|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.|
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.
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|
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! :-)