RC
Optical Systems (RCOS) 32” Ritchey-Chrétien Review
I recently spent a night observing with
the Schulmann RC 32” at Sky Center on Mt Lemmon near Tucson
at one of their public ‘Sky Nights’ sessions. I had
lots of eyepiece time and was able to get a fair impression of this, perhaps
the largest dedicated outreach telescope for visual use and one of the few
that’s really modern (built in 2010).
I originally included some of this content
in the review of the Sky Nights experience under the Travel section, here. But unlike many
other big-scope observing nights, that program includes lots of other good stuff,
so I've split out this review of the telescope.
At
A Glance
|
Telescope |
RC Optical
32” Ritchey Chrétien |
|
Aperture |
32”/0.8m |
|
Focal
Length |
5.6m |
|
Focal
Ratio |
F 7 |
|
Central
Obstruction (incl. holder/baffle) |
~40% |
Data from RCOS/Sky Center.
Design
and Build
This large Ritchey Chrétien was
installed in 2010, provided by the Schulman foundation and built by RC Optical Systems
(RCOS).
RCOS were based locally (to Sky Center) in Flagstaff and mainly produced mid-sized
reflectors optimised for scientific use (i.e. not
really intended for visual), from 12.5” to 36” aperture. R C Optical have since
been acquired by Deep Sky Instruments; this 32” is still available and appears
to be their largest current model.
A few of RC Optical’s
scopes were sold to individuals (I wish) but most in the larger sizes like this
one were sold to smaller institutions such as university physics departments
that wanted something better than the default 14” Celestron
or 16” Meade. Lucky for us this one was acquired for an outreach program!
Like other RCOS products I’ve seen,
this 32” is a beautifully made, all-CNC scope that has many premium features
and is standing up well to lots of hard outreach use.
RCOS made two parallel lines of smaller
Ritchey Chrétiens, one in solid carbon fibre tubes,
the other in open or semi-open truss-tubes like this one. Surprisingly, the 32”
does seem to be a turnkey product – the website shows images of at least two
scopes that are subtly different, so they’ve clearly built others!
Promotional image: RC Optical Systems.
20” RC Optical for comparison.
Optics
Optically the Ritchey Chrétien
Telescope (RCT) is a form of Cassegrain, a true reflecting telescope that uses
neither a sub-aperture nor full-aperture corrector plate. The RCT became
popular with professionals first (Hubble is an RCT) and latterly with amateurs,
due to its wide field and round off-axis stellar images with lack of coma
(though it still has a rather curved field).
Those characteristics, at a focal ratio
faster than most Cassegrains (F7 for this 32” –
faster than the smaller RCs which are typically ~F8) make the Ritchey Chrétien excellent
for wide-field imaging.
The Ritchey Chrétien is harder to make
well than the Classical Cassegrain, because it has hard-to-grind hyperbolic mirrors
(a curve the Classical saves for its secondary mirror alone, the primary being
parabolic like a Newtonian).
The RCOS
website suggests the main mirror is made of a low-expansion substrate, possibly
Zerodur or Astro-sitall,
figured to a minimum of 1/20th wave RMS.
Tube
Like other RC Optical OTAs, this one has
a premium feel – in amateur terms, like a big AP refractor. All of the hardware
is CNC-milled 6061 aluminium with carbon fibre truss-tubes.
The primary is centre-mounted, the secondary
on a 4-vane spider. All Cassegrains are at risk from
stray light and RCOS state the primary has a baffle tube with multiple knife
edge baffles and the secondary a conical baffle.
The OTA houses multiple fans around the
sides of the mirror cell for active cooling.
The petal flaps protecting the primary
are remote operable.
Focuser
For visual
use, a Starlight Instruments Feather Touch rack-and-pinion focuser has been
installed at the visual back. A remotely-operated secondary mirror focuser
comes as standard and would likely be used for imaging applications.
The
telescope also features an electrically operated instrument rotator behind the
focuser with a 4” aperture – the red device you see below.
Massive fork
mount is designed for remote operation of telescopes up to 40”. Also note the
secondary mirror focuser.
Mounting
Unlike other
smaller RCs I’ve used, this one doesn’t sit on an off-the-shelf
mount from the likes of AP or Paramount – it’s just too large and heavy!
Instead, it has RC Optical’s own ‘Professional Series’
fork mount, designed for telescopes up to 40” and for remote or local goto operation.
This is a
proper professional-class observatory instrument and the mount offers pointing
to sub arcsec precision. It weighs over 5500 lbs (2500 kg).
It is
capable of extremely rapid slewing, much faster than would be safe at a public
session!
Accessories
Most of our
viewing was done with a 31mm Tele Vue Nagler giving a magnification of 184x and
a field of view of 0.42°.
A Sky Watcher 150ED
apochromatic refractor is fitted to give complimentary wide-field views.
For imaging sessions, an
SBIG CCD is fitted at the visual back, see below for details.
In
Use – Astrophotography
Unusually we were also allowed to try a spot of impromptu
imaging through the eyepiece with our phones, something I’d never tried before
for DSOs (it really only works with the latest phones that automatically stack
images in low light).
My efforts were pretty crumby (hard to align my IPhone 13’s camera with the exit pupil in the dark), but
the guide showed some truly magnificent snaps he’d taken: with a fancy CCD?
Nope, with a Pixel 3 phone-cam!
You can book the Schulman for some serious imaging, whether
on site or remotely. The website shows some stunning images taken through this
telescope.
The CCD for these sessions is an SBIG STX with a KAF-16803 sensor
measuring 36.8mm x 36.8mm at 4096x4096 pixels (16.8 MP). This may not be the
very latest, but would have been a very high-end ($12000) camera when new.
In
Use – The Night Sky
General
Observing Notes
The viewing experience with the Schulman was amongst the best
I’ve had on a public night. There was plenty of time at the eyepiece on both
scopes and plenty of chairs to sit on in between. Adding to the comfort factor,
the dome does have a small warm room.
Most unusually for a public viewing session, we were allowed
to tweak the Feather Touch focuser for ourselves,
something that really enhanced the experience for me. The focuser was easy to use, even for the
inexperienced. Focus was very snappy with none of the mushiness you can get
with larger apertures.
On its
professional mount, slewing was simple, quick and precise, rock solid when
tracking. You can just choose a new target from an on-screen sky map.
If you’re interested, the full list of objects viewed is as
follows:
Stars
We looked at three bright stars, Deneb, Capella and
Betelgeuse. This might surprise you if you’re a keen astronomer, but for the
novices it seemed interesting and for me handy confirmation of the Schulman’s
optical quality with a quick star test – all showed beautifully pinpoint Airy
disks surrounded by perfect faint diffraction rings.
Planets
I might not choose an RC like the Schulman with a big
secondary obstruction for planetary viewing, but under stable desert skies it
worked fine, just as the 16” version did at Kitt Peak.
Our first views of Jupiter were during full daylight, but the
view was surprisingly sharp, contrasty and detailed.
Later, during the dark-sky session, Jupiter showed a lot of
cloud belt detail at 184x, with lots of tiny belts and storms. Even more
impressively, the large aperture delivered much more colour – a range of buffs
and salmon pinks – than usual.
Uranus showed a small blue-grey disk, but a couple of moons
too, something of a first for me (a week later I saw more of them through the
Struve 82” at McDonald).
Deep Sky
M35
This big bright and familiar open cluster actually looked
best with the lower power and wider field of the Esprit 150ED, but the big
aperture delivered richer star colours.
The Double Cluster
Through the 32”, the cores of the two clusters were just a
fit in the field of view and showed many more stars than I’m used to with smaller
apertures. Again the red stars showed off their
colours.
The Rosette Nebula
I requested this, hoping the large aperture and dark skies
might reveal more nebulosity than usual. Sadly it
didn’t, though I’m not sure why. It’s often said that ‘aperture wins’ – not in
this case!
The Andromeda Galaxy (M31)
This is another standard object for me, but the 32” revealed a
small, dense and bright central core-within-the-core and dark lane differentiation,
hints of the spiral structure, that are different from my views through any amateur-sized
scope.
M1 The Crab Nebula
This large and bright remnant of a supernova in 1054 showed
off its shape and some of the filigree internal structure you see in images in
a way only larger apertures can.
M42 The Orion Nebula
This is a nebula that many of us observe and image regularly,
but here it was different. The limited field just showed the central ‘box’
around the six dazzling main Trapezium stars and numerous fainter ones, but
what a view, with prominent structure in the dark lane, clumps and festoons and
knots in the nebulosity that looked almost 3D and which you only ever see in
images. I could detect more than a hint of colour – pinks and greens – too.
Summary
An F7 RC isn’t
what I’d choose as a large visual instrument. This is really a professional
telescope naturally more suited to use with a CCD or spectrograph.
Nonetheless,
at moderate powers at least (I didn’t have the chance to try higher powers) it
surprised by giving really excellent views of most of the objects we looked at,
even Jupiter. At over 9000ft under mostly stable skies, this kind of telescope
works for visual in a way it wouldn’t under a thick and turbulent atmosphere
(Sky Center quote average seeing for long exposures
at one arcsec).
This is the
largest truly modern telescope that I’ve looked through and a near-unique
experience as such. The public session, despite including other activities,
offered lots of quality telescope time for a very modest price.
A private
session – whether to just slew through your visual DSO bucket list, or
concentrate on getting a set of superb frames for just a few – would be an
amazing astronomy experience, though an expensive one.
The public
viewing night described here was $85 in early 2022. Current prices for private
sessions are $1500 for a full night of either viewing or imaging. A half night
of remote imaging is currently $400. Check the Sky Center
website for latest prices and availability.
The Schulman 32” RC is likely the
largest telescope commissioned and dedicated to outreach. It’s a unique opportunity
to experience a really modern observatory-class instrument. Public nights are
amazing value.
