The attached message from Don York, co-Instrument Scientist, describes the currently achieved performance of the new Echelle spectrograph and some expectations for the future in some detail. I hope that this information will be helpful to those wishing to propose for use of the instrument in 2Q1999. More definitive information will be available later, but institutional proposal deadlines are rapidly approaching and the attached should provide a good basis for writing initial proposals. Obviously the future can always hold unhappy surprises, but at this point, Echelle is still expected to be available for general community use beginning on 1 April (2Q1999). Ed Turner ============================================================================= We are now about 25% of the way through the science commissioning of the ARC echelle spectrograph (aka CARCES-Chicago/ARC Echelle Spectrograph). Things are going very well. Some specifics follow that may be useful to your thinking about the use of the instrument. Use: The echelle is to be accessed by rotating the tertiary mirror of the 3.5 meter telescope. This aspect of operations is not part of the science commissioning, but is expected to be done by the end of March. The instrument is commanded from the Remark interface. The instrument has been tested in a fully remote mode including guiding (but not yet focusing). In normal use, you can think of the instrument as having no moving parts for astronomical targets. A mirror must be moved into the beam to do calibrations. Calibration lamps are built into the instrument. A single flat and a calibration spectrum will require about 6 minutes (mostly CCD readout time.) The slit should normally be the nominal one. Others can be put in by the site staff. There are additionally some filters useful for calibrations and guiding. Performance: Use these values for planning purposes. They are not final. Slit: 1.6 x 1.6 arcsec Wavelengh coverage: 3200 to 10000 Angstroms. Wavelength coverage with signals above 50% of peak: 4000A (range depends on spectral type of target. Resolution: 7.5 km/sec (independent of wavelength) (sampled in two pixels) Stability: better than 1/km/sec per 4 hours. Relative wavelength precision: better than 1 km/sec Efficiency: Assume 3% from the top of the atmosphere to the disk on a night of 1.2 arcsec seeing. This measured number includes slit losses, atmospheric transparency, reflectivity of telescope optics, pointing above 45 degrees to the zenith, spectrograph optics, CCD efficiency. We cannot yet allocate the losses between all the factors, but the spectrograph seems to be operating at the efficiency planned some years ago. Read noise: 7 electrons Relative wavelength response: For an unreddened B star, the efficiency is very low below 3600A and above 9500A, so longer exposures will be needed for features in those regions. Signal to noise: >200/1, when allowed by photon noise. The real number is better, but we do not have adequate reductions to specify the limit. Scattered light: The orders are only 2 pixels wide and are adequately separated. We do not have good numbers on interorder light or intraorder scattering. Neither is an obvious problem and the latter is readily calibrated by use of the telluric lines, once the scattering is understood as a function of wavelength. Note: All of the telluric lines fall in each and every spectrum, within the quoted range. Thus, the wavelength scale, the scattered light and the instrumental profile are readily available in each exposure. Guider: There is a slit viewing camera which is not operating much below ambient. Guiding is done by hand (i.e., human hand on computer). It is hard to see the fuzz falling on the slit plate below 15th mag at the present time, but we are working to cool the camera more (which is the limit of interest.) One can do better with >1 minute integrations if desired. Dark current: ~0.001 electron per pixel per second. I hope to put spectra on an anonymous FTP site in a few days for anyone to look at for planning purposes. However, I am observing this weekend so cannot commit to this for sure. I will do what I can. Neither the science CCD or the guider CCD have any significant defects that affect usage. DON APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 6 in the apo35-echelle archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-echelle/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-echelle@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO