1. Motivation for a 95% Guide Star Acquisition Probability with JWST
Ed Nelan
With help from:
Jerry Kriss
Wayne Kinzel
Peter Stockman
JWST

2. 95% Guide Star Acquisition Probability
What does this mean?
JWST Level 2 Requirement 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.
11/18/2018
TIPS Ed Nelan

3. 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”
11/18/2018
TIPS Ed Nelan

4. JWST, FGS, & GSC-2: a brief history
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.
11/18/2018
TIPS Ed Nelan

5. JWST, FGS, & GSC-2: a brief history
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.
11/18/2018
TIPS Ed Nelan

6. 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.
11/18/2018
TIPS Ed Nelan

7. 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.
11/18/2018
TIPS Ed Nelan

8. Galactic Disk (l,b) = (0,30) spare FGS (redundancy)
2.1’ x 4.2’ FGS FOV
(summer 2002)
11/18/2018
TIPS Ed Nelan

9. North Galactic Pole (l,b) = (180,80)
spare FGS
(redundancy)
2.1’ x 4.2’ FGS FOV
(summer 2002)
11/18/2018
TIPS Ed Nelan

10. 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)
11/18/2018
TIPS Ed Nelan

11. 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
11/18/2018
TIPS Ed Nelan

12. 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.
11/18/2018
TIPS Ed Nelan

13. 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 A
11/18/2018
TIPS Ed Nelan

14. 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.
11/18/2018
TIPS Ed Nelan

15. 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.
11/18/2018
TIPS Ed Nelan

16. Dithering and GS Availability
If the probability of having guide stars in the FGS FOV is high, more than one is typically available.
11/18/2018
TIPS Ed Nelan

17. Dithering and GS Availability
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!
11/18/2018
TIPS Ed Nelan

18. Dithering and GS Availability
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.
11/18/2018
TIPS Ed Nelan

19. Dithering and GS Availability
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.
11/18/2018
TIPS Ed Nelan

20. 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.
11/18/2018
TIPS Ed Nelan

21. 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%
11/18/2018
TIPS Ed Nelan

22. 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
11/18/2018
TIPS Ed Nelan

23. 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).
* using channel A or B with D (SW/TF)
FGS FOV (arcmin2)
Per-pointing GS Acq Probability
Probability of gap-free mosaic
10.8
99 %
95 %
8.4
98 %
89 %
10.8*
69 %
5.4
88 %
32 %
11/18/2018
TIPS Ed Nelan

24. Conclusions
Satisfying Level 2 Requirement 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.
11/18/2018
TIPS Ed Nelan

25. Don’t forget! Total lunar eclipse tonight!
Conclusions
Satisfying Level 2 Requirement 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!
11/18/2018
TIPS Ed Nelan

26. Guide Star Acquisition Probabilities
11/18/2018
TIPS Ed Nelan