Hints, Tips and FAQ
version: 2003-01-15 oh
This page lists various hints, tips, and frequently asked questions. If you don't find a reply to your problem, pls contact us.
- I cannot read my CD / some data are missing from my CDs
- I cannot read my data tape...
- Scripts to read dd tapes
- Conversion of ESO headers to IRAF compatible headers
- Active Optics and Image Analysis
- Observations at Zenith and Alt-Az Mount
I cannot read my CD / some data are missing from my CDs
If it is a recent observing run on a VLT-compliant instrument (i.e. with P2PP), you can send us (to email@example.com) a request. Pls be as specific as you can: we absolutely need the date of the nights and the instrument, and ideally the file name of the missing files. We'll produce a new CD and mail it to you.
If these are data older than a few month, obatained on a VLT compliant instrument, you will have to retreive the data from the Archive, at http://archive.eso.org/
I cannot read my tape...
The La Silla tapes are written with DDS3 (3d generation DAT), which is the standard for any unit less than 5-6 yr old. They are written in native density (no compression), unless you did something strange. The format is standard FITS, either directly as individual files or in Tar, depending how your wrote your tape.
The basic commands to read the tapes are:
- tar xvf /dev/yourTapeUnit for tapes written with tar
- for tapes written with dd (e.g. the wfits or disk2tape commands listed below), use the MIDAS INTAPE/FITS command, or the tape2disk linked below, or a mini script such as:
@ i = 1001 while (1) dd if=/dev/yrTape of=ntt$i.fits @ i ++ end
In case of doubt, you shoud try this
dd if=/dev/yourTapeUnit of=toto.dat bs=28800
If you get something on disk, check the first line of toto.dat.
- If it starts IMMEDIATELY with a fits header, then rewind and use the dd recipe given above.
- If you get a line with a few numbers, tar xvf toto.dat
If the dd command does not work, you are in trouble: we check routinely our DAT unit, so it should be ok, meaning that it is possible that yr unit is:
- dirty: try to clean the tape unit with a special tape
- misaligned: try to read a tape written with another unit.
If you still cannot read your tape, you will have to request the data from the Archive. See below.
Scripts to read a dd tape
If you have a tape that was written with dd (i.e. individual FITS files written to tape), you can read it directly to disk using this script: tape2disk. Ideally, it should be combined with RestoreID to rename the files to their original name (which is stored in the header). Download the scripts, edit them to adjust the name of your devices, and store them somewhere in your path for executables. For competeness, we also have a disk2tape script to write to tape. These scripts were written by James Brewer.
Conversion of ESO FITS headers to IRAF compatible headers
The FITS files produced by ESO instruments make extensive use of the hierachical keywords. Some image processing systems, including IRAF, cannot deal with them. To convert these keywords (e.g. from HIERARCH ESO TEL AIRM START to AIRMASS ) use the hierarch28 program developped by N. Devillard. This program, together with a series of other useful FITS-related programs, is available at http://archive.eso.org/saft. (They are part of the ECLIPSE package, and therefore directly available at the telescope and on any scisoft machine). Alternatively, ESO has developped a set of tools to deal more generically with the FITS headers, the FITS Translation Utility, or FTU, which is available from http://archive.eso.org/ftu .
NTT: Active Optics and Image Analysis
- An image analysis should be performed at the beginning of night (not necessarilly on the first object, e.g. if the latter is a standard star).
- An image analysis should be performed when the seeing is better than ~1.2", and that the telescope moved by more than ~10-15deg in altitude when from the zenith and more than 20-30deg when close to the zenith.
- Whenever possible, the image analysis should be ran in parallel with the observations. If the conditions are very stable, the corrections can be sent to the optics automatically, during the integrations. When the conditions are not optimal, the Telescope Operator will check the results of the analysis before applying them.
- An image analysis takes 3-6min in general
See the NTT active optics page
NTT: Observations at Zenith and Alt-Az Mount
The NTT has an Alt-Az mount; as a consequence, the singularity for the pointing is at Zenith, and not at the pole as it is on traditional equatorial mounts. When observing very close to the Zenith, the telescope, building, and instrument rotators have to rotate very fast to track the object. This will result in a poor image quality, unaccurate pointing and tracking, objects getting out of the slit, etc. In the worst cases, presetting too close to the Zenith can also crash the system.
To avoid any problem:
- Never, ever point within 5deg from the Zenith (-32
- Never, ever observe within 5deg from the Zenith (-32
- In the -32 to -27 dec range, do NOT observe within 15min from the meridian
- in the -50 to -10 dec range, be aware that you may get bad surprises within 15deg from the meridian.
So, adjust your program taking this into account.
As a compensation, the NTT does not have a 6h limit on the hour angle, as on equatorial mounts: you can point below the pole at 12h HA.