Project Cyclops, A Design... - Department of Earth and Planetary ...

Cyclopswouldbesolvedbydesigning aninstrument to
worknotonsinglestarsbutratheronstar.Seldsasin
UBVphotometryandobjectiveprismspectroscopy.
Suchaninstrumentwouldsimplybe positioned to a
succession of fieldschosentocovertheentireskyvisible
fromtheCyclops site. The coordinates of any star would
then be determined from the known direction of
sighting (i.e., the direction of the center of the field) and
the position of the star image in the field. The positions
of known stars could be checked to eliminate systemic
errors. Because the light from as many as a few thousand
stars would be integrated simultaneously, the total
integration time would be correspondingly reduced
If dichroic mirrors were used to split the received
light, different spectral regions could be integrated
simultaneously in a bank of camera tubes, each having a
photosurface optimized for the spectral ranges involved.
Interference filters could be used to select narrow
regions of the spectrum, and prisms could, be used to
disperse these regions. Thus, a wide variety of spectral
analysis methods is at our disposal. It is not clear
without extensive further study just what combination
of techniques and what combination of spectral regions
or spectral lines would be most effective in selecting our
desired target stars and discriminating against giants and
reddened interlopers.
One definite possibility is to use four (or more)
spectral regions in a direct photoelectric photometry
mode to make an initial screening of the stars in each
field. If, through suitable calibration procedures the
measurement errors in the various spectral bands can be
held to -+0.01 magnitude (_ -+1%), good correction for
reddening should be possible and the size of the
confused region for late G and K stars should be greatly
reduced. This should permit rapid classification of the
stars in a given field into three categories-target,
doubtful, and nontarget-with only a small fraction
falling in the doubtful category. These could then be
examined spectroscopically using several telescopes of
the type described earlier, while the next star field is
being classified photometrically.
Another possibility is that several appropriately
chosen wavelengths would permit the unambiguous
classification of all stars in the field on one pass. If so,
such a procedure might be preferable even if longer
integration times are needed. For the present we can
only conclude that:
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1. No rapid method of preparing a clean target list
for Cyclops exists at the present time.
2. Promising techniques do exist, and it appears
likely that with adequate study and development,
a satisfactory automated system could be
designed.
3. Classification of all stars within 300 pcs of the Sun
would be of considerable value in refining our
knowledge of stellar evolution and is therefore of
merit in itself.
4. Consideration should be given to funding a program
to develop an automated system of rapid
accurate stellar classification irrespective of the
imminence
of Cyclops.
THE OPTICAL-ELECTRONIC
INTERFACE
Assuming that a suitable optical spectrum analysis
technique can be developed, a few problems remain in
transforming the optical information into digital form.
Once the information is stored digitally the analysis can
proceed rapidly and reliably with well-known data
processing techniques; but first we must get the information
into a proper digital format without significant
loss of accuracy.
The first problem is one of dynamic range. If we are
examining stars down to magnitude 16 we must accommodate
a brightness range of 2.5 million to 1. This is
beyond the capabilities of any camera tube. If we can
assume that all stars down to magnitude 6 are already
known and classified, we are left with a magnitude range
of 10 or a brightness range of 10,000 to 1. Good
noise-free linear integration can be obtained over at least
a i 0 to 1 brightness range. Thus, we will need to take at
most four exposures of the same field differing in
successive exposure times by at least 10 to 1. Assuming
the readout time is negligible, this procedure increases
the observing time by i1.1 percent over that required
for the longest exposure alone and so causes no real
difficulty. However, the software logic must be designed
to ignore images that have already been analyzed, or that
represent inadequate or overload amounts of integrated
signal.
The second problem is to convert the analog information
stored in the camera tubes into digital information
in computer memory in such a way that precise
positional and amplitude information is retained in spite
of the discrete nature of a raster scan. To do this, the
spacing between successive scanning lines must be small
compared with the image of any star as limited by
atmospheric blurring or diffraction-that is, the scanning
line spacing and the spacing of data samples taken along
each line should be small compared with 1 sec of arc.
A complete field would then be scanned, sampled,
converted to digital form, and stored in a temporary
memory. Assuming 1/4 sec of arc spacing between
samples and 8 bit (256 level) quantization, 1.6× 10 9 bits