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Equipment Discussions >> Classic Telescopes

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Joe Cepleur
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Reged: 03/18/10

Loc: Dark North Woods
Is this Celestron C90 "Classic" or "Used?"
      #5539184 - 11/25/12 06:13 PM Attachment (128 downloads)

I have an opportunity to buy an older C90 from a member of my club. I'm interested if the price is right, but have no idea how much to offer. It strikes me as an excellent birding scope and a better-than-expected, grab-and-go, ultralight astronomy scope. I was impressed upon first reading Uncle Rod's assessment last year, yet had never seen one in person until now. An apparently classic article in "Better View Desired" praises this scope highly, suggesting its a real sleeper. Views are about as good as a Questar's at a fraction of the price, which is actually not surprising.

What's the drill with this scope? If a 4" refractor is suitable for various authors to use in publishing books about the Messier objects, how much different could at 3.5" be? According to the article in "Better View Desired," the C90 offers basically perfect color correction, 3.5" of resolution, 3.25" of brightness, and the contrast of a 60mm. That last may not be much to brag about, but for a lightweight rig used more for practicing finding Messier objects than studying them in detail, may not be so bad. Portability is key here. A C90 would not replace my C8, nor would it need to.

So, here's the first picture. This C90 is a black tube with helical focuser and a 0.965" visual back (if it's called that when it is integral to the scope). One respected opinion is that it was made by Vixen in Japan; can anyone attest to that? The undersized tripod is what I had on hand, but the case would be sold with the scope. No eyepieces or diagonal are included, so I used a set from my Jason 313. It will not focus with the extra back focus created when using a 0.965'-to-1.25" adapter before a 1.25" diagonal, so I'd need to buy a good hybrid diagonal. I suspect the optics are excellent, but can not confirm so by star test with the junky high-powered eyepieces currently at my disposal. I must say, the detail seen a mile and further away in daylight was astonishing!

Before I questioned whether he might like to sell it to me, the seller commented that the focuser was so tight that it was impossible to focus without knocking the scope off its target. I'd think that's a matter of cleaning away old grease and relubing with Mobil 1 Synthetic Grease (any other recommendations?). He also said that he paid little for it, because it "had problems," but I suspect he and the former owner were simply baffled by the need to use a diagonal. It will not focus when viewed straight through. The seller bought his dream Questar Birder, and so has no further use for the C90.

I have found no indication of where the scope was made. Torrance would be cool, Japan perfectly acceptable, but maybe not China in this vintage. What do I have here, a "Classic," or a "used telescope," and how much is it worth?


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539188 - 11/25/12 06:14 PM Attachment (54 downloads)

The "Special Coating" sticker exactly matches those on older C8s made in Torrance, which does not necessarily mean it was made there.

Edited by Joe Cepleur (11/25/12 06:16 PM)


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CHASLX200
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539192 - 11/25/12 06:15 PM

Looks like a much later version compared to when they fisrt came out.

Chas


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Joe Cepleur
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Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539199 - 11/25/12 06:23 PM Attachment (61 downloads)

The 8x20 finder is a bit of a joke, but it works. Might want to replace it with a 6x30. It focuses easily, but has a narrow field of view, and it is impossible not to bonk one's head somewhere on the scope when attempting to use the finder. I have not decided whether that's a hazard of the overall small size, or a result of bad design. This finder was from the spotting scope package. The astronomical package had a 6x30; was it discontinued by the time this scope was made? Anyway, the rest of what you see is all metal. Was it cast from the same molds as the Orange Tubes?

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Joe Cepleur
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Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: CHASLX200]
      #5539201 - 11/25/12 06:25 PM Attachment (52 downloads)

Quote:

Looks like a much later version compared to when they fisrt came out.

Chas




Yes, thanks; note that all of the writing and the orange stripe are on a sticker! No early Celestron would have that!!


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539203 - 11/25/12 06:27 PM Attachment (49 downloads)

Serial Number 887061. Note also that this is the f/11 version, not the later f/5.6 sold for photography with supposedly inferior optics.

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539209 - 11/25/12 06:29 PM Attachment (72 downloads)

Here's a shot for all you Forensic Lenscapologists, which also shows the interior of the case and the business card of the vendor who sold it to my seller.

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539212 - 11/25/12 06:30 PM Attachment (54 downloads)

Forensic Lenscapology being a popular sport, here is the other side.

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539217 - 11/25/12 06:32 PM Attachment (42 downloads)

Nice case, cut to fit a camera.

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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539223 - 11/25/12 06:37 PM Attachment (53 downloads)

It looks very similar Joe.
I paid $100 for mine not long ago and I really like using it. Except I don't like the helical focuser for astronomy, anything else it's fine especially nature. The focusing is very fast, too fast for astronomy but it is just my opinion.
Its a excellent value at what seems to be a regular average price, approximately $100 for the spotter.
I bought a Meade adapter #07036FL that screws on to the port and then you can use your C8 visual back and accessories.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539226 - 11/25/12 06:40 PM Attachment (48 downloads)

The outside of the case. The 32mm Plossl is not included in the sale, but is there to show comparative size.

So, a black C90 with a helical focuser and 0.965" back:

*** "Classic" or "Used?"

*** Torrance, Vixen Japan, or China?

*** What might it be worth?

*** Lucky find, or best to pass?


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5539238 - 11/25/12 06:47 PM

Quote:

I don't like the helical focuser for astronomy... The focusing is very fast, too fast for astronomy...

I bought a Meade adapter #07036FL that screws on to the port and then you can use your C8 visual back and accessories.




I'd imagine the helical focuser could be difficult for astronomy. Still, Uncle Rod's article notes that there is no mirror flop possible with helical focusers, so that this model of C90 can be used as a guide scope!

The adapter sounds interesting. I wonder whether the optics differ between your Orange Tube and this scope. Just extending the back focus with a 0.965"-to-1.25" adapter made it impossible to focus.


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5539240 - 11/25/12 06:47 PM Attachment (59 downloads)

Mine doesn't have special coatings which is very desirable and may make a C90 worth considerably more.
The tube and focuser grip are cast as one and made out of Bakelite on the orange version.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539246 - 11/25/12 06:50 PM

Actionhac, you paid $100 for a clearly classic Orange Tube. Is a later black tube of lesser value, even if the optics proved to be comparable?

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5539264 - 11/25/12 07:04 PM

Quote:

Mine doesn't have special coatings which is very desirable and may make a C90 worth considerably more.




Yes; important point. I've read that in the articles linked in the first post above. I'd love to see the two scopes side-by-side. The difference is said to be appreciable, maybe not so much because it's eye-popping, but because it squeaks the best possible image out of a small aperture scope. An 8" without Special Coatings may be said to work like a 6-inch, but there's less headroom with a C90. Still...

Quote:

The tube and focuser grip are cast as one and made out of Bakelite on the orange version.




An Orange Tube will always be cooler! Great collector's piece, and I'm sure the images are great.


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actionhac
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Loc: Seattle
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539274 - 11/25/12 07:11 PM Attachment (58 downloads)

I wouldn't think the black tube would be worth less given

equal optics. The C90 seems to be more a working telescope

as opposed to being a collector. And really the newer the
better is the general rule when it comes to

CAT's and this may apply to MAK's as well.

The Meade #07036LF adapter is a ETX 90 accessory, I'm not sure if its still available. Basically it is the same as the original LAR adapter for the C90. Here it is:


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H.S.
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Reged: 06/26/07

Loc: Terre Du Lac, MO
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5539640 - 11/25/12 11:39 PM Attachment (46 downloads)

I think mine is a 1976 model. Hybrid diagonal with a 32mm UO Konig. I don't know what the coatings are.
Harry


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Ducky62
professor emeritus


Reged: 10/31/10

Loc: The ATL
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: H.S.]
      #5539664 - 11/26/12 12:10 AM

I've got a black one in the tacklebox case. I bought it on astromart for $120 shipped back when I really needed the finder and a "better" .965 diagonal, Barlow and some EPs for another 'scope.It isn't terrible but I don't like the helical focuser. I'll probably keep it until I find a deal on a nice Orange Astro model. I have a Chinese-sourced Vixen-branded 90mm Mak with a flip mirror which blows it away optically and is my primary grab-n-go telescope.Both were 1/20th the cost of a used Questar but I do love 90mm Maks so one of those is on my bucket list too.

I think my point is I wouldn't give more than about $50-60 for an untested black C-90 with no mount or accessories.
If it performs well and the case is nice the C-90 might be worth a C-note, but no more. JMHO.

(I'd add at least $25 for a LAR but the 'scope in question doesn't have one.)

Edited by Ducky62 (11/26/12 12:13 AM)


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Ducky62]
      #5539672 - 11/26/12 12:24 AM

Now here's a good idea:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5475652/Main...

Robert


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Ducky62]
      #5539674 - 11/26/12 12:28 AM

Quote:

I have a Chinese-sourced Vixen-branded 90mm Mak with a flip mirror which blows it away optically and is my primary grab-n-go telescope.




I've seen that scope advertised. Neat scope. The 110mm version has had mixed reviews. It may be that if people expect more than so small a scope can deliver, they are disappointed; but, it they appreciate its limits, it becomes a favorite grab-and-go. They are inexpensive even with a Porta II Mini mount. Good deal!

Quote:

I think my point is I wouldn't give more than about $50-60 for an untested black C-90 with no mount or accessories.




Good to hear that a comfortable price would also be fair.

Quote:

(I'd add at least $25 for a LAR but the 'scope in question doesn't have one.)




What's a "LAR?"

I just took the scope out with the 21mm Kelner (53x) and the tabletop photo tripod, and succeeded only in proving what we already know, that the mount is half the telescope. Somehow, I managed to find Jupiter. Moons were fairly sharp, and I could see faint cloud bands. Focusing proved difficult, and after I did manage a decent focus, it was too easy to knock it out again!

By whatever miracle, I then found the Orion nebula hand-held, by scanning the skies. Impressive nebulosity, but it was not possible to simultaneously hand-hold and focus sharply.

A better mount and tripod would ease the situation.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5539909 - 11/26/12 07:54 AM

Quote:

Now here's a good idea:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5475652/Main...

Robert




Yes, Actionhac; thanks, that's slick! Funny to see that adapter used successfully, since I could not get the scope to focus with even the added back focus of a 0.965"-to-1.25" adapter. I'll have to try again. Alternatively, since the scope needs a new hybrid diagonal anyway, does anyone make one with a built-in, helical fine focuser?


DREAMING OF ASTRONOMY -- LEARNING IN MY SLEEP

I dreamed last night about the images of Jupiter and the Orion Nebula seen through the scope. They answered the question about the relationship between resolution, brightness, and contrast.

Other than the trouble getting the scope to focus, images seemed sharp and detailed. That is "resolution."

The Moon was bright, so conditions were not ideal. Still, the sky and my targets were brighter through the scope than with the naked eye, a function of "brightness."

Trouble was, stars did not pop against the background, the cloud bands of Jupiter were barely discernible, and the nebulosity surrounding the Orion Nebula, while impressive for so small a scope, did not extend further into the sky. These were problems of "contrast." The image was however bright, but the difference between the brightest and darkest parts of the image was insufficient to make observed objects pop. Still, the view was pretty good for a scope that weighs almost nothing, and is small enough to carry anywhere! Presumably, this is the quality that would be most improved by trading the little Mak for a refractor or off-axis reflector. The contrast would be more suitably appropriate to the other qualities of the view. It was likely good luck for my understanding that I tested this on a moonlit night, when this issue would be at its worst. The views would be much better, and the importance of contrast less obvious, in dark skies.

In sunlight, it's a different story altogether. I'm still impressed with the detail seen on objects between half a mile and a mile-and-a-half away. I can see why birders love this scope without reservation.


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5540092 - 11/26/12 10:59 AM

I like that idea too. It does solve the problem of the camera lens style focusing which is the only thing I don't like about the C90 for astro work. Now for action like animals, sports etc. the camera type helical is great because of its speed and you can hold the C90 like you would a SLR camera.
I guess I need to go to a hardware store and look at the plumbing section.

The contrast problem is probably the secondary obstruction. with any CAt or Mak I feel like I forgot to take my sunglasses off.
I have done side-by-side tests with my C8 and Optical Craftsmen 8" Newtonian and the difference is very obvious, especially the Orion Nebula.
But this is one of the sacrifices we make to have such a compact instrument. Sometimes I'm willing to give up some precious contrast. When I want to observe seated the C8 is great. Just imagine taking a 60X1000 refractor to a baseball game.

Robert


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Ducky62
professor emeritus


Reged: 10/31/10

Loc: The ATL
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5540093 - 11/26/12 10:59 AM

Large Accessory Ring. It is the Celestron part that allows the use of 1.25" accessories. I don't know if the Meade one fits later C-90s.

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Joe Cepleur
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Reged: 03/18/10

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5540193 - 11/26/12 11:59 AM

If the C90's mathematics say it should have the contrast of an unobstructed 60mm, why are the dimmer images of Jupiter's cloud bands through my Jason 313 so much more vivid and beautiful? So much for theory. It's as though the view through the C90 was washed out and overexposed. Still, at the right price, it's a tempting keeper for birding, or for astronomy from the trunk of the car. It's just so perfectly portable.

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Geo.
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Ducky62]
      #5540424 - 11/26/12 02:33 PM

Quote:

Large Accessory Ring. It is the Celestron part that allows the use of 1.25" accessories. I don't know if the Meade one fits later C-90s.




Fits some, tight on others. I've found the helical focuser's action is much better on the later C90's. The early black tubes in f/11 and f/5.6 were for use as camera lenses and were not coated. The OT C90s had a coated menicus. Probably why this post OT period specimen is denoted as special coatings, meaning it was intended as a scope not a lens.


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Joe Cepleur
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Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Geo.]
      #5540620 - 11/26/12 04:30 PM

Quote:

The early black tubes in f/11 and f/5.6 were for use as camera lenses and were not coated. The OT C90s had a coated menicus. Probably why this post OT period specimen is denoted as special coatings, meaning it was intended as a scope not a lens.




Are you saying that all orange tube C90s have the equivalent of "Special Coating," despite not being labled as such? That's interesting, because one of the posters above said he did not know whether his orange tube was coated.


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greju
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5540700 - 11/26/12 05:24 PM Attachment (29 downloads)

*** "Classic" or "Used?" - Used

*** Torrance, Vixen Japan, or China? - Not enough info

*** What might it be worth? - Alot of acc's and nice case, $150.00-200.00

*** Lucky find, or best to pass? - Only you can say, it is not a "classic" and it is not really made for astronomy. Might have good optics though.


I prefer the ones that still used "International" in the name. And a LAR that is the real deal. Try finding one for $25.00 though.


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greju
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: greju]
      #5540702 - 11/26/12 05:27 PM Attachment (29 downloads)

It has been said the optics on the early ones are comparable to Questar. I know they are good.

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: greju]
      #5540733 - 11/26/12 05:57 PM

It would be clearer of me to photograph scopes with only the parts included in the sale. This one actually sells with no accessories other than the case, the lens cap, the yellow rubber cover for the back, and the finder. The diagonal, eyepieces, and tripod are mine. How much does that change the notion of a $150 to $200 value?

No question, this is a birding scope suitable for only basic astronomy duty. It would be suitable for carrying to birding sites, and perhaps for carrying about the yard, dodging the many trees, practicing finding objects already seen in binoculars.

I was out with it this afternoon, testing it on boat houses across the lake 3/4 and 1 1/2 miles away, and was surprised to find myself watching loons stretch their flippers out of the water. Their molting plumage was clearly changed from breeding to winter. Pretty amazing for so small a scope!


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5540910 - 11/26/12 07:52 PM Attachment (30 downloads)

Here I am about to do some sky sweeping for targets. (once it gets dark of course!)Anything I stumble across is game for a check out look see. It can get fairly exciting if I catch a satellite and track it. This set up is probably the favorite thing I do with my C90. Wide field 25mm eyepiece for around 40X. You will notice I have no finder, I'm not trying to find anything. I just want to be lost in space!

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
C90 is "Used" Indeed! Missing the Visual Back. new [Re: Joe Cepleur]
      #5540911 - 11/26/12 07:52 PM Attachment (33 downloads)

Please could someone confirm that the photograph below shows the problem with this scope?

The seller told me that the man who sold it to him had "drilled it... to accept 1.25" eyepieces, or something like that... there are problems with it." The scope has two holes drilled near each other at the end of the baffle tube. One of them has a retaining screw, for gripping a 0.965" diagonal from a refractor. That's the problem! The C90 requires a SCT-style visual back that screws onto the threads on the outside of the baffle tube. A refractor's diagonal is too small when stuffed in there, and so is both off-center and wiggly. The result is a miscolimated scope with weird vignetting at low powers, that never quite focuses correctly, and has a cat's-eye shaped field stop.

Is the visual back for this scope readily available, or an expensive rarity on the used market? Ducky62 so valued the...

Quote:

Large Accessory Ring... the Celestron part that allows the use of 1.25" accessories.




because that's an even better option.

Probably someone bought this as a camera lens (judging by the cutouts in the case), and then later decided to convert if for astronomy, but did not know how to do it right. The optics can not be properly tested without the right visual back, which surely lowers the price I'd pay.

Anyone know where I might buy a visual back or Large Accessory Ring, and how much it might cost?


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Ducky62
professor emeritus


Reged: 10/31/10

Loc: The ATL
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: greju]
      #5540913 - 11/26/12 07:54 PM

Quote:



I prefer the ones that still used "International" in the name. And a LAR that is the real deal. Try finding one for $25.00 though.




You're not helping on the future availability of affordable LARs

Quote:

I'd add at least $25 for a LAR




I was trying to be a bit more subtle


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Joe Cepleur
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Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5540918 - 11/26/12 07:57 PM

Quote:

Here I'm about to do some sky sweeping for targets. (once it gets dark of course!)Anything I stumble across is game for a check out look see. It can get fairly exciting if I catch a satellite and track it. This set up is probably the favorite thing I do with my C90. Wide field 25mm eyepiece for around 40X.




Love it! That's exactly the setup I'd like for trotting about the yard. I see a well-mounted C90 not as a substitute for a C8 or even a classic refractor, but rather as a kind of uber-monocular that combines almost the convenience of binoculars with a much steadier view and higher powers.


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Joe Cepleur
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5540924 - 11/26/12 07:59 PM

So, I need a LAR and then the same visual back I'd use on a C8. Suddenly, this scope has gotten expensive. I'd have to buy it at a bargain-basement price.

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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5540961 - 11/26/12 08:17 PM Attachment (34 downloads)

Here's a picture of mine Joe, does this help?

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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5541053 - 11/26/12 09:11 PM

Joe from what your describing I wonder if the baffles have a problem.
Look at this site and go to regluing light baffle and also look closely at the end of the baffle tube in the pictures and compare to yours:
http://astronomyasylum.com/c90.html

Robert


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Joe Cepleur
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5541139 - 11/26/12 09:55 PM

Quote:

Here's a picture of mine Joe, does this help?




Let it be shouted from the rooftops:

*** JOE CEPLEUR WAS WRONG!!! ***

Your picture clearly shows the same configuration as mine. Same set screw in same place. No aftermarket, kludging debauchery wrecked my scope. Do you slip a 0.965" diagonal in there, the same sort that fits a classic refractor, and secure it with the set screw? or do you screw a visual back to the threads?

Okay; I was partly right. My diagonal is floppy, and that was a problem. Would an ordinary hybrid diagonal solve my problem? or would I be better served to find and screw into place a proper visual back?

Crawmach could machine a LAR, but custom work is pricey.


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greju
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541147 - 11/26/12 10:02 PM

Depends on how much you want to spend. A hybrid diagonal should work fine with the set screw. I wasn't seeing your "problem". Turns out there wasn't one. Great!

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greju
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541170 - 11/26/12 10:14 PM

Looking at yours there is that extra hole in the visual back and if your diagonal does not fit tightly some type of "debauchery" could have been done there. The diagonal that came with mine is not a SCT screw in type and fits in there snugly.

Edited by greju (11/26/12 10:17 PM)


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5541174 - 11/26/12 10:18 PM

Quote:

Joe from what your describing I wonder if the baffles have a problem.




Good thought, thanks, Robert; but, alas, the light baffle is not rattling around inside.


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: greju]
      #5541189 - 11/26/12 10:27 PM

Yes anything .965" is a sloppy fit in there.
I like using the .965" stuff with mine because it fits the small scale of the optical tube.
One thing I've been meaning to do is star test the little bugger! I can't believe I haven't, I guess everytime I use it I'm having too much fun.
Well I mention star test because Celestron probably has a optimum mirror spacing figured out for the C90. Or does it matter with the Maksutov? I suppose it would? The C8's optimum is when you use the supplied star diagonal with the supplied 25mm eyepiece, and infinite focus will set the mirror spacing to the distance with the lease aberrations.
When we start putting visual backs and 1.25" star diagonal etc. on the C90 we may be upsetting the apple cart.

Robert


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5541227 - 11/26/12 10:47 PM

Maybe it would help to wrap the diagonal's barrel in tape, to tighten the fit and center it better. I want to get rid of that cat's-eye shaped field stop and improve the focus.

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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541364 - 11/27/12 12:01 AM Attachment (43 downloads)

Is the cat's-eye shaped field stop in sharp focus or blurry?
There is definitely a problem somewhere. Can you see anything out of place looking through the tube without diagonal and eyepiece?
My baffle tube is melted, probably from solar viewing somehow I guess. Not bad, I marked it with a B in this picture. But check yours for any damage:


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5541707 - 11/27/12 09:20 AM

Quote:

Is the cat's-eye shaped field stop in sharp focus or blurry?




Sharp focus, but variable. Seems to be related to the diagonal being mis-aligned in that big hole.

Another problem is an odd darkening of the edge of the field at low powers, extending maybe 20% from one edge, with a dead straight, sharp line cutting a cord across the outside of the field. This may be a weirdness of the diagonal or eyepiece, or an aberration due to moonlight (unsure about that last).

Quote:

There is definitely a problem somewhere.




Yes; I think so.

Quote:

Can you see anything out of place looking through the tube without diagonal and eyepiece?




No. All looks perfect.

Quote:

My baffle tube is melted, probably from solar viewing somehow I guess. Not bad, I marked it with a B in this picture. But check yours for any damage:




Again, all looks perfect.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541745 - 11/27/12 09:46 AM Attachment (45 downloads)

The Hybrid diagonal works for me for both visual and I just slide my SLR with 1-1/4 nosepiece into the diagolal to take a photo.
Harry


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: H.S.]
      #5541747 - 11/27/12 09:48 AM

Forgot, the above photo is a Coyote at about 200 feet.
Harry


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Joe Cepleur
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: H.S.]
      #5541765 - 11/27/12 10:05 AM

Quote:

...the above photo is a Coyote at about 200 feet.




Amazing! Whatever its compromises, the C90 is a astonishing beast. Weighs nothing, occupies almost no space, yet allows counting the hairs on a coyote's nose from far away.


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541856 - 11/27/12 10:56 AM

Joe a star test may show a problem.
Your not concerned with optical quality right now just the optical alignment.
Try it without the diagonal first. Defocus a star and see if the diffraction pattern is round in the center of the FOV. Start with the pattern in the center of the Field and then check the pattern from positions around the edge like 12:00 6, 3, and 9.
Look for any deformity in the pattern when at the positions around the edge.
You can also use the sun reflection off a distant chrome bumper or power line insulator.

Robert


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5541941 - 11/27/12 11:48 AM

Quote:

Joe a star test may show a problem... Try it without the diagonal first.




Yes, a star test would tell plenty, when I can get one. So far, only specific combinations of my diagonal, 0.965"-to-1.25" adapter, and eyepieces have worked. I may need to borrow parts to get a star test.

Quote:

Your not concerned with optical quality right now just the optical alignment.




Yes; I think that's so. I suspect the optics are good. Just a hunch, based on the kinds of problems I see.

Those three screws on the back of the primary mirror cell... do they affect collimation? I *promise* not to mess with that unless it is absolutely clearly necessary!


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541986 - 11/27/12 12:08 PM

I don't know what the 3 little screws are for. I think you are right though collimation.
If I remember right the meniscus lens up front just has 3 tiny little set screws holding it in the tube, I don't think any adjustments can be made there but a blow to the meniscus could knock it out of alignment very easily.
All these things will show up in a star test.

I went back and looked at your photos and I'm not sure what is holding the meniscus in the black tube. I don't see the set screws on the outside.

Robert

Edited by actionhac (11/27/12 12:17 PM)


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CharlieB
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5542061 - 11/27/12 12:49 PM Attachment (32 downloads)

Quote:

I don't know what the 3 little screws are for. I think you are right though collimation.
If I remember right the meniscus lens up front just has 3 tiny little set screws holding it in the tube, I don't think any adjustments can be made there but a blow to the meniscus could knock it out of alignment very easily.
All these things will show up in a star test.

I went back and looked at your photos and I'm not sure what is holding the meniscus in the black tube. I don't see the set screws on the outside.

Robert




The three screws in the back are not for collimation - they are too short. I have one of the orange C-90 Astro models that I replaced those screws with longer nylon bolts and nuts that CAN be used to move the primary. I had my mirror recoated and while it was apart, I figured that weak spring that is supposed to center the primary might not be the best way to accomplish the task. With the new nylon bolts, you can nail the collimation and it stays in place (assuming you have no issues with the secondary). I also flocked the inside of the tube for good measure. I use .965 eyepieces, but I do have a LAR for it. I seem to recall reading there are two slightly different LARs and they don't always fit properly. With the wedge and motor drive, this makes an incredible little portable scope.


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Pierre Stromberg
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5544878 - 11/29/12 12:35 AM

Joe, I'm just coming into this conversation but wanted to let you know that I purchased used the exact same black C90 from someone on Craigslist for 100 bucks. I use it primarily as an autoguider scope so it doesn't get stringent use from me.

When I've used it for observations, I was pretty impressed. Very sharp images of planetary objects. The helical focuser was a bit annoying at first but now it doesn't bother me much at all and it retains focus quite well. I wouldn't recommend the C90 for DSLR imaging because the small baffle leads to severe vignetting. But overall, for 100 bucks, it's quite a sweet little scope.

Like others suggested, get a LAR (large adapter ring) so you can use other Celestron accessories.

Pierre


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Joe Cepleur
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Pierre Stromberg]
      #5553056 - 12/03/12 11:02 PM

Is there an official way to collimate these old C90s? I saw the retrofit way described in this thread, but would happily try something easier first. Images of stars are not sharp. Daytime images are partly clear, partly slightly fuzzy in different parts of the field (not due to objects being at different distances). Seems it should be able to be better than it is.

What a lesson in the importance of contrast! Images are bright, but not vivid, the compromise for having so small a scope.

Fun gizmo. Hate the finder. Even a tiny scope needs a right angle finder.

Needs a good tripod and mount. Any recommendations? Homebuilt is in the budget, but Vixen Porta Mini might be great. Other ideas? Something lightweight and portable, good for birding or light astronomy.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5553488 - 12/04/12 07:50 AM

Quote:

Is there an official way to collimate these old C90s? I saw the retrofit way described in this thread, but would happily try something easier first. Images of stars are not sharp. Daytime images are partly clear, partly slightly fuzzy in different parts of the field (not due to objects being at different distances). Seems it should be able to be better than it is.

What a lesson in the importance of contrast! Images are bright, but not vivid, the compromise for having so small a scope.

Fun gizmo. Hate the finder. Even a tiny scope needs a right angle finder.

Needs a good tripod and mount. Any recommendations? Homebuilt is in the budget, but Vixen Porta Mini might be great. Other ideas? Something lightweight and portable, good for birding or light astronomy.




Believe me, collimating these little scopes is not for the faint of heart, and it is fairly rare for one to be out of collimation. You can't tell by how "sharp" stars are, you must observe the diffraction rings of a just slightly out of focus star under good seeing with said star centered.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: rmollise]
      #5553822 - 12/04/12 11:43 AM

Quote:

Believe me, collimating these little scopes is not for the faint of heart, and it is fairly rare for one to be out of collimation. 




Thanks for the tip, Uncle Rod! Sounds as though I don't need to worry about collimation, and would not want to learn it anyway -- at least not while testing a fellow club-member's scope. 

Quote:

You can't tell by how "sharp" stars are, you must observe the diffraction rings of a just slightly out of focus star under good seeing with said star centered. 




I was thinking in simpler terms. Often when people bring their Newts to star parties, the images are quite blasé, with a quick collimation making them far better than their owners had ever seen or imagined. I wondered whether miscollimation contributed to the imperfections here. 

To reach focus, the scope requires the length that the diagonal adds between the end of the baffle tube and the eyepiece. At this stage, I should either find a known good diagonal, or perhaps a plumbing fitting that would allow straight-through viewing. Ruling out bad accessories would help. 

As soon as I can get to the club to borrow a suitable mount, I'll report on a star test. 

Uncle Rod, calling the C90 "Little Kitty" in your blog was memorable. I know that referenced the classic, orange-tube astro version, but it was still a C90. 


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5553848 - 12/04/12 12:03 PM

The 3 screws in the front of the tube are to HOLD THE MENISCUS(front glass) IN! don't mess with them, Those usually have pretty decent optics, did you let it cool off long enough?

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: orion61]
      #5553866 - 12/04/12 12:15 PM

I've used the scope both when it was surely cool and when it may not have been. I think I need a really clear night, a certain cool-down period, a solid mount, and a new diagonal or straight-through pipe to test this further. It also sounds as though it would be easy to re-lubricate the helical focuser, so I'll do that. Everyone says these scopes should be good, so whatever is off, this one likely is, too.

For all its imperfections, such a potent little kitty sure is seductive! Love the idea of a miniature, if limited, powerhouse.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5554452 - 12/04/12 06:56 PM

Joe,

You can get an LAR here:http://www.cncsupplyinc.com/index.htm?etxstuff.htm for $29.95 plus shipping.

I bought one for my black C90 earlier this year and it works fine.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: dbledsoe]
      #5554475 - 12/04/12 07:15 PM

Quote:

Joe,

You can get an LAR here:http://www.cncsupplyinc.com/index.htm?etxstuff.htm for $29.95 plus shipping.

I bought one for my black C90 earlier this year and it works fine.




Perfect, thank you! Had no luck Googling for that part, so your referral is truly important.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5558535 - 12/07/12 12:11 AM

Following the link to CNC Supply and emailing the owner (who is amazingly responsive and deserves our business), I found a likely better solution than even the LAR. The site does not allow direct links to specific items (bad programming; hope they fix it), so click the link in its left sidebar for this item:

Rear Port Adapters (Visual Backs) ETX/C90 (34mm)

It screws on to the same threads as the LAR, but accepts 1.25" accessories directly, without the added weight or excessive back focus of large accessories, such as the visual back from a C8. It is better than a 0.965"-to-1.25" adapter because it allows the inserted diagonal to sit about 1/4" shorter in the optical train, and it properly centers and squares the eyepiece. A lot of the problem I've had focusing is related to the floppy fit of 0.965" accessories in the baffle tube. So, this adapter should help the C90 to focus.

I have a sense this is an exceptional small manufacturer. I have a hunch he'd work with us to produce all the adapters we'd ever need, including those coveted gizmos that screw directly into classic 0.965" drawtubes to natively accept 1.25" eyepieces.

It also helps to better understand the odd illusion this scope creates. If I look at a fence or window frames or clapboards a mile away with the C90, they look pretty sharp. The bushes with leaves down for winter look hazy, because their many fine branches are too small to resolve.

Still need to lubricate the focuser and perform a star test. After a long cool-down tonight, the skies turned cloudy.


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CharlieB
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5558800 - 12/07/12 08:16 AM

I used SuperLube on the focuser threads and it smoothed out the focusing and removed all of the focus shift I was experiencing in my older C-90.

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: CharlieB]
      #5560319 - 12/08/12 01:47 AM

I dragged mine out the other night for a few quick looks at Jupiter and M42. Fine focus is a PITA even though it feels perfectly smooth. I still want an Orange Astro (or better yet a brass Astro) but I'm thinking of getting rid of my black f/11. It has the nice Celestron International tackle box, a diagonal and .965 Kellners so I'm not in a real hurry. Anyone use theres as a telephoto? Early 80s Celestron marketing suggested it was a good lens for taking pictures of bikini babes on the beach

Edited by Ducky62 (12/08/12 01:51 AM)


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Ducky62]
      #5685565 - 02/17/13 05:19 PM

Upon reading about "sky-flooding" in another thread, a quick experiment noticeably improved contrast under most daytime conditions. I rolled a newspaper into a tube, and stuck it over the tube like a long dew shield, to block stray light. The newspaper is quite reflective, yet still got results. I'm going to try something similar with something less reflective, such as black construction paper or black felt.

Even with just the newspaper, the effect can be dramatic. Looking across the lake, the forest a mile-and-a-half away can be a wash of ill-defined lines. Pop on the shade, and the individual trees appear.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5685775 - 02/17/13 07:15 PM Attachment (27 downloads)

I'm experiencing "sky flooding" with my new Orion Mak 90mm.
I recognize the problem from my ATM projects, very similar to what happens if you build a refractor and not install baffles.
Its difficult to do anything internal on this new scope of mine but I'll try a lens shade for daytime like what you are doing Joe.
The baffle tube is quite far from the secondary baffle. I think your on target with the lens shade idea.
You can see how illuminated the inside of the baffle tube is in this photo from this angle:


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5686236 - 02/17/13 11:58 PM

An excellent photograph, ActionHac! Hard to capture the inside of these beasts. It shows the problem exactly!

Next, I'll try a better cheap material, such as black construction paper. If the idea works, I envision a good quality shade made from plastic sheeting lined with flocking, tightened around the tube with Velcro. This could be rolled for use or flattened for transport, and would add essentially no weight. The scope would be as portable with it as without it. The only problem comes from the necessary length of the shade. It catches wind, but no worries if it can't be used all the time. I may even make two, long for best shading, and short to catch less wind when necessary.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5686256 - 02/18/13 12:12 AM

The photo actionhac posted only shows the upper end of the primary baffle's inner wall being illuminated. This is not 'sky flooding.'

The latter condition results when light from off axis can pass through the corrector, just outside the secondary baffle, into the primary baffle and thence directly out the back end. In other words, from the focal surface, you could peer at an angle into the rear opening and see a crescent of *direct* illumination framed by the inner edges of the two baffles.

One could think of this condition as resulting from one or both the baffles being not quite long enough.

Actionhac should have aimed the camera so that the line of sight was as close as possible to the secondary baffle's inner edge, so as to reveal a clear line of sight all the way through the scope. If this is not noted, or if a view up the rear opening from well off axis does also, sky flooding is not an issue.

Sky flooding is more likely when the focus is set so as to be near the rear opening. The farther rearward the focus must be set (as when longer light-path accessories are attached), the less likely the problem.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5686285 - 02/18/13 12:33 AM

Thanks, Glenn; I always like to learn to be correct in my knowledge and terms. Still, even if the problem isn't "sky-flooding," a long light shade helps, so there seems to be some sort of problem with stray light.

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5686306 - 02/18/13 12:50 AM

I will look in the rear port tomorrow Glenn in the daylight and see if I can see any light coming straight though between the baffles.
I'll try looking at all angles and see if I can spot anything.
My C90 is fine but its a older orange tube Joe. The black tube like yours may have slightly different baffles.
My Orion has the problem. Very badly washed out with light, like over-exposed film.
Its exactly like a refractor with no baffles and a shinny unpainted tube interior. I've also seen the same problem with a shinny focuser tube.
I also want to make a lens shade, it could help and I need it for dew also.

Robert


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5686970 - 02/18/13 01:28 PM Attachment (21 downloads)

I can see around the secondary baffle when my eye is close to the rear port.
If I move my eye away from the port to the point of focus I can no longer see around the secondary baffle.
One problem I'm seeing is lots of reflected glare from the inside walls of the baffle tube. Check yours Joe. We need to kill that reflection. Some dead flat paint is in order I think, no room for Protostar light trap I can barely get my little finger in the baffle tube.
My drawing is inside focus, the focal plane is approx 4" away from the port.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5687525 - 02/18/13 06:42 PM

You got it, actionhac!

Indeed, even in the absence of sky flooding, an illuminated baffle wall is a source of scattered light which reduces contrast. A C90 I owned about 20 years ago had a thread-on metal lens shade, which helped.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5687602 - 02/18/13 07:16 PM

ActionHac, you are a magnificent artist. Well done!

When I looked at mine earlier today, I did not see directly past the secondary baffle, so I'll look again. But, I did see a blaze of reflected light in the shiny, black baffle tube, which the long sun shade dramatically reduced. Best performance may come from both a long sun shade and blackening of the baffle tube.

How does one get the right amount of paint into so small a tube without making a gummy mess?

Why did Celestron not paint them better at the factory?

I *really* want to like this scope. It's so portable. Perfect to keep in the car for birding, hiking, and whatever impromptu astronomy it can manage.


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5687714 - 02/18/13 08:18 PM

I've used this:
http://www.krylon.com/products/camouflage_paint_with_fusion_technology/
Its about as flat as I could find.
I spray it on a plastic lid off of a margarine tub or similar and use Q-tips to apply, the sloppier the better.
I've done this on Edmund focuser tubes with great success. I did find a water base artist paint that was actually flater but it didn't adhear as well as the Krylon.

Robert


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Datapanic
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5687847 - 02/18/13 09:25 PM

Over in the ATM Forum, there's a thread about Flockboard vs. Krylon Ultra Flat Black and a link was provided to the original non-"Camouflage" Ultra Flat Black 1602 which is said to be more flatter than the Camouflage mix. I've used both and haven't noticed a difference. Apparently, the 1602 mix was replaced with the Camouflage mix but still available, just not off the shelf...

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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Datapanic]
      #5687945 - 02/18/13 10:19 PM

Wow I didn't know you can still buy the 1602. I used the 1602 and when I couldn't find it I reluctantly started using the camo version and I'm OK with it. It is non-reflective and doesn't flake off those brass Edmund focuser tubes.
I really should look for a brush on can rather than spray.
One problem with the Q-tips I forgot about is the paint can pull the cotton fibers off the tip and you might have a few in the optical path. You can push em into the paint as it dries or pull them out.

Robert


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5688135 - 02/19/13 12:37 AM

Try a sponge brush instead of Q-tips. With a sponge, you can also dab it on and get a little texture going to help dissipate the light reflections.

I haven't really noticed a difference between 1602 and the newer Ultra Flat Black, but I haven't done any real comparisons either.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Datapanic]
      #5688699 - 02/19/13 11:26 AM

Sky-flooding, indeed!

I checked my baffle tube more carefully this morning, following ActionHac's model. Surely enough, removing the diagonal and putting an eye to the very edge of the open end of the baffle tube, looking past the secondary, I can see through nearly half the cross sectional area of the baffle tube. That is only the view from one point. Considering that one could see a similar view from any point around the open end of the tube, it appears the stock baffles do next to nothing. A vast ring of light slips past, spoiling the view with veiling glare.

I'll bet that properly blackened baffle tubes and long sun shades could turn old C90s from objects of occasional scorn and frequent questionability into the most wonderful portable scopes one could want. Why would they not be ideal for daytime birding, and better than expected for nighttime planetary and lunar viewing? Because mine is loaned to me, I may not be able to paint it, but I'll work on a better sun shade, and look forward to ActionHac's report on the effect of painting the baffle tube.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5689246 - 02/19/13 04:40 PM

Note that sky flooding always looks worse the closer to the back end and the farther off axis you view from. Technically, if such direct light is not seen from the edge of the field *at the focus for the actual configuration*, sky flooding is not in operation. If that outside-the-field light is illuminating a surface some distance ahead of the focus, some might scatter into the field as contrast-reducing veiling glare.

When set up for visual observation, you can assess the potential for contrast loss simply by examining the exit pupil with a 7-10X magnifier. If you have no such magnifier, use a 20-25mm eyepiece, either with the barrel removed or with the eyepiece in reverse.

You can do this indoors, with the scope pointed toward a well illuminated wall. And a light shining into the OTA from a variety of angles can supply some interesting results.

When examining the exit pupil under magnification, pay attention to the ratio of scattered light surrounding the exit pupil to the illuminated pupil itself. While this is difficult to estimate, if you suppose the scatter to amount to more than 5% (1 part in 20) of the direct ecit pupil illumination, then strategies to lessen it are in order.

You should consider primarily the zone surrounding the exit pupil which equals your maximum pupil dilation. Scattered light outside this circle does little to impair contrast, as it's intercepted by your own iris.

If your pupil dilates to, say, 6mm, you could initially lay a ruler across the exit pupil to see how large a zone surrounding the exit pupil you should concentrate on.

Particularly so for eyepieces having larger field stops, it's worthwhile to examine the exit pupil from angles far off axis, up to the limit imposed by the apparent field. You will be effectively looking up into the scope from off-axis viewpoints, on the focal surface, right up to the field stop.

For this is what the eyepiece does. It forms an image of the scope's innards as would be seen from the focal surface. You are essentially given a bug's eye view, if such a critter were sitting on something like a glass reticle supported by the field stop. As you increase the angle at which you peer into the exit pupil, it's like the bug walking toward the field stop.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5691372 - 02/20/13 05:24 PM

Glenn, I studied the exit pupil with my trusty 7x Bosch & Lomb Hastings triplet loupe. Not sure I'm doing this right, or what I'm supposed to see. By adjusting the placement of the loupe the tiniest bit, I can focus anywhere I want in the optical train, from the dust on the eyepiece to the secondary cell. No question, the walls of the baffle tube are brilliantly lit. In an ideally baffled scope, am I right thinking they should they dark, completely in shadows? With the simple sun shade attached, the walls of the baffle tube are darker, which explains the improvement in contrast from reduced veiling glare.

About that loupe: Is there a classics forum for loupe heads? It's an antique now, and I bought it new!


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5691532 - 02/20/13 06:52 PM

Joe,
You're doing it right! And you have indeed discovered how slightly adjusting the in-out position of the magnifier allows to focus on specific bits inside the scope.

Have you also tried peering into the exit pupil from a rakish off-axis angle? For large field stop eyepieces especially, you will note how your view becomes correspondingly offset, with at least some circular apertures becoming a bit offset with respect to each other. As I'd earlier stated, this has the effect of you viewing the optical system from the focal surface but offset toward the field edge. This is useful for seeing if there are any light leaks getting to the outer part if the field (if you look into the exit pupil from an angle near or equal to the semi-angle of the eyepiece's apparent FOV.)


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5691767 - 02/20/13 09:13 PM

Quote:

Have you also tried peering into the exit pupil from a rakish off-axis angle?... this has the effect of you viewing the optical system from the focal surface but offset toward the field edge... look into the exit pupil from an angle near or equal to the semi-angle of the eyepiece's apparent FOV...




Trying to picture this... You want me to see what an ant would see, were he walking along one edge of the eye lens (where it meets its housing), always peering under the opposite edge, along the barrel inside the eyepiece, between the eye lens and the eyepiece's field stop?

I'd like to learn that trick and see the effects of stray light, but let me guess: The solution will be the long sun shade and blackened baffle tube, which will erase whatever errors known and undiscovered could realistically be fixed?

This is becoming an incredibly interesting thread, because it points at a possible misunderstanding that may have become established myth among scope heads. People say, "Horrors; that scope has a 15%, or 20%, or 30% obstruction, so it's no good; too much lost contrast due to the obstruction." The effects of obstructions are well known, if not well believed. Small ones, in particular, do measurable yet little harm. What would you bet that the loss of contrast so often blamed on obstructions actually stems more from the veiling glare so often entering scopes with obstructions, particularly Cassegrains? The lion's share of the trouble likely stems from veiling glare, with obstructions unfairly taking the blame. This would weigh especially heavily on Maksutovs used without sunshades in daylight, such as the sometimes loved, oft maligned C90.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5692112 - 02/21/13 01:57 AM

Peering into the exit pupil from off axis with the magnifier is simple; merely center the exit pupil in the view while moving the magnifier in a swinging arc to one side. To first see what I mean, try this with a bino, without the magnifier, while holding the bino a couple of feet away. As you rotate the bino in your hand, your sight line becomes increasingly more angled. When that angle reaches 1/2 the eyepiece's AFoV, the exit pupil will become clipped to fairly sudden invisibility by the field stop edge.


The business of contrast among many amateurs is not well understood, perhaps due to confusion between the two sources you mention:

1) What I call *small* scale contrast, as affected by the central obstruction. Here, losses are restricted to within a few Airy disk diameters, and have the greatest impact for brighter targets at higher magnifications.

2) What I call large scale contrast, as affected by veiling glare. This impacts the observation of low contrast targets at all scales, where scattered light is spilling through much or all of the field.

I get the distinct impression that no small number of amateurs mistakenly think that a central obstruction introduces a degradation in contrast at *large* scale as well, affecting the views of all objects and at all magnifications.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5692450 - 02/21/13 10:07 AM

Thanks, Glenn! Chalk one up for ol' Joe Cepleur! I've been interested in the subject of contrast because our club's optician builds Newts cheaply that offer the performance of uber-costly APO refractors. He had me read Suiter's "Star Testing" to see how this was possible. Despite much of Suiter still rolling and roiling right past my head, it is clear that the optician is right: small obstructions make little difference (and more, as you say, at high powers). That does not mean that the legions of astronomers who are leery of bad contrast don't see what they see, so I've been searching for the answer to how the small losses from obstructions got such a bad rap. The C90 reveals the answer. People confuse the effects of obstructions with those of veiling glare. Quite exciting for me to have you second that!

To test how the inside of the eyepiece looks from so steep an angle, I'll first have to find an eyepiece that either lacks a deep rubber eyecup, or has one that is removable. A volcano top would be perfect. Does the focal length of the eyepiece matter? I've been using a 25mm Plossl, since 25mm is supposed to be ideal for the C90's mirror spacing. I could try my old, 16mm Brandon with a completely flat top, with the eye lens flush to its housing.

This much I saw upon this morning's attempt: The various rings of light are not concentric when I look straight into the eyepiece. It may only mean the the baffles are offset, but it could also mean there is a problem with collimation. That, in turn, may not be a problem with the scope, but perhaps with the diagonal. I'll play around with what parts I have and try to determine the answer.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5693132 - 02/21/13 05:00 PM

For off-axis, peering-into-the-eyepiece-at-an-angle testing, an eyepiece having the largest useable field stop is best. And this happens to occur with longer focal lengths, which moreover tend to have longer eye relief and hence a more accessible exit pupil.

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5696273 - 02/23/13 10:08 AM

I've made a crude, extended sun shade from two pieces of paper, one telescoping into the other, to test the necessary length. Peering directly through the baffle tube, the sun shade extends to 14.5" without its outer edge becoming visible. This suggests overwhelming sky flooding when the scope is used without a long sun shade. No wonder the C90 has a terrible reputation for bad contrast! I suspect that, after making a better long sun shade (blackened inside and truly cylindrical), I'll prove that the C90 is perfectly capable of acceptable contrast. The problem is veiling glare, and has nothing to do with the obstruction.

Whether it makes sense to drag around a sun shade that is longer than the scope remains to be seen! I suspect it will, because the entire system remains lightweight to carry, and the shade is easily stored with the tripod when the scope is in its case.

Still looking for a suitable long-focus eyepiece to test for glare at extreme angles.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5697907 - 02/24/13 08:57 AM

Don't know what may or may not have changed, but my Orange Tube most assuredly doesn't have poor contrast, revealing all the detail on Jupiter I'd expect from any scope in this apertue. That said, a dew shield is a good thing.

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: rmollise]
      #5698232 - 02/24/13 01:05 PM

Quote:

(M)y Orange Tube most assuredly doesn't have poor contrast, revealing all the detail on Jupiter I'd expect from any scope in this apertue.




Uncle Rod, have you tried your C90 during the day? I'm finding that contrast is quite terrible when the image is washed out by veiling glare in daylight.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5698334 - 02/24/13 02:04 PM

I have just finished building the improved 14.5" sun shade. It is made of two layers of poster board: a thick, sturdy, outer layer (which happens to be gloss white--good for finding the scope in the dark!), and a flat black lining. Now, the baffle tube and interior of the eyepiece are DARK, except for the inside of the eyepiece's field stop. I suppose that is normal, because the field stop is meant to be overshot by a bit of light on the objective side?

Peering through the screen porch to the white-out snowstorm outside, I am unable to test the scope just now, but will report as soon as I can.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5698673 - 02/24/13 05:31 PM

For such a bright target as Jupiter against a dark sky, a Cassegrain system could probably even do without baffles at all. Seriously.

Jupiter has a surface brightness of 5 magnitudes per square arcsecond. Even a bright suburban sky is about 19 MPSAS. Jupiter therefore is brighter by 14 magnitudes, or a factor of 400,000. That's huge contrast!

We might consider veiling glare as becoming problematic when additional, non image forming light overlying the subject has a surface brightness about 10% that of the subject. This means that for Jupiter, this veiling glare must have a surface brightness 1/10 as bright as 5 MPSAS, which is 2.5 magnitudes fainter, and hence of surface brightness 7.5 MPSAS.

How much sky glow must be taken in so as to result in a veiling glare of surface brightness 7.5 MPSAS? With the sky at 19 MPSAS, we see that it must become brighter by 19 - 7.5 = 11.5 magnitudes. This is a brightness ratio of just about 40,000.

Veiling glare more or less fills the field uniformly when baffling had not been employed. And so the full field must have the sky brightened by a factor of 40,000, in order that Jupiter be seen through a veil of light having 1/10 its surface brightness. And this sky light must come from an extent of sky having 40,000 times the area of the field of view. If the FOV is, say, 0.25 degree, an area of sky 40,000 larger is a diameter of about 50 degrees. As 'seen' from the focus, the angular width of the front corrector of a compact Cass or Mak-Cass is about 20 degrees, which limits the angle of sky contributing to veiling glare accordingly. Twenty degrees of the necessary 50 is an area about 1/6 that required, meaning the veiling glare is 1/6 as bright as required in order to be problematic.

To be sure, *any* non image forming light is unwanted. Hence the installation of baffles to minimize the amount of light from outside the designed maximum field of view reaching the focal surface directly.

Once this is accomplished, the next important step is to make the illuminated portions of the baffle walls, which can be 'seen' from the focal surface, as non-reflecting as possible. So-called micro baffles are usually the best solution.

In the C90 I once owned, the primary baffle was made of shiny plastic. At the inner opening was a kind of 'cap' with a hole somewhat smaller in diameter than the baffle tube's I.D. This was an attempt to reduce illumination of the inside of the primary baffle, not fully successful.

I did determine that with rear accessories removed I could see from well off axis that direct sky light was getting through, but don't recall looking for this with diagonal and eyepiece in place. My best recollection has it that sky flooding for even the widest field was not evident.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5699378 - 02/25/13 07:49 AM

Quote:

Quote:

(M)y Orange Tube most assuredly doesn't have poor contrast, revealing all the detail on Jupiter I'd expect from any scope in this apertue.




Uncle Rod, have you tried your C90 during the day? I'm finding that contrast is quite terrible when the image is washed out by veiling glare in daylight.




I did when I first got it, and if memory serves, it seemed fine.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5699379 - 02/25/13 07:51 AM

Quote:

For such a bright target as Jupiter against a dark sky, a Cassegrain system could probably even do without baffles at all. Seriously.






Well...that's what I tend to look at with a C90: bright objects against a dark sky.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: rmollise]
      #5699630 - 02/25/13 10:54 AM Attachment (17 downloads)

My C90 is fabulous during the day. Thats why I just don't understand what is wrong with yours Joe.
Now my Orion 90Mak has a problem, and its similar to your C90's problem.
When I look in the rear port I can see the reflection from the inside of the baffle tube when I hold the scope up to the sky. This glaring reflection is washing out contrast.
If I do the same with my C90 I don't have the reflection from the baffle tube walls because the sky end of the baffle tube is partly closed off like a field stop. Glenn mentioned this about the baffle tube in a post above.
I have read where photographers have enlarged this hole. Look at yours Joe and compare it to this picture:


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5699830 - 02/25/13 01:09 PM

Good pic; it nicely shows the 'reduced' baffle opening as it was on my old C90.

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5699915 - 02/25/13 02:09 PM

Mine also has the "reduced baffle opening" as shown in ActionHac's picture. Weirdly, it also shows what appears to be the cut end of the metal baffle tube (inside, beyond where the plastic parts are attached). It's a shiny, silvery ring, as though a long tube had been painted black before shorter sections were cut and assembled, leaving an unpainted cut visible from the meniscus. This highly reflective surface is not visible from the eyepiece, but may contribute somewhat to light bouncing off the back side of the "reduced baffle opening." I'd say it would not explain the full extent of the problem.

I have read that based on its obstruction, the C90 should have the resolution of an unobstructed 3.5" scope, the brightness of an unobstructed 3.25" scope, and the contrast of an unobstructed 60mm. Resolution and brightness appear to be good, but the contrast is more what one sees through a 60mm scope at high power, with the image dimmed and lost in its own shadows.

Currently, with the sunlight evaporating newly fallen powder snow off the lake, I can't get a good look at anything far enough away to test my new sun shade truly well. A quick look through the screen porch and heat waves suggests it makes some difference, but maybe not as much as I had hoped. This would, sadly, be exactly in keeping with Glenn's impressive calculation. I hope this doesn't mean I have to call Roland and tell him I've canceled the licensing agreement for Astro-Physics to manufacture Cepleurean paper sunshades.

This is... baffling. I wish there were a contingent of old-style C90 owners locally, so I could see through their scopes to know what to expect. It's not that I can't see anything, but that the image is far less impressive than I would expect from a 3.5" scope.

Next idea: With what is currently in my kit, I can only bring the scope to focus through the Towa 0.965" diagonal that came with my 60mm Jason 313 refractor. My 1.25" diagonal yields a noticeably better image through the Jason, but not so radically better as to likely account for the wash-out seen through the C90. Any votes whether the diagonal could nevertheless be contributing to the problem?

This scope works, but its owner said it had "problems." He did not know what they were, but paid accordingly little for it. As a photographer, I can say that I would not want this on my camera as a lens. Tinkering with scopes so favors the determined mind, perhaps even the compulsive streaks hiding in the personality that can be darker in other contexts. Still, it's such a fascinating puzzle. I'd love to figure out what's wrong.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5700217 - 02/25/13 04:41 PM Attachment (16 downloads)

I'm not sure what you mean by the cut shinny end Joe.
I do have a thought though. Make sure the secondary baffle has not slipped on the inside of the meniscus, it should be glued directly over the aluminized spot like this photo and not exposing the spot edge outside the baffle:


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5700241 - 02/25/13 04:59 PM Attachment (13 downloads)

From the meniscus end if you stand back a few feet everything you see should appear concentric when your eyes are directly in front of the telescope, you may just use one eye as a matter of fact, and close the other one. This off centered shot is showing the basic parts.

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5700354 - 02/25/13 06:21 PM

Thanks, ActionHac! That's my mission for daylight. (It's dark here now.)

Another thought: I took the scope outside today, to test it without looking through the window and screen. From within the shaded woods, my sun shade made only a slight difference. If it's sunny tomorrow, I'll take it out onto the frozen lake, where there is tons of glare. Maybe the trouble comes from excessive glare entering the scope. There may be two ways to protect the view. Sight from within the shaded woods, or use the sun shade on the open ice, with excessive glare attacking from all sides.

Glenn, your calculation was based on the amount of light needed to hurt contrast on Jupiter, which implies viewing at night. Does it also apply to viewing during the day? Surely bright sunlight is far brighter than any skyglow or street lights.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5700479 - 02/25/13 07:50 PM

1) If I understand correctly, Joe, the shiny cut end you described as seen from the front end only and lying a short distance behind the 'reduced' aperture on the primary baffle, should pose no problem because it is not visible from the back end.

2) Regarding the seemingly persistent, poor contrast. Have you verified than no elements have any kind of 'haze' on a surface?

3) The quoted equivalence on contrast, whereby this obstructed 3.5" is like that of an unobstructed 60mm, applies to the small scales made visible at higher powers, and being limited to about a few Airy disk diameters. This in no way is like the effects of veiling glare, which operates at FOV scales.

4) My previous analysis of Jupiter and veiling glare from the night sky is a 'special' case, where the subject is enormously brighter than its surroundings, the latter of which is too dim to contribute injurious veiling glare, even when no baffling is installed.

For a 'general' scene, where what is seen in the FOV is of similar brightness as its surroundings, well, this is an entirely different matter! Whereas with Jupiter it required the light from 40,000 (!) times more sky area than contained by the FOV to flood the field, in the 'general' daytime scene it requires only 0.1 (!) times the outside-the-field light to flood the field. Hence the need for very effective baffling.

And the baffling, from the standpoint of sky flooding, is effective when no direct, non image-forming light can get through to the edge of the largest field stop (or sensor corner) anticipated to use. It then remains to 'trap' any such light which falls outside the field and illuminates other bits such as inner baffle tube walls, front barrels on diagonals, etc., by minimizing the scatter therefrom into the field of view.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5700807 - 02/25/13 11:39 PM

Quote:

1) If I understand correctly, Joe, the shiny cut end you described as seen from the front end only and lying a short distance behind the 'reduced' aperture on the primary baffle, should pose no problem because it is not visible from the back end.




Yes. I could try to get a picture as good as ActionHac's. If this reflection is a problem, it is only a small problem. The ring may reflect light off the back side of the baffle tube's reduced baffle opening, which appears to be shiny, if black, plastic-- not a wholly convincing non-reflective surface.

Quote:

2) Regarding the seemingly persistent, poor contrast. Have you verified than no elements have any kind of 'haze' on a surface?




Haven't noticed any haze, but that warrants a good look in daylight.

Quote:

3) The quoted equivalence on contrast, whereby this obstructed 3.5" is like that of an unobstructed 60mm, applies to the small scales made visible at higher powers, and being limited to about a few Airy disk diameters. This in no way is like the effects of veiling glare, which operates at FOV scales.




I know that I do not wholly understand the relationship and brightness, but I had tried to describe how the scope's image looks much like a 60mm refractor's at high power. Surely you've seen this? The image becomes dim, so the contrast is lost. There just is not much difference between light and dark when everything becomes dark. And yet, the image is not dark. If just looks washed out.

Quote:

4) My previous analysis of Jupiter and veiling glare from the night sky is a 'special' case, where the subject is enormously brighter than its surroundings, the latter of which is too dim to contribute injurious veiling glare, even when no baffling is installed.

For a 'general' scene, where what is seen in the FOV is of similar brightness as its surroundings, well, this is an entirely different matter! Whereas with Jupiter it required the light from 40,000 (!) times more sky area than contained by the FOV to flood the field, in the 'general' daytime scene it requires only 0.1 (!) times the outside-the-field light to flood the field. Hence the need for very effective baffling




Or, the need for a Cepleurean paper sun shade. I'll call Roland again, to tell him we're back in business! It is amazing how little daylight is needed to overwhelm a poorly baffled telescope.

Quote:

And the baffling, from the standpoint of sky flooding, is effective when no direct, non image-forming light can get through to the edge of the largest field stop (or sensor corner) anticipated to use.




By my measurements, the C90 requires a Cepleurian sun shade extending fully 14.5" past the end of the tube. Any less, and one can see from the end of the baffle tube past the secondary. Of course, the light path is actually longer, through the diagonal and ocular. Does this suggest a shorter Cepleurian sunshade may be equally effective? I'd say not, if the idea is to wholly darken the baffle tube.

Quote:

It then remains to 'trap' any such light which falls outside the field and illuminates other bits such as inner baffle tube walls, front barrels on diagonals, etc., by minimizing the scatter therefrom into the field of view.




So, it's normal to see light on the inside of the field stop, but that light must somehow be trapped before it harms the view?

All rather complicated, but I love learning about this stuff, and I plan to be victorious at least in understanding this scope, if not in fixing it.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5701376 - 02/26/13 10:43 AM

Quote:

Make sure the secondary baffle has not slipped on the inside of the meniscus, it should be glued directly over the aluminized spot like this photo and not exposing the spot edge outside the baffle:




Good thinking, ActionHac! If the baffle were shifted, the secondary could mis-direct a lot of light. I checked. My baffle is fine.

Eyeballing the collimation is another matter. Quite a difficult task. Because the mirrors magnify, the slightest shift of my head makes it difficult to assess how well everything aligns. I'll try resting the scope, and my head, on a bed to steady the view. For now, I can only say collimation may be correct and is not disastrous, but I can not say it is perfect.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5701684 - 02/26/13 01:31 PM

Joe,
Your Cepleurian shade could be made a bit shorter if you install at its front end a ring whose opening is equal to or just slightly larger than the corrector's clear aperture. This ring would also have the benefit of somewhat shadowing the inside of the shade itself, making it yet darker. And you could consider adding similar ring baffles behind the frontmost one, to even more effectively shadow the inside surface.

It seems like you wish to go further than merely block sky flooding, extending this to exclude illumination of the interior of the primary baffle. I assume you know how to check for this, but here's my suggestion anyway.

With shade installed, while peering past it into the front end, you must not be able to see any part of the primary baffle's opening, even when your sight line is grazing the shade's edge.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5702308 - 02/26/13 07:16 PM

Glenn, your diligent follow-through on my project makes it fun for a young foagie to try to utterly optimize an old C90! Thank you. It's amazing how much I can learn from projects such as this. Such fun! I am becoming an alchemist of optics, making silk purses from sow's ears. Anyway...

Quote:

Your Cepleurian shade could be made a bit shorter if you install at its front end a ring whose opening is equal to or just slightly larger than the corrector's clear aperture.




Good idea. I see how that would work. Given the greater difficulty of building such a shade, is it likely to make a dramatic difference? Certainly a worthy experiment, and an intriguing problem of design and manufacturing. Rolling a paper tube is easy. Adding rings must somehow become so!

Quote:

With shade installed, while peering past it into the front end, you must not be able to see any part of the primary baffle's opening, even when your sight line is grazing the shade's edge.




That is exactly how I measured the necessary length of the shade. I nested two paper tubes one into the other, popped them over the tube, and slid them along each other to adjust the length until I could just no longer see the edge of the shade from the edge of the baffle tube.

The shade is, as you implicitly note, actually the diameter of the telescope's tube, meaning it is larger than the meniscus. Funny Roland didn't mention that when we discussed licensing.

We had a bright, sunny day, but I spent it overseeing repair of a frozen sewage system. C'est la vie! No further tests today.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5703291 - 02/27/13 11:12 AM

Checking shade length by viewing through the rear port only assures sufficient length to stop direct sky light getting out the back end.

By looking into the *front* end, you adjust shade length until you cannot see the primary baffle opening, the latter remaining completely hidden by the secondary baffle as your line of sight grazes the edge of the shade. This ensures that absolutely no part of the inner wall of the primary baffle can be directly illuminated by outside-the-FOV sky light.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5704336 - 02/27/13 09:43 PM

Oh, my! That sounds like it's the makings of one long, lllloooonnnngggg sun shade! Makes sense. I'll study my creation, and maybe redefine Cepleurean style! It would be interesting to see whether the neo-Cepleurite would work noticeably better than the peleo-Cepleurite. That would be a real lesson in contrast!

Would this mean that the sun shade would be of the right length if, when shining a light into the eyepiece, I could not see the light directly when looking through the meniscus? (Edit, I hope better phrased: When peeking past the secondary baffle from the meniscus with a bright light at the eyepiece, any opening in the primary baffle appears in the shape of a brilliantly lit crescent moon. Would it be okay if the light at the eyepiece illuminated the entire tube with ambient light from my point of view, so long as I could not see even the slimmest crescent at the primary baffle?) It would seem so, because this method merely highlights the baffle tube's opening on the objective end. Such a design may be impractical, but I'd like to try it anyway.

Interesting problem of manufacturing: It would be difficult to keep so long a shade properly aligned with the scope, so as not to partially mask the sky. If it drooped, it would block light from reaching the meniscus.

Edited by Joe Cepleur (02/28/13 08:54 AM)


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5711998 - 03/04/13 07:03 AM

All these days later, the clouds and snow have not parted long enough for a thorough test, but I did haul the scope outside yesterday briefly, in the cloudy late afternoon. The long, Cepleurian sun shade shade remains as it was, no longer and still unbaffled.

The long shade clearly improves contrast, but at least in the light as it was at the time, not so radically as I might have hoped. Colors are richer with it, as though a slightly frosted filter had been removed. Oddly, lines were sharper with the shade, an illusion, most likely, stemming from the improved contrast.

Even without adding baffles or additional length, the shade blocks the vast majority of interloping, undesired light. The contrast seemed worse on the sunny days of fall. I suspect the brighter the ambient light, the worse the contrast this scope shows. Perhaps that is... normal? The image just seems so flat and washed out!


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5712167 - 03/04/13 09:49 AM

Hello, Joe.

Responding to your PM, but I don't know if I want to wade through the entire thread to see what has been discussed and what has not been discussed so I will just enter in "Blind."

First, I have owned an older orange C90. Daytime contrast was poor. I also did not like the helical focuser. In every way, I found it inferior to my C5 and though it was a bit smaller, it was only a bit smaller, so I sold it.

But here is what I found.

First, I took it apart and cleaned the focuser, which improved things, but it was really tedious to use. The standard C5 mechanism is far better.

I also cleaned the primary mirror. It may have been that over the years there could have been some film or something that developed, and there was a bit of dust.

This improved things, but only a tiny bit.

The rear of the corrector appeared to be uncoated. I attribute much of the daytime issues to this.

The reason that coating the rear of a lens is important is because it lets light back out. Most people think of the primary role of coatings as being to increase transmission, but they also serve the very important role of letting light that reflected back towards the rear of a lens (or Meniscus, or Schmidt Corrector) to exit the system so that they do not reflect back to the focal plane.

I attributed much of the poor daytime performance to this lack of coatings on the rear of the corrector. Any off axis light that enters the aperture and is reflected off of the end of the baffle, the rear of the mirror cell, or any other place but the mirror can reflect back to the rear of the Meniscus and then back to the primary and back to the secondary and so on.

This is why Celstron started coating the rear of the corrector of their SCTs a few decades ago.

The star test also revealed considerable higher order spherical abberation.

This is to be completely expected. It is impossible to build an all spherical MCT with aliminized spot secondary faster than about f/15 without accepting some HSA, and at f/11, there is a farily large amount of higher order sphecial aberration (HSA).

Damage for HSA is more concentrated at the lower spatial frequencies, and damage from LSA at the medium frequencies.

Even at f/12.5 (many smaller commercial MCTs are built this fast), the HSA componenet is large enough to prevent the instrument from having a strehl higher than .95 or so even if the optics are otherwise perfect.

A large central obstruction can reduce the effects, but only at the cost of lowering mid frequency spatial response. The designer picks an obstruction size that attempts to "Balance" these factors. Big enough to hide most of the HSA, but not so be that by itself will totaly destroy contrast. But it doesn't eliminate it. It would have to be to big to do so and what would be the point of that if the obstruction itself became to large.

So, my guess is that you are seeing a combination of a lot of bad things.

Reflections off of the rear of the Meniscus scattering light across the face of the primary (not a serios problem at night), a largish central obstruction intended to balance out the very large HSA component of such a fast aluminized spot MCT, and the un-masked HSA, which lowers contrast at the lower (top left) part of an MTF plot (LSA lowers it around the middle, so you combine these and have a system that starts to loose contrast on detail even at 2 line pairs per millimeter, which is pretty sad).

Bottom line? While compact and capable of providing very low power utility use, The sample I owned was completely unsatisfactory for my own needs. I quickly sold it.

My C5 was light years better.

So, I think it may be a combination of these factors. The rear of the Meniscus may be uncoated (mine did not appear to have any coatings on the rear, and this is not nearly as big a problem for night time usage).

The design has severe HSA which lowers contrast at the lower end of the spatical frequency spectrum (large detail) and a large central obstruction (that lowers contrast near the middle of the Spatial Frequency).

As a spotter, these things may not matter so much.

To more critical astronomical telescope users, it will indeed be noticable.

Only my opinions.

The book Star Testing Astronomical Telescopes whas my reference for the details on Higher Order Spherical Abberation, and I have extrpolated from the authors example for the f/12.5 design being limited to .95 Strhel (as opposed to a design that has taken steps to eliminate or better control HSA).

I hope this has been helpful.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Eddgie]
      #5712304 - 03/04/13 11:11 AM

Thank you, Eddgie;

Yes; that was immensely helpful. The owner reports that the dealer who sold this C90 to him said there were problems with this particular scope, although it was useable. Let's suppose we had a hundred C90s to compare. Maybe this one would be among the worst, yet that also misses the point. It may well be a run-of-the-mill, not-so-good C90. Most of its failings are likely due to its being what it was designed to be, with its essential flaws to be expected. This does not mean it's a bad scope in some absolute sense. It is, instead, a profoundly compact scope, with attendant problems of contrast and clarity. At the right price, it may make sense when easy carrying trumps clarity of the view. For birding, it might be a good choice for carrying up a mountain, or for hiking far into the woods.

Reading your response, I had to wonder whether it points toward the reason so many people prefer the Questar 3.5" to the C90, despite all the double-pass auto-colimation tests saying the C90 should be comparable. Checking with our good friends at Company 7, the Questar is f/14.4 in visual mode. That's awfully close to the f/15 needed to avoid the problems you discuss, or at least diminish them. Odd choice of design. Redesign the C90 to be a bit longer than its current f/11, and it would still weigh almost nothing and still be tiny small, yet optically may be far better.

Funny about the problem with one-sided coatings. Does this mean that coatings only help as light moves in the air-to-glass direction (but not in the glass-to-air direction)? Strictly speaking, this means coatings indeed help with transmission. It's just that transmission works two ways, allowing desirable light to enter, and allowing stray light to escape.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Eddgie]
      #5712442 - 03/04/13 12:37 PM

The C5 is better. However a good C90 is perfectly capable of delivering sharp images and not just at low power. It is, however, a 90mm telescope, so what it can do compared to a C5 is limited. The helical focusing? I am OK with it. Properly greased it works well enough, and you don't get any focus shift. But is it as easy to use as an SCT's moving mirror focuser? No.

It was cold (for us) last night, so I got my li'l C90 out into the front yard in the interest of seeing something. I had been completely skunked at the club dark site the evening before. What did I look at? The easy stuff: Jupiter, M42, etc. How was he C90? Very nice. The seeing wasn't that hot, so I didn't try to push the power, but at 100x Jupiter was extremely sharp and showed good cloud band detail. A little better, maybe, than my Pal Junior Newtonian at that power. At least as good as. M42? It was nice as well. The Trapezium was resolved at 40x, but looked better in a 16mm Konig. The nebulosity? About what you could expect from my light polluted environs...adequate, with the contrast being good.

For me, the C90 fulfills a role. It is my prime grab and go scope. It does better in that role than the other scopes I've used for that over the years. The Short Tube 80 has nice wide field, but get to 100x and the images are not so hotsky. The StarBlast takes too long to acclimate, even in our mild climate, and when I want grab 'n go, I want grab 'n go. The ETX 125 is way too much effort. The 3-inch f/11 refractor requires too much dodging of furniture and doorways to make me want to run outside with her. So, for me my Orange Tube C90 is it. Is it a perfect scope? No, but it will do _far_ better than most folks think. The exception? Get much past the Orange Tubes and quality seems to vary fairly wildly, not really picking up till we get to the current C90s.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: rmollise]
      #5712627 - 03/04/13 02:19 PM

Sorry, I wasn't really trying to justify why anyone should or should not own one, so if you took the post that way, or you thought I was questioning why you or anyone else would want one I am sorry about that.

My post was specific to a request in PM that asked me why I thought their C90 might have low contrast.

My anwswer was intended only to give some possible explanations.

My own sample behaved the way the person that asked me to comment.

I did not find mine satisfactory.

That in no way possible can be construed to suggest that they would be unacceptable to anyone else.

We all use what we like.

Happy to hear that you like your C90.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5712713 - 03/04/13 03:09 PM

Well, from a design standpoint, the Questar differs substantially.

First, it is near f/15 so that does a great deal to control HSA.

But there is even more difference. In the all spherical aluminzed spot telescope where the secondary is a simple silvered area on the rear of the Meniscus, you have no choice but to make it the same curve.

If you could have it ground to a different curve than the rear of the meniscus, you get several benefits.

Most germine to this particular thread is that by doing this, you can eliminate HSA. It is possible to build this design at f/12.5 to .993 Strehl at the center of the field.

This, as I recall, is the way that Questar builds their scopes. The secondary is not simply an aluninzed spot with the same RC as the rear. A special curve is ground into the rear of the meniscus and this curve is different from the rear curve of the lens itself.

And my guess, and it is only a guess, is that if you put the Questar and a C90 on an optical bench, the Q90 would come very close to this number, and the C90 would be very far away from it.

And of course the Q90 is exquisitly made.

But optically the design is much differnt and in fact, considerably better.

People I think often doubt that this or that error could really be all that meaningful and taken as a single error, rarely is. Added together though, and some HSA, LSA, and a big obstruction push the design over the line of what would be considered optimal for high contrast astronomica observing.

At f/11 though, the HSA alone is bad enough that for high cntrast observing (not birding or target shooting) you are perhaps over the limit.

And that is why these scopes were sold as spotters and equipped with correct image diagonals. The design is for a spotter and the compromises are acceptable for that purpose.

But the Questar is a much different telscope, clearly built to offer the highest contrast possible, and even though at near f/15 they could have gotten by, the designer was not content to do so. Instead, it would appear that they were seeking "Perfection" and not simply "Good enough for mass consumer use."

From a design stanpoint (not optical quality... Just design), it is comparing a Chevy Cruise to a Corvette. One is for the masses and one is for the the more serious auto driver that will pay more for the extra performance and care little about the extra cost.

And when you do factor in the excellent of fabrication, it really becomes a badly lopsided proposition.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: rmollise]
      #5712730 - 03/04/13 03:21 PM

Quote:

The exception? Get much past the Orange Tubes and quality seems to vary fairly wildly, not really picking up till we get to the current C90s.




Thanks, Uncle Rod! Based on adding your thoughts to everything else I have learned, I'd rest my case saying this scope has something weird wrong optically. It's not that it won't focus sharply per se. It's that even when focused, a superimposed out-of-focus blur wreaks the contrast. The long sun shade helps some, because there is some issue with stray light; but, it can not help enough, because the real culprit is bad optics.

If you and ActionHac so favor your C90s, it is because they are better C90s than this particular, most likely Taiwanese example. At the right price, this might be a utilitarian spotting scope, but it will never yield glorious views of anything, not at night or during the day.

Interesting, isn't it? An uncoated orange tube would trounce this later version with its Special Coating on at least one surface of the meniscus. There is never, never a substitute for good optics.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5712768 - 03/04/13 03:44 PM

Quote:



Thanks, Uncle Rod! Based on adding your thoughts to everything else I have learned, I'd rest my case saying this scope has something weird wrong optically. It's not that it won't focus sharply per se. It's that even when focused, a superimposed out-of-focus blur wreaks the contrast. The long sun shade helps some, because there is some issue with stray light; but, it can not help enough, because the real culprit is bad optics.

If you and ActionHac so favor your C90s, it is because they are better C90s than this particular, most likely Taiwanese example. At the right price, this might be a utilitarian spotting scope, but it will never yield glorious views of anything, not at night or during the day.

Interesting, isn't it? An uncoated orange tube would trounce this later version with its Special Coating on at least one surface of the meniscus. There is never, never a substitute for good optics.




I'd certainly believe there are lemons out there, especially the post Orange Tube C90s, and I certainly believe Ed and you have unfortunately got lemons. My only point is that there are good ones too. The C90 is often maligned, but sometimes the reason is not because of the optics, but that dadgum helical focuing scheme, and because people tend to undermount the little sucker. With the twist to focus paradigm, a shaky mount will ensure you never get it in focus. Yes I've used mine on a photo tripod, but it is much, MUCH better on my Synta AZ-4...

I'll admit I used to be one of the C90's staunchest critics...till I got my own (unlooked for) Orange Tube, and got the chance to get up close and personal with her and see what she could do.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5712876 - 03/04/13 04:41 PM

Gee, Eddgie; I think you've just settled the long-running debate over the virtues of the Questar verses the C90. Makes sense to me. It really is better, and objectively so. For me, a good C90 might be fine given the price of a Questar, but not the C90 currently in my possession.

I've come to love telescopes that are tantalizingly good, yet have something wrong. Through studying their flaws, I learn what makes the good scopes right. It would not be possible to learn that if every scope I saw were perfect!


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Eddgie]
      #5713014 - 03/04/13 05:46 PM

Which C90s were sold as spotters? Recent models, sure, but there was indeed an astro version of the Orange Tube.

As for the Questar, I wouldn't expect the C90 to stand up to the J.R. Cumberland optics of the Q, but it does pretty well anyway.


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: rmollise]
      #5713037 - 03/04/13 06:02 PM

I'm sold on the idea of a C90 as a grab-and-go. If they're good enough for Unk, they're good enough for me. I'd guess that their less-than-stellar reputation comes from the inconsistency of their construction over the years. Sounds as though the best bet is to find an orange tube or a new Synta. I'd much rather a classic orange, since I love antique scopes.

Thank you all for your help. Amazing what I've learned in this thread. I hereby pronounce this C90 "functional, but defective!"


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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5713120 - 03/04/13 06:53 PM Attachment (9 downloads)

Here`s an image of my C90-Astro, Great little scope with Great optics.

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Dave M]
      #5713169 - 03/04/13 07:23 PM

Were the later orange units given coatings on the Mak corrector? My impression is, "no." But I could be wrong...

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5713178 - 03/04/13 07:28 PM

Quote:

Were the later orange units given coatings on the Mak corrector? My impression is, "no." But I could be wrong...




I don't _believe_ so, Glenn, but I am not 100% on that...


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rmollise
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5713184 - 03/04/13 07:30 PM

Quote:

I'm sold on the idea of a C90 as a grab-and-go. If they're good enough for Unk, they're good enough for me. I'd guess that their less-than-stellar reputation comes from the inconsistency of their construction over the years. Sounds as though the best bet is to find an orange tube or a new Synta. I'd much rather a classic orange, since I love antique scopes.

Thank you all for your help. Amazing what I've learned in this thread. I hereby pronounce this C90 "functional, but defective!"




'Twere me? I'd just get the current C90 (but ditch that diagonal). If I didn't have my OT, I'd do that very thing. Part of me wants to anyway...it is one hell of a bargain.


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actionhac
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5713202 - 03/04/13 07:40 PM

I own two orange C90's, a spotter and a Astro. Both of mine have uncoated corrector/meniscus lenses.
I use the spotter the most and its one of my favorite scopes.
My spotter spends very little time doing astronomy because I'm not a big fan of the helical barrel style focuser.
It spends a lot of time out in the garden as a garden scope and for that job it excels.
Colors through the C90 are dead on, its extremely sharp, close focus is around 15ft. So I can just sit there in my yard and bring any flower or animal or insect right up and examine it. Its extremely fun.
For that reason it is one of my all time favorite telescopes.

Robert


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Philip Levine
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: rmollise]
      #5713314 - 03/04/13 08:34 PM

I bought an orange C90 "spotter" and started out mounting it on a camera tripod. Later, I obtained the Celestron one-arm mount, voila now it's an astro version. The Celestron one-arm mount makes observing much easier. There's something special about this astro version, just looking at the setup, "classic" comes to mind. I can deal with the helical focusing mechanism.
I would rate the optics on my orange C90 as very good, this together with the "grab and go" factor, fills a niche.
Phil


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Joe Cepleur
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Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Philip Levine]
      #5713651 - 03/05/13 12:01 AM

I would love an orange C90 astro. It's the classic classic! Very Old World in its cast metal construction, yet Space Age in its conception, and all too easy to transport!

ActionHac, if must be a joy sitting in your garden with the bionic eye. You, like so many others, cite the perfect color of the C90. That was one of the hints that I was not in possession of a perfect C90. If the world were a water color, everything looked as though it were dimmed and flattened with a uniform, soft grey wash.

Uncle Rod, your advice to scoop a modern C90 sounds wise. Hold it for twenty or thirty years, and I'll have a classic!


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Joe Cepleur
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Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5713690 - 03/05/13 12:24 AM

The specs on the new C90 spotter are:

* Multicoated optics
* Focal Length : 1250 mm, f/13.89

Hmmm... two issues that surfaced in this thread! It may not have the Questar's specially cut secondary, but it is multicoated (does that mean on both sides of the meniscus?), and it shares the Questar's long focus. I'm not saying it would equal a Questar, but good manufacturing to similar specifications can't hurt in making an affordable knock-off.

$150 at B&H now!


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GlennLeDrew
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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5713875 - 03/05/13 05:24 AM

Yep, coated on both sides of the corrector.

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

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Re: Is this Celestron C90 "Classic" or "Used?" new [Re: GlennLeDrew]
      #5716134 - 03/06/13 10:32 AM

Just to be complete, here is the report I sent my friend about his C90. He always knew it had problems. Now, finally, thanks to the help of Cloudy Nights, he'll know why.

<><><><><>

It has taken nearly four months of experimenting under the guidance of the wise astronomers of Cloudy Nights, but I finally have a diagnosis for your C90's terribly low contrast:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=classics&...

This report reminds me of Robert Frost's poem, "The Bearer of Evil Tidings," which begins:

"The bearer of evil tidings,
When he was halfway there,
Remembered that evil tidings
Were a dangerous thing to bear."

and ends:

"As for his evil tidings,
Belshazzar's overthrow,
Why hurry to tell Belshazzar
What soon enough he would know?"

That said, I've learned a lot and can answer the question, so here goes: In short, if sadly, your C90 is a lemon.

The thread on Cloudy Nights grew to be quite long as we tested and ruled out many issues. The scope did not need a cleaning; its baffles were not misaligned; collimation appears to be okay; the design does have issues with stray light, but adding a long sun shade does not improve the view significantly. In the end, we now know why the image is so low in contrast, or at least the Cloudy Nights member Eddgie does, and I believe him.

Think of a star test, with an Airy disk centered in concentric diffraction rings. There are aberrations that push the light out from the Airy disk, into undesirably thick diffraction rings. Essentially, every point of light in the scene being viewed creates a point of light in the image, but also and unavoidably, a blur around itself. Choices in design and quality control in manufacturing constrain the blur in better scopes. The C90 is a good design, but this likely Taiwanese specimen has flawed optics. The excessive blur is at once too small to allow other than sharply focused images, and yet so rampant as to smush the image. The result is that if the image were a water color, it looks as though a soft grey wash obscures the tonal range. The forest seen across the lake, over a mile away, literally fades into a cloud of not-so-bright trees hidden by only slightly darker ones.

It's a useful spotting scope. It will tell an ice fishermen whether the flag on his far-away trap indicates he has caught a fish, or the rifleman how well he has scored. It will even show, beyond doubt, that the distant bird is indeed an eagle. What it won't do is show the gorgeous colors on a bird even as close as in the yard. The views will never be as beautiful as the C90 is renowned for showing; the vaunted color correction will never be right, but always dully washed as though with gray.

All told, it's an acceptable utility scope for working in the field, but not an aesthetically gifted wonder when viewing for pleasure. It was also, for me, a truly great teacher, so thank you for allowing me to assess it. It's hard to learn about optics from studying excellent scopes. One learns by studying how specific flaws induce specific problems with the view.

Wonderful concept, anyway, a small, powerhouse scope. Hope you are enjoying the Questar that replaced the C90!


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