2.1 Magnitude limits and choice of observing site

Taking factors such as stray light and transparency of the air into account, the magnitude of the faintest object that can be detected using a particular instrument at a particular location is referred to as the limiting magnitude for that site. This goes some way to explaining the choice of a mountainside on Tenerife as the site for COAST; being 2400-m above sea level means that COAST is above a large part of the Earth’s atmosphere. A telescope the size of COAST can see fainter objects from a clear site like the observatory in Tenerife than it would at ground level – in other words, it has a fainter limiting magnitude.

In this next video Alan reminds us of the importance of dark adaptation and a clear observing site and compares what can be seen with dark-adapted eyes and with binoculars, before considering the benefits of larger instruments such as COAST. (Note: since this video was shot, COAST has been upgraded. The new dimensions are stated in the paragraph below. )

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INSTRUCTOR:

Here at Tenerife, I was out the other night, and a constellation that's prominent at the moment here is Orion. And this is what I could see just with my own dark-adapted eyes at this nice, clear observing site.

So we've got the four main stars of Orion. Rigel, Saiph, Bellatrix, and Betelgeuse. We've got the three stars of Orion's belt. And I can just about make out the sword and a faint fuzzy patch in the middle of Orion's sword. That's the Orion Nebula. With binoculars, immediately I can see the detail of the individual stars in the sword, and as well as that, begin to see the structure of the nebula itself.

So taking this process one step further, we can use the 350-millimetre aperture of COAST to collect yet more light, seven times more aperture than the binoculars. So collecting about 50 times as much light to see those ever fainter objects.

The other advantage that we've got with COAST is that instead of the retina of my eye, we're using a sensitive CCD camera, which can also collect light over a period of time in a time exposure. So again, we're increasing the light collecting ability of our equipment.

So with COAST, we can again increase the magnification and the resolution, but the primary purpose of building bigger and bigger telescopes is this increase in light collection power. And these are the kind of results that you can achieve with a telescope of the size of COAST. And you can see now the incredible amount of detail that we've got in the Orion Nebula here.

So we can take this process to extremes and go up into space with the 2.4-metre main mirror of the Hubble Space Telescope. But as you're about to find out, you'll be able to achieve some amazing images using the 350-millimetre main mirror of the COAST telescope here at this wonderful observing site on the mountainside at Tenerife.

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The theoretical magnitude limit of any instrument can be calculated providing you have knowledge of the diameter of the mirror or lens. However, as noted earlier, the actual limiting magnitude also depends on the conditions at the observing site.

For COAST with a primary mirror aperture of 430 mm the theoretical limiting magnitude would be approximately +16 if fitted with an eyepiece for visual observing. However, COAST is fitted with a sensitive CCD camera which by using long exposure times can collect more light and detect even fainter objects than this.