[ATM] is mirror thermal equilibrium important

[Tom Krajci]

>From: "Vladimir Galogaza" <vladimir.galogaza1 at zg.t-com.hr>

>Imagine, after tons of discussions about indisputable need for mirror
>temperature equilibrium,  e.g.. while on test stand, while waiting for
>observations, while pressing... etc),   one demonstrably
>practical & working solution  is  using forced heating of backside
>and forced cooling of the front side of the mirror while in use.
>Thus warranting lasting temperature gradient across the mirror.

The solution given is for dew prevention...and I see two options:
- heat the mirror above dew point
- don't observe when you get close to 100% relative humidity

Heating the mirror distorts it, and adds problems with non-homogeneous air
in the light path.  This spoils image quality, but if you didn't do it you
couldn't observe at all.  (In a mild dewing situation, the small amount of
heat required will not really do much damage to image quality...I'm talking
about heavy dewing situations, where much more heat is required.  Also,
mirrors and lenses radiate heat to the sky, so they tend to drop below
ambient air temperature unless measures are taken to keep them closer to
ambient temperature, such as active ventilation or gentle heating.)

If heat is applied to the back of the mirror in a *uniform* manner...the
mirror's figure won't change much...what will change is the overall radius
of curvature.  Your paraboloid mirror will essentially remain a
paraboloid...just a different focal length.  Uneven heating of the mirror
will introduce zonal and irregular/local changes of figure.  One example of
uneven heating/cooling of a mirror is the 'edge effect'...the edge of the
mirror cools faster than the center zones...the mirror exhibits a
rolled/turned down edge while its warmer than ambient air.  Edge effect is
most pronounced in plate glass mirrors because this material has a higher
coeff. of thermal expansion compared to Pyrex or other substrates.

The professional observatories often will shut down when relative humidity
approaches 100%.  No point risking dew damaging the coating in an expensive
telescope.

>Thermal equilibrium redefined:
>It does not necessarily mean that
>all parts of mirror are or should be at the same temperature as
>environment.. It may mean that it does not hurt if
>temperature gradient is present if it is temporally and spatially constant.
>Because in this situation mirror figure, whatever it is,  will not change.
>That it will be different than at test stand is another story.

Maksutov had some things to say about thermal gradients in mirrors,
especially those used in solar telescopes, where thermal gradients could be
large.  He did some calculations showing how much the mirror's radius of
curvature would change.  He favored thick mirrors in this regard because the
thermal gradient (temperature difference between front and back faces)
existed across a greater thickness of glass...which changed radius of
curvature less than for a thinner mirror with the same temperature
difference between front and back faces.

Maksutov went so far as to suggest that in solar reflecting telescopes:
- both front and back should be aluminized
- both front and back should be illuminated with sunlight

These steps would help reduce thermal gradients across the mirror's
thickness.

(One of these days I'll have these translations published.  I'm currently
slugging away on Maksutov's 170-page monograph on knife edge testing.)

-------------------------------------------
Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/

Center for Backyard Astrophysics (CBA)
http://cba.phys.columbia.edu CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
-------------------------------------------