Which is Best - Refractor, Reflector, or Catadioptric?
By Ed Ting
Updated 12/30/07
(Complete Version)
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Introduction:
"Ed, what's the best telescope you've ever seen?"
We were sitting around the kitchen table on a rainy day, talking about
equipment. I leaned back in my chair, lost in thought as I do every time
I get asked heavy, philosophical questions like this one. After a few min-
utes, I generated a short list that included the Astro-Physics AP130EDT,
Starmaster's 7" Oak Classic, and the Intes MN61 Maksutov-Newtonian.
Someone else at the table pointed out that I had, consciously or not,
selected one example from each of the major designs: one refractor, one
Newtonian reflector, and one catadioptric. What's more, they're closely
matched in aperture. Which, of course, started a debate: is one design
inherently superior to the others, and if so, which of them would fare best
in a shootout?
We decided to find out. I obtained samples of each telescope. By this time,
I not only knew which models I wanted, I knew which samples of each model
that I wanted for the test (there is one 7" Starmaster in our club, for example,
that outperforms the other two, which are already superb samples.) We com-
pared the three scopes side by side on planets and deep sky. We tested the
scopes' resolution, determined their limiting magnitudes and ran a star test
on each.
Since this was to be primarily a test of absolute performance, we spent most
of our efforts evaluating the optics. Value, cost, and availability (two of the
three scopes tested are not available - or at least very difficult to get - at
this writing) were not regarded as the most important factors, although we
do address the cost issue later on. Read on, and no peeking!
The Venerable Front-Runner: The Astro-Physics 130 EDT
Astro-Physics refractors certainly need no introduction. These telescopes
are so good that the term "Christen Triplet" is virtually synonymous with
excellence these days. The 5.1" f/8 AP130EDT keeps falling into and out
of the AP lineup. An f/6 version is also available. At $3650 for the OTA
alone -- which could buy the other two telescopes combined -- it's by far
the most expensive of the scopes tested, yet it has the least aperture.
In other words, it's a typical high-end refractor.
The 5.1" f/8 Astro-Physics AP130EDT
(Mount: Losmandy GM-8)
This particular sample of the AP130 has perhaps the most perfect star test
I have ever seen. It is completely lacking in chromatic or spherical aberr-
ations, and can be taken up to ridiculously high powers when atmospheric
conditions permit.
This telescope's reputation certainly preceeded it. Going in, many of us
wondered, to varying degrees, how the AP could possibly lose. Was this
blind prejudice, or informed bias based on years of experience? I was an-
xious to find out.
An Incredible Mirror: The Starmaster 7" f/5.6 Oak Classic
At the other end of the aperture/price spectrum lies this unassuming-looking
Dobsonian. Made by Starmaster for several years, these scopes have gained
a cult-like following among reflector buffs. Sadly, they've been discontinued,
and Rick Singmaster tells me he has no plans to bring the scope back. The
world's the poorer because of it. There are only about 150 of these out there,
which makes these scopes even more rare than Astro-Physics refractors.
The Starmaster 7" Oak Classic
All 7" Oak Classics are worth collecting, but the real stunners are the last
40 or so units with Zambuto mirrors in them. This particular sample's mirror
has typically astonishing specs:
RMS Wavefront Error: 1/74
Peak to Valley Wavefront Error: 1/29.6
Strehl Ratio: .993
The numbers only tell part of the story. The views through this telescope
are stunningly vivid, sharp, and clear. Some of us have remarked that this
telescope is like having a 7" apochromatic refractor for about $1000.
Early versions sold for $695, while the last run of Zambuto-equipped units
went for as much as $1000, depending on the options you wanted. On the
used market, expect to pay as much as $1400, assuming you can find one.
Although I expected the AP refractor to have the edge in winning this test,
deep in my heart I was pulling for this little scope. My previous views through
it suggested that it had more than a fighting chance (the owner has elong-
ated the .45 arc-second B component of Gamma Andromedae and has no
trouble seeing eight craterlets in Plato with it.) I mean, wouldn't it be a great
story if a $695 scope managed to beat out a $5000+ refractor rig?
A Great Catadioptric: The Intes MN61
Once a curiousity, Maksutov-Newtonians are gaining well-deserved respect
for providing crystal clear, pinpoint sharp, contrasty images. That they
manage to do this at very reasonable prices makes them even more attractive.
The 6" Intes MN61
(Mount: Losmandy G11)
The 6" f/6 Intes MN61 is also sold by Orion under the "Argonaut" name. The
tube sells for $1150-$1350 depending on who you buy it from and what options
you get with it. You can get Orion's Argonaut version with a GP-DX mount
for $1999. Like the Starmaster, it has a tiny (18%) central obstruction,
and the lack of a secondary support structure does a good job of convincing
you that you're looking through a premium refractor.
None of us have done extensive comparisons with this telescope, yet each
time we look through it, or see it at a star party, it always elicits gasps from
delighted observers. I've looked through some amazing Mak-Newts in my
time, but I knew I wanted to include Allen's MN61 in this test. It is the best
catadioptric I've ever seen.
The MN61 comes into this test as something of a "dark horse" entry. One of
us even cynically said that this comparo was really a test "between Mr. Christen
and Mr. Zambuto" and that the Intes was just along for the ride. Would our
prejudices hold up, or would the Intes surprise everyone and silently walk away
with the crown?
The Stage is Set: The Observers
We tested these scopes side by side. The AP130 was mounted on a Losmandy
GM-8, and the Intes sat atop Losmandy's larger G11. After the comparisons,
we each spent time with the scopes individually to form subjective opinions.
Notes were privately taken, and we took care not to discuss our thoughts
with each other before the test was concluded. At the end of the observing
sessions, I collected and collated the data.
Several observers participated in this comparison.
1) Dan S has twenty years of experience under his belt. Dan is
well-traveled and has been to almost every major star party in the US.
A patient and meticulous observer, Dan's skills came in handy during
the limiting magntiude tests. He's owned over forty telescopes throughout
his observing career.
2) Mike T is our resident refractor nut. He's a skilled star tester (most of
the star testing comments are his) and loves to do high power planetary
work and split tight doubles. Like Dan, Mike has also owned more than
forty telescopes.
3) Chase M is a former club President and an enthusiastic photographer. He
observes planets and double stars through his 4" Celestron ED refractor and his
C8.
4) John P is a big fan of high quality Newtonians and spends every clear
night he can under the stars. He owns two Starmaster reflectors.
5) Me (Ed), the writer. I've been involved in astronomy since 1979. Many of
you know me through my writings, both in print and on the web. I'm primarily
a deep sky observer. Equipment wise, I split my time between high quality
Newtonians and apochromatic refractors.
Observing results
It was a cold night with temperatures that dipped into the single digits. Seeing
was in the "good" range, not outstanding, but good enough to obtain accurate
and repeatable results. All the scopes were collimated and allowed to cool
down thoroughly.
Our first targets were Jupiter and Saturn. We were blessed with a wonderful night
for observing Jupiter, as at least three of the Jovian moons were either in egress,
ingress, or transiting. This allowed detailed comparisons between the three scopes.
Right away I noticed two things. First, the Starmaster's images were much brighter
than those in the AP or the Intes. This was not subtle. Even though it was still
twilight, I was itching to run the limiting magnitude test right away to see how much
deeper the Starmaster would go (luckily, cooler heads prevailed and we waited until
later to do so.)
Secondly, the Intes gave stunningly pinpoint-sharp images of Europa as it egressed
off the limb of the planet. A good telescope should make the moon look like a tiny
pimple off the limb. The Intes, like the others, showed this beautifully. Further-
more, I noticed a wealth of detail on the surface of Jupiter. The red spot was
clearly visible and I could see subtle shades of color on the surface.
Moving to the Starmaster, I was distracted by the too-bright image and the diffrac-
tion spikes off the secondary's spider. The AP gave stunning images too but I
thought the Intes at least kept up. Hmmm...Was an upset in the making?
The eyepieces used were mainly TeleVues:
the 6 mm and 8 mm Radians, the 7 mm Nagler,
and the 19 mm and 27 mm Panoptics
Our answer came a few minutes later. While the MN61 provided outstanding
views, when the seeing settled down, most of us could make out more planetary
detail, and higher contrast, in the other two scopes. In the end, four out of
five of us voted for either the AP or the Starmaster in the planetary section
of the test.
We moved to M74 as a test of low-level contrast. M74 is a galaxy with
extremely low surface brightness and is the bane of every Messier Marathoner.
Here, the refractor stayed true to its high-contrast roots, easily showing the
galaxy as a pretty smudge against a pitch-black sky. The Starmaster easily
caught it too. I could see a brightening towards the center of the galaxy
in the Starmaster that was very hard to see in the AP.
The Intes, with its many optical surfaces, almost failed to catch the galaxy at
all until I pushed the power up, used averted vision, and tapped the tube to
make the image move. Furthermore, I noticed the sky background in the Intes
wasn't as black as in the Starmaster or the AP. The AP and Starmaster clearly
caught the galaxy, but it was nearly invisible in the Mak-Newt. In fact, one
observer declared that he never saw the galaxy at all through the MN61.
Out of curiousity, we set up an Astro-Physics Traveler, the 4" version of the
AP130EDT, nearby. It had no trouble detecting M74.
Moving on to the star test, all three scopes have outstanding optics. The AP
is just about perfect, and the Starmaster has a very slightly undercorrected
mirror. The Intes was slightly overcorrected. No false color was noted in
the refractor. The real surprise here was the Intes, which easily showed
the best star test of any catadioptric telescope I have ever seen (many cata-
dioptrics made today - especially Schmidt-Cassegrains - make a mess out
of the star test.) These slight aberrations are purely academic and have no
effect whatsoever on the in-focus image quality. Call this one a tie.
All three telescopes are superbly corrected for field flatness. Even at low
power, I could focus on a star in any part of the field in any of the three
scopes and could count on the rest of the field also being in sharp focus.
Call this one another tie.
I sent Dan, who is an unusually patient observer, into a corner alone with
his star charts to determine the scopes' limiting magnitudes. The common
procedure here is to use M67, the open cluster in Cancer, but the cluster
is too low in the sky at my 43 degree north location, so we settled on
NGC 1778 in Auriga, which was riding high overhead. The magnification
was kept around 130X in each telescope during the test.
Worthy contenders (l to r): 20" Obsession, AP Traveler,
7" Starmaster, 10" Starmaster EL, MN61
Conventional wisdom states that the more elements there are in the optical
path, the dimmer the images should be. Would practical observation support
this? On this night, the 7" Starmaster reached down to 14.3, while the 5"
Astro-Physics penetrated to about 13.9. The 6" Intes finished last at 13.7.
Yes, the refractor penetrated two-tenths of a magnitude deeper than the
Mak-Newt, despite having less aperture. Running the numbers, the AP
behaved as if it had about 5% more aperture than the Intes, despite act-
ually having 38% less. This does give some support to the theory that
multiple surfaces do have an effect on light throughput. It also supports
our subjective opinions; every observer present commented, at one time
or another, on the dim images in the MN61.
(Update, 3/10/00: On a later night, I sent Dan out with the 20" Obsession
to check out its limiting magnitude on M67. Readers should keep in mind
that this was a different night, on a different object. The Obsession went
down to 15.3, or about one magnitude deeper than the 7" Starmaster.
Interestingly, theory predicts the difference should have been closer to
two magnitudes.)
Later on in the evening, we spent time casually looking through all of the
scopes at whatever struck our fancy. A couple of us commented that the
differences we noted during the formal part of the test seemed to diminish
during casual use. All the scopes were superb. However, I did notice that
people tended to crowd around the AP or the Starmaster. We pointed all the
scopes at M81/M82, M42, NGC2392, M46/NGC2438, M45, M1, the Double
Cluster, and many other objects. At high power, the companion to Rigel was
clearly spotted in the Starmaster and the AP, but only marginally in the Intes.
During this informal part of the evening, I made some mental notes. There
were a couple of consistent comments made. The MN61 seemed dim - at
one point, someone actually thought the Mak-Newt was operating at a higher
power than the others, and asked to drop its magnification. In fact, all three
scopes were operating at about the same power.
Also, all of us were astonished at how good the views were through the Newt-
onian. This is in spite of the fact that many of us already own Zambuto-equipped
telescopes (I guess we needed a reminder!) For myself, looking through either
the Starmaster or the Astro-Physics reminds me of a paint job I once saw on a
Ferrari. The views were deep and rich, and I felt as if I could fall right in.
The Final Tally
After a few hours of this, we decided it was a good idea to get out of the bitter
cold and we retired to a local pizza parlor to discuss the results. The comments
were interesting.
The votes were split between the 7" Starmaster and the 5" Astro-Physics. Which
way each observer voted hinged, without exception, on one aspect -- whether the
observer was bothered by the diffraction spikes of the Newtonian's secondary
spider on bright images or not. A couple of us (myself included) find these
spikes distracting. At one point, I could not locate one of the moons egressing
off of Jupiter's limb because it was caught in the glare of one of these spikes.
To those not bothered by this, however, the Starmaster was at least the equal
of the Astro-Physics.
In the end, the Starmaster captured five out of the ten first place votes on
the planets (five observers ranked views on both Jupiter and Saturn), while
the AP got four first place votes. On deep sky, the Starmaster got three out
of five votes, while the AP got the other two.
And what of the Intes? Though oustanding, the Mak-Newt was downgraded
slightly due to its slightly softer contrast, and its somewhat limited light
throughput. On the planets, the Intes finished last in eight out of the ten
votes cast, although it did receive one first place vote on Jupiter (mine).
On deep sky, the lack of contrast scored it last on all ten of the votes cast.
The Astro-Physics didn't get a single last place vote in any category.
A test like this one tends to accentuate problems. In truth, the differences
between the scopes are small. To illustrate this, we compared all three to a
TeleVue Pronto, an excellent portable semi-apochromat. We like Prontos -
three out of five of us own one. It put up an excellent fight, but the little
refractor threw up some false color on Jupiter and didn't gather enough
light to get M74 from my location. This one was undercorrected by about
1/4 wave. The poor Pronto would have finished last in all the ranked tests.
However, when these scopes are not around, we are quite happy to look
through our Prontos, and do so all the time.
Which would you buy?
This is a loaded question, since two of the three scopes are either not cur-
rently available or nearly impossible to get. At this writing, only the MN61
is "available", either through Intes dealers, or from Orion under the "Argonaut"
name.
Still, I posed the question: If price were no object, which one would you buy?
Four out of five of us picked the Astro-Physics, despite the refractor having
been clearly outscored in the voting. Why? "Contrast", "super-sharp images",
"fit and finish", "resale value", and "mystique" were the answers most commonly
given. "The AP does the least amount of damage to the image," said one observer.
"Sure, I voted for the Starmaster more often," said another, "but if I had to own
only ONE scope for the rest of my life, it would be an Astro-Physics refractor."
Interestingly, the fifth observer said he would pick the Starmaster even if price
were no object -- quite an achievement for a $695 telescope.
There is one other reason to pick the refractor -- with no last place votes in
any category, it didn't come close to doing anything wrong (other than costing
too much.) The Starmaster has the potential problem with diffraction spikes,
while the Intes had the contrast and light throughput issues. The AP is like the
team in the playoffs with the bye week. Let the other two guys beat each other
up, then pick up the spoils.
And with price as a consideration, which one would you buy?
Two of us jumped ship to the Starmaster once price was factored into the equation.
Thus, three of us favored the Starmaster, while the other two declared that the
Astro-Physics was still the one to own, even with price as an issue.
Conclusion
So, what are we to make of all this? Going by the numbers, the Starmaster wins.
It has a killer mirror, gathers enough light to do serious deep sky work, is easy
to set up, and looks great on planets.
Going by the numbers, the Starmaster wins
Factoring desire into the equation, the Astro-Physics wins. It's the most
consistent of the scopes, with legendary optics, incredible contrast, and it's
got that indefinable "refractor mystique" that everyone seems to want these
days. Check with AP if you want one (they make f/6 and the f/8 versions,
but not both all of the time), and be prepared to wait two years or more
after you put your name on the waiting list.
...But everyone wants an Astro-Physics refractor
And what of the Intes? It's still a world-class telescope, with refractor-like
pinpoint sharpness, with superb planetary images. It's highly recommended for
quality-minded observers.
Also, in fairness to catadioptrics, there are superb Mak-Newts from Ceravolo
lurking out there, and Astro-Physics has been demonstrating their 8" and 10"
Maksutov-Cassegrain scopes at star parties for years. This raised some ques-
tions as to how these scopes would fare, had they been available for this test.
Also, the unusually dim images in the MN61 caused one observer to state that
we should have used the 8" version, the MN86, instead. Finally, although I
am confident that these results are repeatable by anyone who wishes to try
this test, it would have been nice to use the scopes over several nights. So
you see, this article is far from the last word on this topic. Use it instead
as a jumping-off point for your own research.
Later on in the evening, other observers showed up, toting two more 7" Star-
masters. An Astro-Physics Traveler, a 10" Starmaster EL, and the 20" Ob-
session were also present. Our ultimate conclusion is that it's really decadent
to have this many good telescopes in one place at one time. But hey...we can
handle it.
Sidebar: Alternatives... OK, so you've just finished reading this article and
now you're shaking your fist at the terminal because you can't get these tele-
scopes. Fortunately, there are some alternatives.
If you're dying to get your hands on one of the APs, you have a few options.
Aside from putting yourself on the waiting list and hoping, you can get a
Takahashi FS128, which has nearly identical performance. The problem is,
the Tak will set you back a cool $5200 or so (yes, it's $1550 MORE than the
Astro-Physics), and you will still need to buy a mount, finder, diagonal, etc.
Also, you can scale back your ambitions a bit and get a premium 4" apochromat
like the Takahashi FS102 ($2500) the TeleVue 101 ($2750) or the Vixen fluorite
(available from Orion, $2500.) The 4" TMB Fluorostar is another fine alternative.
Discounts are commonly available; check before ordering.
If it's the 7" Starmaster you want, check out the larger models in Starmaster's
line, including the 10", 11", and 12.5" EL models ($1900-$2500.) They're not
as portable as the 7", but you still get a Zambuto mirror. Starsplitter makes
a Zambuto-equipped 8" f/8 "APO Eater" Dobsonian for $1400. The Teleports
are becoming available again also, in limited numbers. Finally, you can get
one of the Portaballs: the 8" f/6 sells for $1300+, the 10" f/5 is $1900+, and
the 12.5" is $2900+. All come with Zambuto optics.
The Intes is available from Intes dealers like ITE, Earth & Sky and APM (in
Europe.) Also, you can get the "Argonaut" version from Orion, which comes
prepackaged with a nice GP-DX mount. The 5" f/6 version, the MN56, is also
superb and is a little lighter and cheaper at $795. If you're strong of build and
thick of wallet, you might go for the larger 8" Intes MN86 ($4300).
None of these telescope models exactly grow on trees, but at least you have
some alternatives to look at. Good luck, and happy hunting.
Note: To see my original comments on the scopes in this comparison, access
the Master List & Review Archive, or click on the links below:
Astro-Physics 130EDT
7" Starmaster Oak Classic
Intes 6" Mak-Newt MN61
End Three Way Super Comparo
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