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EIGHT-INCH SHOOTOUT


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EIGHT-INCH SHOOTOUT

A Celestron 8 SCT and 8” Dob go head-to-head against a premium Mak-Newtonian

(with a special guest appearance by a Takahashi FS-152)

by Lawrence Carlino

Any astronomy enthusiast who frequents the numerous forums dealing with the virtues and vices of diverse types and brands of telescopes can easily be overwhelmed by classic information overload.   Actual use in visual and photographic astronomy, past experience, and arcane technical details make it sometimes difficult to arrive at any valid conclusions.

As an amateur astronomer who has owned over 100 telescopes and looked through many more, I still find it hard to trust my own judgment when it comes to evaluating optical quality.  Faulty memory, variations in observing conditions, and inconsistencies in telescope production all muddle the outcome.   Perhaps the only truly accurate method of appraisal is to round up a dozen or so examples of every type and test them against the perceived competition simultaneously - not very practical or affordable.

However, I’ve tried to do the next best thing with three very different 8-inch instruments.

Having obtained an 8” Russian-built Intes-Micro Maksutov-Newtonian from Astronomics last spring, I was anxious to determine if a standard Newtonian and a Schmidt-Cassegrainian could stand up against its well-documented optical quality.  I had owned a Celestron C-8XLT for several years and had recently purchased an 8”Astro-Tech Dobsonian Newt, forming a basis for comparison.  This would not be so much a test of brands as an empirical evaluation of different telescope optical designs. 

THE CANDIDATES

Intes-Micro MN86 Maksutov-Newtonian

This Russian-built hybrid came complete with its fabled optical quality, “built-like-a tank” bulk, rotating tube rings, a fan for the primary mirror, and a very decent 2-speed focuser.  The 50mm finder provided was a travesty and was replaced by an 8x40mm unit from Orion.  This “standard” iteration of the scope is advertised with at least 1/6 wave peak-to-valley correction, and testing revealed it to be that good or better.  I added a dew cap to protect the big front meniscus and built a Dobsonian mount to hold the beast after my Orion Atlas mount proved to be a bit too clumsy for efficient use.

The clear aperture of the Intes is 203mm, with the meniscus being somewhat oversized in order to yield the full aperture.  With an f-ratio of f/6, the optical train forms a nice balance for low and high-power observing, and the tiny 20 percent secondary obstruction promises high contrast and minimal diffraction.  A whole army of baffles graces the inside of the tube to cut down on stray light and glare.  Overall, this is a very impressive instrument.  Unfortunately, there are currently no U.S. vendors for the scope, and it must be obtained from Europe at considerable expense.

Celestron C-8XLT

   

I’ve owned six C-8’s in my lifetime, and this one is clearly the best.  It, however, may very well represent the current state of Celestron’s quality control – in general, very good.

I added a William Optics Crayford focuser to allow finer dialing-in of the image at the expense of lengthening the telescope’s focal length to beyond the nominal 2030mm. A 1.25” dielectric star diagonal of known quality capped off the back end, with a W.O. EzTouch alt-az mount making the whole ensemble light, sturdy, and easy to carry when fully assembled.  Street price for the optical tube alone is in the neighborhood of $900 - $950.

Astro-Tech 8-inch, f5.9 Dobsonian

                              

This AT Dob is a bargain-priced Newtonian seemingly well-suited for both beginners and seasoned amateur astronomers.  At $379, the scope includes a nice dual-speed 2-inch Crayford focuser, cooling fan, 50mm finder and two eyepieces, AND a nicely engineered Dobsonian mount.  The only fault I could find with the package was the “lazy Susan” azimuth bearing that displayed a roll (yaw) rate like a MiG-21 on steroids.  However, this was easily fixed with the installation of felt pads to damp the excessively touchy rotation.   

INTO THE FIELD

This past winter in the Buffalo, New York, area graced us with record mildness, below-average snowfall, and an unusual percentage of clear nights.  Seeing conditions were highly variable, but a few steady nights provided some excellent observing conditions.  My observing buddy, Tom, helped in the evaluation process, as he has a keen eye for determining image quality.  Because this was to be a comparison of the VISUAL performance of the three telescopes, no effort was made to test any of their photo or ccd capabilities, many of which are significantly different.

With the moon, Venus, Jupiter, and Mars all available at reasonable times, I first checked and adjusted the collimation of all three telescopes.  Anything less than perfect alignment has a serious effect on image quality and is often the culprit when an instrument fails to meet expectations.  A Hubble Optics artificial star was used to set the alignment with Polaris employed for a final check.

The C-8 needed no adjustment whatsoever, and the large collimation knobs on the AT Newtonian made for quick and easy work.   The Intes, having come with no instructions and a misaligned primary and secondary, required at least two weeks of frustrating, counterintuitive fiddling to get right – but once collimated, it held alignment well.

As all three scopes were stored in an unheated garage, the time to reach thermal equilibrium was very short.  However, quickly falling temperatures under a clear sky had the instruments playing “catch up” at times.  I tried to limit observations to moments when no obvious heat plumes or runaway thermal waves plagued the image.  The fans backing the primary mirrors of the AT and Intes were used intermittently and did help to shorten equilibration times.

Trying to achieve the same magnification with the Intes and AT Newt was easy, as both scopes had almost identical focal lengths.  A range of 80x to 240x was explored with a combination of TeleVue Nagler Type 6, Astro-Tech Paradigm ED’s, and TMB Planetary eyepieces.  The difficult to-determine efl of the modified Celestron 8 was arrived at by observing the actual field-of-view and calculating the magnification – probably accurate to 5 percent or so.

PLANETARY OBSERVATIONS

JUPITER, perhaps more than any other celestial target, provides a severe test for optical quality.  The low-contrast detail found in the giant planet’s belt and zone system quickly separates an adequate telescope from one with excellence.

Though Jupiter was sinking slowly into the northwestern sky, I did manage to get two or three nights with decent seeing conditions.  A convenient shadow transit of Io helped in determining the “wow factor” that a truly great scope will deliver.  Here, at about 140x, the Intes was a decisive winner.  Io’s shadow was stygian black, elusive details popped out in the polar regions and Equatorial Zone, and the color saturation of the North and South Equatorial Belts was outstanding.   But the C-8 and Newtonian were not far behind.  The Celestron provided a much brighter image than the Mak-Newt and displayed almost the same level of detail with just a bit less visible in the polar regions.  Color saturation and authenticity was very good, and I did suspect that it was actually compromised by its much brighter image – an effect that I’ve often observed with larger telescopes as their extra light grasp can cause a “whiting out”effect.

The AT Newtonian fell just a little behind the Celestron: very nice detail, color, and contrast with all of the major Jovian features easy to see.  Diffraction spikes from the spider caused some annoyance, and the open tube did make heat currents a factor with my own body heat providing some inconvenient calories.

MARS, now shrinking to a disappointingly small disk, still provided some impressive views.  Running the magnification into the neighborhood of 240x, I again found that the Intes delivered the most satisfying image.  The tiny North Polar Cap and collar stood out in bold fashion, and limb brightenings were easy to see.

Syrtis Major appeared very dark and well-defined, and Sinus Meridiani, Sabaeus Sinus, and Mare Serpentis completed the picture along with a bright Hellas region.  The characteristic “salmon pink” of the Martian terrain was quite striking.  The elusive, low-contrast dark markings on the “hard side” of the planet were also visible in moments of steady seeing – not an easy feat with this year’s small Martian disk.

Both the C-8 and AT Newt, however, came very close in revealing the same features.  Contrast didn’t quite match that of the Intes, and I had the impression that the straight Newtonian was slightly better than the SCT despite the intrusion of the spider diffraction.

Nevertheless, the image quality of both scopes would easily allow some serious visual and ccd exploration of the red planet.  There is no “loser” here, but the Russian scope again illustrated the value of a small central obstruction and smooth, well-corrected optics.

VENUS, close to dichotomy and dazzling in the western sky, created an impressive sight.   Darkening along the terminator looked almost identical, but the image was significantly brighter in the C-8 and AT Newtonian.  The Intes seemed to impart a slightly yellowish cast to the image, but it did look a bit sharper with a refractor- like cut-off where the Venusian limb met the dark sky.  Again, the spider diffraction of the Newtonian proved to be a minor irritation, but something one quickly learns to ignore.

THE MOON

Our natural satellite was a striking vista in all three telescopes.  With the moon just past first quarter, the orb was loaded with impressive detail.  The chains of tiny coalesced craterlets near Copernicus stood out beautifully at 140x, but the real treat was the impressively revealed Rima Birt near the Straight Wall.  Each telescope showed the feature easily, but the Mak-Newt provided the “wow!” moment. Actual shadowing within the rille and an almost three-dimensional appearance bested the other 8-inchers.  This seemed to me analogous to what audiophiles describe as “presence” that only a top-of-the-line audio system can deliver.  This is a superiority that is rather subtle, but worth the extra cost for the truly dedicated.  A novice looking through any of these instruments, however, would undoubtedly be impressed by the magnificent level of stark detail.

DOUBLE STARS

I’ve always found that a high-quality refractor provides the most aesthetically pleasing views of double stars, but these three 8-inchers did perform pretty well.

Castor, placed nearly on the meridian and in steady air, was the customary treat.  At 155x, the Intes revealed tight, concentrated airy disks with a delicate first diffraction ring coming in and out of visibility with variations in the seeing conditions.  Both the Celestron and Astro-Tech served up a similar image: very white and considerably brighter than the Russian scope, but with the central airy disk just a bit less tight and well defined.

Eta Geminorum, a good test because of its tiny companion lying nestled close to its bright primary star, was resolved with all three instruments.  Here, at 240x, the Intes pulled noticeably ahead with an easy resolution of the pair, but the AT Newtonian also provided a neat split despite the intrusion of its spider diffraction.  The C-8 didn’t fare as well, as the bloated central disk of Propus made the companion elusive and visible only in moments of good seeing.  It seems that tube currents were at work with the Celestron playing “catch-up” with the rapidly falling nighttime temperature.   However, with the C-8 in full thermal equilibrium on other occasions, I can recall the pair being a fairly easy split.

THE DEEP SKY

The Orion Nebula, always a fabulous object, was very impressive in all three telescopes at about 80x.  Green tendrils swept out from the bright central areas, with the Intes providing the darkest sky background.  However, the Mak-Newt also provided the dimmest image with both the C-8 and Astro-Tech revealing a bit more faint nebulosity.  With the magnification pumped up to about 125x, the ‘e” and “f” stars in the Trapezium were best seen in the SCT, with the straight Newtonian just a bit behind and the Intes in last place because of its inferior light transmission.  Yet, the Intes again delivered the tightest stellar images and a very pleasing view.  It also displayed the least field curvature and edge-of-field coma.

The Double Cluster in Perseus (NGC 869, 884) revealed the same order of magnitude penetration: the C-8 showed the faintest stars; the AT lagging by perhaps .1 magnitude; and the Mak-Newt a solid .3 to .4 magnitude in arrears.  The same pattern was observed in Auriga’s beautiful M37 and the neat NGC 1502 in Camelopardalis.  In light grasp and magnitude penetration, the SCT was a clear winner with the classic Newtonian almost as good.

CONCLUSIONS

Certainly, my own experience with three very different telescopes may not duplicate what others may find to be true.  Variations from unit to unit, less-than-perfect collimation, and tube currents all play a significant role, as does atmospheric turbulence and transparency.

INTES 8” Maksutov-Newtonian

This Russian “bear” is very impressive in its overall optical quality, sturdy build, and ability to deliver almost refractor-like images.  Coma is very well controlled, and the scope can be pushed to absurdly high powers without image breakdown.  However, it is large, heavy, and falls short on the light grasp that other 8-inchers display.  Is it the equal of an apochromatic refractor of equal aperture?  No.  It does stand up well against a top-of-the-line 6” APO while showing superior resolution

and a slightly dimmer image.  Perhaps the “Deluxe” iteration of the scope has better light transmission and even better optics.  Can it be considered a “poor man’s APO”?  Absolutely!   For a dedicated lunar and planetary observer or imager, it’s the real deal.

For deep-sky observing, there are better choices.

CELESTRON 8 XLT

There is a lot to like about this Celestron classic.  At less than 13 pounds, it’s incredibly light and portable.  Effective coatings give it superior light grasp and magnitude penetration while the largish 34 percent central obstruction doesn’t detract much from its image quality and contrast.  This telescope does everything well (except for super wide field), and it is reasonably priced.  The instrument is capable of serious lunar and planetary observation, and it shows about the same level of detail as my 5-inch APO refractor, punctuated with its superior resolution.  Available focal reducers make it suitable for imaging and emulsion photography.  Only the Vixen VMC200, 8” Meade,  and the Celestron 8” Edge HD (which I haven’t tried yet)  fall in the same category. Thermal equilibrium and proper collimation are essential, but if I could have only one telescope, this would be it. 

ASTRO-TECH  8” Dobsonian

This scope was the real surprise of the trio.  Despite its bargain price, this conventional Newtonian optic delivered sharp, high-contrast images.  It would make a fine beginner’s scope but would also serve well for a seasoned observer on a tight budget.  Wide-field views are compromised a bit by the obligatory coma, and the spider diffraction may be annoying to some, but this instrument does almost everything well.  It illustrates just how cost effective a Newtonian can be.  I suspect that a Newtonian with a premium mirror, tiny secondary, optical window or curved spider, and superior thermal management would be even better – perhaps the equal of the Mak-Newt in most respects – but I have no way of knowing this for certain without actual field testing.  This AT scope, however, is amazingly good for its modest price and will not disappoint.

Overall, considering the strengths and weaknesses of all three telescopes, one might conclude that no instrument “blew away” the other two.  That COULD occur with a defective unit, pinched optics, bad collimation, or a thermal disaster – but it didn’t happen here.  In essence, you do “get what you pay for,” and only each individual’s priorities can determine if a premium instrument is worth its purchase price.

postscript: THE REFRACTOR FACTOR

I must admit – I’ve always been a “refractor guy.”  After doing this side-by-side evaluation, I’m once again reminded as to why.  The 6-inch Takahashi FS-152 (a true fluorite doublet) simply delivered sharper, higher contrast images than any of the other scopes.  There were no excuses or rationalizations for tube currents, cooling down periods, bad seeing, or eye fatigue.  The Tak actually provided brighter lunar and planetary images than the Intes Mak-Newtonian with about the same contrast and color saturation.  Only in revealing tiny detail at the theoretical limits of resolution did the Intes pull very slightly ahead.

The C-8 and Astro-Tech, despite their excellent performance, didn’t quite measure up to the refractor’s magnificence, but they still generated some very satisfying images.

Obviously, the cost of ownership is a major factor here.  There is a huge disparity in the cash outlay for a bargain Dob, a premium Mak, and a top-of-the-line APO refractor.  The difference in performance, however, is not immense.  In looking back through dozens of planetary sketches that I’ve made over the past 30+ years, I found about an equal split between a 6-inch refractor, 8-inch SCT, and 10-inch Newtonian.  All displayed a marvelous level of planetary detail that depended on seeing conditions and training of the eye more than the type or brand of telescope.   Certainly, for observers plagued by customarily wretched seeing conditions, a smaller refractor may be a more suitable choice, and a larger reflector or compound scope a benefit in calmer locales.

After this test, I’m convinced that the real key to enjoyable observing is making the most of what one has, sprinkled with a healthy dose of patience and practice.

That, at least to me, means a lot more than the telescope itself.

Clear and steady skies!

Larry Carlino

pps  I have no affiliation or financial interest in any of the manufacturers or retailers mentioned in this review.




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