Last updated: 7 November 2003
Sent: Friday, November 7, 2003 08:27:44 From: firstname.lastname@example.org (Hutchings, Mike) I have been troubled by the variable accuracy of the goto features of my DS90 for some time and have completed all the modifications such as drive electronics, power supplies and leather washers. While minor improvements have occurred it is still hit-and-miss, mostly miss!!! I noticed a problem that may have some relevance while I was taking the telescope apart for the production of a pier mount. I looked at the worm gear and clutch assembly and noticed that while there are adjustments for worm gear backlash, the clutch assembly has no provision. You might say that if the clutch is working there should be no backlash but this is not the case even if you over-tighten the lock knobs; Meade suggest that you just gently tighten the knobs. With the Meade recommendation there appeared to be 2 degrees of Alt and Az movement due to the clutch! Further investigation showed that the two three-toothed washers either side of the worm gear forming the clutch are very loose in the three grooves of the threaded stud, about 0.2mm gap, and this allows clutch assembly movement. It appears that the leather washer modification can affect this if the washer is tight on the thread such that the leather extrudes into the gap so reducing the play; mine was not! My solution was to modify the steel washers such that they are a tight fit in the grooves and this was done using a small hammer to carefully spread the three tongues on the washers, being sure to check for fit. If there was too much spread I used a small triangular needle file to adjust it. I made sure that the washers retained their flatness by rubbing them on fine grit paper on a flat surface. Applying this modification to all four washers (Alt and Az), I have reduced the play to less than 0.25 degrees Az and 0 degrees Alt. The drives now lock with a light touch on the knobs as recommended. This is not an ideal modification as the clutches need redesigning but it does the job. I am disappointed with the Meade mechanics, which contradicts their excellent optical quality as I have had to make other modifications to poor mechanisms such as the finder scope adjustment and the focussing drive that had so much play in it that fine focussing was near impossible without having to then adjust the tracking. My solution for the latter was to pack the rack groves with shims to take out lateral movement and use sticky tape to ensure a tighter fit of the focussing tube in the main tube thus making focussing possible. The finder scope mount was modified to two three-screw adjustments rather than the two-screw adjustments on the original to overcome the problems there. There is no excuse for poor engineering, even in mass-produced items. Unfortunately I have not finished the new pier to try this latest modification on the heavens but I am confident that this has removed another major obstacle to accurate tracking. I will keep you posted. Good observing. Mike Hutchings - email@example.com 52 25"N 0 32E
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