No, the guy writing the note below doesn't have his math correct.
500mm / 90mm = 5.56 (approximately). The common 500mm f/8 mirror lens
only has an objective diameter of 62.5mm, a common size for lots of
small scopes and a lot smaller than 90mm.
And, no, engineering should not be expected to apply a correction factor
since it's called an f/stop and not a T/stop. T/stops are corrected for
light loss through the optical train and are what you'd use for exposure
determination.
Regardless of the light loss caused by the appreciable size of the
central obstruction, depth of field is still calculated based on the
f/stop and not the T/stop.
Despite the very large size of the secondary (at 43mm almost 50% of the
diameter of the objective) the actual area of the obstruction is only
22.8% or a bit shy of 1/2 stop (25%).
Chuck Norcutt
usher99@xxxxxxx wrote:
> Hi, Found this on Pnet (1999) about the Orion Mak 500. Perhaps it
> really is F8. Perhaps someone knows how to calculate the true
> aperture and thus DOF. May have to try one of these things. Mike
> "It's a longer focal length, isn't as fast, but it's a proven design.
> May or may not suit your needs. BTW, if the MAK-500 is a 90mm lens,
> and has a focal length of 500mm, how can it be an f/5.6??? You see,
> it has a central obstruction, so it's light gathering is less than a
> 90mm lens. Marketing did the math, but engineering should have
> applied a correction factor. It's probably closer to f/8 in terms of
> actual light gathering. With TTL metering it probably doesn't matter,
> but I think it's a bit deceptive. OTOH, everybody does it. Also,
> consider the solar filter. In spite of its optical problems, I really
> enjoy using my Meade to look at sunspots. We're near the peak of the
> solar cycle right now. Good luck!
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