The 22" mapstar mirror

[barkis]

I,ve just concluded that the confusion about your mirror size, is due to quaoar's 28" Saturn image, which had a big impression

on the forum. A mobile phone capture through that huge mirror, must have focused my mind on the mirror's diameter of 28".

That's a good excuse Heh?

Ron.

[Stub Mandrel]

You've pretty much sorted out the edges and in doing so left a lump in the middle?

I know absolutely zilch about this but it seems like progress to me! The very regular shape of the 'hill' suggests to me that you must be working pretty symmetrically which is surely half the battle?

This makes scraping a surface plate seem like child's play!

Hang on in there.

[mapstar]

No worries Ron I may attempt a 28" one day but will have to finish the 22" first which is going to take a while ☺

Thanks for the kind words Stu and Stub Mandrel

I'm still working on the middle but it's so out of shape it will take a lot of work to pull back and I may have to work back to a sphere of sorts

Damian

[mapstar]

Quick update to the thread today.

The mirror has been sat under wraps most of the week as I've been concentrating on other less interesting things. It has been a break and I think I needed it, but glass fever is just around the corner again and I will be hopefully back to it next week.

I have today been readying for a trip to see John, and fingers crossed, formulate a plan to get things back on track and see the mirror moving forward.

Damian

[estwing]

good luck mate.........

[mapstar]

Afternoon all,

I made the three hour round trip to see John yesterday with my mirror safely tucked up in the boot.

As ever John opened the door with a welcoming grin and friendly hand shake.

After a cuppa and a chat on various bits of mirror making we moved on to testing my 22" on his auto collimation rig which is extremely sensitive to picking up surface errors.

Initially it was a play around with getting it set up after John had asked what focal length I had aimed for. This gap changed quite a bit but more on that later.

He confirmed that the turned edge had gone and the correction in the outer zone was good, uniform, and had a nice overall smoothness.

Onto the middle and after discussing technique he told me the verdict in his usual straight talking honest style,

Basically I'd not varied the lap enough whilst doing the edge work and cut a channel around the middle due to the circumference being smaller.

First option the long haul with cerium which may result in other problems along the way

Or secondly and rather reluctantly go back to fine grinding and Repolish which would save a lot of time.

I have spent a lot of time getting to where I am now but it hasn't been wasted and I've learnt a lot on the way.

Time to learn a bit more

So it's back to the 400/600 grit and then Repolish

I am still debating on whether or not to do it again myself (third time lucky) or return to John and set it up on the fixed post machine of his to save me some time and get me back to where I should be now. I have already discussed it with John who is more than willing to help.

Hey ho

Damian

Edited June 20, 2015 by mapstar

[Mark62]

It's a shame you are not were you wanted to be with the mirror but it is a learning process.

Which ever way you decide to go be it on ur own or with johns help everyone will support you every step of the way as we have up to now.

Mark

[Physopto]

At least you know now from a well respected friend and professional. As with most things that are worth doing  in life it is a learning curve. But you have gone this far ! 

Whatever you decide good luck.

Still got the Brasso !

Derek

[barkis]

Damian, I applaud your decision to return to the fine grinding to remedy the problem.

It will speed up the correction required, and then your polishing and figuring can be accomplished

now you are aware of the reasons why it all went wrong first time, you are now forearmed with the knowledge John has 

given you.

Don't for one moment think yourself a failure, on the contrary, your upfront reports on this terrifically difficult

project will have given others contemplating making a large objective, an excellent insight into 

the difficulties that await the unwary. 

You might feel a bit disappointed, it's only natural, you have worked long and hard to create a massive

light bucket, and you have learned much, and you will now progress to a successful conclusion.

It is still going to be no easy task, but you are better prepared now.

The old saying still applies.

Those who say they have never  made mistakes,

have never really made anything.

All the guys will remain loyal to your efforts, and will you on  to complete the job you

set out to do. 

Remember, you are not in a race here, you are going to create something you will be immensely proud of,

and when it's complete, and nestling  in it's Dob. base, waiting to take you back through time,

in a splendid way, the rest of us will want a look see too  .

Go for it buddy.

Ron.

[mapstar]

Guys

Thank you all for the kind comments and words of wisdom.

It is without doubt a difficult thing to go back wards again but the best option at present.

I have already made a start and have just moved the grinding table to the garage after cleaning it down and dug out the tile tool ready for the 400grit.

After cleaning the tile tool thoroughly I'll make a start. Hopefully it won't take long to fine grind back to a sphere.

I'll as always keep all posted on the progress

Damian

[RLWP]

Damian, you aren't going backwards. You've had a bit of practice around the use of fine grits, polishing and mirror measurement. Now you are completing the basic shape and finishing the journey much better prepared than when you started out

Per Ardua ad Astra

I think you will be surprised how quickly it goes, most of my foul-up rectifications do anyway

Richard

Edited June 20, 2015 by RLWP

[mapstar]

Damian, you aren't going backwards. You've had a bit of practice around the use of fine grits, polishing and mirror measurement. Now you are completing the basic shape and finishing the journey much better prepared than when you started out

Per Ardua ad Astra

I think you will be surprised how quickly it goes, most of my foul-up rectifications do anyway

Richard

Nicely put Richard

Quite right.I do remember the fine grinding didn't take me long last time.

As the saying goes back to the grind and hopefully by the end of the week I will be something like.

[RLWP]

I'm quite pleased with the RAF motto, it seems to fit your situation very nicely 

Nil Desperandum*

Richard

*Nil Carborundum Illegitimis seems somehow appropriate too

[RAC]

Did John say anything about polishing out with a larger lap?

[mapstar]

Did John say anything about polishing out with a larger lap?

Hiya Raymond

No he didn't suggest it, why do you ask have you read about it somewhere?

We discussed the 18" lap I already have and also polishing via machine to avoid the turned edge.

[RAC]

No I thought you had polished it out with the 10" lap.

[Soupy]

Now the TDE is gone and you have a solid course of action! Excellent work buddy and good luck this week with the 400. Keep us posted!

[Astrobits]

If that's John's opinion then so be it. If, as John says, you have a deep zone around the central hill ( which is presumably below your final parabola ) then reverting to fine grinding is the best way to get back on course.

I find that Ronchi tests are not my cup of tea, it is difficult to tell from your last image just what the profile is. I prefer something a little more positive. Using the fringes at the edge of the knife shadow in a normal Foucault test, for example, to give an approximate depth/height of any zones. That way I can pick up any wayward area before it gets too bad. For the 20" mirrors that I have made I have never used a grinding or polishing lap bigger than 13", but then I used a machine that I had built and a spherometer that reads to 0.001mm ( direct reading of 2 waves although I could estimate to 1 wave ). The spherometer kept the grinding on target and when polishing the disappearence of the pits was closely observed and the stroke changed so as to be even over the whole surface.

Fine grinding and polishing will go much quicker now you have so much experience under your belt.

Nigel

[estwing]

Mr Nichol...

[mapstar]

If that's John's opinion then so be it. If, as John says, you have a deep zone around the central hill ( which is presumably below your final parabola ) then reverting to fine grinding is the best way to get back on course.

I find that Ronchi tests are not my cup of tea, it is difficult to tell from your last image just what the profile is. I prefer something a little more positive. Using the fringes at the edge of the knife shadow in a normal Foucault test, for example, to give an approximate depth/height of any zones. That way I can pick up any wayward area before it gets too bad. For the 20" mirrors that I have made I have never used a grinding or polishing lap bigger than 13", but then I used a machine that I had built and a spherometer that reads to 0.001mm ( direct reading of 2 waves although I could estimate to 1 wave ). The spherometer kept the grinding on target and when polishing the disappearence of the pits was closely observed and the stroke changed so as to be even over the whole surface.

Fine grinding and polishing will go much quicker now you have so much experience under your belt.

Nigel

Thanks again for commenting Nigel

Foucault knife edge testing is easier to look at for overall shape for my inexperienced eyes.

I will have to build myself a proper tester to supplement the ronchi I know Stellafane have an example on their site.

I have managed two hours fine grinding today with the 400 grit . I will continue tomorrow and hopefully move onto the 600 alu ox

Damian

[barkis]

Damian, for what it's worth, I used to add a tiny drop of Washing up liquid to my fine grind wets.

It seemed to help in distribution and lubrication, and prevent locking up of the discs.

Unlikely anyway with the smaller grinding tool, but it could happen.

I've had a full sized tool get stuck fast on the mirror, because I didn't keep the tool moving, It was fatal to stop.

Devils own job getting them apart too  .

You've been through that phase once though, so as I say, unlikely you will experience it this time around either,

but I'm just making you aware of the danger.

Ron.

[mapstar]

Thanks for the advice Ron.

I have read somewhere about the washing up liquid before. I wasn't sure if it was just myth but I am convinced now

The grinding today went well and threw up some interesting insights into the shape I've ended up with.

I shall post an update later tomorrow maybe. Hopefully it will be good news

Damian

[mapstar]

Mr Nichol...

Yeah

[mapstar]

Afternoon all,

Although quiet for a few days I have been busy in the mean time.

The mirror has had the tile tool on again and after 4 hours with 400 grit silicone carbide testing to make sure I had good contact and then a further couple of hours to make sure. Because I'm not grinding large 180 grit pits out the grind didn't take that long.

So with the 400 grind completed I moved onto two hours with the 600 Aluminium Oxide

I will probably do another couple of hours with the alu oxide to make sure.

I have also been tidying up the 18" lap ready for the move to polishing again (third time )

Damian

[RLWP]

This is an exciting development

Richard