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Measuring eyepiece focal length
I am interested in measuring the focal lengths of various eyepieces with
reasonable accuracy, maybe 5% or less. I am aware of that one can just measure the exit pupil but measuring the exit pupil accurate, especially for a short focal length piece seems difficult. Thoughts, suggestions, links... jon |
#2
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Measuring eyepiece focal length
In article , Jon Isaacs
wrote: I am interested in measuring the focal lengths of various eyepieces with reasonable accuracy, maybe 5% or less. I am aware of that one can just measure the exit pupil but measuring the exit pupil accurate, especially for a short focal length piece seems difficult. Thoughts, suggestions, links... I've had a go at using Don Taylor's method which he gave details of in February 2000: http://groups.google.co.uk/groups?th=16bb70d0cac07219 I've written a web page describing a variation of Don's method: http://www.easter-vivian.clara.co.uk...fl_method.html My accuracy is not very good, particularly at the shorter focal lengths. I hope you find a better method. -- Jim Easterbrook http://astro.jim-easterbrook.me.uk/ N51.36 E0.25 |
#3
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Measuring eyepiece focal length
In article , Jon Isaacs
wrote: I am interested in measuring the focal lengths of various eyepieces with reasonable accuracy, maybe 5% or less. I am aware of that one can just measure the exit pupil but measuring the exit pupil accurate, especially for a short focal length piece seems difficult. Thoughts, suggestions, links... I've had a go at using Don Taylor's method which he gave details of in February 2000: http://groups.google.co.uk/groups?th=16bb70d0cac07219 I've written a web page describing a variation of Don's method: http://www.easter-vivian.clara.co.uk...fl_method.html My accuracy is not very good, particularly at the shorter focal lengths. I hope you find a better method. -- Jim Easterbrook http://astro.jim-easterbrook.me.uk/ N51.36 E0.25 |
#4
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Measuring eyepiece focal length
Jon Isaacs wrote: I am interested in measuring the focal lengths of various eyepieces with reasonable accuracy, maybe 5% or less. I am aware of that one can just measure the exit pupil but measuring the exit pupil accurate, especially for a short focal length piece seems difficult. Thoughts, suggestions, links... jon Here is a method that worked very well for me when I wanted to check on the focal lengths of my eyepieces. It gave results accurate to about 0.1 mm in the focal length even with short focal length eyepieces. It basically works in reverse of the classic method of measuring exit pupils. Instead of measuring the exit pupil we place an illuminated grid of closely spaced lines lines where the exit pupil would normally be and use the eyepiece lens to project the grid onto a distant screen. For the grid I used a ronchi grating. Now instead of measuring a tiny exit pupil we measure the vastly larger projected grid spacing on the screen. The setup consists of a flashlight shining through a Ronchi grating (120 lines/mm in my case) which is placed about where the eye would go if it were looking through the eyepiece. Be sure to shield any stray light from the flashlight that doesn't go through the gratiing so that it doesn't fall on the screen. The eyepiece is then carefully moved toward or away from the Ronchi grating (I used an old rack and pinion focuser to aid this but it wasn't essential) until the lines are sharply in focus on a screen (I used a white wall of the room) a couple of meters away. One then measures the center to center spacing of the projected Ronchi lines. For the longer focal length eyepieces I measured the distance covered by 5 lines and then divided that by 5. To find the focal length one first calculates the magnification, M = D/d where d is the spacing between lines on the grating itself (1/120 mm in my case) and D is the spacing as magnified by the projection. I then calculate the focal length of the eyepiece as f = L/(M+2) where L is the distance from the Ronchi to the projection screen. This is an approximation (but a good one) in that it assumes that the separation between principal planes of the eyepiece is small compared to the distance to the screen. Thats why you want the screen to be on the order of at least 100 times the eyepiece focal length from the Ronchi grating. I also tried this method using a transparent millimeter scale instead of the Ronchi. That works OK for the longer focal length eyepieces. If one doesn't have either of these a single wire of known thickness might also serve. The technique can also be used to measure the focal length of Barlow + eyepiece combinations. In that case it gives the effective focal length of the eyepiece as used with the Barlow. By dividing that result into the focal length measured for the eyepiece by itself, one can get the Barlow's amplification factor as used with that eyepiece. A more exact formula for finding the focal length is according to my derivation: f = (L-h)/(M + 2 + 1/M) where h is the separation of the principal planes. I have been trying to think of something that would work as a substitute for the Ronchi grating. A single wire of known thickness ought to work, but I would want to check the thickness with a micrometer. Have fun if you try it, Don Taylor -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#5
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Measuring eyepiece focal length
Jon Isaacs wrote: I am interested in measuring the focal lengths of various eyepieces with reasonable accuracy, maybe 5% or less. I am aware of that one can just measure the exit pupil but measuring the exit pupil accurate, especially for a short focal length piece seems difficult. Thoughts, suggestions, links... jon Here is a method that worked very well for me when I wanted to check on the focal lengths of my eyepieces. It gave results accurate to about 0.1 mm in the focal length even with short focal length eyepieces. It basically works in reverse of the classic method of measuring exit pupils. Instead of measuring the exit pupil we place an illuminated grid of closely spaced lines lines where the exit pupil would normally be and use the eyepiece lens to project the grid onto a distant screen. For the grid I used a ronchi grating. Now instead of measuring a tiny exit pupil we measure the vastly larger projected grid spacing on the screen. The setup consists of a flashlight shining through a Ronchi grating (120 lines/mm in my case) which is placed about where the eye would go if it were looking through the eyepiece. Be sure to shield any stray light from the flashlight that doesn't go through the gratiing so that it doesn't fall on the screen. The eyepiece is then carefully moved toward or away from the Ronchi grating (I used an old rack and pinion focuser to aid this but it wasn't essential) until the lines are sharply in focus on a screen (I used a white wall of the room) a couple of meters away. One then measures the center to center spacing of the projected Ronchi lines. For the longer focal length eyepieces I measured the distance covered by 5 lines and then divided that by 5. To find the focal length one first calculates the magnification, M = D/d where d is the spacing between lines on the grating itself (1/120 mm in my case) and D is the spacing as magnified by the projection. I then calculate the focal length of the eyepiece as f = L/(M+2) where L is the distance from the Ronchi to the projection screen. This is an approximation (but a good one) in that it assumes that the separation between principal planes of the eyepiece is small compared to the distance to the screen. Thats why you want the screen to be on the order of at least 100 times the eyepiece focal length from the Ronchi grating. I also tried this method using a transparent millimeter scale instead of the Ronchi. That works OK for the longer focal length eyepieces. If one doesn't have either of these a single wire of known thickness might also serve. The technique can also be used to measure the focal length of Barlow + eyepiece combinations. In that case it gives the effective focal length of the eyepiece as used with the Barlow. By dividing that result into the focal length measured for the eyepiece by itself, one can get the Barlow's amplification factor as used with that eyepiece. A more exact formula for finding the focal length is according to my derivation: f = (L-h)/(M + 2 + 1/M) where h is the separation of the principal planes. I have been trying to think of something that would work as a substitute for the Ronchi grating. A single wire of known thickness ought to work, but I would want to check the thickness with a micrometer. Have fun if you try it, Don Taylor -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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