When discussing telescopes, it is often necessary to divide them into groups. Refractors versus reflectors, alt-azimuth versus equatorial, expensive versus cheap. After these arbitrary divisions, the author will often sub-divide the scopes and discuss the pros and cons of each group. Ok, so here goes....
LONG TELESCOPES VERSUS SHORT TELESCOPES
Long telescopes have focal ratios of around f12 to f15. What that means is that the focal length of the objective (the big lens at the upper end of the scope, or in the case of a reflector, the big mirror at the bottom of the tube) is 12 to 15 times as long as its diameter. If, for example, in an f15 telescope the lens is 3 inches in diameter, the focal length will be 45 inches. That is to say, that the objective lens creates an image at the bottom of a tube at least 45 inches long whereupon a smaller lens called the eyepiece will magnify the image after a knob is turned to "draw" the eyepiece tube in or out to get the clearest view. This is called focusing.
A lens can be designed to create an image only 4 or 5 times its diameter down the tube. An f4 lens 3 inches in diameter would have a tube only 12 inches long(4 times 3). Such a tube would be much smaller and lighter than the 45 inch long tube, so why not make all telescopes short? There are two advantages in making a long telescope with a slow (high) f ratio.
First of all is error of figure. If a lens is ground to only one umpteen millionth of an inch out of correct figure, this error will be of far less consequence in a slow (f12 to f15) scope as opposed to a fast scope (f4 or f5). The smaller the wavefront error, the better the optics and the clearer the image will be. Many regard the worst allowable error to be about 1/4 wave. Many also believe that any improvement beyond 1/8 wave will yield no observable improvement. So a good astronomical telescope should have optics that are between 1/4 and 1/8 wave accuracy.
(Actually, technically speaking, an error in figure of 1/8 wave will produce an error in the wavefront of 1/4 wave, so bear in mind that you will often find 1/4 and 1/8 wave specifications, but they are often referring to the same degree of precision.)
Here is a quote that should be of some interest:
Optical Quality of Telescopes. "Sky and Telescope" March 1992
"Frankly, I am surprised by the results. Conventional wisdom among amateur astronomers suggests that a 1/ 10 wavefront mirror should easily reveal its superiority over a 1/4. wavefront mirror of the same design. This is not what I observed, and it suggests to that under typical backyard astronomy conditions a 1/4. wavefront optical system is perfectly adequate. On those rare nights of excellent seeing, maybe the 1/10 wavefront mirror would show its stuff, but even then I doubt the difference would be noticed by most observers, except in rigorous side-by-side comparisons on selected test objects. Clearly this exercise suggests that there are huge gains in visual performance as optical quality improves to 1/4. wavefront, and less conspicuous benefits with further improvement.
The 1/4. wave mirrors intrinsic performance in this test was outstanding compared to that of many other telescopes I have used. To me this says two things. First, a good quality 6" f/8 Newtonian reflector- so seldom seen anymore -is a superb instrument for the casual or intermediate level observer. Second, if manufacturers advertise diffraction limited systems and deliver on their promise, everybody but the connoisseur of costly precision optics should be happy."
That umpteen millionth of an inch error in the figure of an f4 scope might produce an error of 1/2 wavelength and create a "soft" or slightly blurry image (even when at best focus), but that same umpteen millionth of an inch error in the figure of an f15 scope might produce an error in the figure of only 1/8 wave and, thus, still produce a tack sharp image. It is simply cheaper to grind a good f15 glass versus a good f4 glass. Faults such as chromatic aberration and spherical aberration are much easier to control in a slow glass. We will touch on aberrations in more detail a bit later.
Secondly, a slower f15 scope is easier to focus than a fast f4 scope due to the narrow cone of light produced by the slower lens. Because of the broader cone of light coming down the tube in a fast f4 scope, you must have your eyepiece drawn to within a very small tolerance to achieve perfect focus. Because of the long narrow cone produced by the slow lens, you can be a little out of focus and not even notice. Also, because of the broad cone of light produced by a fast f4 lens, your eyepiece, because of its depth of field, will actually pick up a bit of the light cone that is just inside and outside of focus and your eye will perceive it as a blurry glare surrounding the object being viewed. A slow lens is much, much less susceptible to glare produced by this phenomenon and hence is considered superior at viewing the planets, whose images can be seriously degraded by glare.
The type of telescope we have been discussing so far, with the big lens at the upper end is called a refractor telescope. Light passing through this positive lens is bent (refracted) inwardly and eventually all the light converges at the point of focus where it is magnified by the eyepiece. All wavelengths of light are not refracted by the same amount as you would know by examining the rainbow that is produced by sunlight passing through a prism. The blue-violet light waves are the highest in frequency and are bent (refracted) the most severely and the lower wavelengths (orange and red) are refracted less severely. This would produce a colorful and uselessly blurry image were it not for the invention of the achromatic objective.
The achromatic objective is comprised of two lenses of different types of glass mounted together in a lens cell. Because each glass has a different refractive index, this lens can be designed to bring the blue-violet and the orange-red light to the same focal point. This chromatic aberration can be controlled by an achromatic objective in a scope as fast as about f10 but for shorter f rations a third element is added. This lens which is corrected for all wavelengths is called an apochromat, and is usually quite expensive.
A telescope with a slow f ratio of f12 to f15 can, however, be made very short by using a combination of lenses and mirrors, but in order to do this we will have to explain folded optics, and in order to understand folded (catadioptric) telescope optics we must first examine the reflector scope.
The objective lens in a refractor is a positive lens, that is to say, a convex lens, that is to say, a converging lens and it is fatter in the center than at the edges, as opposed to a negative lens, that is to say a concave lens, that is to say, a diverging lens that is thinner in the middle and fatter at the edges. A positive lens creates an image by focusing all the light striking it into a focal point. A concave mirror can accomplish the same feat.
NEWTONIAN DESIGN
All the light striking a concave mirror will be reflected back up the tube converging at the focal point. If a small, flat mirror at a 45 degree angle intercepts the cone of light being flectorized back up the tube and diverts it to the side, an eyepiece can be placed into position at the side of the tube and the image can be magnified and observed. This is called a Newtonian reflector telescope because it was invented by Sir Isaac Newton. Since a mirror reflects light of all wavelengths by the same amount, there is no chromatic aberration with a reflector. Since there is no chromatic aberration to deal with, a reflector can be made faster (with a lower f number) than a achromatic refractor. Newtonian reflectors rarely are slower than f8 and are often available in versions as fast as f4.
The mirror in a Newtonian reflector must be figured to a parabolic curve rather than to a simple spherical curve. A simple spherical curve would not reflect parallel light waves coming from far away to the same point. This effect is called spherical aberration and produces a blurry image. A parabolic mirror will focus incoming parallel light rays to the same point.
Grinding the mirror to a parabola rids the scope of spherical aberration. Traditionally, the mirror was ground to an f8 sphere by a machine and then hand corrected to an f8 parabola. The degree of manual labor needed to correct a slow f8 glass is far less than needed to correct a fast f4 glass. In fact, a mirror ground to a spherical curve of f10 is within 1/4 wave of being parabolic, and hence, is usable.
Today's grinding machines are computerized and so mirrors ground by the machines to an f4 parabola are quite common. But, while the chromatic aberration is non existent, and an accurate parabolic curve can minimize spherical aberration, a fast f4 mirror still suffers from many of the same constraints as a fast f4 lens.
The main shortcoming is something called "field curvature" and is a function of the mirror's speed and nothing else. An f4 mirror will have a usably flat field (for photographic purposes) of only about one half inch in diameter whether the diameter is 4.5 inches or 200 inches as in the great giant on Mount Palomar. Yes, that huge scope's field is flat only for one half inch without corrector lenses. The effect produces an off axis effect known as coma. Star images outside of the half inch flat field are coma shaped rather than round. To put it plainly, without special corrector lenses, the 200 inch Hale telescope cannot even produce a flat image large enough to fill a 35mm negative.
MAGNIFICATION AND THE BARLOW LENS
The magnification of a telescope is found by dividing the focal length of the objective lens or mirror by the focal length of the eyepiece. If the focal length of the objective is 45 inches and your eyepiece has a focal length (abbreviated FL) of 1/2 inch (.5 inch), then your power is 45 divided by .5 = 90 power. The shortest focal lengths of eyepieces are around 4 millimeters. One inch equals 25.4 millimeters (abbreviated mm). So in a telescope with a FL of 45 inches and a 4 mm eyepiece, the calculation would by: 45 inches times 25.4mm = 1143mm and that divided by 4 mm equals 285.75 power. But if you had one of those expensive 12 inch long apochromatic telescopes, your 4mm eyepiece would only yield 76.2 power.
In order to observe detail on the planets, you need at least around 150 power and since there are few eyepieces with 2mm focal lengths, which you would need with your short refractor, you will need to employ something called a Barlow lens.
A Barlow lens is a negative (concave) lens that diverges the incoming f4 light cone into a slower light cone. A 2X Barlow will diverge the incoming light cone just enough to create an incoming light cone that would be equal to a light cone produced by an objective of twice the focal length of the telescope into which it is placed. So, if a 2X Barlow is placed into the eyepiece drawtube and the eyepiece placed into the open end of the Barlow lens tube itself, the power is doubled. Our 4mm eyepiece would now give us 76.2 X 2 = 152.4 power. To make it even more clear, the Barlow lens is located in between the objective and the eyepiece and is very much closer to the eyepiece than the objective.
Now, as we have observed, a concave mirror will converge light rays into an image just as a convex lens will. So, it stands to reason, that a convex mirror will diverge light rays much as the concave Barlow lens does. Remember that Newtonian reflector with the flat mirror at a 45 degree angle at the top of the tube near the open end used to divert the light rays being flectorized up from the big concave mirror at the bottom of the tube? If you replace that flat 45 degree mirror with a convex mirror at zero degrees to reflect diverging (or at least a less converging) light cone back down the tube through a small hole right in the middle of the big concave mirror at the bottom, you would have a Cassegrain reflector, so named because it was invented by Mr. Cassegrain.
Because the secondary convex mirror at the top of the scopes multiplies the focal length of the primary concave mirror at the bottom, the primary mirror can be ground to a much faster f ratio and hence be made into a much shorter telescope. Remember the short 12 inch refractor with the Barlow? Well this is the same idea. The curves of the mirrors must be ground into unholy shapes to avoid spherical aberration and the curves are hard to even pronounce let alone actually grind. But enter Mr. Maksutov, Mr. Schmidt and their corrector plates and you have the birth of the modern compound mirror-lens telescope known as the catadioptric.
The Maksutov (abbreviated MAK) was the first of these designs to become popular. The Meade ETX series, the Celestron 4, the Orion Starmax series, and that beautiful monument to yesterday's technology at tomorrow's prices, the Questar are all versions of this design. The corrector plate is a thick negative meniscus lens on the inside of which is "painted" a small dot of aluminum to act as the positive secondary mirror. At the bottom is a spherical mirror and the action of all these mirrors and lenses produces an f12 to f15 scope only a foot or so long; the slow optics being well suited to planetary and lunar observing. Because the surfaces are all spherical and the f ratio is slow, good quality Maksutov optics are relatively inexpensive to produce.
MAKSUTOV (MAK) DESIGN
The Schmidt-Cassegrain (abbreviated SCT) is a more advanced design which uses a thin aspheric corrector plate which can be designed as fast as f6.3 (as in some Meade scopes) and is very good for prime focus astrophotography. These scopes are typically of eight inches aperture and only about a foot and a half long. Compare this with an eight inch f8 reflector with a tube almost six feet long!
Some people have said that Schmidt-Cassegrains are less suited for planetary observing because they have a larger secondary obstruction and produce softer, less contrasty images than the Maksutovs. I personally think that the main problem is that the SCTs have to undergo a two step cool down process every time they are taken out and set up.
SCHMIDT-CASSEGRAIN (SCT) DESIGN
First when taking the scope out into the cool night air, the outside of the tube must cool down so that there are no degrading external thermal air currents to spoil the image. But, after these malevolent dragons of turbulence quieten down, the air inside the tube is still a little warmer than the air outside of the tube. This places slight thermal stress on the thin corrector plate, and while there are no longer any air currents present to degrade the image, the slightly stressed corrector plate will not completely correct for the spherical aberration of the fast f2 primary mirror. The primary mirror is not so affected because it is usually made of pyrex or some other zero coefficient glass that is unaffected by temperature. The corrector plate, since it must transmit light, is made of optical glass, however, and is somewhat more susceptible to temperature stresses. This is only my theory, but I can't think of a better explanation.
Once the air inside the tube cools down and the thermal stress no longer bends the corrector out of shape, the planets snap into very sharp focus. First, I take out my SCT and focus it on a planet and then go inside to let it cool down. When I return, the currents have quietened down but the focus must be reset to adjust for the difference that results after the corrector plate relaxes to the proper shape. After that, all is well.
MOUNTS AND DRIVES
There are two general types of mounts for telescopes. The alt-azimuth mount which is adjustable in altitude and azimuth (up and down and side to side) and the equatorial mount which has its polar axis aligned with the polar axis of the Earth. Astronomical telescopes have to be on a mount to hold them steady. The mounts also help in finding objects and tracking them. When observing an object on a scope without a clock drive, the object slowly rides out of the field of view in a matter of a minute or so. This is because of the durnial rotation of the Earth on its axis once every day. When magnified several hundred times by the scope, this movement is very noticeable. A clock drive that causes the scope to track the object is convenient for the causal observer but necessary for the photographer.
Equatorial drives are tilted so that the axis is aimed toward the North pole of the sky, which is closely marked by the pole star Polaris. East/west movement of the scope is in Right Ascension and north/south movement is in Declination (abbreviated R.A and DECL. Every cataloged object in the sky has coordinates of R.A. and DECL. assigned to them. So, if an object's coordinates are known and the mount has been properly set up, the user can simply dial the coordinates on the mount's setting circles, and the object should be in the field of view. Some mounts have variable speed clock drives and drive motors for the declination axis so that one can use a hand controller to move the telescope. There are focus motors available also so one can, in theory, aim his scope and focus it without touching it and causing it to vibrate.
There are computer driven scopes with pre-programmed sky charts in their memory where the user can just punch in the object he wants to view and the scope automatically skews to it. There are computer drives that do not even need an equatorial mount. Once its position is determined by its internal global positioning satellite system, the telescope's computer can just aim itself on its alt-azimuth mount. There are video attachments that can put the image of what the telescope is seeing onto one's TV or computer monitor. The computerized mount can even be controlled from your PC keyboard. You can observe on a cold winter's night from your easy chair inside your warm house. Your can even subscribe to cable TV and get the Astronomy channel and avoid the expense of buying a telescope at all. Oops, I guess I went too far.
But, seriously folks, how much of a mount you need is up to you. The huge mounts on some scopes assure rock solid stability. When I was younger, many years ago, we spent every cent we had on aperture, but with the rise in popularity of astrophotography, good mounts became a necessity.
Still, there is much difference in what is regarded as acceptable. My Celestron Classic black tube C8 is on an equatorial fork mount. It is so rock solid, that after I touch it to slightly adjust focus at hight power, it only takes about 10 to 12 seconds to settle down. But would you believe that is not good enough for some people who would rather spend a thousand dollars more to get a bigger mount that settles down in only two to three seconds? Go figure, but the choice is yours.
WHICH TELESCOPE REVIEW TO BUY
When shopping for a telescope, you may well search the internet for "telescope reviews." I have looked at these reviews and, while they are generally helpful, some scare me. Some are written buy newbies that are just pleased by the first views they saw through their new instrument. Others are written by professional reviewers who, shall we say, know very well who butters their bread.
One review by a newbie was explaining how sharp his views of the planets were. Then the reviewer newbie stated that the views began to break up at powers of over 100! A professional reviewer was talking about how much contrast the image had and how good the planets looked through the four inch apo he was reviewing. The price tag was several thousand dollars. I would not pay several thousand dollars for a four inch scope, nor would I recommend one to any but the very rich. I have a six inch f8 Newtonian reflector that would probably blow his four inch apo out of the sky. Another reviewer was complaining about the shakes his mount had but the problem cleared up after he added a clock drive. DUH! I wonder if he realizes he still has to touch it to focus it.
Reviews are very helpful when talking about fit and finish, construction and how well the dealer treated the buyer. How good the manufacturer backs up his warranty is something that is important to know.
Some reviewers warn against buying a cheap department store telescope. They say the optics will be so bad and the mount so shaky as to discourage the newcomer. True, the optics might be bad and the mount might be shaky, but if you can get a telescope for no more than about a hundred bucks, go for it. What ever the shortcomings of the scope are, if you are dedicated to the hobby, you will overcome them. If you enjoy yourself, you will soon be buying a better scope and because of your experience with the cheapie, you will better know what you want and what to do with it when you get it. If on the other hand, you don't take to the hobby, you will still have a fine terrestrial scope with which to watch the ships come in. Maybe one of them will be yours.
Are high priced scopes worth the money? That depends. I was watching the Johnny Carson show about twenty years ago, and Johnny mentioned that he enjoyed star gazing and that he owned a Questar, a very expensive 3.5 inch Maksutov. If I saw Johnny Carson show up at a star party, I would expect him to bring nothing less. If I had Johnny Carson's money, I would show up with a Televue 4 inch apochromat with a expensive mount and a bino viewer with a couple of Nagler eyepieces. Probably around five or six thousand dollars worth of stuff. But if I had Arnold Schwarzenagger's muscles instead, I would show up with a 14 inch Celestron SCT, also about five or six thousand dollars worth of stuff. Here is an anonymous review I found on the net for a very expensive five inch refractor:
"The FS128 is as perfect as a 5" aperture instrument as can be. The optics are superb, with perfect star test in and out of focus and excellent high contrast images in focus. Performs quite well on deep sky objects, even with the limited aperture. Performs on deep sky about equivalent to a 7" reflector.
The tube is hefty, well built and has been mistaken by many to be a 6" refractor. Although it looks heavy, it isn't, weighing in at 16lbs, it is very manageable for portability sake. It is the epitome of the optical craft, and is as pleasant to look at as look thru. The focuser is incredibly smooth and accurate. Holds focus dead on, even with heavy eyepieces. As for the finder, it's comparable to a 60mm apo in optical quality. You can actually see the airy disc and diffraction rings in it. The same goes for the polar scope. Quality optics through and through. My only pet peeve is the price. A bit expensive when compared to other 5" apos.
The EM200 mount is as smooth and solid as they get. Polar alignment is simple and positively perfect. The tracking in RA is MUCH better than rated and comes in at about 2-3 arcsec of periodic error. Widefield prime focus photos can be taken virtually unguided! For up to 20 minutes with NO star trailing. INCREDIBLE! It's almost a waste to have an autoguider port on it! The only criticism I have for the mount is that it is much heavier than it has to be and is almost overkill for this optical tube. I now have an EM10 mount under it, and it is plenty stable and enhances the portability of the setup. I use the whole setup for visual use anyway. But, I feel it will hold up when used photographically as well.
Now to the nitty gritty. I've been doing some comparisons, side by side with many other scopes I've owned and those owned by friends and other club members. First, I'll tell you that, this refractor is a marvelous instrument and against like apertures of other designs it wins quite handily on image quality. But, I've discovered as I've suspected all along that quality aperture wins.
In less than perfect skies the 5" seems to beat larger aperture Newtonians and SCT's on planetary detail, but when the skies are steady they BEAT the 128 on resolution and CONTRAST as well. The scopes I've compared to it are as follows: C8, C9.25, Meade 10"SCT, 8" Dobsonian (F6), 10" Starmaster, 11" Starmaster, and a 15" Obsession, all were well collimated and had 1/5 wave or better optics. When the seeing allowed the others won out over the TAK. Granted, 75% of the time the Tak would outperform these others, but when seeing allowed, aperture won.(In Fact it's an aperture related thing, bigger the aperture the better the image)
As for deep sky, when it was at a dark sky site, there was NO CONTEST, the Tak lost handily.
The reasons that many perceive that refractors have better contrast and tighter images are several. I will list some of them: 1) Smaller apertures are less affected by atmospheric turbulence and cut through bad seeing better 2) they also aren't affected by light pollution as much and don't take in as much ambient light, because of their smaller apertures. This darkens the sky background and is perceived to be better contrast. But, this is not the case when looking at the object itself. (i.e. planets). 3)The smaller aperture images a larger airy disc, which looks like a tiny circle, and is easier to see. 4) Reflectors, need periodic maintenance and collimation to produce high contrast images. Many are not maintained by there owners properly and often image poorly as a result. 5) The folded light path of reflectors give tube currents (caused by temperature differences), more opportunities to degrade the image. There are other reasons, as well.
My conclusion with all of this side by side testing is that, there is NO BEST telescope. All designs are excellent if properly made and maintained. That is why I own several. At least one of each type. This is the reason for a 9 rating. YES, I'm a proud owner of a TAK! But, if I could only keep one scope, it would be my C9.25. Why? Because aperture WINS! "
I have no preference for the views seen through one design or another, so I recommend you buy the biggest, affordable scope you can easily carry around. If it is portability you want, get a Maksutov. But don't pay any attention to those that say views through a Maksutov are better than other scopes of similar aperture. What astonishes folks most about a Maksutov is that you can see the belts of Jupiter and the rings of Saturn with a scope no bigger than a can of Burma Shave! If you can permanently set up a scope, get the biggest Newtonian you can afford.
ACCESSORIZE
The first accessories you will want to consider will be more eyepieces. Your scope probably came with only one or two that yield only low to medium power. To get higher power I recommend the addition of a Barlow lens. I prefer an eyepiece of no less than about 10mm and if possible, 20mm. The larger lenses are simply easier to clean. So, in my case, a Barlow is necessary for high power.
Never use Huygens or Ramsden eyepieces. They are cheap, have bad color correction and are found only on cheap telescopes, usually in the .965 inch barrel size rather than the better 1.25 inch barrel size. The lowest quality eyepiece you should consider is the Kellner family that includes Kellners and modified achromats. They give OK on axis performance for the Moon and planets. Orthoscopics and Plossls are even better corrected but cost a little more. For wide angle views consider Erfle eyepieces. If you have loads and loads of money, go for Naglers or Meade series 4000 Super Wide Angles and such with their 80 degree apparent fields of view.
The maximum power usually used is about fifty times per inch of aperture, or 2 times per millimeter of aperture. For an eight inch scope that translates to 50 times 8 (inches) equals 400 or 2 times 200 (millimeters) equals 400, so your maximum usable power for an eight incher would be 400. Anything greater than the appropriate power would probably provide an image that would be too dim and blurry.
Remember that power (magnification) equals the focal length of your primary mirror or objective lense divided by the focal length of your eyepiece. If you have an eight inch f10 Schmidt-Cassegrain then your focal length is 10 times the aperture or eighty inches, which is 2032 millimeters. If you want 400 power you will need an eyepiece with a focal length of 2032/400 equals approximately 5mm. If you have a 3x Barlow lens, you will need an eyepiece with a focal length of 15mm to get 400 power.
The lowest power you should use is about five times per inch of aperture. Anything lower in a reflector will produce a depth of field sufficient to start bringing the secondary mirror into focus. It will appear as a dark blurry spot in the center of the field of view. But, the number five is significant for another reason.
Since the pupil of an adult eye will only open to about 5mm and the exit pupil created by the eyepiece of greater than 5mm will cause some light not to reach the retina resulting in light loss (dimming), then the number five is arrived at by dividing the aperture (in millimeters) by the minimum magnification, that is to say, it is more than just a subjective evaluation, it is arrived at by a scientific approach, the mathematical relationship between millimeters and inches notwithstanding. So there.
For example, the minimum power recommended for a six inch scope would be 5 times 6 equals 30 power. For a 200 mm aperture scope the minimum power would be 200 divided by 5 equals 40, or if you realized that 200mm equals eight inches, then you could say 5 times 8 equals 40. For a 90mm scope, the minimum power would be 90 divided by 5 equals 18 power.
In the case of the six inch scope, let's say it is a f8 scope with a focal length of 48 inches. That is about 1200mm so a 30 power eyepiece would have to be of 40mm focal length. On the other hand, if the 90mm scope was an f14 Maksutov, the focal length would be 1260mm and in order to get its minimum power of 18, you would need an eyepiece of 1260 divided by 18 equals 70mm. I know of no commercially available eyepieces of that focal length in the 1.25 inch barrel size. The longest I know of is about 40mm, so it is easy to see that the Maksutov would not be the best scope to choose if you wanted low power views of dim nebulae.
This might be a good time to explore the concept of field of view (FOV) of an eyepiece. A wide field of view is, of course, better since you can see more stuff than through an eyepiece with a narrow field of view. Needless to say, a very wide field of view costs more than a narrow field of view. You can, however, calculate the maximum apparent field of view of an eyepiece with a known barrel size. We will stick to the 1.25 inch barrel. The formula you need is:
APPARENT FOV (for a 1.25 inch barrel)= 2 ARCTAN (16/EPFL)
The apparent field of view equals 2 times the arctan of the number(16 divided by the focal length of the eyepiece in millimeters)
For example, the maximum field of view of a 40mm eyepiece with a 1.25 inch barrel is: 2 arctan (16/40) = 2 arctan .4 = approx 44 degrees. For a 20mm eyepiece it would be 2 arctan (16/20) = 2 arctan .8 = approx 77 degrees. Only a very very expensive eyepiece would have a 77 degree field of view, but many have fields of view from 50 to 65 degrees. If a manufacturer claims that his 1.25 inch barrel 40mm eyepiece can give a wider field of view than 44 degrees, he his fudging. On the other hand, a 40mm eyepiece in a two inch barrel (instead of a 1.25 inch barrel) can be designed to give as wide a field of view as 65 degrees. Very nice, indeed!
For my 8 inch f10 telescope, a good choice might be:
20mm Plossl (100X)
32mm Erfle (60X)
2-3X Barlow lens (to create 120X, 180X, 200X, AND 300X)
10mm Plossl (to yield a maximum of 400X-600X in conjunction with the Barlow lens, for those rare nights of great seeing when I can use very high magnification)
Finderscopes provided with telescopes often need upgrading. The first thought is that a larger finderscope will be able to see fainter objects. I recommend a 50mm objective for a finderscope. I also strongly recommend a right angle finder. This makes finding things high in the sky much easier. I recently bought a 50mm 8 power right angle finder from Orion and it was one of the best decisions I ever made.
PHOTOGRAPHY
The upswing in the popularity of the 35mm single lens reflex camera back in the seventies, with its interchangeable lenses and attachments, created a firestorm of opportunity for the amateur astronomer. Already well versed in basic optical theory, it took him no time at all to adapt his telescope to his camera and vice versa. The recent development of digital cameras and personal computers have put an incredible arsenal of technical innovations at his disposal. I will focus mainly on film rather than expensive digital cameras in keeping with the theme of the website. (me being a poor man and all). The basics are about the same.
The simplest method of photography is to put your camera up to the eyepiece and press the shutter. Try taking your video camera out and, while recording, walk up to the telescope and aim the video camera right into the eyepiece while looking at the moon. Bring the tape in and play it on your TV for all to see. Neat, huh?. This is called the afocal technique and will work for cheap digital cameras also, provided they have an LCD screen to assist composition and focusing. Other ways of employing cameras are prime focus, eyepiece projection, and piggyback mounting the camera with its own lens-the telescope being used only to drive the entire setup.
PIGGYBACK. A camera with its own lens can be attached to the telescope for piggyback photography. The camera just rides on the telescope and avails itself of the telescope's clock drive. You look through the telescope at a guide star and adjust for drift if necessary. If you have a high enough power eyepiece and your piggyback telephoto lens is not too strong, you may be able to use the slow motion control on your telescope for guiding even if it does not have a clock drive.
The camera is attached to the telescope by means of an accessory called a camera mounting bracket. The problem now is that the telescope is top heavy. There are counter weight assemblies to balance the scope but they are generally somewhat expensive. A second camera mounting bracket is much cheaper and you can mount another camera which will also serve to balance the load. You will balance the telescope tube and be able to take another picture of the same area of the sky perhaps with a different type of film, or a lens of different focal length.
The addition of a dew shield also assists in balancing the tube.
PRIME FOCUS PHOTOGRAPHY. This technique uses the telescope optics for creating the image in the camera. You attach the camera directly to the telescope where the eyepiece would normally be for viewing. Since you are using the full focal length of the telescope itself to create the image on the film, you must guide carefully to prevent streaking star images over the many minutes of the exposure. Off axis guiding devices are available but can be somewhat expensive, and are used for only one purpose.
The addition of a second scope piggyback on the main scope can give you something to guide with or, if you wish, be employed with the camera to give a long piggyback telephoto lens . If you will examine the picture above, you will notice that on the top of the telescope is a 90mm Maksutov telescope tube that I bought mail order from Orion Telescopes in California. I found this by searching the "clearance" section and got it for $170. I can put a high power eyepiece in this scope and use it to guide on a star while my camera is at the Celestron's prime focus.
EYEPIECE PROJECTION. In this technique the eyepiece is used as a projection lens, like with a movie projector, and it projects an image onto the film plane of your camera. The camera is held up to the eyepiece with the camera lens removed.
THE CAMERA IS TO THE RIGHT AND THE EYEPIECE ON THE LEFT IS ATTACHED WITH AN ADAPTER TUBE
This technique is employed for taking pictures of planets or close up views of the Moon. There are attachments available for holding the eyepiece and camera in the telescope, but firing the shutter will often cause vibration. The way I avoid this is to put the camera on its own tripod and simply move it into position in front of the eyepiece. Be careful not to let any ambient light sneak in, center the image of the planet and focus the telescope. The exposure times run anywhere from one to five seconds depending on the magnification and the speed of your film. I recommend using the fastest film you can. Grain has not been a problem for me. I use color print film of ASA400 to ASA1600, with cheap chain store film of ASA800 being suitable.
If you can see Polaris, then either use the polar alignment scope in your mount and follow the directions that came with your scope. If you don't have a polar alignment scope in the right ascension axle, then set your scope to 90 degrees declination, and use your finder to center Polaris. If you can not see Polaris to align your mount, then here is a quick close, one star setup.
Assuming you have adjusted your mount's latitude setting the day you first got your scope then you simply must make sure the tripod is level. My Celestron C8 has a bubble level in the wedge for that purpose. Once I have leveled the scope, I pick a star for which I know the declination (not too near to the meridian). I adjust the telescope to that declination. Then I slew to it and using only the azimuth and right ascension controls, I center the star. This is a pretty close alignment and will do for all but long deep sky exposures.
TELESCOPE Q AND A TO HELP YOU ARRIVE AT A PURCHASING DECISION
NEWBIE: I would like to purchase a telescope to enjoy stargazing. Which one is best.
OLDTIMER: Get the biggest scope you can afford that is portable enough for you to handle.
END OF Q AND A SESSION
But seriously folks....
OLDTIMER: Large scopes must be Newtonian reflectors in order to be affordable. Ten inch Newtonians are available and are capable of gathering quite a lot of light for dim deep sky objects and have the aperture needed to resolve fine lunar and planetary detail.
NEWBIE: But, I have heard so much about refractors giving superior images because of the lack of a central obstruction.
OLDTIMER: Don't believe it. The higher contrast, more relaxed image is more the result of slow f12 to f15 optics rather than the lack of a central obstruction. The Maksutovs have large central obstructions, but yield "refractor like" images. What they share with refractors is (1) a slow focal ratio, nearer to f12 to f15 and (2) like refractors, Maks very rarely need to be collimated. The glare seen in fast Newtonians (f5 or so) is the result of the fast focal ratio which produces a fat light cones which causes the image to be surrounded by glare. And since both refractors and Maksutovs are always perfectly collimated, comparisons with Newtonian reflectors and Schmidt-Cassegrains that are sometimes not perfectly collimated are quite possible. Hence, the misconception that refractors and Maks are better for planetary detail. Viewing fine planetary detail tasks a scope to its limits and any misalignment of the optics can easily show up.
NEWBIE: Can you explain why some scopes are suited for deep sky observing, and others more for planetary observing?
OLDTIMER: The faster the optics, the greater the surface brightness of a nebula for photographic purposes. But, an f5 Newtonian will not be able to completely fill the frame of a 35mm SLR camera. The one exception is the Schmidt-Newtonian design being sold with the LXD55 mount by Meade. It is an f4 system with a corrector plate to accommodate deep sky photography with shorter exposures. Unfortunately, the faster the scope, the shorter the focal length tends to be, requiring shorter focal length eyepieces for high power, and as I said before, resulting in glare due to the fast optics and the fat light cones.
NEWBIE: Must I buy two scopes then, one for deep sky and one for planetary work?
OLDTIMER: No, you can compromise. The Schmidt-Cassegrains like those sole by Celestron and Meade are excellent all around scopes, and despite rumors to the contrary, can resolve detail as fine as any eight inch scope of any design. The bad reputation that SCT's have is partly due to the fact that so many of them have been sold to so many amateurs that have let them get out of alignment (or have never aligned them at all) and partly due to performance claims by extremely high priced telescope manufacturers. These claims are just advertising puffery but, over time, start to be regarded as fact. When properly aligned and properly cooled down, SCT's give wonderful performance. Any glare you see is the result of the large quantity of light being gathered by an eight inch mirror and lower than optimum magnification being used.
Any smaller scope with good optics will give what appears to be a more contrasty image of the planets, but in fact, the smaller scopes are just rendering smaller, darker images. Sort of like comparing a 27 inch TV to a small 13 inch TV. The smaller TV looks clearer, but upon closer inspection, it is just the same. An image through my 90mm f14 Maksutov of a planet looks very sharp and contrasty, but upon closer inspection, one can easily see that the eight inch scope is resolving much finer detail. But if you can not afford an eight inch SCT, then I would recommend an f8 six inch Newtonian reflector. They are making a comeback, and are amazing a new flock of telescope reviewers too young to remember when the 6 inch f8 Newtonian was "king." If you put a $500, 6 inch f8 Newtonian alongside a 6 inch $10,000 apochromatic refractor and look at Jupiter or Mars, there will be very little difference in image quality.
NEWBIE: That is OK for planetary views, but what if I want to view a large nebula with a 6 inch f8 scope?
OLDTIMER: The lowest power you can visually use is a function of aperture, not f ratio. The philosophy to which I adhere says the aperture in inches times the number five, or the aperture in millimeters divided by the number five yields the minimum magnification. For example, the lowest power to use on a six inch scope is 6 times 5 equals 30 power. An f8 six inch scope will have a focal length of 48 inches, which is approximately 1200 millimeters. The focal length of the eyepiece you will need for minimum power is 1200 divided by 30 equals 40mm. 40mm eyepieces are readily available and are not very expensive. You can have your cake and eat it too.
There is another school of thought that says the minimum magnification is three times per inch of aperture. For a six inch scope, this would result in 18 power. At this low power, the central obstruction will start to show up as a dark, blurry circular shape in the middle of your view. I would stick with the five times formula.
NEWBIE: But what about wide field views? Isn't the maximum apparent field of view of a 1.25 inch 40mm eyepiece only about 44 degrees, going by your equation from several paragraphs above?
OLDTIMER: Ok, ok, it is a compromise, but in my opinion, a good one. Nothing beats a Maksutov for a nice looking, relaxed planetary image (due to its f14 optics). Nothing beats a fast Newtonian for deep sky observing (due to its fast f5 optics). But the planetary image of the Maksutov, while more relaxed, reveals no more detail than the faster Newtonian, and while the f8 scope will not give you wide fields of view at low power, you can examine the fields of view in sections with your 40mm 44 degree eyepiece.
NEWBIE: What about department store telescopes?
OLDTIMER: I am not as opposed to them as some people. If you can get one for about a hundred dollars, go ahead. Actually, the 4.5 inch f8 Newtonians that you can get for about $150 aren't bad if you can get one with a 1.25 inch eyepiece holder so you can upgrade to better eyepieces. When I was young, I started out with a 2.5 inch reflector that could not even see belts on Jupiter, but I watched the moons night after night and as the seasons advanced I found some nebulae and star clusters. This gave me occasion to learn the constellations, and now I do not need one of those "go to"gizmos.
NEWBIE: How about the Orion catalogue?
OLDTIMER: There are some nice scopes there. I will review the ones I like from the smallest to the largest.
NEWBIE: You said the Starmax 127 Mak should make a good planetary scope, but doesn't that contradict what you said about different designs being no better than others?
OLDTIMER: Since it will always be perfectly aligned, the Mak can be relied upon to give theoretical performance limits required for the planets without much maintenance. That's all I'm getting at when I say it will be a good planetary scope. Other scopes can be used for deep sky observing without being well collimated.
NEWBIE: Is there any reason for choosing one type of scope over another?
OLDTIMER: OK, here it is. Newtonians are the cheapest. Equatorial fork mounted Schmidt-Cassegrains and Maksutovs have the advantage of the eyepiece always being right there for you. A great advantage for eyepiece projection photography. When these scopes are mounted on German equatorial mounts, this advantage disappears and with a GEM the scope must be swung around to and fro to aim at different locations in the sky. Of all the scopes I have used, I greatly prefer the equatorial fork mounted Schmidt-Cassegrain, for all types of observing. Your budget and physical ability to move it to your observing location should be your only limiting considerations.
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