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Edmund 3011 6" F6 w/ late-70's GE mount - overview

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#1 amicus sidera

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Posted 15 June 2012 - 02:15 PM

My recent acquisition of an Edmund Scientific model 3011 6" f/6 reflector, on Edmund's late-1970's German equatorial mount, has allowed me perform a detailed inspection and disassembly of both telescope and mount. The telescope OTA itself shares an almost identical design, and has most points in common with its larger 8" f/5 sibling, save for aperture (follow this link to the Cloudynights thread on the Edmund 8" f/5 4001); however, a detailed review of this particular mount doesn't appear to be extant on the internet. What follows is a modest attempt to fill this knowledge gap. A few items of interest specifically related to this example of the 6" f/6 OTA will be also be addended.

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#2 amicus sidera

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Posted 15 June 2012 - 02:17 PM

I cannot emphasize strongly enough that I performed all work on this instrument using proper tools of good quality. Attempting such work, or any similar work, without the correct allen wrenches, screwdrivers and other necessary tools, and without sufficient experience in the repair of small mechanical assemblies, can easily lead to damaging the instrument beyond reasonable repair.

The Edmund Scientific Medium Duty Equatorial Mount, part numbers 85311/85312 (4-1/4" and 6" OTA respectively), was available from the late 1970's through approximately the early 1990's. This mount replaced previous Edmund versions of the German equatorial mount found on such models as the 4-1/4" f/10 Palomar Jr. and the 6" f/8 Space Conqueror. This version used the same 5/8" shafts as the Palomar Jr. mount, but with a larger polar bearing area for improved stability. The majority of the parts for these mounts consisted of cast aluminum, in an effort to reduce their overall weight. The newly-redesigned 4" and 6" reflectors, in various focal lengths, found a home on these mounts (the 6" was also available on the heavy-duty fork mount for a time in the late 1970's and early 1980's). The 8" f/5 reflector from this period was never offered on this medium-duty mount.

The photo below shows the clock drive motor side of the mount, on its pedestal base (this mount was also available with a wooden tripod, for use with Edmund's 3" and 4" f/15 refractors (part numbers 85303/85304 respectively). The drive is Edmund's standard: a 1/15 rpm, hysteresis (self-starting) synchronous AC motor, turning a worm and 96-tooth gear combination, and fed via a cork disc clutch.

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#3 amicus sidera

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Posted 15 June 2012 - 02:18 PM

The other side of the mount shows the clock drive shaft preload setscrew as well as the latitude-adjustment bolt, which on some models is instead a large (3" diameter) cast-aluminum knob.

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#4 amicus sidera

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Posted 15 June 2012 - 02:20 PM

The counterweight safety washer, at the end of the declination shaft... this is there to save the telescope (as well as your foot!), should a counterweight loosen and slide down the shaft.

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#5 amicus sidera

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Posted 15 June 2012 - 02:21 PM

In this next photo, I've (carefully!) removed the counterweights and the declination shaft collar, and slid the declination shaft collar/saddle assembly out of the casting... note the two bushings, which have been placed onto the shaft for illustration purposes. These bushing slide into each end of the casting, and then the shaft slides into them. These bushings, along with a similar pair on the right ascension shaft, are most likely made from either Teflon or Delrin. I've found that, smooth as they are, a little light grease helps them work even better.

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#6 amicus sidera

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Posted 15 June 2012 - 02:24 PM

Up where the declination shaft enters the saddle, it is held in place by a single setscrew... needless to say, this is an important setscrew! It shouldn't be touched for any reason, unless it becomes loose (which is unlikely), as it holds the shaft onto the saddle. If loosened, the entire shaft, with counterweights, could slide out of the mount, the freed saddle and any instrument attached to it also falling, with attendant possible personal injury.

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#7 amicus sidera

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Posted 15 June 2012 - 02:26 PM

Next up is a photo of the top of the mount, showing the strap system for securing the telescope tube to the saddle. The straps are similar to those used to hold large boxes together for shipping purposes. While quite strong, they should be inspected whenever used and replaced if found to be worn or damaged in any way. These straps should be tightened sufficiently to hold the optical tube assembly, but not overtightened, as the tubes are made of phenolic-impregnated paper and can be become somewhat brittle as they age (+30 years, for most instruments as of this writing).

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#8 amicus sidera

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Posted 15 June 2012 - 02:27 PM

The saddle tube bearing points on the example shown consist of screws covered by vinyl covers. It's my understanding that previous examples used felt-covered cork, which I feel was a better method, as the bearing surface was considerably larger; reference comments regarding tube material above - that's a lot of pressure at four small points, on an old and possibly brittle tube.

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#9 amicus sidera

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Posted 15 June 2012 - 02:28 PM

The straps are routed in a not-particularly-intuitive manner, as illustrated below. According the the Edmund manual for this mount, the thermally-welded part of the strap should go underneath the flat bottom part of the saddle, as indicated.

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#10 amicus sidera

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Posted 15 June 2012 - 02:30 PM

The tension of the polar axis is regulated by means of a clutch, so that tracking may continue even as the instrument is being moved about. The photo below shows the knob at the rear of the polar axis shaft used to tighten the clutch as desired.

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#11 amicus sidera

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Posted 15 June 2012 - 02:31 PM

The polar tension knob assembly consists of the threaded knob, a washer and a small ball-bearing race, the latter's function being to allow polar axis movement without loosening or tightening of the threaded knob, and hence retaining the desired amount of tension.

Here it's shown partially unthreaded for clarity:

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#12 amicus sidera

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Posted 15 June 2012 - 02:32 PM

The three parts of the polar tension knob assembly.

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#13 amicus sidera

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Posted 15 June 2012 - 02:33 PM

With the knob assembly removed, the polar axis shaft is free to be removed from the main casting... the photo below shows the polar shaft partially removed, with the drive gears and cork clutch exposed in their housing.

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#14 amicus sidera

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Posted 15 June 2012 - 02:34 PM

As with the declination shaft, there are two plastic bushings at either end of the casting, in which the polar shaft rides. Below is shown the bushing nearest the drive gears.

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#15 amicus sidera

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Posted 15 June 2012 - 02:34 PM

This next photo shows the other polar shaft bushing, seen here for illustration purposes protruding from the end of its cavity (in use, it is flush with the end of the casting).

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#16 amicus sidera

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Posted 15 June 2012 - 02:35 PM

A semi-exploded view of the polar shaft and its ancillary components. It's worth noting here that both the worm and gear are precisioned-machined items, and that utmost care should be taken whenever handling or working near them, so as not to nick, dent or otherwise damage them.

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#17 amicus sidera

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Posted 15 June 2012 - 02:36 PM

The cork clutch only lasts so long, depending upon conditions of storage and exposure, but is easily replaced by cutting a new one from thin (approximately 1/16" or 3/32") sheet cork, readily available from craft stores or online. Be sure to obtain the densest, finest-grained cork you can find for this application. The dimensions of the clutch are shown (not to scale) in the photo below.

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#18 amicus sidera

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Posted 15 June 2012 - 02:38 PM

The next photo shows the worm on the motor shaft, within the casting. The worm itself should be directly under the centerline of the polar shaft to mate properly with the gear, as shown, in this case to the far right. Note that the end of the motor shaft is contacted by a small ball bearing, which is held in contact by means of a setscrew accessible on the outside of the casting. Its purpose is to "preload', or take a reasonable amount of play out of, the motor shaft.

Caution! Under no circumstances should this setscrew be tightened any more than is absolutely necessary, as damage to the motor and its reduction gear train may result. These motors are no longer available except as custom-order items, at great expense and inconvenience... tolerating a tiny bit of play is a much better proposition than having to source a new motor!

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#19 amicus sidera

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Posted 15 June 2012 - 02:39 PM

Here's a close-up of the worm... note the tiny setscrews, which use an .050 Allen hex key. Any attempt to loosen or tighten these setscrews MUST be done with a high-quality, properly fitting allen wrench, by someone very well-versed in working on such small parts, as they can strip out very easily. Note also the flat area on the shaft, which the setscrews tighten down onto.

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#20 amicus sidera

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Posted 15 June 2012 - 02:43 PM

I decided to lubricate this motor's geartrain (it hadn't seen oil in 30 years, so thought it a good idea!). The motor must be removed for this procedure, and hence the worm gear must be loosened from the motor shaft to allow it (see above). Two allen hex head screws, used to attach the motor to the casting, along with the wire cable clamp, must be removed as illustrated in the photo below. Note that one of the screws has a green wire attached to a lug underneath it. This is the electrical ground, and MUST be reconnected in exactly the same manner as it was removed. Remember, there is LINE VOLTAGE applied to this motor, with all its attendant hazards and possibility for electrocution. Personally, I never operate any telescope using a 110v AC motor without using a ground fault circuit interrupter (GFCI).

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#21 amicus sidera

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Posted 15 June 2012 - 02:46 PM

Here's an exploded view of the motor and its shaft, the worm, ball bearing and setscrew described previously. I later cleaned and lubricated this part of the drive.

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#22 amicus sidera

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Posted 15 June 2012 - 02:48 PM

The motor itself rarely requires lubrication, but the geartrain (the pear-shaped box below the motor, and attached to it) is another matter. I don't feel that it's necessary to remove the geartrain cover for this operation, as Edmund appears to have very kindly specified a lubrication point on this motor! Note the small hole on the bottom plate, next to the motor shaft... a very light, high-quality oil can be introduced to the geartrain via this hole. In my experience, the oil migrates to all the gears inside fairly rapidly. I also place a drop or so where the motor shaft meets the bronze bushing, as shown below, as well as the two small pinions on both the bottom plate and the other end of the shaft (the latter not visible in the photo below).

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#23 amicus sidera

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Posted 15 June 2012 - 02:55 PM

The factory finder on the 6" f/6 is a 6X30mm convertible model... it can be used either straight through, or with a diagonal as seen here (the factory default method). If one wishes to convert to straight-through use, it is necessary to slide the objective lens forward in its cell to allow the eyepiece to reach focus - I've found that a piece of stiff paper, tightly rolled, serves admirably for this purpose. The eyepiece is then secured using the thumbscrew, and final focus checked.

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#24 amicus sidera

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Posted 15 June 2012 - 03:09 PM

It appears that, sometime in late 1981 or early 1982, Edmund discontinued the use of the high-quality, cast-aluminum-body 1-1/4" focuser for their 6" telescopes. My first 6" f/6, purchased in August of 1981, had such a focuser, while this recently-acquired instrument, purchased originally in April of 1982, has a focuser that is somewhat simplified.

Apart from the drawtube and the focuser knob shaft, it appears to be made entirely of black thermoplastic, and up-and-down movement is accomplished by means of the drawtube moving against a rubber tube which has been slid over the focuser knob shaft, alĂ  Edmund's Astroscan.

Contrary to what might be thought at first, it actually works quite well, providing the rubber makes good contact with the drawtube. Replacement rubber tubing that is too thick exerts too much pressure, and makes focusing a bit difficult, but with the correct piece of smooth (not ribbed) tubing in place, it functions very smoothly. Additionally, the focuser knobs can be adjusted via setscrews to increase or decrease the stiffness of the focuser, by making them tighter or looser where they contact the focuser housing.

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#25 amicus sidera

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Posted 15 June 2012 - 03:15 PM

The focuser drawtube has two "tabs" on its very top, again in the style of the Astroscan, and these are bent outward to prevent the tube from accidentally falling into the OTA, should it be moved too far inwards.

Altenatively, one can be bent inwards to better grip the eyepiece.

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