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loron91423

TESTING THE OPTICS OF YOUR TELESCOPE

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You will gain experience in using your telescope especially if you direct your attention to the main points of quality. You will find in time that you will become critical of imperfections of your telescope, casual or inherent. When you test your telescope's optics, you should be careful and pay close attention to your own eyesight and be guided by proper directions and only then will you have a good idea of the quality of your telescope.

Remember that no telescope will give the most perfect image. Your telescope is good when you can see fine definition and not to worry about the out of focus star image. If there seems to be a problem with the telescope because of the star image, you can not say what the problem is until you make an examination of the out-of-focus image on a night fine enough to allow you to make a star test at high power, unless the fault is because of a astigmatism, maladjustment, or centering of the optics and on a good night these can be seen at focus.

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Thank you for that "Ioron."

Is there any more to your piece on "Testing your Telescopes Optics," or are you simply stating that we will all gain an insight into possible shortcomings in the optics of our telescopes as we actually use the scopes over a period of time?

Regards,

philsail1

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Hmmm, I think this really is too big a subject for a quick post. However, it is an interesting teaser...

I used my first telescope and then moved on, without ever really knowing how good the optics were. I wonder how many people can actually interpret the star images their telescope provides? The first time I used a scope with perfect optics (a TV85), I was puzzled why those circular lines were there! :oops: Of course, I now know that they're supposed to be there! Most people will look at a star image to collimate their scope, but there's other information there too...if you know what you're looking for.

If you Google "star test images" you get some pretty good results, like this one (amongst others):

http://tinyurl.com/2zsy68

I'm not knowledgable enough to be the one doing the teaching here, but I would suggest that if you check a star image only for collimation, perhaps you would find some research in the area of optics interesting? After all, wouldn't you like to know how good your scope really is?

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When the Hubble telescope first opened it's eye on the universe it was soon apparent something was not right. It could not focus the images sharply. The reason was spherical aberration. A condition caused by the paraboloidal shape on the mirrors surface, not extending from the 90% zone, to the edge. This is a vital area on any mirror. It was an error in the final testing stages that led to this problem, and the enormous cost of putting right whilst in orbit, is well documented. So, mistakes in manufacture do happen, and anyone who has a persistent problem with their poor image quality, should have their mirror tested, but only after every other possibility has been explored.

The device to test it with, is not difficult to make in itself, but a complete understanding of how to use it, and interpreting the test findings is required.

Ironically, this simple device, had it been used as a quick final check on the Hubble mirror, would have saved the American taxpayers Millions of dollars. Of course, we all know hindsight is a wonderful thing.

Ron.

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To me, the test of a scope is how well I can see through it, just in the same way that the test of a golf club is how well I can hit it, or the test of a bicycle is how it rides and steers. You can quote numbers at me all day, but not one of them is worth a look at the sky through a scope.

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I hope no-one here turns into one of those poor people that queues up at star parties to peer through a scope just so that they can do a covert evaluation, then tell the owner just how bad their optics perform.

I have heard horror stories of these selfish individuals who, for some reason, get a kick out of testing optics to death and never actually do any real observing.

There's also Mr. "I've got to get my collimation perfect" who also spends 90% of his time under the stars twiddling rather than observing.

What's wrong with just looking and enjoying the equipment that you have.

If you find limitations of your equipment in use, surely that's the time to work out how it performs.

Being in the UK the atmosphere is seldom stable enough for star testing anyway :( I don't ever recall seeing rings around stars just bouncing blobs.

--

Martyn.

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I'm not really into the idea of testing the optics. I think my telescope is pretty good for the price I paid, the eyepieces are pretty ordinary but that's because I haven't spent any money on getting better ones. The laser thingwhatzname helps with the collimation so that's a pretty quick job and then it's into observing or imaging (attempting to image I should say! :().

Is this thread really a tutorial or just a statement about the quality of optics?

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Not a tutorial,paying attention to your own eyesight is kinda obvious i would have thought.

Agree with WH ,the important thing is how you see things through your scope, and as Martyn said LOOK and enjoy...... :shocked: How far does the average amateur astronomer want to go with analysis of their optics?.....If i get good planetary detail and stars which are sharp points of light then i'm happy.

I dont have a top of the range scope or eyepieces and my eyesight is certainly not top of the range :(:D I could upgrade the optics on my Orion Optics Europa from 1/8th to 1/10thpv but where does one stop in the pursuit of the elusive "perfect image".

I'm just a humble amateur who enjoys the view and has a reasonable knowledge of what i'm looking at,without over analysing things scientifically,mechanically or theoretically. :lol:

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I hope no-one here turns into one of those poor people that queues up at star parties to peer through a scope just so that they can do a covert evaluation, then tell the owner just how bad their optics perform.

I don't believe this is the idea.

While I certainly enjoy looking through my scope, I find the telescope (and the technology that resides within), a quite fascinating instrument. I think the interest to delve deeper into specifics, performance, & contruction is a natural tendancy in any hobby. An occasional star test is only one aspect of this.

What's wrong with just looking and enjoying the equipment that you have.

Of course, there's nothing wrong with this. But there's equally nothing wrong with learning more about the equipment you have.

I recently read a book on the history of the telescope. A big, hard back thing...it was a complete overview of the evolution of the telescope, from Galileo to Hubble. Lots of information on telescopes of significance over the years, including Herschel's, Birr Castle, Lick, & Hooker. The actual title and author escape me at present, but I'll have a look. It's a really interesting read.

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I don't think there are many "humble" astromers out there, who simply enjoy the view through whatever instrument they have - otherwise we wouldn't keep buying more telescopes (I ask myself many times, "what am I doing with three telescopes?" - "am I not satisfied with one?").

Like many hobbies, once one gets into the hobby, the tendency is to want bigger, better, faster, more efficient, etc, etc. It must be human nature to "not" be satisfied with what we have got!

It could be that if we were a race of people who were "satisfied" then we might not have progressed from "caves" to where we are now!

I know this is talking extremes, but can you see where I'm coming from - (or where they are going to take me to!).

"Now, how can I get enough money for that 12" scope!!"

Regards,

philsail1

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Fascinating thread, this.

My take on this is "ignorance is bliss". I would much prefer to get a half decent scope and average eyepieces and enjoy taking them out and seeing things that few humans ahve ever seen, rather than notice that my £20-eyepiece shows CA and my scope has spherical aberration, and consequently spend 3x the amount to get a better image.

I started off with standard kellners and plossls and had okay views, then upgraded to £20 plossls and loved them. However, at one point I decided to have one really nice eyepiece, but of course that wasn't enough and I had to upgrade them all to the same standard. Though I love my current eyepiece collection, it's worth remembering that they are worth over £500 of my kit list and I could have been equally happy with £20 plossls.

Testing your optics is a very dangerous act both for your wallet and your relationship with your wife/husband. Enjoy what you have, and if you really don't, ask about it on SGL and we can work out where it's going wrong :(

cheers

Andrew

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This is a very interesting thread, but for many reasons. Who could tell where it would go?

Myself, I'm very interested in the how and why a telescope works, and what makes a good one compared to a bad one. Maybe it's just natural curiosity, I think it's more. An intimate understanding of how something works, to me, makes the use of that item much more enjoyable, especially if there's some detectable flaw. Being able to discern if it's a "user error", or equipment failure or conditions, (seeing), and understanding the differences increases my enjoyment.

Understanding the limits of my equipment only adds to my enjoyment. In this I agree with WH-how well I can see something through a particular scope is what it's about. But knowing what it should look like is helpful, at least to me.

I guess I'm what philsail would call a "humble" astronomer. I've had views through so many scopes of so many apertures and types, the mind reels, yet I've always loved my simple C8 with no frills. Now, I have a 20" Newt, (look elsewhere, maybe in a couple of days for a report), and I find it almost as enjoyable. I kid you not! It's a whole other smoke, so to speak, and will take some getting used to, but it's just an instrument.

The real enjoyment comes from within.

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I agree "Astroman" that one can be a just as much a "humble" astronomer looking through a 20" Newtonian, as someone looking through an 8" scope, however, the fact that you now own a 20" Newt confirms what I was saying earlier - that "once one gets into the hobby, the tendency is to want bigger, better, faster, more efficient, etc, etc."

There is nothing wrong with this! (If I had the chance of a bigger scope, I'm sure I would jump at it (I was looking (drooling!) at the 250mm TAL for sale on SGL!!).

I think the difficult bit for us all, is to be satisfied with the scope's we have. (And this is the very value of the "Beginners" threads on SGL - where it seems the predominant advice to beginners asking which scope they should buy to start out in astronomy, is to go for the biggest scope they can afford - the traditional 8" Newtonian being the best all-round instrument which will keep one "happy" for a long time).

I think that second to the fascination of looking at planets, stars, galaxies and nebulae, is the actual psychology of how and why people choose the scopes they buy - and the reasons why we keep selling, exchanging our scopes for different, bigger and "better" scopes.

Perhaps if any telescope manufacturer were to study this aspect of their customers "needs" they (the manufacturer) might be able to come up with the "perfect" scope! (that would sell millions).

(Is this too "Orwellian"!).

Regards.

philsail1

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Yes, I'm with Astroman (and some others?) on this. Just born an inveterate "tinkerer", I guess? I remember coming across the idea of diffraction patterns in a review article on my ST102 and couldn't resist working out how to see them for myself. From there to taking the Achromat out of it's cell for a quick look-see, "tapping it" (qv), to hopefully "improve" abberations, buying a Cheshire e.p. to "collimate" the scope (even modifying the latter) etc. Did it make any difference? In all honesty NO! - At least within limits of my slightly dodgy eyesight. Or, it MAY have done? Certainly I relieved some over-tightening on the lens-gasket thingie. :(

N.B. Such things are not without risk: I have indeed "knackered" one (or two?) other, thankfully inexpensive, components with my "Cat-like curiousity"! There are my limits too - I ain't going to be taking my beloved MAK127 apart - Ostensibly, 'cos it "works"? But above all, I reckon I've gained some PLEASURE from doing all this stuff - Maybe even a little knowledge too? It has certainly helped me identify (IMO) the more "trustworthy" eyepiece reviewers - Ergo saved me money? And, should I ever "Win the Lottery", I can at least vaguely "understand" (and check) what to look for in a "better" scope too... :D

P.S. I happily aknowledge some of this may be seen as a "Diversionary Activity" - I blame my (lack of) observing site! :evil2:

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There are some live star tests on utube which might help on what the dot and rings should look like. Quality and central obstructions basically take light out of the central spot and put it some where else. That could be between the spot and the 1st ring, into the rings or all over the place. Search astronomical terms. Star tends to bring up the wrong things.

It's not too difficult to get an idea with terrestrial objects either by simply viewing them at very high magnifications. Go up in steps and the effects can be seen. A very good scope will tend to hold the detail rather than gradually blurring it out. Finding focus will get worse too as the mag goes up. That applies to looking at the sky too. Fast easy snapping in and out of focus is a sign of a decent scope. Need to be realistic about max usable magnifications though.

Other ways involve an artificial star which can be made at home if one wants or bought. Much easier to use than the real thing.

It's also possible to do a foucault test on a star. Just point the scope at a very bright star and center it. Remove the eyepiece and look down the focus tube. You'll see the mirror full of light. Then move a knife slowly across the view and the mirror will darken most likely on one side. Move the focuser in and do the same thing again. Then out and do the same thing again. If you have moved it far enough the shadow formed on the mirror will change sides. Some where in between the mirror will darken evenly all over and mirror defects can be seen and even be measured if required. Get it right and you will see atmospheric effects boiling all over the mirror. Need to take no notice of those. Stand a hot cup of tea in line with the scope and they will get worse. The idea is to get the end of the focuser in a position where the hills and valleys you can see look as flat as possible. Need to bear in mind that the knife will be cutting across a spot of light that will only be a some 0.001 mms across so knife movement is a bit critical. Foucault tests are usually done at the center of curvature of a mirror so if you web search that's the sort of thing you will usually find. This way it's been done with the object at infinity which will form and image at the focus. The same thing can be done with an artificial star but it needs to be a good distance away. 30 times the focal length of the scope from memory but it depends on the size of the star. Google might help.

Can also measure mirrors to amazing accuracy with a thing called a bath interferometer. Most interferometers use a thing called a reference element which tends to be a bit expensive. The bath uses a lens and a laser pointer plus some free software to interpret what's seen. There's much info here

http://starryridge.com/mediawiki-1.9.1/index.php?title=Bath_Interferometer - don't really need to understand it - just use it. Even more info available via google.

Added that for any budding do it yourself people out there. :D

John

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Ah! this "testing the optics" thread seems to have re-surfaced again!

That's a good "scientific" piece your wrote John, and I will give that "focault" "knife" test a go when I next come to collimate my 200mm Newt.

However, if I find any "defects" (and I would not really know what I'm supposed to be looking for!) what could I realistically do about it?j

So far over the past 6 years that I have taken a serious interest in astronomy, and in particular, using a high powered telescope to look at objects in the cosmos, I have tended to simply rely on the "Mark1 eyeball" to tell me whether or not something is wrong with the view I'm getting through my scope.

I have found that after many hours of viewing over the years, I can now usually sense if something feels amiss with my scope. I then go through the obvious in order of "ease" and simplicity of making any adjustments.

So far, the only visual problems I've experienced - and have been able to put right are:-

1. Main mirror out of collimation.

2. Accumulation of dust on main mirror.

3. Finding (after much trying to collimate the scope) that I had installed a new

Crayford focuser incorrectly.

4. Finding a "slug" trail across the main mirror after leaving my scope out all

night (to keep it ready for viewing Jupiter).

Happily I was able to correct the above "faults" with relative ease (and I found the slug and put it back in the garden - I just can't kill things!). (hope this doesn't start another debate!).

That is just about all I can do with "tweaking" my scope to try and obtain (and maintain) the best performance that my scope can give within the limits of it's "mass produced" optical parts.

I am at present saving up for a 140mm Maksutov (delux version) from "Orion Optics" (the UK company), and this scope is said to come with a full optical "Interferometer wave" test, and report on the accuracy of the mirror and collimation, before being delivered to me.

I am aware that even if I can understand the report, I would still have to trust the manufacturuer's report entirely. The only real test I will be able to give the scope, is as before, "a visual test" using the only "scientific" "tool" I have - the "Mark1 eyeball!" If I feel anything is wrong, I would have to contact the manufacturuer.

So there you have it! That's my (simple) take on testing the optics on a scope.

Regards,

philsail1

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I mentioned it as a curiosity Phil. It's one that amuses me. I often see comments in scope reviews such as "I did a star test and the telescope was slightly under corrected, sensibly perfect etc ". It's a very macho way of testing a scope that many don't understand as it can be hard to interpret etc. They could have a gadget in their pocket that would enable them to say exactly what the errors were. Anybody could use it. Done properly it would look like an eyepiece and have a screw adjusted knife attached to it. A simple version would be an old eyepiece with the lenses removed and a section of razor blade fixed some way to the underside of the eye hole. The scope drive could be used to knife the beam. There was also an eyepiece about with a ronchi screen in it at one point. Rather than a knife it uses a series of black lines evenly spaced apart. Ideally 4 or 5 of them per mm but less will work. It's even possible to print one on at home on clear film. I don't know how these can be used to measure errors but they can be used for that. They can also be used for a quick assessment of the optics.

On what to do if the errors are bad. Well it's possible to refigure a mirror oneself or pay some one else to do it. Looking at the cost of building a telescope from scratch these days it's probably the most economic way of obtaining a high quality scope. There's also the chance that it might already be one.

John

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Yes I see John. If someone had a "gadget" to test the accuracy of a mirror's configuration, they would need some comprehensive - but easy to understand - interpretation of their findings.

I would not understand what is meant by "under, or overcorrection."

But it is all up to how far each individual wants to go in checking the accuracy of their particular scope.

I would reiterate that perhaps for most amateur astronomers, as long as we can obtain what we think (and are sure of) is a crisp and clear view of whatever we are looking at, it is good enough.

Perhaps in future, we may see some very sophisticated (but easy to read and understand) gadgets becoming available for the "layman" to use. Or perhaps, telescope optics manufacture might become so accurate that we will not require any home testing at all!

Regards,

philsail1

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The under or over corrected bit is easy. A parabolic mirror has a very nearly spherical shape. Your 200mms F5 newtonian has a shape that differs from a sphere by 0.00039mms that's 15 millionths of an inch in old money. It's a sphere that has been deepened about 70% out from the center with a very smooth curve. That tiny amount makes a huge difference to the image. Remove too little glass and it's under corrected, too much and it's over corrected. A pure sphere produces spherical aberration - a distorted image - that's what over and under correction refers to on a newtonian. On a refractor it might also refer to the color correction - how well it focuses white light to a point with/without false color.

Opinions vary on the tolerances needed on mirrors but the usual one is 1/8th of the wavelength of green light, that's 2.7 millionths of an inch (0.00007mms). Some people who specialize in planets feel that 1/15 of a wave is better and can see the difference. The usual tolerance where it's specified by mirror suppliers is 1/10th wave or 1/4. The price difference might amount to £150. Some specify 1/6pv but I think that means 1/3rd wave in the terms I've used. Some quote rms which is square root (error/2). Most quote nothing at all. It's all a bit like specifying central obstructions as a % of area rather than diameter - things can be miss leading.

I think the best thing to say about scopes is that within reason any scope is better than no scope at all because the observer can't afford one or just wants to try the hobby. So if anybody does test their optics bear in mind the price. Rumor, always a dubious thing to trust, has it that takahashi mirrors are manufactured to 1/20wave. If they stamped that on the side of all of their scopes it really would be a tough act to follow. What they should specify is wavefront error at the image. 1/8wave then become 1/4 and it doesn't matter what sort or how many optical parts make up the scope.

John

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I just came across a decent web page on star testing http://www.robertvincent.freeserve.co.uk/testing.html. Straight to the point and no messing about.

There is also some info on the knife edge test (foucault) as applied mirror making on the site. That should give people an idea what's going on when that's done on a telescope pointing at a star. Main difference as against making a mirror is that the mirror should darken evenly all over. Any "shape" represents an error.

John

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Yet another web link that shows what a ronchi test looks like through a telescope. Hope the translate works

http://66.102.9.104/translate_c?hl=en&langpair=de%7Cen&u=http://www.teleskop-service.de/Leistungspaket/focault/focault.htm

As far as I know the is the only company in europe that will test a telescope when it's sold. The charge is reasonable and they speak excellent english.

John

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Hello John!

Sorry I took so long to reply to your excellent explanation of meaning of "under/overcorrection in telescope mirrorS. My wife and I have just come back from a week in Southern Ireland! Never took any telescope with me - wouldn't have had any time to use it with all the trips we went on!

Anyway, back to the world of dreams now! (and I'm dreaming of an Orion Optics (UK) OMC 140 Maksutov!).

I read and re-read your piece on mirror over/undercorrection. It all makes sense to me - but how does one tell if a mirror is under or over sorrected?

I note the Orion Optics advertise their scopes as having either 1/4 wave correction, or on the de-lux versions 1/8th wave.

I see that according to your explanation 1/8th (or more) correction does make a visual difference.

I will look up the other references you give on the testing of optics and the "ronchi" testing. Should be interesting.

Now, I don't expect a reply if you are busy - I know how these messages can run on and on! (But of course if you do have any more information you wish to impart, then it is most welcome John).

Thanks for your replies - I'm sure many others have found the technical information very enlightening.

Regards,

philsail1

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All I can really add is the following. More on what you are buying rather than actual testing.

P-V it seems (Mel Bartell) is the same as rms. That's root mean square. The error might be measured in say 5 places. Each error is squared then they are all added together and divided by 5, the number of errors measured. I believe that some manufacturers have also divided by an additional 2 in the past. I'm not saying that orion is doing this or abusing the calculation. That can be done by say 4 of the errors being 0 and 1 making up the lot. Calculating rms makes the mirror look a lot better than it really is. The 1/8wave I mentioned is a straight measurement of the maximum error.

The latest greatest measurement of telescope performance is strehl ratio (spellin :grin: ). Turns out that this is just a calculation that just uses the rms error I've mentioned. Again according to Mel.

The most direct method of specifying a telescopes quality is the wavefront error. That way it doesn't matter if its' a refractor or reflector or what ever. The max error in that respect is 1/4 wave for the entire telescope. Some russian and others do specify this but going on the mak I bought it's done in a jig not the assembled telescope. There are a number of other reasons why it would be done that way but I wont go into that. It could be done easily in the assembled scope.

This 1/4 wave and the 1/8th for a mirror and for that matter 1/12 wave for the 2ndry comes from a true brit called lord raleigh. In his day there wasn't a standard and that's the one he set. It hasn't ever been effectively disputed other than very experienced planetary observers saying that they could detect the difference between a telescope like that and one twice as good. There is an important thing to grasp here. The errors become more and more serious as the magnification goes up. It seems that there have been some tests done in the states with 1/2 and 1/8th wave mirrors. People who had never looked through a telescope couldn't tell the difference. Those that had experience could.

The other aspect that comes into this is the 2ndry diameter % of the main mirror (Not area which is often quoted). This also tends to degrade performance. 20% is reckoned not to do much harm, 30% on the other hand will be noticeable. This has it's biggest effect on compound scopes - usually this and that cassegrain eg mac etc. However if some one wants a very compact scope they have to put up with it. In my view though smaller scopes suffer more than they need do because they are generally fitted with 2ins focusers. Big schmidt cassegrains also have 30% plus obstructions. Many scope do.

John

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There is one other thing I could mention under this heading and that is testing one telescope with another. It can also be used to collimate a scope but it's not something I've tried myself. One scope is set up so that it produces light in the same way as it comes from a star. That scope needs to be the same size or bigger than the other although I have heard of it being a bit over 1/2 of the size of the other. This scope needs to be set up with an artificial star at it's focus. That could be the smallest round hole that can be made or one of the methods described on the web. For collimation a pin pushed carefully pushed into aluminum foil on glass should do but it must be round. The roundness can be checked by viewing that hole against a led or clear bulb/torch through a pin hole in a piece of card. This will show diffraction rings as well as anyone is ever likely to see them. If the hole is round the pattern will be round. This needs to be lit from behind and placed very close to the infinity focus of the scope. That can be found by focusing the scope on a distant object without an eyepiece maybe even the moon. The image can be viewed on something translucent.

That scope is then pointed at the scope that's been tested. If it's focused in the normal way a "star" will be seen. The scopes need to be squared on to each other. Given that its possible to produce holes in the region of 0.035 mms (.0015ins) or so in aluminum foil this can be a very sensitive test. Once set up it's possible to carry out any of the tests mentioned. Scopes can also be collimated. This is how many types of optics were tested before interferometers came into general use. An optician would use a pin hole and a very accurately made parabolic mirror.

John

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Many thanks for your replies there John. (I could not find an earlier reference to what "rms" is?

You have an excellent grasp and knowledge of the mechanics of optical systems! I am struggling to understand much of what you say, but I can appreciate the relevance of accurate testing, to manufacturers of scopes. (The accuracy of a scope's optics would have a direct baring upon their pricing and sales potential).

The bit about using two scopes to test optics was interesting. Also the "pin hole" in the foil - as an artificial star. Could this method be used to collimate a Newtonian scope - by placing the artificial star as far away as possible (say at the bottom of one's garden), then collimating using a "cheshire," or even a simple film cannister?

I believe "Orion Optics" (UK) use the "Interferometer" method to test all their mirrors for accuracy of "wavefront error." They also offer higher quality (for want of a better word) upgrades to their mirrors - though they say that the accuracy and viewing pleasure through their basic 1/4 wavefront optics is very good.

I am assuming that if you go for the upgraded mirrors (1/6th and 1/8th wavefront error) you would be able to detect the difference in quality of views? (I will have to ask them this).

Do you think John, that to amateur astronomers (and I do not wish to denegrate amateurs in any way), the "minute" differences in the quality of the optics of a "quality" scope, would matter that much. I mean, would one know if their optics were fractionally below par in a particular scope - unless they were able to compare their particular scope with an identical scope fitted with "upgraded" mirrors (or objective lenses?).

Regards,

philsail1

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