Re: ATM 3-Vane or 4-Vane Spiders

["Richard" <richas@earthlink.net>]

Sorry, even with no vanes at all there will be spikes caused by the edge of
the aperture.  However, because the aperture edge is curved, the spikes are
spread uniformly around each star image.  These edge spikes, as well as the
spikes from spider vanes, might be suppressed if there could be a gradual
blended shading around the obstructions.  Of course, there may be other
undesireable consequences.

It's too bad that physics actually works.

----- Original Message -----
From: "William Thornley" <billt051@hotmail.com>
To: <AMB2@pge.com>; <atm@shore.net>
Cc: <jf@arn.net>
Sent: Thursday, October 11, 2001 2:05 AM
Subject: RE: ATM 3-Vane or 4-Vane Spiders


>
> G'day All,
>
> Any subjective test needs to be done 'double blind' (so to speak). ie.
More
> than two observers looking thru more than two scopes, with at least one
> placebo and none of the observers nor the recorder knowing which two vanes
> were flocked (so to speak). Even better, how about a range of flocked
vanes
> None, 1, 2, 3 and 4?
>
> 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!
>
> Regards,
>
> Bill Thornley
> Sydney, Aus.
>
> >FWIW - John Dobson has long recommended three-vane spiders made from wood
> >(e.g., cedar shingles), rather than metal.  He likes three vanes largely
> >because it gives you options for the largest possible unobstructed
aperture
> >when you use an off-axis mask.  As best I recall, one of the reasons he
> >likes using wood is so as to minimize thermal effects.  It does seem like
> >the extra thickness is a down side to using wood, though.
> >-- Andrew Bell
>
>
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