Re: ATM cold air sheaths around spiders, etc.

["Gene A. Lucas" <geneluca@ix.netcom.com>]

Bruce Swayze wrote:
> 
> At 01:06 PM 5/20/97 -0400, you wrote:
> >Gene A. Lucas wrote:
> >
> >snips
> >
> >> Most amateurs also do not consider the skin of cold air that clings to
> >> broad (wide, not necessarily thick), metal secondary support vanes,
> >> increasing the OPTICAL thickness (obstruction) of the vanes,
> >
> >snips
> >
> >I wonder what your theory is about the formation of these cold air
> >sheaths?  I assume it is due to radiational cooling, and also the effect
> >of high [compared to plastics] thermal inertia of metallic components.
> >With a long dewcap - 2 times the tube diameter - is radiational cooling
> >still a problem?
> >
> >Comments from you?
> 
> "Skin" of cold air?  Interesting theory, but after almost 20 years of
> observing, I have never seen or heard of anything like this.  And why would
> air "cling" to a spider vane?  (Or anything else, for that matter?)   With
> all the air moving around and through the tube, I don't see how this would
> even be possible.
> 
> There is an extremely sensitive test that I often employ to look for seeing
> conditions, turbulence, tube currents, thermal effects of any kind, etc.
> This can be done by anyone with any kind of scope in the field under actual
> conditions.
> 
> Point your telescope at a very bright non-stellar object.  The moon works
> well, but it can be too bright.  Jupiter is even better.  Remove the
> eyepiece and rack the focuser all the way in to give you some eye relief.
> When you look through the focuser, center Jupiter in the field and when you
> get your eye just right (at the focal plane), it fills the field of view and
> illuminates the whole mirror.  Any
> atmospheric turbulence will be easily seen whooshing by.  Any tube currents
> will be visible.  Body heat drifting into the optical path will be seen.
> And here's the amazing part:  hold your hand in front of the tube and you
> will literally see the heat waves coming off your hand and drifting around.
> I can't believe the heat coming off my hands even when they feel "cold" to
> me.  If I grab the end of the tube to move the scope, I can see the effects
> of the heat from my hand drifting into the optical path for several moments
> until it dissipates.  This test is so sensitive that if there were any skins
> of cold air clinging to the spider vanes, they would be visible at this
> time.  And I've never seen them.  In any other scope either.
> 
> IMHO, You'll be far better off with thin broad metal vanes than you will
> with thicker vanes made of some other material.  A lot of interesting and
> thought provoking theories thought up in the lab just don't hold up in
> actual use in the field.
> 
> Bruce Swayze
> swayze@europa.com
> http://www.europa.com/~swayze   Swayze Optical Home page
> http://www.europa.com/~swayze/spider.html   Homemade spider page
At the risk of wasting bandwidth by repeating earlier messages, I will
reply this time by including Swayze's message, which jumps ahead with
his own "definitive" answers before I could reply to an earlier
posting.  Now my intent earlier and here is to bring out certain
opinions and ideas _for_ discussion, not to hypothesize about the
physics.  In all cases, one will gather the most information by your own
direct observations.  The purpose of the discussion is to establish an
enlightened course of observation; in other words, suggest a place to
look for information. I will also comment that I have over 30 years of
observations in astronomy, and I have seen certain phenomena that
reinforce my ideas. While I appreciate Mr. Swayze's opinions. ideas, and
information (especially bringing out the thermal current visualization
technique, which I was also aware of), _nothing_ kills an interesting
discussion quicker than the "expert" who jumps in with the "last word". 
This is evidently symptomatic of both the Internet in general and this
net in particular.  And yes, I too am very quick with the "send" button
at times.
That said, I would like to add these replies:
1)  My use of the word "clinging" in reference to any supposed cold air
sheath around metallic spider vanes is probably unfortunate.  My
thoughts were more at the concept of air being cooled by metallic
structures in the optical path; a continuing source of cold air would be
the metallic spider vanes (and/or _any_ metallic structures around the
secondary or in the optical path); the thickness of such a cold air
layer would be relatively constant (as the metal radiates heat to the
sky and stays colder than the surrounding air), thus a "layer"
"clinging" to the vanes.
A dew shield would definitely reduce the radiation to the sky by any
metal parts, thus slowing the cooling of said metal parts.  (See my
earlier posting some time ago about dewing for details.)  Obviously the
colder, more dense air will drain off into the surrounding air in the
optical path.  This air flow may or may not be visualized with the
technique described by Swayze. 
2) I am familiar with the "Schelierin" technique (my terminology and
spelling) air flow visualization described by Swayze - and it is indeed
a sensitive and excellent way to visualize air flow and tube currents
(also the optical characteristics of the system, and atmospheric air
currents).  My comments were originally related to a question regarding
mechanical supports, and the other points were added as a means of
generating discussion.  I guess I succeeded too well.
3)  Everhart's experiments on telescope diffraction in the 1950's were
conducted using a back-to-back 6 inch aperture optical test rig -- a 6
inch mirror threw a collimated beam into another 6 inch mirror; the
image of the 2nd mirror was examined for diffraction effects when
various secondary supports were inserted in the collimated beam.  The
results were compared subjectively and sketched by visual means.  The
experimenters reported some of the results were too faint to
photograph.  This would be an interesting experiment to reproduce using,
say, a laser light source and a video camera, also with a larger
aperture.   Everhart's criteria based on his experiments (as I recollect
the article, which I don't have in front of me) is that a maximum
thickness for straight spider vanes (below which the diffraction spikes
were not detectable visually in the test) was 1 percent of the
aperture.  Now that implies that most amateurs today use very much
thinner spider supports.  I suggest that Everhart's results imply that
we may be able to use much thicker vanes than typical, leading to more
robust secondary supports. 
4)  I do not care to agree at this time with Swayze's statement about
"broader, thin" supports being better (Even regarding diffraction.)  The
purpose of my comments is to solicit _more_ discussion and
experimentation, not "definitive" statements.  Most of the typical very
thin secondary supports on amateur scopes I have seen produce wobbly,
springy supports that allow the secondary to move and sag; thin,
foil-like vanes are also prone to twisting and kinking, which will
produce more diffraction, not less.  IMHO, better to use a thick, stable
vane system.
5)  Swayze's comment about "higher thermal inertia" for metallics I
believe is misstated.  The statement as transmitted implies metals cool
off slower than air, which I believe is just opposite to experience.  
6)  I acknowledge that no matter how thin, a straight vane always has
two edges, therefore, the diffraction produced by this "negative slit"
(terminology contributed earlier by another correspondant) will consist
of two overlapping patterns for each straight vane.

So have at it kids, but don't flame at me.  Right or wrong, my opinions
are mine.  You want me to agree with you, play nice.  I'm going on
vacation to IAPPP and RTMC, and won't be answering for awhile.  (Yeah,
it's chicken, but what the hell, I'm not such an expert and I don't
enjoy getting my tail feathers singed evey time I present an opinion. 
Lighten up, gang!)
Gene A. Lucas (AstroVideoGrapher in Phoenix)