MSU - SOAR Telescope Project Newsletter
February, 1997
Thanks to all of you who have shown an interest in this new telescope.
This is the first of what (I hope) will be monthly updates on our
progress.
For those of you who only left an address but no email address, I will
be mailing this first newsletter out directly but ask that you please send
me an email address (if you want to get it via email).
In any case, the newsletter will also be available (a week after it
comes out - i.e. about the second week of each month) at
/www.pa.msu.edu/soarfunds/news.html. Those of you who wish to
pick it up there and don't want further email can just reply
to this with instructions to cancel your email listing. The advantage
of the on-line newsletter, obviously, is that it can (and will) have
images illustrating some of our studies.
Its early days yet. We have the glass for the mirror in
hand (or actually stored at Corning in upstate New York) but that is
the only physical sign of a telescope so far.
What we do have is lots of design studies going on both here, at Michigan
State, and at NOAO (National Optical Astronomy Observatories, the
organization which will be operating the telescope) in Tucson. Since the
project was first thought of (over ten years ago) there have been several
new 4 meter-class telescopes built (ARC - which you can find out about at
/www.apo.nmsu.edu/ and WIYN - at /www.noao.edu/wiyn/) and new, larger
telescopes in the planning stage (such as GEMINI - /www.gemini.edu/ ).
These new projects have made it clear that we "probably" will want to
build a telescope with a very thin mirror (10 cm thick for a 4M aperture).
An even more "radical" possibility is that of building a telescope
with no obstructions in the light path (a so-called "off-axis" telescope
with the reflecting secondary off to one side - a sketch of this
is on our web page at /www.pa.msu.edu/soarmsu/tel.html)
Both these possibilities have never been done for such a large telescope
and, naturally, we are concerned about their feasibility and the
balance between scientific pay-off and risk which they entail.
In both cases the issue is one of how best to control the distortions
in the telescope caused by both the instrumental profile of the telescope and
what is known as "atmospheric seeing." A lot of progress has been made over
the past 20 years in understanding the causes of these distortions.
For "seeing" distortions most problems are caused by the slight
differences in refraction experienced by light as it passes through
air pockets with different temperatures (and thus different densities
and refractive indices). Temperature differences as small as 0.5 C
can significantly distort the image.
Distortions introduced at the surface of the primary mirror by a
temperature difference between the mirror surface and the surrounding
air can be minimized by designing the mirror so that it cools rapidly enough to keep these
differences below 0.5 C. (The night time temperature drop at a mountain
observatory can be 10 to 15C on a clear night and the telescope must
cool along with the atmosphere.) A "thin" mirror (of 10 cm) cools with a
time scale of about 30 min - enough to keep up with this temperature
change. A "thick" mirror (of 20 cm, which was the original SOAR design)
takes over an hour to cool by the same amount and thus must be artificially
cooled (by refrigerated coils during the day - which also require the surface
of the mirror to be heated to prevent icing!) Applying the KISS principle
(Keep It Simple Stupid), makes using a thinner mirror seems like a
"no brainer." However, a thin mirror will tend to sag under gravity as
the telescope moves and needs a lot more active support (from
"actuators" attached to the back of the mirror and repositioned under
computer control). At issue is the trade-off between a more complex
support system and the advantages of a lower thermal distortion.
Distortions introduced by air pockets well above the telescope mirror,
can be (partly) removed by changing the focus and figure of the telescope
optics (several times every second!) This is "adaptive optics" (or
"active optics"). SOAR will be designed to do minimal corrections of
atmospheric distortion from the start by using a "tip-tilt" secondary
(which takes out the distortions that shift the center of the image).
However, once the telescope is built and up and running, we plan to
do more active corrections and these MAY be done by distorting the
figure of the primary mirror to compensate for the atmospheric distortions.
(A complex task which needs much more computer controls than the simple
ones needed to correct gravitational distortions to the mirror figure.)
The other possibility is that the atmospheric distortions may be corrected
by complex distortion of the telescope secondary mirror. What we are trying
to do before we start construction is sort out the ``best'' way to
build the telescope so it can be easily modified later on to use
these more advanced types of adaptive optics. (Since the entire field
of adaptive optics is in rapid evolution, this is a bit like fortune
telling.)
The other optical design issue we are looking at (off-axis telescope
structure vs. on-axis structure) is coupled with the question of
how best to deal with atmospheric distortion. Classical reflecting
telescopes all have either an instrument or a secondary mirror in the
light path between the sky and the telescope primary. This gives rise
to complex images (for point sources) which have spikes and halos
around them (the classical cross-like star patterns you are accustomed
to seeing on astronomical images.) In addition, the light scattered
by these obstructions into the telescope field of view from bright
stars outside it, increases the sky background is an irregular way.
(Since most parts of galaxies and extended nebulae are well below
sky brightness, this can be a significant problem.) For a telescope
which is limited by atmospheric "seeing" the extra scattering near
the core of the star is not very important, because the star image is
smeared out by the atmospheric distortions. (The large scale scattering
which comes from stars outside the field is ALWAYS important and limits
our studies of galaxies). However, this central distortion by seeing
becomes more important as the image is improved by active optics.
An off-axis telescope does not have obstructions in the primary
light path AND it does have a secondary which is located off to the side
(and may be easier to work with if we decide to modify the atmospheric
distortion by adjusting the secondary mirror.) So seems at first thought
that it makes sense to go with an off-axis design. However, some scientists
fear that such a design might limit the types of observations they already
can do with an on-axis telescope and all of us worry that a radical, new
design might be more difficult to build and thus more expensive.
To address the scientific concerns we have been modeling the type of
images the two types of telescopes would produce, both in an ideal
case (no atmosphere) and in the case where the "tip-tilt" secondary
method of removing some of the atmospheric distortions is used. This
type of modeling is similar to what was used to understand the distortions
in the HST images to help fix the optics for that telescope.
You can see
the results of some of our simulations
HERE:
Next time I will review some "ancient history" to give you an idea
of how the ORIGINAL site surveys for the Cerro Tololo observatory
were done (by burro). Thank goodness that is one problem we do not
have to solve!
Susan Simkin,
Professor of Physics and Astronomy
Michigan State University
(simkin@pa.msu.edu)
Susan Simkin (Simkin@pa.msu.edu) last updated: 29 Sep '97
