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Rule of thumb for using barlow element on EP?

7 replies to this topic

#1 cheapersleeper

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Posted 28 February 2013 - 11:57 PM

Is there any way to even get close estimating what a barlow element will do directly on an EP? Just in the ballpark would be quite dandy.

Any caveats regarding the practice in general?

Regards,

#2 cheapersleeper

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Posted 01 March 2013 - 01:05 AM

Cloudy Nights search did not do much but a Yahoo search gave me this:

http://www.cloudynig...5591995/Main...

I think that covers the topic well enough for my needs.

#3 csrlice12

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Posted 01 March 2013 - 09:57 AM

2X doubles the power, 3X triples the power, etc....(you did ask ballpark!). There was a thread here awhile back where someone had a chart/spreadsheet that would help compute that. Like Baader Rings, the further out of the barlow you pull the eyepiece, the greater the mag...

#4 Karl Fabian

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Posted 02 March 2013 - 12:19 AM

A ballpark way to figure out the barlow amplification factor is to measure the distance from the barlow element to the top shoulder of the barlow tube. For instance if the unit is labeled as a 2X barlow and the distance is 4 inches it is a (-)4 inch FL barlow. That means when the focal plane (field stop) of the eyepiece is 4 inches from the barlow element it is 2X. If you moved the eyepiece another 4 inches out with an extension tube (8 inches total) you add 1x. (2x plus 1x = 3x) If you increase the length only 2 inches to 6 inches total it will add only .5X or become 2.5X. If you have a shorty 2X barlow with 2 inch length tube from lens element to housing shoulder it will become 3x if you lengthen the distance from 2 inches to 4 inches total. Likewise if you make the distance shorter like only 1 inch from the focal plane of the eyepiece it will be 1.5X . The formula for the amplification factor is arrived at by dividing the distance of the barlow lens from the focal plane of the eyepiece by the focal length of the barlow and then adding 1 to the results. With most barlows the focal plane of the eyepiece is assumed to be at the shoulder of the barlow tube when they label the magnification factor, but that can differ among eyepieces. The magnification with a barlow lens can vary depending on how high or low the field stop of the eyepiece is positioned in the tube.

#5 cheapersleeper

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Posted 02 March 2013 - 12:47 AM

Thanks very much for your post, Karl. I have three barlows and am doing a little webcam imaging and would like to be able to change some configurations to get different amplifications. I think that info will give me a close enough approximation to be useful.

#6 Ed D

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Posted 02 March 2013 - 01:09 AM

Brad, all the houses in my neighborhood have barrel tile roofs. What I do is first look at the peak of a roof and count tiles in the field of view, then do the same with whatever barlow I want to test. A simple ratio can give the ammount of magnification. You did say a rough estimate, and this gives me a pretty good idea, not exact to the Nth decimal but good enough for me.

Ed D

#7 cheapersleeper

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Posted 02 March 2013 - 01:25 AM

This is Texas, Ed, no tiles here!

That's a good idea, though... Might do something similar tomorrow.

Thanks,
B

#8 Scanning4Comets

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Posted 02 March 2013 - 02:04 AM

Drift Method...

Not very accurate, but this will give you a "ballpark" figure.

Cheers,

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