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Losmandy GM-8 Mount and Digital Circles/Computer



The Losmandy GM-8 is one of my favorite things. I remember the first time I put it together and admired it. It is just one of those rare acquisitions that have some special aura surrounding it.

First, I would describe the unit itself. Pictures don't do it justice. The unit is substantial. Everything is sized generously, and the rating of 30 pounds, while probably accurate, can most likely be attributed to the fact that if you are using the unit with the legs fully extended, you will probably be pushing the stability limit with that much weight. Two of the optical assemblies that I have used it with however (MN61 and Celestron C9.25), have allowed me to use the tripod with the legs either fully retracted, or extended less than 10 inches, and at that height, I think that the unit could carry much more. I used the tripod with a Celestron CR150 briefly, and to be able to view at zenith, the legs would have to be extended all of the way (and I would still be sitting on the ground!). Because I also had 2" diagonal and heavy eyepieces, and Telrad finder, this configuration wasn't as acceptable. The weight being at the ends of the tube assembly created considerable polar inertia. When bumped or when disturbed by a wind gust, that heavy weight on the ends of the OTA caused a long period oscillation, with maybe five to seven seconds dampening time required. As mentioned earlier, this mount is perfect for a C9.25 or MN61 size instrument.

The workmanship on this mount is beautiful. I love the satin feel of the machined parts, and tolerances and fitment are first class. It is a showstopper in the looks department. It has that machine age elegance that many of the newer goods today don't present. There is nothing wrong with castings and plastic covers and such contrivances that may appear on other goods you might purchase, but the Losmandy mount screams loudly, "I am a precision, highly crafted, fine instrument."

The tripod head is a steel tube, maybe 6" in diameter and about 8" long, which had robust leg attachments. The legs are a rectangular section, and are quite sturdy. Even when fully extended, they offer excellent flex resistance. The equatorial assembly attaches by three large screws that drop into slots in the tripod head, and turn in "L" shaped groves to lock down. A couple turns with an Allen wrench tighten up things. While the tripod doesn't have quite the aesthetic appeal as the equatorial head, it is still nicely finished in a matte black paint that appears very durable.

There is a large knob for setting latitude, and I have found that it is not necessary to tighten the unit in this plane. The latitude adjuster easily supports the weight of the head and telescope, so I would just put light tension on the side screws so I could do fine polar alignment using the latitude adjuster. There is also a nicely sized fine adjustment for azimuth.

Setting Circles are a bit on the small side, and while they were fine for my smaller scopes, sometimes I found that the graduations were not quite fine enough for use with the smaller FOV of my Celestron 9.25. No matter, because I am a digital boy, so I really only use the setting circles to do the initial 90 degree Declination setting when polar aligning the scope. Losmandy instructions for Polar alignment and adjustment are excellent. An important inclusion is that the instructions provide detail for checking to see that your optical axis is perfectly square to the equatorial mounts polar axis. In some optical systems, you might actually encounter this, and if you do not correct the condition (which might require the use of shims between the scope rings and the dovetail plate), you might have problems with pointing accuracy. Again, the instructions to identify this condition are very well written. As a matter of fact, my CR150 required some shimming to achieve this alignment.

Motions are smooth and the drive has several fine adjustment speeds and a "Fast Forward" functions, which allow slewing at the most rapid mode that the drive supports. On the hand paddle, there are reversal switches for both right ascension and declination. This is useful if you want to position the hand controller a certain way, so that if you press the right button, the object moves either left or right in the field of view. I usually sit with the cable of the hand control coming out away from my wrist, and I orient these switches so that when I view through the eyepiece, pressing the right button will move the object to the right in the field of view, and pressing the top button will move the object up in the field of view. As you change positions, it is nice to be able to re-map these buttons so that you don't have to bump navigate.

Tracking is rock steady, though I had to ask Losmandy to replace one of the original stepper motors to achieve this. The original motor "Ticked" in movement. This was only really a problem at high power, but that is where I do much of my observing. Scott sent a new stepper motor and now, even at high magnifications, tracking is smooth.

Several tracking rates are available, with Lunar and Solar, as well ask King rates.

Periodic error correction and a function that helps deal with worm gear play in the declination axis are also included. I am not an astro-photographer, so I cannot comment on the effectiveness of the PEC function, however the function for taking out the lash in the declination axes works fine.

The Losmandy digital setting circles and computer are a little expensive, but have features that seem to make them worth the extra money. While the instructions were very complete, do be aware that there are many small parts involved in fitting the encoders to the polar and declination shafts. You have to wade through a stack of small washers, shims, and bearings, and some of the spacing is important. I have a really strong electro-mechanical background, and I found that I had to pay close attention to the diagrams to feel comfortable that I had the digital encoders and various shims and washers configured properly.

Once the circles are installed, instructions for polar aligning and for initializing the computer have you up and running in minutes. I always do the two-star align, even when I do a polar alignment using the polar finder, as I feel that I got better pointing than when using the one star align, which requires careful polar alignment

I previously owned a 10" Meade LX3 that I bought in 1986, which I fitted with digital circles some years ago, and that computer lacked some of the most useful features of the Losmandy unit. For example, the Losmandy unit offers magnitude filtering, which just suppresses the display of data for objects that the user deems to faint for their location/instrument.

The two functions that are most useful in the Losmandy computer are ones that differentiate it from most other units I have used, including the computer on my new Nexstar 11, and they are features that in my opinion, every computer should have:

  1. The first is an "Identify" function. This function can be set to run in the background. When you are slewing the telescope, either manually or using the drives, the computer is tracking your position and searching the database for any object that comes within .5 degrees of the scopes position. Every time you come within that half-degree of something in the database, the object name or catalog number will pop up on the display. If you want to just roam around the sky, and you stumble across something faint and fuzzy or a cluster, is all you have to do is look at the display, and it will give you the object name, and with the press of a button, the other information (magnitude, description, etc.).
  2. The second function, and the one that I find to be even more important, is the "constellation diamond". On the top line of the display in normal mode, the constellation name appears as a three-letter abbreviation at the right end of the line. Next to it, there is a small diamond symbol. Positioning the cursor under that diamond lets you scroll through objects in the constellation that the computer is currently set at. It shows just about every type of object in the catalog that appears in that constellation (magnitude filtering is in effect), and if you see something of interest, it shows which direction you need to move the scope to center the object. This means that I can go out for an evening of observing, and never have to consult a chart. Believe me, this isn't desirable to me because I am lazy; it is desirable to me because I am an observer, and find no enjoyment in creating a viewing agenda for an evening. Also, conditions in my back yard limit me to a small window for viewing (trees and other obstructions). Consequently, I tend to focus my observing in a small section of the sky at one time. This also allows me to stay in the same position for longer periods. To me, moving everything around to change to a different part of the sky is a pain, so I like to focus on two or three areas in a session, but do them well.

Observing with the GM-8

As with any finely crafted equipment, this mount is a pleasure to use. When you walk out with your OTA and slide the dovetail into the mount, your eyes will always be drawn to the beautiful finish, and the "form follows function" kind of pureness to the design. It looks good and feels good. The motor drive board has a black anodized mounting plate, and the tactile feel of the switchgear is complimentary to the overall feel of the mount. For the mount itself, everything is well laid out, and easy to access. If you have the digital stuff, you will have cables are everywhere, and this could detract slightly from the use of the mount. I had to tape cables into bundles to keep them from dragging or snagging.

The clutches are smooth, with a good sensitivity to pressure, so that you can usually find a tension that allows fluid movement when slipping the mount against the friction of the clutch (a very large plastic-like thin disk, maybe teflon). There is no skipping or binding. The clutch knobs themselves are a natural satin finish (aluminum?) metal that is easy to encircle with the thumb and pointer, and make small adjustments easy. I mounted a C 9.25 with optional 50mm finder, VirtualViewer, 2" Everbrite Diagonal, and heavy 2" Televue eyepieces on the Losmandy GM-8, and the clutches handled everything easily. Also, I might mention that this combination was extremely solid, and offered exceptional viewing comfort. It handles the MN61 I currently use if for with ease.

Motor driving is smooth, with several choices for slew rate. Long time tracking (Visual) is perfect. Hand controller buttons are positive in action, and well spaced for use with gloves. The cord on the hand controller is way long, so if you want to use your long refractor, this won't be a problem.

Note… There are no provisions for slow motion controls on this mount, but I doubt that you will miss them. You either use the motor, or you push where you want to go. The clutch action is so finely adjustable that there just isn't any need for a slow motion control. Once you have the balance set, and the clutches tensioned properly, this thing moves like a dob. Small pressure can cause the scope to move to the desired position. If you are using a combination of heavy and light eyepieces, this can throw the clutch tension off some, however if you tighten a little to correct for this, you can still reposition without difficulty. Also, this action is so fast and easy, that you will leave a "Go-to" drive in the dust when making manual corrections.

I usually use mine with the legs retracted, and used this way it is rock solid. I did have a CR 150 on it for a while, and to use the long refractor at zenith, I would have to fully extend the legs. With a heavy eyepiece, diagonal, finder, and telrad, this thing had some horrendous polar inertia problems. What that means is that the weight was all out at the ends. In this configuration, the GM-8 was less stable. I saw someone with a CR 150 mounted on a G 11 once, and though it seemed kind of silly to have a $500 optical assembly riding a $2000 mount, but in retrospect, it was probably necessary to keep the CR 150 rock steady. Small breezes would disturb my setup, and there was a settling period. I am sure that vibration suppression pads would have helped (what is that weird orange stuff Celestron uses in those things anyway??? It still freaks me out to poke it and pull on it).

Criticisms:

Few. Some are small, and some are maybe not so small.

Small first… Some of the appointments could be better. For example, the cigarette lighter cord that Losmandy includes is flimsy. The wires are quite thin, and the connector that plugs into the drive panel does not have a strain relief, so there is nothing to hold the wires in the screw on cover of the connector: the wires are only held in place by the solder joints at the plug. Normal flexing of the wires will cause them to break at the connector. I have already had to repair mine twice, and it is limping again. By contrast, the cord on the power pack for my Nexstar 11 (an AC wall transformer, with a cord to the plug on the base of the scope) has thick molded cable, with a strong strain relief molding built in. I doubt that it will ever break under normal conditions. The Losmandy one is fragile by comparison.

The hand controller case feels a little flimsy. The box is constructed of lightweight molded plastic. In service, it is fine, but I am worried that if I step on it in the dark it will break like an eggshell.

There are a lot of wires and cables. Unavoidable perhaps, but some of them could have been bundled better. There are several cables that have to go to the computer, and I had to tape mine together to keep them from snagging on things. They are also stiff, and awkward to pull around the mount. A coiled cord to the computer from a single plug on the mount, which splits off the other cables, would be a nice touch.

The standard counterweight is totally out of character to the elegance of the rest of the mount. I've seen nicer boat anchors. There is a machinist that advertises his replacement weights on the web, and they are far more in style with the beautiful Losmandy workmanship than the standard weight.

Now for the not so small, and they basically summarize into one area…

  • Price. Remember, I consider this to be a beautiful piece of equipment, and I am proud to own one, however if you consider the overall cost of ownership, the dollar expenditures can stack up quickly. The mount and drive controller at about $1500 are actually pretty well priced. The pain comes when you start adding accessories.
  • The polar finder is in my opinion, rather expensive, though it does work well. It is a zero power unit, and doesn't even have variable brightness for the alignment reticule (though many other polar finders don't have illumination at all). I have seen it advertised for $145. Finders of the same type for other GEM mounts can be purchased for less than $50, and some competitive mounts come with polar finders as standard equipment. An option is to buy an Illuminated Reticule 8x50 finder scope, such as the Celestron unit ($99 w/o brackets, $189 with mounting brackets for a C-8)… If you were going to upgrade a 6x30 finder anyway, this way you could kill two birds with one stone…
  • The small, anodized metal covers for the motors are a cosmetic must have, but again, you'll pay about $79. The standard rubber covers are functional, but detract from the otherwise elegant appearance of the mount.
  • The computer itself is pretty reasonably priced considering the wonderfully functional software and comprehensive database, but by the time you purchase the encoders ( aprox. $325), and the optional delrin encoder gear covers ($70, which in my opinion are a must have), you're going to spend a bundle.
  • Speaking of optional gear covers, it is easy to grind a cable in the exposed gears of the encoder assembly. I already did this. I didn't think the thing would come without covers, and identified the exposure as soon as I installed the encoders. The covers were already on order when the cable pinch occurred. I could see someone pinching clothing or body parts in there as well. In my opinion, the delrin covers should be part of the encoder package. Moving gears should ALWAYS be covered, as even small ones are capable of generating enormous pinch pressures. In my early career as a field engineer, it was not unusual to hear of other employees loosing fingers in the gear trains of tabulating machines (I am dating myself here, eh?). Though I don't think that the Losmandy encoder gears pose that significant of a threat, I bet they could bite you hard enough to draw blood. I am surprised that Losmandy doesn't consider this a legal exposure. To charge an extra $70 for the covers seems to be really taking advantage of the customer.
  • The saddle plates for most scopes are the wider style as used on the Losmandy G 11. This means that to mount many scopes, you will have to put on the optional saddle plate adapter that allows the GM-8 to accept the wider G 11 style dovetail plate. Almost all of the Losmandy custom plates (for example, the Celestron 9.25 plate) require the larger saddle plate adapter. Also, the various dovetail plates are not exactly cheap (aprox $100). While some of these other things are optional, ya ain't gonna be able to duct tape your scope on here; you'll have to buy a dovetail. I think most people are going to want to put a larger scope on this mount one day, because it can hold it. I think that Losmandy would better serve the consumer by making the saddle plate size a choice at time of order (it is held on by a couple of screws). I had to upgrade mine to use it with the first OTA that I put on it, so the original GM-8 plate that came with mine has never been used, which seems like a waste of money to me….

Summary

I love it. OK, as I lust for larger and larger telescopes, I admit that I will probably sell it to replace it with a larger G11 one day, but I still love it. My criticisms for the most part only state the obvious: quality comes at a price, and (clever?) marketing can sometimes make you pay more for it (by bits and pieces in this case). The GM-8 would be a terrific all around choice for someone that needs to store and move around a medium sized telescope and mount, and that wants a jewel of an instrument that will allow them to mount a wide range of scopes. It does this with a mechanical elegance that is becoming rare in today's injection molded, die cast, stamped assembly junkyard. If it is big enough for the instruments you intend to use it with and you don't think you will outgrow it, it could be the most enduring and most cherished astronomy purchase you ever make. Maybe I will just keep mine and ante up the difference for the G 11. I like it that much.




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