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DIY Off Axis Guider

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  • DIY Off Axis Guider

    Im getting my 10" LX200 ready to use for astrophotography. I plan on using my Photometrics Quantix monochrome camera. To go with this I am using an Optec Filter Wheel and I also want to use a off axis guider for guiding. An off axis guider picks off a section of sky with a small mirror or prism and sends that to another CCD camera. This camera connects to your computer and software watches a star you pick put of the field of view and sends commands to the scope to keep it on target. This is the most accurate way to guide a scope. PHD Guider is probably the software I will use.

    The filter wheel is needed because the sensor on the main camera is monochrome. It is a stepper motor driven wheel that moves filters in the path of the camera. The filter wheel I am using has 5 filters, Red, Green, Blue, Luminance (Clear with IR cut filter) and Hydrogen Alpha. Later on you stack these images and get a nice color image.

    The problem is that I am also using a Moonlight focuser which attaches directly to the 3" visual back of the telescope which holds the field reducer if I choose to use it. I could put the filter wheel and guider behind it but by the time I have all the adapters I am getting pretty long on my optical path. Optec also recommends keeping the filter wheel as far from the camera as possible to reduce the effects of dust. They do make an adapter that will mount it directly to the visual back but the problem is then I would have to put the OAG after the filter wheel which can't be a good idea.

    So I decided to make my own Off Axis Guider. This guider will fit between the visual back adapter and the filter wheel using the Optec 2.4" taper. I also need an adapter that will go from the threaded back of the filter wheel to the 3-1/4", 16tpi visual back threads. Optec was kind enough to give me their taper and thread specs.

    The advantage of this is I keep the OAG in front of the filter wheel and the focal reducer. It also keeps the weight off the focuser. Disadvantage is I am going to have to use quite a long extension tube to get the OAG in focus with the camera hanging off the back.

    Fot the two adapters I started with a chunk of 1"x3.5" aluminum bar. I chopped that in to two 3.5" square pieces. At work we have a lathe with a 4 jaw scroll chuck so a drilled them out, bored the ID to 2.2" and faced on side. I brought the pieces home and used my Monarch 10EE lathe to do the rest of the work since I have a nice Buck 6 jaw for it. I turned the OD down on the pieces and machined them as needed. 3-1/4"-16 female threads and the taper on one and 2.34"-20 threads and 3-1/4-16 male on the other piece. I also turned some fine (72tpi) threads on the bore for anti reflection. The visual back threads on the scope itself are slightly over 3.25 so I ended up just cutting till the piece fit.

    For the OAG I found another chunk of aluminum in my scrap pile about 3" square and about 1.125 thick. I bored that out to 2.2 as well and also made the recess of the male taper. Turned the OD down on the 10EE and cut the male Optec taper. For the optical pick up I got a couple 9mm square right angle prisms from Surplus Shed. I cut down a piece of aluminum to 1/2 x 5/8" and rounded over the edges with a 1/8" radius cutter. Drilled through with a 9mm bit and counterbored 3/8" on the camera side of the pickup most of the way through. On the pickup end I milled out the center to hold the prism and drilled and tapped the flanges 4-40 to hold the prism in place.

    To make the hole in the OAG housing to receive the pickup I used my CNC mill. Simple program to cut out the hole to match the pickup dimensions. I then turned the part 90 and counterbored and drilled for a 8-32 screw to lock the pickup.

    So thats is where I am as of tonight. To Do:

    • Set up the 4th axis to drill for the taper locking screws
    • Make up the adapter to go from the pickup to either a C mount or T2 fitting. Probably C as I can do that on my Monarch, my CNC lathe is having issues threading.
    • Anodize the parts.

    Here are some pics:

    IMG_1221 by macona, on Flickr

    IMG_1222 by macona, on Flickr

    IMG_1223 by macona, on Flickr

  • #2

    IMG_1224 by macona, on Flickr

    IMG_1225 by macona, on Flickr

    IMG_1230 by macona, on Flickr

    IMG_1231 by macona, on Flickr


    • #3
      I got the parts back from the anodizer today. They look real nice.

      Got it all put together and on the scope. My CoStar auto guider camera showed up last week. Still need to make an extension tube for the guider.

      IMG_1519 by macona, on Flickr

      IMG_1518 by macona, on Flickr

      IMG_1522 by macona, on Flickr

      IMG_1526 by macona, on Flickr