Experience and comparison of Off Axis Guder (OAG) with guide scope

To streamline my equipment, I wanted to move away from the guidescope/guiding camera combo to an off-axis guiding system. I chose the standard OAG from ZWO, as it has exactly the right thickness to ensure a 55mm working distance in front of the sensor of my main camera (ASI2600MC Pro) when used with the ZWO filter drawer. The components are arranged in the following order:
Telescope – Flattener/Reducer – OAG – Filter drawer – Main camera.

I initially used my ASI120MM Mini as my guiding camera, which I had previously successfully used with a William Optics Uniguide 50. With the Uniguide, I usually achieved guiding values ​​of 0.5-1 on my AM3 in good seeing conditions.

It'll be exciting to see what the result will be with the OAG, as one reads and hears very different things...
First, I simply inserted the ASI120MM Mini into the top of the OAG, and when assembling the system, I made sure that the OAG arm was positioned on the long side of the main camera sensor so that it wouldn't be shaded. This is very quick and easy to accomplish thanks to the three possible positions and the included thin spacer rings.

In daylight, I then pointed my telescope at a distant object and first focused the main camera as best as possible using the video function in the ASIair (which I use). The next step was to set the guiding camera as the main camera in the ASIair and also switched to the video function. This allowed me to bring the guiding camera into focus by carefully moving it in its mount. That had to be enough for now. The last thing I did was take a few flats to check if the prism was shading my main sensor. This was also the case with my APSC camera, but was quickly and easily resolved by sliding the arm outward a few millimeters.

At night, I first focused the main camera precisely on the stars and performed the polar alignment. Then I moved the guide camera into focus – I had to adjust the position from daytime a little. The good thing is that you only have to do this once! From then on, the guide camera is always in focus with the main camera, regardless of what equipment is placed in front of it.
As usual when guiding with the Uniguide scope, I first set my ASI120MM Mini to gain 60 and 1s, and then started guiding, including calibration, in the ASIair. But then came the disappointment: On my ASKAR 103 with a 700mm focal length at F6.8, guiding stars were only sporadically detected, and the image was generally quite dim. So I increased the gain and also set the exposure time to 2 seconds. Now everything worked much better, much better in fact: Despite the small field of view that the 120mm lens delivers in combination with the OAG and the ASKAR103, the multi-star guiding worked smoothly with dream values ​​around 0.2! A test shot with a 10-minute exposure time had perfectly round stars... So I moved to my subject and started my session with 180-second lights. The guiding stayed around 0.2 for hours, and at worst, 0.4!

WHAT MORE COULD YOU WANT?!

ANSWER:

1. A helical focuser sounds tempting because it allows you to adjust the focus more precisely?! Yes, that may be true – but ultimately, you only adjust everything accurately once and never again. This allows you to easily reach the focus point manually by moving the camera. Therefore, the ZWO OAG is functionally perfectly sufficient as it is.

2. A guiding camera with slightly more sensitivity and a slightly larger field of view when the telescope is pointed into areas with few stars. I was subsequently able to work with an ASI220MM Mini that I found used. With this, I can go to 1 second exposure time with a gain of 250, and there are significantly more stars in the field of view. The guiding values ​​always stayed around 0.2, even with a 1 second correction cycle. An absolute recommendation if you're considering purchasing a new OAG system! But if you already own the small (and rightly popular) ASI120MM, you can certainly use that for now.

My conclusion after 5 nights with the OAG: I've never had such good guiding values ​​before, and this is 100% reflected in the roundness of the stars in my light frames. I, for one, wouldn't want to bother with an additional guidescope anymore and clearly prefer the OAG instead!

Thanks to Jörg for this report.

ZWO Off-Axis Guider for Astrophotography:

https://www.teleskop-spezialisten.de/shop/Teleskop-Zubehoer/Astro-Fotografie/ZWO-Off-Axis-Guider-fuer-Astrofotografie-umfangreiches-Zubehoer::4311.html

ZWO ASI120MM Mini:

https://www.teleskop-spezialisten.de/shop/Astrofotografie/ZWO-Astronomie-Produkte/ZWO-ASI120MM-Mini-Kompakte-Mono-Astrokamera-und-schneller-Autoguider::3922.html

ZWO ASI220MM Mini Autoguider:

https://www.teleskop-spezialisten.de/shop/Astrofotografie/ZWO-Astronomie-Produkte/ZWO-ASI220MM-Mini-Autoguider-und-USB2-0-Monochromkamera-Chip-D-8-81-mm::5972.html

ZWO ASI2600MC Pro / Cooled Color Astro Camera:

https://www.teleskop-spezialisten.de/shop/Astrofotografie/ZWO-Astronomie-Produkte/ZWO-ASI2600MC-Pro-gekuehlte-Farb-Astrokamera-CMOS-Sensor-D-28-6-mm::4460.html

Askar 103APO 103mm f/6.8 700mm Apochromatic Triplet APO Refractor:

https://www.teleskop-spezialisten.de/shop/Teleskope/ED-APO/100-115mm/Askar-103APO-103mm-f/6-8-700mm-Apochromatischer-Triplet-APO-Refractor::6548.html