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NEWSIPS Manual Addendum

II. Revised LWR Ripple Correction

The LWR ripple correction described in section 11.2.1 of the NEWSIPS image processing manual was modified shortly after the Goddard images had been processed. The new correction derived by Cassatella [1] slightly improves the ripple correction shortward of 2200 Angstroms. The new correction was implemented at Vilspa and, in Novemeber, 1997, all the Goddard LWR images were reprocessed with the new correction. As a result, all archived LWR high dispersion NEWSIPS data requested after November 14th, 1997 use the ripple correction described below.

The new LWR ripple correction uses the same expression for the $\alpha$ parameter as defined in Section 11.2.1 of the NEWSIPS manual. The blaze wavelength for order m however is now defined as:

\lambda_{c}(m) = (K / m) + \Delta\lambda(m,T)\end{displaymath}


K = A + Bm + Cm2 + Dm3 + Em4


A =   0.281749635D+06
B = -0.223565585D+04
C =   0.365319482D+02
D = -0.262477775D+00
E =   0.701464055D-03

The $\Delta\lambda$ term is a time-dependent correction to the blaze wavelength defined by the equation

\Delta\lambda(m,T) = a(m) + b(m)date + c(m)date^{2}\end{displaymath}

where date is the observation date as a fractional year (e.g., 1987.1), and the coefficients are defined at the reference orders shown below.

Order a b c
115 35736.699D0 -35.970790D0 0.0090515542D0
111 62433.199D0 -62.859885D0 0.0158223297D0
107 53287.133D0 -53.638642D0 0.0134980459D0
103 42742.7090 -43.014583D0 0.0108219635D0
99 28040.843D0 -28.206111D0 0.0070930274D0
95 10463.439D0 -10.501169D0 0.0026347077D0
91 6223.1919D0 -6.2320255D0 0.0015601476D0
87 4478.2512D0 -4.4662128D0 0.0011134034D0
83 112.32014D0 -0.0566704D0 0.0D0
79 156.16074D0 -0.787367D0 0.0D0

In practice, the $\Delta\lambda$ term is calculated at the reference orders for a given observation date and then linearly interpolated to the desired order. Orders longward of order 115 use the correction for order 115, and similiarly orders shortward of order 79 use the correction for order 79.

The study by Cassatella [1] determined that the changes to the LWR ripple correction did not affect any other calibrations (e.g., the absolute calibration). Therefore, no other calibrations were modified.

To verify images have been processed using the correct ripple correction, a line was added to the HISTORY portion of the primary FITS header. Images processed with the original ripple correction will contain the comment: "LWR RIPPPLE CORRECTION VERSION 1.0 APPLIED". Reprocessed images will have the comment: "LWR RIPPPLE CORRECTION VERSION 2.0 APPLIED".

Note that because the LWR images had already been processed, the LWR ripple correction was applied (at Goddard) using an IDL routine called MXCOR2. The program basically read the net flux vector from the archived MXHI files, re-applied the ripple and absolute flux calibrations, wrote the new ripple-corrected and absolutely-calibrated net flux vectors back into the MXHI file, and updated the comment in the HISTORY portion of the FITS header. At Vilspa, the new correction was implemented in the NEWSIPS processing software. Tests showed the two methods produced identical results.

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Randall Thompson