An Explanation of the Resolving Power of Telescopes
Posted: Wednesday, June 11, 2008
by Will Kalif
Kalif Publishing
Resolving power is not as well understood as the magnification of telescopes. Yet it is more important. This article explains resolution, what it is, and how to calculate it. Magnification of a telescope is easy to understand. The higher the power the closer the look we get at images in the night sky (the bigger they look). Resolution is another critical component of telescopes and it is very easy to understand if you think of it like television sets. Older televisions have a certain number of lines of resolution and the new high definition sets have many more lines of resolution so you get a sharper and more detailed image. The same applies to telescopes; the higher the resolution the more lines of information we get so the better the image. And this resolution has nothing to do with magnification. If you get closer to your television does the image get sharper? No. It just gets bigger. The resolution remains the same.
The wavelength of the light being observed is a factor in resolution. The higher the wave length of the light the more waves a telescope sees so the more information it gets, and the better the resolution. A second factor is the size of the telescope objective. The bigger the lens or mirror the more wavelengths it will gather. And more wavelengths mean better resolution.
Formula for resolution
This relationship between the wavelength of the light and the diameter of the telescope objective gives us, along with a constant, a formula for calculating the resolution in arc seconds. Divide the diameter of the telescope objective by the wavelength of the light being observed then multiply this by 252,000 (which is our constant). This gives you the resolution in arc seconds. (Resolution = 252,000 X (wavelength) /(diameter)). You can see that the larger the telescope the smaller the resolution is in arc seconds. And the smaller the resolution in arc seconds the better. It is like smaller lines on your high definition television. Smaller lines means more lines per inch or centimeter and a crisper, sharper image.
Other Factors in Telescope Resolution
Atmospheric conditions are a big factor in resolution. A turbulent, thick, or unsteady atmosphere will distort the wavelengths coming into the telescope and distort the quality of the light and information. This will reduce the resolving power.
The quality of the telescope is another factor. High quality scopes with good optics will correctly gather good information and give you good resolution. Poor quality optics will distort the images and give you a lesser resolution.
Resolution, while lesser known than magnification is much more important. It is the factor that determines the quality and sharpness of what you see through the telescope and it is the reason why bigger telescopes are better than smaller ones.
You can read a more comprehensive explanation of telescope resolution, with drawings, on the authors website : TelescopeNerd.com This Article has been viewed 3,296 times. (Not updated in real-time.)
Top-level comments on this article: (2 total)Just have a look at some of what was said and the use of English in this article and make your own judgement as to its merits."The HIGHER the wave LENGTH the more waves a telescope SEES and the more information it gets, and the better the resolution. A second factor is the size of the telescope objective. The bigger the lens or mirror the more wavelengths it will gather. And more wavelengths mean better resolution."What a brilliant account, so eloquently put, no wonder the world is falling apart around our ears. For example: for " The higher the wavelength" read " The shorter the wavelength". And since when could a telescope see? It is the observer that does the seeing not the telescope! Do I have to go on? Just a couple of sentences chosen at random so full of misinformation and such a lack of clarity!Do me and yourselves a big favour: stay clear of websites such as this if you are at all interested in optics or astronomy (or, indeed, anything faintly scientific). Instead, go out and buy a good book such as: the Amateur Astronomers Handbook by J.B. Sidgewick. There is more information in that one volume than can be gleened from the internet in a year of searching for 8 hours a day. In addition, you won't have to put up with having to trawl through all the ill-informed nonsense as is shown here. The author's title says it all "TelescopeNerd".I have a feeling this critique will not be posted as the people in charge of the website have, obviously, not a care about publishing quality work if this article is anything to go by.They even have a policy that tells me that my comments will not be published if they are mean or even borderline rude, and that I have to be nice no matter what the mediocre and poorly informed dross that they are pushing!Oh, yes, I mean you! Why don't you show a lttle more discernment in what you publish and give the people out there a real chance of improving their knowledge and stop feeding them nonsense such as this.Hey Ho!John Barton Wood
We suppose that it is understood how the angular measurement system within Total Stations/ tachymeters works, and what it means if an instrument is specified with 0.5", 1", 3", 5",7" od 30" angular measurement accuracy. According to ISO17123 it would mean that a single angular measurement would have the specified standard deviation (e.g. 1").Most manufacturer today follow ISO for defining angular accuracy.But now to the second point: Resolving powerIts generous definition isthe ability of a microscope or telescope to measure the angular separation of images that are close together.The big unknown in this discussion is the eye of the observer and the light conditions during your observation. Based on a resolving power of 3" the human eye can center to any target observed with a factor 3 (worst case) to 10 times better -> that means the telescope ensures an aiming accuracy within 0.3" and 1". Mainly dependent on:• Design of the target (symmetry)• Distance to target• Background light• Ambient light• Heat shimmer• Reticle design (thickness and shape)• Target brightness (recognizabilty)There are some approaches to put this dependency into a formula, however, they are all based on empirical data, hence they are all different. Statistically Speaking it cleary indicate that an average observer can achieve 0.5" having standard environmental conditions. Compared with 2.5" it would mean that they have a range between 0.25" and 0.9" aiming accuracy. Hence for any instrument which specified to achieve angular accuracy of 1” this 0.5” difference in resolving power makes no difference.I hope this is clear to you now. Should you need further assistance please feel free to contact us.
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