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The truth, finally: Conventional Wisdom

Started by astro2575, 06/09/2003 03:38PM
Posted 06/09/2003 03:38PM Opening Post
Due to recent discussions in this very forum, to finally bring the truth and nothing but the truth to light, because I feel controversial today, because it comes up all the time anyways, because it needs to be said, because I just found an excellent flame suit ....
Conventional wisdom (CW) in astronomy:

(1) CW: A larger telescope is more affected by bad seeing
Truth: A larger telescope is affected the same by seeing given same magnification, but it may not live up to its higher resolution potential except on good seeing days (or never, if it's a 24" ). See various threads below smile

(2) CW: Quality APO refractors are the best planetary scopes
Truth: Quality APOs do perform as well or better than other telescopes of same aperture and quality. They are also the by far most expensive scopes per inch of aperture/photon collected/arc sec resolution, bar a few handmade and rarely sold other types.

(3) CW: The central obstruction in SCTs (or most other reflectors, for that matter) greatly reduces contrast
Truth: The central obstruction in SCTs slightly reduces contrast. Consider that practically all professional telescopes built in the last 100 years (ground or space-based) have a central obstruction, and if they are Ritchey-Chretiens, which many are, a rather large central obstruction at that!

(4) CW: Binocular viewers are only good for bright objects because of the light loss
Truth: Quality binocular viewers are good for all objects. The brain makes up for (almost all/all) of the light loss so that only on very dim objects at the edge of visibility there may be a difference in what can be seen.

(5) CW: Beginners need to learn star hopping first
Truth: Depends on if you like the journey or the destination! Seriously, not finding objects is the largest reasons for beginners to ditch the hobby. Not having to star hop does not preclude anyone from "learning the sky" if they so desire. And, most beginners observe mostly from their light polluted back yard, and star hopping to objects never seen before using a few dozen visible stars is very challenging event for experienced observers to say the least!

(6) CW: Aperture rules
Truth: Almost true. Aperture rules as long as the scope isn't to large/heavy to be used. Resolution increases linearly with aperture, light gathering with the square of the aperture, and weight almost with the square of aperture. But: Aperture = power, not magnification = power (as anyone who ever owned a 50mm Tasco refractor knows quite well)

Woohoo! Let's get this show on the road.
And - it's OK if someone else but the three APO musketeers (it's all in good fun, right?) posts/flames/(or dare I say) agrees!

H
Posted 06/09/2003 03:41PM #1
Hinrich, I agree.

I know, I know, not as fun as disagreeing - but your statments are generally correct, IMHO.

-Jake
Posted 06/09/2003 04:04PM #2
I agree with some of the things you said wholeheartedly.

But in my humble opinion, all I know is, for myself I haven't been to every square foot on the planet, have not seen every telescopes on planet, haven't done a 1-on-1 or 1-on-many comparison on every telescopes at every places on the planet, have not watched every nights since the beginning of my life-cycle, to be wise enough to make any sort of generalized conclusion. What works for me best may not work for others. What works for me best where I am or where I will be, may not work for others.

Thus the *generalized* truth *for me* is: keep an opened mind ;-).

Ron B[ee]
Posted 06/09/2003 04:15PM #3
I was going to jump in on this, but I decided I'll go down and do some work in my shop instead. I'm tired of telescope arguments, and think I'll get back to spending my time on ATM projects. Have fun!
Posted 06/09/2003 04:17PM #4
That's very interesting story, Mike! I guess if I were to be so bold as to conclude then that the morale of your story is: astronomy is not a hobby for everyone (some likes collection stamps with stars/moon/planets in stamp collection better ;-)?

Ron B[ee]
Posted 06/09/2003 04:57PM #5
Hinrich!

Six issues at once.... we'll NEVER get out of this board! *grin*.

FWIW - I've started to wonder what REALLY hurts mass market SCTs more... the central obstruction or poorly figured /inaccurate mirrors ( and owners who don't collimate). High end limited production Tak SCTs and RCs all seem to have Very Nice image quality... and correspondingly, I can easily tell apart similar aperture Zambuto/Swayze mirrors and mass market mirrors.

In a similar fashion, theoretical, or achievable "resolution" improves with aperture, all else in the optical train being of equal quality. That may be an obvious qualifier to most, but often unsaid. An 8" SCT will not necesarily have higher apparent resolution than a 5" APO. For that matter, an 8" refractor might not have a higher apparent resolution than a 5" SCT.... it totally depends upon the optics quality/collimation/ad nauseam.

Karl
Posted 06/09/2003 07:33PM #6
Well I don't think I believe either the so-called conventional wisdom or the so-called truth of (4). While two eyes helps, no doubt, dropping the intensity of the light to the retinal rods by a half or more is bound to drop ALL the available detail at the threshold of visibility when viewing without the beamsplitter, and if the rods don't detect it, the brain won't have any of that signal to work with. The CW is more like binocular viewers are good for all objects, but it sure helps to have a little extra aperture to make up for the light loss. Is this not so?
--
Mike
Posted 06/09/2003 09:25PM #7
You wrote:
(3) CW: The central obstruction in SCTs (or most other reflectors, for that matter) greatly reduces contrast
Truth: The central obstruction in SCTs slightly reduces contrast. Consider that practically all professional telescopes built in the last 100 years (ground or space-based) have a central obstruction, and if they are Ritchey-Chretiens, which many are, a rather large central obstruction at that!

My response:
I am afraid I must disagree with your basic assertion. It is true that central obstructions are often blamed for far more than they are responsible for, but the fact remains that large central obstructions really can destroy contrast, particularly on the planets. Where the secondary is small, less than 20% the size of the primary) The impact can be all but negligable, but when it gets up to 35-40% (as it does on SCTs) the image quality drops. I always remember a time we were looking at the GRS through a 6" Astro-Physics, a 4.7" Vixen Neo-Achormat and an 8" SCT with what has been claimed as being above average optics. In the 6" the GRS was easily seen with great definition. In the 4.7" the GRS was readily visible but nearly as well defined.. in the 8" SCT it was barely visible.. .it just blended into the background.

Regarding professional telescopes, it should be remembered that they are used almost exclusively for imaging work. Contrast is not nearly as important in this application because much of the contrast can be restored in processing the image.

--
Bill
Posted 06/11/2003 05:09PM #8
Forgot one (this ought to be good too):
Myth: Red light is best to preserve night vision.
Reality (I shouldn't have called it Truth): Light intensity is more important than color. Thus, the best light to use is *green*, because a much lower light intensity can be used to still be able to see.

http://lvaas.org/flashlight.html (RED IS DEAD... GREEN IS KEEN)
http://www.corvus.com/kniffen.htm
http://www.flashlightavenue.com/colorguide.html (now there's a scientific source smile )
but, arguing for red,
http://www.aoa.org/clincare/aviation-night.asp saying that exactly because the eye is so insensitive to red, red doesn't even stimulate the scotopic vision (but goes straight to photopic system (cones)).

Honestly, this one I'm not sure about. Did anyone experiment with this (i.e., didn't just always use red because that's what everyone else doesn?)