Ajohn

What is chordal ?

Derek

As in the chord of a circle.

John

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I'd still suggest you spend some time cleaning up the centre as you go rather than leaving it all to later.  Just over centre and some mild W and NO overhang until that looks like it will work the edge. It should even out any irregularities the chordal work leaves and give you an area to press the lap on. As the area being worked is smaller it should correct a lot more quickly than the edge. How long - you would need to find out.

Numbers of lines - every  time I see examples shown there are usually around 5 but this gets more tricky as the F ratio gets faster.

John

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This might help Damian inject some science into lapping  or at least provide a bit of  light relief now and again. I found it yesterday, written by some one who is interested in computerised figuring. If I remember correctly that idea was started by a person  called Zambutu who uses a whole series of laps to figure mirrors getting gradually smaller towards the centre. It's here

http://martin-cibulski.de/atm/polishing_simulator/

Despite the comments it does react to changes in lap and mirror sizes. They can be changed from the setup menu  I tried the stroke  I suggested out of curiosity but find I can't attach a video file so I have uploaded it here. Filebin is a useful service at times

http://filebin.net/s9con6ib13

It shows the wear pattern it should achieve and concentrates the glass removal where it needs to be so should do it fairly quickly providing the turned edge isn't too deep.

John

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It might be working Damian but not in the way you expect - trouble with laps. Say you use a chordal stroke  with the edge of the lap just running just past the centre of the mirror. Stroke length so that the max overlap at each end is say 1in.  This would scarcely work the edge at all. If things are working efficiently it should produce a hill in the middle that runs out gently to the cord the lap ran on and then rise up towards the edge leaving your turned down edge. Extend the stroke so that the max overhang is about 1 1/2 and eventually the edge should start cleaning up. You need to work up to that. There's a need for a sense of humour and determination to meet the challenges.

Ronchi isn't something I am very familiar with but I'd guess you need to adjust to see 5 lines with one over the centre of the mirror. That way you may see that you are making progress as outlined above. In fact you can probably look at the lines and guess how things will turn out if you continue. The shape of the lines will relate to errors but wont measure them unfortunately.

Small laps can make big changes as some have mentioned but yours has a pretty large area really. It isn't going to work a 22 in mirror as quickly as say a 15in.  In your case you can get round the problems by doing some centre over centre from time to time with no overlap at all to try and keep the curve that is being worked out spherical. That can be aided by mixing in some mild w's again with no overlap. Eventually when the edge has gone or staring to go that is the sort of stroke that will be needed but this time  with some overlap. Only testing can indicate when you should do this. You might find 1/2hr work takes longer to clean up or a few mins. The time spent correcting can be balance to suit.

I'd guess you already know that it's very important to spin the lap a bit regularly and alter the amount as randomly as possible. Every stroke wont hurt.

If there isn't any efficiency I would wonder about the polishing mix being too thick and in any case add weight to the lap.

However you do it - try it for sufficient time to see what it's doing, clean up and test. At this stage I don't think you need worry about letting things cool. That comes when you are worrying about wavelength sized adjustments. Here you need to lower a substantial area of glass. Waite's video might well be correcting 1/4 wave or less over correction - that really does need doing slowly and carefully. The problem with people like him is that he has done so much of it that he knows how much work will be needed to do what ever he is trying to do - us poor sods have to learn. If the work doesn't do what we want at least we know how long it took to make the change. If there is no change and it should be possible to detect it - something some where is very decidedly wrong.  Some people make notes.

John

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All I'll add is that the 2nd part of the video can also be used to  reduce a turned down edge - with a sub diameter lap - that's the only reason I posted the link - plus some interesting factors about laps.

John

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I would have thought that could only happen  if machine polishing rather fast or really soft pitch. I must admit the Luton polishing surprised me. He might have been pre polishing.

I've wondered if Damian used a straight chordal stroke with his lap say a couple of inches in from the side of the mirror at centre stroke without much overlap at the ends of the stroke it might sort his  problem out more rapidly.  A mixed stroke could be added to prevent the bump in the middle. This should keep the lap in good contact with the final curve.

Anyway while  nosing around on another type of problem I found Waite doing something similar to the above but with a small star lap. He mentions that it can also be used for a turned edge. Keeping a round lap further in should achieve the same thing. Part 2 of this video - on a small mirror but the same idea should work out. He calls them tangent strokes -  I recollect Tex....... calls them chordal.

John

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I've read that pitch that is too soft can cause a turned edge. Maybe Ron has too. I can't figure out why other than the lap tends to follow it so wont completely remove it. Damian can guess how I test pitch for hardness but I would describe the results as pretty stiff chewing gum, doesn't shatter. If this is the reason turned edges can be hard to get rid off the only solution will be to make sure the lap spends plenty if time inwards from the turned edge so that it can keep  the required shape - also why I reckon a larger star lap might fix it. On the other hand that might not be the reason some reckon that soft pitch turns edges.

I saw the mirror making man from Luton at work in his industrial unit.  His polishing machines ran at something like 60 rpm, he offered me one to take away but the things were way too big for me at the time. The drive was interesting though.  He polished till they squeaked and then added more rouge, about maybe 1 1/2 teaspoons for a 10in mirror. Consistency about that of very runny salad cream - plenty of colour. This would last for some time, longer than I was there. Personally I use it a little runnier and spread it out over the lap area with a finger and then get to work. Same when pressing over night other than a little work to spread the rouge. His test for scratches and sleeks was a little single AA torch all most  horizontal across the mirror. Sleeks look like broad highly polished scratches.

John

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Cheers for the questions Nick and no question is daft as it maybe something obvious I've overlooked

At this stage there is no difference in temperature to the touch but it will have raised slightly after polishing. If you handle the mirror, then where your hands have been it will expand causing high spot problems when you polish, and I'm sure some Texereau Know it all will be along soon to point it out! It's one thing I wanted to eliminate before progressing further but there is a piece of anti fatigue matting between the mirror and table so it would in fact thermally insulate it from the table surface. 

You can see the matt in the back ground against the radiator

This one thinks it's best for some one to find out themselves. Many do after a while

John

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 I wouldn't laugh to hard about Texereau's comments on mount weight and scope size and some of his other comments too such as mirror thickness and accuracy too really.

Perhaps Damian should consider using his mirror in a scope at max magnification for viewing say mars and being able to touch and focus the scope or even keep it pointing at it. Most people would quickly find that conditions can mean that the view was hopeless anyway at this sort of size so that mag would have to be dropped way back. Accuracy comes into that as well. Not just the mirror but the entire structure of the scope. He still makes a number of very fair points really but they have to be taken in context. Change to a relatively  low mag light bucket and things are different but not as much as some might think especially in relationship to mirror thickness that will hold the sort of accuracy he is after. The other point about accuracy is that light buckets don't really need it unless they are going to be used at max magnification but on the other hand too serious a departure will spoil the view of a number of things. The term diffraction limited has changed a bit too -  he really means it. With a 1/10 wave PV and not RMS PV and the sort of obstruction sizes he suggests things are very close to it. His book is also more or less totally aimed at visual work but he does touch on the photographic problems of his day. That area is now totally different but the same principles apply.

Making mirrors - nothing has changed really as far as the actually making is concerned. There are numerous methods of testing mirrors but really his comments about ronchi still hold. From memory MB proposes it but follows it with star testing - that ain't as simply as people might think. Some one posted on here ronchi so far and then Foucault.  There is also a variation on Ronchi where a supplementary lens is used to make the lines straight when the mirror is parabolic. Then there is the Dall null test and the other with a different name that is based on it but the lens goes in front of both the knife and the source.  The lens for the latter one is pretty expensive. Sounds great until people realise just how accurately it has to be positioned in either case. Both can be used with a Ronchi screen as well. Some one has also come up with a "Ronchi screen" that consists of circles that stay round when the mirror is parabolic or something like that. Most of this type of testing has the same problem - things are set to give the correct result with a very specific mirror that has to have an exact focal ratio not what it just happens to turn out to be. Foucault software works on the basis of the best fit parabola it finds when the measurements are taken.  Maybe the is a match the ronchi pattern set up where the screen is just moved a precise amount either side of the ROC and software analyses the image and it comes up with the best fit parabola. If so that would be a bit different but there would still be a need to know exactly where the ROC was.

The pro's often use supplementary optics in what is essentially a Foucault test but the optics cause the mirror to null out when it has the correct shape, no shadow reading - eg Hubble  . They usually use several lenses in the tester though not just the usual one used by amateurs. They have also use the Hartmann test sometimes just as a final check. It's probably possible to do that one with a DSLR. 

Foucault - easy to make the gear that is needed and no real drawbacks that can't be circumvented. The usual complaint about Texereau is not taking his suggestions on scope size etc in context and also the sums used to work out the mirror profile. I posted links to several packages that will do the sums. Another comment is sometimes pre Dobson but in real terms that has nothing to do with mirror making His comments about size and usability in typical urban situations are not far off the truth as far as resolution goes. 8in F6 was a bold step in his day. The usual was plate glass 6in F8 and Rubbish instructions as well. No one in the USA had even heard of Plossl's until his book was translated. It's a pity good ones aren't easy to come by and that they had to go super etc. I understand good ones are expensive to make and have little eye relief at shorter focal lengths.

 If anyone does it don't make the source too bright - after several hours of minor figuring and testing I looked in the mirror and found one pupil was the size of pin [removed word] and the other as big as it could be. It  took a while to get back to normal.

 Hope Damian doesn't finish up like this

There are toooooooooo many video's on youtube about mirror testing and making the bits for that and even the mirror. I think there are a few purely on the Foucault test too.  They should aid understanding. There are some good ones. There should also be examples of fixed source and the other type of tester.

 I can't write a book on here. Texereau wrote 2 books. How To Make A Telescope and the 2nd edition of the same book which as far as this thread goes doesn't add anything useful. The book is fairly old now but explains Foucault testing and mirror making better than any other source I am aware of.  There was a copy of the 1st edition on the internet archive. Like many famous books it's been bought up by some one "William Bell" I think and so now any ebook copies are removed. It mentions Ronchi and some other tests but doesn't really rate it as an accurate test - as some one on here has already mentioned perhaps the best option for making a very accurate mirror is Ronchi followed by Foucault. There are other tests as well that use additional optics but like most things there are catches. Perhaps the easiest way of getting to grips with Foucault testing is to do it once the basic idea is firmly in the head. It sounds a lot worse than it is  in practice.

This page and any links off it might help but I don't know of any web source detailing fringes ahead of the knife edge

http://www.telescope-optics.net/foucault_test.htm

I've come across another twist on Foucault but it's past my bed time so will add it later.

John

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Texereau shows a demo of a ronchi using 5 lp/mm but only 4 lines across the mirror using a 40um slit. From the photo diffraction doesn't look to be much of a problem, it's a 125mm diameter with a 2m rad and a turned down edge. The faults are pretty evident including surface finish. This area is a bit of a pass  for me. I couldn't get my hands on one easily on my 1st mirror so knocked one up using fine wire but wasn't impressed as his method actually measures the problem.

Damian could use the same method. The knife would be down below the led but that doesn't matter. Just fasten the knife blade 1/2 over the LED, find something shorter to form the other half of the slit and adjust it by eye with the led on - just look for a narrow parallel slit. Put that some way in from the ROC of the mirror and fringes will be seen. These need to be straight too but the advantage is that the first fringe will be 3/4 of a wave from the knife edge. He also uses this to set the slit width. A very fine slit will cover half of the mirror with fringes and this will also give strong diffraction effects. If the slit is widened so that only 2 or 3 are seen the test can still be accurate and the diffraction effects will be minimised. You hope. Adjusting it isn't as bad as people might think.

The radius of curvature can be measured roughly with a torch or led etc. Wet or oiled when only fine ground. Wave the light around at the side of your eye and look for the reflection back from the mirror. Probably best to start too close. As this moved back the direction the reflection moves  will reverse as it goes past the centre of the ROC. Hone in on the position so that the mirror looks to be full of light and slight movements of the light makes no difference.

Lining a tester up is fairly easy but if it's truly slitless it would be best to put something thin centrally over the led with a small hole or rough slit etc in it.  The mirror will reflect it back and the position can be found with piece of card. Move things around / tilt the mirror until it's where it should be. A slit and knife edge are handy again here - focus a loupe onto the knife edge and adjust to bring the return image sharply in focus with the knife edge about 1/2 way across it. An eyepiece used backwards should make a decent loupe.  25mm will give about 10x. With source stationary set up there is no need have the knife cut into the beam as the knife can be moved independently.

Lining up towards the mirror is a bit more tricky but the return image should remain half masked by the knife through a significant amount of movement. There is a need to worry about this  when shadow style testing is used during figuring. Some people add an x traverse to keep the shadow generation even when source and knife move together and only worry about getting it exact when things are nearly finished. I've wondered about using a camera macro slide/rail for this and making the knob bigger to get fine adjustment. It's not so much of a problem with stationary source as the knife edge is usually swung into the beam via mounting it on an L shaped part. Probably all difficult to understand but his rig is shown here

http://astro-foren.de/index.php/Thread/4323-Foucault-Test-Einrichtung-nach-Texereau/

a better shot of it here

http://www.iucaa.ernet.in/~scipop/Literature/hbt1/fig17000.jpg

The basic idea is that the sliding part runs on 2 V's formed by metal plates running on a piece of bar or tube supported by a screw that runs on something slippery. (I just used a bit of metal rather than glass) That screw is used to move the knife edge but the same set up can be used for source and knife moving together. The table usually needs a bit of weight adding to keep it stable and another one can be mounted on top of it to give x-y movement if needed.

I'd guess source and knife moving together is the most precise method but the fixed source is probably easier to set up and allows the amount the knife cuts into the beam to be easily adjusted. Trouble is it tends to finish up with a wide separation between the source and knife and that needs keeping down to a minimum really. I have seen figures given based on % rad of the mirror but I'm sometimes dubious about info on the web, i've seen a photo of a pro set up used to test mirrors up to and above 60in dia. Slit and source move together and I would estimate were about 6mm apart. It was mounted on a lathe top slide giving X-Y adjustment and modified to read to 0.0001in.

John

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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

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I think I would try a large star lap before deliberately changing part of the mirror Damian.  That way you can work with some overhang without working the edges much and still keep an even figure. The other way is to press the lap over a ring round the circumference. Didn't work for me, I'd guess it depends on how soft the pitch is.

I'm guessing but with an 18in lap I would try 5 or 6 points going say 4 in into the lap and using an over hang of circa 1/2 that. More points = more work biased to the centre. You'll probably find that this deepens the centre but I'd be surprised if it removed all of the glass that has to be removed from there to form a parabola. What I found with laps was they had to be tried to find out if they were going to work out.  A bit suck it and see despite all of the info that is about. The classic small tool on top for getting to a parabola would be 1/2 diameter. Used to the edge to deepen the centre and it's centre run round the edge to correct that part.

John

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 A sharp idea callump, straight to the point. I like that.

John

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Low breaking strain fishing line too but I've never had to do it.

I wanted a cross hair eyepiece for aligning a scope so bought a Chinese 10x microscope objective with cross hairs- Ebay. I will need to turn up a sleeve to convert it to 1 1/4. For the price I was surprised by the quality. It's an eyepiece for a compound rather than a stereo microscope so the field of view is limited to  around 16mm.  Focal length 250mm / magnification so 25mm in my case.  They have an accurate scale on one of the axis so it might be possible to use them for polar alignment. I didn't fancy the price of the astro ones for the amount of use it will have.

John

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Some of my post was for people who might be thinking about making a mirror Damian. I mentioned the full sized lap because of an earlier comment about the centre polishing slowly. The usual way round that with big mirrors on the bottom but generally done with a machine is to  use a smaller lap in the centre and forget the initial pure spherical mirror. It's possible to measure what will loosely be 2 radii this way as it's done to make sure it isn't too deep for the eventual parabolic shape.  I have a similar problem with an F3 mirror for different reasons - the figuring depth. Trouble is I am not sure if it's suitable for hand work as the "cenral sphere" might be off centre compared with the outer one. In real terms they wont be distinct spheres but measurements can still be taken.

RAC's mirror is a nice piece of work. The software he shows is probably one of the ones on here

https://stellafane.org/tm/atm/test/software.html

I know for a fact that I can't make measurements as accurately as RAC can. I don't do it often enough.  I've tried all sorts, masks and shadows touching sticks, centre, 70% and edge correct and the rest smooth - not too bad that one and etc anything else I came across. In the end I followed Schroder's caustic test in ATM III. The others were fine for rough figuring. A lot is made of the difficulty of making Schroder's test rig but really all that is needed is a Tex' one sitting on top of another so that things can be moved in an X Y fashion and the knife is replaced with a wire. I viewed it with  10x eyecup loupe as it was easier to mount than an eyepiece. Nothing ambiguous about this one at all. It's relatively easy to position of the wire to better than 0.001in.  I tried the Dall null test as well. Had problems but later found that it needs setting up pretty precisely by ray tracing it. It's probably better used to null the mirror and work back from the position the bits finish in or to null the mirror looking for zonal defects.

A lot of this area depends on how accurate the mirror needs to be. There is also need to consider the source and knife / wire spacing when high accuracy is needed. Big mirrors help with that a bit.

 My F3 mirror is on the back burner.  It took a long time to come up with a design for a flat field cassegrain.  Finally did it and concluded that I couldn't hope to make some of the small glass optics accurately enough for it to be a good as it should be. It will probably turn into a Dall Kirkam at some point. Not sure.

John

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I'm surprised you haven't made a full sized lap for producing the sphere. It works well especially with mirror on the bottom with a random mix of over centre and mild w strokes with say 1/4 dia long strokes. Plus random steps round and try and be random with the rest as well. Mirror on top tends to bring up the centre quicker though. Tool on top tends to wipe up towards the edges of the mirror which is why it's best to finish off this way to avoid any chances of a turned down edge as most of the light in an image comes from the outer parts of the mirror,  pi r^2 etc. 

Some one mentioned using the fringes ahead of a knife edge in front of the ROC earlier. This work rather well and is probably the best way to check  right up to the edge. It needs a slit source to work well though - around 0.04mm from memory over a hole say 5mm diameter.  When you get to a sphere with a source like that you will find it very difficult to cause the knife edge to grey out the mirror evenly but that too is a very sensitive test but will usually give bright diffraction rings around the rim if it really is spherical making it difficult to see what's going on there. The knife edge has to be set very precisely. Texereau did that by swinging it on an L shaped  arm with M6 bolt on the end and mounting the knife edge so that it could be tilted to square it up with the slit.  I did it the other way round and tilted the slit and as it was separate lost it. The diffraction effects can be reduced by widening the slit but that can also mess up accuracy.

There are various ways of making a lap. Very big mirror makers cast squares and stick them on. I've followed a book by Muirden and it worked for me but it assume the mirror is transparent. The mirror and tool need to be warmed up, not too hot to hold but sort of warm and cosy style. Pitch melted and then stirred while cooling, lift the stick out and let some drip back in. It needs to take a few seconds to melt back in. Pour an even spiral over the tool. Let it cool until it takes a finger print without sticking. Cover the mirror with plenty of polishing powder mix and place it over the lap while keeping it moving around - w and centre over centre stokes etc. If the mirror is transparent lack of contact can be seen. I've managed to get full contact this way. If not and the pitch gets too cold trim up and hot press again etc. I cut the channels with a wet tennon saw.

I've tried the other ideas, very coarse draining board mats when they were available, didn't really help and the channels still needed trimming up with a saw. Also a hot knife but it leaves a lip. One thing I did find which helped was pressing with a piece of fine plastic mesh over the lap. This gives a lot of very small facets and I have done this on any size of lap I've made - onions used to be sold bagged with it. A good coat of polishing solution prevents sticking.

I've never done anything as big as this and suspect I would bite the bullet and cast squares or maybe cast a sheet and cut it up. I think a sheet would be easier to maintain a constant thickness. Cutting it up pass but I suspect I would give it a try. One thing that does help with pitch - it sticks well to warm surfaces.

John

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The point was making about think arms rather than platforms is that it's easier to make a rigid pivot. One way for  instance would be to fit a ball race into a hole in one, few washers for spacers and and bolt it to whatever it pivots on. The 2nd arm needs a sliding contact -  ball race again. Another alternative might be the the bronze bushes bearing suppliers sell. The only problem with those is that they are meant to be driven into holes sized to shrink them a bit. It might be possible to buy say ones with a 6mm bore and ream them out to 1/4 in or some similar idea. Bright drawn metal bar is usually pretty close to the stated diameter. Silver steel these days or drill rod as it's called in the USA isn't as close as it used to be. Some supplier stock ground rod. That will be very close.

I would be inclined to use 18mm MDF even though breathing the dust isn't a good idea. I have also routed aluminium plate in the past. To add a camera all that is really needed is a small platform on the end of an arm with a ball head on it.

  I've been intending to do it for some time but ordered the light weight Ioptron mount earlier this week. I suspect I may get annoyed with the suppliers. If they sell it that should stock it / declare delivery times on their web sites.

There are probably some better web pages about on tangent arm drives than the ones I posted. It's an old idea but I would seriously wonder about alignment if really long exposures are contemplated and wonder if the complication of the 2nd arm was worth while.

John

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You might do better to think about a tangent arm drive rather than a platform. I've tried to do things with hinges and usually run into problems because they are loose. There are all sort of ways of making bearings and that sort of thing is easier on things that look like arms rather than a platform.

This gives and idea of what accuracy the twin arm can give. Use the link towards the top of the page to download it

http://adsabs.harvard.edu/abs/1995JBAA..105...65C

This might help

http://www.mikeoates.org/mas/projects/scotch/hdm/home.htm

An alternative is to drive the nut and and bend the all thread as shown here. This one is over the top for just a camera but it should be possible to include the sort of adjustments equi heads have for alignment. Longer possible exposure times need accurate alignment. I suspect that is why many simple trackers have been made just using one pivot and arm.

http://www.dv-fansler.com/Astronomy/portable_equatorial_mount.htm

Must admit I wondered about a commercial camera platform recently. The max payload is disappointing and they suggest mounting them on a Manfrotto geared head and a hefty tripod. Price - ouch. While I haven't handled a geared head like that I doubt if they allow easy fine adjustment as most equatorial heads do. This is why I drifted away from any form of camera platform but do feel most parts of a tangent drive could be made from mdf or even ply.

John

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Strange sounding name but a good company to deal with. Cheap and quick shipping. For model makers really but many screws etc in smaller sizes.

http://www.giantshark.co.uk/screws-nuts-washers-c-39_144.html

There was a company called giantcod and much to my surprise they seem to have been replaced by giantshark. I assume it's the same people but web suggests they are now dearer than they were.

John

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There are a number of things that might attack the plastic part or glue to make getting the glass out easier. See

rfelektronik.se/manuals/Datasheets/solventguide.pdf

Cement cleaner might be worth trying. It's about 10% or so hydrochloric acid. Nylons are often used for electronics. Other chemicals can sometimes be found by searching the name as it may be more common than you think. The glue is probably a UV cure type so a search for solvents for those might bring something up.

Looks like some help with dcraw could be useful too so I have attached a list of it's options.

John

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dcrawoptions.txt

Being the proud owner of a C9 1/4 carbon fibre tube with a broken corrector plate from some one called Chris in Rome who strung me along until it was too late to make a paypal claim I have been into this is some detail. So in case some one in the future find these posts.

It all depends on the age of the scope. All carbon fibre tubed scope continued to be made in the USA and all up to that point where made the same way. There may be small errors in the corrector plate so these were hand figured out by working on the 2ndry mirror. The change happened when Syntra bought them out. The USA continued to make all carbon tubes and the very large ones. The rest went to China. There they continued to make the scopes the same way for some time but the old 1/10 wave add was dropped and replaced with diffraction limited. At some point they stopped working on them by hand. This doesn't have to mean that the performance is worse. It probably means that they had better equipment for making them. Once a plate has broken this leaves some scopes with a bit of a problem. The more recent just fitted corrector plates are available. In my case £200 that may or may not include VAT. These can be supplied by all Celestron dealers every where and are even ex stock at Hinds. Fit one to the earlier hand finished scopes and it may or may not work out. There is no way of knowing without actually trying it. The way Hind's put it on my scope is that if it is to be as it should be the whole lot has to be sent to the USA for repair. I get the impression from elsewhere that they would make this simple by replacing all of the optics with a matched set. It's not a cheap option.

In the UK the best way to date the scope is to send side and both end photo's to Hind's. Seems they can always date the scope that way.

With the Edge their literature suggests that they are hand figuring the 2ndry mirror again and only making them in the USA. When the 2ndry is hand finished it's no longer a sphere. This means that they have to be centred more accurately. A sphere can always be centred by just tilting it. Actually with the things that can happen to scopes people are probably better of buying the Chinese scopes as sorting out tilt and centring can be a bit of a nightmare. The edge does have centring screws it seems but it's not a task to be taken on lightly.

John

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I'm well used to my comments on forums sparking off primers and some times even web sites. These leave me feeling a bit dissatisfied if they don't get the points across. KIS is the usual reason offered too. No matter. I'm happy providing I've helped clear the mire and started people thinking. Like most people I eventually get fed up of typing the same answer over and over again.

John

I think you could say a bit more about maximum mag. I've toyed with the idea of running through that one here explaining why and showing that it all holds together.

There is plenty of evidence on this board that peoples expectations are far higher than they will get.

It's a sad optical fact that as the exit pupil gets much below 1mm apparent planetary definition deteriorates. At 0.5mms that's obvious on all but the very best scopes and should be reserved for testing and aligning telescope. Even on the best scopes available there will be a noticeable drop in contrast. A realistic limit is something like 0.8mms. As usual with telescopes it varies. Much depends on the optics design/quality and general conditions. From the eyes point of view exit pupils can go a lot lower than that. Comfort has nothing to do with it.

The other factor is scope size. Some where around 8ins scopes working at these levels of magnification start being effected more and more by the atmosphere and general seeing conditions. Much depends on where one lives. While some one with a 6ins scope may be able to achieve maximum magnification they may well be severely disappointed if they move to 10ins and expect to achieve a similar exit pupil. They are more likely to achieve a similar object scale and a brighter image. The later is pointless on planets. It might help them image uranus. I have a 10ins sct that has done that. Not much detail putting it mildly. In practice an 8ins could do it as well. The exposure would just be longer.

Glad to see you have mentioned the cost aspects below something like F8. In my experience it becomes critical at F6. Skywatcher should be shot for supplying those super plossls with them. The old ones were better even at F8.

I wish you luck in finding some one with an 8mms pupil. 6 is a more realistic limit. A few manage 7. Not much chance of either if there is any ambient light. Reduced eye relief helps a lot with that. Some use a sort of light proof bag that fits to the scope. It helps a lot. Pupil size is a well researched area. The main argument against over sized exit pupils is that some of it doesn't enter the eye so light is wasted. Use the correct sort of eyepiece and one can move the eye around in the beam that comes out of it. Could also be that one can see what one wants to look at in the part of the beam that enters the eye. I do that a lot. The effective magnification is low so the contrast is very high.

John