Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

The 22" mapstar mirror


mapstar

Recommended Posts

Cheers Ron and Rick for the encouragement and kind comments.

Hopefully I have got all the details right and the thread make's sense in the way everything progresses.

To quote what a very wise person said "it's a 22" sub F4 mirror and a beast compared to the average mirror so must be treated with the respect it deserve's"

I'll get there in the end and maybe inspire others along the way.

Damian

  • Like 2
Link to comment
Share on other sites

Onto the polishing

Now the mirror was at the stage of polishing I required some more supplies. I trawled the internet for Cerium Oxide and came up with some from a well known auction site. The pitch for polishing was a different matter as it seems to be quite difficult to find a supplier that has some?  A visit to John though sorted that problem and 2kg of Gugolz #55 pitch was winging it's way home with me so I could make the polishing lap.

post-28847-0-70295700-1425108152.jpg

I made up a base for the lap from four 12mm plywood disc's of 17.5" diameter that I happened to have lying about. All glued together I sealed them with a coat of fibreglass resin.

Now actually melting and pouring pitch is a horrible business and not at all nice. I melted the pitch in a pan that I would dispose of later. The pitch is then heated slowly making sure not to overheat it as boiling drives off the volatile's which give the pitch it's unique consistency. Gordon Waite does quite a good video on this and a video writes a thousand better word's than I could so I'll just go through the basics.

The mirror is covered in tinfoil and the melted pitch is poured on top in the middle and slowly spreads out in an even layer. Then with this cooling the pitch lap base is pressed down onto it. The results are a perfect lap or in my case an absolute disaster! I had not quite got the blank level and the pitch started it's way towards one edge and I waited a little too long to press the base onto it as it hardly stuck! the lap wasn't exactly what I was hoping for and ended up a proper messy affair with pitch everywhere.

I then let it cool and chipped it all off to redo and hopefully get it right.

The second attempt was a success but as I've already said pitch is horrible stuff and just get's everywhere so glad I was in the garage doing all this. After cooling I cut facets in with a tenon saw finding out the blade doesn't clog if there is running water on it whilst cutting. I'm sure if there was a missus Mapstar living with me then she would be none too pleased with the use of the kitchen sink for this phase of the lap making! 

I then covered the mirror with grease proof paper and pressed the lap over night on top of the mirror to form a decent contact. Here's a shot of my rather messy lap cold pressing with oxide before polishing operations began 

post-28847-0-72469000-1425110089_thumb.j

The polishing compund comes in a couple of forms, Cerium Oxide the other being Regipol. I initially started out with Regipol optical polishing powder like this

post-28847-0-73916200-1425110600_thumb.j

After suspending it in filtered water (I used a brita water filter) I did my first half hour of polishing and below show's the lap wasn't making contact in the middle so more pressing with more weight

post-28847-0-76799400-1425109549_thumb.j

The lap eventually made full contact after warming it in water and pressing several time's. The bevel around the outside and facet channels were re-cut several times during this as they close up when the lap forms to the surface

post-28847-0-29891300-1425110964_thumb.j

After 12 hours the surface looked to be polishing but the centre was still hazy which I'm not sure can be seen in this shot? 

post-28847-0-04504800-1425111221_thumb.j

I had been polishing without any weight on the back of the lap so added 10Kg and continued. The lap become's quite a chore to push across the surface and I would polish for about an hour then press the lap overnight and carry on.

November by this stage was at an end.

When you first see your mirror polished it is a genuine WOW. I can count it as one of the most satisfying things I've ever done in life. It makes the hard work of the rest of the journey disappear. These two shot show the curvature of the surface with the fluorescent tube reflection. 

post-28847-0-02669000-1425111854_thumb.j post-28847-0-45047800-1425111872_thumb.j

I inspected the surface throughout using a laser to test on the surface to make sure the micro pit's had all gone. I had picked up a couple of scratches near the edge but these would stay.  It had taken me nearly 22 hours to polish which probably would've been a lot quicker with weight on from the start so up to now well over 100 hours I had in this.

It now looked like a mirror so onto the testing  :eek:

Damian

Edited by mapstar
  • Like 11
Link to comment
Share on other sites

Looks brilliant now Damian, a proper mirror!

A question. I get that you have ground it to a target Sagitta, but how do you check the curve/figure itself as you are going on? Does this just naturally follow given the tools you were using or did you test as you went along?

Link to comment
Share on other sites

Looks brilliant now Damian, a proper mirror!

A question. I get that you have ground it to a target Sagitta, but how do you check the curve/figure itself as you are going on? Does this just naturally follow given the tools you were using or did you test as you went along?

Cheers Stu

The pencil test during fine grinding is a good indicator of full contact and a sphere. To accurately measure the surface when I was at John's he checked it for me with a spherometer which is on his website. This one

post-28847-0-11543600-1425118558_thumb.j

The measurement should be the same all over the mirror if you've got things right (i.e. perfect sphere). The only way would be to constantly check with one but as an amateur I don't have one so the sagitta depth is a rough guide and the mirror will end up around that figure. If it was fitting in an already made scope then it needs a lot more precision so I would've bought one. 

The spherometer works on four point's with the three outside fixed and the middle with micrometer measures the depth in between these. So subtracting the depth of a flat surface off the measurement you can calculate the radius of curvature through a bit of maths (which I'm not going to go into) but this is done through a computer program and spews out the focal length (it's magic!)

It's quite strange to see a mirror without a coating in the flesh. As with most things pictures just don't do them justice.

Damian

Edited by mapstar
  • Like 6
Link to comment
Share on other sites

Certainly inspires me to give it a go (and I have a 14" glass blank waiting), but I want to try something smaller first I think.

James

Thanks James,

I can only say have a go you'll not regret it, it's difficult and to be honest at present I still have the hardest part ahead which is figuring but that's another post 

Damian 

  • Like 1
Link to comment
Share on other sites

A thoroughly enjoyable read Damian. It has inspired me to.....

Never ever try this. Lol.

Thanks Jeremy,

It's a lot of work which isn't really portrayed through posts, 100+ hour's (still counting) is a huge investment of anyone's life as everyone can appreciate.The thing about it is though it's a mirror for the rest of my life so worth every second :smiley: 

I'm sure with determination I'll change your mind ha ha :grin:  

  • Like 1
Link to comment
Share on other sites

 I got the Sagitta to 9.4mm for your target F of 3.7 Damian, but 

rarely does one get exactly the hoped for result.

It's looking so good though, and I think fist light is on the minds of most of us, 

but still a ways to go yet, so we'll just have to be patient  :smiley:.

Congrats on the fine job you've accomplished thus far though.

Ron.

  • Like 1
Link to comment
Share on other sites

Actually, with apologies for wandering off on a tangent a little, a suitcase dob is something I really want to think about.  The childrens' swimming club are doing a "club families" trip to Lanzarote (I think -- one of the islands anyhow) in October 2016 and it's tempting to go, but I'd be gutted if I went and didn't take a scope of some kind.  As it's all school-age kids in the club it would be really nice to "spread the word", too.  So a dob that would go on an aeroplane fairly easily would be an excellent thing to have.  Given that I have a year and a half perhaps that should be my first self-made mirror project.

In fact, I've just looked at the BA website (as an example) and the the maximum hand luggage size is 56cm x 45cm x 25cm.  I wonder if it might be possible to make an 8" "cabin baggage" dob?

James

  • Like 3
Link to comment
Share on other sites

I got the Sagitta to 9.4mm for your target F of 3.7 Damian, but

rarely does one get exactly the hoped for result.

It's looking so good though, and I think fist light is on the minds of most of us,

but still a ways to go yet, so we'll just have to be patient :smiley:.

Congrats on the fine job you've accomplished thus far though.

Ron.

Hi Ron

Thank you again and right that just over 9.4mm is the sagitta for F3.7. I ended up at around F3.8 so roughly 9.2mm.

I was aiming originally for F3.9 so a fraction under 9mm but as I said I took the rough ground sagitta to 12mm and because I didn't want to grind much more off I ended up slightly shorter focal length.

Yes there is now the hard part of figuring which is a proper challenge

Damian

Link to comment
Share on other sites

I would hope that by the time the SGL11 Star Party comes around, this telescope will have been commissioned, and fully operational.

I also hope you can find some means of attending said SP, with your creation, as a symbol of what can be achieved by the determined Amateur Telescope Maker.

Plus, if the skies permit, there would be queues around the Lucksall Camp Site, of eager beavers waiting for a A look see at some far distant Star Island  :smiley:.

Ron.

  • Like 2
Link to comment
Share on other sites

I would hope that by the time the SGL11 Star Party comes around, this telescope will have been commissioned, and fully operational.

I also hope you can find some means of attending said SP, with your creation, as a symbol of what can be achieved by the determined Amateur Telescope Maker.

Plus, if the skies permit, there would be queues around the Lucksall Camp Site, of eager beavers waiting for a A look see at some far distant Star Island  :smiley:.

Ron.

I'm not sure that I'll be attending that Ron but will try to make it if I can. It's nice to put face's to name's on here and thank people for the encouragement they give.  

I have some parts of the scope build already complete along the same line's as the 24" I built for a friend. Fingers crossed I plan to have everything done by September so 6 months away but I know that is a small amount of time to get it finished. I've quite a few star parties planned and really want to get out there and use it and as you say show what can be achieved with maybe the odd problem here and there to overcome.

I still have to finish the mirror though which I know is going to be the hardest part as it's something that I've never done before  :eek:  

Damian

  • Like 1
Link to comment
Share on other sites

I'm not sure that I'll be attending that Ron but will try to make it if I can. It's nice to put face's to name's on here and thank people for the encouragement they give.  

I have some parts of the scope build already complete along the same line's as the 24" I built for a friend. Fingers crossed I plan to have everything done by September so 6 months away but I know that is a small amount of time to get it finished. I've quite a few star parties planned and really want to get out there and use it and as you say show what can be achieved with maybe the odd problem here and there to overcome.

I still have to finish the mirror though which I know is going to be the hardest part as it's something that I've never done before  :eek:  

Damian

I'm sure any difficulties can be surmounted with Johns guidance Damian.

My largest mirror completed was a 14".    A self blame disaster with a 16" terminated that effort. 

However, I always felt a huge satisfaction when a project was  finished, be it a  4" or 14".

I cannot see you failing at all, you have too much grit. And I think I deliberately intended that pun.

Besides, you are a Yorkshireman, and the grit is inherrent  :grin:.

  • Like 2
Link to comment
Share on other sites

Onto the first testing

To test the mirror I was going to use a combination of Ronchi screen and the knife edge Foucault test.

Firstly I had to build the testing rig. I managed to pick up a single axis linear stage with 25mm of travel and went about fitting it to a camera tripod. Next up was a piece of ply to hold the LED light source and Ronchi screen. Here's the results

post-28847-0-28436600-1425152267_thumb.j post-28847-0-12679200-1425152283_thumb.j post-28847-0-10553400-1425152336_thumb.j

So with that complete I made up a test board to mount the mirror on via a sling suspended between 2 bolts.

post-28847-0-77901500-1425152359_thumb.j

I was now ready for testing. I was going to perform the test's with a moving light source so here is the test rig labelled up

post-28847-0-71310100-1425152375_thumb.j

The test's are carried out at the Radius Of Curvature (ROC for short) or twice the focal length. There is a lot of info on the net about both these tests and others that are used but I shall stick to just these two as they are probably the easiest for the amateur to use and set up.

Firstly the Ronchi test and I took these inside the ROC

post-28847-0-36882000-1425152460_thumb.j post-28847-0-10971500-1425152478_thumb.j

Then onto the Foucault test 

post-28847-0-66280700-1425152535_thumb.j post-28847-0-32158200-1425152555_thumb.j

Interpreting these image's was my untrained eye, so I exchanged a few email's with John and a verdict was given which I can't fully remember all the details. As can be seen the centre is raised (think it was actually mount Everest in mirror terms) and there were other problems with it too mainly the edge. So John advised a stroke through the centre with lot's of weight on.

So back home and I tried this but again I had problems. I was in the garage so however I warmed the lap it would cool off and basically school boy error No.2. Looking at the surface I might as well have used a brillo pad to polish as when I took the lap off to inspect it there were more scratches than my parent's record collection. The pitch was obviously too cold and the lap just skidded about. You can feel when the lap is working right as it sort of suck's onto the mirror and the polishing action is smooth and quite a resistance to push I should have known.

A return trip to what I thought was the redundant tile tool and another trip through the fine grinding (400, 600) pencil testing along the way and then back to polishing. I moved all the table into the kitchen where I could work at a constant temp and have the lap warm. After cleaning and pressing I tried again with the lap I had to polish but struggled to polish out the centre. The lap had become too thin at the edge's and was never going to form to the surface.

To cut to the chase I made a new lap below and redid all the the polishing operation having purchased more Cerium oxide. The new lap was a world away from the old one with the lessons I'd already learnt

post-28847-0-71260100-1425153762_thumb.j  post-28847-0-81137800-1425153816_thumb.j

And the table in the kitchen with the old lap pressing

post-28847-0-77622400-1425153935_thumb.j

The lap at the correct temperature worked beautifully and in no time at all I was polished again 

post-28847-0-89552100-1425154032_thumb.j  post-28847-0-87852600-1425154118_thumb.j

Again now I could return to testing, but not nice retracing your steps and doing it all again but I should at least be getting goods at it!

Just realised the back door put's into perspective the size of the mirror the table will just fit through the door jamb.

Damian

Edited by mapstar
  • Like 5
Link to comment
Share on other sites

Amazing perseverance Damian. I'm sure I would have got disillusioned by now, all credit to you.

It has been difficult and I have thought about the giving up so many time's.

It's been a big help doing it along side my good friend Rich (crashtestdummy) as we have both kept each other motivated along the journey when things are not exactly going right. The saying "don't struggle in silence"  come's to mind 

  • Like 3
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.