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Brian Engel
member
Reged: 08/25/09
Posts: 89
Loc: Cincinnati,Oh
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I am making a ross null tester and I am planning on using this lens from surplus shed... http://www.surplusshed.com/pages/item/l10266.html
The problem is I am not absolutely certain what glass it is made from (neither do the folks at surplus shed). I am presuming BK7 (which is probably a good guess) but I am curious to know if there is a way to determine this by an amateur without expensive equiptment.
The excellent Ross Null software can adjust for different wavelengths of light based on the glass type but I don't know exactly what it is!
Thanks...
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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First, I would determine the density, that will narrow the glass types. Since it is a lens you can measure it's exact focal length in a monochrome light of known wavelength and using the thick lens formula to back out the refractive index. If you can measure the focal length at a couple of wavelengths that will be even better.
If you have a spherometer, then measure R1,R2 and the center thickness and with the given focal length of 275mm, again use the thick lens formula and back out the index of refraction.
- Dave
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Brian Engel
member
Reged: 08/25/09
Posts: 89
Loc: Cincinnati,Oh
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For a monochromic light source, is an LED good enough?
I also have to ask a dumb question. How do I measure the FL of a lens if the object is not at infinity? This is my ignorance since I have not worked with lenses. Easy with a mirror, measure RoC with knife edge and divide by 2!
I do have a sphereometer so I can back it out that way.
Thanks!
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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An LED is not monchrome enough but you can just look through an a 10nm or less bandwidth interference filter. To measure the focal length of a lens you need a small optical flat and setup the lens in double pass mode. Just use the center of the lens so it is stopped down to about f/10 since it will show a large amount of spherical abberation if you viewed the whole surface. Stopping it down will make it easier to null the center and find the focal length at the wavelength your testing.
The spherometer method is going to be the easiest and the Ross Null program calculates the focal length of the lens from the two radii, center thickness and refractive index. The focal length given by Surplus Shed is most likely for sodium light so use that wavelength and what you measure for radii and thickness and see how close you are to the given focal length using the refractive index for BK-7 in sodium light.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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