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SPACETELESCOPESCIENCEINSTITUTE
TIPS
Meeting
1
Dec
1994
R.
Jedrzejewski
Summary
of
FOC
Activities
for
the
period
3
Nov
­
30
Nov
1994
Operations .
The
F/96
relay
has
been
operating
nominally.
Proposals
Executed
.
Two
GO/GTO
proposals
were
successfully
executed
with
the
FOC:
5469
Externally
Illuminated
Proto­Stellar
Disks
and
Naked
Jets
5437
UV
Spectroimaging
of
Saturn
.
The
following
CAL
proposals
were
executed
with
the
FOC:
5670
FOC
Internal
Relative
Calibrations
5750
FOC
Geometric
Correction
and
Plate
Scale
5767
Fall
Heptathlon
Calibration .
The
results
from
the
F/48
slit­location
proposal
have
been
more
fully
analysed.
It
appears
that
in
addition
to
the
lessening
of
the
background
noise,
the
geometric
distortion
behaved
much
as
it
did
soon
after
launch.
The
plate
scale
only
changed
by
0.5%
with
a
rotation
of
only
0.23š,
almost
identical
behavior
to
that
seen
in
Cycle
1
observations.
The
scans,
however,
were
performed
a
little
differently
than
expected,
along
the
slit
orientation
rather
than
perpendicular
to
it.
This
has
only
increased
the
error
in
the
location
of
the
slit.
TIPS
Meeting
1
Dec
1994
R.
Jedrzejewski
Summary
of
FOC
Activities
for
the
period
3
Nov
­
30
Nov
1994

SPACETELESCOPESCIENCEINSTITUTE
TIPS
Meeting
1
Dec
1994
R.
Jedrzejewski
Summary
of
FOC
Activities
for
the
period
3
Nov
­
30
Nov
1994
Calibration
(cont'd)
.
A
new
set
of
geometric
correction
files
has
been
created
based
primarily
on
data
obtained
from
SMOV.
New
distortion
models
were
determined
for
the
following
f/96
for­
mats:
512zX1024,
512X512,
256X1024,
and
256X256.
These
models
were
determined
using
a
new
technique
involving
a
new
representation
of
the
distortion
based
on
overlap­
ping
observations
of
a
crowded
star
field.
Documentation .
`Deriving
the
Geometric
Correction
from
Crowded
Fields',
P.
Greenfield,
DRAFT
ISR.
This
ISR
describes
the
new
distortion
models
and
how
they
were
derived.
.
`Calibration
Product
Review
for
the
FOC',A.
Nota,
F.
Paresce,W.
Hack,
FOC­082.
This
ISR
provides
complete
details
on
the
calibration
files
and
processes
for
FOC
images.

SPACETELESCOPESCIENCEINSTITUTE
TIPS
Meeting
1
Dec
1994
R.
Jedrzejewski
Summary
of
FOC
Activities
for
the
period
3
Nov
­
30
Nov
1994
Options
for
optimizing
FOC
focus
There
are
three
ways
one
can
adjust
the
focus
of
the
FOC:
1)
Move
the
OTA
secondary
2)
Move
the
COSTAR
DOB
3)
Move
the
FOC
internal
refocus
mechanism
Each
method
affects
the
other
instruments
differently.

SPACETELESCOPESCIENCEINSTITUTE
TIPS
Meeting
1
Dec
1994
R.
Jedrzejewski
Summary
of
FOC
Activities
for
the
period
3
Nov
­
30
Nov
1994
Option
1:
Move
the
OTA
secondary
Affects
all
of
the
instruments.
Primary
long­term
focus
position
change
is
caused
by
desorption
in
the
OTA
(approx.
0.8
+/­
0.2
µ/month)
Instruments
vary
in
their
sensitivity
to
focus
changes:
The
FOC
is
most
sensitive:
small
focus
changes
can
cause
significant
changes
in
the
performance
in
the
near­UV:
the
pseudo­Strehl
(ratio
of
counts
in
the
central
pixel
to
the
total
counts
in
the
PSF)
drops
by
approxi­
mately
50%
for
the
FOC
for
a
2
µ
SM
change.
The
effect
for
the
WFPC2
is
smaller
since
the
PSF
is
not
sampled
as
well
The
large
(by
comparison)
apertures
for
the
spectrographs
make
the
effect
of
a
+/­
2
µ
move
virtually
unmeasureable.
Requiring
the
focus
to
be
controlled
to
+/­
2
µ
would
require
moving
the
secondary
mirror
every
5
months
or
so.

SPACETELESCOPESCIENCEINSTITUTE
TIPS
Meeting
1
Dec
1994
R.
Jedrzejewski
Summary
of
FOC
Activities
for
the
period
3
Nov
­
30
Nov
1994
However:
We
do
not
have
a
good
measurement
of
the
desorption
rate
since
the
ser­
vicing
mission
Breathing
effects
can
typically
apply
an
orbital
modulation
of
the
focus
posi­
tion
by
+/­
2
µ,
with
larger
excursions
of
+/­
4
µ
possible
at
unfavorable
thermal
configurations.
Since
the
initial
SMOV
alignment
and
focussing
of
the
cameras
in
January
1994,
a
single
5
µ
OTA
move
has
been
done
(in
July
1994).

SPACETELESCOPESCIENCEINSTITUTE
TIPS
Meeting
1
Dec
1994
R.
Jedrzejewski
Summary
of
FOC
Activities
for
the
period
3
Nov
­
30
Nov
1994
Option
2:
Moving
the
COSTAR
DOB
Method
currently
used
to
apply
small
corrections
to
FOC
focus
in
between
OTA
secondary
movements.
Also
affects
the
spectrographs,
but
not
to
the
same
degree
as
the
effect
on
the
FOC.
Moving
the
DOB
by
1mm
is
equivalent
to
moving
the
OTA
secondary
by
5.4
µ
for
the
FOC,
and
about
1/2
this
for
the
spectrographs.
Aperture
location
is
altered
by
less
than
0.1
arcsec
for
the
spectrographs,
slightly
more
for
the
FOC
by
moving
the
DOB
by
1mm.
So
far,
the
DOB
has
been
moved
by
0.1mm
(August
1994)
and
0.6mm
(October
1994).
Images
taken
after
the
DOB
moves
showed
that
the
FOC
focus
was
close
to
optimal.
The
next
move
should
be
in
early
to
mid
January
(pending
TTRB
review).

SPACETELESCOPESCIENCEINSTITUTE
TIPS
Meeting
1
Dec
1994
R.
Jedrzejewski
Summary
of
FOC
Activities
for
the
period
3
Nov
­
30
Nov
1994
Option
3:
Moving
the
FOC
internal
focus
mechanism
FOC
has
an
internal
fold
mirror
that
is
movable
by
+/­8mm,
which
corresponds
to
an
equivalent
secondary
mirror
movement
of
+/­
3
µ.
Since
FOC
filter
thickness
compensation
is
required,
the
available
range
is
probably
+/­
2
µ
of
SM.
To
alter
the
FOC
internal
focus
involves
a
PDB
update,
a
PMDB
change
and
onboard
table
load,
taking
typically
1­2
weeks
from
notification
of
the
new
reference
position.

SPACETELESCOPESCIENCEINSTITUTE
TIPS
Meeting
1
Dec
1994
R.
Jedrzejewski
Summary
of
FOC
Activities
for
the
period
3
Nov
­
30
Nov
1994
SUMMARY: Ideally,
the
focus
would
bemaintained
bymoving
the
OTA
secondary,
but
thismust
be
traded
off
against
actuator
lifetime
and
reliability
estimates.Moving
the
DOB
is
a
good
way
to
provide
finer
control
of
the
focus
between
OTA
secondary
adjustments,
and
also
should
improve
the
spectrograph
focus
.
Moving
the
FOC
internal
focus
mechanism
has
limited
compensation
range,
but
might
be
useful
for
fine­tuning
the
FOC
focus
as
long
as
the
adjustment
required
is
small
(<
2µ)