A prototype instrument for the US Air Force's Improved Solar Observing Optical Network (ISOON) is being developed at NSO/Sacramento Peak in partnership with the Air Force Research Laboratory's Space Hazards Branch. ISOON will replace the existing SOON system, which was designed and deployed in the 1970s for space weather forecasting, starting in the year 2000. ISOON will deliver near real time, rapid-cadence solar images to the Air Force Space Command and NOAA/SEC. The ISOON network will consist of four sites including Holloman AFB, New Mexico and Learmonth, Australia. It will be operated autonomously and remotely controlled from Schriever AFB, Colorado.
The ISOON instrument will have a 1.15 solar diameter (37') field of view on 1.1" pixels and will nominally deliver Ha images at a one-minute cadence, a continuum image every hour, and a line-of-sight magnetogram every three hours. A high-resolution imaging mode having 0.3" pixels will also be incorporated into the prototype. The major components of the ISOON instrument will be the evacuated 25cm SOON telescope (which will be fitted with a new objective), a dual Fabry-Perot filter system, and a fast (two-frame-per-second) 2048 × 2048 CCD camera similar to the one used for RISE. The filter system uses two 15cm Fabry-Perots manufactured by Queensgate Instruments, Ltd., one acting as a narrow-band element and the other acting as an "order selecting" element, that isolates a single passband from the narrow-band element. A photograph of one of our Fabry-Perots can be found on page 48 of the May 1998 issue of Photonics.
The Fabry-Perot filter system will require prefilters having FWHMs between
0.4 nm and 1.3 nm, depending on wavelength and prefilter design. The
Fabry-Perot filter system is designed for use between 600 nm and 1000 nm and
may be usable out to 1100 nm. The filter system will operate in two modes: a
f/55 general imaging mode designed to have a FWHM of about 0.025 nm at
H-alpha and an f/108 high spectral resolution mode for obtaining the
magnetograms, designed to have a FWHM of about 0.010 nm at 630.2 nm. In both
modes, the Fabry-Perots will be operated in a telecentric beam in order to
eliminate wavelength variations across the field of view. The prototype
ISOON will have prefilters for operation at Ca I 612.2 nm (photospheric
magnetic line and continuum), Fe I 630.15 and Fe I 630.25 (a pair of
photospheric magnetic lines and continuum), H I 656.3 (chromospheric
H
line), and He I 1083.0 nm (chromospheric line used for coronal diagnostics).
The two Fabry-Perots arrived in February 1998. An observing run in March at
the Sac Peak Vacuum Tower Telescope (VTT) using the high-resolution
Horizontal Spectrograph was used to obtain the basic tuning parameters for
the filter system. The first images through the filter system were obtained
during an observing run at the VTT in May. The image shown
here of a sunspot group and the surrounding granulation field was obtained
during good seeing on 24 May (exposure time 80 ms, field of view 300"
sampled at 0.16" per pixel, wavelength 630.225 nm, FWHM 0.010 nm). There is
some distortion at the edges of the image due to optical misalignment in the
setup. The plot shows
the spectral profile for the region around the 630.15 nm-630.25 nm line
pair, produced by plotting the mean image intensity vs. wavelength from a
97-image scan of the solar disk center, stepping the Fabry-Perots 0.002 nm
between images. The small dips in the profile near
630.20 nm and 630.28 nm are telluric lines. We currently estimate that the
total transmission of the dual Fabry-Perot system is about 20 % at 630.2 nm.
Craig Gullixson, K.S. Balasubramaniam,
Don Neidig, and the ISOON Development Team