Spectroscopy with PACS

M82 PACSline imagingfrom theSHINING team(Contursi et al. 2010First Results workshoptalk)

Phil Appletonand Dario Faddafor the PACSTeam

Important Resources

• NHSC Proposal Planning Pagehttps://nhscsci.ipac.caltech.edu/sc/index.php/ObservationPlanning/HomePage

– NHSC HELPDESK– links to AOT Release notes for each spectrometer mode—very useful. – links to HSpot and Observer Manuals– links to other calibration documents

http://nhscsci.ipac.caltech.edu

Line Spectroscopy• Chop/Nodding (Phil Appleton will discuss)

– Chop rapidly between two sky position– Targeted “Line” scans or broad “Range/SED” spectroscopy available– Single Pointed or mapping on scale < 4 arcmins

• Unchopped scans with “off” position (Dario Fadda will discuss)– Allows for targets that cannot be chopped– Line-scans or Range/SED Spectroscopy– Single pointed or mapping on any scale

Chop/Nod Cycle Each line is visited with one up and down scan per repetition

Example above shows three lines in simple NodA/Nod B pattern

Simple LINE SCAN– GRATING SCANS OVER TARGETED LINE ONLY

Range Scan• User can specify the range of the scan• Useful for known broad or multiple lines• Special version is SED mode where Blue/Red

or Green Red bands scanned• Range Scan is only mode that allows

exploitation of extended second order (see next slide)

Range scans can providebroader coverage ifthere are multiple featuresUser can control the scanrange

Order Selection and Line/AOR

1st is always selected2nd

3rd

You can only select inany one AOR either the2nd or 3rd order paired with1st order

You can observe the same line 10 times (10 repetitions) or 5 different lines x 2 repetitions, or as many repetition-lines not exceeding 10 total per AOR. To repeat the whole sequence you can add more cycles. Note that calibration block is run at beginning or AOR.

Either “Blue/RED”

or“Green/RED”

You can select multiple linesper AOR—for each line scannedyou will get “for free” anobservation in the blue or redband (e. g. If request 2nd/1st ordermodes and you observe [CII]158mm, you wlll “simultaneously” get “blue” observation at 158/2 = 79mmAccessible

in range/SEDmode

For True Point Source

• Know your position well! Place target at center

array and use “Pointed Mode”. Be careful of any avoidance angle constraints (visualize in HSpot)

• Be aware of distortions (rotation) in footprintof NODA and NOD B –only fully aligned at centerof field

Chopped distortion on Large Throw(1, 3, 6 arcmin throw separations)

If you suspect your sourceis not a point-source oryou are very unsure of itsposition to 1-2”, probablybest NOT to use POINTEDmode, but a fully samplemapping mode.

Only the central fewpixels are properly alignedfor largest (LARGE) choppedthrow. Best to use small ormedium throw if possible.

Mapping Slightly Extended SourcesChop-Nod Observations

• First select target• Choose which “blue” order? 3rd+1st or 2nd+1st

Note that if you have target lines in all three orders you will need 2 separate AORs

• Choose a line (manual or from list) –don’t forget to check the box at right!

• Enter continuum and line flux—saturation will trigger different capacitance—warning in Pacs time est. message

• Select the Observational Mode • Set up the mapping parameters

– raster step sizes (follow guidelines of the release )herschel.esac.esa.int/AOTsReleaseStatus.shtml )

Time estimation and S/N calculation”Also warnings for saturation

a

bc

= a+b+c

On slew

IF ENTERED FLUX TOO LARGE: HSPOTWILL REQUEST A NEW GAIN BY

CHANGING CAPACITANCEUSUALLY FOR VERY HIGH FLUX > 10^4 Jy!

(See PACS Observer Manual)

Cycles versus Repetitions versuscomplete AOR?

• One line observation involves a complete up and down scan with the grating—this is called a repetition. 2 repetitions= 2 complete up and down scans

• If you have one weak line (say [NII]205mm) and one strong one [CII]158mm, you can increase the number of repetitions on the [NII] line at the expense of the brighter line. e. g. 9 reps [NII], and say 1 rep for [CII]. If you need more time on both lines you can then increase the number of cycles. (complete sets of repetitions). A cycle does NOT lead to a repeat of the calibration block. If you request a new AOR you will allows get a new cal block.

Repetition v Cycles: large-chopHSpot v5.0.2 (from obs estimate button)

• C/N Single pointing mode 1 line, 2 repetitons Total time = 952s (on source*=688s, cal=129s) • C/N Single pointing 1 line, 2 cycles Total time = 985s (on source 688s) Extra inst.

overhead of 33s!• C/N Single pointing 1 line, 1 rep, 2 AORs Total time = 2 x 586 (on 688s; cal=2 x 129) = 1172s but you get 2 cal blocks..very inefficient..

* Note HSpot reports “On source” time including the time chopped off! Repetitions have less overhead than cycles.

Some considerations withChop-Nod

• Is your source point-like, slightly extended or very extended, or HUGE? (If huge use unchopped scan mode)

• Choose an appropriate chopper throw small=±0.5 medium =±1.5 large =±3 arcmins• Choose the right mapping strategy for your object (Pointed = point source, large-scale map or small map use correct raster step size)• Nyquist spatial sampling is best achieved in instrument coordinates rather than

sky coordinates. Coverage can be less uniform in sky coordinates because of potential telescope pointing drifts

• If you want to ratio two lines, best include in same AOR to ensure pixels fall on same place on sky

Example Dither Pattern(large source)

Worked Examples: (AOR Design one or more AOR which provides minimal coverage along major axis of NGC 4565Map along the major axis of the galaxy NGC 4565 in [OI]63um linewith 5 x 2 raster map-covering with 38 x 38 tile size” redshift z = 0.004 (CHOP-NOD)—left below right below—an unchopped scan—see DARIO

Chop-nod mapping is limited to instrumentplane (chops along z axis). Limited abilityto map a feature without multiple AORs= 4.1hrs 3 x (38 x 38” 5 x 2 raster) one line

Unchopped grating scanin RA/Dec Coords. 38 x 38 “ 20 x 2raster at orientation = 135 degrees= 3.7 hrs clock time

detectorcords

z

y

Unchoppedscan can beprogrammedin sky coords

Offposition