The Washburn Extraction Routine for low dispersion IUE spectra was reviewed. The shape of the point spread function (PSF) in low dispersion spectra is sufficiently well described by a gaussian function. The PSF is in large and small aperture essentially identical and values of sigma are presented. Several advantages of the extraction routine are mentioned

Deboer, K. S.

Koorneef, J.

Meade, M. R.

English

NASA Technical Reports Server

Washburn ExtractionandWidthof IUEPoint Spread.Function
Klaas S. de Boer, Jan Koornneef*, Marilyn R. Meade
Washburn Observatory of the University of Wisconsin
Madison, WI 53706, U.S.A.
*European Southern Observatory, CasiIla 16317, Santiago 9, CHILE
We review the Washburn Extraction Routine for low dispersion IUE spectra.
The shape of the point spread Function (PSF) in low dispersion spectra is
sufficiently well described by a gaussian function. The PSF is in large and
small aperture essentially identical and we present values of o. Several
advantages of the extraction routine are mentioned.
Introduction
The Washburn IUE extraction routine (Koornneef and de Boer ]979, hence-
forth KB) has been used up to now only to extract low dispersion spectra.
The routine basically fits an a priori known point spread Function (PSF) to
the intensity distribution perpendicular to the dispersion. In four iter-
ations the routine Finds the intensity I in the spectrum, the background B as
the baseline of the input data, and the position xo of the spectrum. We find
that the PSF can be represented sufficiently close by a gaussian, hence in
pixels at positions x, the intensities fit to
I(x) = B + _2 exp - _ (
where o is the dispersion of the gaussian function in units of 2V_--x(pixel
length). Crivellari and Morossl (]980) also find a gaussian but they suggest
that haloing in the camera can be accounted for by adding a 15% effect of a
Lorentz profile. Our extraction is per-
-'_--_-- -- -- Formed on a line of pixels diagonally
-_---- _ -- through the image, hence deviating from
__ perfect perpendicular by small angles,
'----_ ,, _------ 6° for SWP, 8° for LWR (see Figl). .
pixel grid,
,I_ ' set of pixels on one diagonal; dot-pixels:_-- _,_ set for next diagonal in Washburn ex-I _ _ traction.
i ,j
771
https://ntrs.nasa.gov/search.jsp?R=19810017443 2020-03-21T12:06:27+00:00Z
Fig• 2: The actual inten-
06 _ ..... '- - sities of the pixels of each
fit are coadded, after proper
3 s0_0 alignment of the centroid
of the PSF, and normalizationO. _ ""
• • of the fitted PSF to unit
" • area; the PSF is gaussian for
w . point sources. This plot
- • shows the sum of 350 extracted
0.2 diagonals in SWP between ]58()
and 2000_, of a MV = 14 BHB
" " star in NGC 6397 (de Boer
" " 1980) The very minor de-
• %% • • lie_ •• • i• •NoD.• e%j • • ••••u oleeemN •
viation from a gaussian in
the lower wings is caused byIltl-ll. lll 1111_.1.
O. 10. 20. 30. _. SO. 60. 70. 80. 90. 1_. the change of o from = ].l tO
P0SI_IOms0 _ 0.9 respectively (see fig.
4).
Based on a prescribed o, we have extracted many images since the devel-
opment of the routine. Coaddition of all the x-adjusted fits provides a
check on the actual PSF. In LWR, the PSF is always gaussian. In SWP images
reduced in 1979, the PSF turned out to be inexplicably asymmetric• We have
reduced the spectra from images of that period which had the flaw in the SWP
ITF, by redressing that error at the pixel level based on the data from Table
I of Holm (1979a). These rereduced spectra now produce also a gaussian PSF
in SWP.
Properties of the Point Spread Function (PSF)
In order to find what o is in the _pectra, we have carried out extrac-
tions with o also as a free parameter. In that case, of course, the result-
ing I is meaningless. From the free-o extraction we have calculated average
o's in 4OA and 50X wide bands in SWP and LWR respectively, representing 34
diagonals in SWP and 27 diagonals in LWR. In the calculation of that average,
o's larger than ].8 and smaller than 0.3 have been excluded, the large O's
because they cannot be real regarding the grand average of o = 0.98 _ 0.03
(2¢'2"-xpixel length);the small ones were excluded because they came from an
accidentally high central pixel with low wing-pixels on the input diagonal.
Before extracting, extreme-valued pixels get low weight (see KB). In
the provided geometric-photometric images all pixels have been offset by 2000
IUE flux numbers. Reseaux then have pixel-values < 1950 f.n.; these get low
772
weight. High pixels (saturated, > 32500 IUE f.n.) also get low weight.
There is another set of pixels which gets low weight. KB showed that the
ITF's applied to the raw images truncated at a certain value. Hence, in
commissioning phase data in LWR no pixel had more than 20000 IUE f.n., re-
sulting in unflagged "semi-saturation" in spectra which had not yet DN near
255. The ITF's employed since May 1978 are better but still have truncation.
Holm (]979b) emphasized this more and gave actual DN's. The spectral ranges
where this is important are:
max f.n. per pixel max trustable standard
(Turnrose 1978) extracted intensity
1300 17740 _ 85000 IUE flux
2800 25220 _ 120000 IUE flux
where the maximum trustable intensities are adapted from KB allowing for
smaller o found presently. Such pixels als0 get 10w weight resulting in
positively improved intensities from the extraction fit (see KB sec 5).
Turnrose et al. (1980) described a numerical extrapolation to the ITF.
As reported by KB, the PSF was essentially equal for large and small
aperture spectra. For the presently discussed 12 images for each camera,
that ratio is close to unity indeed. Figure 3 shows how this ratio behaves
with respect to wavelength.
KB reported that the PSF in SWP had _ = 1.05 constant, and in LWR had o
= 1.3 at 2000A to o = 0.9 at 3000_. For later images, Koornneef derived that
in SWP o gradually decreased to shorter wavelengths, and that in LWR the
change of o along the spectrum was less pronounced. Figure 4 shows how, for
characteristic images, o changes along the spectrum. Apparently, the IUE
Fig. 3: Ratio Ola.ap/Osm.ap. fo[ a
I , I ' I gaussian Point Spread Function ot
SWP LWR
I.I 40_ bonds 50_bends _ IUE low dispersion point-sourcespectra in its relation to wavelength.
I i i mverag e f°r 12 images in each camera'
b_ with standard deviations given. In
_ several SWP images the signal had
vanished at ll8OA leading to a very
-- uncertain ratio in this figure.
b 0.9 e: Images were obtained between Sept.
j l I 1978 and Feb. 1980.
I000 2000 3000
773
).2 I I I I I--- I I' I' 1 I i
SWP _ LWR
m I,O
+ _
t.O-
0
× /
X
0 • ×
0'8-_,,,°o/ • , O_ --
I "'-i" I I I I I I °I 1
1200 1600 2000 2000 2400 2800 5200
Fig. 4: Width o of gaussian PSF in relation to wavelength, for 4 images in
each camera. The thin lines are only meant to aid the eye. The point with
error bar in the corner gives the characteristic size of the dispersion.
optical system has been brought into an optimized condition after a few
months of operation. A plot of _ with image number (not shown) indicates
large o for early images, in time levelling off to values of o as given pres-
ently.
Most likely, there is a correlation between o, i.e. the ]eve] of _, and
the temperature in the satellite. Figure 5 shows for each camera the values
_ at specified wavelengths in relation with the read head temperature THDA
(Bohlin et al. 1980). No clear correlation is apparent.
LW--'_---_ --_'--I [ ] T-- --[-'- 1,2 -----_ .... I-----l---F--r [
x SWP
1.O • •Q
x • •
, x x
o_ Xo X X x x
-- XX 1.0 X
"B. o o • o "& o
_o.,j._.o,".°°..°.°•• o_-'; o.° ...o.o ,o.o,,_.-_,,oo.• O.8F • • _ [ 1500oJ
__l__L _ I I __ ____L_______i, , = I'P°LI
IO 12 )4 16 6 8 I0 12
THDA (°C) THOA (°C)
Fig. 5: For all 12 images, in each camera, we show how o behaves with
respect to the temperature THDA available on the scripts. No clear cor-
relation is indicated. The figure can be used to judge the dispersion of
the o's.
774
Error Estimate
The Washburn routine fits a prescribed PSF to the data points on a line
of pixels diagonally through the image. The best fit is found at the minimum
of the sum of the deviations of all points. This minimum deviation is used
as a quality indicator of the fit. The procedure is set up such that the
final error is calculated from all_ the data points at full weight, hence
saturated data points or points from truncated ITF's get "flagged" in propor-
tion to their deviation from the adopted PSF. The error estimate is used as
weighting factor in combining data from small with large aperture (if avail-
able) and so bad data (reseaux, saturation, etc.) are effectively replaced by
good data, in addition to an overall improvement. The propagated errors are
most important after calculating the ratio of two different spectra (extinc-
tion studies; Koornneef and Code 1980) in order to obtain a fair judgement
of the limited spectral overlap between SWP and LWR spectra.
Spectral Resolution
The Washburn IUE extraction routine has a sampling rate twice as large
as in the standard IUE extraction. We therefore may expect somewhat better
spectral resolution. Often we resolve in low dispersion the strong inter-
stellar Mg II in the direction of the Magellanic Clouds, and the stellar Si IV
doublet.
Spatial Resolution
The main advantage of PSF fitting is that one may offer any spatial
function to give the best fit. A first step has been made by Koornneef and
Mathis (1980) who studied the spatial intensity distribution near 30 Doradus
in the LMC. With such procedures also the shape of the UV light distribution
of globular clusters in the LMC can be determined (de Boer 1980).
Bohlin, R.C., Holm, A.V., Savage, B.D., Snijders, M.A.J., Sparks, W.M. 1980,
A & Ap., in press.
Crivellari, L., Morossi, C. 1980, 2nd year of IUE, ESA, in press.
de Boer, K.S. 1980, This symposium.
Holm, A.V. 1979a, NASA IUE Newsletter 7, at p. 30.
Holm, A.V. 1979b, NASA IUE Newsletter 7, at p. 34.
Koornneef, J., Code, A.D. 1980, Ap.J., to be submitted.
Koornneef, J., de Boer, K.S. 1979, NASA IUE Newsletter 5; and SRC IUE News-
letter 4, p. 41. (KB)
Koornneef, J., MathiS,cJ.S. 1980, Ap.J.,submittedTurnros , B., Harvel, , Bohlin R.-_. 1980, NASA iUE Newsletter 8, p. 32.
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Washburn extraction and width of the IUE point spread function

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