COS Line-Spread Functions

Analysis of FUV spectra obtained during SMOV indicates that the line spread function (LSF) of COS with the HST OTA departs from the Gaussian profile observed during ground testing. It exhibits a broadened central core and strong, broad wings, where a significant fraction of the flux (up to 40%, depending on the wavelength) is redistributed. The reason for the difference is that zonal (polishing) errors on the HST OTA introduce mid-frequency wavefront errors (MFWFEs) into the beam entering the COS spectrograph, which in turn diverts more light from the core of the LSF into the wings.

We have performed optical modeling of the COS+HST LSF with the MFWFEs included, and tested these new models on COS data obtained during SMOV. We found that convolving these LSF models with very high-resolution STIS E140H spectra leads to an excellent match to the observed COS FUV spectra, in contrast to the poor fit resulting from the convolution with pre-launch Gaussian LSFs. The fraction of light diverted into the wings of the LSF is smaller in the NUV channel of COS than in the FUV channel, and decreases toward longer wavelengths, as the LSF wings progressively shrink moving into the NUV. However, the effects of the MFWFE remain significant at all wavelengths covered by COS.

The wavefront errors produced by a typical focus offset are nominally included in our NUV LSF models. In reality, however, the amount of focus offset can vary significantly from visit to visit, producing significant variations in the widths of the NUV LSF around our model values (the FUV channel is much less sensitive to focus variations). With these variations, assigning a single numerical value to the resolving power of the COS NUV channel is less meaningful. Further characterization of the observed on-orbit COS LSF in the NUV is in progress and results will follow in a subsequent report.

Note that in the NUV, our current models predict LSFs with cores that are narrower than were observed during ground testing, perhaps due to limitations in the pre-launch alignment setup. The extent to which these narrower cores are realized on-orbit has yet to be fully evaluated.

We have computed a series of LSF models with MFWFEs included for all FUV and NUV gratings; these models are provided online to the astronomical community. The LSF models can be used by COS observers to assess the impact of the on-orbit COS LSF on their planned science observations and/or to analyze data acquired on-orbit. The models can be found at: http://www.stsci.edu/hst/cos/performance/spectral_resolution/

A description of these models and their impact on COS spectra is described in COS ISR 2009-01(v1)

We are currently in the process of computing LSFs for all FUV central wavelengths using the aberration constants appropriate to each central wavelength setting. We will provide this more extensive library of LSFs to the astronomical community as soon as they become available.

We already anticipate that the MFWFE effects are most important for science with the medium-resolution FUV gratings, G130M and G160M, and with the G185M NUV grating. In contrast, due to the nature of the science likely to be conducted with the low dispersion G140L and G230L gratings, we expect that the MFWFE effects on most science conducted with them will be minimal.