poster

ABSTRACT

        Ever since the British study conducted in 1974-5, the island of Madeira has been forgotten for optical astronomy. And yet, Encumeada Alta (at Madeira) had the best seeing of all sites inspected by then (which included La Palma and Mauna Kea). The downside of this 1800m site is the frequent immmersion in the cloud inversion layer in Winter, translated into two-thirds of the nigths in the year being lost (cf. half for La Palma and Mauna Kea).
        A semi-portable dome-observatory shall be built at Encumeada Alta to:
        i) make the best of the third of the year when the atmosphere is clear and steady
       ii) measure, the nigth-tech way, how good is the seeing down to sub-arcsec values.

MADEIRA

Not far off the african coast (Morocco), the island of Madeira enjoys a mild subtropical climate at the sea level. In the highest peaks (over 1800m) mild storms and some snow are common in the winter. These peaks are distributed in a W-E direction, dividing the island in two. The prevailing winds are from the north, which makes the north of the island very wet and the south very dry. Funchal (250 000 inhabitants) is located in the south.

THE OBSERVATORY AND TELESCOPE

1) OBSERVATORY
The model of the dome-observatory purchased by the University (figure below) is the Bielser 3.5 School Model (3.5m in diameter and 3.2m high), made interely of fibre glass. The dome rotation is electric. We plan to fully automate the observatory in the future.

2) TELESCOPE and INSTRUMENTATION

The telescope is a MEADE 12'' (30 cm) Schmidt-Cassegrain. We have a pier and a giant field tripod as alternative supports. Using a field de-rotator, we preserve the solid altazimuthal mount. The instrumentation consists of a CCD Pictor 416XT (DT= -40º; 0.3''/9µm-pixel) + Autoguider Pictor 201XT; there is a set of standard UBVRI filters (Schüler) which give an efficiency of 25% at 550-850nm (combined with the CCD); finally, we have a Sivo vII spectrograph.

ENCUMEADA ALTA

This is a high site (1770m) close to the highest peaks of the island, in the north (see below). It consists of a small plateau at 16º56'W and 32º46'N. The mountain is steep all the way down to the sea (8 km away in a straigth line), facing the prevailing north winds. The site thus avoids the lee-side turbulence in its southern slope. The site is well away from the light polution of Funchal.

THE CASE FOR ENCUMEADA ALTA

        McInnes (1981) conducted a campaign between Jan 1974 and Apr 1975 to study the quality of this sitefor astronomical optical observations. At the figure below, we reproduce their results, including the year means : at the right we show the number of photometric hours per month: at the left the percentage of time with a seeing better than 1''.
        Concentrating on the year means, Encumeada Alta has an average of 110 hrs/month of photometric nigths (cf. 160 hrs/month for La Palma and Mauna Kea) and good sseing 50% of the time (cf. 40% La Palma; 30% Mauna Kea). The fact that our site has the best seeing of al sites led us to extend McInnes results further, calculating effective good hours per month (hours with a clear and steady sky). We get 57 hrs/month for Encumeada Alta (cf. 66 hrs/month La Palma; 55 hrs/month Mauna Kea) which suggests that Encumeada Alta may be a better site than Mauna Kea and is not far from the quality of La Palma. These results are consistent with a similar extended study conducted by Smith (1981).
        McInnes (1981) used exclusively the noth pole star to determine the seeing at all sites studied. After 25 yrs, we plan to use a seeing tower at Encumeada Alta, to compare the seeing with the best sites in the world, now at a subarcsecond level (down to 0.3'') and correctly update the comparisons as they have been done here. We will also install a meteorological station nearby. After our data are gathered, we should have enough information to decide whether to construct a permanent observatory with a larger telescope at Encumeada Alta or not.