François Ochsenbein

Guido Russo

English

Archivio della ricerca - Università degli studi di Napoli Federico II

ST- ECF Newsletter No 9 May 1988 Optical Disk trials at the ST- ECF Guido Russo f3 François Ochsenbein As reported in a previous issue , we are con-ducting experiments with optical disks (WORM type) and their interfaces for VAX computers. We have recently finished the tests for the inter-faces , conducted on three brands: 1. U.S. Design 1108, 2. Emulex UC04 and 3 . TD Systems TDL-12. While ali these interfaces (the proper name should be host adapters) provide the possibil-ity to connect SCSI devices, t hey differ in the emulation for tbe bost computer. The first two support the DEC MSCP protocol and are used through standard software drivers while the lat-ter uses special software. In terms of reliability, performance and ease of inst allation and config-uration, we found the U.S. Design clearly supe-rior to the others. In particular, the TD Sys-t em interface and the associateci software, al-though able to provide full VMS support, does t his by remapping ali disk writes to consecu-tive sectors on the disk, starting from the be-ginning and thus treating the opt ical disk as a tape more than a disk. Moreover , although we use a µVAX computer for the archive , we do not intend to write onta optical disks in the VMS format (known as the ODS-2), for reasons of in-t ercbangeability and portability. We bave therefore selected the U .S. Design host adapter for use at ESO / ST-ECF. Concerning the optical disks themselves, in ad-dition to the GIGADISC mentioned in the pre-vious note, we bave now also a MAXTOR RXT-800S. This devi ce uses pocket-size ( 5 ¼ inches) disk platters, with a storage capacity of 800 Mbytes ( divided between two sides) at a cost of less than $100 per disk. Disks are easily exchangeable in the drive in a few sec-onds. The software on- line disk structure we use was easily implemented on such a device, and performance comparable to the GIGADISC was achieved. We pian to use this device for data distribution and will provide software to read tbe disks written with our format. Data within the files are written in FITS format , but the MIDAS BDF format is also accepted. We also pian to buy a LMSI (formerly OSI) Laserdrive 1200 optical disk , which is the one selected at ST Sci. However, optical disk is not the only technol-ogy we are experimenting with at ESO / ST-ECF. We have now under test the Gigastore unit, manufactured by Digi- Data corp. This device uses as a storage medium st andard T-120 VHS cartridges, which hold 2.5 Gigabytes of data. The device is now used on the ESO VAX cluster for the daily backup operations which can now run overnight witbout operator inter-vention. Five hours of elapsed time and twenty minutes of CPU time are needed to backup five full RA81 disks; comparable with a high speed conventional tape but at a lower cost for the drive and at a much lower price for tbe medium - a higb-quality T - 120 cassette costs about DM 30 compared with DM 800 for twelve mag-netic tapes. Severa! otber tests bave been made to inves-tigate the possibility of using such devices for data transport. The tape initialization is very slow (90 seconds) as well as rewind or tape-mark searcb operations. Writing on a cassette is also limited to append operations (start writing be-hind the last existing file) or rewri te from tape beginning, i.e. , rewriting existing files is not a l-lowed. The embedded interface is a standard SCSI, like the optical disk one, but via a bost adapter tbe unit looks like a tape to tbe operating - 5 -ST-ECF Newsletter system. The host adapter may be either a DILOG one, or the same U.S. Design 1108 we selected for optical disks: switches on the board No 9 May 1988 allow configuration of the 1108 in severa! ways. In both cases the standard DEC-supplied MS driver is used. 'HST observations' of galaxies at high redshift Thierry Courvoisier 8 Andris Lauberts (ESO) The high angular resolution of HST will allow observations of the structure of small objects. Prime candidates for this type of research are galaxies at moderate or high redshifts. These galaxies are expected to be approximately I arc-sec in size and will be imaged over many res-olu tion elements of the ST cameras. The ex-act resolution will depend on the wavelength of the observation and on the mode in which the observations are made, the highest being ob-tained with the FOC at f/288. Most imaging modes will, however, provide a resolution bet-ter than ~ O.Iarcsec. Galaxies at large redshift will therefore use more than about 100 resolu-tion elements, the worst case being near infrared observations with the WFC. The potential of the WFC for this type of work has been tested in a programme of simulations, using the HST Model developed at the ST-ECF (Rosa & Baade 1987), of galaxies whose images were derived from ground-based observations of nearby objects. The aim was to evaluate the possibilities offered by 'parallel mode' observa-tions and to serve as test images for the prepara-tion of the European Medium Deep Survey key project proposal. In addition to the study of in-dividua_! objects, we bave also simulateci a field of galaxies with a random distribution in the sky piane and in redshift. The high redshift objects were simulateci by shifting the nearby galaxies according to a standard cosmologica! model and applying K-corrections on a pixel by pixel basis, with a value depending only on their colour. No spectral evo-lution of the galaxies has yet been considered. This approach allows an estimate of how redshift modifies the appearance of the galaxies with this instrument and will help in the interpretation of actual observations by allowing the separation of the effects of evolution from the mere colour dependent redshift corrections. One of the obvious results from our simulations of individuai galaxies is that at high redshift they appear to be of later type than at low redshift. This trend is more pronounced for the late type galaxies and for U and B images than for early type galaxies and V and R images. The Hubble classification of galaxies also does not match the high redshift galaxies as well as it matches nearby systems. The field of galaxies was constructed on t he ba-sis of a random field generateci by Cappi et al. (1987) . We have not attempt ed to simulate clus-ters since our field is supposed to b(' in a random position of the sky. For each galaxy in t he field , we used the one in our samplc of locai galaxies corresponding best to the prescribed type, shift-ing it to the proper redshift and scaling its inten-sity to the prescribed magnitude. The resulting field was then observed using the HST Model, taking into account photon and readout noise, blooming (not important for weak extended ob-- 6 -

Optical Disk trials at the ST- ECF

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Optical Disk trials at the ST- ECF

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