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.
is your lead time?
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
-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.
do you require to place an order?
require a 50% deposit for a
new primary mirror,
a 60% deposit if a primary and secondary mirror are ordered, and full
for secondary mirrors ordered alone. This reserves a spot in my
queue. We order glass for primary and secondary mirrors
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
-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
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).
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.
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
without the permission of Lockwood Custom Optics, and will be subject
to additional fees.
-Glass is expensive, and I have to buy glass to
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
mirror for a reduced price. Shipping charges for mirrors are
refundable under any circumstance, and a 15% restocking fee
and/or recoating fee (including shipping for
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
-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.
size secondary should I use in my telescope?
can't answer that question unless we know the measurements of the
instrument, the required fully illuminated field, etc.
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?
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
-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
shipping, that is my responsibility. However, we take no
responsibility for shipment of refigured mirrors. The owner
the mirror should advise if he/she wishes for LCO to insure those
don't you make small mirrors?
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"
mirrors, though we make smaller custom Cassegrain secondaries. We
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
-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
is a superb material for telescope mirrors, and we highly recommend
it. It is significantly 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,
-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
-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
believe that the infinitesimal improvement can be seen visually or in
images. The real reason to use quartz is because it is a
What thickness mirrors do you
recommend? How much do your mirrors weigh?
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
-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
cooling time vs. 2" thick mirror
|0.42 of the time required for a 2"
|1.50" - 1.60"
|48.3 lbs (1.55")
|1.90" - 2.00"
|83.7 lbs (1.95")
|1.00 (same time as a 2")
|2.00" - 2.10"
|113.3 lbs (2.05")
What advice do you have regarding
-For mirrors 8"
to 40" in diameter, I
recommend and use Optical
for enhanced aluminum
Their facility is fairly close - about a half day's drive my
- which reduces shipping costs and time for larger projects.
-We highly recommend and offer coatings from Zambuto
Optical Company for mirrors as large as 24".
ZOC will currently coat mirrors that we have worked on up
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 recommend Ostahowski
Optics. They can coat mirrors as large as 36" at this time.
-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
We only recommend coaters based on experience - I cannot
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
to the coater for recoating. However, I cannot reinburse a
shipping, because coating is not my service. The buyer of a
mirror understands that I am hiring the coater at their direction and
I am not guaranteeing or warrantying their services or product.
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
mirror or any damage to it.
-Do not let your
to the point that they become unusable - this can result
etching the glass, and the degraded coating may be very difficult to
strip, risking damage to the optical surface. If this occurs,
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
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.
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
measures to remove the residue before recoating. This can
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,
you wish to use a different one, choose carefully and you do so at your
-Do NOT clean you mirror with
petroleum-based solvents, such as
turpentine, mineral spirits, gasoline, etc. These solvents
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
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
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
off of the chrome, aluminum will not stick to chrome that has
exposed to the environment, so it is useless afterward. To
it worse, the chrome is not easily strippable without risking damage to
the glass surface through harsh chemical action.
line, if aluminum won't stick to the glass, then the
not clean or it is damaged in some way. Choose your coater
you recommend standard or enhanced aluminum coatings?
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
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
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
and enhanced aluminum coatings for Newtonian primary mirrors.
Some coaters offer enhanced coatings for no additional cost,
some chage a bit more. You will be happy with either.
More coating facts:
-Let's get a few definitions straight here.
coatings will corrode quickly, so it is generally only used on large
observatory mirrors (because the observatory has its own coating
chamber). Its reflectivity when fresh is ~92%.
has a thin layer of something over the aluminum. Technically this
is a dielectric layer. Reflectivity approaches 90%, and is
actually lower than bare aluminum, but it lasts much longer. It
it easily strippable with ferric chloride (which I use) or green river
solution. Neither of these will damage glass.
has several layers of dielectric thin films over the aluminum, which
enhances reflection, thus the name. Reflectivity approaches
94-95%, and the reflectivity can be extended in the UV or IR depending
on materials and the thicknesses of various layers. It is also
easily strippable with ferric chloride or green river.
(as amateurs refer to them) have many layers of materials that make up
a very durable, high-reflectance coating. However, many of the
materials are not removable without nasty chemicals that might damage
glass or even grinding or polishing off the coating, thus ruining the
optical figure. A stack of many of these layers is essentially
permanent. Also, the coating is typically done at high
temperature, and when everything cools this leaves stress in the whole
assembly of coating and glass because the coating shrinks more (I
think) than the glass. So, the coating can and does warp the
shape of the glass. Thus, coatings like this are not suitable for
larger amateur astronomical mirrors, and I do not recommend then on
secondaries of significant size. I'm not even sure where these
can be found.
for amateur coatings, I find any claims of reflectivity over 95% to be
quite dubious and suspicious, and I do not recommend them. I
have seen no differences in performance between overcoated and enhanced
aluminum, no damage to glass from these coatings, and I have had no
trouble stripping these coatings provided the owner does not wait too
long for recoating, thus allowing the degraded coating to react with
environmental chemicals and either damage the glass itself or form
chemicals that are more difficult to remove. Other
opticians and I have seen far worse damage from coaters that used harsh
chemicals or techniques for stripping or cleaning. Sometimes this
actually requires a re-grind of the mirror! So, this is why I
recommend coaters for my clients, so that their mirrors will last for
many decades to come.
regrind my slow mirror into a faster mirror?
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
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?
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/6.3 primary.
What are the advantages and
disadvantages of fast telescopes?
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?
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
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?
telescopes require good structures and mirror cells. LCO works
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.
eyepieces, coma correctors, and collimation products do you recommend
for fast telescopes?
fastest focal ratio for visual use that we recommend is f/2.7.
(We can make faster for imaging, though, and for Cassegrain primary
-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
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
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
then you don't need to adjust it any more. You can switch
without having to tune the Paracorr, like one must do with the
-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
removed (for collimation, etc.) by unscrewing it into the secondary
-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
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
How much will
it cost to refigure my mirror (which you have never seen or tested)?
-We can't give you an estimate of
until we test the mirror.
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
the mirror, and in that case the client will pay for the testing and
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
from less expensive glass that may or may not have been
-To test for strain, we must strip the coating so that I can pass
polarized light through the mirror and observe the result.
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
-The less expensive glasses, such as plate glass or BK7,
higher coefficient of expansion than Pyrex, so they change shape more
while cooling. These glasses are also softer,
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
risk. If we determine that the mirror is unsuitable for work,
owner is still responsible for the cost of stripping the coating and
-Consider this carefully before sending your mirror out for testing -
it is quite possible you may be disappointed.
methods do you recommend and use?
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
-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.
-For decades opticians have attempted to compensate for this
by undercorrecting mirrors so that their star test appears good as the
optic cools, but this has yielded undercorrected mirrors, sometimes with
substantial error. When conditions are
good and these optics approach thermal equilibrium, these optics
produce bloated star images and perform
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
-Thanks to decades of observing and measurement by professionals, 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.
concave mirrors, LCO tests most frequently with knife-edge/zonal
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
mirrors, the zonal testing method is extremely accurate in measuring
correction when used
and provides results that agree with star testing when
the entire mirror is the same temperature as the air.
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
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
test uncoated or coated optics, and we test (and figure) Cassegrain
double-pass autocollimation with one mirror uncoated to null the system.
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
-We value customer feedback - this is a form of testing.
Please let LCO know how our optics are performing! We use
customer comments, with the customer's permission, on our Customer
Comments page. These comments become a permanent part of this web
site for the benefit of LCO and its clients, and comments will not be
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
a 16" mirror and matching flat (diagonal, secondary, etc) mirror,
the basic testing of the mirror's shape will cost will be around $150
Interferometric testing or other more involved testing will
more. Larger primary
and flats over 10" in diameter will
require more time, and thus will cost more to test. Again,
the hour, so testing mirrors with severe problems that require more
complicated tests and analysis will cost more.
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?
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
-In our opinion, you truly do get what you pay for.
provide test numbers for a mirror?
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
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
mirrors are properly supported, equilibrated, collimated, and
clean. When you experience a night that allows this type of
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
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
-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.
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
consult my records to see the specification and verify that we worked
it or made it. You will need to
provide the serial number and the size and focal ratio of the
All engraved serial numbers are unique, and are photographed
kept on record for verification purposes.
will almost always show some residual astigmatism in tests.
horizontally as we must, it is almost impossible for it to be
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
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
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, the minor effect of small amounts
astigmatism will never
seen in images because the effect is so small that it will only be seen
telescope is sent to space! It is difficult
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.
What scratch/dig rating do you provide, and what is your policy on
-For new telescope mirrors 32"
and under, we usually achieve a scratch-dig rating of approximately
-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
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
on the image than a few minutes of dustfall.
-On larger mirrors,
more factors come into play. It is far easier to miss
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
glass itself may have more defects that I cannot control. So,
these larger mirrors, we do our best and typically achieve much better
60-40 scratch dig, but we do not guarantee that unless it is explicitly
-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.
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.
refigured mirrors, we generally improve the polish quality of the
mirror, but don't expect any pre-existing scratches to go away.
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
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 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.
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.
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
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
on most blanks. We believe that this makes mirrors more
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
-Surplus quartz may have small edge chips and dings and bubbles within
the glass that may end up in the optical surface. These
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.
is the difference between Pyrex and 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.
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,
(stuff in the glass), and bubbles.
-Supremax is sometimes available up to
~2.5" thickness, which is more than the ~2.25" maximum of
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
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.
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
by Starmaster, JPAstrocraft, StarStructure, and Equatorial
Platforms. I hope that more telescope builders adopt this
superior mirror cell design in the future.
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?
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.