Please note, due to essential maintenance online transactions will not be possible between 02:30 and 04:00 BST, on Tuesday 17th September 2019 (22:30-00:00 EDT, 17 Sep, 2019). We apologise for any inconvenience.
To send this article to your account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about sending content to .
To send this article to your Kindle, first ensure firstname.lastname@example.org is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about sending to your Kindle.
Find out more about sending to your Kindle.
Note you can select to send to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
The scientific career of Dr. Jesse L. Greenstein covers nearly 60 years, a thousand nights on various telescopes, and several hundred papers. He has made substantial contributions in a number of major research areas including interstellar work, stellar abundances and nucleosynthesis, faint blue stars, white dwarf stars, comets, and quasars. He has combined productive research with distinguished service to our profession in a truly exemplary manner.
The idea to organize a Conference on high S/N spectroscopy came to me several years ago, in the beginning of the eighties, when the first tracings of Reticon spectra of 8 and 9 magnitude stars were published. I suddendly realized that the quality of those spectra was comparable to those we find in the at lasses of the Sun, Procyon, Arcturus and of a very few other very bright stars. I thought at that epoch, probably at the top of Mauna Kea, that when high-resolution spectroscopists will have collected enough high S/N results, then, time would be ripe to discuss the impact of these results on our Knowledge of Stellar Physics.
I. Spectrographs, Detectors, Fourier Transform Spectroscopy, and Radial Velocities
The Hamilton Echelle Spectrometer, recently installed at the coudé focus of the Shane 3-m telescope, is a high dispersion spectrograph optimized for use with large format CCD's. It was designed primarily for high resolution (R=50,000) wide bandpass spectroscopy of point-like sources down to a limiting magnitude of about V=16.5, over the 0.34 μm to 1.1 μm spectral region. Its design features a relatively large collimated beam size, the use of prisms rather than gratings for cross dispersion, minimum order separation, the use of protected silver mirror coatings throughout the system, and a fast (f/1.67) folded Schmidt camera with a flat external focal plane. Together, these features yield a very powerful spectrometer for high resolution stellar spectroscopy. This paper gives a brief description of the Hamilton spectrometer and gives several examples of its performance on astronomical objects.
A compact fiber-linked echelle-spectrograph has been designed and constructed at the Landessternwarte Heidelberg-Königstuhl and was successfully tested during its first observation period in the end of May 1987. The optical design of the instrument is discussed in general terms and preliminary results of the first observations are presented. The reduction of several CCD frames has shown that the real properties of the spectrograph are within a few percent of the calculated ones.
Today even for the most efficient spectrograph combined with a large telescope the light efficiency is only about 0.01 to 0.1 for spectral resolving power R larger than 10000 in optical wavelength band (OWB). Consequently for a very high signal to noise ratio spectral observation of rather bright stars still needs very large telescope. The main reason is that there are too many optical surface with rather low light efficiency and serious light loss at the limited slit width. In this paper we suggest a very high efficiency telescope-spectrograph system which will give an overall light efficiency varied from 0.21 at 400 nm to 0.44 at 700 nm, four fold higher than before. Using this system for R = 100000, S/N larger than 100 the limiting magnitude will be about 15.
A simple high speed image widener is described which allows effective count rates of up to 100hz to be achieved with photon counting detectors thus allowing rapid accumulation of high signal to noise data in bright star spectroscopy.
In our studies of activity in pre-main sequence Herbig Ae/Be stars we are mainly interested in searching rotational modulation of line profile. If the period of star rotation is of the order of one or two days, the data collected from a single site is insufficient. This led us to start correlated observations from two or three sites spread as much as possible in longitude. Our first bi-site observations started in 1982 on AB Aur from two observatories located 11 hours apart.: CFHT in Hawaï and OHP in France. To achieve a high flexibility and to gain access to telescopes without attached spectrographs. We built an instrument that is mobile and specially designed for line profile studies.
The G-HRS is one of four axial scientific instruments which will fly aboard the Hubble Space Telescope (ref 1,2). It will produce spectroscopic observations in the 1050 A ≤ λ ≤ 3300 A region with greater spectral, spatial and temporal resolution than has been possible with previous space-based instruments. Five first order diffraction gratings and one Echelle provide three modes of spectroscopic operation with resolving powers of R = λ/ΔΔ = 2000, 20000 and 90000. Two magnetically focused, pulse-counting digicon detectors, which differ only in the nature of their photocathodes, produce data whose photometric quality is usually determined by statistical noise in the signal (ref 3). Under ideal circumstances the signal to noise ratio increases as the square root of the exposure time. For some observations detector dark count, instrumental scattered light or granularity in the pixel to pixel sensitivity will cause additional noise. The signal to noise ratio of the net spectrum will then depend on several parameters, and will increase more slowly with exposure time. We have analyzed data from the ground based calibration programs, and have developed a theoretical model of the HRS performance (ref 4). Our results allow observing and data reduction strategies to be optimized when factors other than photon statistics influence the photometric quality of the data.
I. Spectrographs, Detectors, Fourier Transform Spectroscopy, and Radial Velocities
Linear arrays of self-scanned silicon diodes have been used in astronomical spectroscopy for over a decade. With care in the flat-fielding and data reduction they can be calibrated to better than 0.1%. They are still the best detector for signal to noise levels >100 when continuous wide-band coverage is needed. CCD's should be capable of this spectrophotometric performance but, for the forseeable future, the lack of a large format and their high cost only make them competitive for spectroscopy of single spectral features or multiple echelle spectra.
Photodiode linear arrays are perfectly adapted for spectral analysis. The TH 7832CDZ bilinear array is a new device specially adapted to low level detection (exposure < 7 nJ/cm2) with a reading efficiency of the photodiode signal better than 97% on all the dynamic range (> 70 dB).
Observations of the composite-spectrum binary HR 6902 around the time of total eclipse reveal absorption features that are due to the chromosphere of the G9 II primary star. By the application of o method of digital subtraction we have succeeded in isolating the spectrum of the stellar chromosphere.
Infrared detector arrays implemented for astronomical use during the past few years achieve performance gains which have profound implications for infrared spectroscopy. Arrays are now available with ∼ few × 103 pixels, each of which is ∼ 102 times more sensitive than previous single element detectors. Depending on the spectral regime, it is now possible to construct infrared spectrometers with limiting sensitivities 10–500 times fainter than in current use.
The multiplex properties of the Fourier Transform Spectrometer (FTS) can be considered as disadvantageous with modern detectors and large telescopes, the dominant noise source being no longer in most applications the detector noise. Nevertheless, a FTS offers a gain in information and other instrumental features remain: flexibility in choosing resolving power up to very high values, large throughput, essential in high–resolution spectroscopy with large telescopes, metrologic accuracy, automatic substraction of parasitic background. The signal–to–noise ratio in spectra can also be improved: by limiting the bandwidth with cold filters or even cold dispersers, by matching the instrument to low background foreoptics and high–image quality telescopes. The association with array detectors provides the solution for the FTS to regain its full multiplex advantage.
We have monitored changes in the radial velocities of 24 bright F, G and K dwarf stars (known spectroscopic binaries excluded) for the past six years at CFHT by imposing the absorption lines of HF gas in the spectra to act as wavelength fiducials. The average external error in the δ(velocities) which are based on some 16 stellar lines is 13 m/s corresponds to 0.6 micron in the spectrum or 0.04 of a diode spacing per line. Reductions are complete for 16 stars. There is no evidence for brown dwarf companions in the sample. Two previously unknown spectroscopic binaries were found, and seven stars show indications of significant, long-term, low-level velocity variations which could be interpreted as purturbations by companions of a few Jupiter masses with periods greater than 12 years except for γ Cep, which may have a period of 2.7 years, and ε Eri. Observing time has been guaranteed for at least two more years at CFHT.