The 22" mapstar mirror

[mapstar]

I could always pop across and do the video Damian, but would have to bring a lovely smooth Irish Whiskey instead of the Scotch!

Ha ha yeah

Maybe a bit more work today but I need to concentrate on other bit's today

[barkis]

This post isn't to try and offer any advice Damian.

I just want to make the point about the incredible attention you are giving this project.

Not just your almost running commentary on the work, but to highlight the extreme patience you posses,

both in applying yourself to the task in hand, and your willingness to pop on SGL every day to reply to everyone's posts,

without an Iota of impatience reflected in your replies.

I know each one of us is willing you success, and it will happen.

The only equation left is being resolved by you, and that is.. Patience x Time + Thought  = Eureka!!!

We'll all share in your elation when that moment arrives.

Keep pushing those glass molecules up the hill to the edge  .

Ron.  

[mapstar]

This post isn't to try and offer any advice Damian.

I just want to make the point about the incredible attention you are giving this project.

Not just your almost running commentary on the work, but to highlight the extreme patience you posses,

both in applying yourself to the task in hand, and your willingness to pop on SGL every day to reply to everyone's posts,

without an Iota of impatience reflected in your replies.

I know each one of us is willing you success, and it will happen.

The only equation left is being resolved by you, and that is.. Patience x Time + Thought = Eureka!!!

We'll all share in your elation when that moment arrives.

Keep pushing those glass molecules up the hill to the edge .

Ron.

Thanks Ron

There's no rush as I've got this far and will get to the end. Great that you guys keep following my slow progress and offer encouragement

John offers great advice and encouragement in the back ground which I've not mentioned much

It's always nice to see the other opinions on what you guys think too.

I may get a few strokes done today but I need to catch up on a few chores first as my cleaner has been busy fighting a turned down edge

Damian ;-)

[Physopto]

Aye,

You have put yersell unda da spotlight a bit haven't you.

Glad it's no me

[mapstar]

With work and chores over with yesterday and today it's back to the mirror campaign.

To start with the lap facet's were looking closed up, so out with the saw and half an hour later the lap channels were cut and I was ready for a bit of warming and pressing.

 

 

Not the prettiest or neatest lap in the world but it's worked up to now. This is the third time I've re-cut the facet's and it happens quite often.

Today I'm going to start the parabolising process as this may bring the edge to heel. On with the W strokes 

Damian

[Physopto]

How long does the pressing take and what temperature do you warm the lap to? Guess work? Or specific temp?

Derek

[Astrobits]

I understand your desire to ge to the end asap but I would not recommend starting to parabolise before getting the edge correct. The reference point for the parabola is the edge and if you don't know where that is it is too easy to overdo the centre. Then you have more edge work to correct a hyperbola and you know how much effort it takes the change the edge of a large mirror.

Looking at your lap there are only 6 facets spanning 10". I would expect to have facets about 1" in size so have 10 x 10 on that lap with sub-faceting with a mesh.

Keep polishing.................

Nigel

[mapstar]

How long does the pressing take and what temperature do you warm the lap to? Guess work? Or specific temp?

Derek

The pressing depends on the temperature and weight you put on. If it's warmed up too much it will just close the gaps up very quickly as will sticking lots of weight on. Smaller lap less weight (5 to 10kg). I normally soak it in luke warm water for 5-10mins so the lap is warmed through and then a half hour press. The lap is left on grease proof paper ontop of the mirror when I'm not working on it so it retains it's shape.

Which reminds me it's been pressing for a good hour now 

[Soupy]

Don't try rushing it now Damian you don't want extra work. As for smaller facets. I have seen someone make these with a mesh type car mat. The facets are much smaller and they pressed them onto a warm lap. Seems easier than having to cut

[mapstar]

There's no rush but I thought I'd try a different tactic.

As you say Nigel I will keep a close eye on the centre as I proceed and if it is going wrong I will revert to the edge. As RAC says it is a large fast mirror and the amount of glass to remove is a lot so I am trying to kill two birds with one stone by working both for now if things start to go awry I can change back.

The lap has a lot of micro faceting on it, I am probably wrong but it seems to be working  o.k. 

Damian

[RAC]

One thing that may help things go faster is scrubbing all that old cerium off the pitch lap with a fine brass brush. You want it to be black not brown. I brush mine after every session and cold press for a few minutes.

[alan potts]

What an amasing amount of time and effort you have put into this mirror so far, I take my hat off to you. I know mine is smaller but John made my mirror which I have not been able to use to it's full as yet due to weather. I don't know too much about what you have been doing but I can see by the one night that I have used my 18 inch that John without doubt know exactly what he is doing and talking about, stunning quality.

Having just read the whole thread I consider what I paid for mine very good value as I am sure even a mirror 4 inches smaller would take ages to make.

I hope all goes well from here on in, ready for Christmas?

Alan

[mapstar]

One thing that may help things go faster is scrubbing all that old cerium off the pitch lap with a fine brass brush. You want it to be black not brown. I brush mine after every session and cold press for a few minutes.

I have been doing the wire brush scrub but not quite often enough by the sound's of it. Great tip and thanks for that Raymond. I will test the mirror later after a cool down and see if things have changed for the better. 

Can I also add that is one lovely Flickr page you have 

Damian

[mapstar]

What an amasing amount of time and effort you have put into this mirror so far, I take my hat off to you. I know mine is smaller but John made my mirror which I have not been able to use to it's full as yet due to weather. I don't know too much about what you have been doing but I can see by the one night that I have used my 18 inch that John without doubt know exactly what he is doing and talking about, stunning quality.

Having just read the whole thread I consider what I paid for mine very good value as I am sure even a mirror 4 inches smaller would take ages to make.

I hope all goes well from here on in, ready for Christmas?

Alan

Hi Alan

I'm glad to hear you've read the whole thread and found it interesting, if only to appreciate the incredible experience and dedication that mirror makers such as John have to create such master pieces.

As an amateur I'm inexperienced and obviously feeling my way along so to say if very slowly.

There is lots of input from mirror makers experienced and other wise from right around the globe here which is good and it does create quite a discussion sometimes.

As with scope's there isn't one that is the same or made exactly the same, although if they were we'd all be looking through Kriege and Berry style dobs and you wouldn't have the Canopus agreed?

I hope to read your first light report Alan as it's a beautiful scope and the one that inspired me to build something a little different and will do in the future.

Thanks again for reading

Damian

P.s. I'm not saying which christmas but I think I'll get lynched if it's not done for november 

Edited March 23, 2015 by mapstar

[Demon Barber]

Damian,you took the words out of my mouth!!!!!!!!!!!!!        

Mike

[barkis]

I have been doing the wire brush scrub but not quite often enough by the sound's of it. Great tip and thanks for that Raymond. I will test the mirror later after a cool down and see if things have changed for the better. 

Can I also add that is one lovely Flickr page you have 

Damian

Eeeek!!  Wire Brush??  Tell me you are kidding please  .

Imagine the myriad scratches a piece of wire could do stuck in the pitch.

Ron. 

[RAC]

That's why i use a brass brush. And also make sure nothing is left in the pitch.

[mapstar]

Eeeek!! Wire Brush?? Tell me you are kidding please .

Imagine the myriad scratches a piece of wire could do stuck in the pitch.

Ron.

Hiya Ron

I know It sounds extreme but that's what John and also Raymond now have advised me to do.

I did it quite a few time's on the larger lap to no detriment but I must admit I'm always a bit apprehensive once I've done it.

I do have a couple of small scratches on the mirror which will have to stay at this point but not caused by the brushing. One will probably be under the shadow of the secondary.

Damian

[nicks90]

I do have a couple of small scratches on the mirror which will have to stay at this point but not caused by the brushing. One will probably be under the shadow of the secondary.

Damian

Damian,

wont those scratches - especially the one in the centre - not dissappear when you parabolise it?

[mapstar]

Hiya Nick,

Depend's on how deep a scratch is but I very much doubt it as even though I may be taking a lot of glass off (in polishing terms) it's a small amount. Fingers crossed though as mine are very light.

I have another couple of hours planned this aft so I shall take some image's later on of the progress

Damian

[mapstar]

I had about an hour's more work yesterday on the mirror, but hoping for quite a bit today and see how things progress. I managed to take a few Ronchi Images of the mirror before I made a start.

Inside ROC

Outside ROC

Time will tell if I'm going the right way. 

Damian

Edited March 25, 2015 by mapstar

[Mark62]

Keeping fingers crossed for you Damian [emoji6]

Sent from my iPhone using Tapatalk

[Ajohn]

Phew - just read all 13 pages. I reckon you are doing pretty well really. The problem you have now is removing relatively large areas of glass to cure a small area error. Slow work it but should happen eventually. Some might just go back to fine grinding again but I can understand why you haven't done that and you do seem to be making progress.

One reason I read the lot is wondering how this happened. I'm at a bit of a loss but some maybe's.

I noticed a comment that the centre polished up more slowly. Things usually work from the centre to the edge. Say a small lap was being used on the whole mirror. Some series of narrow w's would be used across the entire mirror. This means that the centre of the mirror has the lap passing over it more often than the edge - the w's might run left right and then right left. This is what usually happens how ever it's done even during grinding - the centre of the mirror spends more time in contact with the tool.

As the laps are light you might be pressing down to speed things up.  I feel that is a no no. It's better to add weight to the lap. Work height can be used to avoid this as well - if it's a bit higher than elbow height it's harder to push down and easier to just push, pull without varying the pressure on the lap. Less tendency to rock the lap as well.

To  much effort needed to move the lap - only a maybe but perhaps the mix of polishing compound is too thick and needs more water added. Not a subject that is well covered from what I have seen.

As I need to make a new tester I found the testing interesting. The test for a sphere was mentioned where fringes are seen ahead of the knife edge. The knife edge needs to be say 25mm ahead of the light source which should be at the roc (slightly off centre in practice). With a slit source the fringes are strong. The source and the knife edge should be equidistant from the axis of the mirror during any testing. There is some mention of that in places. It's done by catching the return image from the mirror and adjusting so that the source what ever it is in focus and ideally as close to the source as it can be while remaining usable as a tester. It needs to be level with the source as well so that everything is then also square to the mirror. The easiest way to do that is probably to rest the mirror back on 3 adjustable screws or have the base the holder sits on tilted by the same sort of idea. Then things have to be aligned for when the tester is moved back and forth to take measurements.

I spent some time looking around at testers as slitless became more popular.  The impression I had was that people were prepared to spend a lot of time making a mirror but not much on a tester. For me Texereau's basic method of making a sliding table makes a lot of sense and is easy to make even if it doesn't use the same materials. Given that all sort can be placed on it.

I did see one form of slitless that interested me. The usual led in a hole but with a craft knife blade edge of some sort running vertically over it and on centre. Sort of one sided slit where the rest of the blade forms the knife edge while always lining up with it. It would be easy to convert this to a true slit source too. I have my doubts about using large sources. I feel that people in the past that used pin holes or short slits were not idiots and must have had some reasons for not just using larger holes. The reason given how the tests function is probably accuracy.

I don't think I would use a wire brush on a lap - lots of fine detail to catch and retain Rubbish. Any mesh that is finer than the squares of pitch on the lap will provide smaller facets with space round them so that they can work correctly. 

Some one expressed doubts about financial aspects of making a mirror like Damian's. Well Orion would charge from 2 to 3k plus VAT for a mirror of this size. Time is likely to vary according to experience and to some extent luck.

John

-

[Cath]

Still love the dedication Damian, amazing

I myself am wondering exactly what is involved in tooling up for and executing the Ronchi tests that you do - presuming you do these at home. I realise the net is there to find out how exactly to do it, but with such an already detailed mirror making thread, it might be a really nice addition to the thread

[mapstar]

Still love the dedication Damian, amazing

I myself am wondering exactly what is involved in tooling up for and executing the Ronchi tests that you do - presuming you do these at home. I realise the net is there to find out how exactly to do it, but with such an already detailed mirror making thread, it might be a really nice addition to the thread

Hi Cath. 

Thanks for the compliment and question. As you say the internet has lot's of info on the various testing methods that can be employed. The Ronchi is simple to set up and use 

I can only talk about the set up and experience I have at present. My set up consists of the following

Ronchi Screen 133Lpi

Camera tripod

Linear translation stage 

LED and power source

Ply frame to hold everything

Small camera set on night mode

A frame to hold the mirror with a sling and a wedge for adjusting the mirror focal plane (to line it up with the light source)

The Ronchi screen I use actually came from the USA (http://www.ronchiscreens.com/screens.php) I bought the large screen and bought 2 the 133Lpi and 250Lpi although I only use the 133Lpi. The small screen would be o.k.  I paid around £35 for both including the postage

The linear translation (XY) stage is something to accurately adjust the travel in and out from the mirror.

Like the one I have with the 25mm travel micrometer at the back (just an X stage) Fleabay £30

Or a micro scope XY mount about £20

Mirror frame (under Foucault knife edge test) you can see the mirror just hanging in the sling this was before the re-grind and re-polish. Lot's of problems here, It had an under polished centre, lots of finger prints around the edge as well as a hill turned down edge and just generally very poor shape 

You need some way of mounting these to the tripod, and then the screen and the light source to the stage for travel. In the photo I used some ply to hold the screen in front of both the LED and the camera. This is a moving light source tester as the LED travels with the screen and the camera. The LED shines through the screen and is reflected back off the mirror back through the screen to the camera lens. Light source and camera lens are as close as possible  

The mirror has a radius of curvature worked out roughly the depth of the curve by a spherometer so you should know this but it can be determined by getting the reflection of the led in the mirror and steadily working your way backwards till the LED image in the mirror fills the whole mirror.

Looking through the camera you should be able to see the fully illuminated mirror and the lines on it. The lines alter as you move inside and outside the radius of curvature and you can use free software ROnWin20 written by John D Upton to produce matched Ronchi image's of your test mirror by entering the mirror details.

The Ronchi is just a variation of the Foucault Knife edge test. It represents multiple Knife edge's as far as I believe.

Phew hope that's correct, but there are plenty of much better explanations on the web. Not many talk about how to set up for some strange reason and about the moving and fixed light sources 

Damian

Edited March 26, 2015 by mapstar