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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.’

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