You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
one key step to the calibration of the infrared spectrograph ( irs ; houck et al .
2004 ) aboard the _ spitzer space telescope _ ( werner et al .
2004 ) is the calibration of the acquisition images from the red peak - up ( pu ) sub - array on the short - low module ( sl ) . our primary standards , hr  6348 , hd  166780 , and hd  173511
were observed repeatedly during the cryogenic _
mission , and most of those observations began with target acquisition in the red pu array .
we can use these data to lock the overall photometry from these standards , and use that to photometrically calibrate their spectra .
to determine the effective wavelength of the red pu filter , we multiply its responsivity ( at a bias voltage of 2.0 volts ) by @xmath0 , which approximates the long - wavelength behavior of a star .
the centroid of the product is 22.35  , which we will adopt for the effective wavelength of the red pu sub - array .
figure 1 illustrates the process .
we calibrate the red pu data by tying our photometry to the calibration of the 24-  filter on the multiband imaging photometer for _ spitzer _ ( mips ; engelbracht et al .
2007 , rieke et al .  2008 ) .
mips-24 observed all three of our primary standards .
our procedure is to preserve the mean of the mips-24 measurements , but to adjust the relative levels of the individual stars to reflect their brightnesses as observed in the red pu images .
our photometry follows the same procedure as described in irs - tr  11001 ( sloan & ludovici 2011 ) .
pu data were obtained in two sets of three images . for each set , we produced a median image , then performed aperture photometry using a four - pixel source radius and a seven - pixel radius for the sky annulus . in cases
where the photometry from the initial ( acquisition ) image differed from the second ( sweet - spot ) image by more than 1.5% , we rejected the initial image from further consideration .
data obtained after irs campaign 14 have been corrected for the small variations in responsivity described in irs - tr  1101 , using divisive corrections of 0.9826 ( camp .
1534 ) and 0.9929 ( camp .
3561 ) .
lllrcccc +   + * standard * & * spec .
* & & + * star * & * class * & * measurements * & * irs red pu * & * mips-24 * + hr  6348 & k1 iii & 163 & 1934.2 @xmath1 2.1 & 30230 @xmath1 122 + hd  166780 & k4 iii & 79 & 1950.4 @xmath1 4.5 & 30650 @xmath1 884 + hd  173511 & k5 iii & 217 & 2033.2 @xmath1 2.0 & 31650
@xmath1 124 +   + * standard * & & & + * star * & & & * irs red pu * & * mips-24 * + hr  6348 & & & 0.9802 @xmath1 0.0011 & 0.9801 @xmath1 0.0040 +
hd  166780 & & & 0.9870 @xmath1 0.0023 & 0.9937 @xmath1 0.0287 + hd  173511 & & & 1.0328 @xmath1 0.0010 & 1.0262 @xmath1 0.0040 +    table 1 presents the resulting photometry , both in digital units from the images and normalized to unity .
table 1 also presents similar results from the mips-24 data published by engelbracht et al .
( 2007 ) , using their calibration of 6.92 @xmath2 10@xmath3 jy ( s / dn ) . the uncertainties in table 1 are the uncertainties in the mean , and for the pu data , the lower values reflect the large number of ( unrejected ) observations .
lcccc + & * normalized * & * measured * & * ideal * & * ideal * + * star * & * flux density * & * @xmath4 ( mjy ) * & * @xmath4 ( mjy ) * & * @xmath5 ( mjy ) * + hr  6348 & 0.9813 @xmath1 0.0029 & 209.2 @xmath1 0.8 & 209.4 @xmath1 0.6 & 235.0 @xmath1 0.6 + hd  166780 & 0.9881 @xmath1 0.0023 & 212.1 @xmath1 6.1 & 210.9 @xmath1 0.5 & 236.7 @xmath1 0.5 + hd  173511 & 1.0306 @xmath1 0.0029 & 219.0 @xmath1 0.9 & 220.0 @xmath1 0.6 & 246.9 @xmath1 0.6 +    to combine the normalized signal in the 2224  region for the three sources , we took a simple average of the irs pu and mips-24 photometry for hr  6348 and hd  173511 . for hd  166780 , we adopted the value from the irs pu images alone , due to the much higher uncertainty in the mips data .
table 2 presents the adopted normalized flux densities for the three standards , shifted up 0.0011 to preserve a mean of 1.0 .
the mean of the measured mips-24 data is 213.4 mjy , and the product of this mean and our normalized flux densities is the ideal flux density of the standards in the mips-24 filter . to determine the ideal flux density in the irs red pu filter , we scale the ideal mips-24 values by 1/@xmath0 , assuming effective wavelengths of 22.35 and 23.675  .    the mean ratio of the ideal 22-um flux densities to the photometry with a four - pixel aperture gives a calibration of 1.2151 @xmath1 0.0008 @xmath2 10@xmath6 jy ( s / dn ) for the irs red pu array
the samples of irs standard stars observed with the red pu array and the stars with mips-24 photometry observed by engelbracht et al .
( 2007 ) have ten objects in common .
table 3 lists their photometry with both instruments .
the photometry for the three k giants used in the calibration above differs slightly because none of the data have been rejected .
the fluxes span a factor of nearly eight , which allows us to investigate the linearity of the reponsivity of the red pu sub - array over nearly an order of magnitude .
the last column of table 3 gives the ratio of the red pu photometry to the mips-24 photometry , after scaling the mips data by 1/@xmath0 .
lrrc + & * irs red pu * & * mips-24 * & * irs red pu / * + * star * & * @xmath7 ( mjy ) * & * @xmath7 ( mjy ) * & * scaled mips-24 * + 21  lyn & 114.5 @xmath1 0.5 & 100.9 @xmath1 0.9 & 1.011 @xmath1 0.010 + 26  uma & 153.4 @xmath1 0.2 & 132.1 @xmath1 0.6 & 1.035 @xmath1 0.005 + hr  4138 & 121.4 @xmath1 0.4 & 110.9 @xmath1 0.5 & 0.976 @xmath1 0.005 + hr
5467 & 37.4 @xmath1 0.2 & 33.4 @xmath1 0.2 & 0.999 @xmath1 0.008 + hr  6348 & 234.7 @xmath1 0.4 & 209.2 @xmath1 0.8 & 1.000 @xmath1 0.004 + hr  7018 & 37.3 @xmath1 0.2 & 32.8 @xmath1 0.2 & 1.012 @xmath1 0.007 + hd  41371 & 116.5 @xmath1 0.3 & 103.7 @xmath1 0.1 & 1.002 @xmath1 0.003 + hd  166780 & 237.0 @xmath1 0.6 & 212.1 @xmath1 0.6 & 0.996 @xmath1 0.004 + hd  173511 & 247.1 @xmath1 0.3 & 219.0 @xmath1 1.0 & 1.006 @xmath1 0.004 + bd+16  1644 & 77.4 @xmath1 2.6 & 71.5 @xmath1 0.4 & 0.965 @xmath1 0.033 +    figure 2 plots the last column of table 3 as a function of red pu photometry in the top panel .
the line fitted to the data accounts for the uncertainties , and its slope deviates from the horizontal by only 0.3  @xmath8 .
we can conclude that the responsivity of the red pu array behaves linearly over the flux range considered .
the bottom panel of figure 2 tells a different story . here
, the analysis is based on the older s18.7 pipeline output .
the s18.7 pipeline version did not provide the new `` acqr.fits '' files .
instead , it provided `` bcd.fits '' files , which are not processed as thoroughly .
ludovici et al.(2011 ) presented a linearity analysis of the red pu data at the seattle meeting of the aas based on the s18.7 data , and they concluded that in fact the data do deviate from a linear response , with fainter targets showing a slightly smaller response than brighter targets .
our analysis of the s18.7 data reveals a shift of @xmath91.0% .
the slope deviates from the horizontal at a 3-@xmath8 confidence level , which while not definitive does support the earlier conclusion .
we conclude that the new `` acqr.fits '' files released with the s18.18 pipeline show a linear response function , but the `` bcd.fits '' files available with older pipelines may not be as robust .
ludovici , d. , sloan , g.c . , barry , d.j . ,
lebouteiller , v. , bernard - salas , j. , & spoon , h.w.w.2011 , `` characterization and calibration of the infrared spectrograph on the _ spitzer space telescope _ , '' _ baas _ , abstract 254.22 .
we present a calibration of the acquisition data obtained by the red peak - up ( pu ) sub - array on the infrared spectrograph on _ spitzer _ , based on repeated observations of three k giants . 
 this calibration is tied directly to the most current infrared calibration based on data from the multiband imaging photometer for _ spitzer_. an analysis of the responsivity of the red pu sub - array reveals no detectable deviations from linearity in the most recent pipeline version , but older pipeline versions show evidence suggesting possible small non - linearities .