RC
Optical Systems (RCOS) 16” Ritchey-Chrétien (Kitt Peak DSD Program) Review
Kitt Peak is a mountain top in southern
Arizona, an hour’s drive west of Tucson. The observatory there is home to
multiple large professional instruments, including the 4m Mayall
and 3.5m WIYN telescopes. However, Kitt Peak Observatory also has a very active
public outreach program for which two RC Optical Systems (‘RCOS’) telescopes
are available – a 16” and a 20”, both on Paramounts
with smaller refractors piggybacked.
You can book a whole night solo on one
of these telescopes, to do whatever you want (imaging or visual or a mix, one
object or many), including room and board in the same accommodation as the pro’s. That’s an expensive option, but one I intend to try;
as a trial run, I booked onto a half night Dark Sky Discovery (’DSD’) program
in the company of a small group.
Kitt Peak’s DSD program is sometimes
run on the 20” RC, sometimes on the 16”. I got the 16” which is in a roll-off
observatory at the stop of the site (see below). It’s near a couple of working
professional instruments, including the SARA 0.9m, so they don’t allow cameras
of any kind (no matter how faithfully you promise no screen or flash). Consequently,
the photos here are of the otherwise identical 20” which is in a dome attached
to the visitor centre.
The only other difference between the
two setups is that the 20” has a Takahashi FSQ-106 as its auxiliary instrument,
whilst the 16” has a more visual-oriented TEC-140.
Note: this review is of the RCOS 16”
telescope, but also of the DSD experience.
At
A Glance
|
Telescope |
RC Optical
16” Ritchey Chrétien |
|
Aperture |
16”
(400mm) |
|
Focal
Length |
3360mm |
|
Focal
Ratio |
F 8.4 |
|
Central
Obstruction (incl. holder/baffle) |
~40% |
|
Length |
~1.5m |
|
Weight |
~43 Kg |
Data from RCOS.
Design
and Build
RC Optical Systems (RCOS) were based
locally (to Kitt Peak) in Flagstaff and mainly produced mid-sized reflectors
optimised for scientific use (i.e. not really for visual), from 12.5” to 36”
aperture.
Our guide described the RC Optical 16”
as ‘high-end amateur’ equipment, but that may be from the perspective of a
major observatory used to huge bespoke telescopes costing millions. The 16” OTA
alone would have cost perhaps $30,000 and my guess is that most RC Optical’s scopes of this size were sold to smaller
institutions such as university physics departments that wanted something
better than the default 14” Celestron or 16” Meade.
Certainly, the RCOS 16” 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. This one is a semi-open truss-tube
model.
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, as we will see).
Those characteristics, at a focal ratio
faster than most Cassegrains (F8.4 for this 16”, F8.1
for the 20”), make the RC excellent for wide-field imaging. Trouble is the
Ritchey Chrétien is even 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 (still maintained at the time of writing, though RCOS are out of
business) suggests that the 20” at Kitt Peak has high-quality (1/20th
wave RMS) computer-polished mirrors made of ultra-low-expansion Astro-Sitall and the 16” is likely the same.
Tube
This mid-sized RC Optical OTA is of
semi-open design that looks pleasingly like a big observatory instrument. All
of the hardware is made from 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 for active
cooling.
Focuser
For visual
use, an Astro Physics rack-and-pinion focuser has been installed at the visual
back. However, the telescope also appears to feature an electrically operated
zero-shift focusing system that moves the secondary mirror, presumably for
imaging.
Mounting
Both the 16”
and 20” scopes are mounted on Paramounts attached to
isolating piers. This proved to be a good choice, however the sheer weight of
the RCOS and its piggybacked refractor (about 50 Kg for the 16”) means a long weight
shaft and lots of counterweights. Even so, vibes just weren’t a problem.
The
Paramount holding the 16” was apparently an early one and made a lot of sci-fi
warp drive noises on slewing, but it did so fast and accurately which really
helped the flow of objects and make it a richly varied observing session.
Accessories
Various
premium Tele Vue eyepieces are available for the observing session, fitted into
a premium TV diagonal, so you know you’re getting the best view possible.
In
Use – Astrophotography
I didn’t get
a chance to try imaging with the RCOS, but many others have with outstanding
results. Choose the full-night solo program and you could too.
F8.4 makes
for a powerfully fast and wide field at this aperture. I tend to think of RCs
as having a perfectly corrected field. But in fact, though they are basically
free from astigmatism and coma, field curvature is still present, something you
could see through the eyepiece. You’d probably need to choose a middle focus
point for imaging, a trick I’ve used in the past.
The roll-off
roof observatory housing the 16” RCOS is in the centre, between the 4m Mayall and 0.9m SARA domes.
Sunset, with
the roof rolled back and ready to start the night’s observing; along with
Charles, one of KP’s super-friendly guides.
In
Use – The Night Sky
General
Observing Notes
Most of our
viewing was done with a 31mm Nagler and occasionally
a 13mm Ethos, giving mags of 108x and 258x respectively. The Nagler gave a wonderfully expansive view, but was a bit
lacking in eye relief for those of us viewing with glasses on. An even higher
power was used on one occasion, but I’m not sure what eyepiece delivered it.
I had the
chance to do a bit of surreptitious focuser tweaking and it suggested focus
snap and optical quality was first rate.
For many of
the objects viewed, we had the option to take a look at lower power and wider
field through the TEC-140 refractor.
The full
list of objects viewed (there might have been one or two more I’ve forgotten):
·
Venus
·
Uranus
·
M42
·
M41
·
M37
·
Castor
·
Delta Cancri
·
M79
·
NGC 2419
·
NGC 604
·
Bode’s Nebula
·
Eskimo
Nebula
·
Crab Nebula
(M1)
Planets
Our first target was a brilliant Venus hanging in a dusk sky at 258x
with a 13mm Tele Vue Ethos eyepiece. I really wasn’t expecting a good planetary
view from a telescope optimised for deep-sky astro-physical
work, but it surprised me. The all-mirror design meant there was no false
colour from the primary optics and very little from the eyepiece or atmosphere
either (we lucked out with very stable seeing). Venus showed its dazzling
gibbous disk perfectly, with little stray light. I may have caught a glimpse of
variegation in the clouds.
The only other planet around in winter 2020 was Uranus (hold the jokes,
we’ve heard ‘em all before) and it looked better than
it ever does through smaller instruments and at lower magnifications – a misty
ball of exotically icy blue-green. Clustered around the planet were several
stars of similar brightness to one another, all named after characters in
Shakespeare’s plays - actually some of Uranus’ many moons, including Ariel and
Miranda close by.
Deep
Sky
Most of the
session was spent viewing a wide range of different types of deep sky objects,
including doubles, planetary nebulae, HII regions, clusters and galaxies.
Highlights included the following.
NGC 2419 is
one of the most distant globular clusters in our galaxy. Usually it’s pretty dim compared to the brighter globulars
like M15, but the RCOS showed it bright and resolved almost all the way – a
spectacular sight. Another interesting globular and possibly an extra-galactic
one, Messier 79, gave another spectacular view.
Bode’s nebula,
consisting of two different galaxies, M81 and M82, are a favourite object of
mine. Most amateur telescopes only show them as a pair of smudges with
different shape, but the 16” under dark skies revealed much more, including M82’s
central dark band.
The Eskimo
Nebula must be one of the most imaged objects in the sky, but in smaller scopes
it can be a bit disappointing visually. The 16” gave a much more interesting
view of this unusual planetary nebula, revealing the outer envelope, central
star and some of the intersecting shells of gas you see in images.
Delta Cancri
is a beautiful double star near the Beehive Cluster. It resembles Albireo, with
orange and blue components. A much more well-known double is Castor, but there
is a fainter (magnitude 9.8) and more distant 3rd visual component
of the system, Castor C, which has a distinctively orange hue in contrast to
the white main stars of the double. The 16” aperture really made the colour of
Castor C stand out.
The Crab
Nebula, Messier 1, is another well known deep sky object. Smaller apertures and
more compromised skies show it as just a faint fuzzy. The 16” RCOS under such a
clear, dark and stable sky revealed its shape and some internal structure, much
the way it looks in a long-exposure image frame.
Only the
central region of the Great Nebula in Orion fitted into the field of view, but
another much more unusual HII (star forming) region is NGC 604. Despite some
clearly arcing wisps of gas, it didn’t reveal as much bright internal structure
as M42, but that’s because it’s in another (the Triangulum) galaxy – Messier
33!
The Broader
DSD Program Experience
Dark Sky
Discovery is one of Kitt Peak’s more in-depth outreach programs, so it’s worth
noting how the wider experience worked. If you’re only interested in the review
of the RCOS telescope you can skip this section.
Kitt Peak is
a fifty mile, ninety minute drive west of Tucson, the
only place nearby you’re likely to get lodging. Initially it’s a fast drive
along state route 86 but then a left turn followed by a much slower slog up the
twists of a minor road (386) to the observatory. Both the road and the
observatory benefit from spectacular views over the surrounding desert and
mountains, so give it some extra time. You’ll first see two of the biggest
domes from Highway 86, poking from behind a closer peak.
You are
supposed to arrive by 5 pm (I arrived closer to 4 pm) and I was immediately
asked to re-park by the car park fence behind a van with government plates. The
reason is that you all have to leave together at the end of the session,
in-line and following the van and with red covers over your headlights (they
fit them). This is to protect the professional instruments from stray light. They’ve
been operating that way for twenty years, but it does mean you can’t leave
early.
Having
signed in at the visitor centre and enjoyed the gift shop and a film about the
research at Kitt Peak, the program starts with a light meal of sandwich and
cold snacks which you can eat at one of the picnic tables outside. You won’t
get bored waiting because the observatory is chocka with interesting equipment,
including the huge solar observatory with its heliostats (take a pair of binos to check them out).
They give
out red-light torches and a pass, both of which you need to return at the end.
The site really is very dark, with even red lighting kept minimal and dim, so
finding your way around later is challenging. You’ll be needing that red
flashlight.
At sunset (~
6 pm in winter) they lead you up to a lookout, next to the small dome for the
0.9m SARA telescope, for some last photos. Then it’s into the nearby roll-off
observatory with further photography strictly banned.
The roll-off
observatory is set a hundred metres or more from the Visitor Centre, but
thankfully has a toilet and its own warm room / class room. The observatory is
a strange looking building and turns out it was re-purposed, having originally
been built by NASA for tracking sounding rockets.
The
observatory floor is up a flight of stairs lit with a red rope light. There are
chairs around the wall, just enough for the eight or so in the group. The 16”
RCOS is mounted on a Paramount and controlled by ‘The Sky’ software. Slewing
from object to object was quick and accurate, which meant plenty of objects and
plenty of time at the eyepiece too. The pier height had been carefully chosen
so most objects were comfortably viewable standing up.
The small
group meant waits for the eyepiece were never too long and as I’ve said the
mount slewed fast and with perfect accuracy, so moving from one object to the
next wasted no time. The only caveat to mention is that the long weight shaft
could be a bump hazard in the dark, but the guide warned us about that.
The roll-off
roof meant fabulous views of the whole star-filled dark sky, including a spectacular
cone of zodiacal light to the west, but means it does get very cold in winter –
much colder than they’d led me to believe, so much so that some people were
clearly distressed and had to keep popping down to the warm room. Wrap up
warmer than you think you’ll need to, or risk missing some of the show huddled
back in the warm room.
The session
lasted for some three hours until 9 pm, after which we followed a white line on
the tarmac back down to the VC for another go at the gift shop. Then, very
sadly for me, it was time to leave.
They encourage
you to cover your dashboard lights, then lead everyone slowly together down to
the first mile marker in procession. Then they remove the red covers for you
and send you on your way, but with the instruction not to use main beams for
the next few miles. There are a number of turnouts in the road and I got off a
few long-exposure dark sky snaps from one of them, but they warned us about
mountain lions so I didn’t linger too long.
I should
point out that if you’re headed back into Tucson you will need to stop at a
Border Patrol checkpoint on the way.
Start of the
Kitt Peak access road.
Kitt Peak
Visitor Centre and dome for the 20” RCOS.
Cars lined
up with red headlight covers on, ready to leave at the end of the session.
Dark and
starry skies with a few clouds from a turnout off the Kitt Peak road.
Summary
This review
is really two in one: the scope and the experience. Both were excellent.
The RCOS 16”
proved to be a wonderful visual deep sky instrument, much better and more
flexible than I expected (at least under these dark and steady skies, under my
home skies probably not so much). The Paramount slewed flawlessly from object
to object and the scope always gave a great view, generally a much better one
than most amateur scopes would. It was nice to have the TEC 140 for a low-power
perspective.
The
experience was flawless too. The guide was friendly, efficient, helpful and
knowledgeable, the visitor centre well equipped, the facilities and
organisation uniformly good. I really appreciated the warm room attached to the
observatory. The cost of the DSD program ($80 including the packed dinner when
I booked) seems almost indecently cheap for the quality of experience on offer.
Not for the
first time after a viewing experience like this one, an uncomfortable question
pops into my head: ‘why own an expensive telescope and mount when this quality
of viewing is available at such modest cost?’
The answer in
my case is, ‘Trains, planes and automobiles (hotels too)!’ But if I lived
within an hour or two’s drive of a facility like Kitt Peak it’s a question I
couldn’t easily ignore. Strange, then, how few amateur astronomers I typically
meet on this type of evening: most of the group on this one had never looked
through a telescope before. Get booking and change that!
Kitt Peak’s Dark Sky Discovery program,
on an RCOS 16” or 20”, is a fantastic way to enjoy some superb DSO viewing
under dark skies with a professional instrument. It gets my highest
recommendation.’
