Great Objects for Rich Field Telescopes
Much has been written about the Summer Milky Way, the beautiful and bright pathway to the stars that Our Galaxy makes when it is overhead on warm evenings in the Northern Hemisphere. Certainly the Milky Way is at its best as it passes through Centaurus, Scorpius and Sagittarius. But let us not forget that the Milky Way is also overhead in the Winter. Many of the objects through the central part of the Milky Way are large in size and demand a wide field of view to take in the entire deep sky vista around that cluster, nebula or galaxy.
There is no doubt that high magnifications yield beautiful views of the Moon, planets and fine detail in some deep sky objects. However, many objects in the sky are too large to fit into the field of view of a high power eyepiece.
Viewing techniques that will take in a large area of the heavens are useful because some celestial objects will not fit into a narrow field. Also, many large galaxies and nebulae have a very low surface brightness. That means that the object is large and quite dim, therefore they are only going to be seen easily when surrounded by dark sky. Large deep sky objects are more easily observed when there is some contrast between them and their surroundings. Only a wide field of view will provide that contrast. Let's look at several techniques which will yield a wide angle field of view.
The simplest is naked eye. Your eye, when properly dark adapted, is a marvelous instrument. Several techniques will help you to keep your eye at peak efficiency in dark skies. As the Sun sets, keep a pair of sunglasses on until the ball of the Sun is actually over the horizon. That will start the dark adaptation process sooner. As darkness sets in, use a dim red flashlight to read star charts and make notes. Several makers of astronomers' flashlights advertise in this magazine.
Using just the naked eye you can see many lovely vistas in the Winter Milky Way. Bright areas in Our Galaxy are generally large star clusters, many associated with nebulosity. Even though your eye cannot pick out much detail, the human eye can take in an extremely wide field of view along with the brightest stars sprinkled across the face of the Milky Way. If you are looking for detail in that vista, you must use some optical aid.
Stepping up to a pair of binoculars will help a lot. The
added light gathering power of the lens system in a pair of binoculars
will allow you to see much fainter celestial objects. Along these same
lines, a finder scope on most telescopes is a monocular (half a binocular).
This small refractor can provide some excellent rich field views.
The biggest advantage of binoculars is that they are easily portable. What can be simpler than carrying a pair of binoculars out to an observing site? Any set of optics that can be carried on a bicycle with ease has the portability edge. The disadvantage of binoculars is aperture. Common size ranges for binocs are: 7X35, 7X50, 10X50, 11X80, 15X80 and 20X80. Remember that the first number is the magnification and the second number is the aperture in millimeters. In these sizes they are quite affordable and easy to transport. Moving to binoculars with an aperture of 100mm and up will raise the price considerably. For those reasons, let's stick to the convenient sizes for this discussion.
In magnifications of 10X and above the weight of the binocs becomes a factor to consider. As your arms tire, it becomes difficult to hold them steady enough for the kind of view they are capable of delivering. For this reason it is wise to consider some kind of support for high power binoculars.
Many varieties of binocular supports have been described and photographed in magazines over the years. If you don't feel comfortable constructing your own stand then there are several types of tripods available thru advertisers in astronomy magazines or at better camera stores in your area. Most large, modern binocs come with a tripod adaptor which have a standard thread that fits the tripod head. This will allow you to easily mount the binocs on a sturdy support. The support will provide a field of view which will not jiggle as you observe because of tired, cold hands. The next step up from a pair of binoculars is a small telescope.
Moving up to a small telescope will allow you to look at dimmer objects than the binoculars because of the larger aperture. To gather up wide areas of the sky in a telescope there are two approaches. The first is the addition of an adaptor in the light path of a long focal length telescope. These rich field adaptors will change the size of the light cone within the telescope and a wider field of view will result. The disadvantage is that the adaptors introduce some curvature in the field. This distortion can be bothersome.
The other method of achieving a wide field telescope is
to grind the mirrors to a short focal length initially. This kind of telescope
is usually a Newtonian and they are called RFT's for Rich Field Telescopes.
The disadvantage to RFT's is that they do not easily operate at high powers.
Basically, one telescope cannot do it all. This is a common problem in
this world, it is very tough to build a truck that could win the Indy
Many commercial telescope makers are manufacturing RFT's in some form. Both Meade and Celestron sell adaptors for their Schmidt-Cassegrain telescopes. Coulter and Chicago Optical sell RFT Newtonian telescopes.
Now that we have some info on the types of instruments that will provide a wide field of view, let's look at some deep sky objects that will look their best using these techniques and optical aids.
If you are looking for the widest field possible, then a pair of 7X35 or 7X50 binocs will give a field generally between 5 and 10 degrees in size. This is about the size of the Bowl of the Big Dipper. If you are trying to view most of a large constellation or all of a small one, these low power binocs are just what you want.
Getting a higher power pair of binoculars will show you dimmer stars and fainter deep sky objects. So the 10X50 and larger binoculars are great for looking at large nebulae and star clusters.
The same can be said for RFT's. They really shine when looking at large star clusters and bright or dark nebulae that take up big portions of the sky. They bring out some fine detail that is tough in large binocs because of the larger aperture available to most RFT systems.
I have had the good fortune to use several styles of binoculars and RFT's. The simplest way to give you some information about their performance is to provide you with some observations of wide field objects in Fall and Winter skies. Then you can decide which type of optical system meets your needs.
The Andromeda Galaxy is often viewed in many different types of instruments. The galaxy is bright enough to be seen naked eye from many sites. It only starts to show off its' grandeur in medium binoculars. The dark lanes are not visible in my 10X50 binocs, even at the best of sites. However, moving up to a pair of 20X80's makes a big difference. This frames the Andromeda Galaxy nicely with about one degree of sky around the spiral arms. The dark lanes are evident on a good evening and the companion galaxies can be seen with ease. Within the southern arm a bright H II region, NGC 206, is visible in the big binoculars and the rest of the spiral arms sparkle with mottling.
The Alpha Perseus Group is a large, nearby star cluster
that surrounds the brightest star in the onstellation of the Hero. In
10X50 binocs I can pick out 28 stars in the group. They are clustered
in an area about 3 degrees in diameter. The cluster aspect is lost in
higher power binoculars or an RFT.
The Double Cluster in Perseus is one of the most famous objects that did not make Charles Messier's list. It can be seen naked eye as a bright spot in the Winter Milky Way. A pair of 7X35 or 7X50 binocs will show the two clusters framed in a glow from unresolved stars. Large binoculars will resolve about 40 members, some even in the compressed central section. An 8" RFT provides the best view, however. The light gathering ability of the telescope will show off a hundred or so dimmer stars among the two clusters. There are several orange or yellow stars, including one almost between the two groups. Many lovely curved chains of stars wind their way out into the Milky Way from the edges of the clusters.
The constellation of Orion has been a favorite of amateur astronomers for generations. One of the reasons is that so many different types of instruments will provide excellent views of the heavens in this part of the sky. A pair of 7X50 or 10X50 binocs will show off the stars in and around the Belt of Orion, this group has the designation Collinder 70. There are about 45 stars visible in medium binoculars. They form beautiful loops around the bright Belt stars.
The Orion Nebula has been written about for centuries. I will say that I particularly like the region in my 11X80 finderscope with a UHC filter in a 20mm Erfle eyepiece. The UHC gets rid of any color from the lens system and still provides about 3 degrees of field. So the entire Orion Nebula complex, including M42, M43 and several NGC nebulae are in this breathtaking field of view.
The Winter Milky Way goes through Auriga and Gemini, two constellations which pass near overhead for much of the Northern Hemisphere. A glance at the map in the central section of Astronomy will show several Messier clusters in this region. There are many lovely dark lanes that wind through Our Galaxy in this area and set off the clusters like diamonds on a velvet cloth. The 10X50's are just right for a leisurely scan through here.
The Rosette Nebula in Monoceros is another faint glow in
the Milky Way to the naked eye. The coarse cluster in this wreath of nebulosity
is shown in 10X50 or 11X80 binocs but I have never seen the nebula clearly
in binoculars. The RFT's will show off the Rosette at its best. Even my
small 4 1/4" f/4 at 16X will bring out this gas cloud in distant
space. On a night I rated 8/10 for contrast and seeing, I could see a
few of the dark lanes in the nebula.
Because comets will occasionally form a tail of several degrees in length, they often lend themselves best to wide field instruments. I had several nights of interesting viewing when Comet Bradfield passed near M10 and M12 in Ophiuchus. Comet Liller was excellent in an RFT. Binoculars were essential for watching Comet IRAS-Araki-Alcock as it moved across the sky in Draco. This comet passed so close to the earth, that even in the 10X50's it would move against the star background as you observed!
Dave Fredericksen, Chris Schur and I had the good fortune to travel to Australia in 1986 to view Comet Halley. We spent a week in Jim Barclay's backyard near Brisbane and marveled at the southern sky. Taking binoculars was easy and they where on hand for convenient viewing of unfamiliar parts of the Milky Way. The 10X50's were great on the Eta Carina Nebula, The Coalsack and The Magellanic Clouds. However, I will never forget Comet Halley in a pair of 15X80 binocs. We happened to be observing the comet while it was quite active. The gas tail was very prominent and there were several layers of brightness to the coma. The tail changed its' orientation and detail came and went from one night to the next. The big binoculars provided a front row seat to all the action.