as I cannot think of any other reason for it.  When targeting M106 I did interfere with the mount to refocus as the tube was still cooling down, which would explain the first jump.  The second jump may have been due to something as simple as a cable tensioning on one scope relative to the other (dew heater cables, guiding cable, power supply to  camera).


See the AstroDIY photo Album for the actual graphs. Any comments, thoughts or observations which could help me improve the situation would be welcomed.  I have noticed that the problem is significantly worse when trying to guide with the MN190 which has only an 11% increase in focal length.

 

March 11, 2012 at 11:09 AM Flag Quote & Reply

Doug
Moderator
Posts: 28

11/03/2012 - In answer to my own question in the previous post, I could attach a laptop to the SG-4 and monitor the tracking errors.  That would provide the evidence one way or the other to determine if it was differential flexure or something more fundamental.

March 11, 2012 at 1:21 PM Flag Quote & Reply

Doug
Moderator
Posts: 28

29/06/2013 - After a number of disappointing imaging sessions I set about trying to find the root cause of the guide errors.  I have come to the conclusion that the Star Travel 80 with focal reducer is not up to the job as a guide scope; the fundamental problem being the focuser, it is not rigid enough.  I now use a Sky Watcher 9x50 finder scope modified to take a 1.25" nose piece.  Not only is this very rigid, it saves about 1Kg in weight on the mount.  Now I get round stars right up to 1230mm focal length.  The adapter that replaces the mirror and eyepiece has been machined to provide focus within the travel allowed by screwing the objective lens cell in and out.  This is now equivalent to the Orion 50mm mini-guide scope which had trouble focusing with the SG-4 as there was not enough back focus.

June 29, 2013 at 4:54 PM Flag Quote & Reply

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