Places · Observatory
BTA-6
A six-metre mirror that held the size record for thirteen years, and the first big telescope built on an altazimuth mount — the arrangement every large telescope has used since.
The sky from here
This is a zone 2 sky — rural on the nine-grade scale, Bortle 2–3, reaching roughly magnitude 6.7–7.5 unaided. The Milky Way is structured and obvious. Light domes sit low on the horizon over distant towns, and the rest of the sky is genuinely dark.
That grade puts 107 of the catalogue within easy reach of 10×50 binoculars and 107 within a 130mm — 44 of them need no instrument at all. There is nowhere darker worth driving to; this is already among the better skies on the planet.
There is no season here in which the sky fails to get properly dark — the Sun clears eighteen degrees below the horizon on all 365 nights.
The galactic centre tops out at 17° from this latitude — low enough that the busiest part of the Milky Way is always behind the most atmosphere, which is what separates this sky from a southern one. Of the 110 Messier objects, 14 never set from here and every one of them rises at some point in the year.
Of everything in the catalogue, Sunflower Galaxy, Andromeda Galaxy and Cat's Eye Galaxy are the best served here, climbing to within a few degrees of straight up — an object at the zenith is being seen through the least air it will ever be seen through, and the difference against the same object low in the south is not subtle.
Whirlpool Galaxy, Little Dumbbell and Pinwheel Galaxy never set from this latitude, so they are available on any clear night of the year rather than for one season of it.
Read at 43.65°N 41.44°E. Darkness is estimated rather than measured; everything else follows from the latitude and holds along the whole parallel. How accurate this is.
Nearby
Questions
What part of the sky never rises at BTA-6?
Everything south of declination −46°. From 43.6°N that is a cap 44° in radius centred on the south celestial pole, which is 14% of the whole celestial sphere. It is out of reach permanently rather than seasonally: no time of year and no instrument brings it above the horizon. The matching 14% around the north celestial pole is the same geometry inverted, and never sets.
How many hours of true darkness does BTA-6 get?
11 hours 36 minutes on the best night, around 21 December, falling to 3 hours 48 minutes around 21 June. That counts only proper astronomical night, with the Sun eighteen degrees down; the merely-dark hours either side of it are longer. Every night of the year gets there eventually, which is the quiet advantage of not being any further from the equator than this.
When is the Milky Way best from BTA-6?
Around 17 June, when the galactic centre transits at local midnight, and usefully from about May to September — earlier each evening as the season goes on. 43.6°N is close enough to the equator that the sky reaches full astronomical darkness on every night of the year, so unlike sites further north there is no part of the season where the core is high and the sky refuses to cooperate.
How far is BTA-6 from the other sites on this map?
Palmachim Airbase is the nearest thing to it here — 1,440 km to the south-west. The nearest other observatory is Virgo, 2,470 km to the west. These are straight-line distances over the surface, not driving distances, and in this part of the world the two can differ considerably.