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"Fe - Fi - Fp - Fum
I'm on the trail of an ATM"
Adventures with
COULTER OPTICAL
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I first saw Coulter ads for parabolic mirrors in Sky & Telescope in maybe 1968. The specs and prices were very aggressive. I didn't buy any though.
About 1978 or 1979, during my days as an impoverished grad student, Coulter advertised a mirror set for a 4" RFT. The price was ridiculous, something like $20. This time, I bought one. It turned out to be not just the mirrors but a slightly more complete kit. I wasn't entirely knocked out by the secondary, as it was a nasty little rectangular thing, but it worked OK. The kit included a piece of dowel with one end cut off at 45° and the other slotted for whatever the customer could cobble together as a spider vane. There was a drawtube holder made out of mystery aluminum and something like phenolformaldehyde, and a pretty decent tube of thin-walled phenolic-impregnated (maybe) black cardboard with a hole precut for the drawtube. I still have it. That damn tube just won't die—unlike SonoTube or all those mailing tubes we inveterate telescope-making hacks just can't bear to throw away, the Coulter tube didn't get "fuzzy" around the edges.
Time passed. I read a semi-current book about amateur astronomy and found out that while I'd been putzing about in school there'd been a refractor revival, that some guy named Al had discovered that he could get people to spend more for a single eyepiece than any halfway rational person would dream of shooting on an entire telescope, and that another guy named John had blown the aperture barrier all to hell, and for less cash than Al thought I should spend on an eyepiece. Well, hot damn!
The Odyssey
Once again, Sky & Telescope to the rescue. Those distinctive page-edge ads showed that Coulter was still in the game, this time with the Odyssey, a telescope much like the one I'd bought the components for years before, just three times larger. Here's an ad from 1980 (I stole this one from www.philharrington.net), and another from 1988 (this one stolen from http://www.silverbugstudio.com/kirk/coulter/ [Oops, dead link, it seems]). The 1980 ad is for the earlier style, now called the "blue tube" telescope. The 1988 ad is for the later style "red tube" telescope.
So I called up Coulter Optical and spoke with some nice lady who estimated that it would cost me about $200 to ship an Odyssey I. Hey, lousy seeing and high shipping costs from California – welcome to Massachusetts! So I temporized; instead of the telescope, I had her mail me the literature. See below for the whole deal. The at-that-time-current owner's manual was included with the literature package.
Mirrors, it turned out, were postpaid. Well, that's more like it! So I called the nice lady again and ordered a 13.1" mirror and a 3.10" diagonal, all for $334.95; one-third down, please. They were due in six to seven months. It turned out to be considerably longer. But in due course I received notification that my mirrors were ready. That was probably toward the end of 1991.
Coulter was selling this mirror as a 13.1" f/4.5. A Little-Known Fact (that is, one the literature didn't say anything about) was that Coulter's spec on the actual focal length was plus or minus 3 percent of the nominal focal length. Most mirrors ended up with short focal lengths. And it Must Be True, because it came straight from my confidential source, the lady on the 'phone.
A bit of glass with a 13.1 inch diameter and a focal length of 58.95 inches would be an f/4.5 mirror. Strictly a 13.1" piece of glass does not a 13.1" mirror make, as there's a bevel on the edge. So the diameter of the parabolic reflecting surface is a bit under 13.1". Coulter didn't seem to get too excited about this, so neither will I.
The mirror I eventually received was marked on the edge 13.1F/4.5 – evidently the blank size – and F.L.57.3". There was also a mysterious circled -3.
That focal length, 57.3", is 1.65" shorter than the nominal 58.95", or 2.8% short – within the specified tolerance range. However it's more like f/4.37 than f/F4.5.
The circled -3 is a persistent mystery. My WAG (that's techno-speak for "wild-assed guess") is that it is a number Coulter used to match mirrors and tubes when assembling telescopes. For the 13.1" mirror, +/- 3% is +/- 1.77", or a total variation of 3.5" for in-spec mirrors. That's a bit more than an el-cheapo focusser could be expected to accommodate. So I suspect that Coulter made both undersized and oversized length tubes (strictly, not oversized tubes, but longer distances between the mirror mount at one end and the focusser and spider at the other). When assembling telescopes, they would match mirror focal lengths to tube lengths. If they made tubes in 1/2 inch intervals, then a -3 mirror would go in a -3 tube, or one 1.5" shorter than the nominal size. And 1.5" under would be close enough to the actual focal length of this mirror for the focusser to do its thing. Anyway, that's my theory, and we're stuck with it.
But enough of Coulter – on to Bill's Heavy Industries. Of course the obvious problem with anything I put together myself, relative to anything I buy already made, is that I'm always screwing with the design. So at any particular time, whatever-it-is tends to be lying around in pieces, and is not very usable. Just because a Dobsonian telescope is made of cardboard and plywood doesn't mean that the parts can't be arbitrarily complex. That was perhaps John Dobson's real breakthrough – at some point he knew when to say enough, time to stop screwing around and use the damn thing.
So although my 13" telescope works just fine, it's not particularly usable at the moment, as it's in pieces. Again.
I met John Dobson a few years ago. A real character. I never met James Braginton (or maybe it was Jacobson – anybody with two names is probably a character, too). He died a decade ago, and Coulter Optical didn't survive him.
Odyssey sales literature
And Odyssey owner's manual
PDF conversions courtesy of smallpdf.com
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