Why a Fork Mount is Better

Several people have asked for my advice on using a German equatorial mount for photometry. Specifically, they've been interestedin buying a Celestron CGE and since I've been using the CGE-1400 for a year it was natural that I would have an opinion on the matter.

I sure do! Don't! Stick with a fork mount! Here's what I wrote someone recently:


The CGE-1400 OTA is great! No complaints there. FWHM of <2.0 "arc (using CCD) when the air is calm.

About the German equatorial mount, however, you've broached a subject over which I can get "worked up" easily. I didn't have an opinion about German equatorial mounts until my experience with Celestron's CGE. Now I dislike them, especially the CGE (I might consider a Paramount ME, but not the CGE). The CGE must have been developed hastily by Celestron in order to beat out Meade with their 14-inch, which they did by a few months. The "gee whiz" users may not care about its shortcomings, described below, but the photometrist will have many complaints.  Some of tyhese complaints may be unique to my set-up, but for what it's worth here they are:

1) When doing a "meridian flip" I have to walk out to my SRO and manually do the flip while watching for cable catching problems (the Paramount ME allows cable threading through the hollow tubes, thus alleviating any cable binding problems),
2) Then I have to swap the CCD vertical and horizontal image reversal settings (in MaxIm DL),
3) Then I have to invoke the "western half of sky" MaxPoint pointing file (more on this below),
4) Then I have to change MaxIm DL's centering tool orientation,
5) Then I have to re-train my AO-7 image stabilizer's "telescope drive" (which involves hunting for a bright star in the autoguider FOV),
6) Then I have to specify a 180-degree rotated flat field file for on-the-fly image calibration,

and after all this there's enough mirror flop to cause a change in the "magnitude to ADU intensity conversion" that my extinction plots exhibit a small offset at the meridian, which makes "all-sky photometry" difficult.

Because of these "meridian flip problems" I have adopted the procedure of planning a night's observing to be on either the eastern or western side of the meridian, with no planned flips. Bummer!

In retrospect, I wished I had waited for the Meade 14-inch (even though the optics may have been worse).

Other problems with the CGE: many people have complained about bad pointing. I had issues with this from Day 1, and complained repeatedly to Celestron. Their CS guys get only a couple nights of experience before being placed on the phone lines, so their help was useless. The problem is that doing a 1-star or 2-star pointing calibration, plus a "DEC Switch/Cone," still leaves you with 1/2 degree errors, typically. Not all people have this problem, so it might be a quality-control issue with Celestron. My solution is to request a "Quick-Align" (after setting the CGE clock) and then use MaxPoint (like TPoint) do perform the re-mapping of coordinates to achieve 2 or 3 'arc RMS pointing.

It's possible my CGE was damaged during shipment, as Celestron insists on using UPS (they must be the "lowest bidder"). On Day 1 I found a loose RA gear, and I think I had noticeable mirror flop. I returned the scope along with a list of ~6 complaints, and on its way there (via UPS, of course) the corrector plate broke. Celestron's incompetent tech's thought I was returning it for a broken corrector plate, so they replaced that and did nothing else. Finally, my telescope store owner felt sorry for me and swapped my old CGE mount for a new one (at his expense, probably) and the new CGE behaves better (but still not as well as a Meade).

The CGE I now use has slightly better tracking than the original mount. PEC training affords only a small improvement. The new CGE mount had peak-to-peak tracking error variation of ~13 "arc; after training it was ~11 "arc. With a CGE you have to use the hand controller to invoke "PEC Training Playback" after power-up in order to make use of it (with a Meade it defaults to using your last PEC Training result). The improvement afforded by CGE's PEC training isn't worth using, so I overcome this with SBIG's AO-7 tip/tilt image stabilizer. The AO-7 is a real savior.

My dream telescope is the Centurion 18. It's a CCD at prime focus only configuration; no eyepiece and no Cassegrain. Good machining and a fork mount. Ideal for my precision photometry work, and also great for pretty pictures. I'll sell the CGE-1400 as soon as I've saved enough for the Centurion ($28,000).


This site opened:  June 1, 2004 Last Update:  June 1, 2004