To this date, experimental data useful for understanding the dome seeing phenomenon may appear surprisingly scarce if one consider the impact of the problem on the quality of observations (cf. ). There are some reasons for this lack of reliable measurements.
One reason is due to the difficulty of measurements methods. Dome seeing is only one of several sources of image spread, which include in particular natural seeing which has a very high variability. There are no direct methods for the distinct evaluation of the local contribution to seeing which is caused by thermal turbulent inhomogeneities in and around the telescope. Only if a large collection of homogeneous and precise data of image quality together with all relevant environmental parameters are available, it is possible to perform statistical analyses through which the local seeing effects effects can be outlined through the fluctuations of the natural seeing. Local seeing is of course more easily detectable on sites where the average natural seeing is low.
Unfortunately the operating procedures of most astronomical observatories do not lend themselves particularly to such measurements. Astronomical observatories are generally operated as service institutions for the use of individual astronomers. Requests for observing time on the best telescopes far exceeds the availability. A severe selection is done on proposed observing programs and successful astronomers typically obtain observing runs of 2 to 4 nights during which they are hard pressed to perform all the intended observations. Therefore this continuous shift of observers, whose priorities are their own immediate observation objectives, is an operational obstacle to the establishment of research programs on local seeing, which will take a fraction of observing time over long periods, and only offer eventually a perspective for quality improvements.
To this date the Canadian-French-Hawaii Telescope is the only observatory in which image quality, temperature and wind data are systematically taken and therefore provides the best database for studying dome seeing. Thanks to the courtesy of the CFHT direction, the author had access to this important set of data, which was analyzed for the first time in its globality.
Figure: The Canadian-French-Hawaii Telescope
The CFHT is a 3.6-m telescope - see fig. - housed in a large dome with a diameter of 32 m, located on Mauna Kea, Hawaii island, at the altitude of 4200 m. This site is deemed among astronomers to be the best in the world for natural seeing. The observations with the telescope, which started operating in 1979, experienced during the first years of operation a seeing in excess of 2 arcsec [Racine 84]. As a result of many technical actions taken to eliminate heat transfer to the telescope air volume, the average seeing has been improved to 0.6 arcsec [Racine 92]. A main factor in this achievement was the installation of a chilled floor for the telescope volume. The floor creates a stable temperature vertical gradient, thus effectively eliminating sustaining free convection flows inside the dome. It also acts as a powerful cold sink for any heat generated or entering in the dome volume, contributing in particular to keeping down also the temperature of telescope structure and primary mirror.
The author has analyzed the data log files concerning all observations done during the years 1991, 1992 and 1993. They record the FWHM image quality of all observations taken with the two main imaging instruments used on the telescope prime focus, which are respectively named FOCam and HRCam. The latter one has a fast tip-tilt guider aimed at correcting automatically telescope guiding errors (wind induced fluctuations in particular). Thus image quality from HRCam is typically 20% better than from FOCam. The records of the log files include for each observation the zenithal angle of the telescope, the integration time, the number of FWHM measurements averaged in that time, meteorological data of the external environment (wind, air temperature) as well as the temperature measured at several locations around the telescope and in the dome - see table , appendix .
The complete analysis of the CFHT data is found in appendix . We will here outline its main conclusions. The main guideline for the analysis is given by the assumption that the overall image quality can be parameterized as a sum of various terms dependent on few driving quantities such as the zenithal distance and various environment parameters. Recalling that the amplitudes of various seeing FWHM contributions sum up with the power 5/3, while all other contributions to image spread shall be added quadratically (cf. equations () and ()), the total image FWHM can be expressed as:
After a preliminary screening to avoid spurious data, the HRCam data set comprised 1662 HRCam observation records, with a median image spread (FWHM) of 0.61 arcsec. The FOCam records numbered 1446 with a median image spread of 0.75 arcsec.
The temperature data measured at several locations around the telescope and in the dome (see table ) confirm that the CFHT dome environment is almost permanently characterized by a stable thermal stratification. In the vast majority of the observations the air temperature above the primary mirror is lower than the one near the top ring, which in turn is lower than the external air temperature - see fig. C.2, appendix . This stratification effectively "kills" free convection flow patterns and any related turbulence and indeed the analysis does not find any seeing that could be attributed to s between air and floor and dome surfaces.
The temperature of the primary mirror stays generally very close to the one of air just above it: the average of the mirror surface-air temperature difference is + 0.05K, with a rms of 0.47K. The analysis shows that positive values are correlated to higher seeing - see fig. . With the HRCam data this mirror seeing contribution is represented by the relationship
This result agrees very well with the finding of [Racine 92]. From the FOCam data the coefficient computed is 0.3 arcsec/K but this value is less reliable as for there are only three clusters of data each belonging to a same observation sequence (fig. C.9 - appendix C). The influence of mirror inclination was also investigated, but no dependency was found.
Figure: Scatter plot of HRCam FWHM versus the temperature difference between the mirror surface and the surrounding air. Binned median values and the best fit found are also plotted.
In its previous analysis of HRCam data, [Racine 92] noted a correlation between seeing and a positive temperature difference between the dome interior and the outside air with a factor of 0.1 arcsec/K. The HRCam data let guess a trend of this kind, but too few data points exist for to draw a conclusion. The range K is better represented in the FOCam data but in that case no apparent trend is detectable (fig. C.10 - appendix C). Thus we do not have conclusive evidence for the existence of this seeing contribution.
We will still mention here that a correlation is found in the FOCam case between image spread and the external wind speed, parameterized as
As expected, no dependency with wind speed was detected in the data from the HRCam instrument.
In conclusion, the analysis of three years of CFHT seeing records shows that mirror seeing is the only significant local seeing effect generated inside the observatory. It is found when the mirror is warmer than ambient air and is correlated with the surface-air temperature difference at a rate of 0.38 arcsec/K.