Experience report Off Axis Guider OAG versus guide scope

Initial Situation
Roland Winde
At the Fish Ponds 57
I recently made a comment to a well-known astro YouTuber that I was having some problems with my tracking.
This comment was apparently interpreted to mean that the tracking accuracy
wasn't that good. But that wasn't what I meant. My "problems" consisted of the fact that
the guide star was frequently lost.
1)
.
There was nothing wrong with the quality of the tracking itself. The control curve showed deviations of max. 0.25 px in Rect and Dec over long distances, with occasional outliers
of max. 1 px. Given the lack of a comparison, I consider these values ​​to be quite reasonable.

In any case, I was advised to switch to OAG.
This recommendation was confirmed by another well-known astro YouTuber, "Cuiv, the lazy geek," in his video "This SOLVED all my issues, OAG explained."
Both YouTubers were of the opinion that the guide scope wasn't as secure as one might think.
Combined with slight mechanical instabilities in the telescope, both systems could occasionally point in different directions, resulting in "incorrect" corrections from the guide (PHD2).

This prompted me to order a ZWO-OAG from my trusted astronomy dealer.

ZWO-OAG
Integration into the Light Path
The assumption that integrating the OAG into the light path could be achieved simply by replacing the 16.5mm T2-M48 adapter turned out to be wrong.
To achieve a working distance of 55mm, my previous setup looked like this:
Camera / T2-M48 adapter / filter drawer / additional spacer sleeve(s) and 1mm rings.
If I simply replaced the T2 adapter with the OAG, the OAG would be positioned after the filter drawer in the light path, which should be avoided.

The accessories included a threaded insert that allowed me to convert the camera-side M48 thread of the filter drawer to T2.

The new setup looked like this:

Camera / Filter drawer / OAG / additional spacer sleeve(s) and 1mm rings.

However, I had to experiment a bit with this, as the filter drawer wouldn't open after the first screwing!

1
I have since been able to significantly improve this by focusing the guide scope more precisely.

21227 Bendestorf
September 17, 2023

Focusing
I planned to use the OAG on a Newtonian with a 1,000mm focal length.


I was planning to use the OAG on a Newtonian with a 1,000mm focal length. The first, somewhat unpleasant, surprise was the significantly lower number of stars compared to the 242mm Evoguide 50.
The second unpleasant surprise: properly focusing the stars proved impossible.
I managed it reasonably well, but did I make PHD2 happy with it?
The following practical result may have been negatively influenced by this.

Conclusion: It won't work without a helical focuser! It also costs money, of course.

Practical Test
The control curve showed deviations 2 to 3 times greater compared to guiding with a guide scope; i.e., 0.5 to 0.75 px, sometimes up to 1 px.

Conclusion
Even if I give the OAG credit for the blurry stars, I would still expect at most the same tracking quality when using a helical focuser compared to the guide scope.

However, every euro spent on this result would be wasted.
In addition, my initial "problem" (losing a star) is probably exacerbated with the OAG, since there are significantly fewer stars available. (When I always read "Actually, I've always found a guide star so far...").
Roland Winde
At the Fish Ponds 57
After switching back to the guide scope and putting in a little more effort with the focus, guiding is going almost
better than before. I don't even notice lost stars anymore.

R. Winde
PS: By the way, I'm using the ASI 120MM mini as my guide camera.