al insights into the atmospheric struc- ture of hot stars. It is essential, although time consum- ing, to disentangle effects due to binari- ty from effects reflecting the kinematics of the stellar group (11). Especially the lack of recognition of wider binaries with an RV amplitude of the order of 10 kmls might significantly influence the conclu- sions. Hence, repetition of RV observa- tions is necessary and will in turn in- crease our knowledge on binarity in the selected young stellar groups. The spectra will in addition allow a study of stellar rotation in these star groups (12). In many respects, this project forms an extension towards the earliest spec- tral types of the key programmes of Mayor et al. and Gerbaldi et al. (de- scribed in the Messenger 56), even when the primary scientific motivation is somewhat different. In addition to the common interest in radial velocities, the stars in Sco-Cen are also observed by the Hipparcos satellite, providing space motions when combined with the RVs. Using these, we hope to reconstruct the initial minimal volumes in which the star formation occurred which gave rise to the presently expanding Sco-Cen sub- 4180 4200 4220 4240 4260 4280 groups, and to discuss the possibility of wavelength (angstrom) sequential star formation (13). Part of the CASPEC spectrum of NGC 2244-201, a 10th magnitude B 1 V star. The strongest lines are 011 4185, 011 4 190, NII 4242, 011 4254, CII 4267, a complex of 011 lines around First Results 4277A, 011 4283, Slll4285 and 011 4286. Strictly within the frame of the key programme, the first spectra have been obtained on 3 nights in May 1990. How- References 8. Theuns, T.: 1990, in Proc. Xlth ERAM ever, a limited amount of data has been (Tenerife, July 1989), in press. obtained during the last three years 1. Lada, C. J.: 1987, IAU Symp. 115, 1. 9. Fekel, F. C.: 1985, IAU Coll. 88, 335. with CASPEC and has resulted in the 2. Duerr, R., Imhoff, C.L., Lada, C.J.: 1982, 10. Wyatt, W. F.: 1985, IAU Coll. 88, 123. establishment of a stable, accurate Astrophys. J. 261, 135. 11. Mathieu, R. D.: 1985, IAU Coll. 88, 249. wavelength calibration procedure (14) 3. Mathieu, R. D.: 1986, Highlights of As- 12. Hartmann, L., Hewett, R., Stahler, S., and an optimized echelle reduction tronomy, 7, 481. Mathieu, R. D.: 1986, Astrophys. J. 309, method (15). Presently, we can concen- 4. De Geus, E. J., Lub, J., van de Grift: 1990, 275. trate on the proper radial velocity work. Astron. Astrophys. Suppl. (in press). 13. Sargent, A. I.: 1985, in: Birth and Evolu- In NGC 2244 we detected a number of 5. De Geus, E.J., De Zeeuw, P.T., Lub, J.: tion of Massive Stars and Stellar Groups, 1989, Astron. Astrophys. 216, 44. eds. W. Boland and H. Van Woerden (Dor- slowly rotating 0 and B stars (out of our 6. Verschueren, W., Hensberge, H., de drecht, Reidel), p. 5. current sample of 17, 5 had v.sin i Loore, C.: 1990, in Proc. Xlth ERAM 14. Hensberge, H., Verschueren, W.: 1989, <30 kmls), whose narrow lines are (Tenerife, July 1989), in press. The Messenger 58, 51. favourable to a very accurate determi- 7. De Geus, E.J., Bronfman, L., Thaddeus, 15. Verschueren, W., Hensberge, H.: 1990, nation of the RV (Fig. 1). P.: 1990, Astron. Astrophys. 231, 137. Astron. Astrophys., in press.

The Status of the Hubble Space Telescope

As you have certainly learnt from the resulting Point Spread Function is vents HST to reach its "level 1" specifi- news media, the focussing tests carried characterized by a sharp core of about cations (about 70% encircled energy out during last June revealed that the 0.1 arcsec surrounded by an extended within 0.1 arcsec) and has serious con- Hubble Space Telescope suffers from "halo": unfortunately the core encircles sequences on its imaging capabilities, spherical aberration. The amplitude of only about 10-20 % of the energy. The the actual impact on the scientific output the aberration is about half a wavelength forthcoming issues of the ST Scl and ST- has to be evaluated a bit more carefully rms and it has a negative sign, i.e. the ECF Newsletters will contain more quan- than what has been unfortunately done marginal focus lays further away from the titative details on the problem. by the generic press. In particular, any secondary than the paraxial one. The Although the spherical aberration pre- comparison with the capabilities of ground-based telescopes has to consid- relevant data will be made available to performance will be reviewed in the com- er the uniqueness of the UV imag- interested scientists shortly after the ob- ing weeks. ing from space and the fact that, since servation. Concurrently the Guarantied Considerable effort is also being in- the HST Point Spread Function de- Time Observers and General Observers' vested in evaluating the applicability of parts considerably from a gaussian-like proposals are being reviewed for feasi- different image restoration methods. profile, spatial resolutions cannot be bility and modification. More about this ECF staff, in collaboration with ESO col- compared just by using the FWHM as a exercise will be published in the ST leagues, is experimenting with different parameter. Moreover, the two HST spec- Newsletters, in the electronic Bulletin algorithms on simulated images which trographs are less affected than the Board and communicated directly to the make use of the actual, aberrated, HST cameras and most of the scientific pro- HST Principal Investigators. psf. The results will be presented to a grams should still be feasible, albeit with On the front of correcting the problem, specific workshop on the subject which an increase in the exposure times. NASA intends to speed up the construc- has been organized by the ST Science Currently, a Scientific Assessment tion of the second generation instru- Institute on August 21 -22. Meanwhile Team has been formed at the ST Science ments, in particular of the WF/PC II, the ECF continues to maintain contact Institute with the task of preparing an which will include appropriate modifica- with the European PIS who are involved observing programme (to be carried out tions in the optical design to compensate in Cycle 1 observations offering assist- in August-September) which will allow a for the spherical aberration of the tele- ance in the review and possible modifi- better evaluation of the actual perfor- scope. The situation of the ESA Faint cation of their programmes. mance of the scientific instruments. The Object Camera in the light of the HST P. BENVENUTI, ST-ECF

"Matching Error" (Spherical Aberration) in the Hubble Space Telescope (HST): Some Technical Comments R. N. WILSON, ESO

Much consternation has been caused probably the opposite. The distance A0 aberration" which gives the maximum in the astronomical community because is called the longitudinal spherical aber- phase error of the image forming light. of the revelations since the last week of ration and is about 40 mm in the HST. If This is 4.34 ym for the above figures and June that the Hubble Space Telescope the focus for the detector is chosen to an aperture of 2.4 m for the HST. This (HST) has a systematic error giving an be at 0 (the so-called paraxial or Gauss- wavefront aberration is the best physical image with about 70 % of the geometri- ian focus), then the total transverse measure of the error and is, in fact, cal light energy within about 1.5 arcsec spread of the light has the diameter BC. exactly twice the maximum error on the diameter instead of less than the 0.1 It can be shown that, for the basic (so- mirror surface involved, referred to the arcsec predicted from its specification called "third order") spherical aberra- Gaussian focus, which is therefore of "diffraction limited performance" for tion, the optimum focus reducing the 2.17 km. Frequently, the rms (root mean visible light (wavelength 500 nm). The diameter of the light patch to a minimum square) error on the surface has been error has been identified as mainly is at D, one quarter of the distance from quoted which is about one sixth of the spherical aberration due to "matching A to 0. This minimum diameter is called above, or 0.36 km, or somewhat more error". The above quality figure has the "disk of least confusion" EF which is than half a wavelength of laser light of been quoted in a number of reports, but obviously one quarter of BC. Taking A0 0.632 km. The above figures reveal how may include other errors (including re- as about 40 mm in the HST, an exit essential it is to define exactly what de- sidual focus error) of unknown amount. beam of f/24, the diameter BC contain- finition is being used, otherwise serious From more specific information on the ing I00 % of the energy at the Gaussian confusion results. amount of spherical aberration, I have focus is 6.0 arcsec; at the best focus it is Let us now return to the probable calculated below that the spherical 1.5 arcsec. These figures, expressing origin of this spherical aberration error in aberration error alone would give an im- angular aberration, can easily be con- the HST. age at best focus with 100% of the verted into the so-called "wavefront In the technical domain of the produc- geometrical light energy within about 1.5 arcsec diameter. Before considering further the origins of this error, let us look at the meaning of the term "spherical aberration". Elementary text books on optics usually explain it as a "longitudinal aberration" as shown in Figure 1. Rays coming from the central part of the optical system (near its axis) focus at the point 0 on the axis, whereas those from the outer cir- cumference focus at A. The sign of the aberration as shown above with A to the left of 0 is what a simple convex lens would generate, whereas in the HST it is Figure 1 : Path of rays forming an image afflicted with spherical aberration.