EBAY SCOPES - PART THREE - TO BUY BIG OR SMALL? : eBay Guides

IN THE LIST OF GUIDES, THIS PART APPEARS BEFORE THE OTHER 11 GUIDES. DON'T BE CONFUSED, IT'S JUST POSITIONED ON HOW MANY PEOPLE LIKED IT.

FIRST THING TO DO IS FLIP TO THE GUIDE 'EBAY SCOPES OF 1ALIENX - START HERE' AND START FROM THERE. THEN FOLLOW THE GUIDES IN PART ORDER.

AFTER READING THIS PART, FIRSTLY YOU WILL BE MORE CRITICAL THAN BEFORE AND BE ABLE TO THINK AHEAD ABOUT YOUR NEEDS. SECONDLY, YOUR EXPECTATIONS OF WHAT YOU BUY WILL BE REALISTIC. MANY THINK THAT THEY WILL SEE FOOTPRINTS ON THE MOON AND PLANETS IN ORBIT AROUND OTHER STARS. NO WAY !

There are various questions that you should be asking yourself before buying.

Please have in mind that astronomical telescopes produce an upside down image and that is perfectly normal for them.

DOES THE COLOUR OF THE TUBE MATTER?

Probably the best colour is Hammerite silver grey. That balances heat absorbtion and heat reflection quite nicely. Some scopes are sold according to the colour of their tubes. That is OK if, for example, the white one has better optics than the pink one that has better optics than the mauve one, and so on. If the listing is just on the colour of the telescope tube, you should ask the seller about what is inside it, unless you want to look at it and not through it.

WHICH SIZE TELESCOPE SHALL I BUY?

A scope with the bigger lens or mirror will show finer details on the surface of planets, fainter stars (higher magnitudes – a magnitude13 star is a lot fainter than a magnitude 10 star) and separate double stars better. The bigger aperture takes higher magnification than one with smaller main optics because the bigger ones have a bigger light grasp, assuming that the bigger and smaller scopes are of the same quality.

Just to set you right, the faintest star you can see with your naked eyes is a magnitude 6. A magnitude 7 star is about 2.5 times fainter than a magnitude 6 star which is about 2.5 times fainter than a magnitude 5 object.

Better quality scopes are always more expensive than the mass-produced stuff, and the bigger the optics, the bigger the price.

Doubling up on aperture produces a four-fold increase in the light received and doubles the resolving power which is the ability to separate two close points – expressed in fractions of a degree or seconds of arc.

Suppose you buy a scope with double the aperture than another, what does that mean practically? Volume-wise, your scope is now 8 times bigger (2 x 2 x 2) compared to (1 x 1 x 1), and the weight has increased. The mounting must be that more robust and consequently heavier and more expensive. Very quickly, you will find that the bigger scope you contemplate, the less portable and the more expensive it will be.

Let us say that a good quality 4 1/2 inch reflector on an equatorial mounting with tripod costs £180 new (solid mounting and a spherical f/10 mirror). On that basis, the 6 inch would cost around £380 (parabolic mirror), the 8 inch about £800 (parabolic mirrors from now on), the 10 inch about £1400 and the 12 1/2 inch something like £2600. The 14 inch would be £4650, the 18 inch £10500 and the 24 inch, £26000. Anything bigger, you will need to be in a position where you run seven Ferraris of different colours, one for each day of the week.

An f/5 24 inch mirror involves a tube length of 10 feet and the equatorial mounting for that is going to be a specially made one-off and cost roughly £16000 with all bearings, worms, wheels and computer drives.

Why such a hike in prices? Leaving alone a progressively more robust mounting, the mirror blank has to be accurately ground away to support a parabolic film of reflecting surface and each bit of the surface has to be as perfect as possible. That means the whole area of the mirror surface has to be near perfect.

The areas in square inches are:   four and a half inches, 15.9.    Six inches, 28.2.   Eight inches, 50.2.   Ten inches, 78.5.   Twelve and a half inches, 122.7.   Fourteen inches, 153.9.   Eighteen inches, 254.4 and twenty four inches, 452.3 all with as near as perfect a parabola over every bit of the surface. That is, the 24 inch mirror surface area is over 28 times the area of the 4 ½ inch mirror.

452 square inches is the area of a slab measuring over 9 inches wide by 4 feet long that carries a curve accurate over those 452 square inches down to a microscopic level measured in wavefronts of light.

Also, there is no point at all having anything bigger than an 8 inch reflector unless the mirror is hand figured to at least one-eighth wavefront – meaning 40 + hours of work on the spherical mirror with optical testing very, very frequently during the hand figuring to a parabola so as not to overshoot the required focal length and get it as near as possible to the correct wavefront and a Strehl of 0.990. Compare that with the 114 mm spherical mirror where the blank is put on the machine and you come back after coffee to pick it up to put it with others into the aluminizing vacuum chamber that deposits an aluminium coating on it a few thousandths of a millimeter thick.

The question you should ask yourself is 'how big do I need'? As an indication, take these scopes on their mountings with tripods or piers and couple them up with what they cost:

60mm refractor. You can carry it with one hand.

3 inch refractor (76mm). You should use two hands.

4 inch refractor. It will take two people to move it for safety.

6 inch refractor. Two or three strong people are needed otherwise try to do it yourself and prepare for the hernia operation. First make an appointment to see a plastic surgeon, then use a brush and pan for the pieces after it has toppled over with you underneath it and the business end of the finder has lodged itself in one of your nostrils.

6 inch Newtonian reflector, use two hands.

8 inch Newtonian reflector. Consider making a platform with wheels on which to stand it.

10 inch Newtonian. Definitely use wheels. A 10 inch f/4.8 tube can weigh around 30 lbs (12 kg). An EQ6 equatorial head on a steel pier with three feet weighs about 45 lbs (20 kg) and the counterweights on the Dec shaft of the EQ 6 weigh around 24 lbs (10 kg). Try to carry that lot all set up and it will topple because it has a centre of gravity about waist height.

12 inch Newtonian. Don't plan on taking it to star parties without a truck or a big estate car. The disassembly and reassembly will take about 15 minutes. Good vibration and impact protection is essential during transport preferably, made to measure padded crates.

14, 15 or more inches. You are not strong enough and neither are your friends. Keep it on a fixed site. Then, how do you intend to protect it from the weather?

Of course, short focal length scopes on Dobsonian mounts very easily separate into two sections. Please see the First Guide about the disadvantages of the Dob. mounting.

Generally, for the English climate (seeing, etc), anything more than a 10 inch reflector is a waste of aperture. If you live in a desert, go for at least a 24 inch computerized reflector and send me an invitation to stay with you for a few months each year. You will be my friend for life.

ARE THERE ANY ADVANTAGES TO HAVING A SMALLER TELESCOPE [3 INCH (76mm) LENS,  41/2 to 6 INCH (114mm to 150mm) MIRROR]?

You are better able to use smaller magnification and see wider areas of the sky - wide field. Rich-field telescopes are made to do exactly that by having a short focal length mirror of lens that works at anything between f3 and f4.5. It is then that much easier to find star clusters you want to observe or image. Also, you will be amazed when you turn it towards the Milky Way using a maximum magnification of eight (x8).

The rich-field refractor is going to cost you ‘big bucks’. To get one of these that actually works very well, you should be thinking of a Vixen, Tele-Vue or similar, and be prepared to spend £1000 minimum for a used optical tube with no mounting.

For a rich-field refractor something like an 8 inch f/3, I would want to go to Orion Optics and place a special order and pay the price, if they were willing to make it for me. No way would one be bought on Ebay.

Construction-wise, the smaller telescope will have a fairly short telescope tube. A shorter tube is more likely not to flex. In any case, any telescope tube should be totally inflexible and be made of a material that does not vibrate (pvc, wood, etc). Personally, I loathe aluminium (aluminum) telescope tubes, especially the cheaper ones with a big outside weld along the length. Tap it and it rings like a bell and the vibrations can take over a minute to subside. If its inside is sprayed with a rubber solution and then thin soundproofing material is stuck on, much better.

At very high magnification, every slight 'twitch' produced by wind, vibrations or inaccuracies in the equipment will make the image 'dance' or, you will lose the image entirely. Highly frustrating when you spend most of your observing time fighting the equipment rather than ooing and aaing at what you see. Invariably, as soon as you have a fairly steady image, it clouds over.

The bigger scope is that much more sensitive to atmospheric conditions and suffers enormously when the seeing leaves a lot to be desired.

Air refracts (bends) light. If the air is moving, that movement will disturb the image (that is what is meant by the quality called 'seeing'). The smaller scope is not affected by this as much as the bigger scope. Also, the smaller the magnification used, the less you will be frustrated by bad seeing. Keep your scope pointed away from stars near the horizon where you are looking through the thickest layer of air. In bad seeing conditions, try to keep the scope pointing upwards at an angle at least 50 degrees above the horizon, in fact, the steeper angle, the better.

That is why observatories are built on mountain tops. Most of the atmosphere is below them. Light wind is not a problem if the mounting, pier or tripod are good. Even so, a wind speed sufficient to buffet the scope and mounting will frustrate your viewing session.

Put your scope on grass or earth. On a warm day, the stone patio will absorb heat. At night, it will give up its heat. Any scope placed on the patio will be surrounded by warm air rising that will affect the image.  Also, let your scope cool down for one to two hours from sunset by leaving it outside and please do not leave it by a window in direct sunlight where the whole thing can get warm. You can get a similar distortion by having the scope indoors in front of an open window. Heat escapes from the house and the warm air escaping will cause turbulence of the air in front of the scope.

There is a lovely story about two astronomers appointed to have ‘first light’ at a then new American observatory where the giant reflecting scope took years to make and many millions of dollars. Just after dark they decided to look at Jupiter just see how well it performed, opened the shutters of the dome and rotating the whole dome, swung the scope around until Jupiter was on the cross-hairs in the finderscope. They  put their eyeballs to the main eyepiece and burst into tears, grown men crying for some minutes. The scope was a disaster on an equatorial mounting. The image was so broken, it was worse than the worst Ebay rubbish you can buy. They went to the lounge and did not speak. After a few hours, one said to the other that he was going home. The other suggested they give Jupiter another go before filing their report in the morning. In the eyepiece, Jupiter was a magnificent spectacle showing its belts with absolute clarity. They both cried again with relief, and, for several minutes.

You should always let your scope cool down which that one did during those few hours with the dome shutters open letting in the night air. The moral of that is, being over enthusiastic and forgetting the basics makes you cry.

Lastly anyone wishing to be a serious observer should not contemplate anything less than a 3 inch refractor or a 6 inch reflector (and their metric equivalent measurements).

In Part Four, you will read about things like how to keep things clean, can I look at the Sun and a few more topics that may eventually interest you.

At the very end of Part One, there is an aside, so I might as well mention another. If anyone is interested in biophysics, I'd certainly appreciate some feedback on three consecutive papers published on the web. The website will need inserting into the very top box of your Google browser but giving you it fully would break Ebay rules. It is

http://three w's/angmalta.net/clients/alan/existence/