Signal and noise

Hi all, I owe you a blog post but I have actually been busy. The surgery stuff is progressing I still don't have a surgery date and I am still massively frustrated with the disaster we call an insurance industry in this country. However I am not going to complain about that I am instead going to complain about something else.

When you don't have a telescope for astrophotography you spend most of your time researching which telescope to buy. Such things occur naturally with hobbies, since the goal is to chase the dragon for the most part. Its strange the first money out the door is going to be spent on a car, an old Subaru Legacy 5 speed (hopefully) then comes the telescope, then comes the software for the telescope then comes the computer upgrades.

My point is that planning is the only thing I can do and part of planing is research. The problem with research is that because the community is small there is a lot of superstition being perpetrated as fact. The best example of this I have come across is focusing.  Astrophotography is part art part science. The art is in the photographs, the science is in the planning on how to get to the photographs. When it comes to actually taking picture I anticipate trouble in two actions, the first is in polar aligning the mount, this must be done any time you move the telescope to a new location or break it down and set it up. The problem with polar alignment is that it is complicated and you want to do it well, this may mean taking a couple of hours to polar align your mount. The methods for polar alignment are not all that controversial.

However the second place I think I will have issues (and by I, I mean all astrophotographers) is focusing the telescope and keeping it focused. There is a great deal of information about focusing some of it might not be all that useful. The problem comes in that for the most part telescopes bend light so it comes to a focus, how they do this is very much dependent on the length that the light has to travel. The problem is that focusing needs to be exact, the goal is to get stars as small as possible on the imaging medium. You can do this crudely by hand, but most go to some sort of mechanical help for this, then there is the problem on thermal expansion. See most objects length changes with temperature, the result is that your focus (the place where your stars are smallest) moves as the temperature changes. This also means that the spacing of your lenses changes as temperature of the cell holding them does.

This could result in a lot of painful problems, not the least of which is people telling you that you should not get a carbon fiber telescope tube for a refractor since the lens cell is designed to compensate for changes due to thermal expansion. This is however a third order affect, the expense of getting a carbon fiber optical tube is a first order effect on your wallet. The interesting thing is that you should probably refocus periodically anyway because your never going to be sure where the best focus is.

Let me explain, when I was talking with my friend and mentor about this issue he asked me this question, "Where are these people going to get 0.5" seeing?" In order to see the effect of having a carbon fiber tube you would have to have really nice skies, since the is not a uniform object, its temperature and speed vary we get tiny lenses forming in the atmosphere as the local air changes, this is called seeing. Seeing acts like little lenses dancing in front of your objective, resulting in stars twinkling. More than that it is something you can measure with your camera, since even when focused the seeing smears out stars on your image plane.

Seeing effects your images much more than any choice of optical tube material. So don't worry about what your optical tube is made of unless your going to launch it into space. This has also made me leery of talk of a critical focus zone on a telescope, maybe a reader can explain why between actual thermal expansion and seeing I should worry about my focuser moving a few microns?