Dennis,
Thanks for the info! I've received some preliminary information stating that
the formula:

Apparent field of view = True field of view * Magnification

cannot be applied with certainty because of something called "pincushion
distortion"; I have no idea what this is, but I'll find out! It appears that
using this formula will not work, thus the mis-match in my results.

If anyone knows more about this, please let me know.

Paul Markov
Toronto, Canada
10-inch LX200 f10


Dennis Persyk wrote:

> Your results are substantially different from my measurements on three Meade
> 4000 series eyepieces, 5 Paul Rini eyepieces and a Scopetronix eypiece. In
> all cases my measured apparent FOV agreed to within a degree (which is my
> probable error) of the manufacturer's (or distributor's) published values.
>
> My technique is to measure a baseline of at least 20 focal lengths with a
> tape, affix a machinists rule (with graduations to 0.010 inch) to a post,
> measure the span of the FOV, and calulate the subtended arc by triginometry.
> One takes one-half of the span, call it x, divides by the baseline b (in the
> same units), and calculates arctan(x/b) =HA. This is the half angle, so the
> true FOV is TFOV = 2 x HA. From this one can determine the AFOV.
>
> I am aware that the focal length varies with focus distance in as SCT, but I
> believe the difference between infinity and 20 focal lengths is negligible.
>
> Dennis Persyk
> Hampshire, IL
>
> > Date: Sat, 09 Jun 2001 16:11:25 -0400
> > Subject: [M]: Advertised vs Real fields of view
> >
> > Before you can accurately test the centering accuracy of your LX200 you
> > must know the actual field of view (FOV) of your eyepieces. Calculating
> > the actual FOV based on the apparent field of view is a good
> > approximation, but for more precise results follow this method:
> >
> > Point your scope at a star near the celestial equator (ie dec = 0 deg)
> > and near the meridian, turn off the telescope drive and time how long it
> > takes the star to cross the eyepiece's field of view with a stop watch.
> > The actual field of view in degrees is the time you measured in seconds
> > divided by 240 seconds. Just multiply by 60 to convert that number to
> > arc-minutes.
> >
> > Here are my results on the following eyepiece on my 10-inch f10
> > (f.l.=2500mm)
> > 12 mm Meade MA illuminated reticle - 10.5 arc-min
> > 15 mm Televue Plossl - 15.25 arc-min
> > 26 mm Meade Super Plossl - 27.25 arc-min
> > 32 mm University Optics Konig - 30.75 arc-min
> > 40 mm Meade Super Wide Angle - 55.5 arc-min (2-inch)
> >
> > Now I can work the math backward and compare the *advertised* apparent
> > fields of view with the *real* apparent field of view:
> >
> > 12 mm Meade MA illuminated reticle: advertised 40 deg, real 36 deg.
> > 15 mm Televue Plossl: advertised 50 deg, real 41 deg.
> > 26 mm Meade Super Plossl: advertised 52 deg, real 43 deg.
> > 32 mm University Optics Konig: advertised 50 deg, real 40 deg. (15 years
> > old eyepiece)
> > 40 mm Meade Super Wide Angle: advertised 67 deg, real 58 deg.
> >
> > This was quite a revelation for me! It looks like that generally
> > speaking the advertised apparent field of view is about 10 degrees more
> > than the real apparent field of view!! Let us know what your
> > experiences have been!
> >
> > Regards,
> > Paul Markov
> > 10-inch LX200 f10
> > http://home.ica.net/~pmarkov/astro.htm
> >

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