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Intes MK91 (9" maksutov-cassegrain) vs Celestron 11" SCT


Find below a comparative review of a 9" Maksutov Cassegrain (mak) and a new Celestron C11 Schmidt Cassegrain (sct). There is no rhyme or reason for the comparison of these two scopes other then the fact that I found myself the owner of both. If I owned a third scope or my other scope was something then the C11, I would have undoubtedly performed a comparative review on it with the 9" mak.

This review is a non-scientific, flawed, visual comparison of an Intes MK-91 and a C11 OTA. I wish to bring to light several compromises in my review methodology that keep this from being an objective comparison:

1. Since I only had one mount, I could only view through one scope at any given moment. This forced me to compare images based upon my memory of the prior scopes view from 10-15 minutes prior. I attempted to counteract this by rotating the tubes in alternating patterns as detailed below, but this is a poor solution at best
2. The focal lengths were "off" enough to confound attempts at using the same eyepiece on each (2800 for C11, 3200 for mak). Hence, I had to use different eyepiece (sometimes of differing designs) to view the same object at similar magnifications. My eyepiece case is limited and there was nothing else I could do.
3. My love of contrast undoubtedly "skewed" my perception of the performance differences between the scopes.

Take the summary of results within this context. I am but human and have my own preferences and dislikes. Lastly, in the paragraphs below I will be using the phrase "refractor like" to describe characteristics of the mk91. I don't mean to imply that the mk91 performs "as" a larger refractor does or shares the same perfections in the visual field. This is simply a term I use to describe the emotional impact the visual field has upon me personally.

For those who are interested in an objective review of the mk91 I encourage you to read Mike Palermiti's review at http://www.burnettweb.com/ite/report13.htm. Mike is an independent optical consultant and a heck-of-a nice guy to talk with. He also offered to answer specific questions about the designs/merits of both scopes reviewed here if anyone cares to email him.

Since I believe everyone is familiar with a C11, I will give some descriptive information on the MK91. The mk91 is produced by a small Russian firm by the name of Intes (not to be confused with the other Russian company, Intes-Micro). It is resold in this country by several importers (ITE, Earth and Sky, John Biggs) or directly from Markus Ludes of APM Germany. Each piece is hand configured and comes with an optical test report if purchased from Markus or one of his resellers.

The mk91s design differs in many ways from a traditional SCT. I won't attempt to describe or explain the merits of each design, as it would take a very long and exhaustive posting to do so. I encourage you to buy your favorite optical expert a coke and have them go over it in detail. Pertinent to this review is the fact that the SCT reaches focus by shifting the primary mirror, while the mk91 has a fixed mirror and uses a standard focuser to reach focus. I wanted to clarify this to avoid confusion in the description below.

The MK91 is much longer then I had visualized but at F/13.5 this only makes sense. Appearance wise it looks like a C-8 that has been stretched out to 2x It's original length. It will get people's attention and I predict a great number of "what is that" when I bring it to a star party. While 4 lbs. heavier then my C11 (C11 is 26lbs, mk91 30 lbs) I found it easier to carry around. Initially I was puzzled but then it dawned on me that unlike a C11 where 80% of the weight is in the rear cell, the MK91 has the weight evenly distributed between the mirror and heavy corrector. Consequently, it lays across my two arms when I carry it unlike my C11, which is like an unruly b,ottom, weighted toy.

The Mk91 uses a rack and pinion focuser with the knobs situated on top as opposed to the more traditional bottom location. Initial focusing becomes an effort to overcome 2 long-term habits - it's a compound scope so one naturally reaches for the non-existent mirror-focusing knob. Once your mind kicks in and say's don't be silly this is a fixed mirror compound scope and you reach for the bottom of the focusing tube for the rack and pinion focusing knob. But again habit is leading you astray as the knob is on the top. I found myself laughing at my own bumbling antics and "corrected" these habits within the first observing session.

The end of the focuser attaches to an extension tube that mates with your star diagonal. The extension tube is necessary due to the generous amount of back focus that was designed into this scope. This allows one to use a binoviewer, camera or other backfocus "gobbling" device on this fixed mirror mak-cass. The supplied extension tube is a very solid metal tube and even using the monstrous Meade 40mm superwide field did not cause any flexing or stress.

This particular sample of the mk91 is one of the early production runs from Intes. The fit and finish of this scope is identical to the early mk65s that Intes produced a number of years ago. Incredibly solid and functional but no beauty contest winner here. The painted color is what I refer to as "communist hospital waiting room" - a unique off white that I believe Intes has stopped using. In the near future I plan to repaint this scope something more cheerful (yellow leaps to mind). No effort has been made to blend in chassis screws and the name and Intes logo is simply orange duct tape. Other scopes have sexy curves or raze color schemes - the mk91 says "I'm a tool - use me".


The C11 ota that I compared the MK91 to is brand new and after collmination demonstrated very good optics and practically no image shift. I have been very pleased this sample of a C11 and if Celestron is turning out all their SCTs to this caliber, they will create many loyal users and customers. Unlike the mk91, the C11 is a beautiful instrument. Celestron has paid ample attention to the fit and finish of the scope. Appearance wise, it is very impressive saddled up on the CI700 mount.

Shoot out

It wasn't until I began comparing the scopes that it really and truly dawned on me that I was comparing scopes of different apertures. Let's pause and think about this. Both scopes are very good optically and mechanically. I can think of no other type of scope where a comparison between 2 different size scopes would be considered fair. Consider the absurdity of comparing a 4" and a 6" apo or a 8" and a 10" dob. Aperture wins unless the optical quality of one scope is poor. Since the 11" sct has quite good optics, how can the 9" Mak-cass do anything but lose? Would it not be fair to say that the optics and design of the mk91 would have to be superb to even "tie" the C11 on objects? The above paragraph cannot be overstated and I will refer to it again at the end of this review.

Since I only owned 1 mount (CI700) the following comparison was performed by swapping otas on and off the mount to view the same object. Each object was viewed 3 times alternating scopes and then reversing the order on the next object (mak-sct-mak, sct-mak-sct). I performed this feat on 2 consecutive nights under the glare of a full moon. I will not be performing the above ever again as it is very exhausting and prone to accidents. Two nights of swapping heavy OTAs was enough to last a lifetime.


I'm going to deviate from the traditional object by object comparison and comment on several characteristics of a scope.

Points of light

In looking at stars or star fields, stars were smaller points in the mk91. The biggest impression that one receives when looking through the mk91 at stars is the "refractor-like" experience. When looking through the SCT the visual experience is, well, a nice SCT image. Look at the same star field with the MK91 and the visual experience is "like" a refractor. I found my mind doing that "ahhhhh" sigh when I switched back to the mk91. Most folks have experienced this at star parties when switching between a SCT and a refractor - the refractor invariably gives the more pleasing visual experience. I partly attribute this to the blacker background of the sky in the mak. The sky background in the sct was a "light" black while the background in the mak-cass was "deep" black. Better baffling? Less light scatter?


On each target selected over the two evenings, the MK91 outresolved the C11. As a solid example I selected 100x as the comparative magnification for splitting the double double. Both scopes were able to split them at this power but the mk91 did it much cleaner with the black dividing space appearing darker and wider. In crowded star clusters I was able to count more stars with the mk91. The stars that "popped" out were those which were very close to it's neighbor. On dimmer star fields, the C11 performed better as the larger aperture made stars that were on threshold of visual perception in the mk91 visible.


In no other feature does the MK91 show its stuff then on contrast. The background sky is blacker, colored stars are more vivid and the wealth of planetary detail is truly amazing. This was most apparent (and puzzling) when viewing m57 - the ring nebula. At equal magnifications I found the C11 showed more details. This did not surprise me as the C11 is 2" bigger and thought that this pattern would continue though the spectrum of deepspace objects. By accident I popped a lower power eyepiece in and found that the details that were not they're before in the mk91 now appeared. Huhhh . . . Here I ran into one of several flaws in my testing. Let me explain.

Typically when one compares scopes one chooses similar magnifications to equalize the experience. However, this assumes equal aperture. I imagine that the proper formula for comparing scopes of dissimilar scopes would be to use a formula that factored the light gathering ability. X amount of magnification per inch in comparing objects would be a far more fruitful and fair then simply randomly selecting a single magnification for both scopes. By selecting a single magnification one scope is in most likely hood at the wrong magnification to "frame" the object for its aperture.

APO owners are familiar with this concept and having owned 2 and reviewing my experiences, this is how I finally "got" it. APOs can perform suprisingly well on deep sky objects considering their usual limited size. Sit an APO next to a sct and the sct owner will show detail on an object at rather high magnification. The APO owner can show the same detail but only at a far lower power (which will also decrease the apparent size of the object) assuming the APO is within several inches of the SCT. Here the superior contrast abilities of the APO are allowing it to see details that require higher powers in the SCT. M57 is a rather bright deepsky object and I would guess that on fainter deepsky objects, the C11 would handily outclass the mk91.

Cool Down

Here the two scopes differed significantly and to come right to the point, the mk91 has thermal issues. Traditionally, maksutov-cassegrains suffer both from lengthier intial cool downs and difficulties in handling falling temperatures. Recent redesigns of the mak-cass by AP, TEC and Intes-Micro have incorporated some sort of cooling system in recognition of this problem. My sample of the mk91 (which can be purchased optionally with a rear fan) had no installed cooling system and this began to take it's toll as winter approached.

The mk91 would take from 1.5 -2.5 hours to cool suitabley for high power observing coming from an unheated garage. With a typical scope one simply backs the power down while waiting for the scope to equalize. However, at F/13.5 this is a bit difficult. This scope is really intended for medium to high power observing and as such it needs it's cool down time. Additionally, if you live in an area where the temperatures fall for the first several hours of the evening, the mk91 will lag behind and give images that are less then it is capable of giving.

I found that the C11 did not suffer from thermal issues to such an extent. 1 hour initial cooldown with far fewer problems with falling temperatures. The above can not be overstated - if you live in an area with fluctuating temperture, the mk91 will rarely perform to it's full potential without a cooling system while the C11 will a greater proportion of the time. Hence, you may find yourself with a Porche on a road with a speed limit of 30 mph. Not really the reason you bought it in the first place.


1. In comparing a 9" Mak-cass to a good quality C11, the mak-cass will need to be of superb quality just to tie the greater aperture. My sample of mak-cass is a superb sample (twiced verified in two different labs) and it was able to surpass the C11 on several counts. However, a "mediocare" quality 9" mak-cass would surely not compare favorabley to a good C11. Buyers beware.

2. On bright deep sky objects the mk91 can keep up with the C11 (m57) on brigher objects. I theorize that on fainter deep sky objects, the C11 will completely outclass the mk91. If deep space is your hearts desire, there really is no substitute for aperture.

3. The mk91 is able to outresolve the C11 and shows superior contrast. The mixture of these two with the addition of the perceived flat field of the mk91 makes one feel like they are viewing through a large aperture refractor. Undoubtedly, when side by side with a large refractor one would find some distinctions. Lacking access to one I can comment no further.

4. This is not a beginners scope. Anyone new to the hobby or these two very different scopes would judge the brighter apparent image of the C11 to be superior on many objects. The ability to distinguish the merits of the MK91 comes with experience. I would hate to think that someone who is new to the hobby or these designs would make a purchasing decision based upon the above. I can't encourage folks strongly enough to try these 2 types of scopes prior to making any decisions. If you don't have access to a mak and a sct, find someone whose experience is greater then yours and whom you trust and have a long conversation. The point of this review was not to encourage folks to prefer one scope over the other, but to give you some
experiences and information to work with in your purchasing decision.

5. As a pleasant surprise, the long focal length allows one to use inexpensive eyepieces that will perform as well as more expensive ones. Long focal length scopes are becoming a rarity and with it the advantages that such scope bring - a very small cone of light for the eyepiece to deal with. As an example, I have had for sometime a small 14mm homebrewed konig. At faster focal lengths, this little guy performs poorly and becomes useable at F/10. At F/13.5 it was sharp to the edge. What this means is I don't have to spring for Naglers anymore and in fact the few that I've owned are gone.


Eventually I opted to sell the mk91 for the seductive mn86. After being professionally cleaned and collminated, the mk91 passed briefly back though my hands. For 2 hours on one of these nights I had the mn86 and mk91 out and viewing on Jupiter and Saturn at various magnifications. No attempt was made to be truely objective, I simply slapped eyepieces in and out of each in an effort to gain the best views on both targets

In summary, the mn86 was able to pick out more details and plumes on Jupiter then the mk91. This pretty much stands to reason since the mn86 has a central obstruction that is only 20%. However, what surprised me was how close they came. Plumes that were very apparent in the mn86 were fainter and thus contained less detail in the mk91. However, as the evening ran on I found myself favoring the mk91 for reasons that are still unclear to me. It may have had something to do with the fact that I was able to use medium focal length eyepieces to obtain or exceed what I was able to do with the mn86 due to the differences in focal length.

I also "felt" after owning both scopes that the mk91 seemed to be able to absorb higher magnifications without image breakdown occurring. I rarely observe planets in excess of 280x here under the gulf stream but regularly found myself in the 350x range with the mk91. This throws up a larger apparent image which has a more emotional appeal when observing the planets. Lastly, like all cassegrains, the tube is much smaller and easier to handle then a large mak-newt. Viewing is simply more comfortable when using a cassegrain and this may have also factored into my favoring the mk91.

There is not doubt that the mn86 is optically superior to the mk91. Thinking it over the mk91 resides in a niche of it's own somewhere between the optically nice scts and the virtually refractor like mn86s - kind of a middle point compromise. Now if someone could produce a mak-cass with a low central obstruction (say 25%ish) and a reasonable cooling system, then the mak-newst better watch out. However, I'm intruding on future reviews here - so you will just have to wait .

The End . . . . . . . .. . for now :}


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