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The 22" mapstar mirror


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Have you a time scale for the scope?

I'll be hoping to be using it in September Calv but these things have a habit of a drifting timescale
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Hi all, I began this journey way back in Oct 2013 so it has taken a while to get to where I am at present but thought it about time I put my experiences down. Wanting a larger scope than the 8" I had

Hi all, It's been a long time in the creation and the update is a little late so after apologies here it is. As I left the thread last the mirror was in the hands of John and he informed me every day

Hi all, It's been quite a while since I last updated the thread and posted a few images.  So here is what has happened and where it stands at present I spent a total of five days working with John on

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Hi all,

Its been what seems like an age since I added the last update to the thread so here goes with the next installment.

After fine grinding the mirror I re-cut the channels in the lap and left it stood for a while. The warm temperatures have played havoc with the pitch with the channels closing up as it becomes more viscous. I had to remove the lap from press until the temperatures had settled back to the usual British summer.

Having discussed things with John about the re-polish I took the decision to accept his kind offer of getting me back on track by putting it on one of his machines.

So yesterday I took the trip north again after boxing up the mirror and lap to be met as usual by John at the door with his friendly grin and welcoming handshake. In the astronomical community I don't think you'll meeting a more helpful and encouraging person.

After a quick cuppa John checked the lap and mirror which were o.k. and went ahead with warming the lap to do a press with the mirror ontop looking down through it to see how the contact was. Not very good the middle due to the hot temperatures had sunk a little as I'd had the lap face up.

After several hot presses the lap was looking much better so it was onto setting up the machine to polish. A boss had to be screwed to the back of the lap to accept the centre post of the machine.  A few photos will help explain this better so here it is on the table polishing away.

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After 20mins we checked the contact on the lap which was much better. I was surprised at the amount of cerium that John uses compared to me (a lot less  :eek: ) The lap was much different to hand polishing as the impregnated cerium was worn away and as some would say the lap had gone back to its natural black colour. An hour on the machine was I have to admit a lot easier than the 4 hours of hand polishing that would take me to the same point!

After an hour the mirror was removed and roughly checked as it needs a lot more polishing. There is no astigmatism but it will take a lot more to determine whether or not the fine grind was a success.

I have with a heavy heart left the mirror with John for now, as he is going to polish it back to where I should be now so I can bring it home and attempt to figure it.

All for now 

Damian

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Hi Damian,

Glad you are now getting somewhere. Hope it works, just take it as a lesson learned. If you don't mess up you don' t learn anything, I should know, it has happened many times to me on other things.

So when you build your new grinding / lapping machine for the 36" mirror........................................................

Yes I know, I can hear all the flowery words even now!

All the best,

Derek

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Hi Damian,

Glad you are now getting somewhere. Hope it works, just take it as a lesson learned. If you don't mess up you don' t learn anything, I should know, it has happened many times to me on other things.

So when you build your new grinding / lapping machine for the 36" mirror........................................................

Yes I know, I can hear all the flowery words even now!

All the best,

Derek

Thanks for the encouraging words Derek.

John has a lot on his plate at present and I'm really grateful of him getting me back to where I should be.

The figuring is something else and will take a while.

On the scope front the ply is ordered so in the meantime I will March on with that as I at least know how to do those bits

Damian

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It sounds like biting the bullet and letting John help you has freed up your head to get on with the rest of the scope. First light is nearer now than it was!

Good stuff, Damian - I'm looking forward to sawdust covered posts

Richard

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It sounds like biting the bullet and letting John help you has freed up your head to get on with the rest of the scope. First light is nearer now than it was!

Good stuff, Damian - I'm looking forward to sawdust covered posts

Richard

Thanks Richard

I'll put the scope build in another thread which will be a while yet but it should be a good read.

John said he will sort it to the point where I can then continue with the figuring part which in itself is going to be very challenging.

Just have to get my plans together to make a start on the woodwork

Damian

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I know leaving it with John was not you wanted to do but if it gets you back to were you should be then I think it's for the best (not that I know a lot [emoji5]️)

All you have done is saved your self 6 to 8 hours polishing and there is still lots of work for you to do on it [emoji106]

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Great stuff buddy. Takes some pressure off you now to think about the rest of the build. A change is as good as a rest and when you get it back I'm sure you'll be ready to figure it like a pro. John has been a great mentor and help to you, and I'm sure he'll also help you on successfully figuring the monster mapstar mirror. (See that... some alliteration there, always thinking about educating ;) )

That polishing machine looks to be moving fast as well for some reason I thought it would go slowly. :)

Edited by Soupy
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Hi all,

I had an update from John today and after several hours on the polishing machine the mirror is polished out and has a good overall shape. No turned edge and no scratching.

It's not a perfect sphere as due to the high ambient temperatures we've had lately the lap centre has tended to sink a little creating a slightly raised middle which shouldn't be too difficult to rectify (a lot easier than TDE!!) . I will be collecting the mirror at some point this week and will discuss the next steps thoroughly with John so  that I can once again carry on my journey and keep you guys up to date with hopefully some successful progress.

Here are the images that John sent me of how the mirror stands with the ronchi test. The first two images are inside centre of curvature and the last outside

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Many thanks to John for his help here and hopefully I can take this to it's conclusion

In the meantime I am going to be the proud owner of a few sheets of birch ply ready for the build  early next week.

More to follow and thanks for bearing with me guys

Damian 

Edited by mapstar
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I've just started parabolizing my 20" f3.8 using the same method I found best with my 18" and it's working great !!!

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I've just started parabolizing my 20" f3.8 using the same method I found best with my 18" and it's working great !!!

Is that on your machine? If so can you explain how your going about it?
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Yeah the mirror is spinning but only slow like 2-3 rpm. I do W's in 3-4 steps working my way out till lastly covering the full mirror. Each step lasting about 10 rotations. So in effect working only the center out to about the 50% mark then make the W larger to cover out to the 70% mark then 80% then the full mirror and back to the start again. I'm using an 8" lap with the very edge pitch squares pointed to help blend the laps action on the mirror. This is producing a very VERY smooth zone free deepening of the mirror making its way to a rough parabola before fine figuring using Figure XP and a couder mask.

I tried a lot of methods with my 18" and this one I sort of ended up stumbling on myself after doing it for a bit and seeing a nice outcome so I never stopped.

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Interesting method and good explanation thanks.

I will obviously be hand figuring and as ajwawp it will be a challenge although one thing John has already told me is that the figure needs to be there but it's nothing without being smooth.

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I've made a little video for you showing what I'm talking about. You can see I work out to one diameter for a bit then move further out in steps.

You can see how this works if you look a a Figure XP graph of the mirror. I made this data part way through today. The first pic is what the minimum amount of glass removal would look like but it's not realistic as trying to scrub glass off the edge could cause problems as you know. So it's the second pic that we are trying to flatten out with this method. When getting close to a flat line you can modify the method as you go.

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Also here's a pic to show how smoothly it applies correction. No sharp zones just nice curves. This is the only method I have used with this mirror so far ever since finishing spin polishing and cleaning up the center.

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