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Orion Argonaut 6" Maksutov-Newtonian (Intes MN-61)
I've owned achros, apos, mak-cass's, mak-newt's, SCTs and fine Dobs. Although very picky and never optically satisfied, I'm also realistic and consider cost-benefit ratio in acquiring scopes (most of the time!). Living within 10 miles of a million people, I'm forced to spend most of my time on planets and double stars, but would rather be searching out DSOs, the fainter the better.
This telescope is made by Intes in Russia and my version is the black tube model marketed thru Orion Telescope and Binocular as the Argonaut line (recently, the Orion catalog no longer lists this scope, or any Intes products, so that source may be disappearing). I purchased this on Astromart based on its reputation as an apo-like instrument tolerating high magnification well. I had an excellent 90mm fluorite, but wanted a brighter view without sacrificing either the sharpness or 109 lira (Euros?). I intended it to be used for planets and doubles and widefield views only. I also hoped that it would do nice widefield photos while waiting for Godot to bring my new 5" Astro Physics.
It is a 6" diameter, 900 mm focal length OTA with a dew shield, standard
Intes crayford focuser and a utilitarian-but-serviceable 50mm straight-through
finder. It is a heavy, Soviet-style metal tube that is all business and no humor
and I give it 5.4 on technical merit, but 4.0 on presentation. Still, barring
a direct mortar hit, nothing is going to come loose. Because of the weight (over
20 #), a Great Polaris mount is marginal and a Losmandy G8 is a much better
match. Incidentally, I had a 6" Intes Mak-Cass MK-67 at the same time and
they were very similar (exactly the same finish, finder and focuser in fact).
The Mak-Cass was obviously much more compact for mounting and traveling, but
I found that it took longer to cool, had tunnel vision (1800 mm focal length)
and was just a cut below in optical quality. In addition, for reasons I have
never identified, the view thru the Mak-cass (at the same magnification and
field) always seemed dimmer than other 6" scopes including the MN-61. The
coaxial mounting of the crayford on the Mak-Cass also highlighted its weakness
which consisted primarily of "slippage" when using big eyepieces pointed
at the zenith. The right-angle focuser mounting on the newtonian configuration
of the MN-61 is less subject to this problem. In summary, I kept the MN-61 and
sold the MK-67 (although I still miss it for travel), opting as usual, for optical
superiority over convenience.
The MN-61 does have several curious quirks. The worst is that the main mirror on the Orion version has a large plastic plug in it at its center (I have heard that the Intes Micro version of this type of scope does not, but cannot confirm this). I have been told that this is for a fan that screws into the base (there is a treaded socket on the outside of the baseplate), but it appears to me to be involved in holding the primary. Although it is in the shadow of the secondary and therefore does not affect the light gathering or image quality, it completely frustrates the normal collimation process! You cannot center spot the mirror and there is no reflection of your laser back to the focuser. I use a holographic laser for general centering and then a concentric star test pattern and cheshire for fine tuning. Still, it's totally aggravating and never really satisfying.
In addition, the main mirror collimation has three pairs of screws for adjustment, and the scant instructions do not really tell you how they work. If you assume some sort of push-pull system, you would not be correct. In fact, the larger, robust-looking outer allen screw is just a locking screw that does not actually move the mirror at all. The smaller, insignificant-looking inner Allen screw is the real brains behind this operation and moves the mirror in as well as out. When you are done adjusting this micro-screw, you then reseat the larger locking screw that is a passive and peripheral participant in this process. Very weird and counterintuitive. Luckily, this instrument tends to stay in collimation if you can ever get it into that happy state.
This scope is really special optically. The views are truly "refractor-like"and anyone who disputes this I feel has either a bad Intes or is the tailor for the Emperor's new clothes. That is not to say they are identical, but this scope delivers what people are seeking when they buy an apo. Cooldown is much quicker than with larger dobs or my Mak-cass, with about 30 minutes for it to be serviceable (from garage or shed to true ambient temp; usually about a 10-15 degree difference) and optimal in an hour. It is not clear why this should be faster than a 6" mak-cass although the larger tube volume may allow more mirror radiation and shallower temperature gradients. Nevertheless, it is clearly faster despite the larger metal tube.
Going to 300x with either a 6mm radian with 2x barlow or with the new Nagler zoom, images hold up very well on nights of good seeing. Clean airy discs are seen and clear double star splitting is excellent down to the 1.2-1.5 arc second range. Perhaps it is the low f-number (f6 yielding a narrow depth of focus) combined with the excellent optics, but the "snap" of the focusing is superb.
In fact the Intes crayford was just not up to the task with its coarse motion and the best modification I have made was to adapt a Feathertouch focuser to the MN-61. With the fine focus adjust and ultra-smooth action of the Feathertouch (this really is the best focuser on the planet), I can watch the focus get better and better and then hit perfect with no iterative searching at all. This combination is incredibly satisfying and for photography, it is invaluable.
Photography deserves special mention here. I had hoped for a fast wide-field scope for 35mm photos, but this was not to be. The MN-61 minimizes its secondary mirror diameter to optimize the image quality. Therefore, the secondary-to-focal plane distance is also minimized and too small for any standard camera adapter I have found. The Starlight Feathertouch already has a super low profile (about 1.5"), but even a Van Slyke zero clearance camera adapter could not quite bring a camera to focus (I subsequently machined a T-adapter to slide entirely into the 2" focuser tube, just reaching focus). There are other trick focusers that may succeed, but there is another problem. The tiny secondary is so small that there is obvious full frame vignetting.
According to Mel Bartel's on line program for calculating secondary size, the 28mm secondary in this scope results in light loss at the edge of a 35mm frame of more than 2 magnitudes (assuming Newtonian and mak-newt geometry's do not differ significantly). I verified this by sighting a light box while focused at infinity and the camera viewfinder shows the degree of vignetting (see photo), dooming accurate 35mm photography. Still, for planets and other high-magnification objects, photography either with CCDs (which typically have small chips and shorter focus-to-mount distances) or using digital cameras and the afocal technique (using an eyepiece) work fine because the edges of the field are not used. Two examples of the afocal technique through this scope with a 4mm radian are attached and show that high-quality images are possible.
In visual mode, when using a widefield eyepiece such as the Panoptic 35, the wide field views are quite pleasing and sharp and the vignetting not as obvious. The full field with this eyepiece is nearly 2.5 degrees with good edge to edge sharpness. Searching is facilitated by this short focal length, but in suburban skies, once you find a faint fuzzy and switch to higher power, it is not terribly satisfying through 6". This scope's strength shows at high power on bright objects. Planetary views at 250-300x have tremendous snap and sharpness, globulars resolve nicely and I see only the smallest decrement in contrast compared to a top apochromatic refractor. In those fleeting moments of true stillness, the gas giants look surreal and if you could change eyepieces fast enough, it seems like you could go to 400 or 500x with utter clarity.
This is clearly the best alternative I have found to a 5" or 6" APO
for those who cannot justify the financial beating from those toys. Used OTA's
typically sell for less than $1000 and deliver performance superior to 4"
apos and I think better than the 6" mak-cass alternatives. A good mount
and possibly a better focuser is needed to optimally exploit this scope (Try
that Feathertouch-- and Detlef and Brigitte are such nice people!), so the cost
does rise, but still it can show you what the APO owners is grinning about for
thousands less. Will I shift allegiance if my number comes up in the AstroPhysics
lottery? Of course I will. The artisanship of that fine scope is second to none,
the weight and size better and I can hope that the performance will be slightly
better than the particular MN-61 I have. For me the key difference will be in
the photographic possibilities, where I still feel 35mm is the pinnacle of the
art. But for visual use and for the purposes stated, a properly tuned and tweaked
Intes MN-61 can keep even the most critical user happy for a long time.