Celestron's C6-RGT Refractor

Celestron's "Yard Cannon", the C6-RGT
By Jay Michaels

Lower cost computer pointed telescopes have only recently become available to amateur astronomers. These more affordable "Go To" telescopes were first introduced by Meade Instruments in 2002 with the LXD55 series; Celestron shortly thereafter unveiled its own "Advanced Series" Go To capable telescopes. In response to the popularity that the Advanced Series telescopes enjoyed, Celestron has readily expanded the line and now offers two-dozen different Advanced Series models (counting the computerized and non-computerized versions separately). Included in Celestron's line-up are four different refractors, ranging from the portable 80mm ED apochromat to the giant 150mm achromat.

            For amateurs who enjoy a refractor's unobstructed light path and resulting sharp images, an instrument boasting a 150mm lens is very tempting. When such a large refractor is coupled with a computerized mount, it becomes an even more desirable telescope. After wanting a big refractor for many years, I finally placed my order with Astronomics for a Celestron C6-RGT in March 2005. Neither the vendor nor Celestron were aware that the provided sample would eventually be reviewed.

Unpacking & Assembly

            The scope arrived packaged in three boxes. The largest box held the optical tube assembly. A second contained the CG-5 computerized mount, while the third box held the counterweights and tripod. The boxes proved sturdy, and all of the telescope components were well-protected by thick custom fit styrofoam inserts. The packaging stood-up to shipping rigors and no part of the telescope arrived damaged.

The C6-RGT features Celestron's largest refractor optical tube onboard the CG-5 computerized mount. For amateurs not desiring Go To functions, Celestron also offers their 150mm refractor with manual slow motion controls in lieu of computerized drive motors.

all photographs by Jay Michaels

There were no instructions included with the telescope. Celestron does, however, have these available for free on their website as a .pdf file. Instructions were unnecessary for out-of-box assembly, as the tripod, mount head, and optical tube arrived preassembled. All that was necessary during initial setup was fastening the mount head to the tripod, affixing the dovetail bar and tube rings to the optical tube, and thereafter attaching the counterweight bar, counterweights, and refractor tube on the mount. When assembling the telescope for the first time, it is advisable that the counterweight shaft be situated above one of the tripod legs. Failure to do this could cause mount instability, with the telescope potentially tipping over during use. While Celestron's instructions illustrate the mount head having the counterweight bar above a tripod leg, the written information does not specify this.

During the CG-5 computer-ized mount's initial assembly, the counterweight bar should be positioned so that it is over a tripod leg. Having the bar positioned between tripod legs could cause the mount to become unstable, especially when carrying heavier loads.

Another assembly note pertains to the dovetail system. Celestron uses a 13-inch dovetail bar to attach the telescope tube rings and optical tube to the CG-5 computerized equatorial mount head. A separate, smaller, Vixen-style plate is also provided with the telescope. While this can be attached to the larger dovetail bar, it is unnecessary and is not described in the instructions. Apparently, this Vixen-style plate was provided so that amateurs can use other optical tube assemblies on the CG-5 computerized mount.

The CG-5 Computerized Mount & Tripod

            Most of the telescope's components are metal, exceptions being miscellaneous knobs and the mount electronics housing, which are plastic. The tripod legs are rolled steel, with 2-inch diameter outer legs. Between these is a sturdy die cast accessory tray featuring precut holes for both 2-inch and 1.25-inch diameter eyepieces. This tray doubles as a spreader for the tripod legs, increasing tripod stability. A central threaded bar connects the tripod and the mount head, and also holds the accessory tray. This central bar uses two knob assemblies: one for securing the mount head to the tripod and another for raising or lowering the accessory tray. To close the tripod legs, the accessory tray must be lowered. With other commercially available tripods I have used, lowering the accessory tray loosens the connection between the mount head and tripod. The Celestron design is the first that I have encountered to prevent this. By loosening only the lower knob on the central bar, the accessory tray may be lowered without affecting the mount head's attachment to the tripod.

The die cast accessory tray between the tripod legs can hold five 1.25-inch oculars and a pair of 2-inch eyepieces. This tray also serves as a spreader for the tripod legs, helping to dampen any vibrations. The central bar is seen in the middle.

All of the CG-5 computerized mount pieces appear well assembled and without poor fit or noticeable roughness. Mount motions are smooth, but have more drag than what I have encountered on other equatorial mounts. I do not consider this to be a problem, as the extra friction in the mount bearings makes balancing the large refractor optical tube easier. A fine balance is not essential for the mount to work properly.

Considering balance, the C6-RGT comes supplied with two 11-pound counterweights. Contrary to pictures advertising this telescope, the counterweights easily balance the optical tube when situated near the top of the counterweight shaft. Both counterweights have Teflon-tipped locking bolts. This simple feature prevents the bolt from scuffing the counterweight shaft and also makes fastening, removing, and adjusting the weights much easier - something most appreciated during cold winter nights and when it comes time to bring the scope indoors.

The optical tube is easily attached to the mount by the dovetail assembly. The dovetail is held in place by two bolts: a larger main bolt having a plastic knob and a smaller all metal bolt. This smaller bolt acts as a security feature should the main one somehow fail - or be accidentally left loosened. While I did not intend to test this feature, one night I adjusted the optical tube's balance by sliding the tube via the dovetail, and forgot to tighten the main bolt. Worse still, I also forgot to significantly tighten the smaller security bolt. I never noticed this problem until I moved the telescope away from me. I was confused by a strange jittering and quickly discovered my error! Thanks to Celestron's good design with the dovetail catch system, disaster was averted and the C6-RGT optical tube remained situated securely atop the CG-5 mount.

Mount dampening time was good considering it holding a very long, large, refractor tube. With the tripod legs extended, and on a grassy surface, the mount quells even the heaviest vibrations within 4 seconds.

The Optical Tube

            The telescope's heart and soul optics are housed in a 4-foot long steel optical tube. The tube is finished in a glossy black paint that is punctuated by Celestron's vibrant orange logo. While I've seen many company logos affixed to refractors only as a sticker, the Celestron logo is painted on. The main objective lens is tucked safely out of dew's reach by a sturdy metal dewshield that's nearly 8-inches long.

Turning to the optical tube's opposite end, the rack and pinion focuser accepts 2-inch diameter accessories. Racking the plastic focuser knobs finds the focus movement to be tight and coarse, this is common with these imported optical tubes. Removing the thick, sticky, grease around the focuser drawtube helped movements slightly. Just above the focuser is a dovetail shoe accepting the finder's bracket. This dovetail shoe is the same that is used with all Vixen and Orion telescopes, thereby making finder scopes by those two companies interchangeable with the Celestron. But, it is unlikely that a finder scope replacement will be called for. Celestron's supplied 9 x 50mm finder scope demonstrates excellent quality, having bold crosshairs and sharp views. The supplied bracket holds the finder by two thumbscrews and a third spring-loaded tension system. Adjusting the finder scope is easy, and the bracket keeps the finder's alignment between observing sessions.

The C6-RGT sports a 9 x 50mm finder scope of good quality. The Vixen-style finder bracket robustly held the finder's alignment between observing sessions and the dovetail readily accepts other finder scopes offered by Vixen and Orion.

The telescope's main objective is a 150mm (5.9 inches) achromatic doublet. Under bright light, the optics show a green cast, indicative of full multi-coating. Comparing the coatings to other refractors, the Celestron's appear about as dark as those on my Meade 102ED. Both of these telescopes have coatings showing more reflection than those on my Tele Vue 102. The Celestron's lens cell has three collimation adjustment points, useful to correct any misalignment caused by rough handling during shipping. Internally, the 150mm achromat is painted flat dark grey and has two baffles. While the limited number of baffles helps to minimize the optical tube's weight, more internal baffles could theoretically improve the telescope's image contrast.

A bright blue December sky is seen reflected in the Celestron's optics here, but the telescope's objective presented the green tint characteristic of full multi-coatings during inspection.

Into the Field (or Backyard)

            Moving the C6-RGT involves separating the telescope into three items: the optical tube, mount head and tripod, and the counterweights. It is feasible to separate CG-5 mount head from the tripod, but the combined weight of the tripod and German equatorial head is around 28-pounds (12.7kg) - quite reasonable by any standard. The 150mm optical tube weighs in at 18-pounds (8.2kg) while the counterweights each reach 11-pounds (5.0 kg) on the scale. Moving all of this equipment outdoors for an evening observing session takes me about 20 leisurely minutes most of the time. This includes balancing the optical tube.

Celestron's NexStar Go To

            Firing up the CG-5 computerized mount's electronics brings the hand control to a spartan screen requiring the user to press the "enter" button to initiate the alignment procedure. The computer first instructs the user to align the telescope on a series of arrow stickers found on each the right ascension and declination axis. This position has the optical tube is facing north, parallel and above the equatorial mount head. Oddly, the alignment arrows marked on the right ascension axis will cause the optical tube to point slightly east. In astronomy discussion forums, many amateurs report moving these arrow stickers so that the telescope faces due north during initial alignment. I don't see this change necessary, though.

Once the telescope is aligned with the arrow markings, the control requests that the current time and date be verified. Changes can be made by pressing the "undo" button. The electronics do not keep the date and time when the scope is powered down, so users will need be sure to have a watch or clock handy during all observing sessions. Fortunately, the next sequence of requisite data - location latitude and longitude - is permanently stored once entered.

There are four types of alignments the CG-5 computerized mount facilitates: auto align, auto three star align, quick align, and last alignment. The auto align will automatically select three stars for the amateur astronomer to center in the scope's eyepiece during the alignment procedure. The auto three star align allows the user to select the alignment stars form a list. The quick align simply calculates sky object positions based on the date and time. Finally, last alignment is just that: it is an alignment that reactivates the telescope based on whatever previous alignment was achieved. The last alignment feature is useful for amateurs leaving their telescope setup in one outdoor location over several days (such as what might be found at a star party) or for anytime that the mount unintentionally loses power.

For alignment procedures, the auto align worked well, but I found that the computers sometimes choose a star very close to my horizon or otherwise obstructed by trees or buildings. To me, the auto three star align is a quicker way to complete start-up, as it allows selecting alignment stars by a list. For example, during my last observing session with the C6-RGT I choose Rigel, Capella, and Caph (Beta Cassiopeia) for alignment. This alignment procedure is quick and intuitive, taking only about 5 to 10 minutes to complete.

All of the Celestron Advanced Series computerized telescopes come with a NexStar hand control. The control has a two-line 16-character liquid crystal display.  Sleek and futuristic, the hand control is easy to work with and comfortable to hold.

I have been impressed by the CG-5 computerized mount's Go To pointing accuracy. By obtaining only a rough polar alignment and centering alignment stars in a low power eyepiece, the Celestron computerized mount routinely places selected objects well within the field of view of a 26mm plossl eyepiece. Determining the pointing error by centering a called object at 124x and then using the NexStar's "identify" feature found an average pointing error of only 10 arc minutes. For a mount that retails for under $1,000, such computer accuracy is commendable.

The NexStar computer has too many features to discuss in-depth here. Needless to say, all of the electronic functions take time to learn, and many of them are useful, if not fun. One function that I particularly enjoy is the "Constellation Tour." Selecting this option from NexStar's "List" menu provides a list of constellations. Users can select any constellation to then browse objects that are located in that constellation. This is especially handy for those just learning the sky, and will ensure no observing session ends in boredom. For example, when I choose to view objects in Auriga, I was surprised to find an asterism I did not know about listed.

The computer system also offers information about many objects. Centering on the bright open cluster M36 in Auriga yields a scrolling message on the hand control screen: "M36 (NGC 1960) is a rather young cluster containing many hot blue main-sequence star[s]. The luminosity of the cluster is about 8,000 suns. Distance = 4,600 [light years], diameter 25 [light years]." Neat! Having this interesting information at one's fingertips will be useful for public and school star parties. The C6-RGT's owner can simply have the telescope's computers point at and track an object, and thereafter elucidate on the view by reading the information provided by the hand control.

            These good performances notwithstanding, some objects situated near the pole star and zenith were occasionally not located by the computer pointing. Sometimes this failure would be sourced in the mount stopping its movements, having correctly sensed that the optical tube would strike a tripod leg. Other times, though, the reason for the error was unclear. Furthermore, the longer the computers operate after startup alignment, the less accurate they become. Fortunately, Celestron compensates for this by allowing re-alignment during an observing session. Additionally, the mount and hand box operate slower in cold weather. On a chilly night (by Florida standards) with temperatures in the mid-40's (mid-30's Celsius), the drive motors were slightly slower at full speed. More annoying, the hand controller's liquid crystal display exhibited broken text and was difficult to read. After the telescope was powered for awhile, though, the hand control display greatly improved.

Direr, I encountered problems with my C6-RGT's electronics. With delivery, the telescope failed to power-up. I traced the problem to a broken fuse in the supplied 12-volt car battery adapter. Contacting Celestron resolved this issue, and a replacement arrived a little more than a week later. Recently, a more serious problem has developed. During the early autumn, the mount's right ascension motor began squealing. At first I assumed that perhaps my balance was poor, but checking this found no fault on my part. By late autumn the mount started periodically acting erratic, with the drive motors continuing a slew even after the hand control indicated a called object had been centered. These problems culminated with the right ascension drive failing at times during my latest night with the C6-RGT.

I have not found such problems frequently reported with the CG-5 computerized mounts, and am sure my experience is an anomaly. I have peace of mind with my purchase; Celestron backs its refractor and computerized mount with a 2-year warranty.  

The Big Lens

             The objective supplied with my C6-RGT is quite good. Having used the refractor in a variety of temperatures, any internal heat currents diminish after about 20 to 30 minutes of cool down. Star-testing the optics (without a diagonal) found no noticeable issues. Magnitude 1.9 Alhena (gamma Gemini) presented clean airy disks of equal dimension on either side of focus. The optics showed no astigmatism. Celestron advertises the telescope as having a "highest useful magnification" of 360x. In my experiences this seems reasonable if given steady skies. On several nights, I was able to maintain a compact star point with a 4mm orthoscopic (300x). Planetary detail seems to become mushier much beyond 200x, though. I suspect these limits are not so much a result of any lens crafting errors as they are a fact of the telescope being achromatic doublet.

For such a large achromat, false color is well controlled. Only on objects brighter than 1^st magnitude have I found the Celestron refractor's chromatic aberration to be very noticeable. On targets dimmer than 2^nd magnitude, false color virtually disappears except at high powers. For viewing the planets, using inexpensive Wratten color filters greatly aids in suppressing the chromatic aberration and also enhances planetary details. The C6-RGT's strengths seem to reside more in deep sky exploration, and when the telescope is aimed on star fields, any visible false color is quickly forgotten as sharp starry pin-points dominate in the eyepiece.

When viewing star fields and clusters with the C6-RGT, there is no doubt that you are viewing through a refractor. The telescope possesses the "snap focus" refractors are well-known for. That is, when focusing the telescope, the image suddenly snaps into keen sharpness and there is little doubt that ideal focus has been achieved. Contrast is also outstanding with the 150mm refractor; this especially good for amateurs who - like me - regularly observe from a light polluted location. Illustrating this, one evening I decided to observe M79, a magnitude 7.7 globular cluster in Lepus. At the time, the globular was situated just 25-degrees above my backyard's washed-out southeast horizon. Here, the sky is unable to offer stars dimmer than about 4^th magnitude to the unaided eye. These conditions were of little problem for the C6-RGT. The globular cluster was very distinct against a dark background and the cluster showed mottling.

Turning the telescope a little east and to an area where stars around magnitude 5.0 could be glimpsed, I took aim at the Orion nebula. In a 26mm plossl eyepiece, the C6-RGT presented one of the most amazing views of this familiar object that I have ever seen. The four brightest stars in the trapezium hovered like vibrant white salt grains above a ghostly nebulous glow. This nebulosity showed considerable texture with direct vision, appearing three-dimensional. Dimmer nebulosity with adjacent M43 was very apparent, and showed a delicate wispy structure. Taking advantage of technology, a UHC filter provided an even more impressive view - displaying the nebulosity sweeping across the 26mm plossl's field of view and beyond.

With just shy of 6-inches clear aperture, the C6-RGT is an instrument capable for "serious" deep sky work. I've used the telescope to catalog numerous lesser-known open clusters, but the instrument also excels on galaxies. During the spring, I observed Lynx's NGC 2683 with this scope (again from my light polluted backyard). The galaxy's dim 12.9 magnitude surface brightness proved no match for the refractor's aperture and quality optics, as shown in my sketch (below). My notes describe the galaxy as seen through the C6-RGT: "a tantalizing galaxy, constantly showing hints of additional detail. The bright core emerges as a pale-white wedge of light… from the core there occasionally flickers a stellar nucleus. A cusp [of light] seems present near the core region rotating towards the galaxy's nucleus."

Although refractors are usually not considered for galaxy-hunting, the C6-RGT has plenty of potential on these distant objects. NGC 2683, in Lynx, is seen here. The sketch, made with the Celestron telescope and both 16.8mm and 10.5mm orthoscopic eyepieces shows the galaxy's distinct shape and even some details.

Astroimaging

            CCD-imaging was on my mind when I purchased the C6-RGT. Using Ron Wodaski's CCD Calculator, coupling my SBIG ST7E camera with the 150mm f8.0 refractor would yield a 13.1' x 19.7' field of view. This is on the narrow side, but is acceptable for imaging planets, or even smaller deep sky targets - like many planetary nebulae. Truthfully, however, my intentions for imaging were to use only the CG-5 computerized mount. I was planning on using one of my smaller, shorter focal length, refractors for actual imaging. Celestron makes such a change very easy to accomplish. All one needs for interchanging optical tubes on the CG-5 mount is an appropriate dovetail bar (available from Celestron dealers for minimal cost) and tube rings for the telescope of choice.

            Since Celestron advertises the CG-5 mount as being able to accommodate 35-pounds of weight, I thought that I could piggyback my Shortube 80 refractor and CCD imager on the 150mm refractor, thus being able to use the large refractor for visually centering an object for the CCD imager. I quickly found that Celestron's provided tube rings were prone to flexing. The solution for piggybacking was to build a simple mounting plate. For my experiment, I choose to make such a plate out of a cherry plank. With the plank situated between the tube rings, the flexure was eliminated. I did, however, find another concern.

            When the mounting plate, Shortube 80, and CCD imager were all mounted on the C6-RGT, the counterweights needed to be adjusted to the very end of the counterweight shaft. The mount's motors showed obvious strain during operation, being both noisier and slower. This was distressing, as I had added less than 10-pounds of additional weight. Considering that the C6-RGT refractor optical tube tips the scales at 18-pounds, my total load was no more than 28-pounds, well below the Celestron's toted 35-pound limit.

            Another consideration for astroimaging, I find that the mount's slower slewing rates (speed 5 and below) are prone to a brief, but noticeable, delay before the mount responds to the directional buttons on the hand control. Careful attention to balance can reduce this delay, but I found it was always present. Such mount response will make the CG-5 more difficult to use with any visual guiding. I did not test the autoguide functions.

Accessories

            The C6-RGT is supplied with a good selection of accessories. Celestron provides buyers with a 20mm plossl eyepiece, a 2x barlow, a star diagonal and a collimating cap. All of these are 1.25-inch accessories.

The 20mm plossl provides 60x in the 150mm refractor and is a good ocular. I could notice no difference in image sharpness and only hints of lesser contrast when comparing the provided eyepiece with an expensive 18mm wide field one. Certainly the contrast difference may have been more a factor of the 18mm providing slightly higher magnification than anything pertaining to the 20mm plossl's design. The supplied 2x barlow looks like the infamous units supplied with department store telescopes. I was pleasantly surprised to find that the barlow is of better quality than it looks. In side-by-side testing with a "shorty" style barlow, the provided barlow showed no loss save more glare. Looking into the barlow's tube, the glossy interior paint is doubtlessly the culprit for this. Finally, the included diagonal is a mirror-type - not a prism type as the Celestron literature suggests. The diagonal's plastic body causes me to question its quality, yet I have held diagonals of far worse build.

            While certainly enough for enjoying the C6-RGT's views on a variety of objects, users will likely want to expand and upgrade these accessories quickly. The first upgrade on my list would be a 2-inch diagonal. These can be had for under $80, but deep sky aficionados may want to consider a pricier dielectric diagonal, available from many manufacturers. An added bonus of a 2-inch diagonal is the extra weight that will help in balancing the front-heavy optical tube.

            Another accessory that should be considered is a portable power supply. Celestron provides a car power adapter cord, but I've always found plugging a telescope into a car to be inconvenient at best. Amateurs observing from their backyards could opt for an AC power supply (Celestron has this as an available accessory; I've not found it listed by dealers), but this is less portable than a dedicated 12-volt battery. Celestron and Orion both have 7-amp hour and 17-amp hour 12-volt power supplies (about $60 and 90 respectively). I use the smaller 7-amp hour supply to power my C6-RGT, and have always found it reliable.

A rechargeable 12-volt battery is useful for powering the C6-RGT in the field or the backyard. The 7-amp hour model shown here will easily power the Celestron for a night of observing. Amateurs who expect to be at a remote location with their telescope may wish to consider the larger 17-amp hour battery (available from Celestron and Orion).

Long Starry Evenings

            Having been available for several years now, the C6-RGT has been many an amateur's "yard cannon". The telescope's sheer size doubtlessly earns it this title (the dewshield looms more than 6-feet off the ground when the tripod is fully extended). Even with the telescope being so large, the NexStar computer and Celestron drive motors capably move the massive refractor across the sky by naught else but the touch of a button. This has amazed many of my neighbors. One evening I was sharing the views with a neighbor; Saturn impressed him so much that he ran to his house to wake his wife! After they both had enjoyed the ringed planet in breathless silence, I pushed the button for the scope to hone-in on open cluster M35. My neighbors reacted in shocked laughter at seeing the huge telescope swing easily to its new target. This is a fun telescope to share with those unfamiliar with astronomy, and I can't wait to use it at the local school's next star party.

            For those nights when communing with the sky alone, the C6-RGT is great company. The computer's abilities to provide whole sky tours or tours by individual constellation will continually ensure new sights are seen and much enjoyment is had. Amateurs not desiring the computer drives will find the CG-5 mount steady and fluid in movement, with the Celestron 9 x 50mm finder scope an ample pointing device. The scope's plentiful clear aperture will hardly limit views to planets or familiar objects alone. All variety of unusual deep sky targets are within reach of the big 5.9-inch lens. Despite my having problems with my C6-RGT's drive motors, Celestron's two-year warranty and knowledge that my problems are not frequently shared prompt me to feel confident in recommending this telescope to amateurs desiring their own yard-cannon. The C6-RGT is a big telescope, but to me this translates to big possibilities when it comes to long nights under the stars.

Approaching his tenth year as an amateur astronomer, Jay Michaels has no affiliation with Celestron or Astronomics. He hopes to use his "Yard Cannon" to glimpse the ridiculously dim galaxy IC 249 from his backyard.