Jeremy Walsh, Martin Kümmel & Harald Kuntschner , ST-ECF

1. aXe Advanced Topics – becoming more dextrous, using aXe with other instruments, making calibration files, … Jeremy Walsh, Martin Kümmel & HaraldKuntschner, ST-ECF STScISlitless Spectroscopy Workshop 15-16 November 2010

2. Topics • Tilted extractions • Catalogue manipulation • Magnitude limits in configuration files • Contamination - Gaussian and fluxcube • Improving the wavelength zero point • Sensitivity adjustment for extended sources • Complex objects • Blind extraction • Making flat field cubes • Configuration files for other instruments • Simulations for other instruments • Optimal extraction

3. 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

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

5. 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_F1600 11 940.581 10.400 5.3085 ………………………….……… 26.5

6. 3. Magnitude limits in configuration files • The +1st 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 1st order extraction based on experience with real exposures; adjust limits by exposure time • All other orders as a delta on 1st order • If too few sky pixels remain after running aXeprep, then adjust MMAG_EXTRACT and MMAG_MARK values and make mag. limits brighter

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

8. 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

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

10. 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 0.6-2.0μm 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

11. 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)

12. 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

13. 8. Blind extractions • 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

14. 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= a0 + a1xλ + a2xλ**2 + a3xλ**3 + … • a0 holds large scale flat component (L-flat) • Copy an coefficients into 2D x n FITS image ACS had no narrow band ground flats. Used in-orbit filter flat fields instead

15. 10. Configuration files for other instruments • 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 See aXe manual Section 5.2.1.6

16. 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 a3x2 a4xy a5y2 λ0 Δλ X and Y measured wrt reference position. Thus x = X-REFX

17. 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)

18. 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)

19. Optimal extraction • 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 See aXe manual section 1.9

20. Other slides

21. 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