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First mirror - eight or ten inches?

Ags

By Ags
in DIY Astronomer

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Ags

Ags

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I've been looking at the mirror kits on Galvoptics, and my partner and I are going for making our own telescope. Technically it is my second mirror as I got as far as polishing a 6" when I was a teenager many long years ago.The question is... what size? It will be going on a dob mount, and I will lso build a motorized platform for it. The main thing to look at around these parts is planets, so I am looking at either an 8" F6 or a 10" F6. I want to keep the focal ratio easy, and planet-friendly.

Would a 10" mirror be much harder than 8"? Would a focal length of 1500mm make for a very ungainly dob? I'm going up from 4" so either 8 or 10" is a sizeable upgrade. Maybe I am just getting aperture-greedy. 

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k9wis

k9wis

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are you frinding the mirror...hats off to you for grinding a 6".. I was in high school when I ground mine but had professional at the Adler planetarium put the fianl figure on it..He also made me a 10" f/6 and its an awesome scope but a real back breaker to move around..so if your young and strong go with the 10".  you wont regret it.. 

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Ajohn

Ajohn

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I had a 10in F6.2 dob - they do turn out rather large and can be heavy if it's a simple build.  Eyepiece being too heigh wasn't a problem, not that it spent much time pointing straight up. An 8in will be a lot lighter.

As some one else mentioned in another thread I too think anyone  who is going to make a telescope should read Texereau's book. It gives a pretty good description of how to make a mirror that has been followed by an awful lot of people. I still feel it's the best available. Some use different methods of testing but there is plenty about on the web covering that area. His whole book is based round producing an 8in F6 telescope. In real terms the same techniques could be used on much larger mirrors. He does comment on that and it seems one person laughs when he reads about the complications with larger mirrors. The comments have to be taken in context - using telescopes at their max power etc and building with sufficient strength to maintain alignment that will match his ideas of a decent quality mirror with trivial supports. He actually describes a simple mount for his 8in. Even that is hefty. I doubt if you will find blanks of the thickness he suggests - they are aimed at maintaining an accurate mirror how ever it's mounted and are in any case probably a bit over the top.  :confused: I'm not so sure though given the mirror accuracy he aims for.

One thing I should warn people about if they are buying a glass tool as well. I once tried to grind  a cast pyrex blank with a plate glass tool. Let's just say the glass lost and the pyrex won, hardly any impression was made in it for the loss of a lot of tool. If you go for a low expansion glass mirror blank off Galvoptics I would ask about this area rather than just buying a plate glass tool. Most cast a tile tool now or use steel nuts rather than tiles or even roughly hog the curve out with what ever they can lay their hands on. Fortunately I doubt if there are many cast pyrex blanks about now - they sometimes come complete with bubbles. The one I tried may have been poorly produced too.

John

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Ajohn

Ajohn

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There is one other book too that adds something that wasn't popular in amateur circles when Texereau wrote his.  All it really adds is working with the mirror up to finish off polishing the spherical stage. It's best too use very little overhang on the tool, maybe an inch or so on an 8in mirror centre over centre mixed with some moderate W strokes. The same thing can help when using grits too. The same thoughts are needed when ever working this way with any tool at any stage on any size of mirror - the closer the centre of the tool goes to the edge of  the mirror the more likely it is to cause problems.

Texereau describes mirror on top with a full  sized tool. The reason is pretty simple really. Say the mirror centre passes over the edge of the tool and it rocks. All that will do is dig more out of the centre leaving the edge alone, just what is wanted when roughing out.  If a tool on top does the same thing it will take material off the edge and can cause the dreaded turned down edge  problems. That can happen when using small tools on top of  big mirrors.

He does cover small tool on top work after a fashion but only in the figuring section - taking down the marginal areas and deepening the centre - that's all that might be needed really plus a bit of thought at any stage. The parts that pass under the centre of the tool will be the areas that are worked most and that sets what type of stroke is used.

:grin: Not that I am a fan of Texereau though. Of course I am.

There are some complications with small tool on top. True where the centre of the tool passes will receive "work" but say it's edge is some way past the centre of the mirror. It becomes a big tool as far as the centre of the mirror is concerned due to it's area and the area of the tool that is passing over it over a period of time. So really that sets how the shape of a mirror is changed once the tool is used off centre. I'd hate to put some  one off making a mirror but thoughts in this area can be beneficial as things progress to the final stages. It's the reason small tools might give unexpected results.  Star laps can help with this sort of problem if they are needed.

John

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Ajohn

Ajohn

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Nice video but I think if some one is making a smaller scope that will be used at high powers the testing will have to be a bit more sophisticated. Maybe the alignment of the optics as well.

It does show how efficient cerium oxide is when little is used and he  doesn't use long strokes. There are also some videos  by Waite on youtube. He mostly works with a fixed post machine but there is still plenty of good idea on all sorts of things in some of them.

:rolleyes: I must be mad too. Just ordered a 10in blank. Cheating though as there just happened to be some pre milled to F5 ones available on ebay and that is what I want. Saves roughing out which really is often a small part of the work. Worse still I want it in a telescope rather than a dob.  :grin: I really aught to know better but I miss the few mirrors I have made myself.

John

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Ajohn

Ajohn

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A high quality 250mm F5 mirror off Orion UK would cost me about £500, probably more. That's where my interest in mirror making lies. For some it's bigger mirrors. My interest is a bit silly really. I've bought a number of scopes over the years and notice that they aren't really as they could be just testing them terrestrially which isn't a difficult thing to do.  :laugh:  Maybe this doesn't matter when they are pointed at the sky but I still know they have problems and have looked through better. Some reckon that seeing conditions are more important.

I'm hoping 250mm  will turn out to make a relatively light weight scope and that F5 isn't too fast to stretch eyepieces too far. Sort of photo visual. The scope itself will have problems. I'll probably finish up getting into aluminium sand casting or waste a lot machining it away.

John

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Ags

Ags

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I think for the video they just wanted to show the simplest method possible to test the optics; I would certainly be doing more precise testing. 10" F5 might be an option for me too. The main problem with my current scope is the eye floaters I battle with at planetary magnifications (150x - 220x). Other people who use my scope do not complain so much of the issue (or report no issue at all) so I think my eyes have a particular problem. I can see the floaters right now looking at the computer screen. The only way to reduce the floaters a bit is with more aperture, which means with 200/f6 I could get 200x with a 1mm exit pupil, while with 250/f5 I could hit 250x. More importantly, at 150x exit pupil would go from 1.33mm to 1.67mm in the 250mm scope.

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Ajohn

Ajohn

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There are several ways of testing mirrors. Faucault - I've used masks as described by Texereau. One of Waite's video's shows one reading being taken properly. It isn't a quick process. Another is to use 3 masks, centre, 70% and edge then check for smoothness by moving the knife edge slowly and smoothly.  I intend to try another I came across in a book discussing making 60 odd inch mirrors for observatories. Match stick type markers across the centre of the mirror. The measurement point is when the shadows touch adjacent matchsticks.  For some reason he makes every 3rd one longer. Sounds ideal for dense shadows to me. He tests for fine ripple etc using a method that it seems is down to Dall and resulted in the Dall null test and one other more famous one. Put a fast achromat backwards centred in the optical path and move it around until the mirror nulls out. Unlike the other similar ones other than picking up the centre of curvature of the mirror it doesn't depend on precise settings. He's using it to test for small zonal errors. He reckons that the achromat should be twice as fast as the mirror but that probably just reduces the spacing. I believe some use an ordinary plano convex lens as per the other similar tests. LED lighting should help with the achromatic problems and also the problems with achromats as they should be well corrected for green. I suspect Oldham optical use this method.

Fancy an interferometer - no problem as there is the Bath type. Dale, the man who writes and maintains the free analysis software wont mind me nicking this shot of his

post-2035-0-10313600-1429185976.jpg

The optical parts needed can be obtained from places like Surplus Shed or at probably more cost locally. The mounting parts could be made in all sorts of ways. The laser pointer isn't used in lasing mode, just enough current for it to produce a tiny oblong square of ordinary red light. Some people add sliding movements to them. The Texereau style one could be used or anything else that comes to mind. They aren't used for measurement just positioning There is more info and shots of other arrangement here

http://starryridge.com/mediawiki-1.9.1/index.php?title=Bath_Interferometer

The people who are into this hang around on here

https://groups.yahoo.com/neo/groups/interferometry/conversations/messages

They are a very helpful bunch. Ask too many questions rather than just building one is frowned on a bit though as it sometimes results in some one not trying it. I did that as I had doubts if it can be used on F3 mirrors but I've abandoned my cassegrain project now at least for a while. They also feel that this is the most suitable test for a novice. Not as mad as that might sound really - no shadow reading and can be highly accurate.

I have mixed feelings about Ronchi. Most recommend star testing after using that. Fine if seeing conditions are good enough. Some one owns a very precise mirror that was Hartmann tested  - easy to produce an artificial star when something like that is available. "Just" put a pin hole precisely on it's focal plane. It can be used to test mirror up to about twice the mirrors size. It should also be possible to use a reflection of the sun off a small ball bearing but I've only used that to get some idea of how good a scope I have bought is and errors in the ball bearing could mess things up. It's ok for getting some idea of how good diffraction rings should be on the real thing if seeing is good enough.  I recently did this on a small apo using the reflection off the 1mm wire in a bird feeder -  :embarassed: wire, so got oval rings but at least it gave me some idea that the scope is ok.

Some people have used artificial stars say a mile away ideally over grass. At some distance the errors are negligible.  Another method means mounting the mirror in a telescope and using an artificial star some yards away. The distance error problem can be overcome by adding a plano convex lens to the set up. A ronchi screen gives straight lines in this one when things are correct. As it would with the others when test lenses are used Ideally a precision lens is needed off some one like OptoSigma. It's probably more precise than the ordinary Rionchi test but might not be so good at showing rough surfaces etc which isn't easy to set in the normal Ronchi test going on examples I have seen.

:grin: Yawn. Then there is the Hartmann test that can be done with a dslr but it seems that the glass thickness over the sensor can cause problems. The same sort of idea is used for the moving wire rather than knife test. Diffraction bands appear around the wire when it's in the measurement position. It needs some sort of x-y stage.

:eek: There may be more.  :shocked: Now I have written this I too wonder if Bath is the best way but frankly I feel 2 different tests is best even if it's only for reassurance.

John

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RAC

RAC

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For an amerture mirror maker i see no need in using any other mirror testing method other than the Ronchi and Faucault with a couder mask and using Figure XP. The Ronchi test is great for cheacking for zones and as a start to figuring but real fine figuring is when you use Figure XP with a couder mask and a knife edge. A dial gauge and focuser is all you need to make accurate measurements for entering into Figure xp.

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Ajohn

Ajohn

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I wonder if people sometimes have problems making mirrors because they just choose some set of instructions and jump in and get on with it rather than thinking about what the tool will do for instance or where they should apply pressure to it or the mirror. It's not rocket science.

I spent a lot of time on cloudynights getting to grips with an optical design package. A different sort of problem cropped up there fairly regularly. Some one would go out and buy an expensive lens that will null out a parabaloid if things are in the correct position and have the aim of making such and such a mirror. The lens is well known and there is a formulae for setting it up. They ask questions and some one ray traces it and tells them just how accurate the set  up has to be to achieve the super accurate mirror they want. It can come as a bit of a shock and in practical terms might even be impossible - measuring the radius of curvature for instance might need something far more accurate than a steel tape measure. The spacing between the lens, source, ronchi screen / knife might be even worse. The software that is available for ronchi and faucault analysis can be used to see what accuracy will be needed.  The best option is to decide what to make and then use the software to see how feasible it is. Taking Faucault for instance the usual formulae can be used to calculate precise value and entered into the software along with the radius. Values can be changed and the errors they introduce will be shown. If for argument sake a 25mm error in mirror radius measurement introduces a 1 wave error there would clearly be a need to measure more accurately than that. Measuring the other distances to say 0.01mm from that assumed radius with that level of error wouldn't produce a decent mirror by any ones standards. The same applies to the ronchi test only here the way the pattern changes with movement matters too. Many claim that finer line spacing is needed to get round that aspect as mirrors get faster but that may mean less movement which also means higher measurement accuracy. Pass on finer gratings, just read from time to time. I've only recently downloaded some ronchi software to play with.

All of this shouldn't put people off. It just allows people to see how ambitious their aims are and what they need to measure the mirror with. One simple way round the rad problem if it's important is  a stick that is positioned axially on the centre of the mirror held in a stand so that aspect can be measured, then the stand to tester can be measured with a tape, maybe in extreme  cases something similar might be needed at the tester end.   :eek: Some people make up a series of very precise measuring sticks. Bit OTT really if not needed. The software that is available allows people to find out without making or even buying anything.

Some have just just moved a tester by hand and drawn lines on a piece of paper with some care via the edge of the  stand at each measurement position. It's probably more suitable for fixed source moving knife. Might sound daft but measurement down to about 0.005in can be made with some 6in engineers rules. It might even have at some degrees C or F stamped on it. Wish you luck with metric. UK people would probably have to buy a suitable rule from the USA now. A stand that has a slide can be used in much the same way, even via a pointer over the rule.

Personally I prefer screw movement on a Faucault tester. It had struck me that a digital dial gauge could be zero'd making taking readings easier. Paper self zero's and a rule could be slid. All depends on how accurate the readings need to be. But don't forget the mirror rad as that is generally needed with all of these types of test, even Bath.

John

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mapstar

mapstar

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I've been looking at the mirror kits on Galvoptics, and my partner and I are going for making our own telescope. Technically it is my second mirror as I got as far as polishing a 6" when I was a teenager many long years ago.The question is... what size? It will be going on a dob mount, and I will lso build a motorized platform for it. The main thing to look at around these parts is planets, so I am looking at either an 8" F6 or a 10" F6. I want to keep the focal ratio easy, and planet-friendly.

Would a 10" mirror be much harder than 8"? Would a focal length of 1500mm make for a very ungainly dob? I'm going up from 4" so either 8 or 10" is a sizeable upgrade. Maybe I am just getting aperture-greedy.

Hi

You've put together what spec's you want and also what the mirror and scope will be used for which is all a good start.

Will you be doing it just for the thrill of saying you made the optics? or is there another reason for making your own mirror?

If it's a financial one then when you figure in all the equipment, materials, coatings and secondary plus most of all your time then it probably will not be viable, but that's hopefully not the way you're looking at it?

It is a challenge and one I hope you'll take up as there are plenty here that offer support and encouragement.

Damian

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Ajohn

Ajohn

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In case some one gets into a Bath interferometer I must stress a red laser pointer - the others use IR lasers to excite a crystal of some sort and are not suitable.

I did toy with the idea of working on a 2nd hand or cheap mirror. Maybe just refiguring it if needed. There are usually some on Astroboot. There was a 10in recently but I came across ebay 251250299718 and bought one of those as it's pre ground to what I want. If some one wants something else it wouldn't surprise me if they could organise it. F5 is getting a bit fast though. About 2um of glass to remove if mostly removed from the edge. Moving to F6 would virtually halve that. It's not the amount of glass to remove that counts within limits. It's the steepness of the slopes and the rate they change at on the mirror surface that they indicate. A boys own 6in F8 hardly departs from a sphere at all. It goes up with size and decreasing F ratio.

Texereau uses Diameter^4/128*F^3 for the figuring depth. Others use the difference in depth at the centre.  :grin: Trouble is until some one has done it they don't know what it means in practice so it's mostly just another thing to worry about. I haven't tried F5 before so feel it will be harder than F6. Hopefully not too much harder.

Another source of blanks is this one

http://www.stathis-firstlight.de/spiegelschleifen/materialeng.htm

They too may be able to organise pre milling the radius but personally I wouldn't worry to much about hogging a mirror out. The only reason I have gone for a pre milled one is that it's easily available.

John

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Ajohn

Ajohn

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The grinding on the ebay blanks is what I would call so so. I have had much better. It might mean running down from a full set of grits to get the basic sphere correct initially. Normally the coarser ones aren't required if the pre grinding is done well. I suspect I will buy a bag of suitable shot blasting grit to do that if it needs it. The back has been ground as well, finish wise better than the front. It will also need more of a chamfer putting on it.

No rush though as for other reasons I have decided to make a fixed post grinding / polishing machine - mainly so that it might encourage me to pick up my cassegrain project again. I feel that pure hand reworking of 2in dia lenses wont be on.

John

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