Purveyor of Bollocks to the Crowned Heads of Europe
Saturday, 31 May 2014
Averted vision, smoke and mirrors
It was with a sense of liberation that I finally sold my 12 inch Dobsonian reflector telescope yesterday.
'Liberation', because its sale psychologically frees me up to get another one which I will use a great deal more than I used this one.
Despite being warned against it many times by many experienced people, I bought this monster because it was the largest I could cram into the back of a Volvo estate car, and - as we all should know - in the world of telescopes, bigger means better. It's all down to light-gathering capabilities, and not magnification, as many people think.
The really big monsters on the tops of mountains in Hawaii do not magnify any greater than a 12 inch equivalent, but their resolution is so fine that they could distinguish between two drawing pins set one inch apart and side by side, from a distance of about 200 miles. That's what you call resolution. Any magnification is achieved simply by cropping the image, and when the image is measured in thousands of light-years across, you really begin to understand the importance of resolution. Try blowing a photo from an ordinary camera up to make a billboard picture 30 feet wide, and you have another idea about the importance of resolution.
The telescope I will eventually get to replace this one will be small enough to carry under my arm, down the stairs of our compact but adorable city apartment, stick on the back seat of the Volvo and take to a sky which is slightly less light-polluted than the one which shelters Bath on any night without power-cuts.
Of course, there has to be a trade-off between performance and convenience, but I believe that the impact of this compromise is acceptably softened by the Maksutov-Cassegrain design, which cuts the length of the scope down by two-thirds, bouncing the light between a series of internal mirrors which triple the effective focal-length of the tube - meaning that I will be able to carry a 10 inch telescope downstairs, virtually under one arm.
The other advantage is that the whole tube is sealed, and some of them are even sealed with inert gas. With open-ended telescopes like ordinary Newtonians, three things will happen which will seriously deteriorate the image.
They will collect dust, and a speck of dust on the surface of a parabolic mirror focussed on deep space will cover many hundreds of small objects, thousands of light-years apart, depending on how deep you are looking.
The aluminised mirrors of open scopes which are 'silvered' in vacuum conditions will always deteriorate within a few years, meaning that they have to be sent back to be re-coated at a cost of £100 at regular intervals, which is expensive and boring.
And lastly, if you are out on a hot night with a cold tube, the eddies of unsettled air in the telescope will distort the image badly, making it next to impossible to see a feint object clearly for longer than about half a second. This last problem is seldom experienced in Britain, but - I am told - is a big one under the dark skies of the Arizona desert.
My suspicions about the buyer of my 12 incher (stop it, John) were aroused (I said stop it, John) when they - despite me giving them my phone number three times - just sent an email saying they would be in Bath at 3.00 o'clock.
I pleaded with them to call me and eventually they did. I asked what car they were bringing, and she said a BMW. I gave her the dimensions of the scope, and told her to make absolutely sure that it would fit in their car before they set out from the dark sky site of Exmoor where they live.
"Any problems," I said, "give me a call." They never called back, so I took the entire day off work to wait for them.
The doorbell rang at 3.00, and I went down to find the couple standing beside a two-door hatchback with the rear seats folded down. The man came up, took one look at the scope and said, "It's bigger than it looks in the photo..."
Somehow - and I still don't know how - we managed to stuff the thing into their little car, and they drove off back to Exmoor.
I only hope that they have a convenient out-building in the centre of the wilderness to house the thing, but now this is their problem and no longer mine.
The title of this post: If you gaze into the sky with your naked eye and spot a nebulous object like the 'Seven Sisters' star-cluster, you will find that you will be able to see much more of it if you look slightly to one side of it, and not directly at it. It is something to do with the cones in our eyeballs.