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Seeing, usually defined ( in arcseconds) as Full width Half Minimum
(FWHM), characterizes how tight a stars disc is at the focal plane
of the telescope (camera). Research indicates that most seeing
issues are local. Caused mostly by dome, telescope issues and
the local site . Thus the OMI design team has put a lot effort
into minimizing local seeing as much as possible.
The chart above is calculated with the photometric
tool. This data shows how the exposure changes for a 25th magnitude
star when going from 0.5" FWHM seeing to 3.0" for a
given signal-noise-ratio . OMI is expecting 1.0" to 1.25"
FWHM seeing, with frequent sub-arcsec seeing. The importance of
the seeing cannot be overemphasized, it is as important as the
aperture, sky brightness, FOV ,QE and telescope size! For example
a seeing of 0.85" FWHM takes half the exposure as compared
to a FWHM of 1.25".
The diagram below shows the effect of seeing on image quality.
The Hubble Ultra Deep Field (HUDF) here has been reduced to the
image scale of the OMI and a Gaussian filter applied to simulate
different seeing conditions from 0.5" to 3.0" which
it clearly demonstrates the importance of seeing.
Two nearby observatories, the 1.9 m David Dunlap Observatory
(DDO) and the 0.6 m Rideau Ferry Observatory (RFO) report 1.7"
and 1.5" seeing respectively. RFO reports (verbal communication)
that 25% of the time they have sub-arcsecond seeing. These telescopes
are not optimized for best local seeing. We believe that by optimizing
local seeing we can obtain something in the 0.8" to 1.25"
range.
The seeing has several sub-components as follows:
- Atmospheric seeing
- Site seeing
- Ground turbulence
- Local seeing
Local seeing can be further broken down smaller components:
- Dome seeing
- Optical Tube Assembly (OTA) seeing
Atmospheric seeing is mostly controlled by regional weather
patterns and is far away from the observatory. Apart from moving
the observatory to a different geographical area, nothing can
be done to change atmospheric seeing. In our area atmospheric
seeing is expected to be between 0.8" and 1.25".
Site seeing can be improved by situating the observatory
on the highest elevation possible and having no higher elevation
in the immediate area from the direction of the prominent winds,
in our case from the west. The idea here is to provide a smooth
air flow over the general site. You don't want be a in a valley!
The contribution from site seeing is hard to quantity but we believe
it could 0.3" or greater.
Ground turbulence is a very low altitude affect caused
by heat rising from the ground. To minimize ground turbulence
the observatory must be raised by several metres. Two metres would
be a minimum. Going higher above the ground has diminishing affect,
thus three metres was chosen for the OMI. In addition the air
must be able to free flow around the observatory , including above
and below. Thus, it is imperative that free space exist below
the dome, to minimize air turbulence. In addition, it accelerates
the dome ambient temperature tracking thus improving seeing. Evidence
suggest that ground turbulence is a least 0.25" from 2 metres
above ground (see below).
From the University of Tokyo Atacama Observatory
Project (TAO):
From the University of Tokyo Atacama Project (TAO).
Nov., 2006 4 nights (red), median 0.6” @ best
Apr. 2007 4 nights (black) with a 2-m tower to avoid ground layer
turbulence median 0.37” @ best, Remarkably Good!
One can see that just moving up the telescope by two metres improves
seeing by 0.23"!
The key to dome seeing is to maintain the dome temperature,
and everything inside it, as close to ambient as possible. This
is achieved by ensuring the inside of the observatory is the same
temperature as the outside, whih typically achieved by moving
air through the dome. In addition, any sources of heat within
the dome must be minimized and carried away as quickly as possible.
Thermal footprints must be minimized. The mount which usually
has the biggest thermal footprint must be vented. Again dome seeing
is difficult to quantity but some of the evidence at hand (i.e.
DDO) suggest that it could amount to several arcseconds of seeing.
The Optical Tube assembly (OTA) has a complex set of issues
that effect seeing:
- Primary mirror seeing (boundary layer)
- Thermal profile of the mirror, i.e. mass
- Dimensional stability of frame
- Thermal profile of OTA as a whole, i.e. mass
The most damaging thermal issue is typically primary mirror seeing.
The boundary layer just in front of the optical surface is incredibly
sensitive to thermals. The mirror must be maintained as close
to ambient as possible to well within one degree Celsius. The
OMI has chosen to use a Borofloat cellular ribbed open-back mirror
with a total mass of 68 kg. The front optical surface, accessible
from the back and 0.25" thick is vented and can track ambient
in a few minutes to well within 1 degree celcius. In addition
the low mass of the primary, 68 kg is much easier to maintain
at ambient than larger more massive mirror. The mass, dimensional
stability and stiffness of the tube all affect image quality.
This material is extremely light and has a coefficient of expansion
similar to that of the Borofloat mirror. The end result is an
OTA with low mass, high dimensional stability, high rigidity and
a mirror that has excellent thermal stability. This will yield
an optical train that is highly stable with temperature and produce
sharp images over without refocusing. In a poorly designed OTA
the contribution to seeing could be as much has 1 or 2 arcseconds.
We believe that the design, materials and careful attention
to local seeing will result in seeing in the 0.8-1.25" range
at the site. We are planning on measuring seeing in the spring
and summer of 2008.
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