European Southern Observatory New Technology Telescope

General :
Observations :

FAQ, Hints and Tips

Content

• Active Optics and Image Analysis
• Observations at Zenith and Alt-Az Mount
• I cannot read my data tape...
• Scripts to read dd tapes
• Conversion of ESO headers to IRAF compatible headers

Active Optics and Image Analysis

Short version:

• 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

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<dec<-26, HA<00:10:00)
• Never, ever observe within 5deg from the Zenith (-32<dec<-26, HA<00:10:00)
• 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.
 

I cannot read my tape!?!??

The NTT tapes are written with a DDS2 (2d 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.

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) use the MIDAS INTAPE/FITS command, or

      @ 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 yr tape:

• if it is a recent run, let us know (nttt) and we will prepare a new one.
• if it is more that 2-3 month old, pls send your request for a new tape to usg@eso.org, with CC to nttt@eso.org

---

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 headers to IRAF compatible headers

 

The FITS files produced at the NTT (and most ESO telescopes ) make extensive use of the hierachical keywords. While these are standard FITS, 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 NTT and on any scisoft machine.) 

 

Send comments to: NTT Webmasters <nttweb@www.ls.eso.org> Last Update: