Download Paper Title - Mees Solar Observatory

Strategies for Prime Focus Instrumentation in Off-Axis Gregorian
Systems
Roy Coulter, Jeff Kuhn, Haosheng Lin
Institute for Astronomy, University of Hawaii
ABSTRACT
A new generation of off-axis telescopes has been proposed to address a number of high dynamic range problems in
astrophysics. These systems present unusual problems and opportunities for the instrument designer. We will discuss
some of the issues that must be resolved when placing instrumentation at the prime focus. The heat stop and occulter
systems for the SOLAR-C Off-axis Coronagraph will be used to illustrate strategies for solar telescope applications.
Keywords: off-axis, Gregorian, solar, prime focus, coronagraph, heat stop
1. INTRODUCTION
The past few decades have witnessed several abortive attempts to field a new, large aperture solar facility. Projects such
as LEST and CLEAR have come and gone and the McMath and Dunn Solar Telescope at the National Solar Observatory
remain among the largest solar apertures available even though they were constructed in the 1950s and 60s. Those
telescopes share the characteristic of large focal ratios, 50 and 72, and thus are in excess of 100 meters in length. The
currently proposed systems GREGOR and ATST intend to work at small f-numbers for primary imaging and are based
on the Gregorian telescope design. In the interim 2 designs have been built that are bridging the gap to the larger
aperture, fast primary systems. The Dutch Open Telescope (DOT) and the SOLAR-C coronagraph are each .45 meter
aperture systems working at focal ratios around f/4. These systems require cooled stops at the prime focus and, in
keeping with conventional wisdom; one would like to hold the temperature of these stops within 1 degree C of the
ambient air within the light path. DOT accomplishes this by using a reflective, water cooled stop which rejects the
majority of the solar radiation and by allowing a free air path through the telescope to carry away heat. SOLAR-C, as a
coronagraph, must have a closed system to shield against scattered light and we plan to absorb the solar radiation at
prime focus using a heat stop. In addition, a forced air system will be employed to stabilize the temperature of the
telescope interior and aid in mitigation of dust accumulation on the primary mirror.
SOLAR-C and the ATST are off-axis Gregorian systems. To better understand the engineering complexities presented
by these telescopes we should have a basic knowledge of the optical and mechanical configuration of the off-axis
Gregorian. We will present several designs that will, hopefully, illustrate some of the tradeoffs that are required to make
the prime focus instrumentation for these systems viable.
2. CHARACTERISTICS OF THE OFF-AXIS GREGORIAN TELESCOPE
If we take a classical Gregorian telescope and place an aperture on one side of the primary mirror, we obtain an off-axis
system as detailed in Fig. 1. The result is a telescope with an unobstructed pupil and a prime focus that is accessible
without obstructing the return path from the secondary, a good combination for observations that require high dynamic
range such as imaging dim objects next to bright sources. Access to the prime focus provides opportunities for occulting
bright sources before re-imaging occurs and, in the case of solar telescopes, allows the designer to limit the amount of
solar radiation that is propagated to the secondary optical systems.
Figure 1. The off-axis Gregorian configuration.
In the solar case the heat transfer to prime focus is a primary concern and should be taken into account during the optical
design process. Using simple geometric optics we can calculate a few useful parameters for such systems:
The prime focus solar image size (yi ) relative to the telescope focal length ( fl ):
yi = fl * .0093
Using the solar constant G0 = 1.370 kW/m2, the heat transferred to prime as a function of the telescope aperture ( D) is:
QPF = 1076 * D2 Watts
The energy delivered to prime focus (irradiance) can be expressed in terms of the f-number ( fn ) as:
GPF = 15.84 W/mm2 * fn-2
We have ignored atmospheric extinction and are assuming a 100% reflective coating on the primary, so this represents a
‘worst case’ overestimate of the prime focus energy, suitable for conservative engineering.
The SOLAR-C coronagraph is an example of an off-axis Gregorian system and a ray tracing is shown in Fig. 2. For this
system ( f/3.8 ) the prime solar image diameter is 16mm, the heat load will be about 218 Watts, and the irradiance 1
Watt/mm2 across the prime solar image. If we use a 300 arc second diameter stop at prime focus to limit the observable
field then only 4.8 Watts will be transmitted to the secondary optics. By comparison, a 4 meter aperture, f/2 system will
form a 75 mm solar image, the total heat load will be 17.2 Kilowatts, and the irradiance at prime will be 4 W/mm2. In
this case, the 300 arc second stop will transmit a little more than 400 Watts to the downstream optical elements.
Figure 2. A ray tracing of the SOLAR-C telescope. Note that the optical axis passes through the center of the prime and
gregorian images.
Optical Parameters:
.46 meter aperture, f/ 3.8 off-axis Gregorian
Derived from 1.2 meter, f/ 2.4 parabolic parent
Off-axis distance = .14 meter
Gregorian focal ratio: f/ 18 (8 meter EFL)
Plate scale: 38 microns/arcsec
It is readily apparent that the engineering challenges at prime focus increase dramatically with decreasing f-number. At
the same time, clearances required for the mechanisms at prime will drive optical design parameters such as the off-axis
distance and size (and position) of the secondary. Figures 3 and 4 detail the topography of the SOLAR-C prime focus
region. SOLAR-C was designed to deliver diffraction limited images of a relatively large field of view, which requires
that the off-axis distance be kept small. The size of the secondary mirror was also kept to a minimum to facilitate active
tip/tilt correction at frequencies of a few hundred hertz. The result is a rather crowded region around prime focus. Note
the tight clearances between optical paths that make such systems a challenge for the mechanical designer. The ray trace
shown takes in the full 18 arc minute, diffraction limited field (calculated at 503 nm) but, in practice, a field stop
between 5 and 10 minutes in diameter is required for a reasonable mechanical separation between the light paths to and
from the secondary mirror. This stop is called the inverse occulter or, simply, the inverse. Since only a small portion, or
in the coronagraphic case, none, of the solar image is allowed through the inverse we must reject the unused solar light
and heat from the system. The, perhaps inaptly named, ‘heat stop’ must somehow surround the inverse and absorb or
reflect the remaining solar image and efficiently transfer the heat out of the optical system without adversely affecting
the air within the optical path. We will explore the design of these heat stops by detailing two different systems.
Figure 3. A ray trace of the SOLAR-C prime focus shows only a few millimeters clearance between the prime focus
and the return path from the secondary. This is the case for the full 46cm aperture and an 18 arc minute field of view. In
practice, the system will probably be stopped down to a 41cm aperture, to avoid illumination of the primary mirror edges
when off-pointing. In addition, a stop at prime focus will restrict the field to between 5 and 10 arc minutes.
Figure 4. A plan view of the SOLAR-C prime focal plane. The dotted circles represent a few of the possible locations
for the solar image when observing the corona. Note that locations on the northern side of the sun are not accessible in
this configuration.
R.J. Rutten, 2002
The Dutch Open Telescope
In monograph ``Small Telescopes in the Era of Large Telescopes''
Ed. T. Oswalt, Kluwer (Dordrecht), in press