Click here if you are having trouble logging into the forums Cloudy Nights - Official Forums of the 2009 International Year of Astronomy Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User   You are not logged in. [Login] Entrance · Main Index · Search · New user · Who's Online FAQ · Calendar Equipment Discussions >> ATM, Optics and DIY Forum Previous   Index   Next   Threaded   Pages: 1 awallin member Reged: 04/01/08 Posts: 24 240 mm F/6 Newtonian project log       #2550576 - 07/30/08 02:57 PM Edit   Reply   Quote Quick Reply   Hi All, I am making my first mirror and my first telescope - hope to see first light during 2008. I'll try to post updates as work proceeds. I'll probably have a lot of questions along the way! First task was to obtain some 240mm diameter borosilicate (pyrex) blanks. They are about 45mm thick and we started by grinding both sides flat against a steel plate using no60 carborundum. There is a before/after picture here: http://www.anderswallin.net/2008/07/starting-to-build-a-240mm-newtonian/ Post Extras:       awallin member Reged: 04/01/08 Posts: 24 Re: 240 mm F/6 Newtonian project log [Re: awallin]       #2550594 - 07/30/08 03:05 PM Edit   Reply   Quote Quick Reply   Some more work on the mirror yesterday. We noticed some chipping of the edge, so decided to follow all the grinding guides out there and put a bevel on the blank. Usually the guides say this is done with a sharpening stone, but I thought this went horribly slow. Much faster with a small 2mm diamond-tool on a dremel! There's a risk of touching the mirror surface with the tool, so I wouldn't do this kind of beveling very late in the grinding/polishing process! Also started to 'dig a hole' into the mirror grinding glass against glass with no60 carborundum. As advised by a local long-time ATMer we're using a turning table. Straight strokes, mirror on top, with a lot of overhang. We got to about 1 mm of sagitta in ca 2 hours of grinding. http://www.anderswallin.net/2008/07/mirror-grinding/ If anyone has comments on good beveling technique let me know! The next evening we find time to work on this I expect we will get quite close to the design sagitta of 2.5mm. Is it advisable to grind it down to 2.5mm using no60 carbo or should we switch to the finer grades sooner? Post Extras:       ken scharf sage Reged: 02/09/08 Posts: 393 Re: 240 mm F/6 Newtonian project log [Re: awallin]       #2551975 - 07/31/08 08:02 AM Edit   Reply   Quote Quick Reply   Note for us Americans. That's about 9.5" and a 5:1 thickness ratio. That's MORE than a full thickness blank! Post Extras:       awallin member Reged: 04/01/08 Posts: 24 Re: 240 mm F/6 Newtonian project log [Re: ken scharf]       #2559951 - 08/04/08 05:14 PM Edit   Reply   Quote Quick Reply   The place where I got the mirror blanks receives the Pyrex as a square sheet of 50cm x 50cm with a nominal thickness of 45 mm. We had four 240mm blanks cut from this sheet, and by the time the faces are ground down to being flat the thickness is closer to 39-40mm which makes for a 1:6 ratio. Two more hours of rough-grinding today: http://www.anderswallin.net/2008/08/rough-grinding-continues/ with no60 and no80 carborundum we are now pretty close at 2.3mm to the target sagitta of 2.5mm. What is the usual plan people use for achieving the target sagitta/focal-length? All the way to 2.5mm with the coarser grits, and then adjust back and forth with the finer grits? Or slowly approach as we go down 80-150-220 ? It's clear we now need to build a focault/ronchi test rig before proceeding much further. Post Extras:       Pages: 1 Previous   Index   Next   Threaded   Extra information 13 registered and 2 anonymous users are browsing this forum. 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