Observa(onal Strategy

Transcription

1 Observa(onal Strategy

2 Some issues for ACS & WFC3

3 Dithering ACS: 1.8% hot pixels, cosmic rays, interchip gap Mutchler & Cox 2001 & Dither Handbook LINE BOX

4 Dithering Large Scale: Mosaic Dithers Images combined later with AstroDrizzle

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6 Geometric Distor(on: ACS/WFC3 focal surfaces are far from normal to the principal rays varia(on of pixel area across the detector. Distorts the shape of the PSF The square pixels of the detectors project to trapezoids of varying area across the field of view The ACS/WFC geometric distor(on calibra(on is stable with (me to < pixels (Anderson 2007; van der Marel et al. 2007) done using observa(ons of 47 Tuc

7 Charge Transfer Efficiency (CTE) Affects both WFC3 UVIS and ACS WFC White Paper: MacKenty & Smith 2012 Post Flash Technique (Anderson et al) CTE: measure of how effec(ve the CCD is at moving charge from one pixel loca(on to the next while reading out the chip. Declines over (me: cosmic rays damage the silicon lahce: hot pixels and charge traps.

8 Retrieving data HST search form send to FTP staging area

9 Some Issues for COS

10 Fixed Pajern Noise : COS introduced by both the detector and the illumina(on pajern. include ar(facts such as shadows from the quantum efficiency grid wires, hexagonal boundaries due to the mul(fiber bundles on the microchannel plates

11 Fixed Pajern Noise : COS Solu(on: Observe at different focal plane offsets Fixed pajern noise in the COS detectors limits the S/N that can be achieved in a single exposure to: 15 5 per resolu(on element for the FUV 50 for the NUV. To achieve higher S/N ra(os one can obtain a series of exposures, each slightly offset in the dispersion direc(on, causing spectral features to fall on a different part of the detector. For STIS these mo(ons are known as FP SPLITs. For COS these mo(ons are specified by the FP POS op(onal parameter. (Calcos data pipeline)

12 Terminology TIME TAG Mode vs ACCUM mode: COS exposures may be obtained in either a (metagged photon address mode (TIME TAG), in which the posi(on, arrival (me, and pulse height (for FUV observa(ons) of each detected photon are saved in an event stream, or in accumula(on (ACCUM) mode, in which only the posi(ons of the photon events are recorded. COS observa(ons should be obtained in TIME TAG mode whenever possible

13 Mar(n WFC3 Op(cal WFC3 NUV ACS/SBC FUV (Lyman alpha)

14 Exposure (me and Es(ma(ng # of Orbits

15 Signal to Noise Need to jus(fy the required S/N (par(cularly why it is sufficient) In general try to maximize it without satura(ng or hihng some limit. Resolved stellar pops (local) S/N COS HALOS: S/N ~ (FUV)

16 Signal To Noise: Instrument Handbooks

17 # of Orbits: Astronomer s Proposal Tool Orbit is 97 minutes min occulta(on Overhead:guide stars, poin(ng maneuvers, buffer dumps (350s, 2 frames) Avg ~3000s overhead

18 Exposure Time Calculator (ETC) hjp://etc.stsci.edu Example: Glob Cluster NGC 2419 Normaliza(on : Beccari Color Index Reddening: Harris Glob Cluster Catalog Or NED

19 Presenta(ons 30 minutes Paper chosen must use HST data Instrument and observa(onal methodology needs to be detailed (including how they dealt with instrument specific problems) Remember to start with big picture intro for why the topic is important Address *why* they used HST data (why space?)