07-12-2015, 08:11 PM
Pog,
Really good question. Please forgive me if I only get through a bit of this as astronomy students generally have to sit through three or four lectures at university to understand all this stuff, but think we're only talking about image quality and one or two other things, not more complicated economic and political stuff!
Optical and infrared astronomers measure something called seeing. The number is how small a point source (e.g., a star) appears. There are many factors that affect this number, but the smaller the number the better. More detail can be seen plus more light falls in a smaller spot on the detector, so good seeing helps see fainter objects (think magnifying glass and ants).
Seeing is affected by many things. Turbulence in the atmosphere is the biggest effect. However, local turbulence is also a big issue. Mauna Kea is great most of the time because trade winds can simply deflect around the mountain, but some of those winds also blow over the summit. Those winds create turbulence on the summit ridge. Often, those winds will also form a vortex after passing over the ridge, which means there's local turbulence to the east and west.
The topography of the summit region plays a massive part in where that local turbulence occurs as does the wind direction. For example, the CFHT, which is rather exposed but only a few hundred yards from UKIRT, may report seeing twice as bad as UKIRT is experiencing, because UKIRT is a little more sheltered. On the other hand, wind direction and speed means CHFT does better on occasion. On average, however, UKIRT does better, but then things get really complicated, because UKIRT generally works at longer wavelengths than CFHT.
So it's not simple, but basically the local differences are due to the unique turbulence each observatory experiences.
I hope that helps. I'm tired now, but let me know if I need to clarify anything.
Really good question. Please forgive me if I only get through a bit of this as astronomy students generally have to sit through three or four lectures at university to understand all this stuff, but think we're only talking about image quality and one or two other things, not more complicated economic and political stuff!
Optical and infrared astronomers measure something called seeing. The number is how small a point source (e.g., a star) appears. There are many factors that affect this number, but the smaller the number the better. More detail can be seen plus more light falls in a smaller spot on the detector, so good seeing helps see fainter objects (think magnifying glass and ants).
Seeing is affected by many things. Turbulence in the atmosphere is the biggest effect. However, local turbulence is also a big issue. Mauna Kea is great most of the time because trade winds can simply deflect around the mountain, but some of those winds also blow over the summit. Those winds create turbulence on the summit ridge. Often, those winds will also form a vortex after passing over the ridge, which means there's local turbulence to the east and west.
The topography of the summit region plays a massive part in where that local turbulence occurs as does the wind direction. For example, the CFHT, which is rather exposed but only a few hundred yards from UKIRT, may report seeing twice as bad as UKIRT is experiencing, because UKIRT is a little more sheltered. On the other hand, wind direction and speed means CHFT does better on occasion. On average, however, UKIRT does better, but then things get really complicated, because UKIRT generally works at longer wavelengths than CFHT.
So it's not simple, but basically the local differences are due to the unique turbulence each observatory experiences.
I hope that helps. I'm tired now, but let me know if I need to clarify anything.