What we are afraid of, is that we spend all that time and money buying a telescope, and then we discover it’s not “right” for us, and our enjoyment goes with that, doesn’t it? We’ve got to know what Telescope is the best one? The amazing thing is, that despite all the modern day appliances and conveniences that “save” us time, between that, catching up with the news and family, extra-curricular activities with the kids, the latest series on TV and those few other things, we want our recreational time to count and be rewarding! Now, a disclaimer – I already wrote previously about that Astronomy isn’t an instant gratification hobby – like a cheeseburger. It’s more like a week long wedding feast. Sadly, some, expecting a cheeseburger, with wrong expectations, don’t last to the entrée.
Astro’s Blogs try to do more than give just give a straight answer (oh, alright, we do that too – skip to the bottom if your rushed!), we try and give you the background and the reasons that count. So, a best first step is to look at the different types of telescopes, and look at the strengths and weaknesses of them. Of course, there’s little sense doing that unless you know what those weaknesses mean, right? So, let’s look at some of the weaknesses of telescopes first. I’ll tell you right now that there is no “one telescope that is brilliant at everything!” so I’m sorry about that. It’s like buying a car – you are going to need to make decisions based on what is most important to you. This post and the next will examine Telescope Advantages and Disadvantages – (AKA – “Annoying things about various telescopes”) – and we will start with the problems associated with various telescopes.
Telescope Strengths and Flaws Comparison
|Type of Telescope||Picture||Advantages||Disadvantages|
|Achromatic Refractor||Inexpensive, sharp views, solid design and durable. As a refractor, it offers contrast-rich images.||Chromatic aberration evident in f/5 to f/8 designs especially|
|ED/APO Refractor||Great for deep sky-imaging / Astrophotography, bypasses most aberration problems||Very high cost per cm of aperture.|
|Newtonian Reflector (e.g. Dobsonian)||Cheap. You can use a Dobsonian mount with it, which is just made of wood and is also inexpensive.||Due to the wide apertures, you may well get coma at f/5 and below especially. It's a given that you need to collimate this design now and then.|
|Schmidt-Newtonian||Lovely, wide field of view, Coma problems are reduced in this design.||There is a rather large secondary obstruction. Hard to get - they are all out of production.|
|Schmidt-Cassegrain||Compact, you can even attach a camera and do some imaging with it||There is a rather large secondary obstruction|
|Maksutov-Cassegrain||Compact, sharp optics, long focal lengths available. Particularly good for planetary viewing.||Slow focal ratios common, narrow fields make wide-views difficult or impossible. It has a very thick mirror which means long cooldown times.|
|Maksutov-Newtonian||Wide, flat fields with a sharp view and good contrasts. The central obstruction is smaller than the Mak-Cassegrain design||Heavy, especially at larger apertures and this also means long cooldown times. On top of that, they are hard to get and expensive.|
What does it all mean? Let’s explore.
This is false color around bright stars and planets. Jupiter will have a yellow green tinge, surrounded by a violet haze for example, or the moon has a blue and yellow glow around the outside. It happens because colors that pass through a lens are are split at different angles. As a result, colors are not focused to the same point.
It can be reduced or entirely eliminated by doing various things. Firstly, it can be reduced by coating the optics. In fact, the coatings used on optics have a huge effect on reducing these sorts of aberrations. Coatings come in various grades. “Coated” is the least effective. “Multi-Coated” is good, “Fully Multi-Coated” is the best. Look for those key words. Secondly, special glass (“ED” – Extra Low Dispersion glass) can be introduced into the optics which has natural properties of supressing such effects. While natural fluorite is the best, it is also unbelievably expensive, so much so that “artificial” glass has been made to try and imitate some of the characteristics of natural flourite while getting costs down. Known as FPL-55, it is very effective. Lesser grades include FPL-53, FPL-51 and so on. Any Fluorite (natural or not) glass adds greatly to the cost of a refractor. Thirdly, the focal length can be increased, which reduces its effects. They discovered this last detail in the 1800s, and one famous telescope was made 40 meters long, though it had a very narrow view! These days, you can get a fairly well colour corrected telescope even in shorter focal lengths. Fourth, by introducing additional lenes into the telescope. By adding one extra (doublet), or two (making it a triplet) low dispersion lenses, chromatic aberration can be reduced or eliminated almost entirely, making it a “APO” (apochromatic) telescope. In real terms, telescopes that are ED, APO and feature triplet lenses tend to be expensive.
Cost per cm of aperture
This means better telescopes will cost you more. In astronomy, aperture is “King”. There is no substitute for size. Some telescope designs are simply more expensive than others. A reflector is cheaper than a refractor. You “get more for your money”, strictly speaking with a telescope that has a low “cost per cm. of aperture”.
You want aperture. I’ll explain why. Now, the difference between a 70mm wide telescope and a 90mm wide telescope might not seem to such, but those extra 20mm actually allow in far more light. Let’s take two telescopes and assume you are using a 25mm eyepiece. One is 70mm, the other is 90mm. They are both 500mm in length. The 70mm one has a light grasp of 100 with a limiting magnitude (this means the faintest star you can theoretically see, higher numbers is fainter!) of 11.7. The 90mm one, has a light grasp of 165 with a limiting magnitude of 12.31See http://www.csgnetwork.com/telescopemcalc.html to run the simulations – this website is not secure. Now, you might see the first number, and see the difference is impressive. That’s 65% more. What about the second number though? The difference between 12.3 and 11.7 doesn’t seem that great, right? Wrong. It’s a difference in stellar magnitude of .6 – That means you can observe 174% 2291 % is the average increase of stars, 291% x .6 is 174 more stars than before! What difference does 174% make? In this case, 600,000 stars and many extra deep space objects. Of course, if you’ve got money, who cares? If you don’t – this will limit your ambitions.
Certain telescope designs can only go up to a certain aperture (width) and still be effective. This is just physics and cost, really. The really huge telescopes that you see are never refractors, always reflectors with a mirror at the bottom. This is because after certain practical limits, some telescopes develop such flaws (for example, they become extremely heavy, ridiculously long, or have such optical problems) that inventors need to be incredibly creative and engineers exponentially more skilled to get around them. So, if you want to see certain things, you might need to accept a lesser telescope with certain flaws than go for that “one that you wish” which simply isn’t feasible given the technical realities of that design. One important
Coma at the field edge
Some Telescopes are prone to “coma”. A ‘Coma” is a defect in an optical system which results in the image of a point system appearing as a blurred pear-shaped patch with a flared appearance resembling a comet”3Maddison, (1980), A Dictionary of Astronomy, p.36). What does that mean? Well, imagine you are looking at a lovely cluster of stars, concentrated at the centre, and exploding out towards the edge of your view. At the middle, you see things very clearly, stars are like pinpricks in clarity. Slowly, as you look across your view, you notice the stars on the outside are not as clear. They flare, and have a tail. You adjust your focus, but no matter how much you do, how delicate you are, you can’t seem to get rid of it. That’s a coma. It’s common in fast reflector type telescopes, though the situation can be improved or eliminated with a “coma corrector”, which of course, is expensive.
Some Telescopes need collimation – some regularly. Collimation is a procedure users of certain telescopes need to do to align the optical system so that they render a beam of light parallel. If it isn’t collimated right, what you see goes fuzzy. How bad of a disadvantage is that? Well, at the beginning, it can be very annoying to learn. A dobsonian for example, actually has two mirrors that can be shifted. So, collimation tends to be an iterative affair (that means, you do one, then you do the other, then you go back to the other one and get that a bit better, then you go back to the first one which is now out a bit, and so on). So, how accurate does collimation need to be? The larger the aperture of a newtonian, the less forgiving it is and the better your collimation practices need to be. Newtonians need to be collimated now and then, and more often if they are moved. Other designs must be collimated, are done like that out of the factory and shouldn’t be touched again. With some designs (though not newtonians, where regular collimation is just expected), if it is out of collimation, you can’t collimate it yourself. Drop a scope or binocular – and it may do a lot of damage – collimation just one of them.
Some telescopes have a central obstruction – Have you ever seen the top of a dobsonian? Or all those “Schmidt-whatever” telescopes? You know, those things on top that seem “in the way”? Well, they are in the way. There are various views on the effects, but generally a loss of contrast seems to be one of the major ones. You can’t get rid of it, it’s just the way it is, and don’t let it bother you, it’s just the way that telescope design is. The consensus among most users? “Don’t worry about it” seems to sum it up.
Slow Focal Ratios
Some Telescopes have slow focal ratios. Focus we of course understand, when your eye focuses on something, right? Slow focus must mean it takes ages to focus correctly, right? Uh – no. A focal ratio is the ratio of the focal length of a lens or mirror to its aperture. Imagine a telescope 100mm wide, and 1.25m long! To determine the “focal ratio”, you get the 1250 (length), divide it by 100 (aperture) and get? 12.5. So, that is f/12.5. That’s a very “slow” ratio. So, what’s wrong with a slow ratio telescope? Well, for starters, they are long. That means they are more annoying to store and move. The thing they sit on (the mount) also needs to be better, as it is harder to slew around and control. The other disadvantage is that wide-field views are more difficult (at times impossible) to achieve. That matters because some objects are distinctly unimpressive if viewed too close (You did read the magnification blog I wrote, right?). If you have ambitions of connecting a camera to your telescope, a slow focal ratio will cause you problems. Long light gathering periods are crucial for astrophotography. Slow focal ratios mean the object you are tracking needs to be photographed for longer with an open shutter, which introduces more tracking errors and more difficulty in getting the images you want. Advantages of a slow focal ratio include reduced color abberation and coma.
Fast Focal Ratios
Some Telescopes have slow focal ratios. So, say you have a telescope that is 100mm wide, and is 500mm long. You get the 500 (length), divide it by 100 (aperture) and get? 5. So, that is f/5. This is a “fast” focal ratio. Advantages? Well, it’s a short. It’s not heavy. You can just grab it, and go out and look at the stars! (Now you know where the expression “grab and go” comes from!). It doesn’t need a heavy mount. You can view wide sections of the sky very well. Problems? They have more aberration of all kinds. Chromatic, spherical, coma and so on (hover over those to use our glossary). To combat these, techniques need to be applied to them that are expensive, such as full multi-coatings, special ED or APO (Fluorite or “FL glass”) parabolic primary mirrors and so on. High magnifications are harder, eyepiece flaws really show up and they are usually more expensive.
Some Telescopes have narrow fields. No, we are not talking little strips of pasture that cows struggle to get through – a “field” or “field of view” means the area that is visible to a telescope in a setting. As I explained in a previous magnification blog, some of the most exciting views are of wider objects. If you only see them up close, you just won’t enjoy them as much. If you are talking other planets, narrow views, for that matter, even coma – who cares?! They will be fairly tiny anyway in your view. But some things, you really want those wide, sweet views. This is where certain telescope types are not very suitable if that’s what your looking for.
Some Telescopes have long cooldown periods. It’s chilly outside, but you see a clear, dark sky. Perfect, you think. You take the telescope from the nice warm comforts of your home, and set it up outside. You take your first view and shake your head. “What’s wrong with the thing!” you cry out. Probably you didn’t account for the time it takes for the temperature of the insides of your telescope to become exactly the same as the temperature outside. Imagine you heat an apple up in an oven. You then put it outside. After a time, the outside is cool to touch, but can you imagine that the inside may well still be warm? Same thing. A telescope needs to be the same temperature as the surrounding air to work at its best. If you don’t, it might appear out of focus, even when it isn’t. You might even get “dewing” on the lens, like a bathroom mirror. It can take up to an hour or much longer, especially if you have a triplet refractor, or your telescope has a thick mirror. Long cooldowns mean your telescope becomes less usable – shorter viewing times, and more thought to use.
Heavy / Weight / Awkward
Some Telescopes are just horribly heavy and awkward to move. If it weights 20kg, are you really going to want to shift that thing? If it requires a red flag and sticks out of your window, are you really going to take it out? If it needs a Ute, Utility Truck, SUV or a purpose built contraption to move, are you going to want to? The best telescope is the one that gets used the most. You will use it less if it’s heavy, awkward, or difficult to set up and use. This is really underestimated. There are plenty of giant dobsobians on the market because they aren’t that great for stairs or apartments.
Some Telescopes you want – but just can’t get. Look, I’ve had my favourites too, “classic”, lovely telescopes that everyone seems to rave about and no one has and you can’t find. There’s a saying in Germany. “Better a sparrow on your hand, than a dove on the roof”. You will get more joy from the sparrow than the dove, won’t you? So don’t pine for what you can’t have. Go out there and get the “sparrow telescope” you can have and enjoy. Also, once you are enjoying your sparrow, stop looking for the “dove”.
So, now at the end of it all, you want to know what’s best, don’t you? In Part II of this series, we will look at various telescopes, their strengths and weaknesses, and what it means for you. Stay tuned! To get an alert and not miss out on a blog, as well as get the latest news from Astro Dog, sign up to our newsletter!
Now, the question I had for you, is which of these problems seems the most serious one for you? What Telescope did you decide on for you first one, and was it a good choice? Sign in below and leave a comment!