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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