Interrupted views through a reflector telescope

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willpittenger

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Having never looked through a real telescope, I have always wondered something about reflector telescopes. Does the secondary mirror interrupt the field of view? I have to believe it would. As such, I would prefer a scope set up like many pairs of binoculars -- offset the secondary mirror to be outside the light path. If the scope is binocular, doing so might also make the overall structure more compact. You could increase that by folding the light path as is done by a prism in normal binoculars.<br /><br />Also, when too telescopes, like Keck or Gemini, work together, do you get an actual image from them? Or are you limited to spectrography?<br /><br />BTW: The telescope I did look through was a hologram. The weird thing was that when you look through it, you see the Moon -- magnified. (The Moon was visible as the scope's target when you approached the hologram.) In fact, what I saw through the scope changed as I moved my eye relative to the holographic eyepiece. It was cool.<br /> <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>
 
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MeteorWayne

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Will,<br /> The secondary mirror blocks some of the incoming light, but other than dimming, does not affect the image. This is because the secondary is (severely) out of focus.<br /><br />It's a tradeoff, as are most things in telescope design. You gat the bigger mirror (light gathering) of a reflector, then give a little back beause of the secondary.<br />It's a good tradeoff <img src="/images/icons/smile.gif" /><br /><br />The problem with removing the secondary from the light path (called 0ff-axis) is that it greatly complicates the geometry required for the primary mirror. In a standard reflector, the primary is a simple parabola, which can be ground ("figured") extremely accurately, and the secondary is a flat mirror, also making it easy to make accurate. Here's a quick link for basic designs.<br /><br />Fo an off axis design, it is still part of a parabola, but it is not the center. This makes it very complicated to accurately grind glass to the required precision. However, this design is used as in the great picture of the Green Bank Radio telescope since due to the size involved (100 X 110 Meters!) the metallic reflecting surface can be constructed with sufficient precision. <br /><br />Hope this helps!<br /><br />MeteorWayne <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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willpittenger

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I knew that satellite TV dishes commonly use such a design. Seeing the radio telescope you mentioned just confirmed what I knew.<br /><br />As for the out of focus part, I still expect that you would see a blur (or given the lack of light, a hole) where the secondary is. Now if the secondary would take up less than the shortest wavelength visible to the human eye, then you might not see it. That, however, is awfully tiny. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>
 
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MeteorWayne

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TRust me, when looking through my 8 inch dob, a Newtonian with a secondary in the path, there is no hole !<br /><br /><img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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MeteorWayne

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Absolutely! I can spend an hour scanning the surface <img src="/images/icons/smile.gif" /><br />No holes.<br /> I know it's hard to believe, but the secondary is so out of focus it has no effect other than dimming.<br /><br />BTW, with an 8 inch dob, it'a almost necesary to put in a lunar filter (blocks 90% of the light) because it's so bright. A full moon that is. Otherwise one pupil closes up as much as it can, the other one stays open, and you head tilts over because the brightness difference is so great when you try and walk around.<br /><br />Looking at the moon, I like to also use my Astroscan, which is a wide field newtonian reflector, like a dob, since you can easily see the whole moon at once. With a larger scope, the field of view is smaller and you can usually only see part of the surface at higher magnification. With a low power and wide field lens, I can just about squeeze the whole moon in, but it's close. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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