[Coulter CT-100]

Coulter CT-100

A review of a vintage 'scope, by Akkana.

I have a vintage Coulter CT-100 as a "travel 'scope". This little jewel is a 4.25" f/4 Newtonian (focal length 432mm) which disassembles into two main sections: a mirror unit and a secondary unit. These two main sections have dovetail mounts which slide onto a single bar (with 1/4" threaded tripod mount) which comprises the open tube of the telescope. There's a piece of cloth which velcros around the main sections to baffle stray light and to keep the optics clean in dirty environments. It also has a 6x30 finder on a dovetail mount.

When not in use, the two halves nest together and slide into a case about 6" cubed. The center bar screws onto the shoulder strap of the case. The cloth tube cover goes on top of the optics, and the finder and its mount squeeze in on top, so it's all self contained (except for a tripod). The whole thing weighs about 5 lbs, considerably less than a Pronto or ETX.

Some people call this the "coffee can scope", because the mirror and secondary pieces are the same diameter as a coffee can, so close that the dust covers supplied by Coulter are actual coffee can lids. It doesn't really look like it's made out of a coffee can, though -- it's light and strong aluminum, blue anodized.

The telescope cost $99, plus about $20 for the optional finder, back around 1980-81 when it was discontinued. I bought one of the last units Coulter made: I'd been lusting after one for some time, and when I heard they were discontinuing it due to the high demand for their (much more profitable) big Dobsonians, I drove out to Idlewild to buy one in person. I've never regretted it.


I've made a few minor changes to it, of course (I don't seem constitutionally capable of keeping anything completely stock). The worst problem with the CT-100 was that it came without a focuser; just a set-screw, so you focus either by sliding the eyepiece in and out of the tube, then locking the set-screw, or by sliding the mirror or diagonal back and forth on the main truss bar. It's quite difficult to focus precisely that way. Fortunately, one of the good design decisions Coulter made was to put everything together with bolts rather than rivets or welding, so everything disassembles easily. So I unbolted the eyepiece holder, bought a used 1-1/4" focuser, drilled some holes in the right place and screwed it in place.

I've also added a homemade mount for a Daisy unit-power reflex sight. I'd bought the Daisy sight because I thought they looked like neat toys and I wanted one. I still think they're neat toys, but in truth, I like regular finderscopes better (especially the excellent 6x30 which came with the Coulter, which is better than the finder on my Cave 6", though the dovetail mount isn't as good, only having one ring instead of two) and I find I never bother to mount the Daisy sight, or, if I mount it, I never turn it on unless I suddenly feel the urge to see the neat LED light up.

Finally, the other problem with the CT-100's design is that, having only one truss bar, the cloth tube cover tends to sag into the optical path. For a long time I didn't use the cover for that reason, but after the very dusty PAS Yosemite star party and a damp star party at Fremont Peak where the mirror dewed badly, I decided that I wanted a cover. So I got a bunch of bamboo barbecue skewers, which just happen to be exactly the right length, sewed a lot of loops of thread into the outside of the cover (a LOT of loops -- five per skewer, for six or seven skewers; I'm not a great seamstress so this occupied several hours which weren't nearly as fun as the several hours I spent making the brightness control for the Daisy sight), and now I have a semi-rigid tube cover which can't sag into the optical path. When I actually did the sewing, I didn't have any BBQ skewers handy, so I used brightly colored plastic pick-up sticks (a birthday gift from a friend -- my close friends obviously have a high degree of respect for my maturity level. :-) I really liked the look of the pick-up sticks (they reminded me of those colorful plastic fittings kids put on bicycle spokes), and they're easier to thread than bamboo because they don't get splinters, but they weren't quite long enough. Eventually I plan to look for some longer pick-up sticks.

What can I see through a 4.25" f/4 Newtonian?

When I first got the CT-100, I didn't expect that a telescope so small could show much. I was wrong. A 4.25" f/4 Newtonian has an extrodinarily wide field, so you can see views of several objects in the same field which larger telescopes can't begin to see even with the most expensive wide-field 2" eyepieces. The Double Cluster in Perseus is a joy in this telescope. In the summer of 1996, Jupiter spent several months within a degree or so of the globular cluster M22, and it was lovely to see Jupiter (with some banding visible even in the very low-power image), its moons, and M22 all in the same field. Later that summer during the total lunar eclipse, with a 40mm eyepiece I was able to show visitors to Foothill College a view of the eclipsed moon, a star near the moon's limb, and Saturn, all in the same field.

At the same time, a 4.25" has much more light gathering power than the small refractors which are today's common travel scopes. M81 and M82 in the same field are bright and obvious, and a wealth of smaller galaxies, like NGC 4565, are possible. The view of the Rosette nebula, with a 25mm Plossl and a UHC filter is the best I've seen -- bigger scopes gather more light but have focal lengths too long to show the whole nebula and cluster in the same field.

What doesn't it do well? In a word, planets. It's hard to get a good image at powers greater than about 60. Part of the problem may be the relatively large secondary that such a fast 'scope requires, but I suspect that most of the problem is collimation difficulty -- the CT-100 does have collimation screws, but they're of the lock-screw variety rather than spring loaded, which means it can only be collimated when pointing more or less straight up, and tightening down the lock screws can affect the collimation so you have loosen the lock screw and repeat the process. There's a small amount of slop in the dovetail mounts where the mirror and secondary units mount to the truss bar, which presumably also affects collimation, though I'm still not clear quite by how much. And when I attempted to star test the 'scope after the last time I collimated it, I noticed that the out of focus diffraction rings were symmetrical inside of focus, but noticably comatic outside of focus, which I suspect means that I didn't get the focuser quite square when I mounted it (which effect would be magnified outside of focus), a problem which I will eventually fix.

I'm still relatively inexperienced at both collimation and star testing; when I become a collimation wizard, I'll figure out how to get this right, and then perhaps the CT-100 will perform reasonably on planets as well as deep-sky objects.

Why doesn't someone still sell something like this?

Good question, and one which I frequently ask myself. The only similar unit that's currently commercially available is the Edmund Scientific Astroscan, another 4.25" f/4 Newtonian. However, the Astroscan comes in a bulky plastic package which is difficult to mount on a tripod or to fit a finder to, has collimation which is not user adjustable at all, and mounts its secondary via an optical window of perhaps questionable quality, instead of a more standard spider. (Actually, the CT-100 doesn't have a standard four-vane spider either, but rather uses a spider of two thicker vanes, rather like a scaled-down version the Coulter Dobsonians from that era.)

I think there are good Astroscans; I know several experienced amateur astronomers who own Astroscans and like them. I have never seen one, however; I've only had the opportunity to look through a couple of Astroscans, but every one I've looked through has had pronounced field curvature or other serious optical problems. I'm not talking minor subtleties in the shape or number of the diffraction rings, but rather inability to reach focus on large parts of the field. I hope it's just coincidence that I happen to have seen bad examples of the Astroscan, and that most Astroscans are much better than this, but I can't recommend them based on what I've seen.

Aside from the Astroscan, the only option for a 4.25" RFT is to build one yourself. Edmund sells Astroscan mirrors for a reasonable price; I'd like to think that perhaps the problems I've seen with their scopes is due to collimation or problems in the optical window, and not a poor quality mirror. I think there are other suppliers of mirrors this size, but I don't know for sure. Of course, if you're willing to take the time to grind a mirror yourself, 4.25" and 4.5" blanks are easy to come by. Building a tube for a 'scope this small should be easy; or if you want a collapsible backpacking 'scope like the CT-100, you could even use real coffee cans (I haven't investigated the availability of parts to make the dovetail and truss bar parts).

Orion also has a rather interesting 4.5" "short-tube reflector" offering. This is a short focal length 4.5" mirror (I'm not sure exactly what the focal ratio is) with a built-in barlow at the eyepiece to double the focal length. No one seems to know for sure how the optics are on this 'scope, and it's not clear whether the mirror might be usable without the built-in barlow - it may be that the mirror is spherical and the barlow section includes a correction for spherical aberration, in which case the short-tube would not be usable as an RFT without the barlow even if its optics are good with the barlow.

The rest of the "travel scope" options are either refractors, high-end ones like the Tele-Vue Ranger or Pronto or low-end ones like the Celestron 80mm or the Orion 80mm short-tube refractor; or catadioptrics like the Meade ETX, Celestron C90 or C-5, or the Pro-Optic/Orion 90mm f/5.6 Maksutov.

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