STScI Slitless Spectroscopy Workshop November 2010 aXe Advanced Topics – becoming more dextrous, using aXe with other instruments, making calibration files, … Jeremy Walsh, Martin Kümmel & Harald Kuntschner, ST-ECF

Topics 1.Tilted extractions 2.Catalogue manipulation 3.Magnitude limits in configuration files 4.Contamination - Gaussian and fluxcube 5.Improving the wavelength zero point 6.Sensitivity adjustment for extended sources 7.Complex objects 8.Blind extraction 9.Making flat field cubes 10.Configuration files for other instruments 11.Simulations for other instruments 12.Optimal extraction

1. Tilted extractions Default extraction is tilted extraction oriented by the object shape with correction for extraction geometry (Freudling et al. 2008) set by target size For a general object described by an ellipse, extraction slit is along major or minor axis (but not along dispersion axis) Range of options – generally slit length is factor mfwhm x object height along slit Optimal slit orientation (for columns of equal wavelength) not along major axis but tilted Options set in aXecore parameter list

1. Tilted extractions - 2 See the aXe manual Section 1.10

2. Catalogue manipulation Input object list based on SExtractor catalogue Minimum number of columns, #, X,Y Image, X,Y size, PA (θ), RA, Dec, Size in RA,Dec, PA on sky, Mag. Can add – more objects, alter PA, more magnitudes MAG_F ………………………….……… 26.5

3. Magnitude limits in configuration files The +1 st order is by convention that with the highest throughput Two mag. limits – extraction (MMAG_EXTRACT) and mark (MMAG_MARK) - for object extraction limit and contamination limit in Configuration file Mag. limits for 1 st order extraction based on experience with real exposures; adjust limits by exposure time All other orders as a delta on 1 st order If too few sky pixels remain after running aXeprep, then adjust MMAG_EXTRACT and MMAG_MARK values and make mag. limits brighter

3. Magnitude limits in configuration files - 2 All other orders than 1 st as a delta on 1 st order

4. Contamination – Gaussian Typical Parameters – (X,Y) position, axis lengths, PA of major axis, magnitude(s) from SExtractor file Multiple filter mags as MAG_F606 MAG_F792 ….. in catalogue

4. Contamination – fluxcube Filter fluxes from direct images per pixel over object extent (Sextractor segmentation image)

5. Improving the wavelength zero point Typical slitless image has a number of detected stars Late K – M type spectra have strong broad absorption features in μm region Use to check the zero point of the wavelength solution for the whole field Cross correlate slitless stellar spectra with templates (e.g. BPGS; Gunn & Stryker atlas) Depending on number of stars and zero point offset, apply correction

6. Sensitivity adjustment for extended sources Flux calibration established on spectrophotometric standard stars - point sources Applying this sensitivity to extended sources with excess short and long wavelength flux → ‘rising ends’ (‘ears’) Correct point source sensitivity by smoothing with a Gaussian of same size as target in dispersion direction (from input image catalogue)

7. Complex objects Extended objects, such as resolved spiral or starburst galaxy, treated as single object by Sextractor, may justify extraction of sub-regions – e.g. nucleus, HII regions, etc

May be objects not on companion direct image – filter too blue, emission line outside band, etc Add targets to catalogue (with new running numbers) based on external data (other catalogue, X-ray position, etc) with unique ID Run aXe to extract all spectra Examine extra spectra (2D and 1D), refine input catalogue if required 8. Blind extractions

9. Making flat field cubes For WFC3, ground narrow band (40Å) flats were taken in TV3 Normalise each flat to 1.0 then fit λ dependent pixel-to-pixel change with (n-1) order polynomial FF= a 0 + a 1 xλ + a 2 xλ** 2 + a 3 xλ** 3 + … a 0 holds large scale flat component (L-flat) Copy a n coefficients into 2D x n FITS image ACS had no narrow band ground flats. Used in-orbit filter flat fields instead

10. Configuration files for other instruments See aXe manual Section Can create configuration files for other slitless grism or prism instruments Simple ASCII file Set extensions for science, error and DQ Specify flat field cube and readout noise, etc For each order enter beam description, trace and dispersion solution and flux calibration file

10. Configuration files for other instruments - 2 One BEAM per spectral order Slope and curvature of spectra per order Dispersion solution f(x,y) Field dependence of trace and/or dispersion Beam extent wrt reference position ao a1x a2y a3x 2 a4xy a5y 2 λ0λ0 Δλ X and Y measured wrt reference position. Thus x = X-REFX

11. Simulations for other instruments Set up a new configuration file for the slitless instrument For each BEAM: –Set the left and right limits for each BEAM –Set any offsets of the trace relative to the reference position –Set the trace angle of spectra –Set up a dispersion solution for each BEAM. Prismatic dispersion 1/λ terms also available –Make a sensitivity file ( λ(Å), flux/Å)

11. Simulations for other instruments - 2 Simulations in detector coordinates – set pixel size to map realistic source sizes Determine the background e-/pixel/s for the whole telescope+instrument+grism+detector passband Set the transmission of the direct image filter Include the telescope effective area in the configuration file (default is HST)

Within aXe (axecore) sources can be extracted with optimal extraction (a la Horne 1985) Cross-dispersion profile determined from the contamination image Contamination image can have λ dependence of PSF Beware of using optimal extraction for emission line sources – spatial variation of line emission may not be identical to continuum Optimal extraction See aXe manual section 1.9

Other slides

Elements of slitless spectroscopy No slit(s) – each dispersed object forms its own ‘virtual’ slit Effective spectral resolution depends on object ‘size’ in dispersion direction Multiple spectral orders (grism, not prism) Spectra can overlap → contamination Background integrated over whole disperser passband (with gradients in dispersion direction), different from filters Each slitless spectrum must have λ- calibration to be flat fielded WFC3 G141 WFC3 G141 median sky