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Why don't I like non-GEM mounts?

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#1 revans

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Posted 11 September 2009 - 05:57 AM

Hi all,

I started out loving fork mounts, but found that I couldn't get as good of a Polar alignment. I became addicted to GEM mounts and am a GEM mount only person now.

But I suspect that the fault is probably my lack of knowledge and experience.

Someone set me straight on this... I'm not up on the latest available technology and maybe this Polar alignment problem I used to experience isn't an issue anymore...

Rick

#2 jrcrilly

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Posted 11 September 2009 - 06:41 AM

maybe this Polar alignment problem I used to experience isn't an issue anymore...

Rick


Hi, Rick. There are several types of EQ mounts. The two you mention are the most commonly seen, though. There's no inherent reason why one of those two types should be easier to align than the other. I've seen German EQ mounts with poor (or no) axis adjustments, and I've seen wedges with poor (or no) adjustments. Either design (if supplied with the required adjustment hardware) is as easy to align as the other. I suspect that the differences you experienced were caused by one mount being better designed or equipped than the other rather than by one being the German design and the other being a wedge.

#3 urke

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Posted 11 September 2009 - 07:00 AM

I aggree with John.

Have you tried drift alignment?

#4 revans

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Posted 11 September 2009 - 07:07 AM

My experience is with a rather old 1970s fork mount for an orange tube C14. It was really rugged, but I never felt I got a good alignment with it. Much later on in the mid 1990s, I bought a Meade 10 inch SCT on a GOTO mount and it found anything I was looking for very accurately... but it never seemed to achieve the degree of polar alignment needed for long deep sky images at F6.3 or F10. I think it was an early version of the LXD 90, but don't specifically remember its designation. It was on the factory fork mount.

And gradually I moved onto GEM mounts... first a Meade LXD75 and later a Tak EM-200 and finally a Losmandy G11 Gemini. Because I've had over a decade of constant use with one or the other of these mounts, I have become a GEM person almost exclusively and am afraid to go back to the fork mount. Do fork mounts come with polar alignment scopes these days... this is a very useful feature in the GEM mounts... but maybe with GPS and all the latest technology this isn't necessary anymore for fork mounts?

Is there any rugged fork mount out there that can essentially align itself using GPS and new computer technology if you at least get it in the neighborhood of Polaris within a quarter degree or so?

I'd really like to put the old C14 OTA back into action and it likes fork mounting... but I'd like new electronics... the 1970s version is too antiquated for me...

Rick

#5 rmollise

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Posted 11 September 2009 - 08:24 AM

Hi all,

I started out loving fork mounts, but found that I couldn't get as good of a Polar alignment. I became addicted to GEM mounts and am a GEM mount only person now.

But I suspect that the fault is probably my lack of knowledge and experience.

Someone set me straight on this... I'm not up on the latest available technology and maybe this Polar alignment problem I used to experience isn't an issue anymore...

Rick


It's no more difficult to get a good polar alignment with a fork than it is with a GEM. If you are after a dead-on one, you are going to drift, which is done the same way it is with a GEM, and which is no more difficult or less effective. Also, some HCs, like Celestron's NexStars, have built-in polar alignment routines which will get you in the neighborhood to start.

I am not a huge fan of forks for (polar aligned eq mode) imaging, but it's not because of polar alignment. That has more to do with balance issues, and some of the inaccessible positions the scope can wind up in.

OTOH, nothing is better for visual than an alt-az go-to fork.

#6 therocksal

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Posted 11 September 2009 - 10:47 AM

I learned to hate gems. I just dont like spending time setting them up, aligning them, having to rotate my diagonal all the time, etc. A permanent setup spot would help trememdously though.

Give me a mini tower or CPC or DSC's on a good alt/az platform anytime.

For imaging, I keep the Atlas....only for imaging.

For visual, ugh.

#7 Eddgie

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Posted 11 September 2009 - 11:11 AM

You did not give much detail about the fork mount you owned.

If your fork mount was an older one that was used with a wedge, I believe that flexure in these setups can indeed throw off alignment. Most of these old fork mounts were very rubbery. My bet is you can attribute any accuracy errors to that.

If your fork was a later Alt-Az version, sometimes these get less accurate as you get closer to zenith. This is because you start dealing with Gimbel effects. A GEM only needs to turn one motor, and always in the same direction. For the fork, the altitude motor has to reverse direction as the scope passes through zenith. Any play in the gears can cause the scope to loose accuracy.

GEMS have a somewhat similar problem in that Cone error can cause alignment on one side of the horizon to be off from the other side of the horizon.

Good modern mounts correct for both of these conditions by adding a third alignment star. Older mounts however did not.

So, there are indeed some mechanical resons why this may have been the case a decade ago, but most of these issues have been circumvented by software improvments. Now it is possible to build a mount that is sloppy mechanically but can still point with a high degree of accuracy.

Regards.

#8 Eddgie

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Posted 11 September 2009 - 11:14 AM

And FYI. My own preference would be for an Alt-az mounted fork. I had a NexStar 11 and ADORED it, leaving it ONLY for a bigger SCT, the C14. If Celestron made a modern fork that didn't weigh 200 lbs (Meade) where the OTA could be easily removed to facilitate setup, I would have bought one rather than a big GEM. Big GEMS are a pain in the rear.

For a big refractor though, you really don't have any options yet. I keep waiting for the Ioptron "Maxi-tower" for my 6" refractor.. LOL.

#9 Eddgie

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Posted 11 September 2009 - 11:17 AM

Imagine a C14 on a carbon fiber FORK! With Dob-like disks on the side of the OTA that you could simply drop into sockets of the fork! MAN would I LOVE that!!!!

#10 revans

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Posted 11 September 2009 - 06:54 PM

OK... so assume the following...

I enter my long and lat exactly into the mount to negligable error... and I get the altitude of Polaris just right for my latitude...

OK then... I sort of put Polaris into the cross-hairs of the fork mount finder scope. Then I do a 3 star alignment using three stars in the sky other than Polaris.

Am I polar aligned enough to do deep sky photography with R, G and B filter for 40 minutes each filter divided into 3 to 5 minute subs? I bet not ...

If I had a GEM and I put Polaris in the right spot to center the celestial North Pole, I bet I'd be in business even without drift aligning... given a decent guide scope and guide scope camera...

Rick






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