Telescope Tuesday - Eyepieces

It's been a couple of weeks since our last Telescope Tuesday.  Let's just all be glad this one is actually on a Tuesday.  Last time I wrote about the mechanics of reflector telescopes and the optical path within the tube.  Today I'll cover the last bit that path which is the eyepiece.

There is the old astronomers saying "The light travels for millions of miles only to get blocked in the last one".  This is of course a reference to the bane of all astronomers, both professional and amateur, which is weather and clouds.  Seriously, clouds suck.

So lets take an easy example...Polaris (The North Star).  Assuming you can see it (difficult in Chicago at best), the photons from the surface of that star have traveled a distance of 434 light years.  434 light years = 4,105,957,025,100,067.5  (4.1 Quadrillion) Kilometers...which is a very long distance by any standard.  The photons you have managed to capture with your telescope now travel down the optical path of the tube assembly (a focal length of 650mm in my case) and get reflected by the secondary mirror into the eyepiece assembly.  This last little bit is one of the most critical portions of the photons journey.  That last little 2-25mm makes all the difference.

There are scads of different types of eyepieces, each type having a specific use; normal viewing, wide-angle viewing, comma-correction, etc...

The fancier they are, the more expensive they are because of the increase in the optic quality as well as the number of lenses contained within, ranging from $50-$500.  Personally, I only have the two eyepieces that came with my scope *sadtrombone*   The image focusing and other special effects depends on the types and arrangement of lenses inside the eyepiece.

Apart from all the special functions that a high end eyepiece can do, the most basic function of the lenses is to magnify the image so that the observer can make out fine details.  The calculation of magnification is simple and depends only on the focal length of the optical assembly and the eyepiece itself.

So in my case I have a focal length of 650mm in the reflector telescope.  With my two eyepieces; 25mm and 10mm this gives me a magnification of 26x and 65x respectively.  It is a common misconception that the higher the magnification the better.  While this is true to some extent every telescope has an upper theoretical magnification which is 50-60 (depending on your source) times the aperture of the telescope in Inches.  Since my scope is 130mm=5.125 inches, that means there is an upper limit of 256x-307x that I could expect to achieve.  This limit is due to the fact that the higher the magnification is, you are not only magnifying the object you are looking at you are also magnifying the imperfections in the optics, as well as atmospheric disturbances.  At very high magnifications, even air currents inside the scope will cause distortions in the image (which is why you are always advised to cool your scope to the ambient outside temperature).

In addition, the higher the magnification, the smaller the apparent field of view and the dimmer an object will be.  Since I am severely limited to planets and the moon due to Chicago's light pollution, I don't have to worry about this too much as of yet, but when I finally make it to a dark-sky site...a lower power eyepiece will allow me to see more of the overall structure of nebula.  Lastly, if like me you don't have a mount that can track, any object is going to whip across the field of view due to the earths rotation.  When I am able to get Saturn centered in my 10mm it is gone from view in about 7-8 seconds.

I will close this with a segue into next weeks (hopefully) topic...CCD Cameras.   The CCD Camera I have has a focal length of about 5mm as best as I have been able to calculate.  This then means that I have an effective magnification of about 130x.  Needless to say this provides some excellent close-up views of the Moon.