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


Roland Christen
 

You have periodic error in a non-encoder mount. That will manifest itself in a slow back and forth motion in RA. This error is normally a sinusoid which has two places where the star will sit relatively motionless and two places where it moves rapidly. Thus the dumbbell shape of the star. This cycle occurs every 6.4 minutes and the way to compensate for it is to run a PEMPro run, create a PE curve and download it into the mount's CP controller. Then you turn on the PEM playback via the keypad or via APCC. This will eliminate almost all of the cyclical motion that you are experiencing.

Since you moved the spur gear by hand you have invalidated the PE curve that may be in the controller now, so you need to create a new one.

Rolando



-----Original Message-----
From: Bruce McMath <bruce.mcmath@...>
To: main@ap-gto.groups.io
Sent: Fri, Sep 24, 2021 9:51 am
Subject: [ap-gto] 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

--
Roland Christen
Astro-Physics


 


thanks Roland, but the problem is I have already trained the PEC curve just a month ago, and got a really great result.  I ran the mount with the new curve turned on to perhaps refine it and the deviations were so small and random that there was no point in trying to refine it. Turning the spur gear was done before not after making the PEC curve.  Furthermore, not every 6'+  image shows such motion, which is why I am puzzled.  


Roland Christen
 

Just run a guide graph in PHD2 with guide corrections turned off. Run it for 15 or 20 minutes so you can see the tracking rate over several worm cycles. Then you will know what you are dealing with. These older mounts sometimes had small subharmonics which roll thru at different gear angles and do not respond to PE correction. It's probably just a few arc seconds +- but shows up easily on any long exposures.

To do proper trouble shooting requires digging a bit deeper. Just posting a picture with eggy stars only shows the problem, but doesn't show the why.

Rolando

-----Original Message-----
From: Bruce McMath <bruce.mcmath@...>
To: main@ap-gto.groups.io
Sent: Sat, Sep 25, 2021 7:32 am
Subject: Re: [ap-gto] Unguided dumbbell stars insights


thanks Roland, but the problem is I have already trained the PEC curve just a month ago, and got a really great result.  I ran the mount with the new curve turned on to perhaps refine it and the deviations were so small and random that there was no point in trying to refine it. Turning the spur gear was done before not after making the PEC curve.  Furthermore, not every 6'+  image shows such motion, which is why I am puzzled.  

--
Roland Christen
Astro-Physics