It’s taken several months to get this image. Long, painful months, but I’m glad I persevered.
It all started after NEAF in April. NEAF is the Northeast Astronomy Forum, the largest astronomy related trade show in the US. I bought a Stellarvue SV80-3SV there and I was looking forward to using it for imaging.
Alas, the clouds that have dominated 2018 continued unabated.
Eventually, I got a few clear nights scattered over a few weeks and ran into problems with my Losmandy GM811 mount. At first I thought the problems were user error, I couldn’t get the PHD2 guiding program to calibrate in declination properly. It kept complaining about backlash. After much trial and error and without any success I reached out to someone in the Northern Virginia Astronomy Club for help and he helped me set the worm mesh correctly. I think some of my attempts may have been correct but as this was new to me it was hard to be certain. This time I was sure it was correct as he had years of experience with Losmandy mounts.
On the next clear night I tried again only to have the same problem. I ultimately let it run using the planetary nebula M57 as my target. M57 is much too small a target for my focal length but I was really just trying to see if I could get anything at all and not really concerned about getting a decent image. I ultimately let it image without guiding because the guiding was making things worse. Here is what I got:
This wasn’t bad but I couldn’t get that when guiding. My stars turned into eggs.
I spent the next few, rare clear nights trying to troubleshoot this and then my mount electronics developed an odd problem and Losmandy asked me to send it in so they could look at it. I also sent the the declination axis for the mount since it was still failing to calibrate. Two weeks later I had them back and the drive electronics were working but I was still seeing issues with the PHD calibration.
I continued to try to troubleshoot this, picking up a copy of PemPro to characterize my mount’s periodic error and generate a PEC (periodic error correction) curve for the mount. To my surprise I was seeing large periodic error in RA and about every two minutes sharp spikes in RA that lined up perfectly. That told me that the RA errors I was seeing were in the mount, at least in part.
I was beginning to think my mount might be a lemon but I wasn’t 100% convinced that it wasn’t all the mount’s fault. Specifically I was worried about cable management, particularly for the DSLR since the USB cable was so short and the power cable fairly heavy. In a last ditch effort to see if that might be it, I bought a ZWO ASI294MC Pro one-shot color cooled astronomy camera. I had ultimately intended to get a mono camera and filter wheel but my budget wasn’t ready for that and it also added complexity on the mount and right now I needed to reduce complexity. This required me to give up some field of view since the ASI294’s sensor is a 4/3 format chip compared to my full frame DSLR but it had slightly smaller pixels and more importantly set point cooling.
Amazingly, the night after the camera arrived was clear and I set up in the back yard to see what happened. I first ran PemPro again and while the periodic error was large, around 17 arcseconds, peak-to-peak, it was much smoother than it had been. I let PemPro collect date for 7 work cycles and then generate a correction curve and uploaded it to the mount. Checking the results, periodic error was reduced though still highish at around 6 arc seconds peak-to-peak. I’m definitely not a PemPro expert but it clearly seemed to help. The periodic error with the PEC curve applied was fairly smooth and the only question is would PHD be able to cope with this.
And the answer was that it could. My declination calibration problem vanished and my RA guiding error while on the high side was under control at my relatively coarse pixel scale (1.96 arc seconds per pixel). I tried exposures ranging from 1 second up to 3 minutes and the stars came out round on all of them.
Here is the astrobin link to m57.