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- INTERSTELLARUM DEEP-SKY ATLAS (FIELD EDITION) REVIEW
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- Discovery 17.5” Split Tube Dobsonian Telescope
CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.
Intes-Micro MN66 Maksutov-Newtonian
What follows is a review of the Intes Micro Newton-Maksutov (or Maksutov-Newton if you like) MN66. This is the new entry of the Intes-Micro in its MN line and fills the previously existing gap between the MN56 (aperture 5") and the rest of the Intes Micro line (MN76-7", MN86-8" and so forth) with a 6" (152 mm) f/6 telescope.
Will this new, scantly known, MN stand up against its close cousin, the much praised Intes MN61? Having had in the past more than one occasion to look through one of those Intes scopes I was intrigued by the specs tagged to this MN, which, while basically identical to the MN61 from the optical layout point of view, was considerably lighter than the former (7.1 kg including mounting ring vs. 9.5 kg OTA only), sporting a mirror coating with higher reflectivity (95% vs. 91%) and possibly a better meniscus coating as well, all of this while costing the same. The big bonus for me was, however, its weight (or absence thereof) since it had to sit on a Vixen GP, at least provisionally.
The (few) points where the MN66 has (supposedly) an advantage over the MN61, weigth aside, can be listed as follows:
- Better mirror coating. The basic MN61 comes with mirrors having a reflectivity of 91% while the MN66 is rated having 95% reflectivity (both primary and secondary).
- Better antireflection coating on the corrector lens. I don't know whether this matters really much but this is what I was told by the dealer. Should improve on ghosting and parasitic reflections, if proved true.
- Knife edge diaphragms. These should definitely improve on contrast.
- Larger OTA diameter as a consequence of the above. Better handling of thermal transients?
The telescope was ordered directly from the APM of Markus Ludes in the end of August 2000 and 2 weeks later it was delivered to my doorsteps. The rather undescriptive cardboard box contained the OTA itself with the mounting ring (more on this later), the 10x50 finder, its holding bracket and the extra Vixen mount dovetail adaptor I had ordered with the scope (plus an APM t-shirt as extra bonus, thanks Markus). I can't remember whether the dewcap was optional or not existing at all but in light of the weight requirements I had in mind it wasn't even considered.
The OTA is a painted dull gray 880 mm long tube with an outside diameter of 196 mm (by comparison the MN61 outside diameter is meager 178 mm) which could easily be termed *big* for its class, hinting at the presence of nothing less than seven "knife-edge" diaphragms equally spaced inside the OTA itself. The inner side of the OTA and the "knife-edge" diaphragms are painted matte black giving very low light scattering. These are NOT of the kind with corrugated ends soldered to the inner side of the OTA which you can find in its bigger siblings, so the airflow is not that much free to recirculate inside the tube slowing the cooling process down a bit. The business end of OTA is protected by a lid with a felt liner inside which suffered from poor gluing to the lid surface, resulting in a less than optimal attitude at doing its job properly, i.e. that lid never fitted snugly in its place. To fix this doesn't take much of a time so it isn't really such a big an annoyance but nevertheless...
Handling the OTA is pretty easy despite its dimensions thanks to its light-weightiness. When rigging the OTA on the mount I just need a single arm to hold it when tightening it on the support cradle.
The business end of the OTA has two slotted "sockets" (I have no better word for them) one of which is used up by the finder support bracket while the other it's free for other uses (piggybacked camera support, I suppose) or to move the finder itself in a better observing position. A very nice touch indeed. These sockets have a pair of knobbed bolts on one side to fix in position and hold the support bracket. While they proved to be adequate for holding the finder they are IMO inadequate if they are to tighten a bracket supporting a heavier load. They also show a tendency to loose the grip on the finder's holder bracket, eventually forcing re-tightening of the two holding bolts.
Just in between the two sockets lies the focuser, of the Crayford type. The focuser drawtube assembly is made up from three differently sized tubes, one inserted in the other. The first one is physically attached to the focuser base while the second one (2" inner diameter) and the third (1.25" inner diameter) can slide in and out inside each other. The 2" and the 1.25" tubes are provided with a clamping ring to avoid marring the EPs when tightened in place. The focuser body has two "rails" with one philips screw in each to allow for squaring the focuser on the secondary center, if need arises. The focuser has just one knob to adjust friction and it's pretty easy to operate in the dark. With just one control knob there is no way you can pick up the wrong one (it happens to me when using the MK67 focuser!). The focusing knobs are smaller then most others I've seen but nevertheless still big enough to provide sufficient setting precision when focusing at high powers. Still I would have preferred them a tad bigger then they are. Overall, although this focuser smoothness is not up to that of a NFS or a Feathertouch it still does its job well enough for visual use.
The primary cell adjustment screws are of the push-pull slotted screw kind and quite different in size (the push one is three times as big as the pull one). I'll never understand why this must be so (I've seen it in many newtonians) but found that you can operate on both with just the screwdriver for the smaller one. In use I have found them very smooth to adjust. The rear end is also where the primary cooling holes cover is. To remove it you have to unscrew another hard to reach knob, which really needs a pair of prongs to do. Once removed it reveals six large holes in the rear plate exposing a conspicuous chunk of the primary back surface. Cooling is attained by natural convection with the outside air. No provision is left to mount a fan cooler on it.
The secondary adjustment screws are hidden beneath a cap, which screws on the secondary holder. Removing this cap requires a bit of care in avoiding touching the corrector (which I clumsily did) while screwing/unscrewing it (not that this is something you do very often, though) or dropping the cap on the corrector surface. Once the cap is removed the three tiny adjustment screws can be easily reached with the same screwdriver used for the primary cell adjustments.
Apart from the main three collimation screw on can rotate the secondary holder undoing two really small screws on the holder side. Once done you can (not so easily) rotate the holder and mirror using a pair of narrow-nosed pliers.
No other adjustment other than tilting or rotating of the secondary is allowed therefore axial position and offset centering must be set right in the shop or you have to return it. A small black circle marks the primary center, which is something I would consider mandatory in any newtonians worth its name. I found it to be rather smallish for visual collimation but just of the right size if using a laser collimator (which I wholeheartedly recommend to anyone wishing avoiding all the nuisances of newtonians collimation).
The OTA is furnished with a mounting ring similar to those equipping the Takahashi Cassegrains and Mewlons, that is, a large, thin, bandlike clamping ring. This one is a bit thinner than those found on Takas and also flimsier in use. The ring is felt lined inside to allow easy rotation of the whole OTA. During operation I found to my disbelief that you can't tighten the clamping knob enough to hold the OTA when nearly vertical as it slides inside the mounting ring risking hitting the tripod's legs. To fix this I inserted two 1 mm thick brass spacers between the knob itself and the threaded rod on which it screws on and voilà, Bob's you're uncle!
The finder is a generous 10x50 unit in a slender grayish body. The achromat doublet is a MgF2 coated unit with
a conspicuous diaphragm at some distance inside the tube body itself. The dewcap is provided with 2 diaphragms
as well which are pretty useless in operation, as we shall see later on. Focusing is attained by turning the EP
(a kellner) and there is a provision for inserting a reticle illuminator by removing a small threaded cap on the
side. The two-rings mounting bracket is well suited for its job and there are, as customary, three screw per ring
to collimate the finder.
Mounting the OTA onto the GP was just a matter of screwing the dovetail adaptor on the two threaded holes on the flat side of the mounting ring, which is large enough to provide resting surface for the larger adaptors of the G-11s and AP GTOs mounts. The bolts on the GP mount adaptor are of the slotted type which, unless you have a *huge* screwdriver, are really a pain in the neck to tighten properly for the torque loading of such an OTA. Replacing them with a easier to tighten hex bolts is highly recommended.
After waiting away the mandatory 3 days of downpour required when you buy a new scope to expire I finally had the MN66 rigged and properly balanced on the Vixen GP, ready for first light. While searching through the finder for a star to start the alignment process of the SS2000 I found with horror that a *huge* paint speckle was sitting right in the middle of the field of view. Hoping it to be just a paint fleck I removed the ocular (pretty easy, just remove a tiny screw on its side and unscrew it). Once removed the ocular revealed to close inspection to have the reticle glass littered with paint flecks all over. After having made justice of those flecks there and anywhere I could find any offending source of contaminants, I restore the EP in place and began business.
First target was Altair, high in the mid northern sky in September. First inspection with a 32 mm Ploessl revealed far more field coma than expected for the TFOV of 1.5°. Switched to a 15 mm TV Ploessl and inspected the on-axis defocused diffraction image. There it was, coma at first glance. Obviously the scope was in need of a collimation. Popped in a 7 mm ortho for closer inspection and confirmed that it was badly out of collimation. Having made my mind up that further testing was useless with an un-collimated scope I set out to do just that and then my troubles began.
To make a long story short it took me the best part of an entire week, night after night, to find out what was wrong with it and more specifically what was wrong with *my* collimation tools. Here's a brief summary of what I've found:
- Precise centering of the focuser to the secondary cannot be carried out on this scope with the "classic" film canister. It really needs a cheshire with a very long sighting tube.
- During my trials I've found the focuser body rim is just at the edge of the tube focuser hole therefore I couldn't adjust its position without leaving a gap in the tube itself. Fortunately I've also found out that the position of the focuser is set right in the shop and needs only very minor adjustments.
- Preliminary adjustments were carried out with a self-made laser collimator + cheshire EP combo considerably heavier than "industrial" units. I found out that the scope needs to be positioned horizontally as the focuser draw tubes need to be extracted all the way up for better precision and also for the longer than usual sighting tube of my unit. Because of the nature of the focuser (three tube held together by screws), it wobbles slightly under the weight of the collimator if the focuser is inclined (which it is its "natural" position when in use). To have the set up right, the focuser axis needs to be vertical so that the laser beam is really normal to the primary optical axis once the secondary is correctly collimated.
- Collimation with small secondary is tougher than with larger secondaries.
In the end I finally collimated it using for the final adjustments a 2x apo barlow and a 4mm ortho, yielding 456x. Obviously, I had to wait for a moment of quiet in the atmosphere but finally I had the scope as collimated as I couldn't make it better to be.
Finders don't usually get much highlights in reviews, as one is usually more concerned with the optical quality of the main optics to give much attention to them. This one deserves it as it is one of the *worst* finders I've ever seen. This contrasts so much with the exterior appearance of the unit that in a way it makes it far worse than it actually is. The on-axis quality of anything brighter than 1st magnitude stars is seriously affected not only by astigmatism (clear indication, if ever needed, of a misaligned doublet) but also of chromatic variation of coma. Off axis performance is so abysmal that the useful field restricts to about a half of the 5° of the unit. User wishing to use it to actually *find* objects are much better off buying just the holding bracket and find a better finder (pun intended) in the marketplace, it doesn't take much of a search to find one (Celestron C8 7x35 finder is an APO by comparison!). In addition to the optical poverty of this finder, life isn't made any easier by the reticle itself, with small crosshair so vanishingly faint to be of no practical use in dark skies. To collimate the finder with the scope I had to point it to the moon to have enough backlight to actually see whether I had the target in the crosshair or not.
The optical quality of the MN66 main optics belonged, fortunately enough, to an entirely different league. Although I do not cast much objectivity and reliability in the star test as a way to actually *measure* optical aberrations comparison with the inside and outside focus diffraction pattern of the certified 1/5.8 wave p.t.v. optics of the MK67 I have, showed the residual spherical aberration of the MN66 to be better than that, nudging toward the 1/7 wave of total correction. Basically, the outside and the inside focus pattern of a bright star at 350x were *nearly* identical. That is as much correction as I do care to have and pay for, but others may want it to be absolutely perfect (and willing to pay for it).
To test whether the scope really fulfilled the optical requirements I had in mind for it (the best planetary scope short of an equal aperture, high quality APO) I used it every night it wasn't overcast for a month or so. The main targets were, naturally, the Big Two: Jupiter and Saturn. Seeing ranged from fairly bad to good and only one night it was very, very good for my observing place (an 8 in the ALPO scale). I never started observations unless the planets were 30° high in the eastern sky and usually carried on till they were 50°-60° high, weather and eyes straining permitting.
What follows is a brief summary of all the notes taken during that period concerning not only the planetary observations but all what was worth to jot down in my observer' notebook.
Mount and vibrations:
The Vixen GP is rated at 15 lbs (or about) which translates into 7.4 kg max payload and certainly the designers hadn't in mind a 0.9 meter long, 7.5 kg fully dressed OTA when setting that limit. Considering also that the AHL90 tripod is not what one would consider the apotheosis of the stability, I had more than one concern on the real usability of such mount/scope combination for planetary viewing (film or, worse, CCD shots never entered in my mind). To the surprise of many of my fellow AAs I found out that it works and you can push the magnification to 350x and still be capable of focusing and tracking an object through the sky. Damping (worst case) with the above said set-up takes about 7 s. when the OTA is nearly horizontal to something less than 5 s. when pointing to an object 60° above the horizon (the roof of my building prevents me from going any higher in azimuth). It takes a light touch when you focus at high powers on an object like Jupiter but you can make it. Although this is no suggestion that such a set up is a recommended one, it is still interesting to know that you can actually use it.
I replaced the original Vixen AHL90 tripod with a surveyor's tripod (bought second hand as an exposition sample for the equivalent of 50$) and the stability of the assembly improved dramatically. Now rapping on the tube causes vibrations damping in less than three seconds. Good enough for CCD stuff when coupled with a GP-DX head. On the GP you should see only minor decrease in stability.
I must confess my sweet spot for orthos for high magnification views. Cost and weight considerations aside (which however play an important role), orthos can and do, if of high quality, deliver the highest contrast in any eyepiece design available today. I can happily part from wider fields for any gain in contrast and color correction I can get. Never having had the money or the chance of getting hold of the Zeiss ortho series (now, I believe, discontinued) I had to content with a range of EPs of different sizes and make (some bought and someone else borrowed from friends). What follows are brief notes on the pluses and minuses of each of the ones I have used during the extensive 1-month test:
Vixen LV 6mm - One of my workhorses in use with C8, MK67 and a 6" newton, delivering in those 'scopes contrasty high power views albeit affected by some lateral color and bearable ghosting, it failed completely in use with the MN66. Coupling it with either a Meade Barlow APO and a Celestron Barlow Ultima it gave enormous ghosting (up to three different and bright ghost images) when used on Jupiter. Even when used alone ghosting, to a lesser extent, was there. Sold. 20 mm pupil extraction.
Pentax XL 7mm - Borrowed from a friend, gave nice deep sky views lightly affected by field curvature and some lateral colour. When used alone on the planets it gave faint ghosting (three greenish scattered images) on Jupiter. Coupled with one of the APO barlows of above it was a no-go. Contrast seem to suffer a bit and false colors were clearly there, plus much higher ghosting the before. Returned with many kind regards to my friend (saved me the bucks to buy it). 20 mm pupil extraction.
Meade SP 4000 6.4mm - Kind a love-hate relationship with this EP. Wider field than most other high power EPs I have, it delivers enough contrast for its price tag but yet it shows some ghosting (one single reflection) when coupled with barlows and some light scatter. No lateral color at all, even at the edge of the field and blacker skies than most others similarly priced EPs. A keeper, although I hate dodging ghost images. Good pupil extraction, about 6 mm.
Celestron Ultima 5mm - Kind of a disappointment in light of the gorgeous performance of its longer focal length brothers. Unusable on the moon due to a huge ghost reflection (basically another smaller moon) it was only passable on planets (Jupiter taking the higher toll) due to ghost reflections (a big one and a fainter second). Contrast seemed to suffer a bit when coupled with barlows (sky wasn't that pitch black) and also scattered light was higher than in the Meade EP. Adding to this, although it was bought brand new, it had flecks of paint all over the field lens and took me nearly an hour to get rid of them. Sold. Good pupil extraction, about 5 mm.
Celestron Ultima 7mm - As above.
Vixen Orthos 4mm, 5mm, 6mm and 7mm - I believe they have the same Vixen Ortho .925" lens only mounted on a 1.25" barrel. While their performance with all the other scopes I have ranged from fair to good, on the MN66 they showed far too much lateral colour to be bearable, especially the 4mm and 6mm units. The 5mm and the 7mm showed some lateral colour. Contrast was high, up to the edge of the field stop but scattered light seemed to increase at the center of field. Returned the 4, 5 and 6mm units to the dealer. Kept the 7mm. Pupil extraction ranged from scant (less than 3 mm for the 4mm) to good (it goes with the focal length).
Vixen LV 8-24 Zoom - Good overall performance in the range from 24mm to 10 mm it seems to suffer a bit from lateral color and faint ghosting when used at the highest magnification. Plus, due to the many air to glass surfaces when used with a barlow, contrast seem to suffer. Kept, as it is so useful (I really like it). Won't recommend it for use on this scope, though. 20 mm pupil extraction all over the magnification range.
Pentax SMC Ortho 6mm - Can't recommend a better EP short of Taka's Hi-Orthos or Ziess Orthos. Pitch black sky and no false colour nowhere in its 43° AFOV. Basically zero light scatter anywhere you look, with or without a barlow. The best EP of the bunch. A must have. Pupil extraction hovers on the 5mm.
Unitron 4mm Ortho - My workhorse with the MN66 when observing Jupiter (at about 220x is just right most of the times). Performance very close to that of the Pentax SMC, this unit delivers scatters free, high contrast images of whatever you throw at it. No false colour in its 45° AFOV and top contrast right up to the edge. Highly recommended if you can find a dealer having them. Got to buy the 5mm and the 8mm units as soon as I can get hold of them. For a 4mm unit it has an excellent pupil extraction, about 3.5 mm.
Televue 15mm Ploessl - Good overall performance albeit with somewhat less contrast than the Vixen Zoom at the same focal length. Good field correction and edge of field contrast. Main use is for deep sky views. A keeper. Good pupil extraction.
Pentax XL 28mm - Disappointing performance. Field correction far below par. Edge of field stars reduce to astigmatic lines thus reducing the really useful AFOV to a meager 45° of the 55° of this EP. Contrast is however high and with 20mm pupil extraction there's ample room for those wearing glasses. Will sell.
Heyford 32 mm Plossel - Love this one. It never fails to deliver high quality views of the heavens (for its price). In a head-to-head contest with the higher priced TV 32mm Ploessl it gave a notch more contrast when used on several globulars. Field correction is good with only a hint of astigmatism at the edge of field. The Pentax XL28 pales in comparison. AFOV is 43°. Pupil extraction is high but never had problems of blackout.
Vixen 40 mm Ortho - This is the biggest and so far the only EP with a 2" barrel I have. This one is a rare EP to find and I believe you got to go to Japan to find many of them. AFOV is of 45° delivering a TFOV of about 2° on the MN66. Although I have no extensive use to report about preliminary inspection of field correction on M45 declared it a keeper.
I must confess that my review is quite limited in scope. I never tested the MN66 on very close doubles (rarely
the seeing allowed such feats and when it did I was far more interested in looking at the planets) nor did I test
it on Moon features (the Moon isn't in the list of my favorite targets) or on deep sky targets. Really this review
is targeted at what is the main (in my view) attraction of this particular type of scope, its planetary performance.
A last note on my observing place. This is my balcony, in the heart of the city I'm living in, in Northern Italy. A place surely not listing in the "10 best place where you can observe planets" guide, if there is one, for a host of very good reasons. Nevertheless it is the only one I have and if the scope can perform from here, than it can perform from anywhere else.
Staring at the sky with this scope is definitely a pleasure for the eyes. Stars are pinpoints across a wide field of view and the general viewing experience is very much "refractor like". Field correction is very good, with just only a slight hint of coma at the very edge of a 1-degree of field. This is more likely due to field curvature effects than anything else. At the edge of a 2-degree field there was clearly some coma mixed with EP astigmatism but quite subdued considering that this is an f/6 system. Overall, field correction was coming close (but still below) to that of a good APO of equal size without field flattener.
Vignetting takes a heavy toll in this scope, mainly due to the size of the secondary (20.5% of the aperture). For a defocused image of Jupiter to show only marginal vignetting you have to resort to an 7mm 45° AFOV ortho, giving roughly 20' of TFOV. Clearly this scope wasn't designed with wide field photography in mind and this was also clearly stated by the figures given in the APM site for the edge of field illumination drop for the 24x36 format (about 50%) and the fully illuminated field size (slightly less than 1 mm). CCD applications using medium sized sensors should show only little light loss at the edge of field, at any rate.
Contrast can be superb provided that you shield the corrector lens with an appropriate hood or dew shield. It
happened during the very first nights to see stray light reaching the focal plane because of nearby streetlamps
or moonshine via grazing refraction. To circumvent this I built a very effective dew shield made of black thick
cardboard about 30 cm long. With this in place I never had stray light problems again nor had experienced any fogging
of the meniscus. I would recommend such an approach for most environments except in the very moist ones where cardboard
may not last that long.
Jupiter - Average seeing
With this kind of seeing you can expect to see, during brief bouts of steady sky, the following: The EB, the
SEB and the NEB (with the SEB split in its two components), obviously the EB, the STeB and the NteB (the latter
more clearly this opposition), a hint of SSTeB and NNTeB (this is indeed rare, though), the GRS with some details
(darker region inside), a glimpse of a number of festoons (blueish) and the polar caps.
Jupiter - Good to best seeing
This was indeed rare and happened just twice. With that kind of seeing you can expect to see all of the above
but in much greater detail (especially in the SEB and NEB), white oval spots (one), structure within the GRS, marked
colours in the festoons inside the EB, with far greater details than during "normal" seeing, starker
colours (orange to pale red) in the bands.
Saturn - Average seeing
Of course Cassini division, now wide and easy at low magnifications, Rings A, B, and C with some structuring
in the B ring, the planet's shadow basically "engraved" on the rings, the polar caps with a brown-gold
colour and a faint hint of banding in the temperate region. Four satellites are pretty easy with this scope and
one time I had the chance to see 6 of them, albeit with averted vision (it was a night of fairly good transparency
and steady seeing).
Saturn - Good to best seeing
Better perception of the transition between Rings A and B (a hint of the Maxwell gap?), more detail (subtle
banding) in the B ring, the Cassini division stark black and the Encke minima blinking in the ansae. I could also
see fine sub-structuring in the tropical band, a hint of bands. In really good seeing condition the contrast that
can be delivered by the MN66 is really to be seen to be believed (although the MK67 in an observing session high
in the mountains nearly delivered the same contrast).
Addendum: I brought the MN66 to a star party and let a number a friends use it. Seeing wasn't that good (averaging around 6) and clouds most of the times had the planets hidden from view. Testing the scope on the Trapezium revealed (sky magn.6) the fifth star (E) on bouts of steadiness while, as far as I can judge, making out the F component was an impossible task for most (or all) of us.
This the bane of the users of MN and source of much complaints about its suability in colder environments. I
usually allow it to settle to the night temperature for a 1.5-2 hours before actually using it. In one case I actually
measured how fast it equalized to the outside temperature (to the point of allowing 200x magnification on Jupiter)
when moved from my home temperature of 22 °C to the outside temperature of 9° C removing the rear primary
cap, therefore allowing increased heat transfer between the primary and the external air. It did it in slightly
more than half an hour, which is pretty good for such a scope.
Overall I am more than satisfied of the performance delivered by the MN66 and I wholeheartedly recommend it.
It's main inconvenient resides (for me, at least) in the observing posture I have to adopt due to the cramped space
of my balcony. No space to allot to a comfortable observing chair. A tall bar stool may do the trick, though.
A GP-DX is a much needed upgrade in light of a future CCD use with this scope as well as a new tripod or a pier.
- pck76 likes this