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G11 vs EM200 Imaging Payload

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#26 BarrySimon615


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Posted 27 July 2009 - 09:26 PM

Perhaps a standard criteria should be worked out to establish a payload capacity both for photography and for visual use. Some ideas to think about may include:

1) With the supplied counterweight(s) fixed exactly at the mid-point of the counterweight shaft, how heavy a payload can the mount carry without needed to drop the weight further down the shaft?, or

2) How long does it take the payload scope to settle down when the diagonal end of the payload is thumped with a consistent and known force?, or

3) When at a magnification giving a .5 mm exit pupil (high power) will the scope never steady down?

FYI - My Losmandy G-11 (1992 model) was good to up to a 33 lb. payload scope (130 f/8) including guidescopes. This payload had a long moment arm. I estimate the photographic load capacity to be in the range of 35 to maybe 40 lbs. with such a long telescope tube.

Barry Simon

#27 mish


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Posted 28 July 2009 - 10:20 AM

I suspect that part of the problem here is that "allowable weight" is by itself not that good of an indicator of performance, because it refers only to the force component of the force/couple system that the mount must resist. Since the mount must support both the mass of the optical train AND its moment of inertia, there is simply not enough information given when mounts are rated in terms of weight (i.e., mass) alone.

The mass of the telescope is a scalar, so it's just a number that we can (and will) argue about. The moment of inertia (which is the mass equivalent for angular motions) is a second-rank tensor, and it's hard to argue about something so profoundly complicated.

Every mount I've ever used (and I started in this hobby in 1965) had fewer problems due to force (e.g., collapsing under self-weight) and more problems due to torque (e.g., long focal-ratio refractor images bouncing around in the field), so any attempt at judging performance of a mount really ought to include its ability to resist torques, not forces.

And while we can introduce simplifications into measures of that angular resistance (e.g., "moment arm"), characterizing the performance of a mount in terms of allowable moment of inertia tensors is going to be an interesting task, e.g., "my Tak can handle a wider range of I's eigenvalues than your Losmandy can!".

And those folks who build high-quality mounts no doubt understand this, which is probably why this whole subject is as ambiguous as it is in terms of effective specification. The good news is that said ambiguity leaves plenty of room for disagreement, so when the skies are cloudy, we can argue about mount capacity here instead of spending time out under the dark sky.

#28 blueman


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Posted 28 July 2009 - 01:02 PM

I would have to agree that the momment arm of a scope will have a lot of effect on the max payload. As I stated before, a longer scope of the same weight will be more difficult to use on a mount and expect fine guiding. You can have a scope that weighs more but is 1/2 the length and it will likely guide better, because of the momment arm.
So weight by itself can not answer the limit question.

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