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Equipment Discussions >> Classic Telescopes

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GlennLeDrew
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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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Joe Cepleur
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Reged: 03/18/10

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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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GlennLeDrew
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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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Joe Cepleur
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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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rmollise
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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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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Joe Cepleur
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Reged: 03/18/10

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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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GlennLeDrew
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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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rmollise
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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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rmollise
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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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actionhac
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Reged: 08/09/08

Loc: Seattle
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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GlennLeDrew
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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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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actionhac
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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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actionhac
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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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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GlennLeDrew
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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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
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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GlennLeDrew
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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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