Off-axis Guider…the first year

Over the 2021/2022 winter I bought a ZWO OAG. I spent those cold months figuring out how to incorporate this device into my set-up. My post Adding an Off-Axis Guider to my Set-up goes into lots of detail regarding back focus requirements for my two telescopes (Celestron 8” EdgeHD SCT and a Skywatcher BK80ED refractor). It wasn’t until May that I finally got a chance to use my set-up “under the stars”.

Don’t let your imaging camera drop to the ground!

My post “First Light” using an Off-Axis Guider explains my “lucky” start where I managed to avoid having the OAG and the QHY294C fall to the ground as they slowly slipped downward on my 2” Crayford focuser. A great system check before you take your rig outside after storing it for a while is, with it still in the house, point the telescope straight upwards and verify that the imaging camera does not slip downward out of the Crayford focuser. The adjustment screws on the Crayford focuser set the holding friction. Anything new added to your set-up will require more holding friction. You have been warned!

Focusing issues

Initially focusing the OAG camera is a pain in the butt so be sure to pre-focus this camera during the daytime on a distant object. This will not be perfect but it will help. Do yourself a big favor and use the lowest magnification (focal length) telescope you have. Once sunset had passed I had PHD2 looping so I could see the image. I used the handcontroller to very slowly move the scope around until I saw a faint fuzzy on the PHD2 display.

Then I carefully started adjusting the OAG camera focus. This is challenging because the image really jumps around. It’s best to make a small adjustment and then take your hands off the set-up to let the image stabilize. Look at the HFD that PHD2 reports and try to creep up on the best focus. This procedure was made a little easier because I bought a helical focuser with a graduated scale to keep track of my adjustments. The helical focuser precisely moves the camera in and out without rotating the camera. After the initial focusing I am finding that I don’t need to change the focus so I wonder if the helical focuser was worth the purchase. I initially did change the focus when I changed the shimming to cater for the 8”EdgeHD 0.7 focal reducer. Over the summer I got the impression that changing this shimming was unnecessary so, by the end of the summer, I stopped changing the shimming or OAG camera focus.

Differential Flexure…a distant memory

Differential flexure was a constant companion whenever I was using my 8” EdgeHD SCT. Over the years I kept track of this issue so here is a brief history of my experiences…

When I first started imaging I noticed that I always had oval stars when using the EdgeHD scope.    You need to know that I never have PHD2 dither between exposures. Any star movement between images is due to poor polar alignment or differential flexure. I do a good job of polar aligning my CGEM mount so I believe any star movement between images is dominated by differential flexure.

This graph indicates the movement of a single star over the course of an evening. Each coloured dot is the star’s position in a different LIGHT frame. I don’t believe I refocused or had to perform a Meridian Flip so this is just one image after another in rapid succession. I was taking 240 second exposures with my Nikon D5300 camera. That star moved around a lot and it was pretty much unpredictable. Later on I started measuring flexure rates and found that the star movement could be as high as 40 arcseconds/hour.

Eventually I decided to write a program called FlexComp which did improve things. I never did make it public but I did rely on it for all of my imaging. This graph illustrates the benefits of using FlexComp. For this imaging run I was taking 300 second exposures with my QHY294C. The blue dots show how FlexComp was able to dramatically reduce the star shift to less than 10 pixels over the course of an evening. The orange dots indicate how much the star would have moved if FlexComp was not running. The main issue with FlexComp was that it needed a few images to lock-on to the differential flexure. Also, if the differential flexure suddenly decided to change direction, FlexComp would always lag behind as it made adjustments. My post Differential Flexure…My Story has more details on this subject.

Here is what has happened to differential flexure now that I added an off-axis guider. On Aug 27th, 2022, I was taking 450 second exposures with my QHY294C camera. I was using my 8” EdgeHD with its 0.7 focal reducer (F7 1422mmFL). The scale of this graph is +/- 1 pixel. I think this means that differential flexure is GONE! The OAG is definitely the way to go if differential flexure is your issue.


When I was preparing to use my OAG with my QHY294C I thought it was odd that the 8”EdgeHD required a slightly different backfocus depending on whether I was using the 0.7 focal reducer or not. According to the documentation for the 0.7 focal reducer I needed to add roughly 1.5mm of backfocus. I thought this was odd because I bought this scope when I was using my Nikon D5300. I had also bought the Nikon adapter and I had taken many images with this set-up and never seen any issues that could be attributed to incorrect backfocus because of using the 0.7 focal reducer. I never used filters with the Nikon D5300. I couldn’t see how I could find the space in the optical path.

When I bought my QHY294C I also bought a filter slider and an OPTOLONG LeNhance filter. I managed to assemble everything with a fixed backfocus and happily shot lots of astrophotos using this set-up for 2021. This is when I first noticed that there may be a backfocus issue when using the 0.7 focal reducer. It was during that year that I notice that the star colours were not aligned only in astrophotos that were taken using the LeNhance filter and the 0.7 focal reducer. If I used a light pollution filter the star colours were aligned. I go into the specific details of this colour misalignment issue in my Narrowband Color Shift in my Astrophotos post.

When I bought the OAG, I decided I would initially shim for the correct backfocus. Over the year I found that regardless of how I shimmed, the star colours would not be aligned when using the LeNhance / 0.7FR combo. On the Aug 27th, 2022 imaging run I actually added a 10mm spacer between the 0.7FR and the OAG. The colour alignment issue didn’t change so I concluded that the colour alignment has nothing to do with maintaining accurate backfocus. I decided that I would not bother changing the backfocus to cater for the 1.5mm discrepancy. Making this decision meant that the OAG/Filter slider/QHY294C assembly was always the same regardless of which telescope I was using.

I decided that the star colour alignment issue is an optical flaw in the 0.7 focal reducer which is highlighted when using a narrowband filter. The good news here is that the star colour misalignment can be corrected when stacking with Deep Sky Stacker. Check out my Narrowband Color Shift in my Astrophotos post to see how Deep Sky Stacker re-aligns the star colours.

Improvements in Guiding

I did not see any degradation of my guiding when using the OAG. I was always able to find a guide star. Sometimes I had to move the mount an arcminute or two to find a star. I never had to adjust the guide camera (QHY5L-II-M) focus. One of the last imaging runs for 2022 was November 6th. I was using the 8”EdgeHD scope at maximum magnification (F10 2032mmFL). This imaging run took place just outside a big city. There were 3 one hour guiding sections this evening broken up by focusing. Here is a table of the guiding results.  All the rms values are arcseconds.


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