9.25 inch refractor project
Posted 16 January 2013 - 07:49 PM
I spent a lot of time polishing to try removing the turned edge but have seen it was not getting it done in a timely manner, I'd be polishing for hundreds of hours at the rate it was going. After popping off the flake on the flint , that pretty much cinched the decision to go back to fine grinding.
Posted 16 January 2013 - 08:26 PM
Posted 16 January 2013 - 09:31 PM
Posted 17 January 2013 - 12:41 PM
Posted 17 January 2013 - 12:45 PM
Posted 17 January 2013 - 07:50 PM
Posted 19 January 2013 - 02:13 PM
Last time I ground the R2 and R3 curves on the objective by crown on flint since they are the same curves. This time I am going to use the glass tools all the way on all the surfaces just in case i need to go back and fine grind again. I am also going to use them for support during polishing by using the silicone again. The edges will be taped to prevent grinding compound from getting between the support base and lens. The lens will be supported over its entirety that way in order to avoid the turned edge due to flexure.
Posted 19 January 2013 - 02:34 PM
As you grind, monitor both your radii and center thicknesses, and I can keep nudging the design back to optimum with ZEMAX as things change. Watch out to keep your wedge <0.001" on both elements.
Glad to help,
Posted 19 January 2013 - 03:06 PM
I'll be a lot more delicate with the lens materials this time around !
I printed out your program you ran for the different lens thicknesses and did some interpolating from that as to what values to get on the spherometer to keep the objective design on target. It turns out that it doesn't change much per .02 inches thickness change. The chip is 3/4 inch diameter. It really only affects the aesthetics of the lens more than anything but I am going to grind away on the flint anyway and get rid of most of if not all of the "oyster". It is the crown that has the turned edge and needs the fine grinding. The focal length was within .3 inches of what your program predicted and more careful control of the radii will get me closer this time. The color correction was excellent just the sharpness was off due to the turned edge.
Oh, and the edge thickness variation was less than .001 inches on both lenses.
Posted 20 January 2013 - 01:53 PM
Fortunately I have extra pieces of glass and I've already generated a replacement lens, lapped on side and getting ready to start on the second side.
Posted 21 January 2013 - 04:06 PM
I started the re-grinding process to help remove some of the chip and get back to a sphere on 3 of the 4 surfaces. The design is one where the R2 and R3 are equal in radius. Because I used the flint to grind on the crown from 120 grit on down, the glass tools for those 2 surfaces needed 120 grit to work them down to the final grit size. Therefore I had to start out with 120 grit on R2 and R3 even though it did not need to go that coarse for the purpose of getting rid of the turned edges. R1 was taken down to very fine so I do not need to go with 120 on it, I'll start with 320 grit there. The only 3 surfaces I am working are R1, R2 and R3. I know the crown was the lens element with the turned edges from testing with the flat. This time I will be using the glass tools as support on R1 and R2 to eliminate flexure.
Here is where I am at now after regrinding the flint R3. The edge chip has greatly diminished after .02 inches of glass thickness removed. The numbers on the edges are thousandths of inches variation in edge thickness. This now varies by .002 inches max. I'll take care of that in the 220 grit phase.
Posted 21 January 2013 - 04:12 PM
So now Mr. Mike Jones, my question to you is what should the different radii be with the flint at around .550 thickness and the crown at around 1 inch thickness ? Do you still have the data for this BK7 and F2 lens combination for the f/12 design (106.3 inch focal length)
Posted 21 January 2013 - 04:22 PM
Well - well - well, I finally got the melt data for the new BK7 glass. Are you ready Mike ? !
The new BK7 is listed as the following;
d-line = n,d/e 1.51660
V/de = 64.12
e-line = n,d/e 1.51852
V/de = 63.91
g line = 1.52649
F line = 1.52218
F' line = 1.52263
C line = 1.51412
C' line = 1.51452
The flint element is the same as before and so has not changed.
the flint is;
The F2 flint;
Can you get me a new printout of the f/12 design as it relates to the new values Mike ? I will fabricate the objective to those and we will see how it turns out. I am excited to see how this all turns out. Thanks Mike for all your help !
Posted 21 January 2013 - 04:35 PM
Posted 21 January 2013 - 08:01 PM
Posted 21 January 2013 - 09:00 PM
This all looks good Mike and I will adjust accordingly.
Thanks for the update !
Posted 21 January 2013 - 09:58 PM
Posted 24 January 2013 - 10:34 PM
The sphereometer has indicated that the radii are as close as possible to calculated values as they can get using the 1/10,000 inch marks.
The small chip in the flint is quite diminished in size now and will not be a factor in the mounted objectives light path in the scope. It is just a matter of being careful now not to ding them anymore !!! Polishing will commence in about a week after my Gugolz pitch comes in.
Posted 24 January 2013 - 11:25 PM
Post your final ground CT's and I'll tweak the design. Hopefully your radii won't have to change from nominal.
Posted 25 January 2013 - 12:38 PM
Posted 25 January 2013 - 04:03 PM
Posted 25 January 2013 - 06:09 PM
Posted 26 January 2013 - 12:00 AM
R2 and R3 = 38.76
R4 = 171.135"
These are as measured with the spherometer Mike to the nearest .0001 inches.
Posted 27 January 2013 - 02:40 PM
Can't wait to hear about the imagery!