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mapstar

The 22" mapstar mirror

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Hows it going?

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Hi all,

I've been making steady progress with the mirror,  although I have spent more time on the wood work and actually getting out to view lately so been getting up late and then work so progress is slow.

Here's how things stand at present the new lap is working nice and feels great to push across the glass.  

Inside ROC

post-28847-0-55298900-1439461538_thumb.j

Outside ROC

post-28847-0-82882100-1439461570_thumb.j

I have a zone to blend in around 50% radius so have had an hour or so blending. I may have to star the lap (cut sections out of the edges as Raymond says above) to give the curves a smoother transition. I am going to have another couple of hours and see how things progress all the while checking against my test images to make sure I'm not going too far.

Damian 

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It's time to start using Figure xp. I wouldn't personally take one step more without using it.

It'll bring a huge amount of knowledge to you about the exact shape of the mirror and where to work next.

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It's time to start using Figure xp. I wouldn't personally take one step more without using it.

It'll bring a huge amount of knowledge to you about the exact shape of the mirror and where to work next.

I will be using the matched Ronchi test mainly but will sort the figure XP out to run in conjunction as two sets of testing should be better.

Have to sort out my test rig.

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It's amazing how much work goes into the dob structure of one of these scopes. I'm making mine for my 20" but the work just doesn't seem to be stopping and I haven't even made the base yet.

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It's amazing how much work goes into the dob structure of one of these scopes. I'm making mine for my 20" but the work just doesn't seem to be stopping and I haven't even made the base yet.

It is and it's a bit of a shock when you see how much of a monster they become.

Shame to admit that I enjoy the wood work far more than the glass bit although I'm sure when the mirror is done it will be provide the enjoyment far surpassing if I'd bought it in.

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Keep your attention on the  mirror Damian, the  clothes for it were never going to be as demanding to create

as the Optical masterpiece you are striving for, much work to that end is still ongoing.

Doesn't harm to take breaks and do the other things that need doing of course,

but the main focus has to be on the mirror.

What about the secondary by the way, are you buying a ready made, or making one,

and have you made progress on the  Foucault tester you were going to make?

Might help if we didn't ask so many questions hey?  :grin:  :grin:.

Ron.

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Hi Ron

The mirror is quite rightly my main concern although building the other bits is a way of spurring stuff on.

I have modified my tester to make it a slitless tester with enough board at the back to mount the camera. I've tried this a couple of times but I think it needs to be on something more stable (mass wise) to hold the image stable.

The secondary will be from John so I have no problem there.

As for the questions I really don't mind as it shows people are interested

Damian

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Hi all,

Just to update on where my thoughts are at present.

I have only managed another 4 hours on the mirror due to work this is quite a snails pace. With time ticking by I am going to have to move a little quicker as I really want to be getting back to doing some viewing and hunting down those faint targets this winter instead of staring at a pile of bits! 

Things are coming along and I will  make a decent up date to the thread this weekend when a little bit more time is at hand.

Damian

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Dina Rush, Russians make mistakes.

Oh Blow! no it was the Americans.

Inches and millimetres :evil:

Derek

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Damian, we do appreciate you taking time out to keep your audience happy, and we appreciate it.

None of us would complain if you curtailed some of your precious time keeping by keeping us informed.

A summary at the end of a weeks work or more, would more than satisfy us I'm sure.

I know you love to keep us in touch with what is, or isn't happening, but we aren't going anywhere matey,

and we'll hang on every word you write when you do get back to these pages.

Time is very important to you, but you mustn't rush just to make some deadline for our benefit, it doesn't  work like that  :grin:.

Ron.

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yes mate the end is near...there's a saying in snowboarding..one last run...its when the days riding is almost over so you relax on your last run of the day this is when you take your eye off the ball and mistakes happen!!..be careful on the home straight!!

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Just remember good is good enough - don't chase minute errors that won't affect your viewing!

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Afternoon all,

I promised an update to the thread this weekend and here is the progress I've been making.

Since the last update I've done roughly five hours on the figuring with a tight V through the middle then a W stroke across the whole of the mirror with a 1" overhang to try to smooth out the zone that I seem to have in the outer 3.5"-4" 

Here's how the mirror looks at offsets through the Ronchi from inside ROC to outside travelling a total of 17.5mm. Each step is 2.5mm

post-28847-0-00779300-1440342181_thumb.j

post-28847-0-22749300-1440342185_thumb.j

post-28847-0-06002000-1440342189_thumb.j

post-28847-0-05146000-1440342192_thumb.j

post-28847-0-43141000-1440342195_thumb.j

post-28847-0-12173400-1440342199_thumb.j

post-28847-0-59246200-1440342202_thumb.j

I'm not 100% happy with how the surface is changing as it doesn't seem to be following the matched Ronchi images that I have. To this end I have suspended any further work until after Tuesday when I am taking the mirror up for John to cast his eye over. My initial thoughts are that the outer edge's will require work to catch the middle up and blend the zone in.

I've not been idling away though and I've spent the majority of the day in the workshop (o.k. Garage!)  progressing the rest of the build for the upper tube assembly (UTA)

Hopefully Wednesday a plan will be formulated and I can continue.  

Thanks for reading 

Damian

Edited by mapstar
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Damian, first let me say that your PM on another subject is ongoing, and a solution is fairly imminent.

Your mirror surface seems to be reducing in the centre and intermediate zones, but left behind in the marginal ones.

You are wise to cease work until you have discussed with John.

Personally, I would make myself a full size lap, and warm press it with a pliable mesh, to manufacture hundreds of small facets within the large ones.

The purpose being, to make it easier to polish with the mirror on top.

I know it sounds crazy, but I used that technique on a 14" with success.

Mix a little detergent in the Cerium polishing mix, it makes a good lubricant. You get a few bubbles, but what the heck,

it will break the monotony a little.

This is just a remedial suggestion. The strokes should be an average of 2" overlap, and no more.

You will need to press several times to repair the small facets, as they will disappear quickly.

You could also place a plywood  disc over the back of the mirror, it would need an edge to fit around the mirror wall to keep it in place.

A couple of knobs screwed glued to the on the back of the disc to make it easier to push it back and forth.

The pitch should be fairly hard, and stay with centre over centre strokes  4" Max, ie  an average of 2" overlap each side. No need for accuracy, but stay away from too long a stroke. and work slowly, 

This should start to bring your edge to heel, but you might finish up with an Oblate spheroid, but that's easy to cure.

Now, I'll qualify this by repeating, it's what I would do, and have done.  Others may tell you different.  

A large disc like yours is no pushover on top a like sized lap, but multi faceted pith square will reduce some of the friction using the polishing agent with a little washing up liquid.

This is simply a means to an end.

Anyway, good luck, it's a difficult stage you're at, and you will need all of your resolve to get by it.

Ron.

PS, How is your Foucault tester coming along.  I'm sure some  images of the shadows will give a very clear picture of the surface.

Those Ronchi lines, although useful, just make me dizzy  :grin:.

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Hi Damian

Only just noticed your excellent thread and just read it from start to now and reminded me of my own 6" and 9" efforts many moons ago, nowhwere near the epic you have embarked upon :smiley: .

No doubt you have sussed out by now how or where you are going to get it aluminised but years ago I became friendly with one of the lab technicians at our local University who just happened to have access to a rather nice vacuuming setup for blanks (up to 20" if memory serves) and after the promise of a nice single malt he aluminised my 9" for me which as it turned out had two foci on it (but thats another story), but just to say a call to the local Uni might be useful !

regards

Steve

Edited by Gasman
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Damian, first let me say that your PM on another subject is ongoing, and a solution is fairly imminent.

Your mirror surface seems to be reducing in the centre and intermediate zones, but left behind in the marginal ones.

You are wise to cease work until you have discussed with John.

Personally, I would make myself a full size lap, and warm press it with a pliable mesh, to manufacture hundreds of small facets within the large ones.

The purpose being, to make it easier to polish with the mirror on top.

I know it sounds crazy, but I used that technique on a 14" with success.

Mix a little detergent in the Cerium polishing mix, it makes a good lubricant. You get a few bubbles, but what the heck,

it will break the monotony a little.

This is just a remedial suggestion. The strokes should be an average of 2" overlap, and no more.

You will need to press several times to repair the small facets, as they will disappear quickly.

You could also place a plywood disc over the back of the mirror, it would need an edge to fit around the mirror wall to keep it in place.

A couple of knobs screwed glued to the on the back of the disc to make it easier to push it back and forth.

The pitch should be fairly hard, and stay with centre over centre strokes 4" Max, ie an average of 2" overlap each side. No need for accuracy, but stay away from too long a stroke. and work slowly,

This should start to bring your edge to heel, but you might finish up with an Oblate spheroid, but that's easy to cure.

Now, I'll qualify this by repeating, it's what I would do, and have done. Others may tell you different.

A large disc like yours is no pushover on top a like sized lap, but multi faceted pith square will reduce some of the friction using the polishing agent with a little washing up liquid.

This is simply a means to an end.

Anyway, good luck, it's a difficult stage you're at, and you will need all of your resolve to get by it.

Ron.

PS, How is your Foucault tester coming along. I'm sure some images of the shadows will give a very clear picture of the surface.

Those Ronchi lines, although useful, just make me dizzy :grin:.

Thanks for the above post and the update on my PM.

The Ronchi lines are difficult to read sometimes and working out your next move takes some head scratching?

I have an 18" lap although this would need a good press and then a lot of work to get it up to speed.

A very detailed post with a lot of great input and maybe John will say the same although I have kept a constant dialogue and until this week everything was going OK

It may be some remedial work in the outer zone to bring things back but we shall have to see?

I have to admit a full size lap and mirror on top fills me with dread!

Fingers crossed.

Damian

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Apologies Ron I didn't answer about the foucault testing.

The problems I've been having is I like to take images and it's been difficult to do it with the set up I previously had.

I have worked it out that the camera needs to be a distance back from the knife edge to get it to work as advised by RAC and an article on the Web I found.

I have taken some images and it seems that the mirror has the classic doughnut of a parabola.

I shall try to post some images.

Damian

Edited by mapstar

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Hi Damian

Only just noticed your excellent thread and just read it from start to now and reminded me of my own 6" and 9" efforts many moons ago, nowhwere near the epic you have embarked upon :smiley: .

No doubt you have sussed out by now how or where you are going to get it aluminised but years ago I became friendly with one of the lab technicians at our local University who just happened to have access to a rather nice vacuuming setup for blanks (up to 20" if memory serves) and after the promise of a nice single malt he aluminised my 9" for me which as it turned out had two foci on it (but thats another story), but just to say a call to the local Uni might be useful !

regards

Steve

Many thanks for posting Steve and writing about your experiences.

The word epic is a good one and saga may be more in keeping with my efforts?

Good to know there are others willing to help out there even if it does end up costing a little malt to get there.

Cheers

Damian

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So pleased to hear about the ongoing progress, and also about single malt being used as a form of currency. I knew I'd feel at home in this forum. Keep up the good work mapstar and thanks for the ongoing images - really nice to be able to compare. I look forward to hearing the next step advised by John.

As a slight aside were you going to post about the rest of the build in this thread or do I remember you saying you'd do a separate one? If by any chance you have time it would be great to hear about your planning and progress with that and maybe see a bit, though I totally understand if the mirror thread is more than enough for you to be spending time on and sorting out photos for. Either way, good luck and keep going!

Sincerely,

An enthused spectator (Pete)

Sent from my iPhone using Tapatalk

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Many thanks Pete

I intend to create a new thread as I did with the last build to detail the build process.

I have to that end been taking pictures and trying to fit in the details that would be beneficial to others.

At present the mirror thread absorbs a lot of my SGL time.

I will do it soon, as the last one I did was well received and interested people hopefully enough for some to have a go themselves as was the purpose of detailing the making of a large aperture mirror.

I must admit it has been a lot longer than what I thought it would take.

Damian

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Oh and it does seem that money does not carry the same gravitas as malt in the astronomy world!

Or am I just mixing with the wrong types he he

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Hi all,

John Nichol is hoping to hold another one day seminar if there is sufficient interest.

The previous two editions have been limited to mirror making, this time it will include aspects of telescope making. This is because some people would like to make their own telescope without making their own optics. As a result the day will be about 50% mirror making and 50% telescope making. Details of the day are...

Date: 22nd November 2015

Time: 10am til 4pm.

Location: BIllingham, Teesside. (Easy access from the A19)

The day will feature a talk from SGL's Mapstar (Damian).

More details to follow as the timetable is firmed up.

This is not a profit making event, But There will be a charge for this seminar, sufficient only to cover costs of

the facility, and materials etc. 

If you would like to register an interest in attending the day please send me a PM 

Damian.

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Mmmm. With the looks of that I would be making a full or near full sized lap and take it back a few steps.

I'll say it again and it will save you a lot of time, You really need to make couder mask and start using Figure XP! You don't need a camera as you will be getting the numbers visually.

Chasing Ronchi curves on large fast mirrors is NOT going to happen. Making a 10" f6 would be fine but not this mirror.

Once you start using Figure XP you will see the light.

I had my first light for the 20" two nights ago and it is totally awesome, it star tests perfectly and the view is amazing! I could view the moon at 700X and the detail was stunning!

I still need to finish a few things on it.

post-14823-0-92955900-1440480412.jpg

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