This is where I get to prattle on without having to worry about whether other OAFs agree with my opinions. Ah the power! The Power! Bwaahaa Ha Haa!!
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My Rabid Opinions on Telescopes
- Larger aperture scopes show more detail and fainter detail than smaller scopes in *all circumstances*.
- It is a myth that smaller scope do better in poor seeing.
- It is a myth that smaller scopes do better in light pollution
- It is a myth that larger scopes have “diminishing returns” over smaller scopes under light pollution. Actually, the ratio of performance is constant.
- However smaller scopes are better than larger scopes because
- Smaller scopes are more transportable. The scope you have with you is better than any scope you left behind. The scope you use the most is, by definition, the best scope you have.
- Smaller scopes cool down faster. At least half of the time people have complained about “poor seeing”, it was actually tube currents caused by warm optics.
- Larger scopes are more work to collimate. Some small scopes, like quality refractors, never need collimation. Large truss scopes must be collimated every night and in some cases, re-collimated more than once per night.
- Large newtonians, particularly truss-tube newtonians are poorly baffled. So light from local light sources can sometimes directly illuminate the focal plane greatly reducing contrast. On the other hand, small refractors tend to be very well baffled and can show very much better contrast than larger scopes when there are local light sources. (This difference may be the basis of the “smaller scopes do better in light pollution myth”.)
- Small scopes have a larger field of view. It doesn’t matter how long a focal length eyepiece you use. The true field you get on the sky is determined only by the focal length of your primary and the diameter of your focuser. A 25 inch scope cannot get more than about 1 degree on the sky, whereas most 4″ scopes can get over 5 degrees. The result: open clusters look far better in the 4″
- Small telescopes and large telescopes do different things well. No single scope does everything well. One telescope is never enough. (The real problem is explaining to S.O. why you need 13 telescopes. 🙂 )
- An off axis mask, to eliminate “central obstruction” does not improve the view in any seeing conditions.
- Actually, if it does, it means that you’ve got poor optics. (Did I mention this was the place for my *rabid* opinions.)
- If you live in an environment like Ontario, big telescopes need to be actively cooled *all night long*
- It’s a myth that one needs to cool a 20″ or larger dob primary for “an hour” and then it’s ready to use.
- In my experiments where I actually measured the temperature of my 25″ primary and the surrounding air I found:
- # a 1 degree C difference between mirror and air was enough to visibly degrade detail on planets
- # after the mirror came to thermal equilibrium, if I then turned the cooling fan off, it would take only 20 minutes for the air to again out-cool the mirror so that there was again a 1 degree C difference and thefore degraded planetary detail.
- The moral is: learn to know when your mirror needs cooling. (see next rabid opinion)
My Rabid Opinions on Weather
- Many times people complain about “bad seeing” the problem is actually warm mirrors. It is possible to learn to tell the difference if you have a newtonian with an open mirror cell. Try this experiment (the Wirths-Danko demonstration):
- While you look though the eyepiece at an out-of focus star (looks like a donut), have your buddy turn on a box fan which is pointed at the back of your mirror. Any change in the ripply pattern you see in the eyepiece is tube current and not seeing.
- In an out-of-focus image of a star, tube currents move slowly, are swirly and seem to know that your telescope is round.
- In an out-of-focus image of a star, true seeing zips quickly straight across the donut and does not know that your telescope is round.
- The jet stream is *not* the major cause of bad seeing.
- Three years of careful experimental work by Allan Rahill has shown that the presence of the jet stream above is correlated with bad seeing to only 30%
- The other 70% of coreleation is to wind shear in the bottom few thousand feet of atmosphere.
- Don’t depend on jet stream maps plan observing. Use Allan Rahill’s seeing forecast or this shortcut to it.
- Weather for astronomy, in southern Ontario sucks. (That’s the technical term.) The south-west USA and north-west Mexico get 3 to 4 times the number of deep-sky observing hours that we do.
My Rabid Opinions On Observing
- There is a least a 2 magnitude variation in sensitivity of eyesight from person to person. So questions of “limiting magnitude” are at best theoretical.
- There is a wide variation in color sensitivity from person to person. This is hard to measure, but judging by how much more aperture I need to see color than Mike Wirths, I’d say a factor of 2 variation is not unreasonable.
- To me, the difference between astrophotography and visual observing is exactly like the difference between my SO yelling at me for observing to much and a picture of my SO yelling at me for observing to much. (Lurid alternative deleted for family audience.) There is a difference in emotional response. As a result, I will always prefer a poor visual view to a great photograph. However, I’ve often get visual views that stun astrophotographers (I have a 25″ scope). I’m happy to look at other people’s astrophotographs and can appreciate the great effort it takes to make a good image. But I don’t have the patience to do astrophotography myself. I prefer to spend my limited time looking though telescopes. Being a visual only observer is rare in this hobby.
- I don’t keep an observing log. I used to. But then I found that I never went back to read it. I figure there is no point writing something if noone will ever read it. Fairly silly of me. Each of my rabid opinions, above was learned from observing experience. Had I kept my logs, they’d be scientific observations rather than blather.
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