Telescopes

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0><tr> <td valign="top" colspan=2 height=22 BGCOLOR="#330099" TEXT="#080000" bgproperties="fixed" background="040000d2erkqrf.gif"> <div> Derby and District Astronomical Society - Aries Magazine</div> <div> </div> </td> </tr><tr> <td valign="top" width=145 BGCOLOR="#333399" TEXT="#080000" background="03fa2c00erkjzj.gif"> <div> <A HREF="index.htm" TARGET="_top" TITLE="Aries Astronomy magazine on line From th"><U>HOME</U></A></div> <bR> <div> <A HREF="id44.htm" TARGET="_top" TITLE="Summer 2001"><U>Summer 2001</U></A></div> <div> <B>Telescopes</B></div> <div> <A HREF="id48.htm" TARGET="_top" TITLE="Mercury"><U>Mercury</U></A></div> <div> <A HREF="id49.htm" TARGET="_top" TITLE="The Barbie a poem"><U>The Barbie - a poem</U></A></div> <div> <A HREF="sky_notes.htm" TARGET="_top" TITLE="Sky scene June to August 2001"><U>Sky scene June to August 2001</U></A></div> <bR> </td> <td valign="top" height=458 BGCOLOR="#FFFFFF" TEXT="#080000" background="06a92963.gif"> <div> <B>Aries Astronomy magazine on line- From the Derby and District Astronomical Society</B></div> <div> <IMG SRC="04107040.gif" border=0 width="263" height="4" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <DIV ALIGN="LEFT"></DIV> <div>  <A NAME="askjeeves"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A><FONT SIZE="4" COLOR="#FF6666" FACE="Comic Sans MS" >Why not search the internet for more information about telescopes ...</FONT></div> <div>  <IMG SRC="http://service.bfast.com/bfast/serve?bfmid=31103363&siteid=35044309&bfpage=football" BORDER="0" WIDTH="1" HEIGHT="1" NOSAVE > <FORM ACTION="http://service.bfast.com/bfast/click" > <INPUT TYPE="hidden" NAME="siteid" VALUE="35044309" > <INPUT TYPE="hidden" NAME="bfpage" VALUE="football"> <INPUT TYPE="hidden" NAME="bfmid" VALUE="31103363" > <TABLE CELLPADDING="0" CELLSPACING="0" BORDER="0"> <TR> <TD 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</div> </NOBR></tD> </tR> <tR> <tD VALIGN=TOP HEIGHT= 34 WIDTH="466"><div> <I><B>Author: Maurice Batchelor</B></I></div> </tD> </tR> </TABLE></div> <bR> <div> I have often heard the question “where can I get a good astronomical telescope for very little cost?”. Sadly, the answer is that there is no such thing as a good, cheap new astronomical telescope. Occasionally, a good instrument can be obtained second hand, for a moderate cost, although this is largely a matter of luck (and speed! – Ed). Small telescopes intended for terrestrial use are often available at a relatively low cost but have limitations as far as astronomy is concerned. There is, however, one particularly good use to which this type can be put – solar observation <FONT SIZE="4" COLOR="#FF0000" FACE="Arial" >(IMPORTANT SAFETY NOTE – do not look at the Sun through any optical instrument, it will blind you, projection is the safe method to use).</FONT> This will be the subject of my next article for Aries. For general observation, a good pair of binoculars will give excellent results when viewing constellations and the Milky Way. </div> <div> Another important consideration when buying an  astronomical telescope is the availability of storage space. Astronomical telescopes are generally much bigger than their terrestrial counterparts. They also need a firm mounting to reduce vibration, so, some sort of permanent observatory (such as a run-off shed) in the garden is often necessary. An outside observatory is preferable to viewing directly from an open window in the house, as warm air currents from the house distort the image. </div> <div> There are two main categories of telescopes. These are refractors and reflectors. Figure 1 shows a refractor (refracting telescope to give it its full name). </div> <div> <IMG SRC="11b63e60.gif" border=0 width="611" height="230" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> This type of telescope gathers light by means of a lens, called the ‘object lens’ or ‘object glass’ (sometimes shortened to OG – Ed). This passes the light down a tube to a point of focus. This point is naturally called the ‘focal point’ and the distance from the focal point to the lens is called the length. The resulting image is magnified by the eyepiece.</div> <div> Figure 2 shows a reflector (reflecting telescope) of the Newtonian design.</div> <div> <IMG SRC="11e66f30.gif" border=0 width="614" height="243" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> This type is so called because it was invented by the famed scientist Sir Isaac Newton. It differs from a refractor as it uses a concave mirror, known as the main or primary mirror, to reflect and focus the light from the object under study instead of a lens. This then passes, by means of a flat mirror, set at 45 degrees, to an eyepiece. The eyepiece is situated in the side of the tube. Both of these mirrors are silvered on the surface rather than on the back, as is the case with everyday mirrors. The eyepiece magnifies the image from the main mirror.</div> <div> An important consideration with both types of telescope is that the magnification should not exceed the ability of the object glass or mirror to gather light. </div> <div> These instruments as described are ones generally in common use. There are however other types of reflector such as the Gregorian and the Cassegrain. The most popular and widely purchased of these is probably the Schmidt-Cassegrain or Catadioptric telescope. This offers a major benefit of making a good size telescope (150 mm (6”) and upwards) fairly portable. </div> <div> As I have already mentioned, small refracting telescopes are well suited to observing the Sun by the method of projection. The instrument does not need to be a true astronomical telescope, bird spotting and other telescopes designed for terrestrial use are fine. An instrument of this nature usually has an object glass of about 40 to 60 mm (1 ½” to 2 ½”) in diameter and is perfectly adequate to produce an image of the Sun that is about 75 mm (3”) across. This will show sunspots clearly. (please see earlier safety note). By using the projection method, surprisingly good results can be obtained, hopefully serving to stimulate the interest of the beginner who could then, if finance allows, move up to a larger instrument that is capable of showing surface details on planets, such as the cloud belts of Jupiter and the rings of Saturn. </div> <div> It is worth mentioning at this point that true astronomical telescopes produce an inverted (upside down) image. Terrestrial telescopes have extra lenses known as erecting lenses, which sets the image the correct way up. When light passes through a lens system, a small amount is lost (by reflection and absorption). This means that faint objects will not be seen as easily as with an astronomical telescope. In the latter, these lenses are left out as the maximum amount of light is required, with the exception of observing bright objects such as the Moon, or the Sun (by projection).</div> </td> </tr></table></body>