Low Power Objects for a Rich Field Telescope

In a separate article I discuss the Rich Field Telescope or RFT and its construction.

Here I discuss what I use my Rich Field Telescopes for. I do not claim universality for my preferences: this is merely what I enjoy.

Introduction

There are three main uses for low-powers:

  • Wide real field for big objects
  • Wide real field to hold object plus background
  • Bright fields -- big exit pupil.
  • Wide Fields for Big Objects

    This is obvious. The Pleiades are about one degree wide. If you want to look at the whole of the seven sisters, you need at least a one-degree field. With reasonable apparent field, that means at most sixty power. To see most of M31, the Andromeda galaxy, you need 20x or less.

    Related is "Milky Way cruising" or just scanning the skies. You need a wide field otherwise you are looking at too little. You want enough magnification so that objects like globular clusters are non-stellar and interesting. Many people use big binoculars for cruising. I find that regular binoculars are a bit too low-power and I do not own any big binoculars -- and big binoculars are difficult to use with nebula filters. But scanning with, say, the six-inch f/5 and a 35mm Panoptic at 22x and a three-degree field is a pleasure.

    Wide Field for Foreground-Background Separation

    This is a bit different. Many target objects do not show a large amount of internal structure: you are looking at a blob. In this case, if the blob is larger than the field, all you see is "the field seems to be brighter than normal." You need to see the edges to see the object. And so you need a field that will not only hold the whole blob, but hold enough "sky background" so you can easily identify the edges: the edges are your view of the object. If the blob is really big, you may have to concentrate on the edge: off-centering the object so you are looking a part of the object plus edge, and scanning around it.

    Brightness: Big Exit Pupil

    A non-astronomer thinks that the sky-background is black at night. If his is not, he thinks that if he got away from the city, it would get darker and darker until it got black. The astronomer knows that the sky does not get black. Even in Chile or Antarctica, there is "sky background" and the sky is just dark grey.

    So if you are dark-adapted and look at the night sky, you see stars etc on a grey background. If you use a RFT with a full exit pupil (say 6.5mm), the increase in magnification is balanced by the increase in light-collection, and the sky background through the eyepiece is that same grey.

    Because of this, increasing the power moderately does not make it much harder to see faint objects. The object gets fainter, but the background gets fainter too: brightness is lower but contrast is unchanged. And a big blob is easier to detect than a tiny blob. So, for smaller objects that easily fit in the field of view, a RFT operating at a maximal exit-pupil is less valuable than you might think.

    Unless the visible sky-background is supressed.

    That is what a nebula filter does. A nebula filter may transmit a passband of around 10nm. Your dark-adapted eye has a natural passband of around 100nm. So the filter is cutting the background (and continuum sources like stars) by a factor of ten or so. Now you do not see the "dark grey background", you see a black (or very close to it) background. The nebula filter will cut the brightness of the target too, because the target will not be radiating all of its light at one wavelength: it may be a mix of nebula and continuum-sources and even if it purely nebular, there is more than one nebula line. But the nebula filter will cut the brightness of the object by less than a factor of ten: the relative brightness of the object -- the contrast -- will be boosted. All this is costing you light, thus making light-gathering important: a perfect use for a RFT.

    My introductory web-page on the joys of nebula filters is here.

    A Few of My Favorite Low-Power Objects

    This is just a selection to "whet your appetite". These objects do not require nebula filters. See my nebula filter page for an introductory list of nebular objects

  • "North American Nebula" NGC 7000 in Cyg, near Deneb. About four degrees across. Not as nebular as it looks from photos, the North American looks like a chunk of Milky Way sharply delimited by dark nebula that are "the Atlantic ocean" and "the Pacific ocean".
  • The Pleiades. Wonderful with about a two-degree field. Can you see that some of the bright stars seem nebular and others do not? Note which are and which are not and then compare with a photo.
  • The Andromeda Galaxy, M31. This object is much larger than commonly believed. Amateurs with moderate telescopes often think they are seeing "the galaxy" when they are only looking at the bright inner core. The galaxy can be seen to extend over three degrees under dark skies.
  • The Orion Nebula region, M42. Glorious even without filters. Can you see the curved streamer that extends from the main nebula down to the southern star in the sword?
  • The whole Sagittarius-Scutum region. The Milky Way from western Sagittarius up through Scutum is a fine region to just wander in at low power, noticing the star-clouds and dark regions and "sweeping up" occasional open clusters, globular clusters, and nebulae.
  • The "False Comet" in Scorpius. Two open clusters combine to look distinctly like a comet in southern Scorpio, just north of mu Sco. The southern "head" is NGC 6231 and the broader "tail" is CR-316. Fun to resolve this "comet" into stars.
  • Globular clusters. They benefit from higher powers, but the larger globular clusters look good at low powers too. M13 in Hercules is wildly popular, but I like M22 in Sag, M4 in Sco (next to Antares -- a fun pairing) and Omega Centauri better.
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    Contacts

    I would be happy to correspond about observing with interested individuals.

    I can be reached via email at astroayers@gmail.com