STScI Slitless Spectroscopy Workshop 15-16 November 2010

aXe Advanced Topics – becoming more dextrous, using

aXe with other instruments, making calibration files, …

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

Topics1. Tilted extractions2. Catalogue manipulation3. Magnitude limits in configuration files4. Contamination - Gaussian and fluxcube5. Improving the wavelength zero point6. Sensitivity adjustment for extended sources7. Complex objects8. Blind extraction9. Making flat field cubes10.Configuration files for other instruments 11.Simulations for other instruments12.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_F1600

11 940.581 10.400 5.3085 ………………………….……… 26.5

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

3. Magnitude limits in configuration files - 2

All other orders than 1st as a delta on 1st order

4. Contamination – Gaussian

• Typical Parameters – (X,Y) position, axis lengths, PA of major axis, magnitude(s) from SExtractor file

Multiple filtermags asMAG_F606MAG_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 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

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= 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

10. Configuration files for other instruments

See aXe manual Section 5.2.1.6

• 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 a3x2 a4xy a5y2

λ0

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