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I've narrowed down my purchasing choice for a light GEM mount...to either the Orion Sirius EQ-G or Losmandy GM-8...it seems the GM-8 has a good chance of meeting those accuracy requirements...Other postings have said the Orion Atlas was as accurate as the G-11. Can anyone make a similar claim about the Orion Sirius and the GM-8?

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I don't know if this exact question has been addressed yet, weight limits and focal lengths aside. I've used an Orion Sirius mount, but not an GM-8. From past reports on the tracking accuracy of both mounts, my guess would be that the Sirius mount has periodic error in the +/- 10 arc-second range out of the box, while the GM-8 is probably closer to +/- 5 arc second. For comparison, my current CG5-GT has native PE in the +/- 20 arc-second range. With autoguiding any of those mounts should be able to achieve +/- 1 to 2 arc-second PE, if the curve is smooth enough (I have no reason to suspect they shouldn't be, but the more mass-produced mounts do not have as much QC and more tolerance variation).

As Craig has pointed out in his post and article, there are more factors than just weight to consider. How long an exposure one can take is a factor of scope focal length, camera pixel size, polar alignment accuracy, the mounts PE and even where you are imaging in the sky. All those factors will affect whether or not you will get star trailing.

Niels Noordhooek wrote a little program called Star Trails that helps bring those factors together. Input your various camera, scope, and mount characteristics and make adjustments as desired to reduce the "Pixel Curve Size" to be less than 1 x 1. Theoretically, that 1 x 1 figure means that a photon of light is not traveling across more than one pixel during the exposure. The program does not account for the blurring affect of the atmosphere that Craig was talking about or differential deflection between guide camera and imaging, but it can give you a basis for comparing various focal length scopes with different cameras and exposure sitting on different mounts.

For example, with my AT66ED and Canon XSi camera with a guided PE of +/- 4 arc seconds, a 5 minute exposure should be attainable, while anything longer will start to become a problem. Likewise, if I decide to image with my C6 SCT and XSi and at the same PE settings, I'm limited to 45 second exposures without star trailing. I suspect either the Sirius mount or the GM-8 mount should be capable of +/- 2 arc second (or better?) guided PE.

While it may not perfectly illustrate your situation, the program might help you get a grasp on what you're up against with long focal length scopes and long exposures.

BTW, I was very intrigued by Craigs comments on off-axis guiding and hadn't really thought much about using one. I found APM America's Off Axis Guider's webpage and think this could be a viable solution to the traditional 2 scope imaging setup. (I'm adverse to heavy scopes and mount as well. )

Regards,

Patrick

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Celestron CPC 1100 XLT
10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Canon XSi; Meade DSI;SPC900-NC
Vixen GP2 Photo Guider Mount


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