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Posted by Ed Moreno   05/07/2008 12:00AM

The Russians know how to build quality optics… And they DO!

The MN56 is a case in point.

This telescope has a well deserved reputation as being a good alternative to a 4” APO or perhaps one of the newer 4” f/7 ED scopes. And at the PRICE point, that is more than true.

The MN56 boasts excellent quality optics.. Of the small telescopes I have owned, the best optical quality REFLECTORS I have owned were without a doubt the MN61 I owned maybe 7 or 8 years ago, and the MN56 that I owned briefly a month ago….

The MN56 is a 5” f/6 Maksutov Newtonian design. It uses a Meniscus corrector and a spherical primary mirror in Maksutov fashion, but rather than a negative amplification secondary, it uses a 30mm flat diagonal to direct the light cone to a focuser in the classis Newtonian position.

The advantage of the MN over a similar focal length reflector is that this design suppresses coma to a much greater degree than a normal paraboloid mirror. This means that you can have a “Fast” or short focal length telescope and still maintain a fairly sharp, wide off-axis field.

The MN56 I owned was much nicer in overall finish then the old MN61 that I owned many, many years ago.. On that scope (bought used) the paint was poorly applied and was flaking.. By contrast, the MN56 had a very good quality paint that was in superb condition, even though this scope I think has passed through a few hands. I was actually quite pleased with how nice the build quality was. While the OLDER MN61s were often reported as being built like a tank, in fact, there was a seam at the center of the tube that could come loose as the tiny pins that held the tube together loosened and bent causing the joint to become loose. As far as I could tell, the MN56 (and most likely the larger newer MNs) use a single part tube now.

The focuser on this sample was replaced by a Moonlight.. Sadly, the focuser probably cost a considerable fraction of the value of the OTA itself. I can’t comment on the original focuser, but the Moonlight unit worked wonderfully of course.

The telescope is light enough that I was able to mount it to an old Vixen Polaris (not Great Polaris, not Super Polaris, just POLARIS) mount. The clamshell that comes with the scope is a funny little thing. It is like a thin, wide metal strap than a true clamshell. It just opens in clamshell fashion. But it takes a special dovetail, and one was not included. On the outside of the clamshell, there is a flat metal “Box” that has holes in it, so the screws go into the TOP of the dovetail from the inside of the clamshell. I fabricated a piece of oak to adapt the clamshell to the mount and it worked fine. But it does take a special dovetail to use this with a Vixen pattern saddle. These are available for prices similar to standard dovetail bars though, so this is a neutral issue.

Because of the Newtonian configuration, I was able to use the mount with the tripod legs fully retracted. I was able to easily reach the eyepiece with my observer’s chair. Of course the Newtonian configuration means that the eyepiece can get into funny positions, but the single clamp screw on the clamshell makes it easy to loosen and rotate, and the scope is light enough that you don’t need to level it first as I recall having to do with my MN61. According to the web sites that carry the scope, the latest versions have a two ring system that allows EASY rotation and THAT is a good thing. While rotating the OTA in the clamshell is by no means hard, it is still more to fuss with than say pivoting the diagonal of a refractor or SCT.

The Polaris easily handled the weight of the MN, so this is an OTA that would be happy on even a MINIMAL go-to mount like the LXD 55 or a good Alt-Az mount. I think the weight was about 11 lbs or so.

The scope has a rear cover that allows for air circulation.. While I like this feature, the downside is that there is a screw in the middle that sticks out meaning that you cannot set the telescope on the mirror end if the cover is in place. It has to be stood on the focuser end. Now to some, this might not be a big deal, but I didn’t like it… Of course you can’t stand a refractor on the focuser end… So, while I know that it isn’t REALLY a disadvangte, the SHAPE of the scope sometimes makes you FORGET that you can’t stand it on the bottom, and well, you DO… Of course you IMMEDIATELY get reminded of the screw when the scope wobbles like a Weeble (but doesn't fall down)… So for some reason, while most scopes don’t rally allow you to set them on either end, on THIS scope, well, you forget… I will let that go now.

Anyway, the scope doesn’t seem to suffer from unusually long cool down times, but I owned it in the spring and maybe I didn’t have enough temperature differential to really assess that.

So, lets move on to the optical review.

Star testing showed nearly perfect optical correction.. As with my previous MN61, the quality here is irrefutable. The spherical aberration correction looked to be essentially perfect, with even secondary breakout on either side of focus. The smoothness was world class. Optical smoothness is probably one of the KEY differentiators between excellent optics and OUTSTANDING optics. The truly “Premium” telescope makers of the world know this. In optics, EVERYTHING counts, and while a lot of amateur attention is focused on things like tiny amounts of fringing on APOs, more damage is done to the image by optics that have roughness than by tiny amounts of color fringing present even in ED telescopes. And things like this WILL be important to the value proposition of the MN56, which I will address later.

I am VERY happy to report that the COATINGS seem CLEARLY superior to the coatings of my VERY old MN61.. I did a review of the MN61 MANY years ago, and commented that I thought that light throughput was poor for a 6” scope. PLEASE remember, that the MN that I owned was probably one of the FIRST MNs to reach the shores of this country. This was seven or eight years ago, and when I got that scope, it had clearly seen a lot of interstellar flight. As SHARP as that scope was, I had trouble getting sufficient image brightness for viewing at much over 180x… Now I am not a “50x per inch” guy. On planets, I PERSONALLY find that going to an exit pupil greater than .7mm to .8mm causes faint planetary detail to be lost.. But in the 6”, I found that powers much more than 200x seemed to dim the image considerably. Well in the MN56, the image was quite bright. As bright as I think it should be for a 5” aperture. Cleary, the coatings here are much improved over the 10 year old MN61 I owned, and previous reports that have commented on light throughput should be considered dated. In fact, the MN56 seemed very close in brightness to my 140mm Vixen (which I no longer own, and did not have for side by side comparison). So, I rate the light transmission as on par with any similar compound systems. Anyway, light transmission was good. The field was as bright as I would have expected for a 5” compound scope, and honestly, I think it was maybe very close to the old MN61 I owned a decade ago.

As for observing, well, as you would expect, the MN56 with its “Big” small scope for the most part provided an exemplary viewing experience.

On axis, I would say the scope would EQUAL a world class 4” APO for planetary viewing. The secondary, being 30mm, means that the contrast is going to be quite similar (only because the optical quality is SOOOO good). The 20% larger aperture also provides a tiny bit of extra resolution.. What does that get you on the planets? Not much more than a premium 4" APO.. But if you look closely, you can see a few differences. For one, the MOONS of Jupiter might actually be presented as slightly discernable disks rather than point sources. A 4” APO will not QUITE match this. The moon is an example of a BRIGHT target with a lot of HIGH CONTRAST detail, and the slightly larger aperture should help the MN perhaps perform a TEENY TINY bit better than a world class 4” APO on the moon..

Indeed, when I pulled out Buffy, my 6” AP refractor, and plopped the MN56 next to it, I was really thrilled to see how close the little MN came in detecting extremely fine high contrast features. Now SINCE then, I discovered that I had some paint missing in the focuser tube of the 6” refractor, maybe damaging the image a small amount due to glare, but I have to tell you, I don’t think it was a big factor. The OPTICAL QUALITY of the Russian scope simply turned in a brilliant performance on the moon. There wasn’t too much in the line of high contrast detail (tiny craters, tiny rilles) that I could see in the 6” refractor that didn’t show in the 5” Mak Newt. Again, the moon has oodles of high contrast detail, and the MN did a REMARKABLE job of presenting it in the eyepiece. I was DELIGHTED at the MN56s lunar performace.

Saturn was a similar story. The MN actually put up a VERY decent fight to the 6” AP!!! The MN56 provided a view that was almost indistinguishable from my 20somehting year old AP… On THIS target, I would call its performance world class. I think that on Saturn, it would have been hard to see the difference if you put the MN next to some of the finest FIVE inch refractors made today.. Now again, Saturn is to me a rather high contrast target. It doesn’t have nearly the subtle color and detail as Jupiter, and I doubt that it would hold up quite as well in a shootout with even a 5” APO on Jupiter.. But I think it would easily match the BEST 4” APOs made today on Jupiter.

General observing was a true pleasure. The very well corrected field of the MN56 meant that medium-large clusters were easily framed, and the bit of extra light gathering over a 4” scope did add a tiny bit of extra sparkle. The field is also very well corrected. My 8.8mm Meade UWA was BEAUTIFUL in this scope at .86 degree. The field was quite sharp over most of the eyepieces that were suited to this scope… In the 22mm Nagler, (which is actually NOT well suited to this scope), the field was quite sharp right to the edge. Perhaps there was a TINY bit of coma evident, but I am EXTREMELY sensitive to off-axis aberration, and in this scope, it was oh so low enough, that I gave it a B+. It will ONLY loose out to flat field refractors telescopes that utilize field flatteners or tele-compressors in terms of presenting superlative off-axis performance. In other words, for a small fast-ish reflector, this is as good as it is likely to get for wide field viewing.

There was only ONE area where this telescope disappointed me, and in fact, is the reason for its rather short visit to my telescope hanger.

Which brings me to this….Many people say that MNs make great wide field telescopes because of their well corrected field… Well, that is only partially correct. As compared to a reflector of similar focal ratio, the MN does indeed have a SHARPER off axis field. But the fact is that most 6” and smaller REFLECTORS do NOT make all that great wide field scopes because of the issues around field illumination… See, to keep contrast high, the optical designer will resort to a relatively small diagonal. In the case of the MN56, the diagonal is a bit over 23% obstruction.. Now a lot of people would say “Hey, that is not all that small. Don’t MNs have something like 18% obstruction? Well, the LARGER ones do, but the MN56 doesn’t. The diagonal flat is 30mm.

Now BEFORE I purchased the scope, I even called a US Dealer for these scopes. I was not able to reach them on the phone, so I sent an email.. I asked them if the MN56 would fully illuminate the field of a 22mm Nagler. See, the 22mm Nagler has a field stop of 31.1mm. Now if the diagonal were RIGHT UP NEXT to the eyepiece, then the field would STILL not be fully illuminated.. And I KNEW that… But I what I DIDN’T know was: HOW BAD would the field illumination drop-off be? Would it be really evident at the eyepiece? Now a 22mm Nagler in this scope would give a field of 2.34 degrees at about 35x (approximations). You know, that is not a giant field for a scope with a focal length of 762mm,

See, since getting my 20something year old 6” 1200mm refractor, I have REALLY come to understand how DAMAGING field illumination drop-off can be. I have lived with it in my SCTs in BLISSFUL ignorance for 20 years. In the SCT design, drop-off is quite insidious. Drop-off starts IMMEDIATELY off axis, so that by the time you are at the edge of a widest-field eyepiece, the illumination is only about 68% or so. It is EVIL because it is so GRADUAL. In the SCT design, you REALLY CAN’T SEE it happening, but once you know it is there and start to recognize it, you start to really understand that using the widest field eyepieces in an SCT really does diminish the field.. But most of us don’t have really SUPERB frame of reference.

After Aiming Buffy (my recently aqired 20something 6” f/8 telescope) at the Double Cluster with a 31mm Nagler plugged into the light-leak end, well.. Lets just say that I had a TOTAL epiphany. OH my GOD was the field UTTERLY FANTASTIC! The field was MAGESTICALLY filled with stars. There were tiny faint stars seemingly CUT IN HALF at the field stop!. Stars EVERYWERE!!! I was ASTONISHED at how RICH and EXTENSIVE the field of faint stars was around this showcase object. In fact, it was like seeing it for the first time in my life.. But better than that…. I felt like I was OUT THERE in the galaxy, warping-in on it in a starship… I have never felt that feeling before. I learned that 100% field illumination is one of the most important attributes of a TRUE wide field telescope.

So, I was not confident that the 22mm Nagler would be illuminated well enough that I would not EASILY notice the illumination drop-off.

And I was right…. I started with the 31mm Nagler, and the light drop-off was GROSS. Even relatively bright stars disappeared before the edge of the field. You could SEE the dark circle around the outside of the field. Not this is NOT vignetting.. In SCTs, you get Vignetting if you go too wide. In THIS case, what you are seeing is the black OPPOSITE inside wall of the extremely well baffled MN56. The center if the field is bright and filled with stars, but as you get about 2/3rds of the way out, you get increasing blackness where you are seeing the opposite tube wall around the outside edges of the flat. Only relatively bright stars are visible at the field stop.

But I KNEW that the 31mm would do this.. Mostly I put it in to see how SHARP the field would be.. Well, it was SHARP, but not fully usable.

No problem though, because the eyepiece I INTENDED to use in it was my 22mm Nagler. Remember? This is the one that I tried to ask a dealer about…And didn’t get a response…

No cigar. The diagonal is out there in the center of the tube, so the cone of light coming from the diagonal leaves it as a 30mm converging cone. Now the converging rays probably shrink that cone to something in the area of 24 or 25mm by the time it reaches the focal plane. The cone is too small to fully illuminate the 22mm Nagler. and I knew that it WOUND'T be... But it was worse than I expected, and once again, there was a dark band that encircled the very outer reaches of the field.. It was very narrow,but quite evident. Now, the stars were quite sharp over the entire field. I could see some minor barely detectable coma about 3/4th of the way out. Now if you think about it, this is probably an area that is maybe RIGHT at the EDGE of the fully illuminated field… But I found the illumination to have fallen off far enough that I don’t think the 22mm is a good match for this scope

In comes the 17mm Nagler…. Ahhhhhhhhh.. PERFECT.. With the 17mm (and the 12mm Nagler) the field looked 100% illuminated to me, and it is quite sharp all the way out…

If you do the math on it this is equal to 45x and a field that is only about 1.85 degrees wide… I think that if you put in even a 20mm Nagler the field would be fully illuminated to the point that you would not see the dark circle around the diagonal field. I estimate that a 20mm Nagler would probably be JUST fully illuminated in this scope..

So what this means is that the MN56 only has a fully illuminated, tak sharp field of about 2.15 degrees!!!! Now my AP 6” f/8 refractor has a field ALMOST as big even with just the 31mm Nagler… With a 41mm Panoptic, it would have a 2.3 degree field!!! Now would you believe that a 6” 1200mm refractor makes a BETTER wide field scope than a 5” f/5 Newt? Well take my word for it, because I have seen it clearly and without doubt.

To make the fully illuminated field approach the size that could be equal to that of a similar focal length refractor, you would have to enlarge the secondary, which would damage the contrast as compared to a 4" refractor. This is the very heart of optical design compromise, and the compromise the MN56 makes to challenge a 4” APO for planetary performace (which it indeed does do) is to provide you with a SLIGHTLY restricted, but extremely sharp field as compared to say, a 100mm ED f/7 scope (though I am not sure that these designs are going to have superb off axis aberration control)….

In the end, I bought the MN to replace my Vixen 140, which I traded for a mount for the 6” AP… I had hoped to at least MATCH the wide field of the AP, but in truth I wanted an even BIGGER field, so in the end, I quickly turned around the MN56 in favor of a Televue 101, which can provide an almost 4 degree field (Surprisingly, I see that it TOO does not fully illuminate the field of a 31mm Nagler!!!!!!!!).

Now, don’t take this as a big negative.. My own goal was to get a really, really wide, flat, sharp field in a scope that could be mounted on a NIMINAL size GEM mount.. I now realize that refractors are the only scopes that can REALLY do this (but perhaps not all of them are equal in this matter… Televue 101 review coming soon).

But remember this… The MN56 can be purchased used and put on a minimal go to mount for a surprisingly modest sum of money.. I paid $550 for a used one WITH shipping and a FREAKING MOONLIGH FOCUSER for mine. You could buy a used LXD55 for $300 (I just bought one for the Telvue 101). So for LESS than the price of a new 102ED f/7 refractor, you can have an ULTRASHARP, perfectly color corrected, scope that would in my own estimation match or BEAT one of the new 4” ED scopes coming to the market today… It will go a bit deeper on deep sky, and still provide a very compelling moderately wide field viewing experience.

So, as an all-around alternative to a modern 4” ED scope, the primarily advantages here are OPTICAL QUALITY. I am not at ALL confident that the 4” ED scopes coming from China today are much better than “pretty good” in terms of optical quality, while the MN56 is world class in terms of optical quality. THIS is the REAL selling point of the MN56. As I mentioned earlier, I think that the quality of the new generation of ED scopes coming from Asia is probably quite good, and maybe bordering on excellent. Still, I have seen a few LAB tests recently that pitted the best from Russia, Japan, the US, and China against each other and in EVERY case where I have seen LAB testing, even where the Chinese made scope was represented as a “Premium” telescope, the optical quality was not in the same league as the Taks, TMBS, and Televues that have been in these test (two different test published on the web). In the tests I saw, the Chinese scopes were not as well corrected for spherical aberration, had minor zones, and had comparatively rough optics. In other words, while being fairly good, I doubt that they were the same optical quality as an MN56. So, put a $1000 4” ED scope with fairly good optics up next to a $1000 MN56 and aim them both at Jupiter. Don’t be surprised if the MN56 comes away as the superior planetary scope. Put a TRUE premium APO up against the MN56 at $2000, aim them a Jupiter, and don’t be surprised if the MN56 which costs half as much is able to MATCH the performance.

But go WIDE, and even the ED scopes might take home the bacon because of their ability to show perhaps a 3 degree or larger fully illuminated, sharp field (4 degrees for a Televue 101!).

It is a hard argument in the small scope market today… But in the end, Optical Quality at a bargain price still makes the MN56 a viable alternative to the ubiquitous 4” fast ED scopes, ESPECIALLY if you are on a tight budget…

As quality continues to improve for the Chinese made optics the MN56 value proposition will get VERY difficult though… (WILL it really though? Or is the price point/value proposition of the 100ED scopes fixed at providing a quite good telescope at a relatively cheap price enough for them to dominate the marketplace and run the true premium telescope producers out of business?)

But for now, before you buy a new 100ED f/7 scope, I would say that the MN56 still has to be taken as a SERIOUS alternative. If OPTICAL QUALITY matters, then this Russian scope delivers. Don’t be fooled into thinking a 100mm ED scope with fairly good optics will outshoot the MN56. It is that good…. I didn’t do a comparison, to an 100mm f/7 ED, but it held up PERFECTLY against the Televue 101. Put side by side with a random off-the-shelf ubiquitous 100mm ED f/7, and in a take no prisoners planetary shootout, I would put my money on the MN56.

Thanks for reading ,and my regards…