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Thread: Orion off axis guider

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    Default Orion off axis guider



    I was curious why so many people recommend using using the 50mm scope with Orions mini guider package over the off axis guider? I know it will pull some light out out of the image. Are there other thing negatives for using these over a separate attached mini ota? Wouldn't the loss of some light be offset by the reduction and weight and the elimination of any tracking issues resulting from the guide scope shifting or what not? I imagine though focusing would be a two part process, main imager and then the SSAG?



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    Default Re: Orion off axis guider

    I looked at getting an OAG for my setup. One reason I didn't was that the guider on the OAG has a very limited field of view, so finding a suitable guide star can be difficult. More importantly for me, using one with a coma corrector is tricky, because anything inserted between the coma corrector and the camera will mess up the precise spacing required for proper coma correction. In my research, I could find only one OAG with a low enough profile to do the job.

    My reason for investigating an OAG in the first place was to save weight. I ended up getting a mini guider. Because it replaces the regular finder, there is no net weight gain, so it does the job without those extra hassles.

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    Default Re: Orion off axis guider

    I forgot to post in my earlier message that with a medium focal length 714mm refractor more stars should be in view compared some of the higher magnification scopes. Plus these would be limited to objects with a bright star near by. Alnitak would be great when imaging the horsehead or flame nebulas.
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    Default Re: Orion off axis guider

    I agree totally with what Keith said. The Mag Mini just works and works great with both my 1500mm SCT and 500mm APO. Never stuck for a guide star and no sign of flexure related issues.
    At £299 for the kit, one of the best astro purchases I've ever made. I wouldn't consider changing unless I was going to be using a much longer FL scope than the 1500mm, when both FL and flexure would probably become more of an issue.

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    Default Re: Orion off axis guider

    Quote Originally Posted by MLaferriere View Post
    I know it will pull some light out out of the image. Are there other thing negatives for using these over a separate attached mini ota? Wouldn't the loss of some light be offset by the reduction and weight and the elimination of any tracking issues resulting from the guide scope shifting or what not? I imagine though focusing would be a two part process, main imager and then the SSAG?
    The OAG doesn't use light that would otherwise strike the camera sensor. When set up correctly, it is located off-axis, to the side of the sensor chip. This has several advantages. The OAG "sees" the same motions and mount errors as the camera, so guiding is more accurate and it's not affected by mechanical flexure that can occur between a separate guide scope and the main OTA. Another advantage is the guider can operate in front of any filters used with the main camera. The guider will have plenty of light even when a narrow band filter blocks most light to the main sensor.

    The disadvantage is a small field of view that sometimes yields only a few choices for guide stars. Another issue is use with focal reducers. Focal reducers shrink the diameter of the image circle from the main scope and if this gets smaller than the spacing of the OAG from the center, the guide camera will see nothing.
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    Default Re: Orion off axis guider

    ... which is kind of the whole problem with the OAG approach... most people using long focal length OTA's - the ones that would suffer most from flexure issues - also use FR's that make using an OAG difficult.

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