Since the first light of my Celestron C9.25 telescope, it became apparent that I would have to take the plunge and learn how to guide using an Off Axis Guider (OAG). I also do most of my imagine in Narrowband so I needed an OAG so that I could guide in front of my filters. My mentor, Michael Caligiuri, told me that learning how to guide with an Off Axis Guider was a must with a larger focal length telescope but would not be easy so I set out on a quest to learn how to use this critical piece of equipment.
Step 1: Sourcing an OAG
There are many OAG’s out on the market and depending on your needs, there are a few that would be right for you. Selecting the right OAG depends on how much distance your focal train can take. With the OAG 5, the light path was 36mm, which is a fair amount of distance to take up in a focal train but I did some research and found that this OAG was used quite a bit with my camera combination (the SBIG ST-10XME). Knowing which OAG I needed, I starting searching Astromart daily to see if anyone was getting rid of theirs and it did not take too long before I found one at a killer price.
Step 2: Putting the pieces together
Admittedly, this step took much longer than I anticipated. Since the CCD and the OAG are connected to the same light path, the most important aspect to figure out is that the imaging chip and the guider chip need to be the same distance from the pick off mirror that sits inside the OAG. This process involved trying out various spacers that were put between the CCD and the OAG to achieve the right distance between the two. Here is a diagram that illustrates the issue:
Your CCD sensor will have specifications that detail the amount of light path your particular setup has and the trick to obtaining the right distance involves a lot of trial (and mostly error) of fitting spacers between the CCD and the OAG as well as getting the Guide Camera sensor the same distance from the pick off mirror. My process involved mostly error because I did not have the correct spacers between the OAG and CCD. At the time, I was using a 12mm spacer but I could not achieve focus with the Guide Camera so I ordered a 15mm spacer and that did the trick!
Step 3: Field Testing & Finding a Guide Star
Since the distance between the CCD imaging sensor and the OAG was fixed on the X axis, the only way to get both pieces of equipment parfocal with each other was to tweak the distance of the Guide Camera. This involved spending about 3 weeks making minor adjustments to the distance while in Guide Camera focus mode to see if I could get a guide star to show up in the focus window. Most of the time spent in the process, in retrospect, was trying to find a guide star to begin with as longer focal lengths telescopes have less of them to choose from. I remember the feeling when I first saw a guide star show up in the focus window and a feeling of relief and success overwhelmed me. This day also happened to be on my birthday!
Step 4: Test Run
Now that I was able to find a guide star, I wanted to test out the capability and the result of this OAG so I focused on a patch of sky that had a guide star in it and started the guiding process. I was not too concerned with imaging a DSO at this point as I was just trying to figure out if the result of the OAG would yield round stars and if it would work. My first image proved to be a success and here is the first image that came through from this testing process: