Performance

The Astro-1 Observatory, including the Hopkins Ultraviolet Telescope, was launched from Pad 39B at Kennedy Space Center aboard the space shuttle Columbia (STS-35) at 01:49 a.m. EST on 1990 December 2. The orbit achieved was nearly circular at an altitude of 358 km and an inclination of . HUT's initial activation was performed as scheduled at 04:30 hours Mission Elapsed Time (MET), but first light, planned for 18:00 hours MET, was substantially delayed by several Spacelab system problems, including the failure of one of the two terminals used by the crew to command the experiments and the failure of the IPS to acquire targets. HUT obtained first light, observation of the airglow spectrum in the general direction of the Vela supernova remnant, at about 32 hours MET and observed the first source (the Seyfert galaxy NGC 4151) at 35 hours MET. Earlier planned observations intended to focus HUT and check its co-alignment with the other Astro instruments were not accomplished due to the problems mentioned above, but the first several successful observations persuaded us that HUT was adequately focused and aligned to proceed with observations.

Observations continued at least partially successfully until 101 hours MET, when the second terminal used by the crew also failed. A new operational mode was then developed, in which the instruments were commanded from the ground while the crew employed the manual pointing controller (similar to a guide paddle on a ground-based telescope) to acquire and guide on the targets. Successful operation in the new mode resumed at 118.5 hours MET and continued without any major difficulty until the final observation was made with HUT at 200 hours MET. This occurred one day earlier than planned due to concerns about the weather at the shuttle landing site. Landing was at 215 hours MET at Edwards Air Force Base, California, and the payload was recovered in good condition.

During the Astro-1 mission, HUT obtained a total of 106 separate observations of 77 different sources, accumulating 39.4 hours of on-source integration time. Observations were made throughout each orbit and include varying contributions from the emission lines originating in the Earth's atmosphere, the airglow. As expected, observations of the fainter sources and those requiring the larger apertures were best obtained while the orbiter was in Earth's shadow and the airglow lines were weaker, but the brighter point-source observations made effective use of the Sun-lit portion of the orbit. Among the objects observed successfully were quasars, galaxy clusters, active and normal galaxies, cataclysmic variables, globular clusters, supernova remnants, planetary nebulae, white dwarfs, Wolf-Rayet stars, Be stars, cool stars with active coronae, comet Levy (1990 XX), Jupiter and Io.

Detector dark count was measured 3 times during the mission with the spectrograph aperture in the closed position. The rates obtained over the whole spectrum varied from 0.73 to 0.96 counts s, with a mean of (mean error), corresponding to counts s Å. This is approximately twice the rate observed during laboratory calibration, consistent with the more intense radiation environment encountered in orbit. A typical exposure with HUT (2000 s) contains 1 dark count pixel. No observations were made during passage through the South Atlantic Anomaly.

The dominant source of background in HUT observations is due to emission from Earth's geocorona, which was at a high level due to strong solar activity and a hot atmosphere at the time of the mission. The strongest line by far is H I Lyman-, whose intensity was typically 3 kR ( photons cm s sr) at orbital midnight, yielding 12 counts s in the smallest aperture () and 100 counts s in the largest aperture (). During orbital day these rates were typically larger by a factor of 10. Other geocoronal lines observed at night include O I 1304 and 1356, Lyman-, and He I 584 in second and third orders. During orbital day, a multitude of additional lines appears below 1200 Å, mainly due to O I , N I , and N II . The full-width at half-maximum (FWHM) of the airglow lines as measured with the narrow slit ranges from 3.2 Å to 3.5 Å, verifying the e xpected resolution of the spectrograph. The positions of the airglow lines were also used to determine the wavelength calibration in flight, which was found to be in good agreement with our laboratory calibrations. HUT wavelengths appear to be accurate to Å in most cases. Most of this error results from uncertainties in the position of a source in the aperture.

Light scattered by the HUT diffraction grating is another source of background. Unlike the airglow lines, which are localized in wavelength, the scattered light component produces a continuum background. Preflight measurements of the holographic grating, selected for its low scattered light, gave a scattered continuum 50 Å away from a line of intensity I counts s of 10 counts s Å. We verified the low scattered light in HUT during the flight by measuring the apparent flux below 912 Å for each source. Except for the few EUV sources observed, this flux gives a direct measure of the sum of the dark counts and the scattered light. These measurements verified that both of these contributions to the background are very small and consistent with preflight expectations.

An example of the data obtained by HUT on the Astro-1 mission is shown in Figure 2, which gives the observed (raw) count spectrum for the DA white dwarf G191-B2B (). This observation was obtained through the 30 point-source aperture with HUT stopped down to the half-aperture door position. It began at 03:19:31 GMT on 1990 December 5 and lasted for 366 s, during which the mean count rate was 2627 counts s. The star was held within the spectrograph aperture throughout the observation, as indicated both by the constancy of the observed count rate and by the guide star position data. This object was also observed through the EUV filter aperture, and significant flux was measured at Å ([Kimble et al. 1993]). The EUV observation allows us to infer that second- and third-order EUV radiation contributes a few percent of the peak counts observed in Figure 2, and it is straightforward to correct the observed spectrum to obtain a pure first-order spectrum.

The only strong features in the G191-B2B spectrum are the Lyman absorption lines, as expected for an object believed to have a nearly pure hydrogen atmosphere. Lyman- is partially filled in by the strong geocoronal emission at this wavelength, but the higher Lyman-series lines are much less affected. The continuum cuts off sharply below 915 Å, due to absorption by interstellar hydrogen in the converging Lyman series and the Lyman continuum. The small residual flux observed below 912 Å is due to second- and third-order radiation from and Å ( of the observed signal) and scattered light from Å ( of the signal). The apparent rise in the continuum longward of 1825 Å is due to a second-order contribution from 912 Å, which is easily corrected in reduced data. Finally, there is a weak apparent absorption feature at 1600 Å that we have determined to be due to a small dead region in the detector. This feature is not readily apparent in full-aperture observations, while it is very obvious in the -aperture observations. The increased strength of the feature results from the much smaller astigmatism in the small-aperture observations.

This observation of G191-B2B has been used to obtain the primary in-flight throughput calibration of HUT; a more detailed description of the HUT calibration will be presented in a future publication. Briefly, however, the observed spectrum was corrected for second- and third-order contributions, dark counts, phosphor persistence (which causes 7.4% of real events to be double-counted), and for dead-time effects (% at the peak of this spectrum). The resulting spectrum was then compared with a model-atmosphere calculation for G191-B2B ( K, , and ), described by [Holberg 1991] and kindly provided to us by P. Bergeron. Division of the corrected spectrum by the Bergeron model prediction, which has been multiplied by the transmission of the interstellar medium for and ([Kimble et al. 1993] and then smoothed to the HUT resolution, yields the effective-area curve shown in Figure 3. The data in Figure 3 have also been multiplied by the ratio of full-aperture to half-aperture sensitivity, derived separately from observations of several stars that were observed in both the half-aperture and full-aperture modes.

The data in Figure 3 have been used to derive the HUT effective-area curve (shown as the smooth curve). Structure on scales less than 25 Å was removed by smoothing the raw data with a Gaussian of dispersion 10 Å. Before smoothing, 5 Å regions surrounding the cores of Ly, Ly, Ly, and the 1600 Å feature were replaced by linear interpolation of the surrounding pixels. A 15 Å region centered on Ly was similarly replaced. Also shown in Figure 3 are effective areas at several wavelengths as computed from the efficiencies of various components of HUT measured in the laboratory several years before the mission. A full end-to-end laboratory calibration of the assembled instrument was not possible within the scope of the HUT program. The in-flight measurements run 20%--30% below the preflight values, probably due to aging of the photocathode. Our measurements of the on-board calibration lamp over time indicate a decline in efficiency of 24%. We also measured a similar decline in sensitivity (in both magnitude and spectral shape) over several years in the original HUT spectrograph, which was replaced after the Challenger accident. The two low points in the preflight data, at 1280 Å and 1336 Å, were suspect, due to inadequate reference-detector calibrations at these wavelengths. To obtain the effective-area curve shortward of 920 Å, where the observed flux of G191-B2B goes to zero, we have used the preflight calibration points at 835, 879, and 920 Å. A linear least-squares fit was made to these points and scaled down (by 27%) to join smoothly onto the in-flight calibration curve at 920 Å. We consider the overall agreement of the preflight and flight data to be highly satisfactory.

Figures 4 and 5 show the adopted flux fit of G191-B2B to the Bergeron model. Also shown is an independent model calculation for the same and , kindly provided by D. Koester. Comparison of the data and the models gives some idea of the internal consistency of the calibration, though, of course, it yields no information on possible systematic errors in absolute flux calibration. These could arise, for example, from deviations of the real white dwarf atmosphere from the model atmosphere or from unknown errors in our data reduction procedure. At present we believe that the calibration of the HUT sensitivity is accurate to better than % throughout the 912--1800 Å band. Uncertainties in the G191-B2B temperature (derived from Balmer line profiles) translate to less than 5% changes in the model-predicted far-UV flux down to within a few Å of the Lyman edge. Variations in the fitting procedure used to derive the effective area curve from the observation (judgments on how smooth the curve should be) lead to only 5% excursions around the curve we have adopted. Therefore, since the statistical precision is also excellent, if the DA white dwarf model atmosphere is correct, the HUT sensitivity curve is extremely well determined.

Several lines of evidence support the belief that the G191-B2B model flux is accurate to the level cited. A comparison of the observed spectra for G191-B2B and the significantly cooler DA white dwarf HZ43 confirms the self-consistency of the model atmospheres employed. Independent of white dwarf models, a preliminary analysis of the HUT observation of the BL Lac object PKS 2155-304 indicates that, when fluxed with the G191-B2B-based calibration, the spectrum is well fitted by a power-law all the way to the Lyman limit, as expected for this object. Finally, the ratio of the in-flight to preflight calibrations varies with wavelength by only 20%, and this is in a manner that is consistent with the degradation we have previously observed for detectors of this type. There is thus no reason to suspect any large calibration error of the magnitude that has plagued sub-Lyman- spectrophotometric observations in the past (see [Holberg 1991] and references therein). Future papers will detail the comparisons of HUT data for other sources with model calculations and with Voyager and rocket data in order to develop further a reliable calibration for the far-ultraviolet region of the spectrum.

We have recently (January 1992) performed a postflight laboratory calibration of the HUT spectrograph at seven wavelengths distributed across our spectral range. The ratio of the postflight laboratory efficiencies is well fitted by a smooth curve varying from 0.85 to 0.65 across our first-order wavelength range. This degradation is consistent with our previous experience as described above. The preflight effective area data for the full instrument (the diamonds plotted in Figure 3, omitting the two suspect points) are also well fitted by a smooth curve. The product of these two curves yields another smoothly varying function that represents our best estimate for the in-flight efficiency of HUT, based purely on our laboratory calibration, and traceable to standards maintained by the National Institute of Standards and Technology. This laboratory calibration curve matches the G191-B2B-based in-flight calibration curve almost exactly.

The ratio of the laboratory calibration curve to the in-flight curve has a mean value of 1.003 with an rms deviation of 6.6%, and maximum deviations of +12% and -8%. We emphasize that no re-scaling has been done to achieve this impressive agreement. These results provide powerful confirmation that (1) there are no significant systematic errors in the HUT flux calibration, and (2) the G191-B2B model atmosphere calculations provide an excellent flux standard for the far ultraviolet. We believe our observation of G191-B2B constitutes the best existing absolute UV flux measurement (i.e., directly traceable to laboratory standards) of a star that is suitable as a primary flux standard throughout the vacuum ultraviolet region of the spectrum, all the way to the Lyman limit.