All About Optics
OPTICS: Testing and explanations.
You will see that we offer four grades of optics, our Standard range which has a measured accuracy of 1/4 PV wavefront and will give excellent views of all astronomical subjects. Our Professional Grade which has a measured wavefront accuracy of 1/6PV wavefront, our Research Grade which is 1/8th PV wavefront and, the best possible quality level, the Ultra Range. All PV wavefront accuracies we quote on our web site are based on the Zygo laser wavelength of 632.8nm. Testing optics is not and cannot be, an exact science due to slight temperature differences, slight air movements and expansion of optics in the measuring laboratory, one measurement to another of the same optics can and does yield slightly different values. What a Zygo does is minimise these differences because of the repeatability and consistency of the interferometer’s optics, mechanics and software of the Zygo. As a result of these slight measurement differences, we allow a 10% variation on measurements which is typical of interferometery procedures. This means that, as an example, a mirror advertised as 1/8PV could be between 1/9th PV and 1/7th PV and so on. Typically we always try to err on the accurate side.
Why four grades of optical quality when most companies offer just a standard grade with often a somewhat ambiguous statement as to their quality in many instances?
Its simple actually: When optics are made, especially where hand finishing is employed, the surface accuracy of each mirror is slightly different from all others. These differences are incredibly small but, they are enough to allow the mirror to make a good image or an image which, for all intents and purposes, is perfect. When our opticians are hand figuring an optical surface they test frequently as to the progress of the surface’s accuracy. Some mirrors exhibit different optical characteristics which do not allow the highest quality of accuracy to be reached with that particular mirror. At the final stages of hand finishing the experience of the optician plays a vital role into deciding as to whether the mirror he is figuring will allow it to achieve a specific grade. All our mirrors are 1/4PV wavefront as a minimum but, at the final stage a mirror will often show that it will allow it to be figured to a higher standard. When the optician detects this he will spend a considerable amount of additional time concentrating on improving the mirror’s accuracy to allow it to enter the next standard quality range and, if the mirror is capable of it, even further work and testing to achieve a mirror with an incredible 1/10th PV wavefront. It must be emphasised here that not all mirrors will allow this high degree of figuring expertise to achieve a 1/10PV wavefront mirror. The four standards will produce images, under identical conditions of course, which increase in overall quality. Visible detail, contrast, stability, overall less affected by air currents are the major benefits of higher grade optics. Unfortunately, as you would expect, producing optics to this level of accuracy is expensive, the more accurate, the higher the cost.
Optics which are mass produced on machines where no hand finishing work is carried out cannot and never will, achieve levels of accuracy and performance which can be produced from the skilful hands of a master optician. That is one of the main reasons so many companies never put an actual numerical value on the optical quality of their telescopes. We appreciate that most customers have a budget for their telescope, or optics, which has to be adhered to. We give them the option of choosing whether to invest in a larger telescope with standard optics or, go a little further and have professional optics fitted and spend less on accessories, which, can always be bought later. The option of research grade optics is for customers who want the best possible images in the size of telescope they have chosen. Several combinations are available within our telescope ranges, we have published the most common combinations but, if there is a combination we have missed which you are interested in, be assured, we can build a customised model for you at no extra cost to published prices and options. Indeed, approximately 24% of the telescopes we produce are customised in some manner, be it optics, focusers, finders etc. We want you to have the exact telescope to meet your needs, if it’s possible for us to do just that, we will.
The following paragraphs will hopefully give you some insight into optical terms often used to describe a telescopes optical performance or accuracy.
Optics are the heart and soul of any telescope no matter what electronic extras or accessories you buy with your telescope. At Orion Optics we pride ourselves on having some of the finest optical technicians anywhere in the world. The optics we produce are world renowned for being some of the best available. Each optical surface is tested up to 29 times as it passes through its many processes of being turned from a blank piece of glass to a highly accurate figured mirror or lens. Daily, as a matter of routine, our optical technicians produce surfaces with an accuracy of RMS 1/100 wave and PV 1/20 wave. The key to this is our ability to identify the accuracies we achieve by means of our Zygo Laser Interferometers. Very, very few telescopes are made and tested to this level of sophistication and accuracy. We believe it is necessary to carry out the critical testing and hand finishing of optics to satisfy the most critical observer. The results of these procedures are there for anyone to see in our telescopes.
We find ourselves continuously looking at analogies between cars and telescopes. The most probable reason is most people can relate to buying and using a car, so to compare these two areas of what could be expensive and, fraught with danger, seems a relatively appropriate analogy. Of all the people who have read this page and bought a car, albeit a new one or second user one, there can surely be precious few who have handed over their money before carrying out a test drive. On the other hand, ignoring for the moment clients who buy telescopes direct from our factory, there must be precious few who have taken their telescope for a ‘test drive’ before handing over their money. By ‘test drive’ I refer of course to seeing their telescope on optical test.
To carry out a relatively simple but very effective test on a telescope, takes but a few minutes and it will immediately give you confidence in what you are buying. The only car dealers who sell cars without offering a test drive or, have the capabilities to show you the performance of a car, would most probably be shunned by prospective customers, so why do we buy telescopes from people who are unable to test them for us? Most telescopes when they come out of their packing box instil enthusiasm and excitement from the outset. Shiny paintwork, lots of ‘whistles and bells’, high tech appearance, highly reflective mirrors, all go to promoting a ‘high quality’ instrument. Unfortunately, in many, many occasions, nothing could be further from the actual reality of the situation.
Our intentions here are not meant to criticise directly any one manufacturer or distributor, merely to give you an insight as to the way we produce and test our optical components. Optical development is an ongoing process within Orion Optics, new procedures, equipment; ideas, designs etc are continuously being updated, acquired, and applied. We are always looking to improve our quality and reliability to enable you, the customer, to benefit by having improved telescopes and optics. Many telescope manufacturers which appear to be producing high quality products are in effect just producing high quality advertising. The two, most certainly, are not always linked.
Here, we hope will give you a much deeper appreciation into how we make our telescopes and to what degree we rigidly test ourselves to give you the best products available, anywhere.
As this section falls under the banner of optics, we will be giving optics the major consideration in the information following …
Telescopes are offered for sale with optical descriptions which are sometimes, at best, confusing and at worst, down right untruths. Over the next few paragraphs we will try and explain in every day language what statements are supposed to mean and what they actually do mean. It will also be explained how any information, particularly specifications, can effect the telescope’s performance. As you would expect, not all telescopes and designs of telescopes are the same. Indeed, there are many on the market and amongst them, some rather uncommon, obscure examples. The most commonly mentioned quality aspect of a telescope’s specification is, and rightly so, the quality of the optics in that telescope.
‘1/8th wave optics’,
‘1/4 wave image’,
These descriptive phrases supposedly relate to the ‘quality’ of the optics. Before we enter into the world of advertising phraseology, it is unfortunately essential to explain some background, but relatively basic physics.
Light travels in waves. The wavelength of light varies, that is what gives us colours in everyday objects and astronomical objects. For an optical surface to be used effectively in a telescope it has to reach a minimum standard of accuracy. if it does not reach this standard, the telescope is not useless, it just means it is not as good as it could be and, it is not as good as it should be. Many years ago an English astronomer, Lord Rayleigh, both calculated and demonstrated that if optical quality achieved this level of accuracy, the surface/telescope under question would work very well. This is a very shortened description and I am sure purists will wish to comment but, this is not a lesson in physics, it is a brief note on what is required of optics interpreted into everyday language.
What is this level of accuracy? Well, it is surprisingly very easy to describe but, with today’s mass produced telescopes, incredibly difficult to achieve. It is also very difficult to prove or disprove without the use of very specialised and very expensive equipment. This is one of the major reasons there are some disgraceful telescope offerings on the market. The quality of the optics in a telescope are for the most part, invisible, you cannot walk into a showroom and see the optical quality nor can you touch it or measure it. You just have to believe what you are being told by the salesman or reading on a web page. Similarly, we are asking you to believe what we are telling you. The big difference between Orion Optics and virtually any other major manufacturer or dealer is quite simple, we will show you the quality, we will measure it for you and we will let you see the quality at work, literally. At work in our factory and your telescope at work on an optical test table in our laboratory.
Getting back to terminology: Lord Rayleigh’s discovery of over 100 years ago is still correct, it’s longevity is due to its correctness both in theory and practice. The accuracy level Rayleigh determined as being required is that a minimum percentage of light in the formed image of a star in a telescope should be held in the central star’s diffraction disc, or Airy disc as it is often called. For anyone not familiar with an image produced in a telescope of a star on extremely high magnification, the following drawing should help.
The central Airy disc is surrounded with a far less bright diffraction ring. Within small differences all the terms listed above in red describe the same accuracy when applied to a telescope’s optics. However, it must be pointed out that slight changes in these descriptions are very misleading when discussing a telescope. When buying a telescope it cannot be emphasised enough that you are given the correct accuracy of the telescope. To be told the telescope is 1/8th wave means absolutely nothing at all, indeed it is more than misleading, it could be described as fraudulent if intentionally misquoted. The figure of 1/8th wave means different things to different people. Some may look at it as 1/8th wave surface accuracy optics. Some may look at it as 1/8 PV wavefront error. The two are completely different. 1/8th wave surface accuracy will produce a 1/4PV wavefront error. The implication could be placed on the description ‘1/8th wave’ as to being the wave front error which suggests optics which are twice as accurate.
PV wavefront is a general term now widely used, if used in the correct way, it tells you what the highest error and lowest error on your telescopes wavefront is. Unfortunately this could apply to two very small areas of perhaps 1% or even less of the area of the image. The remaining 99% could be very accurate indeed. At Orion Optics we ensure all wavefronts have maximum errors of 1/4 PV wavefront and the smoothness, termed Strehl value, is between 1/5 th and 1/7th of the PV wavefront error. a similar value to RMS.
Strehl is becoming more common place nowadays and it does give a more realistic value of what the telescope’s performance on the sky will actually be.
When buying a telescope always, always, ask the following question of the salesperson:
‘Please tell me what the wave front accuracy of this telescope is’ and, if possible, if the sales person seems able to answer that question satisfactorily, ask ‘Could you also let me know its RMS or Strehl value’
You will receive back various answers, we know, we have tried it for ourselves on unsuspecting dealers and manufacturers. We posed on the telephone as amateur astronomers with a modest amount of optical knowledge. The replies we had back were not to put too fine an edge on them, rubbish in most instances.
‘ . . . our telescopes give excellent images’
‘Pin sharp stars’
‘1/8th wave optics sir’
and so on . . .
When asked to quantify in terms of PV wave front accuracy, not one actually revealed what the telescope’s optics were supposed to be. (A favourite reply nowadays is ‘Diffraction limited’. Unknowingly salespeople are then saying the telescope produces a 1/4PV wave front image. Something which can be measured in an optical laboratory.)
We need to say here that not one of the above, or any of the other similarly adjective embellished descriptions, are of any use what so ever, they are useless. PV wavefront accuracy is a numerical value and virtually 100% of any serious quality telescope manufacturer or dealer will be able to offer a value for the telescope on offer. If they do not offer this world wide accepted system of giving a telescope’s performance, you must think why!
There are only two possible reasons:
1. They do not know the accuracy of the optics or,
2. They do know them but are not prepared to divulge the figures.
Which ever of the two possibilities it is, if you buy the telescope, no matter how well it is described or offered, you are buying an unknown quantity. ‘Caveat emptor’ was never more appropriate.
When we carry out final tests on our telescopes and individual optical components prior to assembly, we use incredibly powerful measuring equipment. This equipment is featured in all the worlds leading optical companies, laboratories and high quality optical and telescope manufacturers. This equipment is grouped under the wide umbrella of Laser Interferometers. These pieces of equipment are capable of outstanding accuracy measurements and are the basic tool for determining the wave front performance of telescopes. There are other methods of measurement such as Foucault test, Caustic test, Ronchi test etc., etc. but all are insignificant when compared to a laser interferometer, in particular a Zygo Laser Interferometer. Why a Zygo? Well its simple. If there is a better system on the planet for testing and measuring telescopes, it is being kept a well guarded secret. Before choosing a Zygo, we now have two models, we visited the National physical laboratory in Middlesex several years ago and asked them to test some of our telescopes and components. Without hesitation they did every test on their Zygo which revealed results which we did not think possible. Every small error, zone, mark etc was clearly visible. None of these errors were visible in our existing test equipment at that time and as such, unless any manufacturer is using an interferometer, they will be invisible to them. Our immediate response was to buy a Zygo the following week.
Why do not all telescope manufacturers use Interferometers?
A one word answer . . . .Cost. To set up an optical workshop with a Zygo and its required accessories will cost a minimum of 70,000 – 90,000. That is expensive by anyone’s standard so it has to be a worthwhile purchase. If any company is happy producing average quality optics without wanting to really find out what they can really make in terms of high quality, don’t go to the expense of a Zygo. Two reasons for this: one it will frighten you to death almost, by showing you exactly what you are making, not what you think you are making. Two, your findings will without doubt prompt you to spend more money and time investment on other ways of improving your quality. We costed the total investment we have made since the visit to the NPL a few years ago and it is virtually 143,000 in optical test equipment and complementary measuring equipment.
What have we gained from this investment?
We are in the enviable position now of being able to test all of our optical surfaces and telescopes with a laser interferometer of which we now have four. Two Zygos and a Fizeau for testing our flat, secondary mirrors.
We are also in the position of being able to analyse extremely accurately both our polishing techniques and hand figuring expertise with the use of the Zygos. The Zygos will show you even the differences in the quality of optical polish you are using. They can even detect the ‘signature’ of which optician in our lab hand figured the optic. Small amplitude zones, previously invisible with normal test equipment, stand out in bold relief enabling us to modify machine settings and hand work procedures to eliminate them. The sensitivity of our interferometers to show us how to improve our products was one of the biggest steps forward Orion Optics has ever made.
It is now quite well known that Orion Optics has a suite of Zygos and we are regularly contacted by many individuals and companies to carry out checks on both individual optical components and telescopes. Indeed, if any reader wishes to have their telescope Zygo tested, provided you pay for carriage, we will test any competitors telescope for a modest fee. Contact our Technical Department [email protected]
We could add many more pages to the manufacture and analysis of optics but for most people we think the above should be enough to give them a good insight as to what is important and, a good direction in which to go the next time you buy a telescope, even, even if its not from us. If you feel you would like us to elaborate on any of the above, give some more examples, add some more involved information, or if you disagree to any points raised, please feel free to contact us using this [email protected] . . . All communication will receive a reply.
We hope no one has been offended or upset with any of the above facts, our intention was not to do so, merely pass on information we have gathered over almost 30 years into the subject of telescope optics.