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mapstar

The 22" mapstar mirror

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Hi all,

Not much change today I'm afraid and more work planned tomorrow. Also going to continue with the shell build as I need to get that something like.

An update with a few images tomorrow afternoon.

Damian

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Hi all ,

I did a couple of hours on the glass again during the weekend working both the edge and the centre to try to smooth things down.  The edge is reducing further but from the experience I had last time with this I know the last bit is the hardest to remove.

Here's the latest image 

post-28847-0-76640800-1442936399_thumb.j

I have done a lot more work on the shell build which seems to be coming along quite well. More work on the mirror but time is marching on and if I intend getting this sorted for the up coming events I have planned I need to speed things up.

Last time I had the turned edge problem I went about trying to chase it with the middle of the lap to the edge which is what I have done this time. The main difference is I have spent more time inside each zone moving ever so slightly out as the kink has. Last time as others have said I was creating the turn as fast as I was trying to remove it by working nearly on top of it. A lot of wasted effort and many thanks to those that were trying to tell me that.

It is all experience. Just to theorise (just my thoughts) because of how a person has random motion with stroke, pressure and stance I can see why it is more difficult to remove a turned edge than by a machine which has a set path and pressure so eventually the zone around the edge will polish down  and blend in the offending edge.

More work soon and I know I've been slow with the updates so I will try to add more 

Cheers

Damian

Edited by mapstar
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Hi Damian,

As much as it is nice to read your updates, they are not the be all and end all of your build. We can wait, more important to concentrate on feeling good about your own work on the mirror or the shell of the scope. For your own sake forget us and just take your time. It is not our time, money and effort, it is yours and we are just the interested bystanders. Feeling rushed is probably the worst thing. Just take it easy. I built a model boat a long time ago (nearly 30 years ago). Took me 3 years and £2000+, every time I rushed it went wrong!!!!

Derek

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Hi Damian,

As much as it is nice to read your updates, they are not the be all and end all of your build. We can wait, more important to concentrate on feeling good about your own work on the mirror or the shell of the scope. For your own sake forget us and just take your time. It is not our time, money and effort, it is yours and we are just the interested bystanders. Feeling rushed is probably the worst thing. Just take it easy. I built a model boat a long time ago (nearly 30 years ago). Took me 3 years and £2000+, every time I rushed it went wrong!!!!

Derek

Thanks Derek.

I remember the model boat story and the bits where the wife fitted in.

I'll stay focused and hopefully get there but as you say it's only me setting the time scale.

Damian

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Hi damian,

Its looking good and I admire your patience so im sure it will finish up fine. As others have said just take your time but I was wondering how many hours have you spent on the mirror so far ?

Cheers

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I have to admit I've stopped counting. It's been a lot and to hazard a guess would be nearly 300?

Scary to think

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Scary thought indeed but will be well worth it when you have a scope you can be proud of in the end buddy.

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Fortitude and Stickability, the bywords of Newt. mirror makers.

One has to have it, or failure will result almost for sure.

You have both attributes Damian, so success is probably just  around the corner.

This was a monumental task you set for yourself, and that realisation dawned on you as time

went on I'm sure, but you have battled on, and all credit to you for that.

I hope to see a nice Xmas gift for you, in the form of the  completed Mirror, and I think that will be so.

I had the pleasure of viewing through a 20" f4.7  last week, and mind blowing just doesn't cover it,

so I hope one day to put my eye to the Mapstar Scope  Focuser one day, and get the awesome feeling 

of being back in time again.

Like all in this thread, We're willing you to get this done, but in your own good time.

Ron.

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 Took me 3 years and £2000+, every time I rushed it went wrong!!!!

LOL! I  made a model boat that took 16 years and cost me about £20  :grin: 

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LOL! I  made a model boat that took 16 years and cost me about £20  :grin:

That confirms my Fortitude and Stickability theory. If you have it,

You'll never ever give up. :smiley:

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I have been reading this with interest and I've every admiration for the perseverance and dedication shown on this project, it would have defeated me long ago!

One question I have is how commercial telescope makers overcome mirror issues such as turned edges, central mounds/hollows etc. Is this dealt with on the basis of computer controlled grinding and polishing, where pressure, stroke and speed is governed very carefully to a specific program?

Wishing you all the best for a succesful project completion!

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I have been reading this with interest and I've every admiration for the perseverance and dedication shown on this project, it would have defeated me long ago!

One question I have is how commercial telescope makers overcome mirror issues such as turned edges, central mounds/hollows etc. Is this dealt with on the basis of computer controlled grinding and polishing, where pressure, stroke and speed is governed very carefully to a specific program?

Wishing you all the best for a succesful project completion!

For grinding and polishing the fixed post method would be the most common but I maybe wrong. It's easy and it works great. If I was to put Damian's mirror as it is on my fixed post machine with a 70ish % sized lap I would say you could have it back in good shape right to the edge in about 3-4ish hours so long as the lap was in good shape and playing the game.  When using a fixed post setup the edge never really is a problem if your offset is right, In most cases it will leave a small hill in the middle but normal figuring strokes take care of that anyway.

You can cut through a lot of glass with ceruim some weight a good lap and a fixed post grinding machine.

Figuring is a little different and requires constant testing nearing the end so most would do this by hand.

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I have been reading this with interest and I've every admiration for the perseverance and dedication shown on this project, it would have defeated me long ago!

One question I have is how commercial telescope makers overcome mirror issues such as turned edges, central mounds/hollows etc. Is this dealt with on the basis of computer controlled grinding and polishing, where pressure, stroke and speed is governed very carefully to a specific program?

Wishing you all the best for a succesful project completion!

Thanks for reading and commenting.

Raymond above has already answered the question you've posed in his above post.

I had thought about this in one of the recent posts above why machine polishing doesn't seem to encounter the turned edge problems I have encountered along the way.

Because we as humans are very random it is quite easy to subconsciously apply uneven pressure and movement to the lap when trying to polish an edge problem out and make it worse.

Experience counts for a lot too and last time my edge problem got to a point where as others noted I was polishing it in as fast as I was removing it.

Doing it via a fixed post machine it has constant pressure and path but can make its own problems but turned edge if it is set up right doesn't seem to be one of them.

As Raymond says it tends to polish non spherical and can put in zones (bumpy surface) but these are probably easier to deal with than TDE.

I've yet to get anywhere near but will keep trying.

Damian

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Anyone contemplating making their own Newtonian mirror, and especially if it's a first time  project,

I recommend doing lot's of homework before you start.

There are so many variables that come into play during the task, it pays to be fore armed with certain knowledge.

The degree of difficulty is much  more than directly proportional to the size of the disc.

In the case of very large diameters, the full size lap becomes a liability, and therefore smaller tools have to be employed.

These can be very difficult to use without introducing  some serious anomalies that are difficult to correct.

Without extreme care, they are positively dangerous, and in not too long a time at that.

Pitch consistency, allied to temperature changes can also cause havoc. 

Turned Edge is the Bane of mirror makers, but the best cure is not to let it happen in the first place.

Read about what the causes are, and do your best to avoid them, many do, because they educated themselves as to why it can happen.

It isn't easy to have a place of work where a constant temperature can be maintained.

Working with the mirror face up also enhances the risk of scratches, small particles floating about and landing on the 

mirror face is all takes, so cleanliness is another must.

Not intended to dissuade anyone from having a go at this, but be aware it takes much consideration, and a determination to succeed.

A first time creator, would be advised to take on a smaller disc, about 200mm diameter is  good size for a beginner. No more than a 250mm if

you are brave. Those sizes will permit you to work with a full sized lap for polishing and figuring stages,

and although it make some cringe, allow working the mirror on top of the lap.

It is easily controllable,  and with small alterations to the lap facets, parabolising can be accomplished too.

You can make two full sized laps, the second one can be  a repair tool if it becomes necessary

It is a very rewarding thing to complete precise optical objective, I hope amateur mirror making stays healthy,

If you have the mettle to do this, you will likely become a very good observer too.

Ron.

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Ron makes some good points in his post many of which I can relate to.

It is a difficult task and one of course I cannot say I'm successful at. I hope to one day but it will be a little longer.

Thanks for the kind words and advice Ron for all those that I hope will take up the challenge of trying to craft a mirror.

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I just hope nothing I said hurt your feelings in anyway Damian, that piece was a generalisation, and not targeted at anyone at all.

I was reflecting   :smiley: (No Pun), on my own experiences over the years, and my own lack of so many fundamental preparations that resulted in

many problems, and the need to retrace a lot of work that could have been avoided with some forethought.

I would encourage anybody to take it up though, It has it's many rewards, and not least the utter satisfaction of a job well done.

It's never easy, if it was, there would be a shortage of the necessary materials  :grin:.

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I just hope nothing I said hurt your feelings in anyway Damian, that piece was a generalisation, and not targeted at anyone at all.

I was reflecting :smiley: (No Pun), on my own experiences over the years, and my own lack of so many fundamental preparations that resulted in

many problems, and the need to retrace a lot of work that could have been avoided with some forethought.

I would encourage anybody to take it up though, It has it's many rewards, and not least the utter satisfaction of a job well done.

It's never easy, if it was, there would be a shortage of the necessary materials :grin:.

Not at all Ron, there are lots of sound words in there many of which I should've heeded before attempting my monster.

I am my harshest critic and the only person I have to answer to which is probably a big advantage when it comes to the time I have spent on the project.

More work soon and I am hoping to progress everything to a successful conclusion

Damian

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More work soon and I am hoping to progress everything to a successful conclusion

Damian

You will my friend, you will....never rush perfection,time is on your side.

Rick M

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Thanks Rick the build is at least looking good. The mirror will progress lots over the coming couple of weeks.

Damian

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Hi all,

I have been quite busy with the whole project this week so here is a bit of an update. Sorry their are no pictures to add to go with the progress descriptions.

The build is well under way and on the mirror front I have had made the trip north again to see John for a little more help and input.

The mirror has been worked for a further 16 hours up to now and the results have been a big improvement.

The edge has been sorted so that it no longer exhibits any signs of turn.

The mirror now shows an Oblate sphere for the outside 75% but still has an issue in the middle which is like a depressed ring zone.

Hard to explain but I will upload the photos once I get time.

All good progress and I am enthusiastic about what the coming days will bring.

Damian

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Look forward to seeing the photos and progress you have made buddy! :)

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Cheers Paul. Hopefully up date everything on Sunday evening.

More progress though today and things are looking a lot better.

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Morning all,

The last 3 days have been somewhat of a revelation as I have been working closely with John and plundering his knowledge to bring what has been a troublesome project back on track.

Having worked the edge of the mirror with an 8" lap centred on the 75%/70% zone for quite a few hours the Images were indicating an Oblate sphere with what looks like a small centre issue approx 4"-5" in diameter.

If you were to think of the shallow crater ontop of a hill analogy this would probably describe it better.

A lot of thought went into the next stage of the remedial work and the conclusion was to return to the 10" lap and work with that with the lap centre having an elliptical stroke that went through the 25% zone overlapping the mirror centre. The stroke started and ended with the edge of the lap roughly 1/4" from the edge.

This would work the zone but also the surrounding mirror to blend things in.

After 3 hours work the mirror centre issue has all but disappeared with just a small amount evident.

The mirror is now showing a smoothly under corrected figure ready to make a start on parabolizing again.

All in all great progress and extremely satisfying to see something go the way you want it to for once.

Again apologies for the lack of ronchi images to go with the posts but I will add these in later as it better shows the surface changes than any of my descriptions.

Thanks for reading and more updates soon

Damian

Edited by mapstar
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Cracking news Damian. Sounds like you have renewed vigour and passion for what was once grinding (excuse the pun) you down!

Can't wait for it to get figured and for you to start your star testing! :)

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Nice to know you are getting to grips with it Damian.

Proceed with caution here on in, work a little, and test often.

Don't let anything nasty creep up on you, as you are likely on 

path to  "Eureka".

Ron.

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