There is a reason we chose to build an observatory. In order to capture detailed images of the night sky requires a couple of significant things.
Dark skies
Precise polar alignment
Long exposure photography
These are inextricably linked. The darker the sky, the shorter the exposure, which sounds counterintuitive. The longer the exposure the more ‘noise’ the camera will collect. Noise is simply not light or photons, the best modern CCD sensors suffer from a small amount of amp glow and electrical noise, although they are getting better.
Precise polar alignment was the key factor in deciding to build an observatory. The decision was made nearly 30 years ago. When you begin observing the stars you see all the wonders of the universe in black and white. It is because our eyes are receiving insufficient amounts of light to resolve colour. Our eyes are the best light sensors available, but they cannot collect light! So after visual observations, technology with 35mm SLR cameras started making the adaptors to fit to the rear of telescopes. This would mean if we could collect enough light – we could see the true colours of these wonderous things.
On average the minimum exposure at say 400 ISO for Kodak slide film would be at least 2 minutes. So this raises a new issue. If the telescope is not tracked you have to use the 500 rule to establish your maximum exposure time. My telescope at this point in 1990 was a 12″ Meade LX200 Schmidt Cassegrain. A reflector with a 3mm focal length. So 500/3000 = 0.167 s to 3 sig fig. So a very short exposure and no where near long enough to capture anything bar the brightest of supernova!
So the telescope has to be tracked – why? The earth is rotating at approximately one rotation per 24 hours relative to the sun, its slightly less with regard to the stars 23 h 56 m. So when you look through an untracked telescope, the stars move across your view.
In order to counter this you need to polar align the telescope. This requires skill and patience. On my Meade which had a heavy field tripod, once practised was a 20-40 minute exercise. Drift aligning as it was called, you were first required to position the telescope to point north. Then you would set the equatorial wedge to match your latitude. Now we would be observing Polaris, is circumpolar, it only traces a very small circle around the celestial pole as the earth rotates under it.
It required some adjustment of altitude and azimuth in order to polar align. Once achieved and stars no longer ‘drifted’ in the viewfinder you could switch out the eyepiece for a camera. The slightest knock on any part of the telescope could potentially put the telescope out of alignment. I remember thinking – I need an observatory, once mounted and aligned the telescope need only very minor adjustments to check alignment and deep space imaging could begin.
Fast forward 30 years. We found a house that had pristine dark skies (Bortle 1), great low horizons and room for an observatory. So there were other considerations but ostensibly a massive concrete slab weighing in at 8 tonnes was poured, with steel anchors cast into it. The observatory was built on the slab. The pier was attached to the slab. The telescope fixed to the pier. Now all I needed was a clear night. Almost immediately we had a clear night and I could precisely polar align the telescope. It is done digitally these days, with far greater precision than I could ever had dreamed of years ago. On my first attempt I achieved alignment to within 12 arc seconds. 1 arc minute would have been great but this was outstanding accuracy. Since then I have only had to re align once after a pier extension was fitted. Each time I ask the telescope to ‘go find’ a target it will centre the target on its first attempt. I generally limit each exposure time to 60 – 120 minutes, which are actually 60 x 1 min or 60 x 120 min exposures depending upon the target and its apparent magnitude. This means on a clear night in winter I can image a dozen deep space targets. The only limiting factor now is the weather, and that’s a whole other “kettle of fish’!