[ATM] Thin Spiders & Diffraction - Some Practical Considerations

["Don Ware" <mimas@charter.net>]

While in theory, the thinner the spider the smaller the diffraction effect
you will see,
there are some practical issues that should be considered if you're
fabricating spiders
from sheet stock.  For instance, how accurately can you fabricate and
install a spider
where all the vanes are absolutely parallel to the optical axis through
their entire length?

In the following tables I calculated the apparent width for spiders of width
1" and 2"
angled at only 1/10 degree from parallel.

1" spider width

spider thickness      apparent thickness

.010                        .0118
.005                        .0068
.002                        .0038
.001                        .0028
.0001                      .0018

2" spider width

.010                        ,0135
.005                        .0085
.002                        .0055
.001                        .0045
.0001                      .0036

In practical terms I doubt you buy yourself anything with spiders thinner
than .002" and
as 1/10 degree is probably optimistic, spiders thinner than .01" probably
don't buy much
either.

Regards,

Don Ware
http://webpages.charter.net/dware1/


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