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Equipment Discussions >> Classic Telescopes

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CharlieB
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Reged: 12/11/07

Loc: Southern NH
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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Ducky62
professor emeritus


Reged: 10/31/10

Loc: The ATL
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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Joe Cepleur
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Reged: 03/18/10

Loc: Dark North Woods
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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actionhac
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Reged: 08/09/08

Loc: Seattle
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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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]
      #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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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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actionhac
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Reged: 08/09/08

Loc: Seattle
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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Reged: 08/09/08

Loc: Seattle
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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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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Reged: 08/09/08

Loc: Seattle
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
Carpal Tunnel
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Reged: 10/17/09

Loc: Tucson, Arizona
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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Reged: 08/09/08

Loc: Seattle
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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Datapanic
Carpal Tunnel
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Reged: 10/17/09

Loc: Tucson, Arizona
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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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GlennLeDrew
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Reged: 06/18/08

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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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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