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View Full Version : calculating FOV and Exit pupil when using barlow.



deejayry
2007-Nov-03, 08:48 PM
i'm just writing a list of calcualtions useful in astronomy.

I'm aware how to work out the true FOV and Exit Pupil when not using a barlow, but i'm under some confusion about making these calcualtions for when a barlow is used.

would someone be so kind as to clarify it for me?

Hornblower
2007-Nov-03, 10:44 PM
i'm just writing a list of calcualtions useful in astronomy.

I'm aware how to work out the true FOV and Exit Pupil when not using a barlow, but i'm under some confusion about making these calcualtions for when a barlow is used.

would someone be so kind as to clarify it for me?If you know your barlow's magnification factor, just divide the original FOV and exit pupil diameter by that number.

Suppose a given eyepiece gives a 1o field and a 4mm exit pupil. The result of combining it with a 2x barlow will be 0.5o and a 2mm exit pupil.

deejayry
2007-Nov-03, 11:22 PM
Thanks

That is what i thought but i wasnt sure if there were any additional calculations to make

RickJ
2007-Nov-04, 12:08 AM
The problem is that a barlow's power changes depending on where the camera or eyepiece is located. The farther from the lens the greater the power. Two non-parfocal eyepieces will show a different power. Two camera's with a different back focal length will also have a different power. The shorter the barlow the worse this problem is.

If you want accurate results with an eyepiece you have to time a star crossing the fov [15" of arc=1" of time at the equator, if not at the equator then 1" of time equals 15" of arc x cosine(declination)] With a CCD you can easily measure it with any astrometry program.

Rick

Dave Mitsky
2007-Nov-06, 09:14 AM
Due to a phenomenon known as exit pupil throw out, the eye relief of an eyepiece, particularly one with a long focal length, tends to increase when it is Barlowed.

http://www.televue.com/ask_al/barlows/0059.html

http://www.astrosurf.com/luxorion/reports-barlow.htm

Dave Mitsky