Previously I had been imaging with a guide scope – my Skywatcher ED80 refractor, paired with the Lacerta MGEN auto guider. As I was not getting good results with this setup mounted on top of my Astrotech 8″ Ritchey Chretien telescope, I have finally taken the plunge and bought the Orion thin off-axis guider to improve the guiding.
Previously with the ED80 guide scope, I was managing round stars in at most up to 2 1/2 minute subs which was rather disappointing. This was not going to be sufficient for me to get great images of Deep Sky Objects. I suspect this was partly the problem arising from a lack of mechanical rigidity of the coupling of the guide scope to the main scope due to differential flexure. But other factors such as my polar alignment not being accurate good enough and the flexure of the CGEM tripod, would have contributed to this as well.
Having dealt with the polar alignment and the stability of the mount with the addition of the pier, the next step was to totally take out the guide scope and guide through the main scope itself – thereby avoiding any flexural issues completely. So in comes the Orion thin off-axis guider (TOAG). The reason I chose the thin version, is because it will still work with a fast Newtonian telescope, as it is my aim to upgrade to the Orion VX12 12″ F4 1200mm telescope to allow for more light grabbing which will allow for shorter exposures and lower ISOs.
I chose the Orion TOAG as it comes with the various accessories and allows me to use it straight away with my current setup. It will also allow for the use of the Baader MPCC III coma corrector for the Newtonian telescope, which means it will be future-proof for me.
Looking at the profile, it is indeed very thin and will be usable in scopes with short back focus distances such as the Newtonians; this is not an issue with my Ritchey-Chretien astrograph which has plenty of back focus.
There have been reports that the early version of this off-axis guider had some stability issue with wobble in the stem holding the mirror prism. But as can be seen, this has been dealt with by Orion who have fixed two small allen screws to secure this on two levels. There is indeed no wobble in the stem at all.
Here I have mounted the Lacerta MGEN auto guider camera onto the TOAG.
The whole setup is then mounted onto my telescope. This TOAG, with the guide camera and my imaging camera together weigh very little indeed, and should put less strain on my CGEM mount.
The distance that is required for my auto guider to achieve focus is about 25.5mm on my Astrotech focuser. I have tightened the holding screw on my telescope, as it means that I no longer need to fiddle around with this.
Now I have chosen to use my Hawk Factory Canon to Micro 4/3rds focusing helicoid to adjust my imaging camera to achieve perfect focus, using the Bahtinov Mask and adjusting focus on a bright star in live view on my Olympus E-PL5 until the pattern looks perfect. No need to fiddle about with taking test shots to check focus as this is completed prior to imaging with the camera.
For my Olympus E-PL5, I do need to extend the focusing helicoid by about 7mm. But this I will check each time prior to imaging with the Bahtinov Mask, as there is no way to fix the focusing helicoid to prevent unwanted shift of focus when setting up the telescope and auto guider.
What is good about the Orion TOAG?
1) It is reasonably priced, being less than the TS TOAG, which does not come with the accessories like the Orion TOAG.
2) It is light but well made and feels solid. The stem holding the mirror prism is very steady in the version currently sold.
3) It comes with probably all the necessary accessories to be able to start using it (even with a coma corrector), although adjusting the focus for both the auto guider and the imaging camera may need some extra extender rings.
4) It does work well as the guider and imaging camera see through the same telescope, so any movement noted by the auto guider and corrected will keep the stars nice and round on the imaging camera.
5) As I do not need an extra guide scope, I save on setup and the weight of my imaging train.
What could be improved in the Orion TOAG?
1) The camera adapter mounts to the TOAG via two flanges which are gripped by the two set screws (one on each side). If this is not set correctly i.e. at an angle, it could affect the sideways adjustment of the mirror prism.
2) The instruction manual is not particularly helpful, being vague about how adjustments are done to find a guide star etc.
But I have to say that even on my F8 Richey-Chretien telescope I am still able to find guide stars, although sometimes I do need to do some searching by slewing about a little. And when I find a star, and there are no clouds to affect the guiding, then I get nice round stars and very good images of DSO. Images I’ve managed to capture using this guiding system can be seen in the thread on the Lacerta MGEN here, where I have managed 8 minute subs with pinpoint stars.
Verdict: Recommended. If I had to start from scratch, I would buy this again. But I would not read the instructions but analyse the TOAG myself and work out the best way to set it up and use it.