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  1. #11
    AustinPSD's Avatar
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    All things being equal, a wedge-adapted alt-az mount and a GEM mount, both properly polar aligned and guided can achieve equivalent results.

    However, all things are very seldom created equal.

    I'm going to focus on wedge-adapted alt-az, dual arm fork mounts like that used on the LX-90, or the equivalent Celestron SCT's. For reference, I have an N11GPS, which was originally mounted on a Celestron Alt-Az fork mount, and equipped with a wedge (it is now used on an Orion Atlas GEM mount).

    Concentrating on the basic dual arm alt-az mount, the mechanical construction uses a central spindle fixed to the lower half-section of the mount's base, along with two sets of bearings, one on the spindle at the top section, and a second set along the lower section, larger in diameter and seated in a race retaining groove in the bottom half of the mount. Both Meade and Celestron use similar, but not identical designs. The spindle, bearings and drive motor/drive reduction system in this portion of the mount form the RA/az portion of the drive. The DEC/alt components are DC stepper motors in one of the fork arms.

    The mount's "normal" intent is that it is used "level", and with the spindle perpendicular to the ground in alt-az configuration. When wedge adapted, the wedge tilts the mount base in order to polar align it. This tilt causes the spindle and bearings, originally designed for carrying the fork and OTA load in a vertical orientation to carry load in both vertical and horizontal axis.

    The horizontal loading increases bearing stiction, and contributes additional angular error on the RA motion of the mount.

    In contrast, a GEM is designed from the onset for loading in this fashion, and a quality GEM won't suffer any increase in angular error as does a wedge-adapted alt-az.

    The error is to some extent correctable, but not entirely through PEC and backlash settings. Guiding becomes an absolute necessity.

    The Celestron and Meade dual arm alt-az mounts are in general lower precision, higher angular error mounts than quality GEM mounts (i.e. CGEM as an example). Celestron and Meade use them because they are lower cost, acceptable for visual use and short exposure duration AP. They allow the "marketing" offer of a large SCT tube on a reasonably stable mount at a lower price point to the consumer than the equivalent OTA on an appropriate capacity GEM. This only surfaces as an issue for long-exposure AP, where the overall accuracy/mount precision and need for polar alignment/equatorial orientation arises - the wedge must be added, extra effort is always required for polar alignment, and results can be equivalent, but almost never are, to a GEM mount.

    It is usually more time-consuming and difficult to achieve equal precision polar/drift alignment on a wedge-adapted alt-az than a GEM mount (but not impossible).

    The intrinsic mechanical imprecision and error rate is higher in a wedge-adapted alt-az than a GEM mount.

    Taking the two aforementioned factors into account, it is harder (but not impossible) to achieve equal performance with a wedge-adapted alt-az vs. GEM mount.

    There are other confounding factors, including long imaging train interference and obstruction with the mount base, balance issues (no counterweight arm, forcing counterweights on the corrector plate end of the OTA), tripod stability, "wobble" induced by the wedge, and so on.

    The debate becomes sort of pound-for-pound, effort-for-effort, it is generally easier, less frustrating, and offers a larger operating envelope to use a GEM mount for imaging.

    Unfortunately, few manufacturers choose to publish all the necessary performance information for their mounts. None of the "commodity" OEM/distributors do - one needs to be able to compare angular and other tracking errors, true capacity, etc. for various mounts. This data isn't readily available, so it becomes a more experiential, subjective bit of folklore rather than an engineering oriented, qualitative exercise.

    So while it is possible to achieve equivalent results with either mount, it isn't really likely, easy, or a fun path to go down.
    CGEM 800 HD, NexGuide, Orion XT8 Limited Edition, Oberwerk BT-100, Canon 20D/20Da/T3i/60D/5D Mk III, various eyepieces, adapters, geegaws, widgets, and tiddlybits

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  3. #12
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    As I have come to expect since I joined about three months ago, nothing short of spectacular in regards from your response.

    Thanks Austin. Makes much more sense now.

  4. #13
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    Hear, hear Blaise. That explanation from Austin is really good. I totally understand now, even though I didn't know what a Gem mount was before!

    So thanks guys and no worries in hijacking the thread - I've ordered a Telrad today so that's sorted!

    I've had my 'scope cooling outside the house for an hour now - it's -8degC here, midnight ......so just nipping out to have a look at Mars. Oooh yeahhh.

    Mike

  5. #14
    blaise's Avatar
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    Something you might want to consider, Mike...

    the #909 Accessory Port Module that Austin mentions is no longer being made. I called Meade to get a better grasp on autoguiding solutions and according to their very "helpful" customer service rep, there is no solution. So then, if you own an LX series scope, according to the guy at Meade, autguiding is not in your future.

    I will need to better understand the process however. I seriously doubt that there is no way to place a short tube on the LX with an autoguider to assist in long exposure.

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  7. #15
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    Indeed Blaise.
    I'm drifting over (geddit?) to the opinion that my LX90 8" is going to be my starparty/ travel scope so will purely be for visual, and I'm going to get something a little bigger to permanently mount on a GEM. in a future home obs.

    Just a thought/dream maybe.

    Mike

  8. #16
    blaise's Avatar
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    Mike:

    I'll say this, the more I use the LX90, the more I enjoy it. There does seem to be some type of limited support for this scope from Meade itself (gimping some of its options for long exposure by Meade is inexplicable), but as a whole, it's extremely portable and the optics in my opinion are great (not to mention the autostar and alignment features-they're so convenient).

    I've been using the Meade DSI IIC with both my Sky-Watcher 10 and the LX. Last night I gave some real effort at using my Canon Digi Reb on the LX90. I was amazed by what I was able to do in comparison to the DSI. Although my focus was off, I was still somewhat flabbergasted by the result.

    So for me, I am looking forward to having more time and clear skies to use the DSLR/LX90 together as is with no immediate modifications.

    I'll eventually get a wedge and adapter plate, and maybe an EZ focus kit eventually, but I won't be putting any more money into the LX after that., I'm of the opinion that you are. Time to start saving for something hardcore

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  10. #17
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    This may help you guys in terms of being a suitable replacement/substitute for the discontinued 909 APM:

    Meade 909 clone: assembled and kit version, full specs, schematic, source

    ScopeTronix may also have the OEM unit in stock at their warehouse. You can contact them here:

    http://www.scopetronixinc.com/contact.htm
    Last edited by AustinPSD; 01-15-2010 at 03:33 PM. Reason: added link
    CGEM 800 HD, NexGuide, Orion XT8 Limited Edition, Oberwerk BT-100, Canon 20D/20Da/T3i/60D/5D Mk III, various eyepieces, adapters, geegaws, widgets, and tiddlybits

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  12. #18
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    Cheers AustinPSD,

    you're a mine of information!

    Mike

 

 
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