“First Light” using an Off-Axis Guider

Finally the weather cleared and warmed up. I had been fiddling with my off-axis guider(OAG) set-up during the daytime and I made one revision which I really like. Originally I was using a 1mm diameter wire as a spacer between the OAG and my filter slider. This wire seemed too flimsy and it was not big enough to cater for using a 3mm plastic shim. There was just barely enough threads left on the M42F to M48M zero clearance adapter to thread onto the filter slider.

I had ordered Baader T-2 M42 adjustment spacer rings and when they arrived I noticed that they are not very wide…maybe 2 to 3mm. I decided to use the thickest spacer ring (1.4mm) as a replacement for the 1mm wire. I like this set-up because I can tighten the M42F to M48M adapter against it with enough threads left on it to thread onto the filter slider…even when I am using a 3mm plastic shim for the 8″EdgeHD + 0.7FR configuration. You may need to first read my earlier post about my OAG to fully understand what I am talking about here.

May 4th, 2022

I decided to start out using the lowest magnification which is my BK80ED 80mm refractor and it’s 0.85FR. This setup has a focal length of 510mm. This telescope has a Crayford 2″ focuser with a Moonlite Focuser High Resolution Stepper motor. I do all of my imaging camera focusing indoors using some software I developed. For some weird reason I couldn’t focus the QHY29C. Some times the focuser looked like it was responding and some times it made little difference.  It turned out that the BK80ED focuser tube was slipping. I suddenly realized that the CAMERA/FILTER/OAG/0.85FR arrangement could have slipped out of the focuser and crashed to the ground…YIKS!  There are two Allen screws and one knurled knob on the focuser. When I tightened the knurled knob this stopped the slipping for now. The next day I tightened the two allen screws. I suspect this started happening because of the added weight of the OAG and guide camera. Certainly a key point to verify whenever I add weight to whatever hangs off of a Crayford focuser. During the daytime this problem never arose because the telescope was always horizontal as I focused on a distant hydro tower. Under the stars the telescope can be pointed almost vertical which, in my case, was exactly the case.

-The QHY5L-II-M was out-of-focus and it was difficult to find a bright star. Eventually I found a fuzzy star and was able to focus. Amazingly, I had to pull out the camera to reach focus.  This is good news…I can always move this camera further out but not further in without sawing off some of the prism stalk. I would like to think that if you can reach focus during the daytime that you will be able to reach focus during the evening by slightly moving the camera out a couple of millimetres.

I had set-up three equipment profiles with PHD2. I used my OAG_BK80ED+0.85FR equipment profile. This gave me a pixel scale of 1.52a-s/px. I noticed that the guide star profile was oblong. I had heard that oblong guide stars while using an OAG are not uncommon. Hopefully I can improve this in future. My CGEM mount has, at least, 50a-s of declination backlash so I told PHD2 to use a calibration distance of 50px. This should move the guide star roughly 75a-s so I should be able to see the effects of DEC backlash in the PHD2 log file calibration section. My software looks at the PHD2 calibration data to determine the size of the DEC backlash. The calibration step size was 1050mS which, at 50% guide speed works out to roughly 7.5a-s per step. PHD2 calibrated successfully but it alarmed that I did not pick a suitable star. DEC was 68 Degrees and PHD2 prefers to calibrate with a DEC of roughly 10 Degrees. I had dealt with many issues by this point and I was pushing ahead knowing that I won’t get much imaging done that night. By this point it was 10:45PM.

I finally got PHD2 guiding for approximately 20 minutes before I decided that I was too tired to continue. The total RMS error was 0.73a-s which wasn’t bad for a first attempt. I was using 1 second exposures. There were other issues that had arose with my software. I had made lots of changes to my software over the winter and, unfortunately, it showed.

I also got outside, with this same set-up on May 7th. Things went better. I calibrated PHD2 at 9:20PM and by 9:40PM I had started an imaging run. I had PHD2 guiding for 1.5 hours before I shut things down at 11PM. I was imaging M81 & M82 under very bright skies so I didn’t expect good results. My total RMS error was 0.62a-s. I was using 2 second exposures.

I like to configure PHD2 to use the Hysteresis algorithm for DEC guiding. This makes the algorithm the same for RA and for DEC. This seems to work the best with my Backlash Assistant software. On May 4th I didn’t get a chance to make this change before I shut down. On May 7th I used the Hysteresis algorithm for DEC. For both RA and DEC I use an hysteresis of 0.7 and an aggression of 0.5 to 0.6. This tends to make PHD2 sluggish and chase only slow deviations. I believe this allows me to use up to 1 second exposures without having PHD2 “chase the seeing”.

May 9th, 2022

This night I had switch over to my 8″EdgeHD scope and it’s 0.7FR. This setup has a focal length of 1,422mm. The pixel scale was 0.53a-s/px. I had PHD2 calibrated by 9:22PM…primarily because I got lucky finding a guide star. I did have to adjust the focus of the QHY5L-II-M guiding camera to cater for the 3mm plastic shim. The helical focuser I use with the OAG has a scale graduated in millimetres and I had to move the camera out by roughly 1mm. The helical focuser has a range from 0 to 6mm and I tried to set it up for focus positions close to 3 or 4mm. I really like the convenience of this helical focuser on the OAG.

I had PHD2 guiding by 9:30PM and shut the system down at 11PM. The total RMS error was 0.63a-s. I was using 1 second exposures.

This particular telescope configuration was giving me issues with RGB color shift that increased radially. This tends to be an indication of an improper back focus (too close). I think it is less that last year…when I was not using an OAG and was not paying much attention to back focus. I will experiment as the summer proceeds.

May 12th, 2022

This night I used the 8″EdgeHD without the focal reducer. This has a focal length of 2032mm. I had PHD2 calibrated by 9:44PM. The pixel scale was 0.76a-s/px. The QHY5L-II-M was binned 2×2. Finding a suitable guide star was difficult. All the available guide stars were very faint which is something I knew was common when using an OAG at high focal lengths. I redid the PHD2 calibration because I didn’t tell PHD2 to move 75a-s.

I had PHD2 guiding by 10:08PM and continued guiding until 11:30PM. The total RMS error was 0.9a-s. The guide star was very faint and the SNR jumped around a lot. I was using 2 second exposures.

Differential Flexure

I was pleased to see that differential flexure has all but disappeared. I will be creating a separate post when I compare my experiences with differential flexure over the years…and how an OAG makes it all go away!


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