Telescope Reviews: GEM VS FORK SCT

For visual observing, GEMS are not hard to set up at all. In fact, I can set up my G-11 in a few minutes for this. My LX200 takes some time to go through the automatic procedure for leveling, finding north, and getting a GPS fix. You can roughly align a GEM with a polar scope to a degree of accuracy which is more than sufficient for visual observing. The object will not drift perceptibly unless you stare at it for a very long time. The demands of AP require a more time consuming setup.

Fork mounts are good for visual observing, but they have some drawbacks. I don't find the viewing position to be particularly comfortable for objects near the zenith. Also, if you add a third party crayford focuser behind the rear cell, then you have the problem of not being able to slew all the way to the zenith because there is not enough clearance for a focuser and a diagonal, the latter of which may slam into the fork base as you slew in altitude. This doesn't damage the diagonal in my experience, but it causes the dec clutch to slip and throws off the alignment to an extent that requires you to go through the alignment procedure again. A GEM does not have this problem.

For AP, there are more obvious advantages to a GEM. Put a camera and a focal reducer on the rear cell and you are even more limited in your altitude range. If you want to do AP, you need to get a good quality wedge ($500-700) to eliminate field rotation. I don't find it particularly easy to polar align a fork mount on a wedge to a degree of accuracy that is required for long-exposure AP. If you practiced enough you could probably get quite good at it. In general, fork mounted scopes tend to have much higher periodic error than a quality GEM which is designed with the astrophotographer in mind. You can guide it out to some extent.

Fork mounts are heavy, but so are quality GEM's. If you do buy a fork mounted scope, you are more or less stuck with it unless you want to do a deforking (which I have done). If you decide you want to do AP and prefer a GEM for this, you either have to defork your SCT OTA leaving you with a mount that can't be used with any other scope. You might find a buyer occasionally looking for used replacement forks, but the fork mounts alone are not easy to sell used without the OTA (trust me).

That said, there are a lot of things to like about fork mounts. If somebody out there could design a fork mount with a quick release (like the Questar duplex) which would preserve orthogonality between the mount and the optical axis, I think it would be a welcome development.

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WO FLT-110 f/6.5, TEC optics
Losmandy G-11 Gemini
Meade 10" LX200 GPS/UHTC
Questar 3.5" Standard
Vixen VMC-110L
Canon 450D, unmodified
Assortment of TV Panoptic, TV Radian, Vixen LVW, and WO eyepieces