NEWSIPS Manual Addendum

III. High Dispersion Heliocentric Velocity Correction

Wavelengths for all high dispersion images except nulls and onboard calibration lamp exposures (i.e., object classes 98 and 99) are routinely reduced to a heliocentric frame of reference. The algorithm used in NEWSIPS is basically the same as that used in IUESIPS and is based on programs originally written by Howard Cohen and Arthur Young from Indiana University and described by Harvel(1980) [1]. Some minor differences however do exist.

The velocity components of the earth and IUE in a righthanded rectangular equatorial coordinate system (+x is toward the vernal equinox, +z is toward the north celestial pole) are computed using the routines described in Harvel (1980). The computed net radial velocity of the IUE spacecraft toward the object is given by the expression:

$$V = V{x} Cos(\alpha)Cos(\delta) + V{y}Sin(\alpha)Cos(\delta) + V{z}Sin(\delta)$$

where

Vx = Vx(Earth) + Vx(IUE)

Vy = Vy(Earth) + Vy(IUE)

Vz = Vz(Earth) + Vz(IUE)

and

$\alpha =$right ascension of object,
$\delta =$declination of object.

The corrected wavelengths are then defined as:

$$\lambda{corrected} = (1 + V/c)\lambda{uncorrected}$$

where c is the speed of light.

The calculation is such that a net approach of the IUE spacecraft toward the target requires a positive net radial valocity correction to the heliocentric reference frame, following the standard convention. The individual IUE and earth velocity components and the net radial velocity correction used are documented in the image processing history portion of the primary FITS header. Note that typically the correction for the earths motion is roughly 30 km/sec while the correction for the spacecraft motion is between plus or minus 3.1 km/sec.

One difference between the IUESIPS and NEWSIPS velocity corrections is in the calculated observation times. The IUESIPS correction was based on an estimate of the midpoint of observation which was calculated by subtracting half of the estimated exposure time from the observation end time. This estimate was known to introduce some errors though because the exposure times stored in the IUESIPS label were occasionally incorrect. The NEWSIPS velocity correction however is based on the observation start time! (This may have been an inadvertent error since NEWSIPS also calculates the time of the observation midpoint.) This can lead to a less accurate velocity correction depending on the orientation of the spacecraft orbit with respect to the direction of the target and the length of the exposure. Presumably the error would be small, amounting to a NEWSIPS error of less than 1-2 km/sec.

A second difference is that IUESIPS used a single set of spacecraft orbital elements (obtained on November 22, 1979), while NEWSIPS uses the latest set of orbital elements available before the observation was made. Because the spacecraft's orbit gradually changed with time, updated orbital elements were calculated every few weeks. This difference could cause an IUESIPS error of up to 3 km/sec depending on the orientation of the satellite with respect to the target and the observation date Taylor(1993) [2].