A new geometric distortion solution for the STIS NUV

MAMA

Jesús Maíz-ApellánizLeonardo Úbeda

TIPS

17 July 2003

Why?

• Original motivation: NUV-MAMA objective-prism utility

• Current implementation based on Walsh et al. 2001 gives large errors

• New approach: use well-known PC geometric distortion to obtain solution for NUV MAMA

Data• Central region of NGC 4214• WFPC2

– GO 6716, P.I.: Stecher F170W F336W angle 1: u4190101r, 02r u4190103r , 04r angle 2: u4190201m, 02m u4190203m , 04m

• NUV MAMA – GO 9096, P.I.: Maíz-Apellániz CN182 CN270 angle 1: o6bz02isq, iwq o6bz02j7q , jbq angle 2: o6bz01afq,(akq) (o6bz01b1q),(b3q)

Positions of the PC fields

Positions of the NUV-

MAMA fieldsPC

angle 2

PCangle 1

MAMAangle 1

MAMAangle 2

N

E

F336W PC mosaic

Technique• Find stars and measure positions (and

photometry) with HSTphot in PC data• Measure rotation and displacement between

PC fields and merge lists• Measure rotation and displacement between

PC and MAMA fields• Find stars in MAMA fields using merged PC

list and centroid positions• Calculate and test geometric distortion

Geometric distortion model

• Direct (pixel sky )

– xc = i=0,k j=0,i ai,j·(x-xr)j·(y-yr)i-j

– yc = i=0,k j=0,i bi,j·(x-xr)j·(y-yr)i-j

• Inverse (sky pixel)

– x = xr + i=0,k j=0,i ci,j·xcj·yc

i-j

– y = yr + i=0,k j=0,i di,j·xcj·yc

i-j

• xr = yr = 512

• k = 3,4,5

Testing• Consistency check of the PC solution• F336W vs. F170W PC data• Polynomial degree• Weighting schemes

– Distance between 2 PC positions– Magnitude cut

• Single-field vs. multi-field solutions• CN182 vs. CN270 differences• Comparison with Walsh et al. 2001 (ISR)• External testing

Consistency check of the PC solution

F336W vs. F170

W data

o: F170W + F336Wo: F336W

N

E

Polynomial degree

Testing• Consistency check of the PC solution• F336W vs. F170W PC data• Polynomial degree• Weighting schemes

– Distance between 2 PC positions– Magnitude cut

• Single-field vs. multi-field solutions• CN182 vs. CN270 differences• Comparison with Walsh et al. 2001 (ISR)• External testing

CN182 vs. CN270 differences

Detector coverage

Comparison with Walsh et al. 2001

Walsh et al. 2001

Comparison with Walsh et

al. 2001

uncorrected

Comparison with Walsh et

al. 2001

originalcorrection

Comparison with Walsh et

al. 2001

corrected

Comparison with Walsh et al. 2001

uncorrected original correction corrected

Plate scalesResult x scale

(mas/pixel)y scale (mas/pixel)

CN182 24.53 ± 0.04 24.79 ± 0.04

CN270 24.54 ± 0.01 24.83 ± 0.05

Walsh et al. 2001

24.53 ± 0.12 24.83 ± 0.13

External

testing

Added bonus:

testing PC photometry

Summary• New geometric distortion solution for NUV

MAMA• Provides positions with median uncertainties

of 0.4 MAMA pixels (10 mas)• No wavelength dependence detected in plate

scale or distortion but testing not quite complete

• PC astrometry and photometry in the UV is quite precise after all corrections are applied