Telescope Barlow

The SkyWatcher Explorer 130P is, in my opinion, a very good entry level telescope for someone who wants to get into astronomy on a fairly limited budget. Priced in the range £180 to £200 (UK Sterling), the telescope should hopefully be affordable to most people.

Unpacking and setting up the telescope was very easy - I had everything assembled within about half an hour. There is a one-off requirement to set up the latitude where the telescope is going to be used, which would only need to be re-done if you move the scope several miles from this location. The user guide provides clear instructions on how to do this.

Each time you switch on the auto-tracking Alt-Azimuth mount, the scope needs to be horizontal (zero on the mount's scale) and pointing north. This enables the scope to track objects in the sky. This is really useful because objects have a tendency to move out of the scope's field of view very quickly if you are not tracking them using a motorized mount. The only thing I don't like about the motorized mount is that the power pack does not have an on-off button, meaning that it needs to be switched on or off by screwing or unscrewing it from the mount.

The telescope has a focal length of 650mm and comes with two eyepieces (I guess this might be different in different countries) and a Barlow lens. One eyepiece has a focal length of 25mm, giving a magnification of x26 (650mm / 25mm), and one has a focal length of 10mm, giving a magnification of x65 (650mm / 10mm). The Barlow lens can be inserted in front of the eyepiece to double the magnification, giving a magnification of x52 (2 x 26) and x130 (2 x 65).

The 130mm aperture captures quite a bit of light, producing bright, sharp images. One thing to remember though is that viewing an object through a 130mm telescope is not going to produce images like those we see in books. For example, viewing Saturn at a magnification of x130 will produce an image that is about 2mm at arm's length. So, hold a ruler at arm's length and look at, say, the distance between the 150mm mark and the 152mm mark - this is how big Saturn will appear through the SkyWatcher Explorer telescope at a magnification of x130.

The highest practical magnification is x260 (2 x the aperture (130mm)), but it is really difficult to get sharp images at this level of magnification unless the viewing conditions are really good. (I bought a 5mm eyepiece which, in conjunction with the Barlow lens, gives me a magnification of x260.)

The SkyWatcher Explorer is easy to pick up and move about, which is essential if like me you store your telescope in the house. I normally use my telescope in the back garden, and I never have any problems moving it around.

All in all I'm really pleased with my purchase and would recommend the telescope to anyone who is just starting out in astronomy.

About the Author: John Dixon is a web developer working through his own company John Dixon Technology Limited. The company also develops and supplies a free accounting software tool called Earnings Tracker. The company's web site contains various articles, tutorials, news feeds, and a finance and business blog.

what is a barlow eyepiece used for on a telescope?

The short answer is, a barlow increases the magnification.

You see, in a telescope, the magnification = (focal length of the telescope)/(focal length of the eyepiece)

A telescope eyepiece that has a focal length of 12.5mm offers more magnification for the same telescope, than a telescope eyepiece with a 25mm focal length.

But, if I use the 25mm eyepiece with a 2x barlow, I will get the same magnification that the 12.5mm eyepiece provides, because it is equivalent to reducing the focal length of the eyepiece by 2, since 25/2 = 12.5

The advantage of doing this, rather than using the 12.5mm eyepiece is, it's easier to look through the 25mm eyepiece as the lens on it is much larger. It offers more eye relief.




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