The Darkest skies in Southern Canada

The Milky Way as seen from the site of the OMI. The image was taken just a few km north of Mallory Hill. It is a mosaic of 9 images taken with a 50 mm f/1.4 lens and 20 minutes exposure at f/4 with a Canon 350D. The visual appearance is similar (except for the colors and number of stars) and shows much detail in the hub. This view of the Milky Way is perhaps one of the best wide-angle images ever taken, it shows the remarkably dark skies in this area. Image courtesy of N. Chamaillard ( 2007).

The importance of having a dark skycannot be underestimated. CCD cameras can acheive good images in less than perfect skies but the same cameras can go much deeper with the same exposure in a darker sky. Here you can see the effect that skies of different brightness have on the imaging performance.

The table is calculated for the OMI using the OMI photometric tool. 21.82 mag/arcsec² is the sky brightness at Mallory Hill. (MMO= Mont-Megantic Observatory, RAO=Rothney Astrophysical Observatory, DAO= Dominion Astrophysical Observatory, DDO=David Dunlop Observatory, EO=Elginfield Observatory, sky conditions are approximate only).

OMI exposure times (derived with the OMI photometric tool) with signal-to-noise ratios (s/n) 20 under different sky conditions using the r' (visual) filter. The darker the sky the shorter the exposure for the same s/n, Mallory Hill is situated at the far right at 21.8 mag/arcsec2.

Much effort was expended to locate the best possible site for the One Metre Initiative. Several criteria were established to determine the location.

1. Low sky brightness (i.e. a very dark site).
2. Good seeing
3. High elevation
4. Access
5. Long term viability
6. Lowest possible latitude.

By using the Sky Brightness Map (Cinzano, Thiene, Italy 1998) superimposed on Google Earth, it becomes obvious that an exceedingly dark peninsula exist between Ottawa and Toronto in an area known as the Madawaska Highlands. The area, centered on the town of Denbigh Ontario, has an elevation of ~400 metres, amongst the highest in Ontario. This is the most southerly area in Canada with exceptionally black skies, with a sky brightness of ~21.82 magnitudes/arcsec².

Upon examining the topographic maps to determine a precise location with the highest possible elevation with a relatively easy access it was determined that the only site that met our criteria is located at a place called Mallory Hill. A name that is only seen on topographic maps. It is located at an latitude +45° 01' 37.40" and longitude -77° 05' 57.40" with an altitude of 382 metres. The site is located on a hill top with no higher elevations within the immediate area, especially in the west, the direction of the prevailing winds. This will ensure the smoothest possible air flow over the observatory and improve the seeing.

A daytime visit in early October 2007 confirmed that the site would be suitable for an observatory. There is a good paved road that goes right by the property. The site has clear unobstructed views of all horizons with a treeless area of around 30 acres on top of the hill.

Nighttime visits on December 12/13, 2007 and again on February 28/29 confirmed the incredible darkness of the site. With a limiting magnitude of 7.1, the sky was filled with stars to the point were some constellations were hard to make out. The Milky Way was some 60 degrees wide at its broadest in the Cassiopeia area. Numerous dark patches in the northern Milky Way were quite obvious. Something we had never seen before. M33 was easily seen straight on, and you could tell that it was a broad round object. The area around Orion was pitch black and filled with stars an incredible sight to behold. The zodiacal light obvious on both occasions, visible as a diffuse light pyramid in the west. A Sky Quality Metre (SQM) reading on May 28/29 gave a reading of 21.82 magnitudes/arcsec². A perfect sky is considered 22.10 mag/arcsec².

The night sky at Mallory Hill is considered a truly dark sky. For example the zodiacal light, a very faint light created by solar light reflecting off dust particles in the plane of the solar system, is considered a most difficult phenomena to see. It appears in the west as a very faint diffuse pyramid shape glow aligned with ecliptic and fades within 1 hour or so after the end of twilight. The zodiacal light has been seen on every visit to Mallory Hill. People around you are barely distinguishable and disappear all together when more than a few metres away. Clouds are not visible until the stars start disappearing or above a light bubble. The Milky Way is seen all the way to the horizon and some almost 60 degrees broad in some areas. These are all common sights at Mallory Hill.

The most significant light bubble is from Ottawa. With a maximum height of around 10 degrees and a width of some 20 degrees. Smaller bubbles of less than 5 degrees could be seen from Kingston, Belleville and Toronto. Just barely visible 2 degree bubbles were visible from Smith-Falls+Perth and Renfrew.

Mallory Hill is located about 2.5 hours from Ottawa and about 4.5 hours from Toronto. The largest nearby town is Denbigh, about 20 km to the northwest with a population of about 250. Bancroft with a population of 4000, is 65 Km to the west. Belleville and Kingston are about 100 km to the SSW and SSE. Ottawa is about 120 km away in the ENE and Toronto is 240 km away to the southwest. According to the Sky Brightness map Mallory Hill is located in the gray area which has a Bortle rating of 2, which limiting magnitude of 7.1-7.5. There is a very broad buffer zone of anywhere from 50 to 100 km of blue (Bortle 3) that will give Mallory Hill a least 100 years of exceptionally dark skies.

Next: Sky Brightness Map