## searching a formula for a barlow

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soldevilla
Posts: 571
Joined: Thu Dec 16, 2010 2:49 pm
Location: Barcelona, more or less

### searching a formula for a barlow

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
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Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:
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
Posts: 571
Joined: Thu Dec 16, 2010 2:49 pm
Location: Barcelona, more or less
Many thanks, Rik. This is a big help for me.

Thanks again

soldevilla
Posts: 571
Joined: Thu Dec 16, 2010 2:49 pm
Location: Barcelona, more or less

soldevilla
Posts: 571
Joined: Thu Dec 16, 2010 2:49 pm
Location: Barcelona, more or less
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
Posts: 8557
Joined: Sat Mar 14, 2009 3:58 am
Location: Near London, UK
At SS you can search on "cylindrical", but only a couple of lenses come up.
Chris R

rjlittlefield
Posts: 20987
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:
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
Posts: 571
Joined: Thu Dec 16, 2010 2:49 pm
Location: Barcelona, more or less
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