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ATM [Fwd: spiders]

Harold Suiter wrote:
> 
> ATM Spider Thread Participants
> 
> I have been reading this thread with great interest. During the past year,
> Bill Zmek and I have written an article about spider diffraction that
> parallels much of the discussion in this list. (Mr. Cook mentioned it in a
> recent post.) We calculate many spider patterns using Fourier transforms of
> the pupil.  We discuss wire spiders and thermal inertia (we came to largely
> the same conclusion as Bratislav; wire spiders don't diffract less but they
> have small mass and--significantly--are hard to paint). At the risk of
> giving the article away prematurely, let me sketch out our conclusions:
> 
> 1) If you're going to retain straight vanes on the spider, keep them as thin
> as possible. Not only will you diffract less light, you'll throw it farther
> from the core of a stellar image. In dark-sky fields, the spikes will be
> much less apparent. Try to minimize the area of the mirror covered by the
> spider. Three vanes, if you can get away with it, is better from an area
> viewpoint. Diffracted light is a function of area covered.
> 
> 2) If you want to curve the vanes to reduce spiking in one direction, go
> ahead, but realize that this stratagem won't improve the contrast of
> extended objects (like the planets or especially the moon). Make your
> curved-vane spiders with odd numbers of vanes, a single vane going through
> 180 degrees or 3 vanes going through 60 degrees.
> 
> 3) Windows require excellent wavefronts to beat a very thin-vaned spider.
> Where this is more likely is in smaller sizes.
> 
> 4) Thick Couder-style "spindle" masks cover the spikes, all right, but they
> scatter so much additional light that the cure is worse than the disease.
> 
> Dick Suiter

-- BEGIN included message

Mel,
Would you put the following on the ATM list.
Dick Suiter

-------

ATM Spider Thread Participants

I have been reading this thread with great interest. During the past year,
Bill Zmek and I have written an article about spider diffraction that
parallels much of the discussion in this list. (Mr. Cook mentioned it in a
recent post.) We calculate many spider patterns using Fourier transforms of
the pupil.  We discuss wire spiders and thermal inertia (we came to largely
the same conclusion as Bratislav; wire spiders don't diffract less but they
have small mass and--significantly--are hard to paint). At the risk of
giving the article away prematurely, let me sketch out our conclusions:

1) If you're going to retain straight vanes on the spider, keep them as thin
as possible. Not only will you diffract less light, you'll throw it farther
from the core of a stellar image. In dark-sky fields, the spikes will be
much less apparent. Try to minimize the area of the mirror covered by the
spider. Three vanes, if you can get away with it, is better from an area
viewpoint. Diffracted light is a function of area covered.

2) If you want to curve the vanes to reduce spiking in one direction, go
ahead, but realize that this stratagem won't improve the contrast of
extended objects (like the planets or especially the moon). Make your
curved-vane spiders with odd numbers of vanes, a single vane going through
180 degrees or 3 vanes going through 60 degrees.

3) Windows require excellent wavefronts to beat a very thin-vaned spider.
Where this is more likely is in smaller sizes.

4) Thick Couder-style "spindle" masks cover the spikes, all right, but they
scatter so much additional light that the cure is worse than the disease.

Dick Suiter

-- END included message