It’s nice to read some posts about observing and dark sky conditions on the forums. They got me to thinking about some my past 24 years of observing with big Dobs, so I thought I might reminisce a little. I just started reading some posts during the last few months, and have to say that it has been interesting - well, perhaps other than that long discussion about observing with or without a skirt on your telescope. Of course that topic depends on telescope design, personal preference, and local conditions - among other things. Personally I never observe without my jeans or skirt. Anyway, back to the subject at hand…
Dark skies are one of the top contributors to the best observing experience, but not the only one. Under truly dark skies even the smaller telescope will show some truly amazing sights. A few years ago Jeannie and I spent the summer in the Rockies, and one night at a campground near Leadville (at 10,000 foot elevation) our 8” f/6 travel scope was trying to act like it was a 20”. Or was it really? What would a 20” have done under the same skies. At this point in time we will never know, but I suspect that the 20” would have shown us that the sky turbulence was nasty, and the images would have been quite bright but with little resolving ability. There are few bad nights in a small scope. My 12” f/6 travel scope almost never sees bad seeing!
One of my more memorable observing weeks was during one of our early visits to the Texas Star Party. I had an 18” f/6, and the transparency of the west Texas skies was excellent, even if the seeing was usually pretty bad. My good observing buddy Vic Menard spent the week with a 20” right next to me, and although both of us were more used to observing under water at home in Florida, we often commented that the superb transparency did not always show all the detail that the steady seeing that our home state showed - even with the very dark skies.
During future visits to the high western desert, with my first 36” - the Yard Scope, only one time did the scope ever live up to providing the view that we were expecting from the super dark, high transparency skies of the west. During that week the scope was set up in the parking lot of McDonald Observatory’s visitors center, and the seeing was much better than the usual rougher seeing from down in the valley at the Prude Ranch. A trend seems to be forming from these observing trips.
The opinion has formed itself in my head, that large scopes of 24” or larger seem to provide many of their most memorial views when both the seeing and transparency are both equally good. However, some of the finest all time views from large scopes have been from under skies that are not altogether “pitch black” but more surprisingly, when the air is super steady.
Now I have to admit that my two finest full weeks of observing ever, were first: with my 25” during a week at Ayers Rock in the middle of the Australian outback. The skies were as dark as possible, and the seeing probably averaged better than one arc second all week. The second best prolonged observing experience was with my old 20” at the Oregon Star Party. Dark skies at 6000’, and good seeing again provide memories of night-after-night great observing.
So what is this sacrilege that the most memorial week long observing experiences were not with my later 36-inchers, or even my current 42 inch, affectionally known as “The Beast.” Well, let’s be honest here. Of course my finest ever views have been in either one of the 36 inchers, or the 42”, but good views in a big Dob are rare. (Everyone has their own opinion, but to me big Dobs start at 24” and over.) They never happen night after night. To be honest, the largest scopes only provide the finest views under the very best nights of seeing, and those nights are very rare. While I would love to have a Star Trek Transporter built into my dome, to occasionally move the biggie and its house to Mona Kea, or Pic Di Midi, or especially LaSilla, for some reason or other the astronomy gods do not allow us poor amateur astronomers to use the best sites on the planet for our regular observing sites. Darn!
We are stuck with what we have available to us. While my home at Chiefland is not as dark as it was 20 years ago (What site is?) we still have a few good nights now and then, and when a night of decent darkness and good seeing combine themselves with a moonless night, the star light becomes more like photon torpedoes and you see sights that you will never forget.
Many a night I have walked around on the club observing field and seen some great images in smaller scopes up to 18”, but more often than not, the first views of the night through the 42” are miserable. When the seeing sucks, so does the observing in large scopes.
The 42” truly is a beast, and truthfully, so are the 36s too. When you ask a scope to look through four times the air of its smaller brethren, the seeing seems to always to be six times worse. When we have one of those rare perfect nights when the sky seems to be extremely dark, as often as not you may not have the really good seeing to go with it.
I have overheard those who do not understand how important seeing is, talking in the dark with their friends, and commenting that they looked thru the 42”, and it really does not seem to have star images as as good as their 15 incher. Then other times you will hear voices saying that they just had the observing experience of their lifetime, and they will never be happy with small scope views again. Both opinions are occasionally true, but more often than not the star images in my 24” travel scope are far more pleasing than those in the 42”. It’s just not fair!
It is interesting to read posts on the web forums, of telescope makers who are trying to do everything possible to trick out their scopes: the best eyepieces, mirror cells, comparing slings to other types of edge supports, observing with the latest baffling tricks, etc.,etc.!
However, those who have been in this hobby awhile eventually come to realize that while all the small tricks of telescope performance help somewhat, when it comes to obtaining the best observing times of our life, what really matters to Big Dob users are those nights when the atmosphere settles down. There are very few bad telescopes on the nights when the seeing gets good. Everyone is happy with his scope’s performance.
A friend of mine has a 12” Meade Dob. He’s a bit of a newbie. I’m sure the scope does not have the best mirror on the planet, and definitely not the smoothest mount. We have made some changes to make the motions better, and other minor improvements, but the bottom line is my friend the newbie loves his scope. It takes him to places he never dreamed about in his earth-bound days. My friend spends a fair amount of his time in the dome with me and the 42, but I just love watching him on his own, zooming his small scope from object to object, and hearing his exciting comments as he finds object after object. Working with the newbie, and getting him interested in learning about telescope making is fun, and listening him talk about getting his next larger instrument is even more fun. The point I’m making is that he doesn’t care if his scope is not the best in the world. There is almost never a night of bad seeing in a 12”. And when the sky is at it’s darkest and most transparent, the 12” shines.
And I guess that the point I’m trying to make that no matter how good or bad the mirror on the 42” is, or how good or bad it’s mirror cell is, or if there are tube currents in my big ugly square tube, or whether or not the dome is creating seeing problems on its own, the one thing we have no control over is the seeing. When the Astronomy Gods give us good steady air, the scope shines.
It is interesting how suddenly, when the air gets steady, the collimation problems are gone, the tube currents disappear, and the bad mirror starts acting like it was just refigured and recoated. On those nights the views are the stuff dreams are made of!
The astronomy gods are very cruel to those of us with large amateur telescopes. I talk to them more and more often as I get older, but they usually ignore me…
Tom Clark
Chiefland, Florida
