GEM vs SCT

[MortonH]

I'd like to do a bit of astrophotography, mainly wide field stuff with a camera and lens, or maybe with the ZS80 ED. At the moment, my scopes are all alt-az, so I'd need a driven mount.

I could spend close to $2k on an HEQ5 or EQ6, but for a bit more money I could get a used 8" LX90 with an equatorial wedge, which would give me another scope and would be the guide scope if the ZS80 was piggybacked.

Obviously the advantage of a GEM is that it can hold larger scopes (e.g. my 8" Dob), but are there any other major advantages of a GEM compared to using an SCT as the mount?

Opinions welcomed.

Thanks

Morton

[iceman]

I think you mean a fork mount? The SCT is fork mounted.

The fork mounts are great to start with, but for long exposures you'll get field rotation. The advantage of the GEM is that you get no field rotation.

[[1ponders]]

Morton, even if you piggyback your camera on an SCT on a fork mount you will still get field rotation.

[[1ponders]]

Damn he's quick :lol:

[iceman]

The GEM is also heaps better and you'll be able to mount your 8" newt on it for planetary/lunar photography with a webcam.

To avoid field rotation with a fork mount, you need to get a wedge - which is half the price of an EQ6 anyway.

[MortonH]

Sorry, I should have specified that I meant a fork-mounted SCT with a wedge. Have updated original post.

Seems to me that the SCT option is a bit less hassle in terms of setup time, but then I've never tried to accurately polar align any scope!

Morton

[[1ponders]]

I guess it's more what you become used to Morton, plus the quality of the equipment. I had an LX200GPS 8" with the smaller 8" wedge. I found it a right royal pita to try to polar align simply because the wedge wasn't up to it. I have also used a 10" LX200R with the Ultra wedge and that was much easier and much more successful.

Having said that the same thing applies to EQs. Trying to accurately polar align and EQ1/2 is far more difficult than an HEQ5 or EQ6, simply from a mechanical standpoint.

I guess the one thing to remember is that there is no polar alignment scope with an sct so if you move to an unknown site there is no plonk down, rought align and then fine tune with the polar scope.... its all drift aligning with an SCT (which isn't really a problem once you get comfortable with it. ;) I use it all the time as I don't have a polar scope and wouldn't know how to use one to it's best).

Personal preference; I would go for the EQ every time. Not just because I find it much easier to setup, but because I have one less piece of awkward heavy equipment to muck around with.

EQ: tripod and mount and counterweights

Fork mount: tripod, wedge (they are fairly heavy), mount and counterweights (yes you will most likely need them.)

[citivolus]

Some of my logic for my equipment change from a fork to gem:

LX200GPS 10" with tripod, Milburn wedge, piggyback, counterweights, ED80, cameras: approx. 70kg (150 lbs)
Celestron CGE with C9.25 OTA (or similar), Megrez 90, rings, side by side mount, dovetails, counterweight, cameras: 80KG? 175 lbs? (estimate)

Weight advantage fork mount, just barely. Too close to call, really.

From my perspective the win to the GEM is the ability to break down into smaller (lighter) components, the polar scope, and versatility of loading. If I wanted to piggyback image using a camera, all I need to mount is the camera with lens and the guide scope.

Of course if the SCT OTA is larger than about 11" even a GEM won't make it a one man job to set up.

Also, I'm paying out the nose sideways to get the CGE. I could go G11, but I don't like the handbox. At least I will have 30kg of instrument capacity, unlike with the 10" fork mount SCT.

Another gem plus: Purchase a second OTA & you can mount it on the gem, unlike with a fork where you would need another set of forks.

Eric

[Shawn]

You didnt specify if portability was an issue, ?

S

PS


I love fork mounts...:P

[MortonH]

Portability is an issue. I live in a unit so I have no back garden to observe/image from. Seems that the SCT would probably be better for me generally, except for polar alignment.

Of course, to keep cost down I could go with the HEQ5 standard (non-pro) version and forgo GOTO, but I think I read that the motors aren't as accurate in the standard version. Is this true?

Morton

[rumples riot]

Hey Morton, no need to apologise I read the equatorial wedge with the lx90 8". No field rotation with that. You can get good images with them too. Guide dog works a treat with meade fork mounts..

[Mark]

Morton,

IMO. YMMV.

The LX90 is a great instrument. It's very portable and the goto and tracking in Alt-Az mode are simply brilliant (at least mine was). I found using it with the standard wedge to be a sometimes frustrating exercise. So much so, that I sold of the LX90 and went EQ6.

I'll be blunt:

You will need a very solid wedge. The standard Meade is barely adequate. For piggy-backing the ZS80 and camera gear I'd suggest next to useless. You may be able to do it, but the extra mucking around to get it right will negate any advantage you may have had with the fork mount over a big GEM.

The forks on the LX90 will give you bugger-all clearance between the SCT rear port and the scope base. You may have to attach your guiding camera to a diagonal. This becomes even more of an issue if you decide to image through the SCT.

You can't escape counterweights with a fork mount. You will need to balance it just like a GEM. A 2D or 3D system may be required.


Now...

The GEM will allow you to attach ANY instrument. The ZS80. An SCT. Your Newt. Your camera! Each by it's lonesome or possibly with any combination of the above. Freedom and flexibility will be yours to know. Ha!

Don't get me wrong. I like forks. Forks are good. LX90 forks? Maybe not so much for imaging. For purely visual work, they are brilliant.

Cheers,
Mark