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#1
posted to rec.boats
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Boat telescope 2.0
Progress while waiting for Fay to go away. Finished rough grinding
yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror) and finished the #120 grit and have started on the #180. My magnifications will be a low of 15 X an a high of about 30X. |
#2
posted to rec.boats
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Boat telescope 2.0
On Tue, 26 Aug 2008 19:31:21 -0700 (PDT),
wrote: Progress while waiting for Fay to go away. Finished rough grinding yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror) and finished the #120 grit and have started on the #180. My magnifications will be a low of 15 X an a high of about 30X. How do you know when you have the shape just right? What do use as a blank? |
#3
posted to rec.boats
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Boat telescope 2.0
On Aug 26, 10:52 pm, Wayne.B wrote:
On Tue, 26 Aug 2008 19:31:21 -0700 (PDT), wrote: Progress while waiting for Fay to go away. Finished rough grinding yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror) and finished the #120 grit and have started on the #180. My magnifications will be a low of 15 X an a high of about 30X. How do you know when you have the shape just right? What do use as a blank? At this stage, one simply uses a depth micrometer (or more simply a feeler guage under a straight edge) to measure the depth of the "bowl". The one grinds the "tool" (another glass blank) against the mirror in such a way that they naturally form a concave and convex spherical surface. Later, to change from spherical to paraboloidal, one uses an amazingly simply shadow test called the Focault test to measure the figure errors. It is so amazing that this actually works and was figured out over a hundred years ago. |
#4
posted to rec.boats
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Boat telescope 2.0
wrote in message
... On Aug 26, 10:52 pm, Wayne.B wrote: On Tue, 26 Aug 2008 19:31:21 -0700 (PDT), wrote: Progress while waiting for Fay to go away. Finished rough grinding yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror) and finished the #120 grit and have started on the #180. My magnifications will be a low of 15 X an a high of about 30X. How do you know when you have the shape just right? What do use as a blank? At this stage, one simply uses a depth micrometer (or more simply a feeler guage under a straight edge) to measure the depth of the "bowl". The one grinds the "tool" (another glass blank) against the mirror in such a way that they naturally form a concave and convex spherical surface. Later, to change from spherical to paraboloidal, one uses an amazingly simply shadow test called the Focault test to measure the figure errors. It is so amazing that this actually works and was figured out over a hundred years ago. My father-in-law does this as well. Surprisingly (to me anyway) hand ground mirrors are amazingly accurate. Because if the randomness of a human doing it. So much so that duplicating the level of precision with manufacturing machinery is quite difficult. As I recall it's also very messy and time consuming. You sit with a piece of thick round glass in your lap and dish it out by hand using various grit sizes of compound and another piece of thick round glass as a tool. Once you finish, you send the glass off to have the mirror coat applied. On the smaller sizes if you apply a value to your time it's cheaper to buy a telescope. On bigger sizes like 8 or higher you can save a fair amount of money making your own mirror. Of course there is the satisfaction of knowing you made it your self. For a boat I might try one of the cradle style. My daughter has a 4 inch one. You can take it out of the socket and just sit the ball in your lap. http://www.telescopes.com/telescopes...rtelescope.cfm |
#5
posted to rec.boats
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Boat telescope 2.0
On Aug 27, 8:07 am, "jamesgangnc" wrote:
wrote in message ... On Aug 26, 10:52 pm, Wayne.B wrote: On Tue, 26 Aug 2008 19:31:21 -0700 (PDT), wrote: Progress while waiting for Fay to go away. Finished rough grinding yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror) and finished the #120 grit and have started on the #180. My magnifications will be a low of 15 X an a high of about 30X. How do you know when you have the shape just right? What do use as a blank? At this stage, one simply uses a depth micrometer (or more simply a feeler guage under a straight edge) to measure the depth of the "bowl". The one grinds the "tool" (another glass blank) against the mirror in such a way that they naturally form a concave and convex spherical surface. Later, to change from spherical to paraboloidal, one uses an amazingly simply shadow test called the Focault test to measure the figure errors. It is so amazing that this actually works and was figured out over a hundred years ago. My father-in-law does this as well. Surprisingly (to me anyway) hand ground mirrors are amazingly accurate. Because if the randomness of a human doing it. So much so that duplicating the level of precision with manufacturing machinery is quite difficult. As I recall it's also very messy and time consuming. You sit with a piece of thick round glass in your lap and dish it out by hand using various grit sizes of compound and another piece of thick round glass as a tool. Once you finish, you send the glass off to have the mirror coat applied. On the smaller sizes if you apply a value to your time it's cheaper to buy a telescope. On bigger sizes like 8 or higher you can save a fair amount of money making your own mirror. Of course there is the satisfaction of knowing you made it your self. For a boat I might try one of the cradle style. My daughter has a 4 inch one. You can take it out of the socket and just sit the ball in your lap. http://www.telescopes.com/telescopes...pes/tasco30x76... I know, I can buy one for less than what my time is worth. I just wanna do it for the love of optics and math. However, I can relate this to boating even more. In one of the Patrick Obrien books, Jack Aubrey grinds a telescope mirror being tutored by Lady Herschel if I remember right. He uses the "Finest Pomeranian Sludge" for polishing whatever that means. At that time, they were still making such mirrors from metal because they either couldnt polish the glass well enough or they couldnt make a front surface mirror. I remember from somewhere that the best metal for this was called "Bell metal". In a later Jack Aubrey book, he attempts to rig his telescope from the ships rigging in such a way that it stays oriented regardless of the ships rolling in order to view the moons of Jupiter using them as a timepeice to avoid the need for a chronomator. This didnt work in the book as it did not work in real life. I think the reason was that they had to use very long focal length telescopes because they did not have the Foucault test for parabolizing a mirror and the diff tween a sphere and parabola for long focal lengths is minimal. Long focal lengths mean high magnification so any movement of the ship would be magnified. |
#6
posted to rec.boats
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Boat telescope 2.0
On Wed, 27 Aug 2008 08:07:19 -0400, "jamesgangnc"
wrote: You sit with a piece of thick round glass in your lap I have read the few books on the subject, and perused the telescope making column in Sky and Telescope regularly for decades, and I have never heard that one. Sounds like doing it the hard way. You creat an immovable stand, somewhere in the area of waist high, and fix the tool to it. Occurs to me you might make the stand from concrete blocks. The mortar doesn't have to look pretty. When finished chisel it apart, or just break it up with a big hammer. You walk around it while rubbing the mirror blank over it. Never heard of any other way. You have to keep good control of the grit, or a piece of 180 will show up while you are doing the final figuring. You keep your clothes away from things, so that you don't transfer coarse grit into a fine grit stage. I bet Edmund still has stuff of interest. Casady |
#7
posted to rec.boats
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Boat telescope 2.0
On Aug 27, 10:38 am, (Richard Casady)
wrote: On Wed, 27 Aug 2008 08:07:19 -0400, "jamesgangnc" wrote: You sit with a piece of thick round glass in your lap I have read the few books on the subject, and perused the telescope making column in Sky and Telescope regularly for decades, and I have never heard that one. Sounds like doing it the hard way. You creat an immovable stand, somewhere in the area of waist high, and fix the tool to it. Occurs to me you might make the stand from concrete blocks. The mortar doesn't have to look pretty. When finished chisel it apart, or just break it up with a big hammer. You walk around it while rubbing the mirror blank over it. Never heard of any other way. You have to keep good control of the grit, or a piece of 180 will show up while you are doing the final figuring. You keep your clothes away from things, so that you don't transfer coarse grit into a fine grit stage. I bet Edmund still has stuff of interest. Casady I built a stand weighted down with two concrete blocks. When I was 13, I ground a 6" mirror from an Edmund kit and I think the Edmund kit was crap to comparison to what I have now. Edmund's largest abrasive was 180 Aluminum oxide and they didnt send enough of that. By comparison, my three largest abrasives now are #80, 120 1nd 180 Silicon carbide, MUCH faster. You do not use Al2O3 till the finer grades. My wife who is the usual clean freak around the house marvels at my sudden interest in cleanliness in the utility room. |
#8
posted to rec.boats
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Boat telescope 2.0
On Tue, 26 Aug 2008 22:52:24 -0400, Wayne.B
wrote: On Tue, 26 Aug 2008 19:31:21 -0700 (PDT), wrote: Progress while waiting for Fay to go away. Finished rough grinding yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror) and finished the #120 grit and have started on the #180. My magnifications will be a low of 15 X an a high of about 30X. How do you know when you have the shape just right? There are optical tests with simple equipment. What do use as a blank? Two discs of pyrex from a mail order house. One is the mirror, one the tool. You put grit on them and rub them together. Casady |
#9
posted to rec.boats
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Boat telescope 2.0
On May 28, 5:56*pm, Richard Casady
wrote: On Tue, 26 Aug 2008 22:52:24 -0400, Wayne.B wrote: On Tue, 26 Aug 2008 19:31:21 -0700 (PDT), wrote: Progress while waiting for Fay to go away. *Finished rough grinding yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror) and finished the #120 grit and have started on the #180. *My magnifications will be a low of 15 X an a high of about 30X. How do you know when you have the shape just right? There are optical tests with simple equipment. What do use as a blank? Two discs of pyrex from a mail order house. One is the mirror, one the tool. You put grit on them and rub them together. Casady The action of grinding naturally produces a spherical mirror. You can measure its focal length by wetting it and seeing where it fills with light from a small bulb. The proper shape for a telescope is a paraboloid and for a very long focal length, say f10, the diff tween a sphere and paraboloid is too small to notice. For a short focal length one like what I am making, you have to "parabolize" it. This is done as part of the polishing process using a very simple device called a Foucault tester. The test can literally be done with a candle flame and knife edge. In this test, variations in shape from a paraboloid are enormously magnified so you know where to concentrate polishing. This test (or a variation on it) is normally done on ALL telescope mirrors and this is what Perkin -Elmer neglected to originally do with Hubble necessitating the corrective optics. The entire mirror making process is a beautiful excercise in applied trigonometry and patience. |
#10
posted to rec.boats
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Boat telescope 2.0
On Thu, 28 May 2009 18:38:14 -0700 (PDT), Frogwatch
wrote: On May 28, 5:56*pm, Richard Casady wrote: On Tue, 26 Aug 2008 22:52:24 -0400, Wayne.B wrote: On Tue, 26 Aug 2008 19:31:21 -0700 (PDT), wrote: Progress while waiting for Fay to go away. *Finished rough grinding yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror) and finished the #120 grit and have started on the #180. *My magnifications will be a low of 15 X an a high of about 30X. How do you know when you have the shape just right? There are optical tests with simple equipment. What do use as a blank? Two discs of pyrex from a mail order house. One is the mirror, one the tool. You put grit on them and rub them together. Casady The action of grinding naturally produces a spherical mirror. You can measure its focal length by wetting it and seeing where it fills with light from a small bulb. The proper shape for a telescope is a paraboloid and for a very long focal length, say f10, the diff tween a sphere and paraboloid is too small to notice. For a short focal length one like what I am making, you have to "parabolize" it. This is done as part of the polishing process using a very simple device called a Foucault tester. The test can literally be done with a candle flame and knife edge. In this test, variations in shape from a paraboloid are enormously magnified so you know where to concentrate polishing. This test (or a variation on it) is normally done on ALL telescope mirrors and this is what Perkin -Elmer neglected to originally do with Hubble necessitating the corrective optics. The entire mirror making process is a beautiful excercise in applied trigonometry and patience. Very interesting. Thanks. I'd have said 'geometry' instead of 'trigonometry', but you're the expert at lens grinding here. -- John H |
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