Enhanced Fiber Multiplexing for Significantly Increasing the Efficiency, Target
Throughput and Science of APOGEE
Steven Majewski, Fred Hearty, Michael Skrutskie, Nick MacDonald
Summary: APOGEE-2 targeting, like that of APOGEE-1, is designed around the survey imperative
of probing a large range of distances — and therefore magnitudes — along each line of sight. While the
current scheme employing multiple plate designs cycling through cohorts of different magnitudes is meant to
improve efficiency by only collecting enough flux to achieve the minimum S/N = 100, the cohorts still include
a sufficient magnitude range that large fractions of targets accumulate more than necessary integration, which
means that integration time is being wasted. A number of science applications would benefit from making
use of the extra fiber hours delivered by limiting the number of “overcooked” targets by sampling even more
targets. Other programs would benefit from increasing the time series sampling of all targets. Two schemes
are proposed here that address these issues, and that could realistically increase the APOGEE sample sizes by
a factor of two or more with no increase in telescope time. One of the schemes is perhaps better considered
as an AS4 opportunity. However, the second could, for a relatively modest cost, be implemented during
SDSS-IV, particularly for APOGEE-2S, where the cartridges are still under construction and where limited
survey nights already adds pressures on targeting in the South. Even a limited experiment (one Southern
cart for now), may afford significant gains and targeting flexibility within APOGEE-2, or just as a test study.
Thus, we feel it is worthwhile to share these ideas now.
1.
Background
Figure 1 from Zasowski et al. (2013) illustrates the current APOGEE cohorting scheme designed to
make efficient use of the 300 instrument fibers for targeting sources spanning from H = 7 to 13.2 in a
“12-visit field”, through use of four distinct plate designs that cycles through four sets of short cohorts
spanning H = 7 to 12.2 (each given three visits), two sets of medium cohorts spanning H = 12.2 to 12.8
(each observed for six visits), and one long cohort spanning H = 12.8 to 13.2. The magnitude ranges for
each cohort are matched to the number of cohort visits to ensure that the faintest star in the cohort receives
the survey minimum S/N = 100. Nominally this means that every star within a cohort brighter than the
fainter magnitude limit will collect more than the minimum required S/N. The larger the magnitude range
of a cohort, the greater is the degree of excessive integration. In this case, the Short cohort is the most
extreme. In principle, more cohorts more finely dividing the magnitude range could help, at the expense of
more required plate designs, plates drilled, and stress to the targeting team!
But, for example, one could imagine a set of “Ultrashort” cohorts visited only a single time, which could
include sources from H = 7 to 11. One reason this scheme was not generally implemented is the separate
survey requirement that each star must be visited three times, as a means to identify radial velocity variables
(likely binary stars). An exception was APOGEE-1 targeting of the bulge, where the desire for a reasonably
large in spite of limited sky access overruled the mulit-epoch requirement.
2.
Motivations for Improved APOGEE Targeting Efficiencies
Desire for More Visits: The example of the APOGEE-1 bulge fields mentioned above is illustrative.
Exceptions in standard survey procedures — i.e., single epoch observing and H < 11 plates — were imple-
–2–
Fig. 1.— Organization of observed targets in plate designs and on physical plates, using the
field 180+04 as an example. This field has 12 anticipated visits, which are covered by four
designs (indicated by blue, yellow, green, and orange). Each design has stars from one of four
short cohorts (S1, S2, S3, S4), one of two medium cohorts (M1, M2), and the long cohort (L);
that is, stars in the long cohort appear in all four designs, and stars from the medium cohorts
appear in two designs. At least one plate is drilled for each design, and some designs (here,
the first two) are drilled on multiple plates. Most frequently, this occurs when a field is to
be observed at different hour angles (HAs), as in this example. (Caption from Zasowski et al.
2013.)
–3–
mented for APOGEE-1 bulge targeting to ensure a statistically meaningful bulge sample could be obtained
within the airrmass-constrained observing access. Had we had more access to this low declination part of
the sky, we would have preferred to have satisfied our 3-visit requirement. For those H < 11 plates, these
three visits could have each been only 20 mins in length; however, such visits would be very inefficient, given
the substantially larger overhead/integration time ratio, and limitations in the numbers of APOGEE carts
does not favor having such short visits as a matter of course.
Limited du Pont Access: Of course, the above problem of limited visits for bulge targeting will
be remedied for APOGEE-2S, where the bulge is much more favorably situated for sustained observing.1
However, SDSS-IV has only a fraction of the time on the du Pont telescope (nominally 75 nights per year),
and there is immense pressure to accommodate many competing science objectives within those nights.
Clearly it is valuable to consider every potential improvement in observing efficiency, which is equivalent to
increasing the effective number of observing nights. Obviously the same efficiency arguments can be made
for the APOGEE-2N, but the situation in the South is arguably more acute.
Larger Samples Possible: It is worth noting that the single visit, H < 11 APOGEE-1N strategy did
allow us to reach the bulge for the brightest giant stars. While APOGEE-2 is moving in the direction of
spending fiber-hours on deeper bulge targeting probes to reach smaller samples of bulge red clump stars, if it
were possible, at little expense, to accumulate more bright bulge giants as well, it would be an enormous help
and allow the Southern bulge to also be explored in a homogenous way with the Northern bulge. Indeed,
any direction of the Galaxy with sufficient target availability can benefit from the strategies explored in this
white paper — e.g., the Galactic bulge and disk, Magellanic Clouds, and some star cluster fields.
High Demand Fields: The Kepler field continues to be heavily oversubscribed and continues to be
mostly observed with single-visit designs to H = 11. The demand is so high that other parts of the disk
sharing the same LSTs continues to be under sampled compared to other parts of the disk. Meanwhile,
APOGEE-2’s ability to exploit the K2 asteroseismology/gyrochronology program for getting stellar ages
for fields in both hemispheres is a significant science driver that will be greatly enhanced by more efficient
observing.
The Value of Time Series Data: The unique time series strategy employed in APOGEE-1 and -2
targeting (one not implemented by any previous or contemporary Galactic archaeology survey) is paying great
dividends beyond the humble motivation of identifying “problem” binary stars. Spectroscopic variability has
been observed in emission line stars (Chojnowski et al. 201x), allowed the discovery of rare stellar classes
(Eikenberry et al. 201x), enhanced the astrophysical value of double-lined spectroscopic variables (Fleming
et al.... ), helped check and confirm Kepler KOIs (ref), and — by exploiting the unexpected stability of
the APOGEE spectrograph — allowed the discovery and characterizations of huge numbers of binary stars,
brown dwarfs and exoplanets (Troup et al. 2016). Indeed, APOGEE-1 has delivered some of the science
payoffs that had been hoped for from the MARVELS program. It is very likely that the long term scientific
value of the APOGEE instruments will come through their legacy ini creating an enormous database of long
term time series data on not just stars, but stellar systems, across the Galaxy. The value of these data are
significantly strengthened with more epochs. This science forms the basis for the ATLAS program described
in a white paper recently submitted to the AS4 Futures Committee.
Overcoming a Possibly Shortened SDSS-IV: In the event that a sixth year of SDSS-IV is not
1 The
APOGEE-2S targeting plan includes not only multiple visits to each bulge field, but, indeed, very deep, 24-visit probes
aimed at reaching the bulge red clump.
–4–
funded, it would be valuable to consider strategies for increasing the APOGEE-2 target yield. This would
be particularly valuable in the South, where observing time is already limited.
Remaining Competitive in the Long Term: The APOGEE spectrographs are ground breaking
instruments. High resolution infrared spectroscopy multiplexed by 300 fibers has made it possible to contemplate samples of Galactic stars across all parts of the Galaxy for up to half a million stars. Clearly the
idea has great appeal, and competing wide-field instruments are in the works, but with even higher degrees
of multiplexing: the optical 4-MOST instrument (with 812 fibers working at R ∼ 20, 000) on the VISTA
4-m, the optical WEAVE instrument (with 1000 fibers working at R ∼ 20, 000) on the 4.2-m WHT, and
the infrared-arm of MOONS (with 1024 fibers at R ∼ 20, 000) on the VLT. APOGEE-2N still benefits from
the largest FOV of all of these instruments, but with robotic reconfiguring, more fibers and larger aperture
telescopes, these other instruments will mean that APOGEE will lose its cutting-edge standing and have
significantly lower target etendue. It is not practical to consider increasing the latter via modifications on
the spectrograph end, but it is worthwhile considering strategies for increasing throughput that with the
instrument as is. The compelling potential of the APOGEE spectrographs for obtaining complementary data
for Gaia and TESS targets only reinforces the desirability to consider improving multiplexing capabilities,
especially if they can be done for relatively modest cost.
3.
Method 1: Parallel Cohorting with a Fiber Optic Switchyard
The essence of this method is that all cohorts for a field are observed on every visit through introduction
of a switching capability between the heavily plugged plates and the current spectrograph long links. This
method delivers two advantages. For fields with many planned visits, Method 1 is designed to deliver an
enormously enlarged sample of stars with large numbers of visits suitable for searching for stellar companions
(see Table 2). Method 1 is conceived as a strategy for maximizing opportunities for time domain science,
such as the proposed ATLAS project for AS4.
However, in addition to optimizing around time series, the method delivers greatly expanded samples
of stars in short, 1- and 3-visit fields, by eliminating over-observed targets through real-time switching to
other plugged targets during a visit. This is done by breaking up the huge magnitude range of these plates
into new cohorts (called here Very Short and Ultra Short cohorts) and cycling through these during each
visit (Table 1).
The proposed scheme arose out of many discussions to come up with a means to achieve an efficient
parallel cohorting scheme using an automated positioner. No way to attain this without tangling fibers or
taking inordinate times to reconfigure fibers has yet emerged from these discussions. The scheme proposed
here, however, achieves these goals using the current manual plate plugging, and therefore minimal changes
to the existing spectrograph and other infrastructure. The primary difference with the current process is
that plugplates are now drilled with holes for all stars from all cohorts and all of these holes are plugged at
every visit to the field, but during a visit, Very Short and Ultra Short cohorts fibers are swapped into and
out of connection to the spectrograph long links through a fiber optic switchyard at appropriate cadences to
yield the total required S/N , and not more, at the completion of a field’s total visits.
As a simple model2 , take the example of the 12-visit field shown in Figure 1, and imagine that we have
2 All
calculations here ignore sky and calibration fibers for simplicity.
–5–
100 Short, 100 Medium and 100 Long Cohort fibers collecting photons at any given time on-sky, and these
are directly associated with the bright-medium-faint-faint-medium-bright fibers on the long-links into the
spectrograph. But on the plugplate end there are actually 400 cartridge fibers plugged into Short Cohort
target holes, 200 cartridge fibers plugged into Medium Cohort target holes, and 100 cart fibers plugged into
Long Cohort targets (Table 2). All 700 fibers are plugged by the day staff (less than what they do for eBOSS
now). In the simplest scheme, we have no additional fiber management than we have now, with red, green,
blue sheathing to discriminate bright, medium and faint targets on the plate. But every four cart fibers on
Short Cohort targets are able to connect to a single bright long link fiber, and every pair of cart fibers on
Medium targets are able to connect to a single medium long link fiber. Then all we need to do is build a
fiber optic switchyard, to enable us to cycle through cohorts during a visit to a field. In this scheme, all
faint targets are always collecting flux. The end result, after 12 visits to the field, is that all targets from all
cohorts in the field obtain 12 epochs of data, not just the long cohort, as is the case now. Note that the per
visit S/N achieved for each star is expected to be sufficient for obtaining a precise radial velocity necessary
for orbit fitting companions.
In the case of the 3-visit fields, a different and remarkable degree of target etendue is possible with this
scheme. In the example shown in Table 1, the targets on a 3-visit field are broken up into Short, Very Short
and Ultra Short Cohorts and multiplexed to bright, medium and faint long links by 1, 3 and 9 short links
on the carts, respectively. After three visits to such a field, instead of obtaining three visits for 300 targets
(most with more than needed total S/N ), one can obtain three visits for 1300 targets in nominally the same
amount of visit time (Table 1)! Of course, this advantage is obtained for brighter stars, but it should be
noted that in APOGEE-1 we were already observing the equivalent of Very Short” cohorts exclusively for
observations of the Galactic bulge and the Kepler field.
Figure 2 shows an example of a commercial fiber switch made by DiCon Fiberoptics, Inc. that is
used in a number of laboratory science applications. A brief discussion with a member of the technical
staff established that they could make switches with the fibers we use for APOGEE, that the switching is
accomplished extremely reliably, that light losses can be minimized through AR coating fiber ends and with
use of reimaging microlenses on fiber ends at the switch, and that the unit price for a nominal switch of
interest to us would be of order $3-4k for a single unit, but with substantial quantity discounts possible.
However, they would only deliver switches with short pigtail ends to be integrated into our system (rather
than make switches that would incorporate the APOGEE long links directly as the “common fibers”).
Thus, additional losses from additional connectors can be expected. Exploration of the net light losses and
discussions with other vendors is obviously necessary. Nick is discussing the concept with CTech, who may
be interested in conceiving their own design for a switcher.
In summary, the infrastructure modifications needed to implement this scheme include:
• introduction of additional anchor blocks in the carts;
• appropriate changes to the fiber mapper;
• introduction of the optical fiber switches in a modestly environmentally-controlled environment (at the
Sloan 2.5-m, possibly in the area below the telescope floor?);
• changes in the connection between the short and long links;
• modifications in plate design, mapping, and SOP software;
–6–
Fig. 2.— One fiber optic switch concept, the VX Stepper Motor Switch from DiCon Fiberoptics,
Inc. The “Common Fiber” would be the spectrograph end and the multiple ”Channels” are
on the plugplate end.
• changes in observing times to enable AB dither pairs to be collected for the short integration times of
Ultra Faint Cohorts (these times of course are experienced by all other cohorts on the plate).
Advantages: In summary, the system delivers enormous increases in the numbers of (brighter) stars
in the nominal survey 3-visit fields, and allows huge gains in observations of crowded, low latitude fields.
But the system is designed primarily to achieve highly efficient time series data for vast numbers of targets,
a huge boon for the exploration of stellar companions. Another advantage is that fewer survey plates would
need to be drilled, because all cohorts are on a single plate design. Only variations for hour angle would be
needed.
Disadvantages and Concerns: The additional costs include perhaps something like 200 × $2k =
$400k in optical switches for each hemisphere. There are additional light losses to be expected in the system,
many of them unknown at present. However, in the North, these losses may be made up for through the
changing of the spectrographic corrector, where substantial H-band light is being lost now. The switching
technology needs to be further explored/developed. Alterations in the gang connectors at the switchyard
may be needed. On the science end, because we drill all holes on a single plate, one loses the ability to
overcome fiber collisions by using different plates. This may be a disadvantage for cluster fields. On the
other hand, one can always drill more designs and return to the current observing procedures as needed.
4.
Method 2: Series Cohorting with Multiple Gang Connectors
A simple and cheaper strategy to achieve larger numbers of targets is by cohorting in series. The idea
here is to triple the number of short fibers and gang connectors on the carts. Cohorting is achieved by
observing them in series during a visit, with each cohort having its own gang connector, that is swapped by
the observers during each field visit.
For example, for a 3-visit field (Table 1), the targets can be distributed between 300 Short, 300 Very
Short and 300 Ultra Short cohorts, each with its own gang connector. All are plugged for each cart by the
–7–
pluggers (still less fibers than for eBOSS). For each visit to a field, the observers would load a cart as usual,
start with a 6.5 minute exposure on the Ultra Short cohort, change the gang connector, observe the Very
Short Cohort for 20 minutes, swap gang connectors again, and then collect a nominal 60 minute visit for the
Short Cohort. (Of course, the order the cohorts are observed doesn’t matter, and offers an additional level of
flexibility — e.g., in dealing with twilight.) While this process makes each visit to a field longer (lengthened
by 27 min for extra integration time and a few minutes for swapping gang connectors), the overheads are
also amortized over more targets. Thus, while the total telescope time for a 3-visit field is about 3×72m
= 216m for 300 total targets, the proposed scheme would take 3×101m = 303m for 900 targets, triple the
number at all magnitudes, effectively doubling the overall efficiency for collecting targets.
Much larger samples of targets would be possible in higher visit fields, if a similar observing strategy
were adopted. In the example in Table 2, the Medium and Long Cohorts are kept the same as we currently
use in APOGEE observing, but the Short Cohort is broken up as for a 3-visit field to include Very Short and
Ultra Short Cohorts. With visits to fields lengthened to 101 minutes as before for gang connector swapping,
as many as 3100 different targets could be observed (though with the extra targets being bright stars) at
the expense of only 50% overall telescope time. In the is
In summary, the infrastructure modifications needed to implement this scheme include:
• triple the number of anchor blocks and gang connectors on the carts;
• a nominal increase in fiber management through introduction of two more fiber sheathing colors should
the strategy in Table 2 for long fields be utilized (for Ultra Short and Very Short cohorts – which can
be plugged randomly within the group);
• modifications in plate design, mapping, and SOP software.
Advantages: All technologies exist and proven for SDSS — only additional gang connectors need to be
added to each cart. Note that three gang connectors were already used on carts during MARVELS-APOGEE
cotargeting in SDSS-III. No changes to the gang connectors needed – only more are needed on the cart ends.
Modest cost — Nick estimates the cost for tripling the number of short fibers and gang connectors on the
carts at $25k. This strategy could be implemented now for APOGEE-2S carts, as they are being built.
Disadvantages: Not as efficient as Method 1 (e.g., faint fibers ”wait” to accumulate flux). Observers
spend more time outside, but make fewer cart changes per night. The extra targeting time is spent to
accumulate many more targets in fewer fields — fewer overall fields can be scheduled in the survey.
5.
Implementation Strategy
Clearly implementation of Method 1 involves a major reworking of the long fibers and gang connectors,
requires significant funding, and has too many unknowns to consider for APOGEE-2. However, Method
2 requires no change to the long-links, uses known technologies, and only requires modification of carts.
Despite this relatively simple set of modifications, it is perhaps only sensible to consider implementing the
strategy for the Southern part of APOGEE-2, given that the APOGEE-2S carts are still being built and it
is the South where there is the greatest pressure on targeting.
Obviously there is the concern that in the near term the APOGEE team is already fully occupied
with executing APOGEE-2S plans in hand without adding still more layers of complexity to the hardware
–8–
Table 1: Example Three Visit Field. Overheads (ov) for cart swap, acquisition, dome flats, etc
are assumed to be 12 minutes. APOGEE integration times are modified to ensure that full AB
pairs are collected for each cohort. In the table, d=designs, f=fibers, v=visits, m=minutes,
in=integration time.
Scheme
Current
Parallel
Series
U.Short Cohort
H < 10
1d.100f.3v.60m
9d.100f.3v.6.5m
1d.300f.3v.6.5m
V.Short Cohort
10 < H < 11
1d.100f.3v.60m
3d.100f.3v.20m
1d.300f.3v.20m
Short Cohort
11 < H < 12.2
1d.100f.3v.60m
1d.100f.3v.60m
1d.300f.3v.60m
Total Targets
300=100U,100V,100S
1300=900U,300V,100S
900=300U,300V,300S
Visit Length
60in+12ov=72m
60in+12ov=72m
87in+14ov=101m
Table 2: Example Twelve Visit Field
Scheme
Current
Parallel
Series
U.Short Cohort
H < 10
V.Short Cohort
10 < H < 11
4d.300f.3v.6.5m
4d.300f.3v.20m
Short Cohort
H < 12.2*
4d.100f.3v.60m
4d.100f.12v.15m
4d.100f.3v.60m
Medium Cohort
12.2 < H < 12.8
2d.100f.6v.60m
2d.100f.12v.30m
2d.100f.6v.60m
Long Cohort
12.8 < H < 13.2
1d.100f.12v.60m
1d.100f.12v.60m
1d.100f.12v.60m
*For Series case, the magnitude limit would be 11 < H < 12.2.
Scheme
Current
Parallel
Series
Total Targets
700=400S,200M,100L
700=400S,200M,100L
3100=1200U,1200V,400S,200M,100L
Visit
60in+12ov=72m
60in+12ov=72m
87in+14ov=101m
Comment
only Long Cohort gets 12 visits
all stars obtain 12 visits
only Long Cohort gets 12 visits
Comment
all *’s 3 visi
all *’s 3 visi
all *’s 3 visi
–9–
and observing. There is plenty to worry about already with numerous unknowns on a telescope that is
less familiar and itself undergoing a major overhaul, and that in the South we have a somewhat different
observing/plugging scheme to work out with crews that are new to Sloan-like observing. Obviously, we need
to walk before we can run with APOGEE-2S.
Still, it would be a shame to pass on consideration of a potential opportunity to enhance efficiency on
a relatively short term. At minimum, we might ensure that the APOGEE-2S carts as built do not preclude
the possibility for future expansion with additional anchor blocks and gang connectors.
Alternatively, we could consider building the carts with the multiplexing capability from the start, even
if not utilized immediately for initial observing. Once routine observing is mature, the additional gang
connectors could be slowly folded into operations.
A last option is to consider building just one of the APOGEE-2S carts with the extra capability, as a
test case. Again, we can do the actual observing tests later, when the observing crews feel ready. Or we
could slowly retool carts over time with the extra capability or only do some, but not all, of them — and then
rely on clever scheduling to match highly multiplexed carts to appropriately densely drilled plates. Even
having one cart with the triple fiber capability would be useful, potentially enabling two fields per night to
be observed with the extra multiplexing.