Motivation for a 95% Guide Star Acquisition
Probability with JWST
Ed Nelan
With help from:
Jerry Kriss
Wayne Kinzel
Peter Stockman
JWST
95% Guide Star Acquisition Probability
James Webb Space Telescope
 What does this mean?
JWST Level 2 Requirement 3.2.15.2.2 states:
“The Observatory shall have a greater than 95% probability of acquiring
a guide star and maintaining pointing stability for any valid attitude
within the instantaneous field of regard”.
In other words:
There must be a 95% probability that a star suitable for guiding will be in
the FGS field of view for any pointing of the telescope.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
2
James Webb Space Telescope
JWST, FGS, & GSC-2: a brief history
 Originally, NIRCam was to provide the guide function. The FOV in
which to find a guide star was expected to be ~16 arcmin2 (the
Yardstick NIRCam)
• A. Spagna (2001) concluded that the well calibrated portion of
GSC-2 (JB < 19.5, RF < 18.0) should be a suitable source of guide
stars for NGST (assuming the 16 arcmin2 “FGS” FOV).
• Guide function subsequently moved from NIRCam to a dedicated
FGS to be provided by CSA.
 CSA proposed an FGS with an 8.4 arcmin2 FOV (summer 2002)
• Two units for redundancy, one operating, the other in “cold
storage”
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
3
JWST, FGS, & GSC-2: a brief history
James Webb Space Telescope
 Smaller FGS FOV prompted two studies:
• Assuming GSC-2 is inadequate, what are the operational work
arounds for getting guide stars (Nelan et al.) ?
• Not pretty
• Can JWST meet the 95% GS Acq. probability using GSC-2 if the
FGS FOV is 8.4 arcmin2 (Kriss & Stys)?
• Yes, but need entire catalog (down to plate limits in BJ and RF)
& an FGS with the sensitivity to use stars down to JAB < 20.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
4
JWST, FGS, & GSC-2: a brief history
James Webb Space Telescope
 Smaller FGS FOV prompted two studies:
• Assuming GSC-2 is inadequate, what are the operational work
arounds for getting guide stars (Nelan et al.) ?
• Not pretty
• Can JWST meet the 95% GS Acq. probability using GSC-2 if the
FGS FOV is 8.4 arcmin2 (Kriss & Stys)?
• Yes, but need entire catalog (down to plate limits in BJ and RF)
& an FGS with the sensitivity to use stars down to JAB < 20.
 May 9, 2003 CSA proposed a different FGS. The new design may
reduce total FOV, sensitivity, and redundancy, putting the 95% GS
acquisition requirement at risk of not being met.
• time to re-examine our motivation for achieving a 95% guide star
acquisition rate with JWST.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
5
James Webb Space Telescope
Requirements and the FGS design
 Achieving a 95% guide star acquisition probability requires that the
FGS FOV is large enough to contain, on average, three stars (from
GSC-2) for even the most sparsely populated regions of the sky.
• Having 3 stars “on average” implies only a 5% chance of having
none if the stars follow a Poisson distribution.
• The surface density of stars at “high” Galactic latitudes (|b| > 45o)
constrains the minimum size of the available FGS FOV.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
6
James Webb Space Telescope
Requirements and the FGS design
 Achieving a 95% guide star acquisition probability requires that the
FGS FOV is large enough to contain, on average, three stars (from
GSC-2) for even the most sparsely populated regions of the sky.
• Having 3 stars “on average” implies only a 5% chance of having
none if the stars follow a Poisson distribution.
• The surface density of stars at “high” Galactic latitudes (|b| > 45o)
constrains the minimum size of the available FGS FOV.
 Why not just increase the size of the FOV?
• by increasing the pixel scale
• decreases the accuracy with which guide stars can be centroided, resulting in
degraded guiding.
• add additional detectors
• exceeds cost constraints.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
7
James Webb Space Telescope
Galactic Disk (l,b) = (0,30)
spare FGS
(redundancy)
2.1’ x 4.2’ FGS FOV
(summer 2002)
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
8
James Webb Space Telescope
North Galactic Pole (l,b) = (180,80)
spare FGS
(redundancy)
2.1’ x 4.2’ FGS FOV
(summer 2002)
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
9
James Webb Space Telescope
North Galactic Pole (l,b) = (180,80)
spare FGS
(redundancy)
2.1’ x 4.2’ FGS FOV
(summer 2002)
JAB ~ 18.5 (F4 at 10 kpc)
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
10
James Webb Space Telescope
FGS & TF Science Instrument
revised FGS (May 2003)
A
B/C
D
three 2.3’ x 2.3’ FOV units
• A & B are dedicated guiders
• C is LW/TF (dichroic, same FOV as B)
• D is SW/TF and guider w/10% through put
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
11
James Webb Space Telescope
FGS & TF Science Instrument
revised FGS (May 2003)
A
B/C
D
Achieving the 95% guide star acquisition rate requires
any two of units A, B, or D to be operating.
• Redundancy against single unit failure is preserved.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
12
James Webb Space Telescope
The 95% requirement
Why is it so important to achieve a 95%
guide star acquisition probability?
• Scientific motivation
• Operational impacts if we fail to do so
“Motivation for Meeting the 95% Guide Star Acquisition Rate
with JWST” (Nelan, Kriss, Kinzel) STScI-JWST-TM-2003-0007A
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
13
James Webb Space Telescope
Scientific Motivations for a high
probability of access to guide stars
 Given JWST’s discovery potential, astronomers will want to observe
objects with more than one (perhaps all) of the Observatory’s Science
Instruments. Each visit will have a different bore sight pointing and
spacecraft roll angle. Different guide stars are needed for each.
 JWST will mosaic regions of the sky that are perhaps several times
larger than the FGS FOV. To avoid gaps, guide stars are needed for
all of the tiles.
 Long term monitoring of targets (e.g., high redshift SNe Ia) requires a
high probability of guide stars being available at every orientation of
the telescope for a given pointing.
 Targets of opportunity must be observed in a timely fashion. Can’t
wait for the date when a particular roll range(+/- 5o off nominal) allows
access to a guide star. TOO success rate is closely linked to the 95%
objective.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
14
James Webb Space Telescope
Scientific Motivations for a high
probability of access to guide stars
 NIRSpec observations with the Micro Shutter Array (MSA) will be
highly roll constrained to optimally place the target field in the array.
Optimal orientations can be used routinely only if there is a high
probability of having access to guide stars at all orientations.
 It is best (but not absolutely necessary) to execute large dithers
(~20”) using the same guide star, especially for NIRSpec MSA
observations (avoid complicated SI target acquisitions and
wavelength calibrations), and perhaps coronagraphic observations
(MIRI target acquisition).
• The higher the GS Acq probability, the more likely same GS can
be used for large dithers (generally, more than one GS will be in
the FGS FOV to choose from).
• Conversely, the lower the GS Acq probability, the less likely a
guide star will become available in the small amount of new sky
that enters into the FGS FOV if its needed.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
15
James Webb Space Telescope
Dithering and GS Availability
A
B
If the probability of having guide stars in the FGS FOV is
high, more than one is typically available.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
16
James Webb Space Telescope
Dithering and GS Availability
A
B
If the probability of having guide stars in the FGS FOV is
high, more than one is typically available. Select the one
that works for dithers!
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
17
James Webb Space Telescope
Dithering and GS Availability
A
B
If the probability of having guide stars in the FGS FOV
is low (loose a guide channel, e.g.), more vulnerable to
problems with dithers.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
18
James Webb Space Telescope
Dithering and GS Availability
A
If the probability of having guide stars in the FGS FOV
is low (loose a guide channel, e.g.), more vulnerable to
problems with dithers. Probability of new GS becoming
available when needed is low.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
19
James Webb Space Telescope
Operational Motivations for a high
probability of access to guide stars
If there is a high probability that guide stars are available for any pointing
and orientation:
 vast majority of proposals will schedule without guide star problems.
• minimizes the need for schedulers to iterate with the GOs
• important for proposals with large dithers, mosaics, NIRSpec MSA
 proposers will not need to be concerned about guide star availability.
• STScI will not need to provide GOs with guide star selection rules.
• STScI retains the responsibility for selecting guide stars, and
therefore the scheduling of proposals
 proposals that are not roll or time constrained can be scheduled when
best for LRP and the Observatory’s efficiency.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
20
James Webb Space Telescope
Consequences of Declining Guide Star
Availability
 Risk from catalog contamination. As overall the guide star availability
declines, the percentage of visits that are scheduled with just one
candidate GS increases. These are vulnerable to acquisition failures
due to contamination of GSC-2 (~10%).
acquisition
probability
visits with
one GS
visit failure
rate
95%
15%
1.5%
90%
25%
2.5%
85%
33%
3.3%
80%
40%
4.0%
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
21
James Webb Space Telescope
Consequences of Declining Guide Star
Availability
 Increased risk for successfully completing multi-visit programs that
use different pointings and/or orientations. The probability of being
able to mosaic a field of area AM using an FGS with FOV AFGS with
out gaps is:
Pm = Pon
where Po is the probability of having a guide star for a single visit, and
n = Am / AFGS
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
22
James Webb Space Telescope
Consequences of Declining Guide Star
Availability
 The table below shows the probability of being able to mosaic a 7’x’7’
field without gaps as a function of guider FOV. It is assumed that the
guider can use stars down to JAB < 20, except for the SW/TF
channel (JAB < 17.5).
FGS FOV (arcmin2)
Per-pointing GS Acq
Probability
Probability of gap-free
mosaic
10.8
99 %
95 %
8.4
98 %
89 %
10.8*
95 %
69 %
5.4
88 %
32 %
* using channel A or B with D (SW/TF)
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
23
James Webb Space Telescope
Conclusions
Satisfying Level 2 Requirement 3.2.15.2.2 is a very important objective.
 GOs can use observing strategies to optimize scientific returns
without constraints imposed by guide star availability.
 Simplifies operations, keeps cost down.
 Facilitates the generation of an LRP that makes the most efficient use
of the Observatory.
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
24
James Webb Space Telescope
Conclusions
Satisfying Level 2 Requirement 3.2.15.2.2 is a very important objective.
 GOs can use observing strategies to optimize scientific returns
without constraints imposed by guide star availability.
 Simplifies operations, keeps cost down.
 Facilitates the generation of an LRP that makes the most efficient use
of the Observatory.
Don’t forget! Total lunar eclipse tonight!
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
25
James Webb Space Telescope
Guide Star Acquisition Probabilities
A NASA
Origins
Mission
5/28/2016
TIPS
Ed Nelan
26