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A few years back, I was convinced that planetary and extended field
objects required a long focal length objective, preferably F/6 or even
longer. A long-time associate of mine refers to this attitude as
"paradigm paralysis," a condition suffered by many amateur observers, then and
now. He humorously characterizes this "condition" as one in which ATM's
labor under the pernicious delusion that short-focus reflectors (4.5 or
shorter F/ratio) are unsuitable for serious planetary observation and
must be used with an off-axis mask to perform adequately at all. Contrarary to
this, the best views I've seen, of objects such as planets, Jovian satellites, and other
extended-field objects, have been through large telescopes, with focal ratios faster than F/4.5,
at full aperture.
Some background now: About ten years ago, I was looking for refractor-like planetary performance on a reflector budget. It occurred to me that a really
conscientious optician could make a short-focus mirror in approximately a
16" size. I had the opportunity to purchase a 15" F/4.9 mirror that
had good RTA and wavefront numbers, and although I was skeptical because of its
short f/ratio, I decided to go for it because the price was right, there was
much to be gained if it worked, and not too much to lose even if it didn't fully
satisfy my expectations. Plus, the seller said he would stand by the warranty if
I was disappointed at all. At worse, it'd be a decent scope for deep-sky work, I
reasoned, and a big step up from my old Coulter 13.1".
Since that time, I've gone into business producing optical systems under the Starmaster
label. This work has included star testing literally hundreds of
mirrors that passed through my shop and observing site, from F/4 through F/6, in
sizes from 7" through 24". The results: The short-focus,
large-aperture optics disclose detail on planetary disks that are unrivalled by
smaller aperture refractors with apochromatic design, and are equal to
longer-focus reflectors of F/5 or longer F/ratio, given the same aperture.
Conclusions: When it comes to large aperture reflectors, There is no reason
whatever to use an F/ratio greater than F/4.1 to achieve the finest possible
planetary performance, PROVIDED that the system uses a coma-corrector to create
a wide diffraction-limited field.
So why does this paradigm (that large, fast reflectors are bad for planets)
continue to prevail among many amateurs? If this is so, how come the word hasn't
gotten around? Well, for one thing, there are additional criteria that need to
be satisfied to make a short-focus primary realize its potential, and most
frequently they are not so optimized. These include:
Cell Design
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Edge supports seem to provide a far better method of preventing
deformation than the traditional sling support. It appears that the sling
tends to deform the lower edge of the primary by pulling it against the
lower support points at different elevations. Although I haven't done any
simulations in software to prove this point, the empirical results seem to
indicate solid edge supports work better. There is no image shift at any
elevation, and no astigmatism when the telescope is near-horizontal. I
can't say the same thing for telescopes through which I've observed that
use a sling mount for the primary.
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Stability and freedom of movement of the support points. The supports
must be constrained to prevent rotation of the triangles, yet allow equal
freedom of accommodation in all axes.
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The cell design we use with "equal loading"
four-point lower edge supports is fashioned after the test setup used by
both Pegasus Optics and Zambuto Optical Company to test their mirrors.
Neither Pegasus nor Zambuto uses a sling to
support their optics during testing.
Optical Quality.
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Figuring. A properly figured primary mirror, whether F/4 or F/8
or in-between, will stand up to high magnification, seeing conditions
permitting. Short F/ratio primaries require more precise figuring because
the figuring tolerance is inversely and proportionally tighter in shorter
f/ratios.
- Primary thickness.
The thinner the primary, the better it
equilibrates. I've found by side-by-side comparison of 1.6" and
2" telescopes, that a 1.6"-thick primary equilibrates about 35 -
50% faster than a 2" thick one, depending on the temperature delta
during the observing session, particularly in the earlier part of the
session. The 1.2" thick 14.5" and 16.5" primaries made by Mike Lockwood
cool even faster than the 1.6"-thick mirrors!
It should be noted that figuring a thin primary is considered a
more challenging task than for a thicker one because of the tendency to
flex during figuring. However, Carl Zambuto (1.6" thick mirrors) and
Mike Lockwood (1.2" and 1.6" thick mirrors) have proven to be up to the task.
- Secondary quality.
Before I acquired a steady source of reliable
secondaries, I had to spend a great deal of shakedown time on completed
telescopes assuring that the secondary introduced no error into the
system. Many secondaries from previous sources had to be returned, usually
because of astigmatism along the major axis. Now, Mike Lockwood tests all
secondaries that go in Starmaster Telescopes, and he has rejected or refigured
quite a few. Moral to the story: Spending a few extra bucks on a known
high-quality secondary that was thoroughly tested is well
worth it. Anything less than the best quality secondary jeopardizes the
quality and performance of even the best primary mirror.
- Coatings.
It is my belief, and that of many other knowledgeable
observers, that a "standard," single-layer coating has less
potential for variation and roughness than a multi-layer metallic
coating. My experience bears this out. I've had excellent experience
with an ion-deposition process with quartz overcoat on the primary, and a
dielectric-overcoated enhanced coating on the secondary.
Drive System.
A drive system that keeps the object centered in the "sweet
spot," along with a coma corrector that makes the sweet spot a larger
and therefore easier target, will assure that the optical rays are
concentrated to a size comparable to that of the diffraction spot diameter
in the critical part of the image where resolution is most needed. Part of
the problem associated with an undriven system is the need to shove the
scope from the top end until the object is in the ideal "drift"
position, let its vibration die down, and hopefully by the time the image
has stabilized, it crosses the approximate center of the field. Most of the
time, during undriven viewing, the object will be out of the best part of
the field, especially if no coma corrector is in use to extend the diameter
of the useful field width.
With the Sky Tracker drive system, the incremental steps applied to the base
of the telescope create no observable backlash or instability when the
altitude or azimuth motors engage, and the movement of the telescope is
imperceptible. Even at powers over 1000X, the object stays in view with an
occasional nudge of the buttons on the hand-held control paddle. Because the
object stays virtually centered, 100% of the observer's time and attention
is spent concentrating on the object, instead of diverting 75% of his energy
(and time) to pushing the top end.
I hope these comments don't cause refractor fanciers to take umbrage, and
realize there's a qualitative difference in large reflector images compared to,
for example, larger apochromatic refractors, which typically are about 8"
aperture. The images through such a refractor are pristine, subtle, and are much
to be appreciated for their optical quality, not to mention the value of such an
instrument. However, the 2.7X difference in resolution and 7X greater light
grasp provided by (for example) a 22" scope can provide commensurately more
visual information, detail and brightness. From what I have seen, this is all
too obvious when observing conditions permit this kind of comparison. I believe
it is true that all would agree that the planetary images through such a
large-aperture system are absolutely stunning, by any standard, when you see
them for the first time or even the fiftieth time.
Planetary images are not where these telescopes end. As deep sky instruments they
excel just as well. Recently, Joe Porter, who owns an excellent 12.5" F/5.6
telescope and is an accomplished observer, was observing M2 with me through a
24" F/4.2 telescope with a 1.6" thick primary. Joe commented, "It
goes through this side, then through the center, and then right out the other
side!" and he meant it was completely resolved like he had never seen it
before, which he commented on at length. And, at a recent star party in my area
where there were two 22" F/4.1 telescopes with identical configuration as
well as several other large aperture fast F-ratio scopes, ranging from an
18" F/3.75 to the previously mentioned 22"ers. The incredible seeing
at the time allowed the use of over 1100X on Saturn. More was possible if I had
not been too tired to try it, being near death's door from sleep deprivation by
4:00 a.m. Numerous comments from various "old hands" as well as
newbies to the hobby at this event, in response to what they observed in these
scopes, boiled down to an unspoken conclusion that the old paradigm is not yet
dead, but it's about time to hold an appropriate ceremony to bury it. It
logically follows that the new paradigm shift should be to the concept that
large reflectors, when properly configured, do an excellent job on planetary and
extended object detail.
Rick Singmaster
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