Re: ATM Spider Design

[epabcc@epa.ericsson.se (Bratislav Curcic)]

Stephen Dillinger wrote :

> Being a bit snide aren't we?  I recall awhile back getting pilloried by you
> when discussing sperical aberration based on "theory" instead of practice.  Now
> you hammer on Ken for basing his question on his personal experience instead of
> a rigorous heat transfer calculation.

Sorry but I can't quite recall the "pilloring", so i can't comment on that.

In Ken's case, it was _him_ who suggested that theory may be the empty
one, so I gave the parctical advice (use thermometer) . If you want
full theory of a heat transfer and thermodynamics, I'd suggest a good 
book on physics.

> A good question, how good is good enough?  Having used Ken's scope I can attest
> to it's performance.  Everyone should have a scope that performs as well as
> his.  There's no doubt that four vanes can be made stiffer than three vanes,
> but do you really need to do it?

Well, YOU are the only person to answer that question. If we start talking
about differences in diffraction effects from using thinner spiders and 
different materials to suppres refractive effects, it is quite obvious
(at least was to me) that we are trying to extract EVERY little bit of
performance from our telescopes. The differences may be small, the 
differences may be invisible for most, but then why would someone insist
on a 1/20 wave optics when most of the nights most of the people won't see
the difference between it and a smooth 1/4 wave scope ?
The solid spider is of paramount importance for photography/CCDgraphy.
If you don't want that, fine. If a little wobbly spider if OK for you,
fine again. But do not preach that it is EQUAL to well designed one, and 
that will be good for everybody and for every purpose.
And it costs so little to make a proper one that is simply a shame NOT to
do it properly. Commercial manufacturers including.
  
> Worrying about thermal currents from
> over cooled spider vanes seems rather absurd compared to all the other problems
> we encounter.  Perhaps it's a real problem for some, but I'm with Ken on this,
> I've never noticed anything either.

Again, if you feel happy, I'm even happier for you. But remember that not
seeing something DOESN'T mean that it doesn't exist.

And now on cold spiders and refractive effects.

I think that we all agreed here that spiders get COOLER than an ambient
air during the night. The air in immediate vicinity will be of very
similar temperature, and we will have a smooth gradient as we go
further from the spider towards open air. In the state of equilibrium
(i.e. when radiative heat transfer from spider equals the conductive
heat that it absorbs from the surroundin air), there will be very
little, if at all air movement in the tube. Now, the refractive index
of air is function of TEMPERATURE, among the other things. The gradient
we have next to the spider will form a weak cylindrical lens. This lens
will divert the incoming light and focus into a broad line that is
coincidental with a diffraction "spike" that we have from "normal"
diffraction effects. In effect, this cooler air will "bend" light
around the spider. Result ? We have stronger diffraction spikes. Thing
that prompted me that these effects (indeed initially decribed for large
telescopes) may be plaguing amateurs too is spectacular success of some
of wire spiders. There is NO other reason that I can think of for wire
spiders to perform better, except thermal effects, as wires are of
comparable thickness to normal vane spiders, and should produce equal
amount of diffraction.

Again, if you think that this is heap of B.S. and it doesn't affect you,
fine. If you are happy with existing design, and it serves you well,
fine again. But there may be a small group of people INTERESTED in 
trying everything to improve the performance of their telescopes.
And for a suggestion that what we try to do is useless because you and
your friends think so, or "do not see" - well, you can expect a little
passionate reaction, that's for sure.

Bratislav