Sunday, May 29, 2011


Getting Deforked

I’ve probably mentioned this in passing a time or three, but it’s something I get asked about a lot. To wit: “Unk, I want to take my SCT off its fork and put it on a German equatorial mount, a GEM. How do I do that? Can I do it at all?” The answer is “yes,” and deforking your Schmidt Cassegrain is not scary at all; if fact it is amazingly easy.

Why would you want to do such a thing? Many reasons. Maybe your beloved CAT was purchased before go-to and computers came ‘round or at least became popular. Maybe you already have a computerized mount, a fork mount, but it’s an older one with few features, or it’s given up the ghost in a cloud of smoke. Perhaps your fork is too shaky or too hard to balance. Could be you hate mounting your heavy SCT, fork, and drivebase on a wedge for imaging—or for anything if it doesn’t have a computer. Why did I do the deed, why did I remove my 1995 Ultima C8, Celeste, from her beautiful mount?

One night at the 2001 Peach State Star Gaze, I decided I was over hunting deep sky objects. Like I said last time, what I am interested in now is seeing as much of the Universe as I can in the years left to me and the way to do that does not involve squinting through a finder and at the pages of a print star atlas, especially not with my fading eyes. Heck, at that star party I found I could be more productive using cotton-picking analog setting circles. I would yes, go goto. Finally.

However...Unk, as he has <ahem> been known to do, procrastinated. There was the matter of deciding on which goto scope it would be...I had always been a Celestron man, and Celestron was still in the process of getting its house in order regarding goto.

But a couple of years later, the stars aligned for me in the shape of the NexStar 11 GPS. Big Bertha quickly convinced me go-to had arrived; that it was reliable, practical, and, given my goals, necessary. Using goto, even if I gave every single object sufficient eyepiece time for your silly old Uncle to absorb its gestalt, I could still easily double the number of objects I could visit in an evening. When I used my Stellacam II deep sky video camera with Bertha I could at least quadruple that number.

After I got the 11, she was practically the only telescope I used for months and months. The thrill of seeing deep sky objects in detail with the aid of her beautiful optics did not wear off, but, like it says in the Good Book, eventually the spirit was willing, but the flesh was weak. After a while, my back began to complain about hefting 66-pound Bertha onto her tripod every time I wanted to observe. Good thing I still had my trusty C8. Well, I thought it was a good thing.

As I wrote last time, I felt a little guilty about not using the Ultima 8 more and hauled her out to the Deep South Regional Star Gaze for a week under the stars the old-fashioned way. I found out real quick the only thing I could say for that old fashioned way was that it got awfully old after a night or two. Contorting my aging bod to reach the odd positions an equatorial wedge put the eyepiece in didn’t seem like homage to our glorious amateur astronomy past, but homage to Torquemada. I loved what I was seeing in the C8; I just didn’t like the way I had to view the wonders, or the time it took to get to them.

I still needed an 8-inch, though. There’s a reason the good, old C8 is still the most popular Schmidt Cassegrain: it is relatively light, it is very versatile, and it is surprisingly capable. An 8-inch telescope will show all the basic wonders of the Messier and NGC visually, and when you put a camera on one, especially a deep sky video camera, an humble C8 can go incredibly deep. Problem was, I had just ponied-up for the C11 and wasn’t in the mood to buy yet another C8 (three C8 OTAs curently reside at the ol’ manse).

I considered just putting a digital setting circle (DSC) computer on Celeste, but she’d still have to be used on her wedge, and I was definitely over that. There was nothing for it. Beautiful as the Ultima 8’s massive fork was, it would have to go. I’d buy a new go-to equipped German equatorial mount. That was when I hit the first rut in the road: which go-to GEM?

There were numerous GEMs that fitted my requirements, namely that included full go-to and sold for about 3 grand max—and I do mean “max.” The first one I eliminated was Vixen’s then-new Sphinx. I was mightily drawn to the mount’s cool StarBook computer, but what I was hearing about the Sphinx's quality and performance left me dubious.  I did get to see one in person at the Cherry Springs Star Party in P.A. But... While the mount looked OK, it didn't seem like anything special considering its fairly high price.  Worse, I was less than impressed by the demeanor of the folks selling Vixens at the time. They made clear they didn't want to answer my questions and didn't give the smallest fig whether I bought one of their mounts or not. 

How about the much-loved Losmandy G11? In addition to being at the top of my price range, and a little heavy for the at-a-moment’s-notice type use I planned for the C8, a buddy’s complaints about the under-unfriendliness of the mount’s Gemini go-to system made me leery. Same went for the company’s GM-8, which also seemed a little pricey for its payload capacity.

I loved the Celestron NexStar hand control firmware, so how about the CGE? It was as expensive as the G11, as heavy as the G11, and had a few problems, I’d heard. Especially with the mount’s external cables and their connectors, which tended to fail and cause all sorts of bad behavior. Nevertheless, I was sorely tempted.

Alrighty then. In typical Unk fashion we were heading straight for the bargain basement. Imported Chinese mounts. I didn’t even consider Meade’ s LXD-75. At the time, it was not available separately, and I didn’t need a Meade OTA. Honestly, though, if I’d felt the mount was of reasonably good quality, I probably would have sprung for one with a 6 or 8-inch Schmidt Newtonian onboard. The LXD-75 had numerous QA problems at the time. Worse, I had the experience of seeing an SN-10 OTA fall to the ground off the mount’s direct ancestor, the LXD55, due to a poorly cast/designed dovetail. 

That left everybody’s fave Chinese vendor, Synta. I strongly considered the Atlas/EQ-6 GEM. But it really was way on the heavy side (I did buy one a few years later). The next one down, the Sirius/HEQ-5, was a possibility, but it seemed a wee bit expensive not to be much higher in payload capacity than the final candidate, Celestron’s Synta-made CG5.

Not only did the CG5 use the standard NexStar hand paddle and firmware, a huge plus for me, it was cheap enough, being priced at not quite 800 bucks (it’s even less expensive now). It was also reasonably portable but reasonably sturdy, with a very nice tripod with 2-inch diameter steel legs that was actually better than the one Synta shipped with the Sirius/HEQ-5 at the time.

A review of the mount’s Yahoogroup, “Celestron_AS,” revealed that, while the CG5 had had a few teething pains, most users’ difficulties were of the pilot-error variety. After a couple of firmware bug fixes by Celestron, it appeared the CG5 was making itself known as a remarkably robust and capable mount given its modest price.

I was still skeptical this inexpensive GEM’s go-to would be anywhere near as accurate at my NS11’s, but if it even approached that I figured it would be workable. If it didn’t turn out to be quite good enough? That would be OK, too. The CG5 would be a test-bed to see if I really wanted to use a GEM-mounted C8. If the answer was an affirmative one, I’d put the CG5 on Astromart and buy more-better-gooder. I rang up one of my fave gear dealers, Anacortes Telescope and Wild Bird, and placed my order for a CG5.

While waiting for the mount to arrive, I needed to get the dovetail business sorted. These days, most folks mount their SCTs to a GEM by screwing a dovetail plate onto the tube. You could use tube rings, but those make mounting accessory rails and stuff on the tube awkward or impossible. The CG5 used a Vixen compatible dovetail bracket, one of the two most popular—the other being the wider Losmandy-format dovetail—so I’d need one.

Vixen-type dovetails are available from numerous suppliers. Losmandy and ADM being the two at the top of the heap. Their gear is fantastically well made, but it’s also kinda high for a cheapskate like Unk. I saw my favorite accessory dealer,, had Vixen dovetail kits for the C8 for considerably less, so I took a chance and ordered one.

When the dovetail bracket arrived, I was impressed. Maybe not Losmandy quality, but strong and attractive, and it included all the screws I would need as well as detailed installation instructions. Before I could put the dovetail bracket on Celeste, though, I had to perform that forkectomy, I had to remove her from her fork.

For folks contemplating moving their beloved SCT to a GEM, deforking is a frightening prospect, but the first time I did it, it turned out to be laughably simple, the job of less than half an hour. The pictures and my comments apply to Celestron scopes, but the procedure for removing a Meade from its mount is just about identical.

Step one? The fork arms of most mounts are attached to the drive base with Allen screws, usually four of them. You want to loosen these a smidge so the arms can be spread apart a little when you remove the screws that hold OTA to fork. Don’t do that and you run the risk of scratching your tube on the fork arms as you remove it, like the impatient Unk did by not loosening them quite enough. Just a tiny scratch that BUFFED RIGHT OUT, but it warned me to slow down. A few telescopes may have screws that are recessed enough and fork arms deep enough that you need to go to the hardware store and pick up some long-handled Allen wrenches, which was the case with the Ultima, but that’s the only possible catch. Just loosen all the screws a few turns.

Arms loose at the base, it is time to DO THE DEED. Almost all SCTs are attached to their forks in simple fashion, most often by four screws threaded into the rear cell (above). All you need do is remove the screws and the OTA will come free. In case of accidents, you might want to put a few layers of soft towels down on your worktable. If you own a NexStar GPS or an older Nexstar 8/5, the process may be slightly more involved. See this for information on deforking a GPS scope.

With the OTA off its mount, proceed to install the dovetail bracket, following the manufacturer’s instructions. Usually, that involves fastening the back end to a pair of accessory holes on the bottom of the rear cell, and the front end to one hole on the corrector assembly. A single screw is more than sufficient for the front, since there is usually also a pair of “outrigger” screws that press on the corrector assembly and keep the front end stabilized. Your kit should have come with screws, since the factory screws occupying the accessory holes will probably too short for use—they are just placeholders. Save them in case you need them again sometime.

Only thing to beware of? If your dovetail did not come with screws and you have to furnish your own (not likely, but maybe), be darned sure not to use screws any longer than absolutely necessary. Inserting too-long screws into the rear cell is a good way to stop your mirror from moving and thus your telescope from focusing. I have even seen folks chip their mirrors by using too long screws in the rear cell.

Removing the screws that held the tube to the fork will have left some unoccupied holes. You probably want to close these off to prevent dust/moisture intrusion. If you are picky about such things, you can get plastic plugs from McMaster-Carr. If you are, like your old Uncle, not so picky, you’ll just cover them up with squares of black (electrician’s) tape. Hey! That humble tape has worked fine for me for seven dadgummed years.

So, my C8 now had a nice Vixen-style dovetail bracket on it. All I needed was the mount. Which turned out to be a more convoluted proposition than I expected. Anacortes had given me a tracking number, but after the mount left Washington the projected delivery date never showed up. Nothing changed. After a couple of days I became concerned and called Herb and company. Turned out the UPS tractor-trailer bearing my mount had crashed and burned on the Interstate (or so I was told, anyhow). AT&WB immediately dispatched another CG5, and in just a few days I had it in my hot little hands.

Mostly, I was impressed. In the decade since Synta’s “EQ4” Great Polaris clones began flowing in from China they had obviously improved ‘em a lot. Not just with the aforementioned excellent tripod; the fit and finish of the mount was, if not perfect, substantially better than I had expected. The hand control looked identical to the one I received with my NexStar 11. Maybe this would work out. I was still skeptical, though. I did a fake go-to alignment of the mount in the living room and everything seemed to work fine, but the real proof would be in that heaping helping of backyard pudding.

CG5 set up out back, it was time to mount the OTA on the GEM. I was a mite paranoid at first, clutching the C8 to me like a little drowning person grabbing a lifeline. I needn’t have worried; nothing bad happened. Loosened the main saddle mounting bolt, loosened the “safety screw,” slipped Celeste and her new dovetail into place, and tightened both bolts. There she was, looking mighty fine on a shiny new GEM. I almost stopped feeling guilty about relegating her beautiful fork to a dusty corner of Chaos Manor South’s massive Equipment Vault.

Next order of bidness was getting the C8 properly balanced, which I figured would be important to ensure good gotos. Cool thing about a GEM as opposed to a fork? You can balance one easily without adding weights. I moved the RA axis until the tube and counterweight were level and moved the mount's counterweight up and down the declination shaft until I achieved R.A. balance. I then moved the tube forward and back in its cradle, carefully loosening the two saddle bolts, until the OTA remained stationary with the declination lock off.

The good folk on the CG5 Yahoogroup had told me the mount did not need a close polar alignment for good go-to performance. I was happy to hear that, but I knew the mount would need to at least be in the neighborhood of the Celestial Pole for acceptable tracking. When Polaris finally peeped into view just above the roof of the old Manse, I moved the mount in azimuth (I’d already lined it up roughly on north with a compass) with its two push-pull bolts, and adjusted altitude with the single elevation bolt till Polaris was centered in the hollow polar bore.

“Single bolt? Unk, your mount should have come with two altitude adjuster bolts; one forward and one aft.” Actually it did, Skeezix, but I had to remove the forward bolt so I could depress the polar axis enough to hit Polaris. At my latitude, 30-degrees north, the forward bolt interfered with the R.A. motor housing. Removing the front bolt didn’t hurt my ability to align the mount at all. The CG5’s counterweight keeps enough tension on the altitude axis that a single bolt is more than sufficient for either up or down adjustments.

“And why didn’t you buy the polar scope, Unk?” The CG5, I’d heard, had a computerized polar alignment feature in its hand control that made the polar scope completely unnecessary. Based on my experience with the borescope that came with my non-go-to EQ4 mount, I wouldn’t have spent a dime on the thing even if the HC software hadn’t included a polar alignment routine.

Only one more thing remained, lining the mount up on its “start marks.” The CG5’s computer needs to know its starting position, and you tell it that position by moving the mount in right ascension and declination until two pairs of marks line up. Some folks worry whether their marks might not have been properly placed at the factory—there seems to be some variation in how they are positioned—but it really doesn’t matter. The mount just needs you to start from the same position every time; it doesn’t matter exactly where that is.

Hokay, rubber-meets-road time. I fired the mount up, entered time, date, time-zone, and lat-lon just as I would with any other go-to scope and began the three-star alignment (this was the original firmware; new CG5 HCs offer a 2+4 star alignment procedure). The mount came up with its first alignment star choice, I hit “Enter,” and Celeste began slewing that-a-way. I won’t say the GEM was quiet, but it wasn’t any noisier than the LX200s of my acquaintance.

The scope stopped a fair but not outrageous distance from the star, I centered it in the finder and in the eyepiece and did the same for the next two alignment stars the HC picked. The NexStar display announced “Alignment Successful,” and it was time to put the mount to the real test. With a bit of trepidation, I hit the “M” button to choose a Messier for the first go-to object.

Altitude and azimuth adjustment...
Even seven years ago, the sky from Chaos Manor South’s backyard was almost completely blocked by our semi-tropical vegetation. But on this spring evening I did have a decent clear patch to the east. What to choose? Good old M63, the Sunflower Galaxy, was bright and would be smack in the middle of the open space. I keyed in M063 (M63 will not work with a NexStar), and, with shaky fingers, hit Enter.

Celeste’s new mount made its weasels-with-tuberculosis sound and started slewing for what I could tell was roughly the right place. When she stopped and the little cursor on the display ceased rotating (unlike the Meade Autostar, the NexStar HC has no beep), I put an eye to the 26mm Plossl. AND THERE WAS THE SUNFLOWER! Centered and looking good. How about M53, then? The glob was also in the middle of the eyepiece. So were M87, M64, M110, and any of the other bright Messiers I tried. Shazam! The darned thing actually worked.

For a while, anyhow. The next night, the mount’s slewing seemed awful labored, and kept on in fits and starts till the first alignment star was left way behind. I hit the big switch. “Aw, for cryin’ out loud. Just what I figgered. This cheap piece of junk didn’t even last 24-hours!”

That was when one of my few remaining brain cells fired. I dimly recalled a couple of folks on the Yahoogroup mentioning that the CG5 was surprisingly power hungry. I hadn’t bothered to recharge the jumpstart battery; usually it would do for a couple of short backyard sessions with the NexStar 11. Could the CG5 actually need more current than the NS11? I ran inside fetched my other jump-starter, which was fully charged, and fired the mount up. Worked perfectly, sending Celeste to all the targets we’d visited the previous night and a few more visible between tree limbs. Whew!  

After my fright, it was darned sure Rebel Yell time!  After...I did the other thing I'd been told about by the CG5 boffins online. Using a jeweler's screwdriver, I carefully spread apart the two halves if the mount-side power connector. I'd been advised this split pin often didn't make good contact with the DC power cable.  I had no further strange behavior from the mount in all the years I owned it.

In addition to dealing with the power connector and learning I'd need to operate the CG5 off a fully charged battery, I heard about a few other CG5 quirks. Like the need to always do final centering of alignment stars using the up and right keys on the HC only. That lets the computer take the mount’s backlash, of which it has a healthy but not absurd amount, into account. Oh, and to be prepared to eventually replace the ridiculous little power switch, bypass it, or leave it permanently in the on position. It will fail—mine did after four or five years. Yeah, a few quirks, but nothing that can’t be lived with.

Given the mount’s sterling performance over the last seven years, I don’t mind living with any of those things. In addition to being just as accurate as the NS11 when it comes to gotos—after I replaced the hand control with one with the 2+4 routine, anyway—I found the danged thing would even take pictures! I shot the best images of Mars, Jupiter, and Saturn of my life with the CG5 and my SAC7B CCD/webcam. But that was just the beginning. I found if I slapped a focal reducer on the C8, my ST2000 would happily auto-guide the CG5 for as long as I wanted to go, delivering round, pretty stars.

So…would I do it again? Yep. As I’ve said more than once, if the CG5 went belly-up tomorrow, I’d go right out and buy another one. Don’t get me wrong: the CG5 is not the queen of all mounts. She is light for anything heavier than a C8 (though sufficient for visual use with a C11), and one look at her fit and finish shows she ain’t in the AP-Losmandy-Bisque league—not even close.

And yet, and yet... The little mount works incredibly well with a C8 and can be had for just over 600 bucks (!) these days. I will happily admit I’ve seen more objects—way more—and taken more and better images since Celeste has been on the GEM than I did the entire time she was on that beautiful Ultima fork. It’s possible Celestron is about ready to replace the CG5 with something new. If they do, I hope it is as good, even half as good, as the CG5, which is a real and true modern classic, muchachos.

Nota Bene: Don’t mourn for the Ultima fork. I put it back in service several years ago. I felt bad about it collecting dust, and when a late 80s vintage C8 OTA fell into my hands, on the U8 fork it went.

Next Time: Depends on what the weather gods decreed, but my little girlfriend Charity Hope Valentine is complaining I never take her anywhere...

Hello 'Unka' Rod

The old Ultima OTA is getting pretty old... did you also resilver/refurbish/re-whatever it when you performed the forkectomy?

Nope...and the optics on the 1973 C8 we have at the department (Physics) here are still fine. ;-)
What a very timely post Unk. I too have been pondering upgrading my U8 PEC with either new DSC or going for a GEM. Your review sure has me leaning towards the GEM. One thing I was wondering about was the polar alignment, your description makes it seem very simple indeed.

John K.
Hi Rod,

Thanks for this very useful post.
I've just pulled the trigger on a CG-5 for my Nexstar 8SE. I want to try some low effort imaging (minutes not hours...).

Hi Rod,

Your blog is great. I have a 79-80's era orange tube that see's almost no use, except to show my kids craters, planets, and the occasional neighbor. So I realize I need to do my homework. This CG5 looks like an affordable way to retrofit my scope, and make it much more useful. Is it a good idea to go this route with this scope? Or has this tube faded into obscurity? I admit, I need a thesaurus in reading this tale, but would like to know if this is a viable alternative. Just looking for some insight. Thanks.

The orange tube is still as great a scope as ever and will serve you well on the CG5...
Thanks for that. I need to do some research. Without asking you to figure it out for me. Is it fair to say in addition to the CG5, I need the dovetail kit, and that's it? How do I confirm the dovetail kit will fit my old scope? There is a "warning" on Besides your blog (which I discovered a long time ago, but only rediscovered) what are the best resources to learn more about my C8? Despite owning it for over 30 years, I know little about it (my wife informed me that if I want to invest in an upgrade, I better know what the hell I was doing). Thanks again!

Shouldn't be much of a problem. My advice? Call Mr. Scopestuff, Jim Henson, and tell him your Uncle Rod sent you to him for help.

Yes, all you need is the CG5, the dovetail, and (2) counterweights.
i will, thanks!
Dear Uncle Rod,

I am about to defork my CPC800. Have been reading reports on the subject. I have a question: it may be a odd question, hard to answer afeter so many years, but,what is the lenght of the Vixen dovetail that you fit to your C8?

Thanks in advance, Fernando
This is the one I used:
Hi there, nice explanation of the deforking. I have a dual fork arm cpc 1100. It is computerized and has a go-to functionality but is alt-az and cannot do eq mount unless I purchase a heavy wedge. So opening on deforking it to mount on my new cgx mount. Any tips and pointers?
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