TheSkySearchers

Here’s something I haven’t seen before.

I was imaging a target close to the meridian with the Mach 1 DEC balanced camera-heavy. The PHD2 log showed a lot of oscillating corrections in DEC during the first 10min exposure, decreasing with time. They’re red in the screenshot from PHD2LogViewer, highlighted with the white lines. (There were four dithers during the session, marked by the spikes in the guiding corrections.)

My take on this is that the session started when the target was passing through a zone where the camera-heavy balance in DEC was shifting from south to north, and it took about 10min for it to settle to one side. DEC backlash correction was on, but I’m thinking that guiding was busy keeping up with the shifting balance between north and south. Even with this sketchy beginning, the total RMS error for the session was 0.74”. (I usually do better!)

There’s radial asymmetry in my OTA/focuser, so I’ll rotate the OTA to keep the DEC balance to one side, even for a few hours past the meridian. And dream about getting the Mach 2, which has encoders….

Does this seem like a plausible interpretation?

Thanks,

David

Hi David,

Based on the graph I would say you are on the right track. It there a way you could pause the imaging for 10 minutes or so before and after a median flip? The other option is to know it's going to happen and add a couple of frames to throw out. Other than that I would think a re-balance after the flip.

I keep my setup pretty well even balanced (I have a fairly light setup) and let PHD2 do the work with the backlash. It's worked fairly well that way.

Sorry I'm late to the party. I'm too lazy to look through my pile of Astro Physics manuals for a clarification, but I recall in one of the manuals stating it is not necessary to shift either camera side or counterweight side. Just make sure both sides are equal to each other and therefore balanced. Again, I can't confirm my statement as fact, but either I'm dreaming or I remember it because the topic of shifting one side or the other has many saying the proper thing to do is shift the counterweights slightly in order to keep the gears in contact. I'd better look into this as well. I have owned my never used 900GTO for four years, it is a 2010 model and the original owner (believe it or not) never set it up either. My excuse is medical, the mount is just too heavy with my physical limitations from a bad motorcycle crash that I never fully recovered from. I finally came to the realization that I should sell it and move on. However, every time the thought of actually doing that subsides each time I look at it, what a wonderful piece of mechanical art.

I agree, i get mirrord results on a flip, my solution is to toss the first few frames if they are bad, track pass the meridian with a good 20-30 mins ,or go out and shift all the weights... i have a lot of of set stuff i need to reconfigure, ie the finder scope is off to the side, focuser controller is not centered on the scope aswell ..