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Review of the Celestron C5 “Classic”

Posted by Ed Moreno   09/29/2007 12:00AM

Review of the Celestron C5 “Classic”
This is my review of the Celestron “Classic” C5. This is the second one that I have owned. I owned one about 3 years ago, but it never really wowed me. I felt that the scope was only OK optically, and the aperture simply was kind of small for me. But mostly, it just didn’t seem to be anything special optically.

I sold that one, and the buyer complained that he had owned one previously, and that the one I sold him seemed a little soft to him. I refunded his money. I knew the correction for spherical aberration wasn’t all that great on the scope, but at the same time, I have seen worse in terms of spherical aberration correction in SCTs. My C9.25 was borderline to bad, and it was the telescope that made me want to understand optical quality. These scopes had such a cult-like following that I knew that it must have been something with that particular sample, but I traded it away before I determined the real cause of the problem.

Anyway, I knew that the C5 that I took back had only fair spherical aberration correction, but it wasn’t until I got serious about evaluating optical quality that I really got a handle on the problem. In Suiter’s book on optical evaluation, he dismisses the Ronchi test as being inconclusive regarding SA testing. I had given up using a Ronchi grating because I was hard pressed to see any sufficient bending of any lines to judge overall spherical aberration even in telescopes that I knew suffered from this condition, and this is exactly what Suiter says.. But in attempt to get a better handle on the C5s performance, I grabbed my Ronci grating and put it over the diagonal… Bang… Turned Edge. Not a horrible turned edge, but sharp enough to be visible RIGHT at the edge of the mirror. I would have estimated maybe only 3 or 4 percent. Anyway, I realized that the combination of Spherical Aberration and Turned Edge probably put that first C5 I owned right at the edge of Celestron’s “Just passes muster” threshold from a quality control standpoint. I mean it was OK, but at the eyepiece it never dazzled me… I resold the first C5, pricing it aggressively and disclosing the presence of the turned edge in the For Sale listing.

I bought an 80mm ED scope after that. Everyone was raving about 80mm ED scopes, so I figured I would jump on the band wagon… At first, I was pretty under whelmed by the 80ED.. Oh my sample appeared to have really excellent optics, but it just didn’t gather much light, and I kind of missed the C5 for quick looks because my quick looks usually turn into an hour of high power observing. But the optics were so good that the little scope won me over… For a while anyway. In the end, the 80mm scope was proving to be simply too small in terms of light gathering for me to do the kind of observing I like to do… I noticed the big hit in light gathering on the 80mm scope from the C5, and knew that a 4” scope simply wouldn’t have the bump-up that I needed, and a 6” SCT or Mak just seemed to me to be too close to a C8 in terms of size and mounting requirements. After looking at all the alternative, I decided I would try to find another C5, but that it would have to be a superb example to make me happy.

I posted on Astromart several times and had a few people that offered C5s up, but none could really attest to the optical quality, and frankly, C5s seem to be rather expensive!!!
I was patient though, and finally I was rewarded with a contact from a seller that assured me that the C5 he was selling was a superb example. The price was right ($300) ESPECIALLY considering that it came with a like-new metal dew shield AND… ready for this??? A BEAUTIFUL Takahashi 1.25 inch Prism diagonal! It was my first experience with Takashi equipment, and I will simply say that I am impressed by the quality. Anyway, the deal sounded good, and as luck would have it, I would actually be able to pick up the C5 in person.

Visual inspection of the C5 showed it to be cosmetically fairly good. It had a few minor scuffs and paint nicks as is common for an older scope, but otherwise, it didn’t make any rattles when I hit potholes. Upon doing a “Bright Light” inspection though, I DID see two very faint, parallel marks on the primary!!! Now I doubt that the seller even KNEW these were present. They were VERY faint and only showed up under very bright, oblique lighting. They were about 2 millimeters apart and about 1.5 centimeters long, in kind of a gentle squiggly pattern. I have to tell you though, they looked VERY minor, and while I USED to be OVERLY OBSESSIVE about this kind of thing, I have actually come to realize that in most cases, these kinds of almost invisible defects simply do NOT affect the overall performance. Sellers often panic when they see this, and discount the scope all out of proportion to the optical damage that it is likely to cause (which is usually almost NONE). I LOVE those kinds of sellers!! Anyway, I elected not to mention it to the seller because frankly, I wasn’t worried about it. And even with the marks, the deal was an excellent one, especially considering the inclusion of the dew shield and diagonal.

For those that aren’t familiar with the Classic C5, let me tell you the general characteristics. The weight is bout 6 or 7 pounds.. Maybe a bit more.. I don’t know. But hey, this thing is so light and small that what difference does it make? It will practically fit in a shoebox. Anyway, they are short, and the OTA has an odd shape in that the corrector cell and OTA tube are a smaller diameter than the mirror cell. This means it is somewhat problematic to mount it using rings. The dovetail is also a bit odd in being very short…Maybe four or five inches long. The significance of this is that it makes it hard to mount on anything but a mount equipped with a Vixen/CG5 type of saddle. More on that later.

The next notable attribute of the C5 is that the central baffle is small… Very small. In my first C5, I used a two inch diagonal and a 35mm Panoptic. While this did vignette slightly, I still though I was getting about a 1.2 degree field, but I never measured it. I now realize that I was probably only getting maybe a 1.1 degree true field with that eyepiece. Even a 32mm Plossl will SLIGHTLY vignette in a C5. I guess I have become generally intolerant of vignetting now, so fact is, if you want the C5 field to be sharp and fully illuminated, then you are really limited to using something like a 25mm Plossl or 18mm wide field eyepiece. But you know, this is fine with me. I simply found that even with the excellent little 80ED scope, I tended to use VERY high powers for MOST of my observing. I can remember spending 30 minutes with the C80ED funneling into a 5mm Nagler and never going to lower powers. So really, the fact that the C5 can’t generate more than about 1.05 degree field has become a non issue to me. I simply don’t CARE anymore if my quick-look scope can’t do wide fields. MOST OF MY OBSERVING IS AT HIGH POWER!!! And if I REALLY want to do MAGNFICIENT wild field viewing, well, I have a Vixen 140 refractor for that. So…The 80mm ED scope wound up unused. It simply wasn’t very inspiring in my central Austin back yard.

I mounted the C5 scope on a Vixen Polaris mount. I owned a Polaris previously, and I loved it but my first one had fixed, short wooden legs, and this wasn’t working so I sold it and had been using the 80mm scope on a Vixen GP with HAL 110 legs. Overkill, but hey, it was a killer combo in terms of usability and stability. But I needed another mount. I mentioned this to the C5 seller, and guess what!!!! He HAD a Polaris, and he offered to SELL IT TO ME CHEAP - $100!!! This was because it was missing the legs. But it DID have a Celestron Super Polaris drive and clutches on it.. Now I like the SP drive much better than the drives that used to come with the Polaris because it has a clutch on the drive gear. The Polaris motor was coupled directly to the shaft meaning that you couldn’t use the manual RA slow motion control.. The SP motor eliminates that problem. If you are not using the drive and want to use manual slow motion, you back off the clutch just like with a SP or GP. Also, I LOVE the fact that the SP motor will run for several hours off of a SINGLE 9V BATTERY that goes IN THE HANDSET!!! No external power cords or battery packs. HOW COOL!!!

Well, there were no legs for the mount, but the price was right.. The seller and I had a lot in common, and we chatted for a good half hour, and just as I was packing up to leave, the seller said “Hey, you know, I think I have a set of aluminum legs from a Televue GEM mount around here somewhere. Do you want them? They might not be in great shape, but I will give them too you.” Freaking WOW.. A lovely little Polaris on heavy duty adjustable aluminum legs WITH a self contained RA drive for C-note!!! AM I A LUCKY GUY, or WHAT!!! The legs were not the thin, soft Chinese variety, but rather the thicker, harder aluminum alloy kind that Vixen makes (were they MADE by Vixen?? They are dead ringers for AL 130s). Anyway, the combination is ROCK SOLID. Even at 250x, NOTHING induces shakes. Even without vibration suppression pads, if you bump the mount, dampening is instantaneous. That is, IF you can get it to shake. The focuser on this scope is very light, so focusing introduces NO movement. Even my 80ED on the GP with HAL 110 legs is not this solid. Part of this is due to the fact that most of the mass of the C5 sits VERY close to the vertical axis of the mount. The Polaris design helps because the distance between the RA axis and DEC axis is very small. In essence, there just isn’t much here to wiggle!

So now I head home with a complete “New” setup, and of course I KNOW that the assistant astronomer will just be THIRLLED when I bring it all HOME!!! Yeah, right…

Remember the odd dovetail? The Polaris had mounting tabs and was designed for use with rings. I wanted badly to avoid using adapters to mount the C5 on the Polaris in order to keep the scope close to the mount, (less counterweight, less vibration). What to do, oh what to DO… Well, as it turns out, modifying a Polaris to take a dovetail is a trivial task. Well, not for the faint of heart, but for combat veterans, it is a snap. A little hacksaw work, and a ¼- 20 thread tap, and Viola! I have a Polaris that will take a dovetail. So now in addition to using the C5, I think that it can handily carry the C8 on the occasion that I don’t want to bother with Go-To, but still want a tracking mount…

Ok, so NOW the question was: “Is this C5 REALLY optically superb”. First “third or forth owner” light was on an oak tree leave. I have gotten pretty good at discerning optical quality from doing this. Really excellent optics just have a way of showing themselves in this kind of test. My initial impression was “Oh, My!” The detail in the leave was astonishingly good.

But hey, who looks at leaves at night???

My very first celestial target was a 6 day old moon. Insert 8mm Radian… Twist Focuser.. BANG. Razor sharp. Superb detail. Better that I could have imagined for a 5 inch SCT. I guess after using the 80mm and reading a lot of reviews that said that this or that 80mm beat out this or that small SCT or Mak in a shootout, I wasn’t expecting much, but the image was “Holy Cow” impressive. I IMMEDIATLY ran into the house and grabbed the 80mm and GP mount and in minutes was doing a side by side. Well, small refractor owners, I will tell you that as impressive as the 80mm ED scope was, in MY shootout, the accolades clearly went to the C5. Everything seemed crisper in the C5. Contrast was excellent, and faint detail like the area around Triesnecker was more easily visible in the C5!

On to JUPITER!!! Now on the moon, I had kind of expected that a really excellent C5 could edge out even a really superb 80mm scope because of its greater resolution capabilities. But Jupiter is a tough target, and the large central obstruction of the SCT means that even a perfect C5 would theoretically have slightly hampered contrast as compared to an excellent 80mm refractor. The C5 has about 77mm of clear aperture, so on PAPER, the Modulation Transfer Function lines should be rather similar. Still you have to remember that the C5 has about 50 percent more resolving power. Now much of the detail on Jupiter is actually relatively high contrast, and on a target like that, RESOLUTION IS IMPORTANT… Anyway, I was expecting it to be really close. I was SHOCKED. Jupiter was CLEARLY showing more detail in the C5. The dark belts were more prominent and more clearly defined along the edges, and as an added treat, there was a shadow transit taking place, and the little bullet hole shadow was harder and crisper in the C5 (High contrast feature which SHOULD look better in a scope with higher resolution and it DID!). I studied Jupiter in both scopes using various powers and came away clearly convinced that the C5 had handed the 80mm refractor its lens cover on a silver platter!

I was suddenly determined to continue the shootout in my back yard. I carry the C/5 Polaris back in one trip, motor still running… LOL… I run back for the GP/80mm.

Fist target is M57 in the 80 ED. Beautiful. I have in a 22mm Nagler so I can home in on it, and the 80mm scope shows its wide-field advantage by sucking up half the constellation in the eyepiece. Can’t do THAT in a C5. But I know where M57 is, and can hit is first time simply using the 6x30 finder and a 25mm Plossl in the C5. My immediate impression is that at low power, the entire field is sharper in the 80mm. There is astigmatism caused by the plossl at the edge of one degree field in the C5, but the giant field in the 80ED behind the 22mm Nagler is, well, SHARP!!!!!!!!!!!!!! But the Ring Nebula itself is much more prominent in the C5. My first thought is that this is because the higher magnification in the C5 is suppressing the light pollution, so I amp up the 80mm…. Doesn’t matter. The C5 is still showing a brighter, more defined shape. The central void looks more prominent in the C5. I shove an 8.8mm Meade Ultrawide (old version) into the C5 and I am amazed by how much detail I can see. I try a couple of different eyepieces in the 80mm scope, but simply can’t close the gap. The C5’s light gathering, resolution and optical excellence combine together to simply overpower the smaller scope.

I find a few clusters. Same result. The clusters are brighter in the C5, and the limiting magnitude must be about a half magnitude better in the C5. Now the FIRST impression is that the C5 is running away with the show because everything appears distinctly brighter in the C5. But limiting magnitude tests show that the difference is NOT all that great. In fact, when you look at the field in the C5, some of the dimmer stars don’t initially show in the C80, but if you REALLY study the field in the smaller scope, a surprising number of them DO start to show up. Not ALL of them, but a bunch of them… I see this in refractors. Their ability to press SO MUCH of the total energy into the very sharp spike at the center of the Airy Disk means that often the dim stars WERE there in the refractor, but you had to look much harder to see them.. But see them you would, young Skywalker.. Sharp. Infinitesimally small. But in the end, The excellent optics in this C5 allowed it also to preserve a lot of energy in the Airy disk, and at the very edge of limiting magnitude, the stars looked almost equally tiny and sharp, and the C5 was going at LEAST a half magnitude dimmer!

I spent another hour scouting out doubles, clusters, and even some galaxies. In every instance, the high power views in the C5 appeared to be esthetically almost equal to the refractor, and the C5 displayed MORE DETAIL in EVERY TARGET.

I actually saved the star test for last. The 80ED has an impressive star test, but I am not very good at reading refractor star tests. It is hard. Much harder than reflectors. But the in-focus Airy disk in the 80ED is almost textbook for being well corrected. I go to Deneb to test the C5. I defocus until the inner bright diffraction ring breaks out of the secondary shadow and the structure is just about PERFECT. On the other side of focus, the breakout is equally perfect. Large defocus shows even rings with fairly good contrast on both sides, and no zone. A Ronchi test for turned edge (which I do now for all telescopes) is clean. Most notably, the optics of the C5 are incredibly smooth. Smoother than the 80mm scope for sure. IN fact, I have only seen two other non-“Premium” scopes with optics this smooth: My C14, and the MN61 I used to own.

Now at medium powers, the C80ED shows a distinctly sharper field when using Naglers. The SCT design has a fair amount of field curvature, and when using something like a 12mm Nagler, stars bloat out of focus near the edge of field in the C5, while they tend to be much sharper further off axis in the C80ED before trace astigmatism shows up. Still, most of the field in the C5 will be sharp at medium powers. It is really at high powers, where a big percentage of the field is diffraction limited, that the C5 starts to really show its tiny little claws, and in every comparison I made at higher powers, I came away preferring the C5 over the 80mm refractor. I want to be very clear on this point: MOST of my OWN observing is done at very high powers. THAT is why I tend to gravitate to GEM mounts with motors. The only telescope that I use without a drive is the Vixen 140 refractor, but even here, I usually break down before a session is over and go get the power tank. The fact that the 80mm refractor can generate a wide field does not seem to interest me any more. Maybe if I lived under darker sky conditions where the sky wasn’t so washed out, I wouldn’t feel this way, but 98% of my observing is done in my back yard under moderately bright skies that wash out a lot of the view. I am simply no longer interested in having a small scope that is great at wide field viewing because nothing smaller has been as satisfying as my Vixen 140. If however, you live in an area with less light pollution or travel to one frequently, maybe an 80mm refractor would be a better choice. But not for me. Of the thousands of observations I have logged, 98% have been at powers in excess of 120X. In fact, the vast majority of observations I have logged have been at 200x or greater, and since acquiring my C4 the number of observations I have logged at 450x has exploded. So I am a high-power observing guy…

SCTs often receive a lot of negative comments, and I can tell you first hand that some of it is deserved. I believe that the bell curve of SCT quality is pretty wide, and some of the negative reports you read are probably due to someone having a sample that comes from the low quality end of the curve. The difference between a scope from the peak of the bell curve (typical SCT) and one from the high quality end is not always easy to see at the eyepiece. The reality though, is that the SCT design DOES start life with a large central obstruction, and if you push the optical quality to the lower end of the bell curve, the difference between one sitting at the low quality end of the curve, and one sitting at the high quality end of the curve will become VERY apparent at the eyepiece. When I read a review that says someone was able to beat this or that SCT with a scope of much smaller aperture that SHOULDN’T have beat the SCT, then I believe it. And when I read a different review where this or that SCT beat a scope that people thought the SCT SHOULDN’T have beat, I can believe that TOO. This is that exactly what the bell curve of SCT optical quality suggests SHOULD happen. Only excellent SCTs interest me anymore.

The Classic C5 I recently acquired will replace the 80ED because it suits MY individual observing style better. It also re-enforces my belief that SCTs do vary in quality, and that really excellent ones can perform QUITE well indeed. And to tell the truth, I recently had been craving a 5” APO. But after putting the C5 up against the 80ED, I have become very skeptical that a 5” APO would necessarily provide a compelling challenge to my optically excellent C8!!! I have always suspected that my C8 could hold its own against an excellent 5” refractor, and would beat it on most targets, maybe only matching it on planets, but maybe NOT.. I had been told that a C5 couldn’t beat a top quality 80mm refractor on tough targets, and now I know first hand that it CAN…

Remember though, my preference is high power viewing. I use my C14 routinely at 450x and 500x. I use my C8 at 280x. These are my TYPICAL observing powers, and some of the negatives of the SCT design (Field curvature and coma) are largely absent from the field at these powers. If you find yourself using medium powers (maybe 100x in a 3” to 5” scope) then maybe a smaller refractor might give you better service.

Overall, I am absolutely thrilled with my “New” Classic C5, and if you can find one with optics this good, and like small, ultraportable telescopes capable of doing serious observing, then I give it my highest recommendation. I consider mine to be the best small telescope I could hope to own. Find an excellent one, and I give it my highest recommendation. This is no small Cat, it is a tiny tiger…

As always, my regards to you, and my sincere best wishes to our men and women at war. And for you Leathernecks, stay covered…

Click here for info about an interesting current alternative. -Ed.