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Except at the very best observing sites, light pollution and air turbulance will often a major factors in the degredation of telescope images. These factors must be taken into account in any decision to purchase a telescope for use at a particular site.


Light pollution is the light which escapes into the atmosphere from man-made sources, street lights, auto headlights, etc. It would not be such a problem if this lighht pollution simply disappeared into space. Unfortunately, light pollution usually goes hand-in-hand with air pollution. Air pollution contains particulate matter which reflects much of the light pollution back to earth, brightening the view of the sky. This sky brightening masks the astronomers view of celestial objects, particularly faint objects like nebulae and galaxies.

Light pollution is a big problem for the Urban Astronomer, and many commute to dark sky sights for their observing. That's fine, if you have the enthusiasm for it. And hopefully your enthusiam will not wane.

But we should not give up on urban astronomy -

First of all, light pollution affects our view mainly of extended objects like nebulae and galaxies. It affects certain waveleghts of light more than others, so it is possible to filter out much of the unwanted light pollution, and improve views of extended objects to some extent. But light pollution has little effect on the viewing of bright objects (such as the Moon, the planets and stars) using higher magnifications. So there are wide areas of interest here, that are available to the urban observer.

Secondly, there is very little light pollution in the extreme red end of the light spectrum and into the infrared. So, by using a camera sensitive to infrared, you can filter out and avoid nearly all of the light pollution effects. Downside, is that you will be imaging in monochrome infrared, not colour, but images can be impressive, none the less. Dedicated CCD cameras can be expensive, but cheaper cctv cameras (such as the Mintron) can be used effectively, and these have the benefit of live video viewing. But there is a challenge here. If the Urban Astronomer embrasses that challenge, he/she will find the results just a rewarding as the pretty pictures obtained easily from a dark-sky site - perhaps even more so.


The second problem is caused by atmospheric turbulence. During the day, the ground is heated by the Sun. This heat is stored in rocks and earth. Trees and vegetation are poor storers of this heat, but city buildings are very efficient at storing the Sun's heat. At night, this stored heat escapes into the cooling air, and in winter, heating in buildings adds to the problem.

Clear skies mean that heat is escaping into space, but with the heating effect of the earth, this cooling down does not take place uniformly. Pockets of air form, each at slightly differing temperatures. Differing air temperatures mean that the refractive index of the air is also different in each pocket. As these pockets of air swirl around in the atmosphere, they cause a scintillation of the star images viewed through the telescope. This affects the sharpness of the image (the resolution) of the telescope, and the problem would be worse in towns normally.

In steady air, the resolution of good telescopes would be determined by their aperture. A 4 or 5 inch aperture telescope would resolve to 1 second of arc. Larger telescopes ought to resolve finer detail. Air turbulence does vary from time to time and from place to place, but, from even the best viewing locations on Earth, resolution is rarely less than one arc-second of arc. That figure was the average noted at a professional mountain-top observatory in the Canary Islands, over a 4 year period.


In areas with light pollution, you're better off with a telescope of long focal ration. This will darken the sky, and will be better for high magnification viewing of bright objects.

If however, you want to image extended objects with filters, then you will need the largest aperture possible, and short focal ratio. F4 Newtonian, and F5 or F6 refractors would be a good choice. I would avoid Cassegrains and maksutov, because that focal ratios (F10 to F14) are not ideally suited ti deep sky imaging.

As far as air turbulence is concerned, it is important to note that telescopes of over 5 inch diameter are only rarely able to perform to their theorectic resolution. A bigger aperture will always produce brighter images, and that is important. The implications of air turbulence is that, as you choose bigger and bigger scopes, so the optical quality becomes less of an issue.

The FWHM of the seeing disc (or just Seeing) is usually measured in arcseconds, abbreviated with the symbol ("). A 1.0" seeing is a good one for average astronomical sites. So the best resoluton would be achieved with a telescope of about 4 inches aperture. Using a larger scope would not provide sharper views, except for perhaps short glimpses of greater detail.

The seeing of an urban environment is usually much worse. On the other hand, the very best seeing conditions occur at the best high-altitude mountain-top observatories. But even there, the seeing will very rarely exceed 0.4" - that is, the resolution of a good 250mm (10 inch) telescope .

MEASURING ATMOSPHERIC TURBULENCE - Atmospheric Coherence Length, r0

Another measure of atmospheric turbulance is the "Atmosheric Coherence Length" or "Atmosheric Coherence Diameter" and is denoted by r0. Sounds complicated, but this is easy to explain.

The "Atmospheric Coherence Length", r0, indicates the size of a telescope which can just operate at its diffraction limit for a visual observer. Any telescope with aperture larger than r0 will not operate at its diffraction limit.

For example, suppose that at a particular site and time, r0 = 50mm.

If your telescope aperture (d) is also 50mm, so d/r0=1, then your telescope will operate at its diffraction limit.
See image on the left above.

If your telescope aperture (d) is 150mm, so d/r0=3, views through your scope will be degraded, your telescope will not operate at its diffraction limit.
See image on the right above.

Under a standard sky is, the coherence diameter is roughly 40-100mm (1.6" - 4"). The smaller value would be typical in built-up areas, and the larger value at really dark sky sites. The value changes from night to night and even from hour to hour. But only at a few exceptional places on earth (with exceptionally good seeing) will the coherence diameter exceed 100mm.
Measurements taken over a 4 year period at the William Herschel Telescope in the Canary Islands, showed a median value for r0 = 149 mm.

Under Poor Seeing Conditions (r0 = 50mm)
the wavefront accuracy of telescopes is limited to the following values

Telescope Aperture Limit on Wavefront
50 mm 0.25 (λ/4 P-V) 0.82
100 mm 0.4 (λ/2.5 P-V) 0.60
150 mm 0.5 (λ/2 P-V) 0.45
200 mm 0.7 (λ/1.5 P-V) 0.21

Under Excellent Seeing Conditions (r0 = 100mm)
the wavefront accuracy of telescopes is limited to the following values

Telescope Aperture Limit on Wavefront
50 mm 0.15 (λ/6 P-V) 0.93
100 mm 0.25 (λ/4 P-V) 0.82
150 mm 0.3 (λ/3 P-V) 0.75
200 mm 0.4 (λ/2.5 P-V) 0.6
For Central obstructions, Strehl Number=1-(σ2/0.6), where σ is central obstruction as decimal of diameter
derived from
figures based on formula etc in

These resolution limits are based on averages, and so will have the greatest affect on medium and long exposure astrophotgraphy. Good news for visual observors, the average resolution will be the same, but there will be brief glimpses in which the resolution might be higher, closer to the theoretical 1/4 wave P-V level.

More good news is that using a video camera with your telesope to image the brighter objects like the Moon and planets, will almost certainly enable you to capture diffraction-limited images.


What all this means is that, if you will do your viewing from an Urban or Suburban site, then 1/4 wavefront P-V accuracy (or even less) is more than adequate for telescopes of over 100mm aperture, especially if your main interest is long exposure imaging. The larger the telescope, the less important the wavefront accuracy is.

Exceptions to this rule are -
(i) if you want the sharpest possible views, even though they will be brief glimpses, or
(ii) if you are going to use a video camera/webcam with your telescope to capture images of bright objects (Moon, planets, etc).


Advice commonly given to amateur astronomers is that it is always essential to let your telescope cool down to outdoor ambient temperature before use. This advice is misleading, as in many cases there is no advantage to be gained in letting your telescope cool. If you mostly observe deep-sky objects at low to medium magnifications, then you probably aren't being bothered much by telescope thermals.

If, however, you're a lunar and planetary observer who wants to use high magnification, then, yes, your telescope may perform better if you allow it to cool. But only if the seeing conditions at the time are very good. In average to poor seeing conditions, and especially in towns, atmospheric turbulence will degrade your views to a far great extent than telescope thermals. Unfortunately, the two often go hand in hand, and have a common cause. For example, in winter my telescope needs to cool because it has come from a warm house into the cold winter air. And it is warm air escaping from warm houses in winter that causes much atmospheric turbulence.

So my advice is, ignore the advice! Make up your own mind. Use your telescope immediately, and decide yourself whether there is any improvement as your observing session progresses.


The first requirement of any mount is that it must hold the telescope steady enough for the purpose. To see detail, you must have a rock-steady view. The second requirement is that the mount must allow you to track objects smoothly as they appear to move across the sky (as a result of the Earth's rotation). You could do this manually, by turning one, or two, knobs on the mount. Or there are various levels of automatic tracking mounts available.

Next question is, how will you find the objects you want to view? You could do it manually, the old-fashioned way, with the aid of a printed star map. This can be fun, or very frustrating, particularly under poor urban skies. The alternative is a "GOTO" mount which will find objects and point the telescope automatically, using a huge digital database. Obviously, these mounts require carefull setting up before each observing session. But there are now mounts will will automate much of the setting up procedure for you. All this, of course, comes at a cost.

All these aspects, and more, will be dealt with on the details page (see menu on left). Appologies if this is still under construction.


Under this heading, you must decide where you will use your telescope and the ease of portability and set-up you require. The size and weight of the telescope and its mount are considerations here. If the telescope is to be used at home, then a permanent, or semi-permanent site may be possible to save frequent carrying etc.

The questions to answer are: 1. Where will the telescope be kept, and where will it be used?

If the telescope is to be sited in its own purpose-built shed or observatory, then there is no problem, the telescope is always near ready to use. Where the telescope is kept indoors and is to be used in the garden, say, then the time and effort needed to transport the telescope, assemble it, and set it up, must be taken into consideration. Only small telescopes can be carried out in one go, complete with tripod and mount. Bigger scopes will necessarily have to be carried out in parts, and assembled in situ. For the largest scopes, an amount of heavy carrying is inevitable.

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When complete, this website will aim to provide the most complete guide to the telescope, and to give simple unbiased advice about the choice of telescope and accessories.

As you will see, the website is under construction.
Thank you for visiting, and please come back again soon!

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