HST–ESA issues (Fosbury and Burrows)

HST—ESA issues Bob Fosbury & Chris Burrows , July 1990 Introduction Following the realisation, during June, that the imaging performance of the HST Optical Telescope Assembly is seriously degraded by spherical aberration—most probably caused by the primary mirror, there has been some discussion about the options *** for the rescue of the lost components of the scientific mission. Since NASA will have its own public image very much in mind during this debate, we feel that it is important to set down our understanding of the nature of the problem and to outline the broadest range of possible solutions. This should help ESA to start informed discussions with NASA and take a stand which will protect both the European investment in the project and, we hope, as large a number as possible of the original scientific capabilities.

The inherent danger in the current situation is illustrated by NASA public statements implying that most of the scientific capacity of the observatory will be recovered by replacing the WF/PC in three years time with an optically corrected clone. Whilst such replacement is undoubtedly an attractive proposition which will restore a substantial part of the ‘spectacular’ (to the public at least) performance, it will leave the ESA FOC seriously compromised and so not allow the exploitation of the ultimate resolution of the telescope, particularly in the ultraviolet. In the following sections, we set down a concise statement of the nature of the optical aberration and its effects on performance, the options for improvement in both the short and the longer term and, finally, some considerations regarding time allocation as it affects European astronomers. Status of the OTA ----------------- The telescope is, with the exception of the optical problem working essentially perfectly. Early problems with pointing disturbances particularly at the terminator, and bit flip errors in the fine guidance electronics are all expected to be fixed with flight software changes. This means that if the optical problem can be overcome, we will have an observatory fully capable of meeting all its original goals.

Characterisation of the aberration and description of the PSF ------------------------------------------------------------- The telescope suffers however from overcorrected spherical aberration. Rays of light that hit the edge of the primary mirror focus to a point about 43 mm behind the focal point of rays that hit near the center of the mirror. This means that there is no clearly defined best focus, and all possible focus settings (obtained by moving the secondary mirror) are compromised. The minimum diamiter geometrical spot (circle of least confusion) has a size corresponding to 1.5 arcseconds when projected on the sky. This should be compared to the level one requirement of 70% encircled energy in a 0.1 arcsec radius in the visible. In reality, to obtain 70% of the energy, one needs a circle of radius about 0.8 arcsec (depending on the precise focus chosen).

The image does have a sharp core with dimensions comparable to the diffraction limit, that contains about 15% of the energy. This percentage can be increased somewhat by changing the focus setting, but then the 70% encircled energy point moves out. In the ultraviolet, the proportion of energy in the core will decrease dramatically. (Bob, we could include Encircled energy curves, MTFs PSFs etc from my FOC IDT presentation for example. -- Let me know) No UV images have yet been obtained, so this and all statements concerning UV performance are predictions.

In the visible, the low frequency information passed is comparable to a good ground based site. The high frequency information (> 2 cycles per arcsec) passed by the system is about a factor of 4 lower than expected at some frequencies, but is essentially zero over much of the spatial frequency space. In the ultraviolet below about 300nm, essentially no high frequency information is passed compared to the diffraction limit. These observations imply that for all but the brightest objects, deconvolution techniques will not substantially recover the lost performance.

The image aberration is expected to be field independent across the focal plane (after SI correction for the predicted off axis astigmatism). FOC - Faintest sky limited exposures infeasible Image resolution lost, except possibly on the brightest objects after deconvolution. Long slit severely compromised. WFPC- All faint object science lost. FOS - High spatial resolution observations lost. longer exposures HRS - Loss of efficiency, or spectral resolution. FGS - Largely unaffected (transfer function is insensetive to Spherical aberration)

How accurately can it be modelled? --------------------------------- The modelling of the PSF is crucial to understanding what observations can be done, interpreting scientific data, and implementing any fix. The models are presently adequate for the first two goals in most cases. Any full aperture optical fix must involve knowledge of the wavefront to about 1/20 wave RMS. We presently have about 1/2 waves RMS spherical. Measuring the aberration to this accuracy is demmanding, and has not been done yet. At this point, for planning purposes, it is reasonable to assume that such a characterization will be possible (either from ground based historical data, or on-orbit data), but there is no guarantee. In particular, the results from different instruments may not agree for a variety of reasons to the accuracy required. Independent measurements are possible with the WFPC/FOC and the presently ailing wavefront sensors. (the optics control system is swamped by the magnitude of the aberration, and presently is adversely affected by the attitude jitter).

If a characterization to the required accuracy cannot be achieved, then options involving the primary mirror actuators, extra corrective optics on a wheel, or even adaptive optics would need consideration.

Options for improvement ----------------------- Tuning up the performance now Choosing a ‘best’ focus Instrument considerations Other considerations Options for improving the OTA In orbit The six scientific instruments—which include the Fine Guidance Sensors—are all replaceable in orbit (ORU) and so the opportunity is available to substitute either an optically corrected clone—as WF/PC II would be—or a completely new instrument such as the already approved 2nd generation devices NICMOS and STIS. Whilst the OTA is not designed to have components replaced or added in orbit, the possibility of effecting a ‘global’ solution by correcting the telescope rather than just the instruments is so attractive as to merit serious consideration. The notion that WFPC II solves the problem means that effort may not be directed towards exploring this possibility unless pressure is applied to NASA either by the scientific community, by ESA or both. Stopping up/down Effect on imaging performance Effect on aperture throughput in sparse and crowded fields Correction Full aperture Pre-instrument Replacement of secondary Return to ground Plans to return HST from orbit for refurbishment on the ground were dropped some years ago. Although this is clearly a risky operation, both technically and politically, and would involve a loss of even the compromised scientific opportunities while it was out of orbit, such an option has the attraction of completely correcting the telescope and allowing the fitting of, perhaps, a suite of new instruments together. We know that there already exists a replacement primary mirror, made by Kodak, and so this could be done relatively quickly. Replacement Given the existence of the second primary mirror, the option of building and launching a new HST ‘shell’ has its attractions. This shell could be fitted with the 2nd ESA solar array (on the ground) and some of the existing instruments (in orbit). As a way of recovering the full capabilities of the existing FOC without the risk of returning it to the ground it should be seriously considered. It is, incidentally, an option which was mentioned this week by Riccardo Giacconi as one of the more appealing radical solutions. Options for improving the instruments Cameras For the WF/PC which contains re-imaging Cassegrain optics, the correction of the spherical aberration will be relatively straightforward. The possibilities for the correction of the FOC were discussed last week by the IDT and will not be repeated here. While there are possibilities to make optical corrections to the existing FOC, to replace it with a corrected ‘clone’ (as is happening with WF/PC II) or to build an entirely new version of the instrument, all of these will be expensive options which will not necessarily recover all of the scientific capabilities which were anticipated, e.g., the long slit spectrograph. Solving the problem with the OTA would therefore be most attractive and something which it would be reasonable for ESA to press for. Spectrographs & HSP FGS and astrometry The GO & GTO programmes Use of GTO time GO Time Cycle 1 Later cycles Recommendations The ESA contribution to the HST project has been, it appears, most successful. The hardware works superbly well and the ESA staff play major rôles. We believe, therefore, that the Agency is in a strong position to press NASA to adopt a strategy which will restore the scientific capability of the observatory as a whole rather than make a ‘quick-fix’ for one or two of the instruments. Some of the options which may appear radical now may, in the long term, prove to be better for scientific performance and cheaper (e.g., involving fewer shuttle launches over the lifetime of the observatory). For the IDTs, who have spent so long working in the project, we are concerned that the best use is made of their guaranteed time. While not personally associated with any of the IDT, we see that there is a danger of the FOC guaranteed time being much less effective than was anticipated. This will depend very much on the type of solution adopted for HST by NASA. While the GO, of course, have the option of applying for time on any of the instruments, we believe that their interests will be best served by restoring as broad a range of the HST capabilities as possible.