I have already written a bit about planetary and general high resolution observing but I would like to encapsulate the essentials here for those who need a quick picture before actually purchasing a telescope or beginning a telescope assembly or design/construction process. It is my firm belief that most amateur astronomers become over-concerned with items that have relatively little impact on high resolution observing. I see many spend their money where it doesn't help deliver the best image, particularly new comers who must feel overwhelmed in the sea of gizmos and doodads, both electronic and optical, dangled before them. The fact is that most of these attachments and appliances are of relatively little value and do almost nothing but empty one's wallet. And since most of us have a limited budget I'd like to help those who just want an exceptionally good visual scope and want to get a really good view of the planets, moon and binary stars without going broke.
The Kind of Telescope It is my belief that a reflector with a good optical train and a sound design will work as well as a refractor. A reflector can always be made with a larger objective and relatively shorter tube than a refractor. Why serious interest remains in refractors for instruments over four inches aperture is a mystery to me. They are very expensive - far more expensive than a reflector of, say, two inches more in aperture made to exacting standards with first rate materials. The problem with most reflectors is that they are made with substandard materials and then compared - unfavorably - with refractors. No big surprise. Spend a little money in the right places and a reflector will give you the best image you can get with any telescope. Also, don't expect a 10" reflector four feet long to work like a 6" refractor four feet long. Please be reasonable. Please try and keep things in proportion. A bigger telescope has to be proportionally longer than a little scope, but a reflector will always have the edge in compactness.
The Objective Mirror and Secondary Mirror To have a first class telescope you need to have an excellent primary mirror, the heart of your telescope. Without this, you have nothing; everything else becomes superfluous. If the mirror can not deliver a wavefront image of Strehl .95 or better (nominal 1/8 wave), you will not see things at their best. Buy a mirror from me and you can be sure that this condition will exist. A 10" f/6 mirror from me will cost about $800. Now, go out and price an 8" apochromatic objective. After you catch your breath realize that an 8" apochromat will deliver an inferior image when compared to a 10" f/6 parabolic mirror made by me. Think about that - please. If you can get beyond this you will have the beginnings of a great telescope. Fine telescope mirrors have to be made with great care and with no concern about anything except producing the best product that will deliver the ultimate image. Of all the optics I have made for amateurs as well as professional, industrial or governmental use, nothing is more challenging than making a good telescope mirror. They require the most exacting standards; the finest geometric figures, the smoothest surfaces. Such work takes time, and, yes, even devotion. They don't come off a production line.
The diagonal secondary is a bit easier to make; its smaller and not aspheric; but it is not be despised and treated offhandedly. You need to get a good one or my work will be destroyed! It needs to be as good as the primary (See CALCULATING THE SIZE OF YOUR DIAGONAL and The Impact of Diagonal Surface Errors) and it needs to have a good coating. Mounting is also very important. I strongly recommend Protostar for the best diagonals at any price. I and my customers have had excellent results with these diagonals and their spider supports and mounting cells.
Eyepieces and other Essential Accessories If the mirror and secondary is where the amateur tries to skimp, the eye end is where he loves to spend money. What feels better than a new eyepiece in your pocket at the trade show? Right! The reality is that it will largely do nothing that the ones you have already do. Look, this is like addiction treatment. We live in a world where we are tantalized at every turn, from fast food to new cars. It just feels so good to spend a little money. We're a nation of box openers and shrink wrap rippers. If its packaged well it must be good. Unfortunately this does not work with astronomy. I suggest you save your monster eyepiece money and buy one really good mirror. I have bought very few eyepieces over my life time. I routinely use three or four Plossls, and two Barlows for an evenings work - that's it. Large, multi-element eyepieces have been designed for low to medium power wide field work, not for high resolution observing and high powers. They have too much glass, too many surfaces and scatter too much light. Not good for the planets.
My approach to eyepiece use is based upon an understanding of how optics work. Higher powers are best developed using a Barlow lens. And you do not need a very powerful one, a 2X or 2.5X Barlow is very good. Higher power Barlows are very sensitive to misalignment and have been reported to deliver less than desirable images. Nor is an apochromatic Barlow necessary. An ordinary two element achromat is plenty good enough. Barlows don't work that hard. And they should not be made to work hard. The basic idea of a Barlow is to gently bend the incoming rays outward so as to effectively slow down the primary mirror. An f/6 mirror optically becomes an f/12 or f/15 mirror. You can see now that the f/6 mirror takes on the characteristics of a slow classical refractor. The rays converge less obliquely, the mirror imaging system is now more relaxed and the eyepiece refracting system can more easily handle the job. You need to have less aberration correcting power built into the eyepiece - in other words, the eyepiece can be simpler with fewer elements, fewer surfaces, less scatter. What I have just described is the very essence of understanding the optical essentials of producing a fine stellar image. Keep the elements as few and the rays bent as little as possible. There is no more to it than that.
Bearing this in mind, a 10" f/6, or 8" f/6, is maximized for high resolution observing with the following accessories: 25mm, 12.5mm and 8mm Plossl eyepieces; a 2X and/or 2.5X achromatic Barlow; and a set of color filters such as a are sold by Orion or similar distributors. With these the following magnifications on a 10" f/6 can be realized:
Eyepieces alone: 60X, 120X and 190.5X
Eyepieces with a 2X Barlow: 120X, 240X and 381X
Eyepieces with a 2.5X Barlow: 150X, 300X and 476.25X
If you want to go crazy on a very good night and split a difficult double, pile the Barlows on top of each other and you can get up to 952X. And, believe it or not, with a good mirror, on a very good night, a star will hold its image! It won't be pretty, but it will remain as a well-defined spot and not explode into a fuzz ball. Try that with a mediocre mirror - it won't work.
Lastly is the business of filters. On planetary and lunar work they are essential to really good observing. I use orange on the moon, it seems to increase contrast but mostly it reduces the amount of light. If you have too much light entering your eye it essentially becomes stunned and unresponsive. Even the planets are mostly too bright. Since the moon is a monochromatic gray, after a while the obtrusive orange appearance will begin to disappear and you will seem to not notice much of anything except the nice flat and sharp lunar features. You should feel that the lunar landscape near the terminator is razor sharp and exciting to look at, not dull and soft. Planets benefit from specific colors: Jupiter reveals much with a blue filter, Saturn is especially beautiful with a yellow-green filter and Mars with blue and yellow and orange filters is good for showing clouds and enhancing surface features. Much has been written about this subject, but, sadly, I see almost no one using filters on the planets. Magnifications used should be moderate for planets, for a 10" about 200X to 250X. As discussed above, double stars can stand a lot of power when required for splitting a close pair, but their beauty dissolves with increased magnification. The moon, being an extended object can handle fairly high magnification, perhaps as much as 500X with a 10" on a very good night and 250X on most any night
Thursday, December 4, 2008
What you really need to get the most out of high resolution observing Without wasting your $$$ resources.
at 3:39 AM