Celestron 8" EdgeHD

Posted by Ed Moreno   01/08/2010 08:00AM

I sold a C8 a year or two ago and once again went down the 4” APO road as a quick look scope, but I missed the aperture of the C8. Plain and simple, once I got past the appeal of the perfect little stars in a 4” scope, I realized that I was still looking though a small scope.

I started looking for another C8 and tried a few, but I was looking for one with as perfect optics as I could find. One 8” SCT in particular was actually a VERY nice sample, but it wasn’t quite as good as I would have liked, and while I KNOW that it is almost impossible to tell a very good 8” SCT from a perfect 8” SCT at the eyepiece, I guess I have just become so anal that I CHOOSE to own as high quality equipment as I can find.

Now along the way, I started thinking about the off axis performance of these scopes. In the last year, I have stopped using Naglers in my C14 simply because after using large refractors with their sharp edge of field performance (the MAIN reason I personally like refractors), I became distracted by the off axis coma and field curvature of the standard SCT design. Even the f/11 C14 still shows a fair amount of coma and field curvature. I had been using a 35mm Panoptic with this scope and honestly, I did not notice it so much until the Assistant Astronomer gave me a 31mm Nagler for my birthday a couple of years ago. Also, in recent years, I have lost the ability to visually accommodate all but the smallest amount of field curvature. I simply could no longer keep the entire field in focus at once with the 31mm Nagler was used in the C14.

I adore this eyepiece, but the more I used it in the C14, the more dissatisfied I became with the softness coming from the coma in the scope and my own inability to accommodate the outer portion of the field. Eventually, I went back to Panoptics for this one scope, this time to the 41mm, and this made the edge of field performance MUCH more acceptable to me. It is my believe that this is because the blur is not as magnified so the comatic tail is harder to see, and the lower power does not tax my visual limited visual accommodation ability as much.

Anyway, when the right 8” SCT didn’t present itself, even though I am cheap an rarely buy anything new, I knew that if I really wanted to get the best possible ACROSS-the- field performance possibe in the incredibly small, light C8 format, it would be necessary to buy one of the new coma free designs. The EdgeHD came on to the scene recently and while it was a LOT of money, in the end, I decided that even if the scope was not perfect at the CENTER of the field, having a nice SCT with excellent edge performance so I could use all of my Naglers in it would be an acceptable tradeoff. I kept waiting for the first 8” EdgeHDs to appear on vendor shelves, and finally I spotted one. OMG was it painful to pull the trigger from a cost perspective when you can buy really nice used 8” SCTs for $400, but I knew that in the end, I would regret it if I DIDN’T take the chance and buy the Edge HD.

This is only my INITIAL impression. I have not had a chance to do long term observing with this telescope, but having owned a dozen SCTs (two of them C8s) and knowing their characteristics fairly well, I feel that even with one night’s experience, I can render a meaningful early impression.

First, the finish is BEAUTIFUL. The paint is more like a metal-flake than a pearlescent paint. In the pictures I have seen it appears more like a cream color, but in person, the metal-flake finish makes it seem VERY pale grey. The trim is in black or extremely dark grey and the accents are in anodized Celestron Orange. This is an incredibly handsome telescope. Even the EdgeHD decal is understated and tasteful. I recently purchased a used Vixen ED100sf and the assistant astronomer just about barfed on the big red “Vixen” lettering on the dew shield. I had to scrape it off to make her happy. She LIKED the badge on the Celetron! She said it looked just right for the scope. What pleases the assistant pleases me.

The scope seems to weigh only a tiny bit more than the standard SCT. In addition, it has a very ergonomic handle at 6:00 O’clock on the rear cell. The standard dovetail for this scope is a CGE/CGEM dovetail. I contacted Celstron Support and asked them if it could utilize the older Celetron CG5 style dovetail and they said no, that it needed 1.75” hole spacing, but me thinks that they did not LOOK before providing this answer, because when I removed the CGE dovetail, in the rear, I found the two closely spaced holes (filled by hex head screws) that I remember from the old C8 dovetail. Since I will eventually use this scope on a CG5 ASGT, I will replace the CGE dovetail as soon as the Losmanday VSCT13 arrives. By the way, I had asked Losmanday and they said that the 1.75” radius blocks would NOT be needed, and it looks like they were correct, so I don’t know why Celestron support botched the answer to this one. Still, I haven’t actually installed a CG5 dovetail, but I see no reason why it should not work. Your mileage may vary, because I have read that some later scopes will NOT take the old dovetail, but from what I can see, this one will. The dovetail should arrive in 4 or 5 days after I post this, and if the dovetail does NOT fit, I will ask the webmaster to post a correction.

The next thing is that the instructions don’t really explain how the tension clutches for the mirror work. When I got ready to use the scope the first time, I turned the little clutch knobs CCW several times. I guess I was thinking that they were some kind of threaded rod jack screws that had to be unscrewed several turns to be able to move the mirror. After studying the cut-away drawings in the manual though, it appeared that the rods are affixed permanently to the rear of the mirror support. They pass though the rear of the mirror housing and THOUGH the clutches. When you turn the knob a half turn or so, the clutches un-pinch these shafts allowing the shafts to slide inside the clutch housing. When the mirror is moved to a new focus point, only a half turn of the knobs is need to re-pinch the shafts to hold the mirror in place. This is an EXCELLENT design. With two very quick twists of the clutch knobs, you should be able to lock the mirror in place even after moving it many turns of the focuser knob.

Another nice design point is the way the finder scope mounts. Previous SCT finders that I have owned required you to remove the finder from the rings to screw the ring assembly to the rear housing. This meant that when you removed the finder assembly for transport or storage, you lost the finder alignment. In the new version, there is a base that attaches to the rear housing. The ring cage then mounts in a dovetail fashion to this base which you can leave permanently in place. This way, you can remove the finder cage and finder, and when you re-install it, your co-axial alignment SHOULD be preserved. This is similar to having a Telrad base in that you can replace and remove the Telrad and keep your alignmen. This is another EXCELLENT design point.

Finally, the corrector cover is different from the previous Celestron C8s I have owned. ON those scopes, the cover was not FLAT. They usually had the little Celestron-orange knob at the center. I like to set my telescopes down on the front, and the old design made it impossible to do this with the cover on, and I would NEVER set it down this way without the cover. As a result, I had to always lay the telescope on its side. This takes real estate and you have to be carful that the scope doesn’t roll. The new design is not flat, but rather has a broad concave recess so that it CAN be left in place and the scope can be set down on the nose without a problem. Furthermore, this cover locks in place with pins which project inward from the front ring and mate with little cam grooves in the cover. With about a 20th of a turn, the corrector cover latches firmly into place. I hope you are not getting tired of hearing it, but this is yet ANOTHER excellent design point. It is clear to me that the designers at some point either TALK to users or ARE users of these products and have figured out that these little details are important. I know in an early review of the CGE I was very critical that the setup was not tool-less, and I noticed that new mounts are showing up these days that do not require ANY tools for setup or adjustment. I also complained that the EQ6 review that the designers missed the mark by not having a Losmandy/CGE type saddle and I notice that the CGEMS came to market WITH a CGE saddle. Now, I don’t know if Celestron is listing to me alone, so I can’t take credit, but clearly they are listening to SOMEONE! And the EdgeHD C8 seems to be proof that they are addressing these little usability issues.

Ok, now for the cooling vents. Lots of discussion on forums about the cooling vents. The pictures portray them as not being very big. The bad news is the pictures make them appear BIGGER THAN THEY REALLY ARE…LOL. I was surprised that I could LITERALLY cover each vent with my tumb! Now, I don’t know if they work or not, but I will say this. There are TWO of them, and they are positioned opposite one another at what appears to be about the level of the face of the mirror. I bet that it is only a matter of weeks before someone comes out with a sucker fan that clamps over one of the vents, allowing you to draw air ACROSS the mirror. Because the vents are small, it will not take a lot of fan power to get VELOCITY, and for cooling, I would think VELOCITY IS GOOD! Since the vents are directly across from one another, the stream of air would move straight across the mirror. Even though the vents are quite small, I have a gut feeling that with even a small sucker fan, cooling may be SURPRISINGLY fast. Also, there are screws that appear to hold the grill (and possible the filter element) in place, so it may be possible that a special cooling fan that simply plugs into the port is in the works. I do not KNOW, and I don’t intend to take mine apart, but if I am right, I think that these vents may prove to be surprisingly effective WITH MOTOR BLOWING. Otherwise, I have my doubts that it will be MUCH faster than an unvented system on still nights. But on breezy nights, it HAS to be an improvement. How much? Anything helps, and this solution is at least cord and vibration free. At the worst case it has to be SOME improvement over an unvented tube.

Before I get to the initial performance review, let me give one last point on the general comments. While I really like the finder bracket, the finder itself is pretty awful. Only when a star is at the center will it be sharp. Now for scopes that will be used with Go-To mounts, you really only need the finder to do initial alignment so I GUESS this is OK. I used it to align the CGE and it did work for that task. Otherwise, though, it was pretty useless. Now, having ponied up the purchase price of the telescope, I am feeling too poor to buy a Telrad for this scope, and I realize that a cemented doublet really can’t be made coma free, but this finder could have just as simply been a 6x30 unit for the utility it will provide. I can do initial alignment just as well with a 6x30. An unsharp 9x50 (or whatever) is just extra weight once you get past initial alignment. Celestron should just put on a 6x30 and pass the 5 dollar savings on. I would prefer to use less counterweight than have this almost useless finder on my scope. Better yet, they could create their OWN zero power finder. Even the little Stellarvue Red Dot finder to me is all you need for alignment on a Go-To scope. For the price of the finder, they could probably provide a decent zero power unit and let the people that still use poor quality 50mm finders go out and buy their own. I know far more people with Go-to scopes that remove the traditional finders and stick on Telrads or Stellarvue Red dots than people that stick 8x50 finders on their scopes.

Ok, now for the optics.

The VERY first thing I did with this scope after aligning was to star test it on Capalla. As an aside, I recently received a copy of the Second Edition of Harold Richard Suiter’s Star Testing Astronomical Telescopes. Over the years, there has been a lot of doubt regarding the validity of star testing for "Complex" telescopes. My own experience though was that the SCTs that seemed to have the star tests that best matched the patterns for good spherical aberration correction in Mr. Suters First Edition of that book also gave the best low contrast performance. In the Second Edition, he modeled a star test for a "Perfect 8” SCT” (the actual 8” SCT itself, and not just an 8” scope with a 33% obstruction, but rather an actual model of a star test on THAT SPECIFIC DESIGN). He shows a pattern for the scope in the book to SHOW that the intra and extra-focal slices for a perfect C8 would be IDENTICAL, especially with respect to the ratio of the size of the secondary shadow. He then modeled the star test for the C8 to show different levels of spherical abbearation. In these models, with increasing amount of spherical abberation the shadow ratio between inside and outside of focus became progressively bigger with increasing amounts of spherical abberation (and to be fair, this has to be the ONLY aberration present… Some other aberrations can make the shadow ratios appear SLIGHTLY different). Anyway, when I aimed at Capalla, put in a high power eyepiece, and racked though focus, it was like looking at the picture in Sutiers book for his perfect C8! The secondary shadow ratio looked almost identical. Expanding the rings further produced intra and extra focal patterns that looked essentially PERFECT! Even a slight turned edge or higher orders of spherical aberration will change the contrast of these rings on one side of focus as compared to the others. On my EdgeHD, the rings between the innermost and outermost ring were of exactly the same uniformity and contrast inside and outside. While seeing wasn’t perfect, it was easy to tell that the optics were among the very smoothest I have ever seen. Further expansion of the rings for zonal errors showed none. This particular sample tested on par with some of the best eastern European Catadioptric scopes I have owned. Hands down from an optical standpoint and based on my extensive (but amateur) testing, this was THE best mass produced commercial catadoptric telescope I have ever tested, and take out the “mass produced” and based on my own star testing experience, one of the best PERIOD! I was STUNNED at how excellent this scope star tested. It was like looking at the pictures in the book!

On doing the star test, it is important to note that in this sample there was considerable mirror shift. It was (for Austin, Tx) bitter cold and the scope was left outside for 2 hours prior to testing. With the fresh grease of a brand new telescope combined with freezing temps, I think the viscosity was so high that it was causing the mirror to stick in place. When I would turn the focuser, I would see stars shift in the field, but then in slow motion, they would appear to shift back. I believe the angular pull of the focuser rod pulled one side of the mirror to me, then as gravity overcame the viscosity of the grease, the other side of the mirror would settle so the stars would move back into position. Small movement of the focuser did not cause undue shift, but quick, larger turns of the knob did. It could be that once the grease between the mirror carrier and the baffle is better distributed, the stiction will lessen. Otherwise, the focuser felt nicely weighted and smooth.

Ok, now for the important part: What was the viewing experience like? Anyone that knows me knows that I have become obsessive about edge of field performance in recent years. It is true. My vision is 20/15 and maybe I can just resolve the defocused comatic tail in the SCT view better than most people. And with my inability to accommodate the field curvature, the edge of the field of the normal SCT when used with low power Naglers just didn’t look all that sharp to me. Now, when you consider that FAR more area in the field is in the outer half, and you start to look more critically at that area, you realize that in MOST reflecting systems, that big area out there is not nearly as crisp and sharp as the small circle at the center where we put the “Target.” The target item though has become to me only PART of the experience of viewing an object. To me, it has become JUST as important that the ENTIRE field, which gives “Context” to the target, be as sharp as possible. My 6" refractor taught me to appreciate FOR more than JUST the center of the field.

The seeing was not perfect, but one look into the 31mm Nagler with the scope aimed at M45 was all it took to know that I was not looking though a standard SCT. I picked M45 because it has many bright stars over a large area. Any off axis aberrations present become far more noticeable when you use bright stars. With the 31mm Nagler in the C8 stars across the field looked VERY uniform. I could not detect any field curvature at all. Only when the brightest stars were placed right at the field stop of the 31mm Nagler could I see a TINY amount of aberration, and I think it was a tiny bit of astigmatism. Perhaps there is still an almost trivial amount of field curvature present (I am an older observer and cannot accommodate much curvature visually anymore). Celestron does not say the scope has a FLAT field. On their web site, it says that the standard SCT has three times more curvature than the EdgeHD. This would say that the field is not PERFECTLY flat. By comparison my 6” APO I think IS flat.. I saw a ray trace for a similar design and it said that the radius of curvature was infinite. With the 31mm Nagler in the 6” refractor, the field is 100% in focus from sea to shining see. So, my guess is that there IS a TINY amount of curve in the EdgeHD, but it SOOO small that 99% of observers will never see it. I simply have better than normal eyesight and worse than normal visual accommodation. And remember, this was a BRIGHT star RIGHT NEXT to the field stop of a freaking 31mm Nagler! Overall, I would say that the field looked GREAT! It was the BEST off axis performance I have ever seen in a reflecting telescope when using a 31mm Nagler.

Next, I went to the Double Cluster. Ah… As compared to my older optically excellent C8, the field around the Double Cluster members just looked BETTER. It looked “Richer.” I already knew what Celestron meant in its web page when it showed pictures and made the claim that you could see more stars at the edge of the field with the EdgeHD scopes. I have seen these views with telescopes that do not suffer form off axis aberrations and they always looked richer than in my SCTs. In the EdgeHD, I got the experience Celestron said I would get with an EdgeHD scope!

If you want to know what I mean, look at the pictures on the Celestron web EdgeHD web page and note the picture where they show a blow-up from a photograph taken with a “competing design.” In the pictures on their web page, stars at the edge of the field in the “Competing design” actually showed up as little Fresnel patterns. I COULD SEE THAT IN MY OWN SCTS! With the 22mm Nagler in my C14, I could actually see the comatic Fresnel patterns at the edge of the field from the field curvature of the scope. Yes, I could actually resolve the blur to a Fresnel pattern and in the 22mm Nagler used in my C14, I could COUNT the diffraction rings! Like I said, my eyesite is 20/15, but on top of that, I consider myself to be a darned good observer. When you are used to eeking out the finest planetary detail available, you get to where you simply can’t HELP but see these tiny details! No offense intended, but I think that for most people, when they say that this or that is pinpoint, I just think they aren’t as critical as an observer as I am. I can resolve he beginning of a commatic blur starting half way to the field stop of a 22mm Nagler in a C14!

Well, in the EdgeHD, the field was BEAUTIFL! With the 31mm Nagler (and no bright stars at the edge of the field), visually, the field was pinpoint right to the field stop. The field CONTEXT of the clusters was that they weren’t just clusters adjacent to one another, but rather that they were clusters that appear to us against the rich background of the Milky Way. The EdgeHD C8 showed them as they SHOULD be seen! It was JUST LIKE the web page comparison shown on the Celestron web site. The EdgeHD delivered EXACTLY the experience I was expecting. The field was GLORIOUS!

By the time I got to Jupiter, seeing had not improved enough to make more than a quick look, and Jupiter was too low for anything but showing major detail, but I know that when I DO get to see planets, this scope will show them well. Other scopes that have tested this good have always been good performers, and I am convinced this one will be too.

My initial impression is that this is going to be a crowd pleaser telescope. Numerous design improvements, some small, some large, would have made this a better SCT even if it weren’t an EdgeHD. The off-axis performance though, is in my own opinion, worth every penny of the price premium over the standard C8. And in my own sample, the premium quality optics all add together to make for what is perhaps the most exciting new telescope I have purchased in 20 years. Here we have a package that represents all of the wonderful things that made the C8 perhaps the most well known telescope ever made, and eliminates the primary shortcomings (Coma and field curvature) of the design.

I like to rate things, and in spite of the fact that this scope is STILL an SCT with a largish central obstruction, considering it’s now superb across the field performance, its size and weight, the amount of aperture, and its price, I am going to bestow on this one telescope the highest praise I have ever given ANY telescope. Long after I stop being able to mount my 6” refractor or my C14, I think I will still be using this scope. It is the scope of a lifetime. I give it a Billion Star rating, because in my lifetime, I am sure I will see that many stars with it.

At last, the PERFECT small SCT.


1/15/10 - Just an update on the Dovetail situation. After inspecting the EdgeHD C8 upon receipt, I conirmed that it did have screws that match the hole pattern used on the Losmandy VSCT13 Dovetail.

There WAS however, an issue that I need to alert other users about.

The Losmandy dovetail came with two screws of the same thread pitch as the rear screw holes on the EC8 rear housing. I believe that in older C8s, it is possible that there were centerline holes both front and rear. Or maybe my kit did not contain the proper screw.

At any rate, one would think "No Problem, I will use the factory screw." The problem though is that the recess slot milled into the front hole of the VSCT13 Dovetail is not wide enought for the head of the factory screw to fit into the recess slot of the Losmandy VSCT13 dovetail.

Being a well equpped modern man that owns a drill press and is not afraid to use it, I was able to quickly and easily drill out the slot to the proper width using a 11/32nd drill bit and factory screw slipped right in.

I contacted Scott ad Losmandy, and he said that in future runs of the VSCT13, he will mill the front slot out to 11/32nds so that EdgeHD ownders will be able to convert and use the factory front screw along with the either the Celestron rear screws or the ones provided by Losmandy (the thread and length are the same).

The EdgeHD C8 freshly mounted on the CG5 ASGT looks great (though I confess I LIKED the look of the orange dovetail) and I can't wait to try it out!