The 20 " grind has begun

[Astrobits]

I would say that you have three choices.

1. Use a smaller lap. This is what I would do as it would keep the 10" lap available for further work and you can switch from one to the other at will.

2. cut the existing lap down and/or make a star edge.

3. use the lap as is but set the stroke so that the edge of the lap is roughly over the centre of the mirror and press down on the edge as it goes through the mirror's centre. The lap will loose the profile of the mirror but that can be regained with pressing when required.

Nigel

 

[JRM]

Thanks for reply Nigel, much appreciated

   guess I forgot about my 5" starlap, will work the zone with this and see how it goes, I am

using zerconium oxide now so it may be a tad slower, will keep you posted.

Rick

[Ajohn]

Wait's may have an answer to this. Mixed strokes that depend on conditions. Might be 2 on the centre zone followed by a W and then 2 on centre a w and so on.

I managed to dream the same sort of thing up when trying to correct a figure with a small lap. That's the sort of thing he is using it for. Should work with a sphere too if needed.

He makes another important point somewhere as well. Relative area. Sub diameter laps work a lot more quickly in the centre than they do on the edge. Makes an odd sort of sense and may help me in the future. Might also mean more than on W.

John

-

[Astrobits]

As in Ajohn's post, mixed strokes to blend in to the rest of the mirror.

When using small laps the glass will seem to disappear very quickly so short sessions and frequent testing is the order of the day.

Don't forget that the central area of the mirror will be in the shadow of the secondary and the figure in that area is irrelevant to the final performance of the telescope. But as a mirror maker you will probably want it perfect.

Nigel

 

[JRM]

Okay,

using the 5" starlap I did a short 10 minute session to see how things would progress,with coc and W strokes to

keep things smooth, tested this morning and the hill is getting smaller, so more tonight and test again,thanks guys.

will keep you posted

Rick

[JRM]

Hi All,

 I can report the cental hill is gone and looks nice a smooth but...  what I had thought was a raised zone 1-1/2 inches in from

the edge is in reality a low zone so now my next mission to correct.

Zirconium oxide is what I have for polishing compound and though it seems slower it does appear to give a smoother surface.

I have a couple pounds of the stuff , lets hope I finish before it runs out  lol

I am thinking of using the 10" lap with a couple inch overhang to the side for awhile then moving inward to keep from making

more zones, the central area will raise up again but that can be corrected later, the lap is pressing, will try to get as much time

in as possible then test, keep you posted...

Rick

[JRM]

did two 15 minute sessions this afternoon only because I rotate the glass every 15 or it would have been just one 30 minute session.

both sessions were 7-1/2 minute 1-1/2 inch overhang and 7-1/2 minutes lap flush with edge, working a 10" lap on one side as table

turned 3 rpms, will test this evening see how things went, want to keep tabs on central area, will use the 5" lap for it during my sessions

as well if it starts to raise up again, which I am sure it will,will keep you posted.

Rick

[JRM]

Hi All,

 have been working with the 10" lap to correct the low zone out near the edge,might take a bit of work but did a 15 minute session

yesterday 1-1/2 over hang then tested, things look to be going in the right direction so another 15 minute session was completed tonight,

will test and go from there.

Rick

 

[JRM]

done with the 10" lap,remaking the 15 and going from there, tired of going in circles

Rick.

 

[Astrobits]

Not what I would be doing.

A 15" lap will struggle to maintain or generate any parabolic changes in the curve under it. You will likely be going back towards a sphere not forward to the parabola you want.

Don't forget that each zone on your ( parabolic ) mirror has a different ROC and ideally needs it's own lap. However slightly larger laps can be used that cover a few zones but your 15" will cover all zones from centre to edge trying to bring them to one ROC.

Circles---that is what mirror making is all about.

Nigel

[JRM]

true Nigel, point taken but I have some zones and somehow lost the edge yet again, returning to sphere to get things back in order

is the easy solution and then try again from there, learning things every step of the way albeit sometime the hard way.

have stripped the 10", the pitch is heating, will pour and set the 15" lap tonight and go at it again tomorrow.

Rick

[Astrobits]

One of the problems in figuring is identifying whether a depressed zone is above or below your target profile. It can easily be a depressed zone on top of a generally raised area. Without actually getting behind your tester I can only suspect that you still have a raised central area and have not yet got down to target all over the mirror leaving a low edge which will go once the rest of the mirror is lowered.

I think that you should carefully test your mirror and work on those areas that need lowering to reach the parabola. Do a full Foucault test and plot out the results.

Nigel

[JRM]

Have been trying to correct the edge problem with the 15" lap well awaiting more pitch but things have been moving

slowly, the extra pitch arrived this week so today I stripped the 15" and am currently melting pitch to make a new 20"

fullsize lap, this should move the edge correcting along a tad faster.

I have alsao picked up 6 aluminum pole 8 feet long, they only had the six so will have to get two more on another trip.

they are thicker then needed at 3/32 but all I can get so they will have to do.

Also ordered a spider and secondary holder with offset from astrosystems, should be her next week or so, figure I will pick

things up  bit by bit well getting this mirror done.

can not get applyply plywood here so it will be 3/4" thick baltic birch for the build when the time comes, but for now correct

the edge and go from there.

Cheers

Rick

[Physopto]

What between you and Damian it just goes to show the determination and shear resolve to get it right. Good luck with your endeavours.

Derek

[JRM]

Thanks Derek, it will be nice to reach the finish line sooner rather then later, but whatever the time line is it will be done, no rush

but hoping this summer.

Rick.

[mapstar]

On Sunday, April 10, 2016 at 20:15, JRM said:

Thanks Derek, it will be nice to reach the finish line sooner rather then later, but whatever the time line is it will be done, no rush

but hoping this summer.

Rick.

I've got my fingers crossed for you Rick, we all have 

Keep up the good work. We're cheering you on and looking forward to seeing the progress  

[JRM]

okay, after hours of channeling, then even more hours of pressing to shape and then pressing the mess

for micro faceting the 20" full size lap is good to go, I have finally started pushing pitch again to correct

a T.D.E that has been an issue for awhile.

I did 1 hours worth of CoC stroke, lap on top, varying between 1/4 to 1/3 stroke lenght.

it will take more work to correct the edge but first I want to make sure everything is going in the right

direction, will test in a few hours and see if it is getting better or worse and go from there this evening.

I forgot how hard it can be pushing a full size lap on the 20, real easy to work up a sweat.

will let you know when edge problem is corrected. Cheers

Rick

[mapstar]

20" lap that's some beast Rick.

Hope it does the job quickly for you and then return to figuring 

[JRM]

14 hours ago, mapstar said:

20" lap that's some beast Rick.

Hope it does the job quickly for you and then return to figuring

A beast it is Damian, and I hope so too but an added bonus is I may

have popeye the sailer man muscles by the time I am finished

 

[Gina]

I'm in awe at you people who make your own mirrors, such patience a dedication amazes me   I like to think I have a lot of patience but I don't think I could tacle that.  Great stuff

[Ajohn]

Nigel gave you some good advice Rick. Hope I have the right one but fringexp will allow you to play with the mirror rad and put errors where you want them to be or put another way allow you to minimise the work that needs to be done. The only catch really is the need at some point to but the knife edge pretty precisely on the mirror rad.

John

-

[JRM]

todays testing shows the t.d.e. problem is gone, the surface is showing signs of roughness and is not a perfect sphere but close,

so this evening I plan to touch up the lap channels, press with mesh,then polish with normal W strokes for one hour to smooth things

then round three of figuring begins.

6 hours of work with the 15" lap, then 12 hours of work with the 20" lap but alas it is corrected once again,cheers...

Rick M

[mapstar]

Nice Update Rick.

We're all still here eagerly awaiting the next instalment  

[JRM]

Hi All,

Before beginnig parabolizing I did a cold dry press with mesh, marked a seven inch circle centered on the back of the glass to use

as a guide for the stroke length and width, wet the lap with polishing agent, pressed and got at it.

My first session consisted of six trips around, mirror on top full size lap,full strokes out to guide lines with 12 strokes across from side to

side, (six forward and back) rotate and continue, after which I tested and noticed the center area was digging out much quicker then the

rest as was to be expected, not wanting to dig the center out to quickly my next session was four trips with less side swing to work the outer

area more and keep the center at bay, tested this morning and it worked nicely, now to move the correction out towards the edge and

keep everywhere else in check well doing so... but for now round three is under way.

just fyi, I do two rotations which take ten minutes then let the mirror press for ten before beginnig the next round,always doing an

even number of rotations, drag is heavy to say the least but then the glass is twice as heavy as the pitch lap so that is to be expected,

but a half hour session, (six trips) is enough of a workout for me, will keep you posted as always.

oh btw, my spider and secondary holder for astrosystems arrived yesterday as well, very nice indeed.

Cheers

Rick M

[JRM]

Hi All.

 Still slugging away, mirror on top, short sessions, things are moving slow but steady, had a few days off  to get away for the long weekend but

back at it, cold pressing after 5 days was a chore to no avail this evening even with 65 lbs weight, could not get full contact, will press with mess

tomorrow and then things should be back in full contact, curve is coming along well, have to push it out more to the edge so less parabolizing strokes

with center of mirror near edge of lap and concentrate on more strokes nearer to center over center, work a little, test a lot and devise a plan to

move forward after every saession, that in a nutshell is how it gets done, cheers.

Rick M