fred@inuxe.UUCP (Fred Mendenhall) (02/13/86)
*** REPLACE THIS LINE WITH YOUR MESSAGE *** The question of observing and best telescope optics is not an easy one especially when talking about using the human eye as the sensor. I'll try to share some of my "opinions" based on observing with a sample of telescopes.(Scopes used: Meade 2080, Celestron C-8, Celestron C-90, home built Newtonians 12.5" f6, 10" f5.6, 6" f8) Some of the factors that come into play: 1. Objective Diameter 2. Aberrations 3. Contrast 4. NightGlass Effect 5. F number The objective diameter of the telescope is the dominate factor in deciding whether you can see anything at all of a dim object. In other words, if the telescope can't collect enough light for your retina to sense the object, none of the other factors matter. Once your cross that threshold of sensing the object the other factors greatly affect the perceived quality of the image. Aberrations are the distortions of the image caused by the optical design. In the case of the Newtonian, the major aberrations is coma in the off axis field. This aberrations is more apparent in low F number (photographically fast) Newtonian systems than in higher F number scopes. However, if your primarily interested in observing comets or nebula, everything is fuzzy anyway so this aberrations doesn't matter much. Contrast is particularly important when observing the planets and looking for details as opposed to hunting up the fuzz balls in the night sky. Contrast is greatly effected by the physical obstructions in the optic path, i.e. spider and secondary mirrors. The effect is observed in telephoto photography. You can place the camera lens next to a screen or get water drops on it and if you are focused at infinity, the screen or water drops are not visible. However, the image is much softer because the contrast has been reduced. The Celestrons and Meades suffer somewhat from this problem. They have oversized secondaries for photography and in a side by side comparison with a 6"f/8 designed for 1.25" O.D. eyepieces looking at say Jupiter or Saturn the smaller scope has the superior view. Looking for comets or deep space object the 8" Celestron or Meade win easily. The night glass effect has to do with the way the eye works. Normally you would expect that the magnification that transmits the most light into the eye would give the best image of a dim nebula. That turns out not to be true. Seems for a dim object projected against a black background the sensitivity of the eye is controlled more by the angular extent of the object than by the energy transfer so frequently you "perceived" a better image using medium magnification instead of low power at maximum exit pupil. The night glass effect peaks out at around 1 degree then the object gets dimmer again. This is why some objects like M-31 look better in smaller scopes, because at low power in a smaller scope the image is about 1 degree on the back of you retina. Its interesting to note that ever the Worlds largest scopes, 200" Hale, present an inferior image to the observer than what most amateur equipment presents, at least for a few objects. (When you start talking about anything but visual observing the pros have us beat anyway you want to measure it) What this all boils down to is that there is no one best telescope design. What is best very much depends on what you want to observe. I like comets and deep space objects. I like to see light that started on its journey to me before my ancestor crawled out of the sea, therefore I'm fond of the fast Newtonian light bucket designs. If I wanted to observe planets, I'd look real hard at the new refractor designs. If photography is what you really want, (I caution anyone who hasn't been an amateur observer for at least a year not to spend a lot of money on an astrophotgraphy rig until you find out if observing is really how you want to spend nights) either the C-8 or Meade 2080's are an excellent choice to start with. Clear Skies Fred Mendenhall