The Joys of Low-Power Viewing
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-excerpts, used with permission, from an article by Al Nagler in The Best of Amateur Telescope Making Journal, Vol. 1, Edited by William J. Cook, Copyright © 1992-2003 by Captain's Nautical Supplies, Inc. and published by Willmann-Bell, Inc. More about these fine books can be found at: http://www.willbell.com/tm/batmj/index.htm. (Original article published in Issue 7, Spring 1995 of Amateur Telescope Making Journal.)

The majesty of the universe has fascinated me since I was a teenager in the '50s. To me, "celestial", or heavenly, connotes wide angle star-field vistas.

Let's look at what low power means, its relationship to field of view, the meaning of "RFT", and what specific low power views (in my experience) are particularly exciting.

Field Considerations

How large a field can you get with a telescope? The field of any telescope is determined by the eyepiece field stop and the focal length of the telescope. Divide 47 mm by the focal length, and you have the approximate "tangent" of the true field. Therefore, a 500 mm f.l. telescope could give you 5.4°.

f/# Considerations

A fast f/# telescope objective will produce the same brightness as a slow telescope with the same aperture and magnification. For visual observing, the fast f/# implies a short f.l., which does give the most field. Visually, therefore, we can say that a "faster" scope has more potential field, but no more image brightness than a slow scope of the same aperture!

Exit Pupil Considerations

If you increased the magnification, let's say by using a higher power wider field eyepiece, you would see more detail over a similarly sized field. The view will generally improve. Why? Because the sky background dims at the same rate while contrast is maintained between the background and subject. For best low-power viewing, use the highest power that properly frames the subject. You will see the most detail and best contrast with the least contribution of eye defects.

Low Power Limits

While the example for a refractor shows that there is no arbitrary limit to how low you can go, that is not true for reflectors or SCTs that have central obscurations. So ironically, a fast SCT is not ideal for low power wide angle viewing. An even better choice would be an f/4.5 Dobsonian or Newtonian with the secondary obscuration limited to about 20%. Remember though to consider a coma corrector such as the Paracorr.

To me, no view is spectacular if the stars look like blobs.

"RFT"

The Rich Field Telescope is simply one that has sufficient field and aperture to provide exciting Milky Way and other rich field views. As for which RFTs are best, of course I am slightly partial to fast APO refractors, particularly if they have flat-fields. Larger aperture f/4 to f/5 Dobsonians with a coma corrector are every bit as good if the mirror is well-made.

What to View

We all have our favorites. Here are some of mine in no particular order:

  • M24. This region is filled with stars of all brightnesses and a lovely embedded cluster. Choose a magnification that allows it to fill the field so that the sky background is darkened to enhance contrast. At a low enough power, M17 (the Omega) can be seen in the same field.
  • The Sagittarius star cloud.
  • The Scutum star cloud (with M11, the Wild Duck cluster).
  • The Double-Cluster in Perseus.
  • Anywhere in the Milky Way (especially Cygnus).
  • Large open clusters. Aside from the usual Pleiades or Beehive, try the Hyades and the Coma Berenices region.
  • A special favorite of mine is the region around NGC 6231 in Scorpius. At very low power or with the naked eye, it looks like a comet, but at higher powers, it appears like a spray of diamonds as I have seen it follow the arc above the treetops at the Riverside convention.
  • Galaxies, such as Andromeda, M33, and the M81-M82 pairing.
  • M8 and M20 in the same field.
  • The North American Nebula and all three sections of the Veil Nebula in one view. (you must use a nebula filter).
  • The southern sky, with Eta Carinae (dwarfing the Orion Nebula), the Magellanic Clouds, Coal Sack, Jewel Box, and the entire Milky Way (so bright it casts shadows on the Australian outback).

Well, I'm tired of writing. I'm going outside to view. Care to join me?

— Nagler, A. "The Joys of Low-Power Viewing." The Best of Amateur Telescope Making Journal, Vol. 1, Willmann-Bell, Inc. (2003): 261-265.
Reprinted with permission. © 1992-2003 Captain's Nautical Supplies, Inc.

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