I wanted for quite some time to go into the OAG business, mainly due to weight reduction, but also to find out if this is working well enough and how easy it is to set it up. I was also curious if I see any improvements by eliminating the oh so dreaded flexure issues that a lot of people think plays a major role with guiding problems.

Here is a short summary of my experiences:

1. The setup itself seemed to be easy. The first challenge though came right away when I added my flattener into my train. Question No 1: where to place the OAG in the train? After some inspection which adapters I do own this was answered quickly: I had to place the OAG between scope and flattener since my flattener reduces the M48 connection to T2 (42mm with 0.75 mm thread pitch). I am not so sure if this is the best setup (why I believe that it is not see below), see version B.

2. Ok, the basic setup was done during daylight, wanted to get the focus setting done as well at daylight but ran out of time, so I did it right away with the stars in the sky 😊. First of course focus with the main camera and Bahtinov mask, then look at the guiding camera output (obtained the ASI 290 for this since I heard that you need a more sensitive camera using an OAG). As expected, I saw nothing, even with the scope centered on Caph (Cas). I tried the little screw that the manual marked to be the focus screw, but I really did not see any changes. So finally I losend the screw that fixes the camera and shifted the camera in the holder with my hand. Wow ... I had to move it outside of the "eyepiece" holder to see the first stars! Luckily I had some 1.25" extensions, but even with them I had to shift it almost completely out of the eyepiece holder and then tighten the screw that holds the camera, a pretty shaky setup to say the least. I believe the main reason is that I had to place the OAG in front of the flattener, causing the distance to be that huge. I will need an adapter that converts T2 back to M48, not sure if I saw one of those being available, have to search for this.

3. Guiding: After calibration with PHD it was time to take the images. Unfortunately the first night clouds came in early so that I did not obtain many Hα images, but good that the next night was clear so that I could get quite a few OIII data. In general the guiding seemed to be not better than using a guide scope, the mean error was around 1" or a tick higher, which is worse than what I could obtain when the mount was new. Definitely tells me that I still did not fix the gear as well as it was when I received the mount, what a nightmare. Even worse though and not explainable to me is that some of the images "jumped" (see version C of an original image). This was one of my goals to find out why those jumps happen, and I wanted to make sure that flexure is NOT the reason, which I believe I proved. Looking at the guiding graph those jumps are not really visible. The dec axis has once in a while a tendency to move away from best guiding (especially if the scope points close to the zenith!) and then after some minutes jumping back to best guiding, this has to cause the problem. Makes me of course not happy since this means that the mount is the main source of guiding problems.

4. Flats: Well, a new setup requires new flats, which I took in the morning after the night session. I was more than surprised to see that the OAG makes a big difference (see version D). I knew that the mirror would block parts of the light, but seems to me a bit much. Maybe this is as well caused by the position of the OAG in the train. Hopefully I will be able to obtain another adapter so that I can place it closer to the camera. Once this is done I will see what impact that has, I will keep you posted.

5. Final image (version A): After "blinking" out all those images that showed the bad jumps I processed the image as "usual", creating a HO-H-O image (SII images will be obtained tonight, weather forecast is good 😊). Version A shows a highly cropped part of the PacMan to show how accurate the guiding was at least for the remaining images. I definitely can say that this looks better than images that I tried before, I will process one of my older images to have a direct comparison.

Conclusion: I like the OAG since it reduces wait. One thing that I did not mention above: there was NO problem to find a guide star, as soon as I increased the gain of the ASI 290 Mini there were plenty to pick. Will it help with the guiding: well ... at least it did not make it worse, and it might be slightly better than using a guide scope. Hence I will stick with it for my refractor. I currently use the finder scope really to find the main bright stars when I startup my session to calibrate the mount, which is kind of nice since first I haven't looked at stars through a scope for some time and it always looks nice 😊, and second, no switch of camera with an eyepiece. Only concern so far: the light reduction at the mirror location. Good flats help, but are not correcting 100%.

Sorry for the long description, wanted to pass as much information to anybody who is considering using an OAG.