Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/ftp/instrument_news/FOS/postscript/tips_0196.ps Äàòà èçìåíåíèÿ: Fri Dec 20 00:22:13 1996 Äàòà èíäåêñèðîâàíèÿ: Sun Mar 2 06:58:36 2014 Êîäèðîâêà: IBM-866 Ïîèñêîâûå ñëîâà: annular solar eclipse

SPACETELESCOPESCIENCEINSTITUTE Summary:
21
December
1995
í
17
January
1996
Instrument:
Nominal
operation
Acquisition:
12
successful
(8
ACQ/BIN
and
4
ACQ/PEAK);
0
failures
Pointing:
64%
within
1
arcsec
Science:
10
GO/GTO
programs
Calibration:
4
programs
(Darks,
YBASES,
Polarimetry,
HDFíparallel
Earthshine)
Publications
and
Communications:
HST
Data
Handbook
Other:
IS
Reviews,
Cycle
6
Phase
II
GO
support
TIPS:
18
January
1996
FOS Koratkar

SPACETELESCOPESCIENCEINSTITUTE Description
of
FOS
Detectors:
FOS
provides
digitized
spectra
over
the
wavelength
range
1150
Š
íí
8500Š.
Spectra
detected
on
either
of
two
available
Digicon
detector
assemblies
which
have
their
own
optical
paths.
Digicons
are
a
category
of
image
tubes
that
use
semiconductors/semiconductor
arrays
to
detect
photoelectrons
from
a
photocathode.
These
are
photonícounting
devices. Digicons
are
capable
of
detecting
single
photoelectron.
Digicons
are
characterized
by
a
wide
dynamic
range,
low
noise,
high
photometric
accuracy
and
high
reliability.
TIPS:
18
January
1996
FOS Koratkar

SPACETELESCOPESCIENCEINSTITUTE Description
of
a
Digicon
Detector
Assembly:
The
Tube:
The
two
FOS
Digicons
differ
only
in
their
photocathode
and
face
plate
materials.
Blue
detector:
Bialkali
photocathode
on
aMgF2
faceplate
(1150Š
í
5000Š).
Red
detector:
Trialkali
photocathode
on
a
quartz
faceplate
(1700Š
í
8500Š).
Diode
array
of
512
diodes,
each
diode
is
50²m
wide
and
200²m
high.
Magnetic
and
Electric
components:
Deflection
coils
to
selectively
image
the
photocathode
active
area
onto
the
diode
array.
The
optical
image
is
focused
on
more
than
one
location
on
the
photocathode
(YBASE
location).
The
deflections
can
be
a
fraction
of
a
diode,
which
provides
higher
sampling
to
realize
the
full
spectral
resolution
(NXSTEPS),
or
a
full
diode,
which
smooths
out
diodeítoídiode
variations
(OVERSCAN).
TIPS:
18
January
1996
FOS Koratkar

SPACETELESCOPESCIENCEINSTITUTE The
permanent
magnet
focus
assembly
is
used
to
provide
an
external
magnetic
field
for
single
loop
focusing
of
the
electron
image
onto
the
diode
array.
Uniform
electric
field
to
accelerate
the
photoelectrons
onto
the
diode
array.
The
uniform
acceleration
and
single
loop
focusing
provide
a
good
oneítoíone
mapping
of
the
photocathode
onto
the
diode
array.
Magnetic
shield
to
protect
detector
from
external
magnetic
field.
Pulse
Detection
Electronics:
Each
diode
is
connected
a
preamplifier,
amplifier,
discriminator
and
counter.
Other
associated
components:
Heat
pipes. Temperature
sensors.
TIPS:
18
January
1996
FOS Koratkar

SPACETELESCOPESCIENCEINSTITUTE Counting
the
photons:
Incident
photonsî
photoelectron
Photoelectron
detected
by
diodeî
current
pulse
Current
pulse
amplified
and
counted
if
it
exceeds
a
preset
discriminator
threshold
TIPS:
18
January
1996
FOS Koratkar

SPACETELESCOPESCIENCEINSTITUTE Retrieving
the
Flux
information:
Location
of
the
spectrum:
A
selective
area
of
the
photocathode
ismagnetically
deflected,
hence
knowledge
of
where
the
spectrum
is
formed
on
the
photocathode
is
required.
Geoímagnetically
induced
motion
correction:
The
magnetic
shielding
around
the
detector
assembly
does
not
fully
protect
the
detector
from
the
external
earth's
magnetic
field.
Paired
pulse
correction:
If
an
object
is
bright,
the
photoelectrons
arrive
at
the
diode
array
at
a
rate
such
that
the
current
pulses
are
generated
at
a
rate
faster
than
the
electronics
can
count.
Background
correction:
Along
with
the
target
photons,
cosmic
rays
can
hit
the
photocathode
and
produce
photoelectrons.
These
photoelectrons
are
also
detected
by
the
diode
array.
TIPS:
18
January
1996
FOS Koratkar

SPACETELESCOPESCIENCEINSTITUTE Flat
Field
correction:
The
photcathode
can
have
blemishes
which
couldmimic
or
affect
themeasurement
of
the
line
profile
or
line
flux.
The
photocathode
material
has
granularity
which
needs
to
be
smoothed
out.
Inverse
sensitivity
correction:
Photocathode
material
has
certain
sensitivity
to
the
dispersed
photon
and
emits
the
photoelectrons
accordingly.
A
translation
which
converts
the
observed
counts
to
number
of
observed
photons
then
provides
the
flux
information.
TIPS:
18
January
1996
FOS Koratkar

SPACETELESCOPESCIENCEINSTITUTE Summary:
21
December
1995
í
17
January
1996
Instrument:
Nominal
operation
Acquisition:
12
successful
(8
ACQ/BIN
and
4
ACQ/PEAK);
0
failures
Pointing:
64%
within
1
arcsec
Science:
10
GO/GTO
programs
Calibration:
4
programs
(Darks,
YBASES,
Polarimetry,
HDFíparallel
Earthshine)
Publications
and
Communications:
HST
Data
Handbook
Other:
IS
Reviews,
Cycle
6
Phase
II
GO
support
TIPS:
18
January
1996
FOS Koratkar

SPACETELESCOPESCIENCEINSTITUTE
TIPS:
18
January
1996
FOS Koratkar