Wavelength Calibration and Image Motion Corrections

The HUT wavelength scale is well described by a simple linear relation between pixel number and wavelength. For pixel numbers running from 1 to 2048, the wavelength is given by ${\rm \lambda~=~827.36~+~0.51336 ( n - 1 ) }$.This was verified to be stable to within $\pm 1$ Å in flight during Astro-1 using airglow lines.

The HUT observing apertures are quite large, and typical image motions can induce shifts in the zero-point of the wavelength scale for point sources. Offsets of 1$^{\prime \prime}$ in the dispersion direction correspond to wavelength shifts of 0.33 Å. Nominal performance of the IPS during optical hold is 1$^{\prime \prime}$ rms radius, with excursions of many arcsec during thruster firings for station keeping. Nearly all Astro-1 observations, however, were done under manual pointing control. The payload specialists were able to nearly match the expected performance of the IPS when suitable guide stars were visible in the field of the HUT acquisition TV camera, but most observations of bright stars had no visible guide stars, and pointing errors in these cases typically span the entire width of the aperture.

Timescales for the jitter in optical hold or under manual pointing control is of order several seconds. Pointing information from the Image Motion Compensation System (IMCS) is available at a 50 Hz rate, and HUT data acquired in high time-resolution mode can take full advantage of this information. In practice, we have found that corrections on the 2 second timescale of the histogram data mode is adequate to compensate for nearly all the image motion. Thermal drifts between the ASTROS Star Tracker (AST) and the HUT boresight introduces an offset of the IMC reference direction of several arcseconds on timescales of tens of minutes.

To correct for the wavelength shifts induced by image motion we first correct the IMC reference signal for the slow thermal drift by fitting a low order polynomial to the relative change in the AST pointing direction. Next, we compute the mean offset over the 2 second integration interval for a HUT data frame from the IMCS data. This is translated into a wavelength offset, and the data frame is shifted by the corresponding integer number of pixels before accumulation into the buffer for the observation.

These wavelength corrections in effect remove most of the spectral smearing induced by the image motion and permit one to recover the full instrumental spectral resolution of $\sim3$ Å for a point source. Airglow lines, whose emission fills the entire aperture, are correspondingly smeared into even broader features, however. To permit subtraction of the smeared airglow lines, a Ly$\alpha$ template profile is shifted in the same manner as the observed spectrum. This profile can then be used to model and subtract airglow emissions from the spectrum.

Wavelength offsets are still possible in the corrected data if the observation was started with the target mis-centered in the aperture. Corrections for such mis-centering must be determined off-line by examining the positions of guide stars (if available) relative to the slit center in the acquisition TV images.