Re: [ATM] Favorite Spider Designs

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Michael Wheatley wrote:
> I had thought that the spikes around stars, diffraction or otherwise, 
> were caused simply by diffraction around the edges of the spider so 
> that a wire spider with several times as many edges would move more 
> light into the spikes.
 > I expect the diffraction would be relatively predictable while the
 > refraction would depend on temperature differences and the
 > reflection would depend on ambient light and surface treatment.  I
 > can see why the wire spiders may be best overall.

The fairly well established rule that I go by is the amount of light 
diffracted is roughly proportional to the area of the aperture blocked 
by the spider vanes.   This has correlated with my observing 
experience reasonably well.

This makes sense when you think about it this simple way - the more of 
the aperture you block, the more light is not going into the image. 
This light would go into the image in an unobstructed system. 
Therefore, the more aperture blocked, the larger the difference from 
the unobstructed system.

The spider mentioned in the paper posted by Vladimir is interesting 
(and it reminds me of a similar scheme described in an article in one 
of the Willmann-Bell books).  While an interesting paper, the sources 
used were monochromatic, so I wonder how things change if one uses a 
non-monochromatic source.  Or is the diffraction only "reduced" for a 
smaller range of wavelengths near the He-Ne laser they used?

> I have since read Texereau's reference to a stable layer of air 
> heated or cooled by conduction along the surface of spider vanes that 
> throws light into the spikes by refraction.  I expect this is why the 
> lack of surface would give the wire spiders an advantage.

The advantages I see from wire spiders are
1) Light weight
2) Low price (wire is cheap)
3) Fast, simple construction
4) Almost zero wind profile (they aren't sails)
5) Spider vanes have small cross-sectional area
6) Very low thermal mass (as you mention above)

Disadvantages I see are
1) Not as sturdy as metal vanes (if you whack it it may break)
2) People always ask the same annoying questions about them ("Does 
that really work?") and then I have to answer them.  They still seem 
not to believe me.  :)

As a maker of four moderate-aperture telescopes with four-vane wire 
spiders (wire thickness of 0.020" to 0.010"), I notice four fairly 
thin diffraction spikes at lower powers, and they can show minima and 
maxima in brightness as you move away from the star.  As the power is 
increased, they tend to become less noticeable.  A three-vane spider 
would be slightly better in terms of diffraction, but the difference 
has not been enough to motivate me to change the design.

Want to play with spiders and see the effects?  Borrow a refractor, 
aim it at a bright star, and experiment with various masks and fake 
spider vanes.  Or add a mask to your Newtonian.

Mike Lockwood
http://bi-staff.beckman.uiuc.edu/~melockwo/index.html


_______________________________________________
ATM mailing list http://www.atmlist.net/