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C90 by the #s, relative to other small scopes...

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#1 jrbarnett

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Posted 19 November 2012 - 07:51 PM

So what's a C90?

The C90 is a Celestron-branded, Synta-mase Maksutov-Cassegrain of the Gregory variety. It has a 90mm aperture, and a 29mm diameter central obstruction (10% by area). The focal ratio is f/13.9 and the focal length is 1250mm. The OTA is 16" long and weighs about 5#.

It typically comes bundled with a thinly padded bakcpack, 8x20 finder scope, a 32mm Plossl eyepiece, 1.25" visual back and a 1.25" 45-degree amici prism diagonal. Celestron classifies this as a spotting scope rather than an astronomical telescope. This actually may work in the buyers favor; spotting scopes have a lifetime limited warranty and astronomical telescopes have just a 2-year limited warranty. The bundle runs between $150 and $160, if you shop around a bit.

It sounds like a heck of a bargain, but before reaching that conclusion, it's important to consider how it stacks up, by the numbers, to other small scope alternatives. Because it is a catadioptric, it is not particularly efficient at passing through the light it captures. Due to losses from the central obstruction, the two multicoated surfaces of the corrector, and the two quartz overcoated aluminized mirrors (aka "standard coatings"), the actual throughput is equivalent to a perfect 100% transmissive scope of 2.95". It's important to note, though, that for purposes of resolution it acts as a 3.5" scope.

Given its effective light grasp of just 2.95" is seems as if it should be compared to 80mm refractors. In fact, if we run the numbers for the latest Stellarvue SV80 triplet, that scope's aperture is 3.15" and it light loss is minimal from its six multicoated air-to-glass surfaces, providing an effective "100% transmissive scope" throughput of 3.1". For resolution purposes, though, it behaves as a 3.15" scope. Figure an 80mm multi-coated ED doublet like the Orion ED80 would fare even better on throughput as it lacks two surfaces relative to the SV80.

So for practical purposes (being color free, etc.) the C90 does match up with 80mm ED doublets and triplets, by the numbers. Of course with an f/13.9 focal ratio, it won't behave like these f/6 and f/7.5 scopes in terms of field of view. The effective maximum TFOV of the C90 is about 1.3 degrees (as it is limited to 1.25" eyepieces). The effective maximum TFOV of the SV80, on the other hand, is about 5.6 degrees (with a 2" 40mm 68-degree eyepiece).

So they really are different kettles of fish. So for certain purposes (planetary and lunar observing and brighter double stars where there's plentiful light from the target) the two types of instruments (faster ED 80mm doublets and triplets and the C90) perform almost equivalently. For wide field targets, though (the Veil, Pleiades, M35, M44, etc.), not so much.

But then you still have the price. The C90 is less than half the price of the ED80 doublet and less than 1/7th the price of the SV80 triplet. From a value perspective, assuming the optics are good, the color-free C90 is a value alternative to these other common small "grab and go" scopes.

Before anyone gets too excited and says "Yeah, but what about a C5 or C6 instead?", bear in mind that a C5 or C6 costs almost 3x as much. Remember, "value" is the key for the C90. From a value grab & go perspective it's stellar.

To use it effectively for astronomy, you'll need to chuck the amici diagonal in favor of a 1.35" 90-degree star diagonal. You might also want to swap the tiny straight through finder scope (if you plan on star hopping with the C90) in favor of an Orion or GSO right angle finder. If you already have a GSO or Orion Dob, you can just borrow the finder from that scope when "grab & going" with the C90.

I'm using mine to hold up the "3" class" in an article I'm working on about the democratizing effects of technological advancement on astronomy gear. That is, it is holding up the 3" aperture duties when compared to historical instruments of similar aperture used by amateurs in the 19th and early 20th centuries.

Being that St. Nick is just around the corner, the C90 is a decent under-tree option that can fill the grab & go niche affordably, and partner up with a larger more cumbersome scope.

Posted Image

Regards,

Jim

#2 stevenf

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Posted 19 November 2012 - 09:39 PM

Thanks very much for this Jim, an enjoyable read. There's just something about these little scopes isn't there, not quite sure what. I have the Skywatcher blue tube version, pretty much exactly the same as the C90 (has the same thick metal front and back). And small enough that when I travel I don't have to decide between high magnification or wide views. I just bring both the 90mm mak and an ST80. Best of both worlds.

#3 jalesi

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Posted 19 November 2012 - 10:42 PM

I have the C90 too. A nice little scope, despite the fact I couldn't find Jupiter last night. But that's ok. I know what I did wrong, and I will find my target the next time out. (I didn't have any finder scope attached) I have seen the moon with it a few times and it's a joy on the moon.
I use it on my 6/8SE mount and that makes for a great grab and go set up.

I've only used my Pentax EP's with it, the xw 20 and 5. Which to me have fine views. It's funny that the EP's cost twice as much as the scope!

It is a fine little scope that is easy to use and carry and were something to happen to it, no big shakes, they're a bargain.

#4 Jon Isaacs

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Posted 20 November 2012 - 06:45 AM

So they really are different kettles of fish. So for certain purposes (planetary and lunar observing and brighter double stars where there's plentiful light from the target) the two types of instruments (faster ED 80mm doublets and triplets and the C90) perform almost equivalently



My math, based on the fact that this apparently has standard coatings and a 32% CO, says that the light through put would be comparable to a 76mm refractor. In terms of planetary viewing, the 32% CO would reduce the planetary contrast significantly more... the old rule of thumb suggests that it would be the equivalent of a 60-70mm refractor...

Jon

#5 Eddgie

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Posted 20 November 2012 - 09:49 AM

I have tried a couple of 90mm MCTs.. One was Celetron C90 (previous model with helical focusing) and Meade ETX.

I was not at all taken with either one.

Both of them had small baffles and could not well illuminate even a 26mm Plossl.

Both of them had rather poor contrast.

I did not feel that either of them made particularly useful astronomical telescopes. The view was simply to dim and the field was too narrow.

I did sell the C90, and was going to sell the ETX, but the Assistant Astronomer thinks that it is cute and asked if she could have it.

I found the C5 to be a far, far better telescope for just about every possible application. And while it is not as small, the fact is that it will fit in a shoe-box, so while not "As small," as 5" aperture telescopes go, the size is diminutive.

In fact, the C5 is the smallest telescope I care to own. I have gone though half a dozen small refractors up to about 4", and none have managed to stay around long. They just don't seem to have the deep sky performacne I desire. Even showcase objects look dim in an 80mm aperture.

But the C90 and the ET-90 were worse. Even the Orion Nebula looks poor in thesee scopes.

I sold a C5 about 6 years ago and came to regret it badly. The little ED scope I replaced it with had a wider field of view, but was outclassed by the C5 in just about every other way, with the C5 going deeper and giving more pleasking deep sky and planetary performance.

And now that I have been using it with the Binoviewers, it has been given a second lease on life.

And if you are going to have to use a 1.7x GPC to binoveiw a small refractor, you get stuck with a telescope that has the same narrow field of view, but half the light gathering of a C5 with a Binoviewer.

To me, the C5 is the king of small telescopes. Once you get over the fact that you are never going to get a wide field out of it, you are still left with one of the most (size wise) efficient telescopes ever made.

I just didn't get the utility out of the small MCTs that would make me want to have another one.

My advice... Better to shop for a used C5 and get a much more capable instrument than clutter up the house with another small telescope that is not of much value for general observing.

#6 Binojunky

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Posted 20 November 2012 - 10:23 AM

Well over the years I have purchased the ETX90, Apex90, the previous version of the C90 (rubber covered with a flip mirror) and the latest version C90, this was bought after reading Ed Tings review, they have all gone for various reasons, unemployment, lousy optics, thinning out the herd, etc. For a bit more money you can buy the Apex 102 or 127, these set up just as quick, are nearly as portable and are capable of a lot more.
With the 90mm maks I go through the this is cute phase to this is frustrating phase very quickly, if I need a day/nighttime multi purpose spotter then my Apex 127 fits the bill better.DA.

#7 jrbarnett

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Posted 20 November 2012 - 11:31 AM

And yet, head to head field comparison of the C90, AT72ED and Carton 60mm f/16.7 achromat, and AT60EDT show this not to be the case. The C90 outperforms all three of them on double stars and planets. :thinking: I have several more 3" and 2.4" refractors at my disposal, as well as a C5. Maybe I had bad luck with mine, but the C5 is a dog. My C6, on the other hand, is a very decent scope.

- Jim

#8 curiosidad

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Posted 20 November 2012 - 11:37 AM

Jim,
His analysis has made me fall in love with this telescope C-90!

#9 Eddgie

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Posted 20 November 2012 - 04:24 PM

Well, if you are happy that is all that matters.

To be fair, I find little joy in any small telescope regardless of type.

But in particular, I found the C90 and the Meade 90 ETX to be joyless. Narrow fields, poor contrast, and low light thoughput.

And maybe your C5 was a dog. But mine isn't. Spectacular optics in a tiny package.

When you can have the same true field and double the light gathering in a scope that weighs less than 7 lbs and fits in a shoebox, is there really any benefit in going smaller?

But hey, you want to preach the gospel for this scope, that is good by me.

I would just preach the gospel for a bigger scope, and for me, a bigger scope is almost always a better scope and of physically small and ligth telescopes, the C5 has for me been the best of the best. A decent amount of aperture in a tiny package.

I would rather spend the money that a C90 costs on a new binoviewer eyepiece pair.. LOL.

#10 Jon Isaacs

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Posted 20 November 2012 - 06:44 PM

And yet, head to head field comparison of the C90, AT72ED and Carton 60mm f/16.7 achromat, and AT60EDT show this not to be the case. The C90 outperforms all three of them on double stars and planets. :thinking: I have several more 3" and 2.4" refractors at my disposal, as well as a C5. Maybe I had bad luck with mine, but the C5 is a dog. My C6, on the other hand, is a very decent scope.

- Jim


I only commented on the planetary contrast/detail. In your post you compared the C-90 to 80 mm apochromats... not 60mm achromats and 70mm EDs... If the C-5 was not far better than the 90mm Mak, I can only surmise that the C-5 was seriously flawed.

YMMV

Jon

#11 moynihan

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Posted 20 November 2012 - 08:14 PM

And yet, head to head field comparison of the C90, AT72ED and Carton 60mm f/16.7 achromat, and AT60EDT show this not to be the case. The C90 outperforms all three of them on double stars and planets.


My experience also in direct comparisons (jupiter) to 60mm, 76mm very good long focus achros and the at66edt. I have also directly compared it to my 80ed (yes, it is collimated), at similar magnification, side by side. The 80ed seemed to be a tad brighter, and the edge of Jupiter was sharper. The c90 did show a bit more low contrast detail.
Of course, i lucked out on the c90. Collimation is spot on, and its star test using an artifical star, is textbook.

#12 t.r.

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Posted 21 November 2012 - 08:23 AM

Years ago, 1990ish, I wanted a small travel scope to take with me on my deployments for Uncle Sam. I had my eye on two scopes, the Celestron rubber armored C90 and the Celestron SS80 made by Vixen. I was at that time already enamored with refractors, so I let the sales person talk me into the SS80. It did indeed perform well for its aperture and for its intended purpose of wide-field sweeping. It had a fine star test. It showed me many sights north and south of the equator. But its planetary performance broke down at about 100x and its resolution came up a bit short. I always wondered about the C90 I passed on.

Last year I took a test drive with that old rubber armored C90 and much to my suprise...It blew away the 80mm. Really! I was amazed at how much better it resolved planetary/lunar detail and its ability to go to 200x was appreciated. Color free to boot. In retrospect, I wish I had gone with this little gem of a scope. It would have traveled well and probably showed me quite a bit more than the SS80 has. I think the real point Jim is making is not that there aren't larger scopes available, but that you would be hard pressed to find one that performs to this level and do all the other things...for $150 bucks!!! I wish it still came with rubber armor. It is on my Christmas list for Mrs. Claus! I'm thinking about that little rubber armored C70 too...just because! :praying: :grin:

#13 Skooter

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Posted 21 November 2012 - 08:39 AM

I'm getting great views of the moon in poor seeing with a 4.7mm eyepiece (266x) in the C90. In good conditions...who knows? Scope blows the old 50x per inch rule to smithereens. A good refractor might be able to do this as well; I'm too inexperienced to know for sure. I'm just happy to do it for 150 bucks. :grin:

Needs a better finder though. Would like to split some doubles but don't think I could find any with the supplied finderscope.

#14 jrbarnett

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Posted 22 November 2012 - 12:29 AM

The finder shoe on the C90 is compatible with the Orion RACI finders as well as the GSO 8x50 RACI finders. This is unlike other Celestron scopes, such as the C5, C6, C8, C9.25, etc., which do not natively support such finders. The Orion 6x30 actually has a larger TFOV than the 9x50, and is less costly. Given the smallish aperture of our C90s, the 30mm finder goes plenty deep enough for the C90.

Regards,

Jim

#15 Skooter

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Posted 22 November 2012 - 07:27 AM

Thanks for the info, Jim. I'd like to use the 6x30 off my C8, then get a 8x40 or 9x50 for the C8 but don't know how to do it without buying two complete finder/mount setups plus a dovetail base. Too bad the mount pictured below won't fit the C90 directly. And the C90 rings are too small for the 6x30 tube.

Attached Files



#16 Skooter

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Posted 22 November 2012 - 07:39 AM

Once Jupiter edged above the treeline last night, was able to get a nice 313x out of the C90. Was too much for my slow motion adapter/camera tripod, but with good tracking would have been highly usable for detailed study at that magnification (and beyond) with good seeing.

#17 stevenf

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Posted 22 November 2012 - 10:27 AM

I use a simple red dot finder (came with the scope) on mine. But I've just ordered a GSO 8x50 that I'm considering using. Overkill on the 90mm maybe but my thought is maybe it might provide some wider field views on it's own, and give me some additional capability with the mak.

#18 Ouranos

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Posted 22 November 2012 - 11:03 AM

I binoview my C90 on the moon and find the views amazing. And, I use it as a solar scope - just came in from using it that way - I must say it is sharp in that regard as well. I do not expect it to provide the views my dob does or my 102 refractor. But, for what it does do, pretty amazing for the price. I also shoot some pics through it, but that is another story.

#19 Sarkikos

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Posted 22 November 2012 - 04:56 PM

Jon,

My math, based on the fact that this apparently has standard coatings and a 32% CO, says that the light through put would be comparable to a 76mm refractor. In terms of planetary viewing, the 32% CO would reduce the planetary contrast significantly more... the old rule of thumb suggests that it would be the equivalent of a 60-70mm refractor...


I ought to take out my 90mm Mak and 70mm f/12.9 achro and do a comparo one night to see how true this is. So far, though, I favor the Mak because it's just easier to deal with: shorter, lighter, less cumbersome, can go out on a simple tripod instead of a GEM or astro alt-az mount, and much more comfortable to look through. For every 20 times the Mak sees the sky, the 70mm refractor might see it once.

Mike

#20 Sarkikos

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Posted 22 November 2012 - 05:08 PM

I've seen some surprising detail on Jupiter, Mars and Saturn with my 90mm Mak, but nothing that can keep me entertained for more than about ten minutes. For bright planets, I bring out my 8" or 10" Dobs or my 6" Mak.

My meager experience as a double-star observer doesn't hold for much, but if I were to get seriously into that niche of the hobby - hasn't happened yet - I'd at least take out my 5" Newt for a wider range of doubles and better color saturation.

IME, 90mm Maks are best as grab-n-go Moon scopes. I've spent many hours on many nights with my little Mak viewing the Moon. There are thousands of lunar features within range of my 90mm Mak, including many fine rilles and lunar domes.

Mike

#21 Sarkikos

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Posted 23 November 2012 - 01:38 AM

t.r.,

Last year I took a test drive with that old rubber armored C90 and much to my suprise...It blew away the 80mm. Really! I was amazed at how much better it resolved planetary/lunar detail and its ability to go to 200x was appreciated. Color free to boot.


I'm not surprised that the C90 blew away the SS80. This seems like a very similar comparison to my 90mm Mak vs my ST80. Guess which one still gets taken outside on a regular basis and which one I'm thinking about selling?

:grin:
Mike






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