I've been trying to recap a little on what has been said and also to form a picture of your particular night sky and timings but first, a couple of issues from the foregoing posts which perhaps you can confirm/clarify ?
1) Balance. At one stage you state that your 8SE's OTA is fairly well balanced; maybe a little backend heavy. The statement suggests that you are looking for good balance which, in fact, should not be the case.
The 'scope's OTA should have a significant imbalance (but by no means severe), in order to put a load on the gears. Thus the OTA should be placed either to provide fore-end heaviness, (when fully loaded with all usual accessories), or rear end. Experimentation, (and I know you've already done this), should then be resumed to determine whether a Positive or Negative Alt. GoTo approach is better suited to this imbalance.
I DO APPRECIATE YOU'VE ALREADY BEEN "THERE" !
It looks as if you are currently using a Negative Altitude GoTo Approach setting. Can you confirm this just for the record ?
2) For an object to move out of the field of view of a 24mm EP in 15-30 minutes, mediocre to poor tracking is indicated. This may be attributable to poor anti-backlash settings and/or the phenomena I described in that article I wrote for CN in 2007; (see above).
Can I therefore ask you describe in some detail, how you set the anti-backlash for your 8SE ?
Now to that model of your particular sky.
I have tried to model this using Stellarium's planetarium on the current evening sky as seen from Los Angeles, (owing to the fact that I don't know where you live specifically in Ca. but on the basis that the Los Angeles sky will differ very little if at all from yours).
Thus, and correct me please if I'm totally in error, I estimate the following:
1) Viewing Window: Approximately (dusk to dawn) 8:00pm to 6:00am PDT.
2) At ca. 8:00pm Duhbe lies at ca. Alt. 19-20 degs. just west of north and reaches its lowest Alt. point (ca.6 degs. at ca. 12:30 am.(due north).
3) It then proceeds to climb in the sky reaching its highest visible point, (26 degs). at dawn around 6:00am.
If this is a reasonably accurate model with which you can agree, may I ask at what time were you making your observations when you mentioned that, when slewing to Dubhe, your OTA finished its slew, some 15-20 degs. below Dubhe. It sounds as if it was either virtually pointing to the horizon or below it considering the 15-20 degs "off" that you originally mentioned.
Also, from what you have described and if my viewing window is correct, you seem to have targetted objects which lie west of the north/south meridian line. Have you yet tried objects to the east of the line and if so, with what effect ?
Please don't take it that I'm calling into doubt anything you have stated, Philippe, I'm just trying to obtain a more complete picture of what is going on, hopefully for all of us involved here who are trying to help solve this enigma.
Hi Tel - I'm in Vancouver, BC, Canada, so I'm close to what you see but Dubhe still is much higher (I think 25° @ 10 pm). So yes, the error in pointing Dubhe make the pointing close to the horizon, but not to a point that is below. I made these tests between 8:30-10 pm. I've not tried a lot the eastern
meridian because it is very blocked from my site, but I'll try to find a solution tonight.
About balance, in the picture I've added, you might see a black sticker which is the balance point of my OTA. The OTA is on the mount on purpose not exactly at that balance point, but a bit further left and in fact I'm now doing the tests with Pos-Pos approach and the scope being front heavy a bit (I changed that to try something different). My understanding of the process, and maybe I got that wrong, is that the approach should finish "against" the load of the OTA so that OTA "weights" on the gears for better lock. So my gotos finish right and up (as well as my alignments).
For the backlash setting, once I've done a fake alignement, I'm pointing to anything small I can clearly view, I set speed to 4 and then set a Alt backlash Pos and Neg at 10. Then moving the Alt, changing Pos and Neg values to the max value I can have before the start or stop shows jerkyness : stop is "dead stop" and start, even if delayed, is smooth. Is this correct ?
I'm not taking any "offense" of your suggestion and I appreciate a lot the help of everybody here, especially yours. This would not be the first time I do silly mistakes and will certainly not be the last time, hence I'm asking for collective wisdom. I'm experienced in electronics & embedded sw (basically this is my job) but I'm still a very green beginner in astronomy. In any domain, there is always something to learn from the others !
Thanks a lot