Discmounts DM-6: Über-altaz

Alt-azimuth mounts are more compact than their GEM counterparts, are easy to set up and are wholly adequate for most applications with richfield telescopes (read: small apos). As such, the second wave of the apo revolution has been the proliferation of alt-az mounts designed to support small refractors. The DM-6 has acquired the reputation of being among the best of these.

ACQUISITION

The Discmounts DM-6 and its little brother, the DM-4 are made and distributed by Tom Peters of Davie, Florida. No secondary dealerships exist and all orders must go through www.discmounts.com. Tom is usually available by phone and will talk your ear off about astronomy. He loves the hobby is building mounts as a labor of love, having retired from his other job. I spoke with him many times over 4 months before finally purchasing the mount. He is never in a hurry and does not pressure a sale.

It is hard to find a used DM-6. Occasionally, one will show up in the Astromart classifieds, but they sell quickly. I purchased new. Total time from order to receipt of my mount was less than 3 weeks. Tom has his mounts professionally packed and uses UPS. Despite external damage to the shipping container, the mount was completely unscathed.

FEATURES

The '6' in DM-6 (and the '4' in DM-4) refers to the diameter, in inches, of the bearing surface on each axis. Larger bearing surfaces translate to more stability with weight changes. The DM-6 is designed to not require rebalancing of the telescope despite a one kilogram change in weight at the diagonal. Obviously the length of the tube matters, so the DM-6 will handle most 5" refractors and SCT's up to 11".

Tom makes his own tripods. It a word, the tripod is beautiful. Neither my photographs nor those at the website do it justice. You can choose between a standard (47.5" at the center of the altitude axis) or tall (56.5" at the center of the altitude axis) tripod. I opted for the tall tripod, as I think most refractor users will do. Even with the tall tripod, I still get "down and dirty" when observing near the zenith. Subtracting half of your scope length from the dimensions above will give you a reasonable estimate of the minimum eyepiece height.

When making recommendations on his mounts, Tom is very emphatic in saying that the mount and the tripod are an integrated system. For lighter scopes (most 4" refractors and down), his wood tripods are an excellent choice. For larger refractors, he recommends heavier-duty tripods or piers. With my NP-127, I considered the tall DM tripod and a G-11. He was sure of the stability with the latter, and reasonably confident with the former.

Personally, I find the tripod stability to be adequate for my scope. A Lapides-style spreader system would be a great addition, but this is not available. In order to fully collapse the tripod, you need to remove three braces, which can then bolt on to one leg for easy transport. The procedure takes a couple of minutes. I leave the tripod intact and carry the mount and telescope out my backdoor in one piece, but if you are considering speed of setup in your decision, the DM tripod is "low fuss", not "no fuss". A polymer tripod tray drops into place between the braces, and is worth the cost.

Those who have researched the DM-6 will notice immediately that mine is "right-handed". Normally, the altitude bearing is on the left side of the mount. Many refractors with two-speed focusers have the fine focuser on the right side. Having the telescope on the right side of the mount allows for better clearance near the zenith. As the second photo shows, I can get there, but again, the length of your scope is a factor. If you do not opt for DSC's, changing the orientation of the mount from standard to "right-handed" is as easy as a 180 degree turn. If you have DSC's, this maneuver will result in your Sky Commander pointing away from you. You have to reverse the top shelf. Unfortunately, this is not as easy as it sounds since the top plate and shelf are precut for nearly invisible wiring of the DSC's. You can cut the shelf off of the top plate and reglue, but if you anticipate using your scope on the right side, I would recommend mentioning this to Tom pre-order. He can set up the system either way. I am somewhat surprised that he doesn't make a point of discussing this on his website. I suspect that he will in the future.

Speaking of DSC's, Discmounts offers the DM-6/DM-4 with pre-installed Sky Commander. Reviews of this product may be found elsewhere. Suffice it to say that they work, and work well. This is a testimony not only to the DSC's themselves, but also to the orthagonality of the mount. It's an extra $490 for the Sky Commander and encoders (installed), but those considering this mount probably aren't looking at price as a primary factor. Two star alignment takes 2 minutes or less.

PERFORMANCE

Let me begin by saying that a 660mm focal length scope under Minnesota skies probably does not constitute a stress test for the DM-6. 30x per inch of aperture is actually a pretty good night here. I have run up to 220x a number of times on the mount, but the seeing was so poor that there were larger issues to deal with than hand-tracking. Can the DM-6 perform at such powers? Yes. Focusing is not difficult and tracking can either be done with a hand on the diagonal or by periodic nudging. With good initial alignment, the DSC's remain effective at this magnification. Perhaps someone under better skies can comment on the attributes of the mount at 300+ power. A bump on the tube stabilizes in less than 2 seconds. Tom Peters likes to keep this under a 1/2 second, which is why he is so apt to advocate heavier tripods. For non-astronomical reasons (specifically, the ability to keep the scope set up in the family room), I opted for his tall tripod instead of the likely more stable G-11. I find the platform stable enough, but YMMV. Keep in mind that the NP 127 only weighs 14 lbs. As such, it is lighter than some 4" scopes, and all 5" scopes except the Borg 125ED.

Adjusting the tension on the axes is easy. The top and side panels of the mount slide out to allow a quick wrench adjustment. I recommend starting a little bit tighter. I began by trying to go as loose as possible, but a little bit of tension actually works better for tracking as you bond with your mount. You can go from the 31mm Terminagler to a 5mm plossl without making fore-aft adjustments.

The hype on the DM-6 is that it is an "invisible" mount. In other words, you forget about the mount once you start to observe. Without a doubt, the DM-6 is a strong, stable performer. Invisible? No. Any mount that you need to push around is going to remain part of your consciousness. If I were predominantly a planetary observer, I'd probably put up with a GEM in order to gain tracking function. I prefer to say that it is easy to use, and well made. The most invisible mount I ever had was actually a fork mounted Celestron 8" SCT. I could remain seated all night from horizon to zenith without so much as a wiggle. In truth, refractors are harder to mount due to length. For all but extreme zenith viewing, the ergonomics of the DM-6/NP127 combination are fairly stable. If you need to get to the zenith, you also need to get to the ground. On the other hand, eyepiece orientation doesn't change, so you can do without rotating focusers or awkward viewing angles.

ANALYSIS

I am happy that I purchased the DM-6, and would further recommend the DSC's to those who are considering this mount. I don't use them all the time, but I have never regretted having them. I used to scan reviews for some sort of a statement about whether the owner would ever consider selling the item. The truth is, I sell a lot of equipment as my observing styles and interests change, so this is probably not a valuable measure. What I can say with confidence is that the DM-6 will hold its value should I ever decide to sell.

There is much to like about the DM-6. First, it is lighter weight than any comparable GEM. The mount head itself is 16 lbs, and there is no need for counterweights. Discmounts offers a tip-in saddle plate which is excellent. The tripod legs are furniture grade. The Sky Commander and Tom's "no-wire look" are a strong addition to the mount. The stability is more than adequate. You can change eyepieces without repositioning the scope. There is no need for a power supply. Slewing is as fast as you can push the mount. The DM-6 can reasonably be carried with the telescope attached. Ergonomics are about as good as you can do with a refractor.

I don't really have any gripes about the mount, but I would offer some suggestions. First, a more quickly collapsible tripod would improve the already easy set up. The Lapides spreader for AP tripods comes to mind, but he doesn't make them anymore. I can't help but think that there is a market waiting to be filled. Second, and far less likely, I would love to see a ServoCat option for star parties (newbies can lose an object in a hurry) or longer, higher-powered looks (planetary). I actually suggested this to both Gary and Tom. Gary sounded quite interested, but Tom really sees ease of set up as much of the charm of the DM-6. Adding power supplies and other necessities would wipe out many of the advantages of the mount (but would still eliminate the hated "GEMnastics"). He's probably right, which is why I'm the guy who writes the check, and he's the one who cashes it. There are alt-az plus tracking options on the market, but most if not all are bulkier and counterweighted. It's good to ponder how a mount could be improved, but for now, I think this is the finest alt-az on the market. Test drive one if you get the chance.