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Celestron G-5 (300 x 125mm) Telescope

Posted by gloria under Telescopes

Great Scope for Less (Update 04/29/02) Pros: Light Compact Great Images Low Cost Cons: Not as rugged as the G3 (C90)The Bottom Line: Your money goes into the optics and it shows. This is a fantastic general-purpose telescope. This is really a minimalist setup for a C-5 telescope. More general information on getting a telescope is in my article on Picking a Telescope. A newer version the Omni with a larger mount has just come out and I hope to see one soon. I am dividing this review into several sections since it turned out I actually have some ground to cover on this product: Background Versions Performance Fixes for some things General Comments Background: The G5 first appeared in the 1972 as the C5 which was intended to be a smaller and cheaper version of Celestron’s extremely popular C8 series of catadioptric telescopes inroduced in 1970. Since then it has been in and out of production in several variants. The original optical tube was a miniature of the C8 even to the point of being mounted on a small twin-arm fork mount just like the larger telescope. The C8 in turn had changed the nature of amateur astronomy in 1970 when it debuted in a world where Newtonian reflectors were the only affordable option for a large telescope for amateur astronomers. The Schmidt Cassegrain Telescope (SCT) is a hybrid telescope using some elements of earlier designs to produce a multi-purpose telescope. This compromise is most visible in the SCT’s ratio of focal length to diameter or focal ratio. The focal ratios for these instruments are close to f/10 which makes them a compromise between two contradictory viewing regimes. Telescopes optimized for low magnification/ high light amplification deep sky wide field telescopes typically operate at f/6 and telescopes designed for high performance planetary or stellar viewing at high magnifications which often operate at f/14 and higher. The intermediate ratio allows the SCT to do something at both ends of the viewing spectrum- it can get decent planetary views and it can produce a fair image of a nebula or galaxy. However it will not do as well in either regime as telescopes designed specifically for either kind of viewing. The main appeal of this design was for people who had varied interests but could only afford one really nice telescope. The original orange painted C5 fell out of production for economic rather than practical reasons. Celestron discovered the decreased size made tolerance and machining for the smaller optical tube more demanding than for the larger C8 (machining tolerances are in absolute dimensions but mechanism performance depends on the ratio of sizes- thus if the parts get smaller their dimensions have to be more carefully controlled). This made the C5s actually cost more to produce than the larger C8 telescopes they were supposed to be an inexpensive alternative to. So although the first C5 was popular mobile astronomy hadn’t become common enough for compactness and low weight to be strong motivations for buyers. The SCT originally gained a strong following because it effectively deals with several persistent problems all telescope designs face: First the reflecting design eliminates the chromatic aberration problem refractors experience. Second the corrector plate design eliminates the curved field and coma problems Newtonian designs have. However the unusual figure of the corrector requires special production methods and many of the earlier examples have only mediocre optical quality. As a result by the early 1980s many felt the SCT design had reached its maximum potential and began to explore other alternatives. Among these were telescopes such as high performance refractors using exotic lens arrangements made with special fluorite glass. Although these telescopes are extremely expensive compared to their aperture they produce beautiful images and have one very different characteristic from the 8 and larger SCTs- they are easy to transport. Celestron reintroduced the C5 in 1992 as an astronomical and spotting scope after redesigning it to be easier to manufacture. This new design is the direct ancestor of the variants now in production. These C5s had white tubes with black writing. The astronomical version came on a single arm fork mount with an integral clock drive and was made to be easily detached for use as a spotting scope. Many people feel this version was the most useful ever produced while others have pointed out the single arm mount wasn’t as stiff as the earlier twin arm mount and so it isn’t as good for photography. This model was discontinued when the NexStar 5 was introduced and were around $1000 a copy. These and the spotting scopes came with a large handle mounted on the top which also served as a camera mount rail. The C5 SCT has grown up and become a very good telescope. The G5 is a C5 mounted on a lightweight equatorial mount and tripod. This is by far the most affordable astronomical version of this telescope ever offered. The setup has the option of attaching a motor to the mount to counteract earth’s rotation. The mount and telescope are available as a set for $750 list or $699 for the telescope and mount with no drive. At the moment Woodland Hills Camera has an ad in Sky and Telescope offering the G5 with motor for $699 (their site www.telescopes.net does not mention this promotion). I have periodically seen it on Adorama (www.adoramacamera.com) for as low as $649. If you want to get the optical tube and nothing else look at the listing on www.astroptx.com. About the time the G5 came out the handle/ camera mount was deleted from the spotting scope so the various versions had finally converged to one mechanical design with differences only in paint used and accessories included with them when sold. I ended up getting the version currently sold as a spotting scope and then mounted it to the same mount as is used in the G5. The reason for this was I already owned the mount (I had been using it with a C90- if you want that story see my review on the G3 or the C90) and Adorama Camera (www.adoramacamera.com) had fantastic deal going on the C5 spotting scope. The only difference between the C5 spotting scope and the G5 is the C5 spotting scope is painted white with black lettering on the side while the G5 is black with orange lettering on the side. The only real difference is in the peripheral components sold with them. As a side note the optical tube is also the same as the NexStar 5 which has a steel colored metallic painted tube and no lettering. Otherwise the three telescopes are the same optical device. Interestingly the mounting bracket for the C5 spotting scope is the same as the one on a G5 so it readily mounts to a CG-3 tripod with 2 1/4-20 bolts (it even has a clearance hole for the mount head bolt machined in it). The NexStar 5 cannot be removed from its mount without voiding your warranty. During the 1990s competition from other designs such as recently introduced Russian medium to large size Maksutov telescopes which while more expensive than SCTs have higher image quality at high magnifications have threatened the SCT’s dominance in amateur astronomy. Recently produced SCT telescopes I have looked through all had far better image quality than any I have seen in the past. Others have commented on this as well and I would guess the appearance of both the apochromatic refractor and the high performance Maksutov telescopes can most likely be credited with the recent improvements to the SCT. Also note- do not hesitate to adjust collimation- the difference in image quality between an almost collimated scope and a collimated scope is night and day with these things (see the fixes section below for some tips on this). Performance First and foremost this telescope has met and exceeded all of my expectations for it. On one end its resolution is so good I have started going through Sky and Telescope’s double star list for summer and discovered it can be quite a bit of fun to split doubles. At the other end of the viewing spectrum I have been quite surprised to discover its contrast is good enough to see M57 (the ring nebula in Lyra) in the city (between a Home Depot and a 20 screen cineplex) without a light pollution filter. The C90 is really a neat little telescope in a lot of ways but the C5’s performance is in another league entirely. The moon has become an entirely new field to me with this telescope. When I was 7 years old my parents bought me a Japanese made 60mm refractor from Sears. The first view of the moon I had with it amazed me as I saw the craters in relief for the first time. Much later a 5.5 Newtonian and the C90 showed very sharp views of the moon but I had forgotten what it had been like to see it for the first time. This telescope has given me that experience again. On the moon the detail has been nothing less than astounding. At 166X I have discovered fine crevasses in the lunar surface and seen the shape of low lying ridges as well as mountains and other features. In short it has felt like seeing the moon for the first time again. During the winter I have been observing Jupiter Saturn the Orion Nebula The Pleiades some double stars and the usual winter sky crowd. I have come to really look forward to using the C5- it is still light enough to just pop outside with but it is also giving beautiful images of the planets and other objects. One tip- boost magnification on nebulas to bring out the structure- yes it goes against the popular wisdom but when you get to around a 12mm eyepiece the structure suddenly becomes distinct from a light polluted sky. Mars showed quite a bit of detail during its 2001 close approach. The polar cap was visible as were some of the shadow features. One should remember Mars is a very difficult low contrast observing target; the imagews you see in print are highly processed with boosted contrast. The actual planet is a pale peach color. All in all I have been happiest with the images of Saturn which is positively 3-D with beautiful striping and detail in the rings. Jupiter is quite pretty as well. The stripes and other details refeal their inner twists and knots when the seeing is good. The great red spot (which is as many have pointed out less great and quite beige these days) is visible in this telescope. Jupiter’s moons are not only obviously little balls of different sizes but their colors are visible too. The sulfur covered moon IO is a bright amber color for example. The design of the C5 uses a moving mirror focuser arrangement where the entire primary mirror shifts forward and back to focus. A knob on the back of the telescope pushes on an arm mounted to a sleeve carrying the primary mirror along the central baffle tube. This makes the tube extremely compact and eliminates the need for moving the primary mirror for collimation. The C5 can reach focus between 20 ft. and infinity while using a large variety of optical devices ranging from eyepieces to binocular viewers to cameras. Unlike the Meade SCTs which have simple machined metal knobs the C5 has a nice large focuser knob with a rubber grip which is easy to find in the dark The range from minimum to maximum focus is about 40 turns of the knob so extremely fine focus is possible. The only real difficulty with this type of focusing mechanism is moving from forward to reverse while focusing has some lag and the image moves slightly. The commonly recommended method for focusing an SCT is to move the mirror well below focus and advance it to focus and stop. I have found getting sharp focus isn’t a very big problem with this telescope since it doesn’t have a noticeable shift when reversing the focus direction. However there are companies such as Apogee (www.apogeeinc.com) which offer separate refractor type focusers for SCT telescopes. This makes the telescope longer and heavier as a single unit but if you really don’t like the way SCTs focus you do have an option. The mount with the G5 version of this telescope is the same one as used with the smaller G3 telescope. The tripod I have been using has wood legs with the G5 mount head (the G5 sold today has aluminum legs). The mount uses a simple spring-loaded screw thread for the declination axis (north to south) which allows about 15 degrees of travel in that axis. I have found I really don’t like this method that much (I’d rather have a real worm gear) and I have put a marking on the screw shaft with a magic marker so I can judge when I’m about to run out of travel. To use the mount you need to point the hub roughly at the pole star and then release the two axes to move the telescope to a viewing object. In practice this is a lot easier to do than it sounds at first. The telescope mount needs to have its axis tilted to your observing site’s lattitude. To use the telescope simply walk outside with the tripod and set it down with the hub pointed roughly north (eyeball it). This will be good enough for visual observations. If you want to do photography you will need to follow a procedure for precision polar alignment. Celestron tells how to do this on their website at www.celestron.com. You will also need to have a motor to do this. However the amount of drift error while visually observing an object is usually pretty low on the time span of a few minutes of viewing before going to another object. The mount is very easy to use. The equatorial axis has a worm gear with steel gears. The main mount adjusts via simple tightening knobs. To point the telescope at an object I have developed a technique which seems to have overcome the usual problems with sighting on an object. For this to work the finder scope must be aligned with the main telescope as outlined in the instructions. What I have been doing is looking at the object I want to see in the telescope with both eyes. Then I look through the finderscope with one eye and keep the other eye open looking at the object. It is surprisingly easy to tell where the cross hairs are pointed while doing this. What I do then is simply push the telescope until the crosshairs appear to move to the object. The object will pop into the finderscope’s field of view just as the crosshairs approach it. Then all I have to do is tighten the axle releases on the mount and then use the worm gears to fine-tune the pointing at the object. This technique actually works well enough to center objects at 125X. So it is generally very easy to find objects with this telescope setup and the mount is actually pretty easy to use this way. If you are looking for a dim object select an easily visible star nearby and sight in on it. The mount has an interesting feature where the counterweight arm rotates in synch with the declination head. As a result it should be possible to attach a camera to the arm in place of the counterweight to make a pretty neat photo-guide scope setup that is very light (even without the tube mount rail the earlier C5s had). However the arm on mine was loose when new and tightening it up proved to be somewhat difficult since it has a threaded end with three set screws from the sides holding it in place (yeah- it’s really strange. I have no idea why it is made this way). The possibility of mounting a camera is still there although I haven’t seen a mounting shoe for doing this (there must be a part out there that does this though- maybe for something made to mount on a lab ring stand). The tray included for holding eyepieces and other parts is split up between .96 accessory holes a couple for 1.25 oculars (one of which is blocked by the tripod attachment tab). In practice the tray as supplied is worthless since items set on it can easily be knocked off and lenses with caps on both ends can’t fit into the holes. All in all this mount is easy to use. The quality is OK although I am prompted to wonder how much it would cost to do the few small extras like a full declination gear a better weight arm attachment and maybe a simple polar alignment scope. On the other hand other mounts are available and if you want another mount the telescope can be adapted to it. In the meantime this has turned out to be a light weight and easy to use system. As before with the C90 I can take it outdoors for 20 minutes of looking around and come back in. No time is needed to set it up only one assembly has to be moved and there is no need to go through a setup routine for a guidance computer before it will do anything. That kind of simplicity has a lot of appeal to me. Something also to keep in mind when comparing this telescope to expensive computer driven models is the use of a Go-To telescope disqualifies the user from getting Messier or other object certificates from the astronomy league . The astronomy league sponsors an activity to certify people who have found all 110 Messier objects as well as other object catalogs on their own through their local observing groups. Use of a telescope with the ability to find these objects automatically defeats the purpose of this activity which is to encourage people to master the layout of the skies. In the case of using a telescope with children or others the moment of discovery when one finds something for themselves is a large part of the fun as well as being a great builder of both confidence and bonding between people. Issues with Design The only real issue with the design I have found is one faced by all catadioptric telescopes with glass corrector lenses in the front- dew. The large glass corrector lens cools down and makes an ideal spot for dew to form. A dew shield is essential for preventing the formation of dew on the telescope optics which can bring your evening viewing to an abrupt end unless you have a blow dryer along. There are heater tapes sold for this but they are usually not that good an idea because the tape causes convection currents in front of the telescope which degrade the image. The dew shield also keeps stray light from outside the area you are looking at from entering the telescope and decreasing contrast in the image Fixes Telescope: The only serious problem I’ve had with the telescope itself has been dew fogging the corrector lens in front. So I made a dew shield for it. I found some 2mm thick 8.5X11 sheets of black foam sold at Wal-Mart in the crafts area for $0.66/ sheet. I bought two of these. The outer diameter of the C5 optical tube is 5.75 at the front and a little math shows the circumference is 18 so I cut two sheets and put them together with a 1 overlap using hot glue (use the low temp setting). The resulting cover just slips on the front like a sock and a little bit of adjustment makes it circular. So far it has kept the dew away and the naturally flat black interior is perfect for reducing glare. Best of all it is really really cheap (Usual going price for a C5 dew shield is $30). This shield is also a bit longer than the usual dew shields so it should be more effective in any case. Collimating SCT systems is simply a fact of life- as the instruction manual says it makes a huge difference but is very easy to do under the night sky. Collimation is done by looking at the image of a star in the sky and adjusting the secondary mirror to correct it. This is far easier to do if you have a motor drive so the star doesn’t keep drifting out of the field of view. I upgraded the forward cell with a finger-adjustable version of the collimation screws called Bob’s Knobs (www.bobsknobs.com). Just specify what the telescope is and they are $16 delivered- and well worth it. Make sure you follow the instructions to the letter when installing and it is easy. With these a screwdriver isn’t needed to collimate the scope; instead you just look in the eyepiece and then turn the appropriate screw by hand. Tip- always point your diagonal and eyepiece in the same direction when collimating. For example have them pointing opposite the mount shoe. The reason for this is after you have done this a couple times you will know what turning any one screw will do. As a result you will be able to do this very quickly. If you don’t sharply jar the scope it can hold collimation for a very long time. Tip for the folks at Celestron: Talk to Bob about making his knobs the standard equipment on your SCTs and they will be better for it. Observation on losses of collimation: Everyday use and jostling causes the collimation to degrade. Repeated temperature changes (such as taking it outside in summer or winter) will cause a little bit of drift also. This is enough to visibly degrade the image after a few months of use. However recollimation if done frequently only consists of very minor adjustments. I recommend doing a quick check now and then. Note- over-tightening the collimating screws does not help hold collimation- it causes the secondary mirror to be warped (this is called Pinch and degrades the image. Remember the images are never so clear as they are with a fresh alignment so it will be a minute well spent. Mount: The biggest problem I had with the mount was the rather pathetic tray it came with. I made a tray enlargement out of a gray plastic utility box which I cut to fit with a Dremel tool and then drilled holes in the tray and mounted the utilit box using its original screw-holes. The box is just big enough to fit all of my eyepieces into with their caps and the depth of the tray doesn’t let them topple out while carrying the assembled telescope. The only other modification I made to the mount was to add a marking to the declination axis axle so I could tell if it was at one end or the other of its range. General Comments: The C5 does everything it is supposed to and does it extremely well. To someone who has never owned a telescope before I know the prospect of dropping $700 on one seems daunting. In the case of the C5 I have been pushing its performance in every regime I can think of and after I have been using it for a while I feel it is an incredible bargain. The smaller 90mm telescopes are small enough to take backpacking but 5 or 127mm of aperture combined with very well made optics makes for a huge improvement in performance. Compared with larger telescopes than this one they perform poorly in the light pollution of the city and are far more difficult to move. This telescope is a very practical answer for someone who wants good performance and portability rolled into one. There are other telescopes such as 4 apochromatic refractors which can fill this same roll but they cost four times as much. In the case of this telescope I would now have to say it would be a far better first scope for a beginner than a small computer-guiided scope since the money goes into a very good optical tube assembly and practical mount instead of a limited 80mm or 90mm scope with a difficult to use electronic drive system. Unlike a Dobsonian this scope can support interest ranging from planets to nebulae to photography to daytime observing. Recommended:Yes