----- Forwarded Message ----- From: daniel turner <outwest112@yahoo.com> To: Siegfried Jachmann <siegfried@jachmann.org> Sent: Thursday, August 27, 2015 12:56 PM Subject: Re: [Utah-astronomy] Making a 36" from many smaller mirrors To make the Keck work they needed dozens of electric motors that would bend the cell to realign the mirrors every time the telescope was moved in elevation.  It took a long time to develop and troubleshoot the computer system that controls all those motors and alignment sensors.  This would be beyond the abilities of amateurs to perfect. Another way would be to build a mirror cell stiff enough to hold the mirrors in place against gravity.  The best material to do this would be a giant piece of glass.  Only glass is stiff enough and strong enough.  Of course then you would be gluing your mirrors to a giant piece of glass.  So why not just grind and figure the big piece of glass? DT   From: Siegfried Jachmann <siegfried@jachmann.org> To: daniel turner <outwest112@yahoo.com>; Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Thursday, August 27, 2015 10:48 AM Subject: Re: [Utah-astronomy] Making a 36" from many smaller mirrors Yes but, the Keck telescopes are multiple mirror telescopes. Multiple mirror telescopes do work. Future space telescopes, including the James Webb and the proposed 12 meter, are designed to multiple mirror telescopes. It's not just the weight and sagging. It's the accuracy with which they must be aligned. Typically mirrors must be no worse that 1/4 wave from ideal. That also applies to multiple mirror scopes. SJ  On Thu, Aug 27, 2015 at 9:36 AM, daniel turner via Utah-Astronomy <utah-astronomy@mailman.xmission.com> wrote: The figure of the mirror is held in place by the stiffness of the glass.  With multiple mirrors you need a massive mirror cell or dynamic motors to constantly fight against the sag from gravity.  In space it is easier because gravity is taken out of the equation.  There was a professional telescope called the MMT which was arranged in a six gun fashion.  But that was just to have something to do until the big onepiece of glass was ground, figured, and polished. So there is no real advantage in the long run. DT          From: Jay inUT <jayleads@gmail.com>  To: Utah Astronomy <utah-astronomy@mailman.xmission.com>  Sent: Wednesday, August 26, 2015 6:58 PM  Subject: [Utah-astronomy] Making a 36" from many smaller mirrors Ok, I got approached by my boss at work about the possibility of making a 36" mirror for observing from say several 8 inch mirrors (think an arrangement like James Webb).  In thinking about this to me it seems that there are issues here about positioning the mirrors and ensuring that once ground, they form a parabola etc.  So what are the thoughts on this? Can an amateur make a large mirror from smaller mirrors and what would it take? Just curious and I know what my reply was but I said I would inquire. -- Thanks! Jay _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options". _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options". -- Siegfried