Binoculars

Advertisements shout about the high magnifications available using their telescope. 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. These objects demand a wide field of view to appreciate their beauty and delicate form.

There are several ways to view large areas of the sky. The simplest is naked eye. Your eye, when properly dark adapted, is a marvelous instrument. 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 binoculars 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 binoculars 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 Telescope Making Magazine 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 Magazine or at better camera stores in your area. Most large, modern binoculars come with a tripod adapter which have a standard thread that fits the tripod head. This will allow you to easily mount the binoculars on a sturdy support. The support will provide a field of view which will not jiggle as you observe because of tired, cold hands.

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 adapter in the light path of a long focal length telescope. These rich field adapters will change the size of the light cone within the telescope and a wider field of view will result. The disadvantage is that the adapters 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 500.

Many commercial telescope makers are manufacturing RFT’s in some form. Both Meade and Celestron sell adapters for their Schmidt-Cassegrain telescopes. Coulter and Chicago Optical sell RFT Newtonian telescopes.

If you are looking for the widest field possible, then a pair of 7X35 or 7X50 binoculars 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 binoculars are just what you want.

Getting higher power binoculars will show 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 binoculars 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 binoculars, 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 the 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 constellation of the Hero. In 10X50 binoculars 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 binoculars 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 binoculars 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 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 binoculars 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 within 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 binoculars. 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.

There are lots of objects that demand a large field of view to see them at their best. The list will provide you with some more deep sky vistas to observe. I hope this will start you on your way to getting a wider view of things astronomical.

Clear Skies.

Steve Coe

Observing list of wide field objects in the Fall and Winter skies

Obj	Con	Type	R.A. (2000)	Dec	Mag	Size(min)	Notes
M 31	AND	Sb	00 41.8	41 16	4	160×40	Great Andromeda Galaxy
M 44	CNC	OC	08 40.1	19 59	4	90	Beehive, 200* to mag 14
M 37	AUR	OC	05 52.4	32 33	6	24	very rich, about 500*
M 35	GEM	OC	06 08.9	24 20	6	29	NGC 2158 1/2 deg SW
M 45	TAU	OC	03 47.0	24 07	2	120	Pleiades, look for neby
M 42	ORI	GN	05 35.4	-05 27	6	66×60	Orion Neb, magnificent
NGC 7293	AQR	PN	22 29.6	-20 48	6.3	15×12	Helix, large, diffuse
NGC 457	CAS	OC	01 19.1	+58 20	6.4	13	rich, 80*
NGC 7789	CAS	OC	23 57.0	+56 44	6.7	16	very rich, many dim*
NGC 2244	MON	OC	06 32.4	+04 52	4.8	24	Rosette, OC + neby
NGC 869	PER	OC	02 19.0	+57 09	4.3	30	Double Cluster w/NGC 884
NGC 884	PER	OC	02 22.4	+57 07	4.4	30	350*
NGC 253	SCL	Scp	00 47.5	-25 18	7.1	25×7	Large, mottled spiral
Fornax Dwarf	FOR	Ep	02 39.7	-34 17	9.0	20	Low Surface Brightness
Sh2-276	ORI	EN	05 48.0	+01 00	—	600	Barnard’s Loop
Mel 20	PER	OC	03 22.0	+49 00	1.2	185	Alpha Per Cluster
NGC 1499	PER	EN	04 03.3	+36 25	—	145×40	California Nebula
Scl Dwarf	SCL	E0	00 59.9	-33 42	10.5	75	Low Surface Bright
Cr 70	ORI	OC	05 36.0	-01 00	0.4	150	Incl Orion’s Belt *s
Cr 140	CMA	OC	07 23.9	-32 12	4.0	42	naked eye
Mel 25	TAU	OC	04 27.0	+16 00	1.0	330	Hyades
Mel 20	PER	OC	03 22.0	+49 00	1.2	185	Alpha Per Cluster
Stock 2	CAS	OC	02 15.0	+59 16	4.0	60	Easy in Binoculars
Tr 37	CEP	OC	21 39.0	+57 30	5.1	50	30*, Invl in IC 1396

 

References:
Astronomy	Aug 81 pg. 51
	Dec 85 pg. 44
	Nov 82 pg. 39
	Dec 86 pg. 70
	Nov 86 pg. 75
	Feb 86 pg. 62
Deep Sky	July 80 pg. 8