Survey Options Session1 and 2 Particular focus - special cadence - - PowerPoint PPT Presentation

Survey Options Session1 and 2

Particular focus

• special cadence requirements or
• pportunities that deserve more (or less)

attention than they have received

• candidate “tools” for WFD and mini-

surveys

Sessions by topics

• Session 1 – morning

– Standard visits: 2 exposures vs 1 – Non-standard visits: greater depth in u? – Survey uniformity: depth, seeing,…. – Survey area – trade against number of visits

• Session 2 - afternoon

– Rolling Cadences: what are the objectives, trades, and constraints? – Dithering: translation, rotation – Length of the observing season: denser sampling vs longer time series

Probable contributors

• DESC cadence needs (Dan Scolnic, Humna Awan)
• 1 vs 2 image visits (Chris Stubbs)
• Strong Lensing (Phil Marshall, Aprajita Verma, ....)
• Galaxies (Eric Gawiser)
• Simulations for cadence options (Lynn Jones, Owen Boberg, Tiago

Ribiero)

• Rolling cadences (Peter Yoachim)
• Solar system cadences (David Trilling, Henry Hsieh)
• AGN (Gordon Richards, Neil Brandt)
• Intelligent exposures, dithering (Tony Tyson)
• Dwarf galaxy cadence needs (Steve Ridgway)
• SN cosmology (Renée Hlozek, Nicolas Regnault)
• Comments (Robert Lupton)
• Variables (Fed Bianco,….)

Overview of Session 1 Topics

Exposure times and Visit Counts

• Standard visits: 2 exposures vs 1
• Non-standard visits: greater depth in u?
• Survey uniformity: depth, seeing,….
• Survey area – trade against number of

visits

1x30 sec exposure?

• Typical 2x15 visit interval 39 sec

– Save 1 readout (2 sec) and 1 shutter cycle (1 sec) – Efficiency gain – 7.5%

• As a fraction of LSST construction cost ~$50M
• Considerations

– Reduce data bandwidth and archive volume – Lose science potential of very short gap images – Data loss due to cosmic rays, satellites, glitches

• Comments

– Chris Stubbs – Robert Lupton – Tony Tyson

Increase read time?

• 3-4 sec instead of 2 sec
• Improved detector performance
• Comments?

– Robert Lupton

Visit pattern?

• Multiplicity strategy

– Facilitate study of moving, rapidly varying targets – Confirmation vs characterization – Same filter, different filter – Temporal pattern – gap length

• Multiple visits compete with cadence frequency
• Visit pairs reduces number of “epochs” by 2X

Fans of Multiplicity

• Solar System (David Trilling, Henry Hsieh,

…?)

• Variables, fast transients………

Increase u exposure - rationale

• Improved photon detection efficiency and
• bserving efficiency
• Some science benefits from improved u

depth – e.g. improves discrimination between faint stars and distant galaxies

• Compromise – either decreased number of

u-band visits, or reduced time for other filters

Increase u-band depth

• With fewer visits*

– Yes

• Milky Way, Variable Objects

– No

• Astrometry, Transients, GRB, AGN

– Maybe

• MW Halo, Cepheid ML, Variable Objects, SN, Large

Scale Structure, Cosmology

• Preserve the number of visits

– Yes – Determining impact on schedule efficiency requires simulation

* “Votes” from Survey Strategy paper

Increase u exposure - Simulation kraken_1045

• Double exposure time, retain number of

visits

• Increase u depth 0.5 mag
• Decrease other bands 0.05 mag

Possible champions for increased u-band sensitivity?

• Variables?
• Milky Way?

Survey Uniformity

• Uniform data sets are convenient for

science

• Observing conditions are variable
• Strategies for achiving uniformity

– Statistical (no selection or control) – Control of cadence for conditions – Selection of filter/field for conditions

• Example – uniform depth

Baseline2018, WFD Square Degrees at Visit Depth

Uniformity by control - depth

• Can be actively controlled by adjusting

integration time

Relative number

• f visits

0.57 for median = 0.72 sec 0.95 for mean = 0.85 sec

Baseline2018, Square Degrees at Stacked Depth

Baseline2018, WFD Square Degrees at Visit Depth

Baseline2018, Square Degrees at Stacked Depth – Overlay Random Dithered Healpix

Uniformity by Selection

• Uniformity in other parameters: image quality,

sky brightness, zenith distance, parallax

• Dithering is a profound complication to

selection

• One approach - reserve specific dithers for

particular observing values, and/or to track conditions at all dithers and constrain or repeat dithers as needed.

• If rotation of dither must be considered, it is

more challenging and probably less efficient to achieve

Interests in pursuing uniformity by selection?

• DESC?

Survey Area

• Some science benefits from increased sky

coverage possibly with reduced cadence

– Higher count of targets vs incremental gain from stacking more visits – Better sky coverage

• Study distributions on sky - e.g. MW dwarf galaxies

– Not all science needs full power of WFD survey

• Extend area with limited filter set/cadence

Overview of Session 2 Topics

• Rolling Cadences: what are the objectives,

trades, and constraints?

• Dithering: translation, rotation
• Length of the observing season: denser

sampling vs longer time series

Rolling Cadences

• Why needed?

– Low revisit rate in universal cadence

• Some of the trades

– Shorter inter-visit gaps vs longer seasonal gaps – Rolling cadences can be very complex

• Considerations

– Cadence less “universal” – Or even heterogeneous – Survey “closure” interval

Why Rolling Cadence?

• Ten year survey, 800 visits in pairs means

– 40 epochs/year (all filters) – 10 epochs/year (r or i filter)

• For an observing season of 8 months

– 6 day phase gaps (all filters) – 24 day phase gaps (r or i filter)

• Concept – redistribute visits for more

dense coverage some time and less dense coverage other times

Potential reduction in phase gaps

• Example – assume that half of all visits to

a region are available and are deployed to enhance sampling

– For one pass in 10 years, 6x reduction – For two passes in 10 years, ~3.5X reduction – For 3 passes in 10 years, ~2.7X reduction

• Additional flexibility – ½ season length, no

multiplicity

– 24x reduction

Comments on Rolling Cadence

• SN cosmology requrements (Renée

Holzek, Nicolas Regnault,….)

• Rolling cadence simulator developments

(Peter Yoachim,…..)

Rolling Cadences can be complex

• Spatial region definitions
• Duration of cadence segments
• Selection of filters
• Cross-talk with regular WFD
• Multiplicity of visits
• Survey status for annual releases
• Some science needs both small phase gaps and

long time series

• But implementation can be simple – candidate for

scripted schedule segments

Phase coverage desert

• Time constants larger than 30 minutes and

less than 4 days (any one filter) are not well served by uniform cadence or by general purpose rolling cadences

• Option – visits deployed as “micro-

surveys”.

– E.g. rolling cadence season of 30 days applied to each sky region for one roll

Dithering

• Relevant discussions at LSST2017

breakout on Sky Tiling

Overview of LSST2017 Sky Tiling Breakout

TESTING LSST DITHER STRATEGIES FOR SURVEY UNIFORMITY AND LARGE-SCALE STRUCTURE SYSTEMATICS Awan, Gawiser, Jones, Zhan, Padilla, Arancibia, Cora, Yoachim

• Tested random, haxagonal, spiral dithers
• Examined spatial structure in coadded depth
• Conclusions

– Favor per-visit and per-night dithers – Most dither methods improve estimated number of galaxies – Most methods reduce spurious structure in galaxy counts below statistical

SKY TILING, ROTATIONS, OVERLAPS Chris Stubbs

Attaining good sky coverage (<1% gaps) with fixed centers implies roughly 80% of sky gets single-coverage roughly 20% of sky gets double-coverage Open Question

• What’s the interplay between photometric calibrations, frame

subtraction artifacts, and dithering/rotations?

LSST FoV in MAF Peter Yoachim

• Addressed FOV placement as packing

problem

• Dithering in simulations
• Is there any science planned for the
• verlap regions? Variability on 30s-10min

timescales? Need metrics.

LSST FoV in MAF Peter Yoachim

SEVERAL TOPICS Steve Ridgway

• Possible efficiency loss with dithering
• Difficulty of achieving uniformity in various
• bserving parameters with dithering
• Benefits/costs of achieving uniformity in

stacked images, and when during survey

• Randomizing optics angles

Open dithering issues?

• Dither memory and make-up rules
• Defining quantitative requirement for

rotational dither

• Dithering on field edges (1-2% of WFD)
• Cross-talk between dither and temporal

sampling for short time scale events

Comments on Dithering

• Requirements (Tony Tyson)
• Simulations ……..

Observing Season

• Trading intervisit gaps against length of season is

quite particular to, e.g.

– target characteristics – fast, slow – science objectives – catalog, characterize – likely follow-up strategy – lsst follow-up vs external facilities

• Some survey objectives tend to shorten season

– Improve temporal sampling, lower airmass

• Some tend to lengthen it

– Characterize slow events – Maximize certain discoveries – Maximize parallax baseline

Indicator of season length

Competition with season length?

Comments on Season Length?

• Strong Lensing (Phil Marshall, Aprajita

Verma,…)

• AGN (Gordon Richards, Neil Brandt,…?)