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<P style="TEXT-ALIGN: center; MARGIN: 0in 0in 10pt" class=MsoNormal align=center><B style="mso-bidi-font-weight: normal"><FONT size=3><FONT face="Times New Roman">Note: This article is also posted on the Forum</FONT></FONT></B></P>
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<P style="TEXT-ALIGN: center; MARGIN: 0in 0in 10pt" class=MsoNormal align=center><B style="mso-bidi-font-weight: normal"><FONT size=3><FONT face="Times New Roman">MIRROR MAKING – PHILOSOPHY<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p></FONT></FONT></B></P>
<P style="MARGIN: 0in 0in 10pt" class=MsoNormal><FONT size=3><FONT face="Times New Roman"><SPAN style="mso-tab-count: 1"> </SPAN>When one decides they want to make their own telescope, it is best to start off simple. In that, I mean when it comes down to a refractor (achromatic lens) versus a reflector, there is no contest. A reflector is much simpler.</FONT></FONT></P>
<P style="MARGIN: 0in 0in 10pt" class=MsoNormal><FONT size=3><FONT face="Times New Roman"><SPAN style="mso-tab-count: 1"> </SPAN>The principle behind mirror making is to take round piece of glass, flat (or relatively so) on both sides, and grind a bowl shape into one side while leaving the other side flat. On the other hand, a refractor involves two lenses. Not only do you have to grind curves into both sides of each lens (that’s four individual curves, though in some refractor designs, one side may be flat but that’s not as easy to do as it sounds), the curves from one lens have to match up to the curves from the other lens. Another little complication is choosing the best and clearest flint and crown glass, because the light has to pass through them unhindered. Additionally, the curves on each side of each piece of glass have to be perpendicular to each other. Even the slightest difference can cause what is called wedge which will throw the light rays off. On the other hand, with a mirror, your only concern is one surface and it doesn’t matter how clear the glass is, as no light will be passing through it.</FONT></FONT></P>
<P style="MARGIN: 0in 0in 10pt" class=MsoNormal><FONT size=3><FONT face="Times New Roman"><SPAN style="mso-tab-count: 1"> </SPAN>Not to discourage anyone from making an achromatic lens, but it is wise to get your chops on something simpler before you jump into the deep end!</FONT></FONT></P>
<P style="MARGIN: 0in 0in 10pt" class=MsoNormal><FONT size=3><FONT face="Times New Roman"><SPAN style="mso-tab-count: 1"> </SPAN>Though you do not have to look through a mirror, the glass still must be within certain tolerances or you will never get a good surface on the front. A telescope mirror is a very thin reflective (usually aluminum) coating backed by a thick piece of glass. The glass is there to provide a stable surface for the aluminum coating to stick to. If that glass backing isn’t shaped right, the aluminum coating will never reflect the light to the proper spot so you can focus it. In other words, when you make a mirror, you are actually making a sturdy backing for the thin aluminum coating which is the actual “business end” of the telescope.</FONT></FONT></P>
<P style="MARGIN: 0in 0in 10pt" class=MsoNormal><FONT size=3><FONT face="Times New Roman"><SPAN style="mso-tab-count: 1"> </SPAN>The piece of glass must not have any bubbles near the surface to be ground, must not have any major occlusions (cracks, large internal ripples, impurities), and must be relatively flat on both sides to start with, especially the back side. You do not want to grind a mirror that has lumps and an uneven back on it or it will throw off the grinding and polishing on the other side. Also, the glass should be as thermally stable as possible. Most commercial mirror blanks take all this into account but if you are improvising, you need to know this.</FONT></FONT></P>
<P style="MARGIN: 0in 0in 10pt" class=MsoNormal><FONT size=3><FONT face="Times New Roman"><SPAN style="mso-tab-count: 1"> </SPAN>A word about thermal stability. Many mirrors have been successfully made using standard plate glass. The usual recommendation is Pyrex. Some will go for fused quartz, Astro-Sitall, or Zerodur. These high-end materials are much more expensive, though they are more thermally stable. A little known fact is that when you spring for a Zerodur mirror (for instance), you are paying up the nose for an optic that is actually easier for the mirror maker to fabricate. It has little to do with actual observing conditions in the field. By that I mean that since Zerodur is more stable than plate glass, it is easier for the mirror maker to get a precise figure on the mirror in the shop because they don’t have to deal as much with thermal currents throwing off the test results. Therefore, theoretically, they can polish and figure a more precise and cleaner surface on the mirror. However, with a bit of patience, you can do the same thing with a plate glass mirror. It takes longer, but can be done just as well.</FONT></FONT></P>
<P style="MARGIN: 0in 0in 10pt" class=MsoNormal><FONT size=3><FONT face="Times New Roman"><SPAN style="mso-tab-count: 1"> </SPAN>What does that mean out in the field? A Zerodur mirror will be ready to use a bit sooner than a plate glass mirror (it will stabilize to temperature sooner). However, with good fans, plenty of ventilation, and a good mirror cell, your plate glass mirror will perform just as well if the time is taken to do the figure right.</FONT></FONT></P>
<P style="MARGIN: 0in 0in 10pt" class=MsoNormal><FONT size=3><FONT face="Times New Roman"><SPAN style="mso-tab-count: 1"> </SPAN>In any event, the standard mirror blank is Pyrex and that will put thermal stability in-between both extremes. The simplest route is to buy a mirror kit which will include the Pyrex blank, a tool, and all the grits and polishing compound you will need.</FONT></FONT></P>
<P style="MARGIN: 0in 0in 10pt" class=MsoNormal><FONT size=3><FONT face="Times New Roman"><SPAN style="mso-tab-count: 1"> </SPAN>In the next article, we’ll explore the planning of your basic mirror.</FONT></FONT></P> <br /><hr />Hotmail: Trusted email with Microsoft’s powerful SPAM protection. <a href='http://clk.atdmt.com/GBL/go/201469226/direct/01/' target='_new'>Sign up now.</a></body>
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