Re: ATM 3-Vane, 4-Vane and No-Vane Spiders

["Bell, Andrew" <AMB2@pge.com>]

I find real advantages to using an off-axis mask, both for planetary
observing and for looking at close double stars, above and beyond
elimination of the diffraction spikes and even under the very best seeing
conditions.  To some extent, this may be a reflection on the quality of my
mirror [no pun intended][!]; it is a 14-inch f/5.3 that was figured by an
impatient 15 year-old [me, many years ago], and I know that it is not real
good at the edge.  But I've also surprised a couple of friends with the
improved clarity on their own scopes [at least one of which has a very
well-figured mirror] when using an off-axis mask to split tight doubles.

There are a couple of things that contribute to this.  By avoiding the
central obstruction and the spider vanes and any less-than-perfect optics at
the very edge of the mirror, you do get very noticeably improved contrast.
Just as importantly, you will also be doubling or tripling the effective
focal ratio, and long f-ratio optical systems are in many ways much more
tolerant than faster ones.  Suiter's book has a good table that shows focus
tolerance (in thousandths of an inch) as a function of focal ratio; if I
remember correctly, it improves with the cube of the f-ratio.

Under good seeing conditions, I can routinely get nice crisp images [and
readily split 1.2 to 1.5 arc-second doubles] at 400x magnification when
using my scope as an unobstructed 4-inch [at roughly f/18].  As an
experiment, I've also tried using some really tiny off-axis masks -- I have
been able to go down as far as about an inch and a quarter [f/50][!] and
still split both halves of Epsilon Lyra, at about 150x.  [At that long an
f-ratio, the zone of best focus was nearly an entire turn of the coarser of
the two knobs on my focuser.]

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> On Thu, 11 Oct 2001, William Thornley wrote:
> 
> Also, not having bothered to use an off-axis mask yet, what do to the
> spikes look like then? One would assume no spike at all!
> 
And William McHale responded:

> Yep, there would be no spikes, but of course your aperature would be
> limited to your unobstructed aperature.  Unless you are doing planetary
> work and the atmosphere limits your seeing to such an extent that you gain
> nothing by increasing aperature above the size of your aperature mask I
> don't see much reason to use one.  I doubt I have ever seen one used on a
> 6-8" scope becuase at those sizes the resolution is not nearly as limited
> by seeing.
>