Cometron CO-114 4.5" Newt: Vixen optics & GEM
Posted 12 November 2012 - 10:15 PM
The Polaris had never been used because of a defect in the manufacturing; the counterweight shaft nut had never been tapped! It was easy to tap the nut for M12-15 threads. The DEC axis was incredibly stiff, due to the interference fit of the bearing. I took off a couple of thous, cleaned the old glue out and relubed, and now the mount is simply glorious to use. I did fit a Vixen saddle to the Polaris, so can now use it with a variety of smaller ‘scopes. I've not fitted the MD-5 drive (also unused) to the RA axis because the mount is so nice to use manually. Another modification made was to use a Super Polaris counter-weight shaft which is a direct replacement for the original threaded rod. This enables me to use the heavier counterweights from a SP.
I don’t want to get distracted by the mount, as this article is about the Cometron OTA, or as I call it, the "Vometron". The tube had been stored upright for the past 30 years, and obviously had fallen many times against a table or something, because there is a whole series of dents in the OTA at about the same part of the tube. A quick panel-beat of the tube knocked out most of the dents, but it is still ugly. I thought about bondo'ing the dents and painting the tube white (see my avatar to the left), but the grunge-allure of the battle-ship grey tube with dents is too much to resist.
The original optics were trashed. Stored upright, the primary collected dust which corroded the aluminizing. I tested what was left of the Circle-V primary and it is parabolic. I thought of having the 4.5" mirror stripped and aluminized by OWL, but Cary would not respond to my emails. In the interim, I found a set of optics that were rescued from a C4.5, so I snatched them up. In subsequent tests, the replacement mirror is parabolic (Ronchi testing) and diffraction limited (star testing). More on that later.
The Cometron had optics by Vixen, so I suspect that Celestron used exactly the same mirrors as the C4.5. But there any similarities end. My Vometron was equipped with an extraordinarily poor 0.965" plastic focuser. The 5x24 finder is plastic, including the lens. The mirror cell is plastic. The spyder is the same as the C4.5, but the secondary holder is plastic. The 135mm diameter battle-ship grey tube is thin steel with a crimped seam. The C4.5 uses a tube diameter of around 145mm. Another cost-saving exercise by Cometron. When I bought the replacement mirror, it also came with secondary, spyder, metal 1.25" focuser and primary cell. The primary cell would not fit my grey tube, but the focuser and aluminum secondary holder bolted right up. I had an old but nice Cometron 5x30 focuser from a CO-62 and stalk lying around and that bolted right up. I mounted the rings to a Vixen-style dovetail so I can use it in the modified Polaris saddle.
First light was surprising. I used first-generation Celestron Silvertops for this review, arguably some of the nicest Plossls ever made. The Vometron easily split Epsilon Lyrae (the "double-double") at 90x. I could easily make out the ring of M57. Only a handful of M13 stars could be individually resolved. I was not able to crack Cassini's, but Saturn's elevation of less than 15 degrees may have been a contributing factor. The Moon was absolutely gorgeous even at 240x (7.5mm Silvertop w/2x TV Barlow) with no image break-down. No false color anywhere. Star testing produced text-book rings in and out of focus.
In my experience this appeared to be a really a nice 'scope with the modifications that I made. Cool-down time was about as long as it took me to fetch the eyepieces after carrying outside the whole rig. Definitely a grab-and-go, this rig was easy to haul around as a complete unit. I'm looking forward to testing this against my also-nice 90mm f/11 achro.
Posted 13 November 2012 - 12:57 PM
Posted 13 November 2012 - 01:02 PM
Posted 13 November 2012 - 01:04 PM
Posted 13 November 2012 - 01:11 PM
Posted 13 November 2012 - 03:34 PM
Looks the same to me. The original Cometron spider vanes are the same as the metal C4.5 spider vanes, but the secondary holder was plastic.
Do you have a picture of the rear of your scope for comparison?
Posted 16 November 2012 - 06:33 PM
Posted 16 November 2012 - 09:30 PM
I'd be surprised if yours was truly a dog. Probably needed a serious collimation.
Posted 20 November 2012 - 08:30 AM
The tube cap on the C4.5 has Japanese script on it (presumably the usual disclaimer "Do not look at the sun", etc), whereas the Cometron cap has no writing, and is from a much simpler mould.
Posted 20 November 2012 - 09:17 AM
Posted 29 July 2014 - 11:21 AM
Posted 29 July 2014 - 12:23 PM
Posted 29 July 2014 - 01:48 PM
Another alternative would be to find a focuser from the Celestron C4.5, which should fit and was originally 1.25 to start with. Astro Parts Outlet might be a quick way to find alternatives.
Posted 30 July 2014 - 09:27 AM
Also, thanks for the C4.5 suggestion, I will keep an eye out for one so I can take the focuser from it, they look like they still go for a pretty penny though.
Posted 30 July 2014 - 02:06 PM
Posted 01 August 2014 - 02:37 PM