Unguided dumbbell stars insights


Looking for theories to explain occasional dumbbell star images in unguided exposures.  I have a long standing variable star monitoring project run through DC3 Scheduler.  Recently changed from a C11 to a Meade 12 ACF (in hopes of mitigating mirror flop), both running around F7.3 on an AP900.  All images are unguided and some run over six minutes.  Most images are reasonably tight given the length of exposures and lack of guiding or encoders.  But, occasionally, I get backlash like dumbbell images in which there is a star image, a thin line, and another star image - always in RA.  The worm is snug, I use the approach from the same direction option and the PEC training is the best I have ever had - verified by the tightness of most images. Yet, occasionally, I still get the dumbbell image.

I once concluded that there might be a tiny irregularity in the worm gear, having felt a tight spot once when testing the mesh by turning the large spur gear with the finger as described in the manual.  However, since I have not been able to replicate that, despite driving all the way around the worm gear with the same method, I was left concluding the earlier experience was related to a foreign object - grain of pollen or dust.  

Yet, the other night I witnessed three dumbbell images in a row, which seems to belie the worm gear flaw as well as a foreign object theory, but not rule it out entirely as it is possible that all three just happen to place the worm at the or a critical point on the gear.  

At this point, it is more curiosity than anything, as not likely I can do anything about it.  But, perhaps others know exactly what this is.  

James (Bruce) McMath

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