FoucaultFringes

Frequently Asked Questions

- Please read these before you send me an email, post a question on my Yahoo group, or place an order.
- This FAQ also contains and serves as my Terms of Sale.
- I reserve the right to change my policies listed here at any time, and without notice.
- I apologize for the length and density of this document, but these are important facts to know concerning what can be very expensive optics, and they should be understood before a client makes a purchase.


GENERAL QUESTIONS


What is your lead time?
-Currently it is ~4-8 months for 14.5"-24" mirrors, ~6-12 months for 26"-32" mirrors, and ~12 months for larger optics and optical systems.
-Refiguring lead time is ~3-6 months for a small-medium size mirror refigures, and may be more for large mirrors and more complicated optical systems.
-All lead times are estimates, and are not guaranteed.  Actual time may vary depending on number of orders, delivery times for glass, or events that may be out of our control.  We reserve the right to schedule mirrors in the most efficient manner for the machines and tools that we have on hand.

What do you require to place an order?
-We require a 50% deposit for a new primary mirror, a 60% deposit if a primary and secondary mirror are ordered, and full payment for secondary mirrors ordered alone.  This reserves a spot in my queue.  We order glass for primary and secondary mirrors immediately upon receiving the downpayment to ensure that the cost of the glass does not change.  See the section below regarding refunds.
-Pricing for services that are done outside of Lockwood Custom Optics and are thus out of our control (such as shipping and coating) may change over a long lead time.  Because LCO is having these services done on the client's behalf, increases in the cost of these services will be added to the cost of the mirror at the discretion of Lockwood Custom Optics.
-The balance of the purchase price is due upon completion of the mirror, prior to shipment to the coater.  For international orders, shipping to the coater is included in the purchase price, but international shipping is not.  An estimate of the international shipping cost is obtained and added to the balance due at the time the mirror is completed (before coating).
-For refigures, if the cost is relatively low we will ask for full payment upon completion of the refiguring, prior to shipment to the coater.  If the cost is higher, such as for a large mirror, we may ask for a deposit based on the estimated cost of the work.

Are deposit payments refundable?
-No.  Under certain circumstances, at the discretion of Lockwood Custom Optics, if a new buyer for the mirror can be found, the deposit may be refunded, less a restocking fee of a minimum of 15% of the mirror's purchase price, not including shipping costs.  Orders are not transferrable without the permission of Lockwood Custom Optics, and will be subject to additional fees.
-Glass is expensive, and I have to buy glass to make a mirror.  This process takes time, too.  Consider this carefully if you are ordering a custom mirror that may be unsuitable for other people's projects, and therefore hard to sell.  I will not sell you a partially-finished mirror for a reduced price.  Shipping charges for mirrors are not refundable under any circumstance, and a 15% restocking fee and/or recoating fee (including shipping for recoating) will apply if a mirror is returned for resale or if an order is cancelled.

What forms of payment do you accept?
-For US customers, we accept personal, business, and cashier's checks, and postal money orders.
-For international customers, we accept wire transfers and certain checks that are drawn from US banks.
-We do not accept credit cards, Paypal, Bitcoin, gold, etc.

What size secondary should I use in my telescope?
-We can't answer that question unless we know the measurements of the instrument, the required fully illuminated field, etc.
-The telescope builder usually determines and specified the size of the secondary mirror, though LCO can make a recommendation if that is required.
-For an estimate of some secondary sizes, see my old page here:  Table of Secondary Sizes

How should I pack my mirror?
-To see how we pack mirrors, read this installment of "In the Shop".  If you have more questions about how to ship your mirror, please email us at the address found on my information page.
-To see how we don't recommend packing mirrors, read this installment of "In the Shop".  (We will probably add more bad examples.)
-We are responsible for shipping a new mirror.  If it is damaged in shipping, that is my responsibility.  However, we take no responsibility for shipment of refigured mirrors.  The owner of the mirror should advise if he/she wishes for LCO to insure those shipments..

Why don't you make small mirrors?
-LCO is not set up for it, and others are properly equipped and will do a good job.  Our equipment is mainly for ~14.5" mirrors through 60" primary mirrors, though we make smaller custom Cassegrain secondaries.  We also supply tested elliptical flats to go with our primary mirrors so that the client can be sure that the entire optical system is up to our standards.
-We do occasionally finish up small blanks that we have, so please check my In Stock page for a few smaller mirrors.
-We will refigure mirrors from 10" and up in aperture, assuming the glass is of reasonably good quality and anneal.  We reserve the right to decline to work on smaller mirrors and inferior quality glass, no matter what the size.

What is your opinion on quartz as a mirror substrate?
-Quartz is a superb material for telescope mirrors, and we highly recommend it.  It is more expensive than other glasses like Pyrex or Supremax, but it changes shape less while cooling, reducing cooling-induced overcorrection of primary mirrors, and reducing focal length shift compared to Pyrex/Supremax.  If you can afford it, use quartz.
-Contrary to what some claim, quartz is not that much stronger than Pyrex/Supremax, and we do not recommend greatly reducing the thickness of a mirror simply because quartz is being used.  Our thickness recommendations for mirrors will be similar for most sizes of mirror, except in cases where thin quartz blanks are readily available in certain sizes.
-For secondary mirrors, using quartz can produce small gains in image quality, especially during cooling.  If you use a quartz primary, then we recommend a quartz or other low-expansion secondary if they are available.  Others claim that quartz is polished to a slightly smoother surface on a micro-scale, and while this may be true, we do not believe that the infinitesimal improvement can be seen visually or in images.  The real reason to use quartz is because it is a lower-expansion material.


What thickness mirrors do you recommend?  How much do your mirrors weigh?
-For apertures under 28", there is no reason to use a 2.0"-thick mirror in a visual Newtonian, alt-azimuth telescope for apertures less than 28".  Using unnecessarily thick glass adds weight and this means the glass holds more heat and will take longer to cool.  Because thermal equilibration is usually the limiting factor in telescope performance, it is highly desirable to keep the glass as thin as possible.
-It is important to understand that cooling time is not related linearly to mirror thickness.  The physics of a cooling plate tell us that the cooling rate is actually four times faster for a mirror that is half as thick as another mirror.
-Mirror aperture, LCO's recommended thickness, approximate weight, and approximate relative cooling time are contained in the table below.  Weights are calculated for an f/# of f/3.3.  Cooling times are difficult to predict, and depend on other variables other than thickness.

Mirror Aperture, inches
Mirror Thickness, inches
Approximate mirror weight
Relative cooling time vs. 2" thick mirror
16"
1.30"
18.6 lbs.
0.42 of the time required for a 2"
18"
1.35"
24.2 lbs.
0.45
20"
1.40"
30.7 lbs.
0.49
22"
1.50"
39.6 lbs.
0.56
24"
1.50" - 1.60"
48.3 lbs (1.55")
0.60
28"
1.90" - 2.00"
83.7 lbs (1.95")
1.00 (same time as a 2")
32"
2.00" - 2.10"
113.3 lbs (2.05")
1.05




MIRROR COATINGS

What advice do you have regarding mirror coatings?
-For mirrors 8" to 40" in diameter, I recommend and use Optical Mechanics, Inc. for enhanced aluminum coatings.  Their facility is fairly close - about a half day's drive my shop - which reduces shipping costs and time for larger projects.
-We highly recommend and offer semi-enhancede aluminum coatings from Zambuto Optical Company for mirrors as large as 24".  ZOC will currently coat mirrors that we have worked on up through 24" in size, but are only coating their own mirrors and mirrors from a few select OEMs.
-We recommend Galaxy Optics.  They can coat mirrors as large as 25".
-We also recommend Ostahowski Optics.  They can coat mirrors as large as 25" at this time, and possibly larger sizes upon request.
-It is not a coincidence that the coaters listed above have all done optical work, and understand how to strip old coatings safely and treat a polished surface without damaging it.
-Disclaimer:  We only recommend coaters based on experience - I cannot guarantee a coating or be held responsible for a problem coating because I do not coat mirrors.  If there is a problem, I will help facilitate the return of the mirror to the coater for recoating.  However, I cannot reinburse a client for shipping, because coating is not my service.  The buyer of a mirror understands that I am hiring the coater at their direction and discretion, and I am not guaranteeing or warrantying their services or product.

-IMPORTANT NOTES:
-It is the mirror owner's responsibility to have a mirror recoated in a timely manner, and by a good, reputable coater.  For a mirror purchased new, we bear the responsibility for the first coating so long as one of my recommended coaters does the work.  For refigures and recoats, we do not bear responsibility for the mirror or any damage to it.
-Do not let your coating degrade to the point that they become unusable - this can result in chemicals etching the glass, and the degraded coating may be very difficult to strip, risking damage to the optical surface.  If this occurs, the mirror may need to be repolished and refigured to polish away the remains of the coating and the possible damage to the glass, and this is done at additional cost to the owner.
-If there is an issue with stripping the coating, the coaters listed above will let me know, and we can perform the work necessary to restore the figure of the mirror.
 Again, this work is done at additional cost to the owner, so don't let the coating go too long.
-If you wish to have your mirror coated or re-coated by a company not listed above, we strongly request that you consult us first.  If something goes wrong with the stripping of an old coating or a new coating, some coaters may take extreme measures to remove the residue before recoating.  This can destroy the figure of the mirror.  After the initial coating, LCO cannot and will not be held responsible for damage to the optical figure/surface caused by a coater, so if you wish to use a different one, choose carefully and you do so at your own risk.
-Do NOT clean you mirror with petroleum-based solvents, such as turpentine, mineral spirits, gasoline, etc.  These solvents can remain in small crevices in the mirror and they can come out under vacuum conditions and ruin a recoating attempt.  They are extremely hard to remove, and should not be used.  Use other solvents such as acetone or alcohol to remove residues such as velcro adhesive.
-We do not recommend silver or other metals for telescope mirror coatings.  Some coaters may not be used to removing these metals before recoating, and they may damage the glass.  While silver may initially offer higher reflectivity, we hear that if often fails earlier than an aluminum coating, especially in environments near the ocean or in high humidity.
-We do not
recommend or specify chrome undercoats for aluminum coatings.  Chrome is sometimes applied first, in the vacuum chanber.  In this environment, with no air to react with the chrome, aluminum sticks very well to it.   So, if the aluminum won't stick to the glass, chrome may used as a "band-aid" - but only once.  While the aluminum can be stripped off of the chrome, aluminum will not stick to chrome that has been exposed to the environment, so it is useless afterward.  To make it worse, the chrome is not easily strippable without risking damage to the glass surface through harsh chemical action.
-Bottom line, if aluminum won't stick to the glass, then the glass is not clean or it is damaged in some way.  Choose your coater carefully.

Do you recommend standard or enhanced aluminum coatings?
-LCO has never had a bad experience or seen any degradation in performance with the normal enhanced coatings offered by the coaters listed above.  In theory, the extra layer in a properly applied, simple enhanced coating should make it a little more durable than a non-enhanced coating.
-Even if there is a coating issue, these coatings are easy to strip off without harming the optical surface (if stripped by a competent individual or coater), and are easily recoated after that.  NO coater is perfect, but the ones we recommend above stand behind their work.

-We recommend enhanced coatings for secondary mirrors, and for telescopes that have three or more mirrors to reduce light loss.  Ideally for a Newtonian secondary mirror, the coating is designed for reflection at 45 degrees, and will be different than one used at normal incidence (0 degrees).
-We recommend both standard and enhanced aluminum coatings for Newtonian primary mirrors.  Some coaters offer enhanced coatings for no additional cost, and some chage a bit more.  You will be happy with either.



FAST OPTICS

Can you regrind my slow mirror into a faster mirror?
-Unless the mirror has massive problems, it does not usually make financial sense to destroy the value of an existing optical surface just to make a new one.  The client will usually come out well ahead in terms of money if he/she sells the mirror that they don't want and simply buy a new one from me.

Do you only make and refigure fast mirrors?
-No, we will work on mirrors of just about any focal ratio.  We enjoy making or refiguring a slower mirror now and then, and we am quite capable of making them extremely accurate.  LCO has worked on a hyperbolic f/1.2 primary, an f/30 Cassegrain system, 28" and 32" f/2.8 primaries, 30" f/2.0 primaries, and a 36" f/3.6 primary.

What are the advantages and disadvantages of fast telescopes?
-Fast telescopes are advantageous because they are shorter, and this keeps the eyepiece closer to the ground.  This is safer and more convenient for viewing.
-A shorter telescope is also significantly more resistant to wind, allowing observing when longer telescopes would have to shut down.
-In our experience, there is no compromise in optical quality compared to slower mirrors in the sizes that we make.  In fact, it is LCO's belief that the new fast mirrors are optically superior to many of the slower mirrors of the past because optical standards have risen in the 21st century.  This also is true for secondary mirrors.

Are slower telescopes better than faster telescopes?
-While slower (higher f/#) mirrors can be made with less labor than faster (lower f/#) optics, if all other factors are the same (collimation, cooling, etc.), then the faster and slower mirrors will produce practically identical images.  The effect of central obstruction is often greatly exaggerated by those who insist that slower telescopes are better, when in reality the effect is small.  Even the very fast telescopes that LCO makes optics for have 25% obstruction or less.  The true obstructions of fast telescopes are often exaggerated by people who don't like them or who don't know better.
-Basically, some people will say anything because of their outdated, incorrect bias against fast telescopes.  Do your own research.  It may be difficult to find a quality fast telescope to use before you buy, and if you can't find us let LCO know and we'll try to put you in contact with someone who can show you some views.

What are your recommendations for fast telescope construction?
-Fast telescopes require good structures and mirror cells.  LCO works with vendors who use modern mirror cells and good telescope building techniques such that their telescopes won't bend my optics and will hold them in good collimation at most altitudes.  See our Clients Page for telescope builders and this article on mirror cells for more information.
-LCO does not want LCO mirrors, which are often thinner than the typical competitor mirror, placed in a mirror cell that is intended for a thicker mirror, or that is constructed with an outdated mirror cell design.  Doing so can lead to substandard images.

What eyepieces, coma correctors, and collimation products do you recommend for fast telescopes?
-Currently the fastest focal ratio for visual use that we recommend is f/2.7.  (We can make faster for imaging, though, and for Cassegrain primary mirrors.)
-For f/2.8 to f/4.0, we highly recommend TeleVue Ethos eypieces, TeleVue Delos eyepieces, and the Paracorr 2 coma corrector, preferably in a SIPS.  They provide superb wide-field views, and work well at fast focal ratios.  For slower instruments and at high powers, TeleVue Naglers work very well.
-LCO recommends the Starlight Instruments SIPS.  The SIPS is a 1.5"-travel FeatherTouch focuser mounted to a special stationary version of the Paracorr 2.  More info on these is found in the SIPS instruction manual, (written by Mike Lockwood), a diagram of the SIPS, and on the Starlight Instruments SIPS page.
-Following the installation procedures, the SIPS lenses are positioned at a specific distance away from the primary mirror when you install it, and then you don't need to adjust it any more.  You can switch eyepieces without having to tune the Paracorr, like one must do with the tunable-top Paracorr.
-LCO uses the SIPS on our own telescopes, including a 20" f/3.0, and find it to be much more convenient because we change eyepieces frequently.  The lens group is quickly and easily removed (for collimation, etc.) by unscrewing it into the secondary cage.
-NOTE:  Other companies have recently all but copied the TeleVue Ethos and Nagler designs, and are selling these eyepieces at a lower cost.  LCO has tested these on occasion and found the TeleVue eyepieces to be superior in contrast and to have an edge in sharpness.  We also prefer to support American companies such as TeleVue whenever it is possible when foreign companies attempt to copy products that were originally designed and engineered in the USA.
-LCO recommends Howie Glatter laser collimators with or without the barlow lens.  We have found that these lasers maintain their alignment (the laser alignment to the body of the collimator itself) better than other lasers that we have compared with in the past.  We also recommend the CatsEye Collimation products for non-laser alignment and collimation.

Should I worry about central obstruction?
-Kept within reasonable bounds, the effect of central obstruction is quite minor compared to other factors.  However, it is often obsessed over by telescope owners and builders when really the energy spent worrying about it would be better used thinking about cooling strategies or improving collimation. 
-The larger the telescope, the less the fixed dimensions like focuser height figure into the secondary size equation.  So, while a small f/3 scope may have a 30% or larger obstruction, my 20" f/3 has a ~25% obstruction, and larger scopes may be even a bit less depending on the desired size of the illuminated field.  A 30" f/3 can have a 23% obstruction (using a 7" m.a. secondary).  Bottom line, the larger the scope, the more you may be surprised how small the obstruction is for a fast telescope. 
-Comparing telescopes OF THE SAME SIZE, yes, larger obstruction will degrade contrast somewhat, but it will still perform better than a smaller instrument.  Aperture trumps central obstruction.  For those that worry about building, say, a 25" f/3 due to the central obstruction, LCO says make it a 28" f/3 instead because that will more than make up for the tiny loss in contrast going from say f/4 to f/3 at that size and have more light gathering and resolving power too!
-Keep in mind, for visual use, there's not much point in going faster than f/3.6 at a 16" size, so we are talking about larger telescopes than that.



TESTING AND REFIGURING

How much will it cost to refigure my mirror (which you have never seen or tested)?
-We can't give you an estimate of cost until we test the mirror.
-Refiguring costs are done by the hour, so cost depends on how much time that the project requires.  We believe this is fairest to both LCO and to the client.
-After testing, LCO will provide an upper and lower bound for the cost, and the client can either proceed with work or we can return the mirror, and in that case the client will pay for the testing and shipping.

Can you make my cheap import mirror into a mirror that meets your standards?
-That can't be answered without strain testing the glass.
-In the past, some import mirrors were made from borosilicate glass (like Pyrex), and some of the glass had very good anneal.  However, recently made import mirrors are increasingly made from less expensive glass that may or may not have been annealed properly.
-To test for strain, we must strip the coating so that I can pass polarized light through the mirror and observe the result.  The results range from acceptable strain to unacceptable levels that could cause the shape of the mirror to change in the future, or which could cause noticeable astigmatism while the mirror cools off and equilibrates.
-The less expensive glasses, such as plate glass or BK7, have a higher coefficient of expansion than Pyrex, so they change shape more while cooling.  These glasses are also softer, and requires slower, gentler polishing to maintain a smooth optical surface.
-So, while some mirrors have responded well to refiguring, others have not.  It is like rolling the dice, and work is done at the owner's risk.  If we determine that the mirror is unsuitable for work, the owner is still responsible for the cost of stripping the coating and testing.
-Consider this carefully before sending your mirror out for testing - it is quite possible you may be disappointed.

What testing methods do you recommend and use?
-LCO understands that there is no one test that can completely characterize a mirror, and we utilize a variety of tests to pinpoint a variety of possible issues and conditions.  Often others make the claim that certain types of testing are superior to others and offer more accurate, complete results, but the reality is that no test report, interferogram, or piece of paper is a complete guarantee of a mirror's shape.  In fact, the best guarantee is the optician's knowledge and understanding of various testing methods, and what they each do best.
-Thanks to decades of observing and measurement, LCO understands that borosilicate optics change shape as they cool, and LCO also understands exactly what type of distortion of the optical figure occurs with cooling because LCO has actually tested cooling mirrors.
-Other optical shops have attempted to compensate for this cooling-induced distortion by undercorrecting mirrors so that their star test appears good as the optic cools, but this has yielded undercorrected mirrors with substantial error.  When conditions are good and these optics approach thermal equilibrium, these optics produce bloated star images and perform very poorly.  Thus, while it is possible to produce a good small optic or an optic from quartz or a substrate with even lower expansion using star testing, LCO feels it is bad practice to do so because of the many other variables involved, and because no numerical data is generated from this type of testing.  Additionally, smaller aberrations and figure of revolution errors may be missed completely due to the test being seeing limited, while lab tests do not have this limitation.
-Thanks to decades of observing and measurement, LCO knows that lab testing is superior.  Properly lab-tested optics produce superb, high-contrast, sharp images, and outperform undercorrected mirrors when conditions are good.  Thus, buyers should be highly suspicious of claims of star testing being used to produce high-precision optics.
-Therefore, LCO uses a variety of tests under carefully controlled lab conditions to produce what LCO has found to be the most accurate large, fast optics that are available for a reasonable cost.  LCO also produces smoother optics than other shops that may charge far more for the same optic.
-For concave mirrors, LCO tests most frequently with knife-edge/zonal testing, also known as Foucault testing, to measure the correction of a mirror.  We have found, and it has been verified with state-of-the-art, very accurate modern testing systems, that, even for fast mirrors, the zonal testing method is extremely accurate in measuring correction when used properly, and provides results that agree with star testing when the entire mirror is the same temperature as the air.

-LCO uses interferometry and a proprietary, highly-sensitivity test to check the figure of revolution of each optic at multiple stages in the work.  If astigmatism can't be readily seen with these tests, it will not be seen under the sky if the mirror is mounted properly.
-LCO currently has 12" and 38" flats, which are used for autocollimation and other tests.
-LCO has 10" f/12.5 and 10" f/6.4 reference spheres that can be used to test flats to high precision.
-LCO tests all blanks for strain with a crossed-polarizer test.
-LCO has two Buccini MIC-1 spherical wavefront interferometers, both capable of phase-shifting measurements, a variety of certified divergers from F/1.7 to F/10, and certified coated and uncoated reference mirrors.  We can test uncoated or coated optics, and we test (and figure) Cassegrain systems in double-pass autocollimation with one mirror uncoated to null the system.
-Ultimately, we ask that you use our mirrors under the sky, support them properly, collimate them well, cool them to equilibrium, and evaluate them by observing the sharpness and clarity of the images at high power.  So far no one has been disappointed with the images formed by our optics when they are properly supported, collimated, and equilibrated.
-We value customer feedback - this is a form of testing.  Please let LCO know how our optics are performing!  If you have an issue with your telescope containing Lockwood Optics, we are here to provide advice and support to help you get the superb images that you deserve.

How much does testing cost?
-For a 16" mirror and matching flat (diagonal, secondary, etc) mirror, the basic testing of the mirror's shape will cost will be around $150 plus shipping.  Interferometric testing or other more involved testing will cost more.  Larger primary mirrors and flats over 10" in diameter will require more time, and thus will cost more to test.  Again, this is done by the hour, so testing mirrors with severe problems that require more complicated tests and analysis will cost more.
-Please clean your mirrors before sending them to me so that LCO will not have to.
-Strain testing of glass is necessary before refiguring, and this required the coating to be stripped so that polarized light may be passed through the glass.  Coating stripping and train testing will incur additional charges.

Others test mirrors for free - why don't you?
-First, charging a testing fee shows that both the owner and the optician are serious about doing the testing.
-Second, others don't have a backlog of work - we do - and time spent testing is time not spent polishing.
-In our opinion, you truly do get what you pay for.

Why don't you provide test numbers for a mirror?
-Inevitably test results (or second-hand accounts of test results!) end up on the internet, where "experts" will "debate" them without consulting LCO or understanding how the results were obtained or the implications of how they were obtained.  If those optics are tested by someone with less testing experience than LCO and the results do not agree, then we are compelled to spend time defending our well established test methods, a process that does not benefit us, but can end up educating our competition!  Because we do not wish to do this and do not have time to do it, we simply choose to avoid it.
-For telescope mirrors made by us to our highest standards, we guarantee performance to 50 power per inch of aperture on nights when the atmosphere is steady enough to allow this, so long as the mirrord are properly supported, equilibrated, collimated, and reasonably clean.  When you experience a night that allows this type of use of our optics, you will forget all about the numbers.
-For flat secondary mirrors we guarantee that the mirror, when properly supported, equilibrated, collimated and reasonably clean, will not limit the quality of the image produced by the primary mirror.  Secondary mirrors should be smooth and fairly flat, and should lack obvious errors of curvature (power).  They do not need to meet ridiculous specifications for flatness, specifications which are often not met when tested by a third party because achieving ridiculous specifications is extremely difficult.  Also, secondary mirrors made from borosilicate change shape with temperature changes just as primary mirrors do, and this makes their testing more challenging.
-Mirrors ordered with a less stringent specification (which is often appropriate for non-astronomical uses such as LIDAR) are guaranteed to that specification and should not be used for applications requiring more accurate optics.
-I cannot guarantee that a Cassegrain secondary (convex) mirror will be perfect to its edge.  In certain cases these mirrors are highly aspheric, and achieving a perfect figure to the edge is extremely difficult and time consuming.
-If you have a question about a mirror that we have made, such as in the case that you are buying a used mirror, please contact us so that we can consult my records to see the specification and verify that we worked on it or made it.  You will need to provide the serial number and the size and focal ratio of the mirror.  All engraved serial numbers are unique, and are photographed and kept on record for verification purposes.

-Large mirrors will almost always show some residual astigmatism in tests.  Testing horizontally as we must, it is almost impossible for it to be completely detected and removed, especially in large mirrors, which are almost always thin mirrors.  It is caused by anisotropy and variation within the glass itself - glass can bend more in one direction or in one area than others, and by other minor variations during fabrication.  We can't measure every piece of glass for this type of variation and then select the best - that is very difficult and cost prohibitive.
-Because we can measure the correction of a mirror (how close its curve is to the ideal parabolic shape) very accurately, astigmatism is usually the largest source of P-V (peak-to-valley) error, which would skew the P-V numbers and would make a superb, smooth, accurately corrected mirror (that performs superbly under the sky look) far worse than it really is.
-However, in large mirrors, over 24", the minor effect of even modest amounts of smooth astigmatism will never be seen in images because the effect is so small that it will only be seen if the telescope is sent to space!  As a rough rule, divide the aperture in inches by 100 to determine the amount, in waves, of permissible astimatism.  You won't see that amount, and it is difficult for me to see it in testing.  This is, in fact, why we employ our figure of revolution test - if we can't see significant astigmatism there, you won't see it in images if the mirror is supported properly.
-If you require a large mirror  with almost no measurable astigmatism, please let me know.  This requires extensive and costly interferometric analysis, probably a more expensive substrate, and it will raise the cost significantly over that of a normal telescope mirror.

What scratch/dig rating do you provide, and what is your policy on polish quality?

-For new telescope mirrors 32" and under, we usually achieve a scratch-dig rating of approximately 20-10.
-Also, almost all new mirrors that we make in these sizes have no scratches or sleeks, but we do not guarantee that.
-It is impossible to completely avoid the occasional cosmetic blemish in the form of a few small residual pits, bubbles, or sleeks, so for normal telescope mirrors, we do not promise a perfect, blemish-free surface.  Any blemishes that are present are merely cosmetic, and while they may be visible on the surface of the mirror, they will not affect the images formed by the telescope, so there is no reason to be overly concerned about them.  They have less effect on the image than a few minutes of dustfall.
-On larger mirrors, more factors come into play.  It is far easier to miss blemishes, and since the polishing time is longer and the area of the mirror larger, the chance of getting a sleek increases.  Sometimes scratches happen due to simple bad luck, or the glass itself may have more defects that I cannot control. So, for these larger mirrors, we do our best and typically achieve much better than a 60-40 scratch dig, but we do not guarantee that unless it is explicitly stated.
-It pays far greater dividends, in terms of image quality, to spend time tweaking the figure of the mirror, which will affect image quality, than it does to worry about some cosmetic blemishes that will not affect image quality.
-In certain glass, especially surplus quartz blanks and cast cellular blanks, bubbles and inclusions are common, and we cannot guarantee a bubble-free optical surface because we cannot reject an expensive and difficult-to-get blank just because it has bubbles.  In surplus quartz, minor edge chips and even some small edge fractures may be present that will not affect optical performance.  It is surplus material, and this is part of why the price is lower than "new" quartz.  Bubbles may hold small amounts of contaminants what may tarnish a coating around them over time, but this will not affect image quality.
-For refigured mirrors, we generally improve the polish quality of the mirror, but don't expect any pre-existing scratches to go away.  If there are sleeks or scratches that you wish to have removed, then this generally requires many, many hours of polishing or regrinding, which will greatly increase the price of the work compared to the price of simple refiguring.  This must be agreed to in advance.
-If you have a scratch/dig requirement for a particular application, then please specify it when requesting a quote and we will quote appropriately.
-If no value is specified, then we will assume a value appropriate for the use of the mirror.  For mirrors that are simply collecting light, a perfect polish is not necessary, and will unnecessarily raise the price of the optic.  For telescope optics, the mirror will be thoroughly polished and will have very low scatter.



OTHER QUESTIONS

What other things should I know about your optics?
-See this article for a summary of the process of making a primary mirror and our standard procedures and features.
-For new mirrors, we grind the back of the blank smooth with very fine abrasives before optical work is begun.  This makes them look better, allows them to be cleaned more easily, and the mirror will then "slide" nicely on mirror cell supports without binding.
-We also scribe, permanently with a diamond-tipped scribe, a series of small circle in the glass, exactly centered on the mirror, after carefully centering it on a turntable.  This helps you locate your centering triangle, ring, square, or other shaped sticker on the mirror for collimation.  We do this for all new mirrors and refigured mirrors unless we are instructed not to do it.
-Our standard practice is to add a curved bevel to the front and back edges of the mirror.  We usually round-over the 45-degree bevel that comes on most blanks.  We believe that this makes mirrors more pleasant to handle, and also reduces the chance of "dings" should the mirror accidentally hit something.
-We check the anneal of each mirror blank that we work on.  If we find a level of strain that is concerning, then we send it back for re-annealing.
-Surplus quartz may have small edge chips and dings and bubbles within the glass that may end up in the optical surface.  These will not harm the figure or performance of the mirror, and are due to the surplus nature of the material, and are one reason that it is prices lower than "new" quartz.

What is the difference between Pyrex and Supremax?
-Supremax is basically the same as Pyrex in terms of chemical composition, coefficient of thermal expansion, and how it reacts during optical work, polish quality, etc.  It is made by Schott.
-Supremax is a higher-quality product than some sheet Pyrex was, though the Pyrex I have used was always very good in quality.  Benefits (compared to poor quality Pyrex, which I never used) include fewer striae, inclusions (stuff in the glass), and bubbles.
-Supremax is sometimes available up to ~2.5" thickness, which is more than the ~2.25" maximum of Pyrex.  That's good for me, because I make big mirrors.  The largest mirror that can be cut from a sheet of Supremax is ~44" in diameter.
-Supremax is readily available - Pyrex is no longer available in large sheets.
-The end user will notice no difference in most mirrors, except somewhat higher prices for mirrors because the material costs more.

What type of mirror cells do you recommend?
-See my mirror support article.
-We require that our optics be placed in a "moving-frame" cell.  This is a cell that has a frame that holds the mirror and all parts that touch it (triangles, edge support, retaining clips), and the whole frame moves when collimation is adjusted.  These are currently used by Starmaster, JPAstrocraft, StarStructure, and Equatorial Platforms.  I hope that more telescope builders adopt this superior mirror cell design in the future.
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For mirrors up to 32" in diameter, we strongly recommend a whiffletree-type edge support with rollers for contact points.  JPAstrocraft was the first to use rollers, and they make a significant difference.  StarStructure now uses these, and others are considering it.
-For mirrors larger than 32", a cable sling will work very well for edge support, but we strongly recommend that the sling be attached to the moving frame of a cell, and not to a part that does not move with the mirror.  Even if the end of the sling can slide and position itself, if the mirror is tilted for collimation, the sling can still become non-parallel to the plane of the center of gravity of the mirror, and this can result in undesirable distortion of the mirror's shape that will harm images.
-For secondary mirrors, the mirror must fit properly in the holder shell.  It should be able to move very slightly laterally, and should not be kept in place by tightly-packed stuffing material.  It should also contact the lip of the shell uniformly all the way around, and not just at one end of the mirror.

Can I send you a blank to work on?
-Yes, but then because we do not know its source, we cannot guarantee its quality.  In the event that a problem is discovered with the blank after work is done, the cost for any work done on the blank is non-refundable.  We also may be unable to insure your glass.  Certain types of glass are difficult or impossible to replace.  All work is done at the client's risk, though appropriate care will be exercised to minimize the possibility of any damage.  Consider this carefully before sending me a blank, and purchase your own insurance policy for a valuable piece of glass if you require it.



This FAQ also contains, and serves as, my terms of sale.  I reserve the right to change my policies listed here at any time, and without notice.

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