Re: [ATM] The best time to Foucault?

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> IDEAL (but ignoring the beveled edge loss):
> If I had an 8" mirror the surface area would be 201 square inches.
> Subtracting the loss for a 1.5" secondary (7 square inches), I get 194
> square inches of light gathering.
> Limiting magnitude 7.5 + 5 LOG (20.32 cm aperture) = 14.03
> 
> MASKED (and still ignoring the beveled edge loss):
> If I masked 5%, just less than 1/4 inch all the way around, I would now
> have
> a 7.6" mirror and the mirror would be 181 square inches.  Subtracting the
> same secondary, it's now 174 square inches of light gathering.
> Limiting magnitude 7.5 + 5 LOG (19.30 cm aperture) = 11.90
> 
> TOTAL LOSS:
> 20 square inches of light gathering (10.3% of the usable mirror surface)
> and
> 2.1 magnitudes of celestial objects.


Something went wrong with the math!  A 1/4 inch turned edge on a 8 inch
aperture scope results in 0.05 (1/20) magnitude light loss - about the same
light loss you'll have after a few months when your coating degrades.

Limiting mag on a 8 inch is closer to 15.5 magnitude.  Remember to adjust
the equation for a smaller exit pupil than 7mm (unless you intend for your
scope to be a RFT).  Going to a 1 mm exit pupil gains you about 2.5
magnitude.

Mel Bartels


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