I thought this would be easy to find on the WEB, but no luck. How do I calculate the total magnification when I stack 2 or more Powermates? Is it different if they are upstream or downstream from the diagonal? Thanks for any info...
How to calculate Powermate/Barlow magnification wh
Started by Roughneck-AM, 07/26/2006 02:18AM
Posted 07/26/2006 02:18AM
Opening Post
Posted 07/26/2006 05:00AM
#1
ed manspeaker said:
I thought this would be easy to find on the WEB, but no luck. How do I calculate the total magnification when I stack 2 or more Powermates? Is it different if they are upstream or downstream from the diagonal? Thanks for any info...
It isn't easy to find on the Web, because it's not a simple formula. What you get depends on the spacing. The amplification you get depends on the spacing between the Barlows/Powermates, and also between the rear Barlow/Powermate and the eyepiece that goes into it. Since those aren't fixed, neither is the formula.
If the Barlows are parfocal with each other and with the eyepiece, and you don't put anything else in between (like a diagonal), then you should be able to multiply the amplification factors together and get something like the correct result. But otherwise, all bets are off.
Posted 07/26/2006 02:17PM
#2
Use a tape measure and measure it in the daylight.
Setup a tape measure on something like a fence, far enough away to reach focus. Measure the field of view with something like a medium focal length eyepiece, a plossl should work. See how many inches you see - lets say you are at 100 inches.
Then using the same eyepiece, see how many inches you see of the tape measure, then use the double barlow and see how many inches you see.
There is a chance that you will cut off or vignette some of the field of view, but assuming you aren't vingetting, the ratio of the field of view should equal the magnification ratio.
You can do the math knowing your first magnification of the unbarlowed eyepiece since magnification = telescope focal length / eyepiece focal length.
Lets say you have a 1000mm focal length scope, and your eyepiece is a 10mm eyepiece, which equals 100X. Your first measurement is 100 inches of tape measure.
Then you barlow and see 50 inches, so take 100/50 = 2X, and you are at 200X. With two barlows, lets say you see 20 inches which would then give you 100/20 = 5X, so you are at 500X.
Does that help?
Timm
P.S. I got this from the book "Astronomy Hacks" - a very worthwhile read, especially if you own a reflector, in particular a Dobsonian.
Setup a tape measure on something like a fence, far enough away to reach focus. Measure the field of view with something like a medium focal length eyepiece, a plossl should work. See how many inches you see - lets say you are at 100 inches.
Then using the same eyepiece, see how many inches you see of the tape measure, then use the double barlow and see how many inches you see.
There is a chance that you will cut off or vignette some of the field of view, but assuming you aren't vingetting, the ratio of the field of view should equal the magnification ratio.
You can do the math knowing your first magnification of the unbarlowed eyepiece since magnification = telescope focal length / eyepiece focal length.
Lets say you have a 1000mm focal length scope, and your eyepiece is a 10mm eyepiece, which equals 100X. Your first measurement is 100 inches of tape measure.
Then you barlow and see 50 inches, so take 100/50 = 2X, and you are at 200X. With two barlows, lets say you see 20 inches which would then give you 100/20 = 5X, so you are at 500X.
Does that help?
Timm
P.S. I got this from the book "Astronomy Hacks" - a very worthwhile read, especially if you own a reflector, in particular a Dobsonian.
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