searching a formula for a barlow

soldevilla · Post by **soldevilla** » Sat Jan 12, 2019 12:24 pm

Many years ago I used a simple formula that allowed me, knowing the diopters of a negative lens and its position before the focus, to know how far the resulting focus is away. As in a telescope barlow ...
I know that maybe it is not an issue of this forum, but I think that here there should be friends who know that formula and can help me. I am trying to repair a telescope and I am blocked.

Thanks

rjlittlefield · Post by **rjlittlefield** » Sat Jan 12, 2019 7:17 pm

Let -DB be the diopter power of the Barlow lens, and DIF be the distance from the Barlow to the initial focus, in millimeters. Then at the position of the Barlow, the light will be converging with a power of 1000/DIF diopters as it enters the Barlow, and converging with a power of 1000/DIF-DB diopters as it leaves the Barlow. The new focus point will be at 1000/(1000/DIF-DB) behind the Barlow.

For example, suppose that a Barlow lens of power -20 diopters (-50 mm focal length) is placed at a position that is 33.333 mm away from the initial focus point. Then the light will initially be converging at 1000/33.333 = 30 diopters, changed to 30-20 = 10 diopters by the Barlow, giving a new focus point that is 1000/10 = 100 mm behind the Barlow. The magnification of the Barlow is the ratio of new versus old focus distances, so 100/33.333 = 3X in this case.

If I've done everything right, this is just a different (and much simpler) analysis of the lens setup shown at https://en.wikipedia.org/wiki/Barlow_le ... low_02.png , which is referenced from https://en.wikipedia.org/wiki/Barlow_lens .

--Rik

soldevilla · Post by **soldevilla** » Sun Jan 13, 2019 4:21 pm

Many thanks, Rik. This is a big help for me.

Thanks again

soldevilla · Post by **soldevilla** » Mon Jan 14, 2019 1:28 am

soldevilla · Post by **soldevilla** » Tue Jan 15, 2019 12:52 pm

Does anyone know where to buy simple lenses, flat concaves, cylindrical lenses to correct astigmatism? I bought in Surplus Shed but since they changed the website I can not find anything ...

Thank you

ChrisR · Post by **ChrisR** » Tue Jan 15, 2019 1:55 pm

At SS you can search on "cylindrical", but only a couple of lenses come up.

rjlittlefield · Post by **rjlittlefield** » Tue Jan 15, 2019 3:35 pm

Check also with your local eyeglass maker. Once upon a time, many years ago, I needed a special corrective lens for use in a CRT-to-film movie recorder. The first couple of quotes I got were painfully expensive, but then it occurred to me that the specification looked a lot like an eyeglass prescription so I asked my local optician. It turned out to be a custom part, but even so, going that route the price was so low that I bought two lenses just to have a spare, using cash that was already in my wallet.

--Rik

soldevilla · Post by **soldevilla** » Tue Jan 15, 2019 4:10 pm

Thank you. I am talking to a friend about eyeglass lenses. I tried, many years ago also, a long telescope of 2 meters to have the vision offered by the old single-lens telescopes. I used a lens for myopia, but the vision was horrible, it looked like a fresnel. That's why I was looking for another alternative. But you're right, the eyeglass store lenses are so cheap that I can do a test without my VISA getting angry with me.

rjlittlefield · Post by **rjlittlefield** » Tue Jan 15, 2019 4:30 pm

soldevilla wrote:I used a lens for myopia, but the vision was horrible, it looked like a fresnel.

I'm thinking that was a "bending" issue. The lens probably had a strong meniscus, designed to be used by an eye only a short distance behind it. When used as a telescope, forming an image far away, it would have had a lot of spherical aberration. If you can find a good optician, one who really understands how the light works, then explain to him what you're trying to do and maybe he can make a good match.

--Rik