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On-axis imagining of obstructions mystery
There is something I've been trying to figure out but not sure what caused it. Set up an SCT with eyepiece on and aim at bright object like moon or daytime targets. Stay 1 feet away from the eyepiece and look at it. You can see the secondary obstruction of the SCT in the eyepiece and as you get closer ot the eyepiece. The secondary obstruction would get lighter and at the proper eye relief distance of the eyepiece, you can still notice a very faded image of the secondary obstruction causing contrast lost in the center of the image. Now the mystery is what causes it? Equipment tested is an 8" Celestron SCT done many times few years ago and still at a lost to explain it. Parallel light rays enter the objective at different angles and the rays converge to a point at the focal plane and the eyepiece. So theoretically you are not supposed to see or notice the secondary obstructions when viewing thru the eyepice, yet you can in a very faded sense. Anyone got any theory why? Thanks. Teni |
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On-axis imagining of obstructions mystery
On Fri, 02 Nov 2007 01:42:14 -0700, Tenifer
wrote: Parallel light rays enter the objective at different angles and the rays converge to a point at the focal plane and the eyepiece. So theoretically you are not supposed to see or notice the secondary obstructions when viewing thru the eyepice, yet you can in a very faded sense. Anyone got any theory why? A telescope in an afocal instrument: it has no focal point. You've got the first part right, that parallel light rays enter the objective at different angles, but you neglected to consider the role of the eyepiece, which is to cause those same rays to exit in a similarly parallel fashion. What comes out the eyepiece of an unobstructed telescope is basically a solid cylinder of light rays, and what comes from an obstructed telescope is a cylinder with a shadowed center region. When you use the telescope during the day, or on a bright object like the Moon, your eye's pupil is constricted. If it is constricted to the point that it is smaller than the central shadow (which is common if you are using a low power EP that itself has a large exit pupil) you'll see the shadow, either entirely or as lowered contrast. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
#3
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On-axis imagining of obstructions mystery
On Nov 2, 10:28 pm, Chris L Peterson wrote:
On Fri, 02 Nov 2007 01:42:14 -0700, Tenifer wrote: Parallel light rays enter the objective at different angles and the rays converge to a point at the focal plane and the eyepiece. So theoretically you are not supposed to see or notice the secondary obstructions when viewing thru the eyepice, yet you can in a very faded sense. Anyone got any theory why? A telescope in an afocal instrument: it has no focal point. You've got the first part right, that parallel light rays enter the objective at different angles, but you neglected to consider the role of the eyepiece, which is to cause those same rays to exit in a similarly parallel fashion. What comes out the eyepiece of an unobstructed telescope is basically a solid cylinder of light rays, and what comes from an obstructed telescope is a cylinder with a shadowed center region. When you use the telescope during the day, or on a bright object like the Moon, your eye's pupil is constricted. If it is constricted to the point that it is smaller than the central shadow (which is common if you are using a low power EP that itself has a large exit pupil) you'll see the shadow, either entirely or as lowered contrast. _________________________________________________ Chris L Peterson Cloudbait Observatoryhttp://www.cloudbait.com While it is true that parallel rays exit the eyepiece but as they pass thru the eyes. They are converged back to points as the light beam reaches the retina. Now in the case of 8" SCT at 80X magnification using 25mm eyepiece, EP exit pupil is 80/25mm=3.2mm. If you use this at daytime with your pupil constricting to 2mm. The exit pupil of the eyepiece is still larger than your pupil and the obstruction of the parallel rays from the eyepiece is only 3.2mm/3= ~1mm. With your eyes at 2mm and the obstruction at 1mm, the rest of the 2.2mm light rays (or specifically the 1mm) at the periphery of the parallel rays coming from EP can still reach your eyes and converge to a point at the retina so you are not supposed to notice the obstruction (because it is our retinas that detect photons and not the cornea). Yet there are obstruction seen. I think besides the parallel rays entering the objective lens. The objective lens itself can somehow inpinge on the eyepiece. In other words. Even though you are viewing thru the eyepiece. A part of your eyes is directly looking at the objective lens and that's why you can notice the central obstructive even as faded low constrast trace at the center of the eyepiece. What do you think? Teni |
#4
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On-axis imagining of obstructions mystery
"Tenifer" wrote in message oups.com... On Nov 2, 10:28 pm, Chris L Peterson wrote: On Fri, 02 Nov 2007 01:42:14 -0700, Tenifer wrote: Parallel light rays enter the objective at different angles and the rays converge to a point at the focal plane and the eyepiece. So theoretically you are not supposed to see or notice the secondary obstructions when viewing thru the eyepice, yet you can in a very faded sense. Anyone got any theory why? A telescope in an afocal instrument: it has no focal point. You've got the first part right, that parallel light rays enter the objective at different angles, but you neglected to consider the role of the eyepiece, which is to cause those same rays to exit in a similarly parallel fashion. What comes out the eyepiece of an unobstructed telescope is basically a solid cylinder of light rays, and what comes from an obstructed telescope is a cylinder with a shadowed center region. When you use the telescope during the day, or on a bright object like the Moon, your eye's pupil is constricted. If it is constricted to the point that it is smaller than the central shadow (which is common if you are using a low power EP that itself has a large exit pupil) you'll see the shadow, either entirely or as lowered contrast. _________________________________________________ Chris L Peterson Cloudbait Observatoryhttp://www.cloudbait.com While it is true that parallel rays exit the eyepiece but as they pass thru the eyes. They are converged back to points as the light beam reaches the retina. Now in the case of 8" SCT at 80X magnification using 25mm eyepiece, EP exit pupil is 80/25mm=3.2mm. If you use this at daytime with your pupil constricting to 2mm. The exit pupil of the eyepiece is still larger than your pupil and the obstruction of the parallel rays from the eyepiece is only 3.2mm/3= ~1mm. With your eyes at 2mm and the obstruction at 1mm, the rest of the 2.2mm light rays (or specifically the 1mm) at the periphery of the parallel rays coming from EP can still reach your eyes and converge to a point at the retina so you are not supposed to notice the obstruction (because it is our retinas that detect photons and not the cornea). Yet there are obstruction seen. I think besides the parallel rays entering the objective lens. The objective lens itself can somehow inpinge on the eyepiece. In other words. Even though you are viewing thru the eyepiece. A part of your eyes is directly looking at the objective lens and that's why you can notice the central obstructive even as faded low constrast trace at the center of the eyepiece. What do you think? Not sure if you were actually experimenting with the 25mm eyepiece, or just using that value as an example, but, at F10, a 25mm eyepiece gives a 2.5mm exit pupil (eyepiece focal length divided by focal ratio of objective), or calculated another way (which I suspect you intended), 200mm aperture / 80x = 2.5mm (aperture divided by power) think binoculars (10x50 = 5mm exit pupil). In any event, your eye is a part of the focal system, and the light entering the eye in a parallel manner is convergent on the retina. As the exit pupil to eye pupil ratio approaches unity from either direction, the more efficiency is obtained from the overall system. If the eye pupil is equal to or larger then the exit pupil, more light around the "donut hole" is available to come to focus on the retina, diminishing the impact of the central obstruction. If the eye pupil is smaller the the exit pupil, then not only are you losing the light rays of the central obstruction, you are also losing light rays from the outter region of the exit pupil. If the exit pupil exceeds the eye pupil by a large enough margin, the central obstruction becomes obvious. Take that to an extreme, and use a 30% CO, a 10mm exit pupil, and a 3mm eye pupil. The central 3mm of the 10mm exit pupil will be obscured, and the eye pupil will be passing only the rays from the obscured area (up close). As you step away, then it is possible that some miniscule amount of the 35% peripheral light exiting the eyepiece on any "side" of the CO will fall on the retina and the edge of the CO will be seen as well. The question is then, what percentage of the eye pupil can be centrally obscured, before it becomes noticeable, and/or visible? If the exit pupil exceeds the eye pupil, the CO effectively gets larger and larger than that of the objective. Either way, the 34% obstruction of a 2.5mm eye pupil is going to allow the full complement of the remaining 1.7mm (66% unobstructed) worth of light to enter the eye, which is plenty to form a nice bright image. HTH, Steve P. |
#5
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On-axis imagining of obstructions mystery
On Nov 3, 11:41 pm, "Steve Paul" wrote:
"Tenifer" wrote in message oups.com... On Nov 2, 10:28 pm, Chris L Peterson wrote: On Fri, 02 Nov 2007 01:42:14 -0700, Tenifer wrote: Parallel light rays enter the objective at different angles and the rays converge to a point at the focal plane and the eyepiece. So theoretically you are not supposed to see or notice the secondary obstructions when viewing thru the eyepice, yet you can in a very faded sense. Anyone got any theory why? A telescope in an afocal instrument: it has no focal point. You've got the first part right, that parallel light rays enter the objective at different angles, but you neglected to consider the role of the eyepiece, which is to cause those same rays to exit in a similarly parallel fashion. What comes out the eyepiece of an unobstructed telescope is basically a solid cylinder of light rays, and what comes from an obstructed telescope is a cylinder with a shadowed center region. When you use the telescope during the day, or on a bright object like the Moon, your eye's pupil is constricted. If it is constricted to the point that it is smaller than the central shadow (which is common if you are using a low power EP that itself has a large exit pupil) you'll see the shadow, either entirely or as lowered contrast. _________________________________________________ Chris L Peterson Cloudbait Observatoryhttp://www.cloudbait.com While it is true that parallel rays exit the eyepiece but as they pass thru the eyes. They are converged back to points as the light beam reaches the retina. Now in the case of 8" SCT at 80X magnification using 25mm eyepiece, EP exit pupil is 80/25mm=3.2mm. If you use this at daytime with your pupil constricting to 2mm. The exit pupil of the eyepiece is still larger than your pupil and the obstruction of the parallel rays from the eyepiece is only 3.2mm/3= ~1mm. With your eyes at 2mm and the obstruction at 1mm, the rest of the 2.2mm light rays (or specifically the 1mm) at the periphery of the parallel rays coming from EP can still reach your eyes and converge to a point at the retina so you are not supposed to notice the obstruction (because it is our retinas that detect photons and not the cornea). Yet there are obstruction seen. I think besides the parallel rays entering the objective lens. The objective lens itself can somehow inpinge on the eyepiece. In other words. Even though you are viewing thru the eyepiece. A part of your eyes is directly looking at the objective lens and that's why you can notice the central obstructive even as faded low constrast trace at the center of the eyepiece. What do you think? Not sure if you were actually experimenting with the 25mm eyepiece, or just using that value as an example, but, at F10, a 25mm eyepiece gives a 2.5mm exit pupil (eyepiece focal length divided by focal ratio of objective), or calculated another way (which I suspect you intended), 200mm aperture / 80x = 2.5mm (aperture divided by power) think binoculars (10x50 = 5mm exit pupil). Yes I was actually experimenting on it. I used a 25mm eyepiece at an 8" SCT. As you calculated, the exit pupil is 200m// aperture/80x = 2.5mm. I tried it many times at daytime. My eye pupil is 2mm. Now the obstructure area of the EP exit pupil is 2.5mm/3= 0.833mm. This means there is enough light to form image at the retina because 2.0mm-0.833mm = 1.167mm of the light is passing thru the eye 2mm pupil. Therefore you are not supposed to see the obstruction at the eyepiece. Yet I noticed a faded center in the eyepiece where the contrast is not good which concides with the central obstruction. What I'd like to understand is how come the central obstruction still form a faded center image at the eyepiece. It's as if the black central obstruction can mix with the pure image at the center creating contrast lost. Is this it? Teni In any event, your eye is a part of the focal system, and the light entering the eye in a parallel manner is convergent on the retina. As the exit pupil to eye pupil ratio approaches unity from either direction, the more efficiency is obtained from the overall system. If the eye pupil is equal to or larger then the exit pupil, more light around the "donut hole" is available to come to focus on the retina, diminishing the impact of the central obstruction. If the eye pupil is smaller the the exit pupil, then not only are you losing the light rays of the central obstruction, you are also losing light rays from the outter region of the exit pupil. If the exit pupil exceeds the eye pupil by a large enough margin, the central obstruction becomes obvious. Take that to an extreme, and use a 30% CO, a 10mm exit pupil, and a 3mm eye pupil. The central 3mm of the 10mm exit pupil will be obscured, and the eye pupil will be passing only the rays from the obscured area (up close). As you step away, then it is possible that some miniscule amount of the 35% peripheral light exiting the eyepiece on any "side" of the CO will fall on the retina and the edge of the CO will be seen as well. The question is then, what percentage of the eye pupil can be centrally obscured, before it becomes noticeable, and/or visible? If the exit pupil exceeds the eye pupil, the CO effectively gets larger and larger than that of the objective. Either way, the 34% obstruction of a 2.5mm eye pupil is going to allow the full complement of the remaining 1.7mm (66% unobstructed) worth of light to enter the eye, which is plenty to form a nice bright image. HTH, Steve P.- Hide quoted text - - Show quoted text - |
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