Posted 10 January 2013 - 11:29 PM
Posted 10 January 2013 - 11:33 PM
Posted 11 January 2013 - 04:18 AM
Even with 360 grit you will change the focal length up to a couple inches.
Posted 11 January 2013 - 11:43 AM
I've seen too many mirrors by amateurs that are not fully polished at the edge. Dam shame with the price of aluminizing what it is.
Posted 12 January 2013 - 03:25 AM
Posted 12 January 2013 - 11:53 AM
...I am a believer of nailing the fl on #80 and getting a decent sphere...
...I've seen too many mirrors by amateurs that are not fully polished at the edge...
I added a bit to the f.l. when grinding my 6" f5, following the advice in texts that stated the f.l. would shorten during polishing. I haven't found this to be the case-- it hasn't budged a bit, despite all polishing MOT. So, I'd suggest nailing the f.l. at 80, and maintaining it with MOT and TOT to the end, too. I wish now that I had done mine that way.
re: edge polishing-- I think it may be helpful to frequently re-distribute the grinding medium between new abrasive charges. It seems the edges tend to push the grains towards the middle, an effect compounded by a slightly deeper center. Though I ground each grade for an hour or more each, I'm finding the edge VERY slow to polish out. It's distinctly possible that I need better distribution much earlier. At this point, I'm not 100% certain my edge is going to polish out, and I may have to backtrack to fine grinding. So, watch that edge!
Posted 12 January 2013 - 12:57 PM
The readings say it is hyperboloidal, so as you get close to R, shorten strokes/alt. You can do a lot of glass removal at your next grade once you know where you are and where you're going.
Posted 12 January 2013 - 04:10 PM
By the way, I don't think that #80 will ever get you a sphere, because the grit is too large.
Posted 14 January 2013 - 11:25 PM
Posted 15 January 2013 - 03:13 PM
How deep do you think the pits are? For #80 this surely is more than 0.001...
Posted 19 January 2013 - 01:09 AM
Posted 19 January 2013 - 05:02 PM
Since I generally build the telescope after making the mirror I don't worry too much if I am a little long or short especially with a Newtonian telescope. I just adjust my OTA dimensions as needed.
Posted 20 January 2013 - 02:47 AM
I usually move to #180 when sag is approximately OK and only then focus on getting a nice sphere. Once sufficiently sperical I wet the mirror and use a flashlight to measure F. Use longisch MOT or TOT to shorten resp lengthen.
Posted 20 January 2013 - 08:29 PM
Last grits, really overdo the time spent and you'll have an easy time polishing.