Starlight Xpress Slimline OAG

SBIG STF-8300M, Starlight Xpress 7 position 36mm USB filter wheel, Starlight Xpress OAG w/QYH5L-II camera, M48 adapter, 6.4mm M48 spacer, Astro Physics CCDT67 Telecompressor.

The off-axis guider (OAG) is another item I have been wanting to get for a while. Since I had the Starlight Xpress (SX) filter wheel I decided to get the Starlight version of the OAG. The SX filter wheel is recessed a bit and the OAG is machined to fit perfectly onto the filter wheel. I just removed my M42 T-thread fitting from the filter wheel and installed the OAG using the supplied screws and then installed the M42 fitting onto the OAG.

I quickly noticed a problem. This configuration would be just fine with a CCD camera with a small sensor and a filter wheel using 1.25" filters, but placing the prism far enough into the light path to work properly was covering up too much of the filter and quite probably my cameras sensor. To fix this I ordered a Starlight Xpress M48 filter wheel adapter. They offer these adapters in many sizes including M54, SCT, and others.

It was tough getting a shot of the prism with the Telecompressor installed but maybe you can make out how I currently have the prism in the light path. I should have removed the Telecompressor for this image. Everything seems to be working just fine with it adjusted like this.

It was a couple weeks before I could try the new OAG out, but it wasnt because of the weather. Anyone who owns the QHY5L-II camera and is considering this OAG should know that it will NOT reach focus as it comes. The OAG comes with c-mount threads and I could not get the camera far enough in to reach focus. This QHY camera comes with a 10mm thick C-mount thread and the camera body has 1.25" filter threads. I had to order a 5mm thick CS-mount and with this I was able to get the camera to focus with the OAG. I got the CS mount from Astrofactors and I believe it was about $15. Here is a picture showing the C and CS mount camera adapters.

On the left is the c-mount that comes on the QHY5L-II and on the right is the cs-mount I ordered from Astrofactors.

Upon receiving the cs mount it was just a matter of getting things adjusted. If the prism isnt far enough into the light path the stars do not look good at all. I wouldnt call them egg shaped or even football shaped, they were much worse. With the stars looking like this PHD had a hard time calibrating. When I lowered the prism down the stars started to look much better and I was able to calibrate and start guiding. The stars on my PHD screen still are not round but it seems to be guiding and I hesitate to lower the prism any further into the light path.

Once you realize how things are adjusted its fairly simple to get things set. The long thumbscrew on the bottom is for adjusting the depth of the prism into the light path. The shorter thumbscrew on top is for adjusting the focus of the camera. For focusing the camera the c-mount slides up and down on the stalk of the OAG that the prism is attached to. The thumbscrew aides in getting the focus set and then there is also a 1.5mm allen head set screw that needs tightened once you have it set where it needs to be. Before you tighten down focus you can rotate the camera to the desired orientation.

The first night of testing went well although my PHD graph was not as smooth as what I was used to when I was using the 50mm mini guider. This is to be expected since I was guiding at around 1150mm instead of 160mm. I hope some further tweaking with the settings in PHD will smooth out the graph but as can be seen in this 100% crop of my test image (NGC 7380) it seems to be doing its job quite well. FWHM numbers were between 4.09 and 4.53 for the individual subs.

100% crop

One of things I like most about the OAG is that my stars do not drift at all. Flexure is no longer an issue! This image of NGC 7380 has not been cropped at all and you cannot see any stacking artifacts.

To show the problems I was having with flexure, here is the uncropped image of the Cocoon nebula, my most recent image done using the 50mm mini guidescope.

Stacking artifacts in luminance data seen to the left and bottom of image

One other thing I should mention is the backspacing with this setup. The SBIG STF-8300 camera has 17.5mm of backspacing and the SX filter wheel is an additional 29mm with the fittings on each end. By adding a short spacer I could achieve the recommended 55mm of backspacing for my MPCC (newt) and the Astro Tech 0.8X reducer/flattener. With the OAG added to the imaging train I am now out to about 60-61mm and can no longer use these correctors. The AP Telecompressor uses a lot of backspacing so I was good with it by adding an 6.4mm M48 spacer. Teleskop Service in Germany makes a field flattener with M48 threads that needs a ton of backspacing that I will need to purchase and use with my apo.

I would recommend the SX OAG to anyone with the SX filter wheel. I hope this post has been of some help to anyone considering this form of guiding. Feel free to contact me if you have any questions and I'll do my best to answer them.

Clear skies and happy imaging! :-)