Zero expansion optics are made from a glass-ceramic. The glass properties are formulated to result in a near zero coefficient of thermal expansion. This means that the glass will not change shape due to changing temperatures and is therefore desirable for use in telescope optics. Some examples of this type of material are Zerodur, Astro-Sital and ULE. These materials are more expensive than Pyrex and for this reason are most often found in research telescopes. Optical Mechanics, Inc. uses zero expansion glass in all of our Nighthawk series telescopes.

Why use Zerodur or Astro-Sital?

Using Zerodur or Astro-Sital helps to maintain focus stability with thermal variations during an observing session.

What size telescopes do you manufacture?

Optical Mechanics, Inc. manufactures a wide range of telescope sizes. Our standard sizes are 0.4 meter (16 inch), 0.5 meter (20 inch), 0.6 meter (24 inch), 0.8 meter (32 inch) and 1.0 meter (40 inch). Other sizes are available upon request, please call for more information.

What focal lengths?

Optical Mechanics, Inc. Nighthawk telescopes come standard as F/10 systems with F/3 primary mirrors. Other focal lengths are available, please call for more information.

Should I get a Ritchey-Chrétien or Classical Cassegrain telescope?

Optical Mechanics, Inc. produces both Richey-Chrétien and Classical Cassegrain telescopes. Our Nighthawk series telescope come standard as a Classical configuration. The final choice of telescope design will depend on the end use of the telescope and customer needs.

Do you offer other size/focal ratio telescopes?

Optical Mechanics, Inc. has the ability to design both optical and mechanical systems in house. We can manufacture telescopes with custom apertures and focal ratios. Please contact us for pricing information and to discuss your projects needs.

What cameras does you software support?

Optical Mechanics, Inc. telescope control software, Talon, is currently supports Santa Barbara Instrument Group (SBIG), Finger Lakes Instrument (FLI) and Apogee CCD cameras. Other cameras can be supported but will require the development of a camera driver to be used with the Talon software.

How do you adjust focus?

The Nighthawk series telescopes reach focus through motion of the secondary. The focus shaft is captured by bearings and does not effect collimation or create image shift. The step resolution of the secondary motion is variable and typically set so that there are multiple steps needed to go through focus. The depth of field is many times larger than the step resolution. The focus axis also has an encoder attached and is repeatable to plus/minus 2 encoder counts; well with in the depth of field. The encoder resolution is variable and can be changed depending on customer requirements. Once focus has been reached the focus motor produces very little heat as it is only holding position. The heat generated has not been shown to effect imaging.

How stable is the focus position over temperature change?

The optical tube consists of a lower primary mirror assembly and an upper secondary mirror assembly. The two assemblies are connected using carbon fiber trusses that have a near zero thermal expansion rate. Refocusing is typically not required as the dimensional changes due to temperature change during an observing session are within the depth of focus of the standard Nighthawk telescope.

Why are there cooling fans on the telescope?

Optical Mechanics, Inc. Nighthawk telescopes use cooling fans in the primary cell to reduce localized thermal currents caused by the difference in temperature between the ambient air temperature and the temperature of the optics. The fans help the optics stay closer to the ambient air temperature during observing sessions.

Why do you use an open truss design?

An open truss system allows the primary mirror to cool much faster than a closed tube system. Open truss systems also tend to not suffer from “tube currents” that can affect local seeing conditions. Since open truss systems have a much lower surface area they are less sensitive to wind loads.

What about stray light in an open truss design?

The telescope is baffled such that there is no direct light at the focal plane with a fully open enclosure. The baffles are coated with low reflective dark black paint with reflectivity of less than 2% independent of the angle of incidence. The baffle system consists of a primary baffle with internal knife edge glare stops and a secondary baffle with internal glare stops.

How does collimation affect my images?

Improper collimation will result in aberrations in the image plane, such as astigmatism and coma. The telescope optics should be well collimated to achieve the best image performance possible.


Optics

Will an enhanced coating change a mirrors figure?

With regard to the coating changing the figure of the mirror, consider the application of each layer of material. The layer most likely to change the mirrors figure is the aluminum layer as it forms the surface from which the light is reflected. The SiO2 and TiO2 layers are transparent. They act only to enhance the reflectivity. If they lack uniformity their primary effect will be a variation in the reflectivity across the mirror.

The aluminum layer is about 900 Angstroms or approximately 1/6-wave thick at 5500 Angstroms. A loose tolerance on coating uniformity is 5%. This is equivalent to 1/122-wave at 5500 Angstroms. A more realistic target for uniformity is 2% or 1/300-wave. A properly applied coating will not change the figure of the mirror by any significant amount. It would have to be a very poor job in deed to ruin a mirrors figure.

How can I tell if my coating needs to be replaced?

Null Foucaultgram of a failed coating Null Ronchigram of a failed coating

I've worked with many different coating vendors. The ones we've had longevity issues with did not use ion assist. They all look fine out of the box but non-ion assist failures exhibit signs of moisture absorption in the over-coat layers early in their useable life. When a porous coating absorbs moisture it starts to look hazy. As time goes on the moisture in the over-coat layers breaks them down further and accelerates the deterioration process. This allows the elements to get through to the aluminum. Eventually, you'll see blotches in the aluminum. If you clean the mirror, the spots where the blotches are will eventually wash away leaving bare glass. By this time, the rest of the coating will look very hazy. It will have deteriorated to the point where it scatters an incredible amount of like.

It's not that all non-ion assist coatings will inevitably fail this way. This only occurs when something goes amiss in the process. It's tough to detect this problem right out of the chamber because the coating looks fine to the eye. You can detect the problem by measuring witness samples that are coated with each batch of mirrors. If you expect your reflectivity to be X and it's significantly less than X you probably didn't apply exactly what you wanted. In this case you should run samples of each material as if you were coating a mirror but run only a single layer of each material. With the proper instruments you can measure the actual thickness you applied and the refractive index of that particular material. If the index is off by too much then you have a problem to solve.

There are many factors in a process that can drift and affect the quality of the coating. That's why it's important to measure witness samples from every run. In addition to the reflectivity and index measurements you can do destructive testing on the samples like; the tape peel test, eraser rub test, salt spray test or leave the sample exposed to the elements for long term tests. Modern coating machines keep a log of everything they did for each run. If you see a problem developing in your witness samples you can look at the logs to see what is drifting and. With this information you can set things right and keep the product within your QC parameters.

Why do you use Pyrex?

Pyrex is readily available and capable of being polished to very high accuracy. It is also a lower cost option to the zero expansion glasses available. For these reasons Optical Mechanics, Inc. uses Pyrex in our line of Newtonian mirrors up to 30” aperture.

How do I know my mirror is good quality?

Any mirror you purchase should include certification of optical quality. The most common values used in optical certification are RMS wavefront error and Strehl Ratio. Optical Mechanics, Inc. optics will not be certified unless they have a RMS less than or equal to 0.712_ at 633nm and a Strehl ratio greater than or equal to 0.8. Optical certification is done using interferometry.

Why use interferometry?

Interferometry provides higher test resolution than the zonal Foucault test. The zonal Foucault test assumes that the mirror has perfect rotational symmetry. This is never the case so Foucault results tend to be over-optimistic. Interferometry measures the entire mirror surface. It has limitations because it sees everything, including air currents and test sling errors so it tends to be more pessimistic. There are ways to remove some of the random and systematic errors in the interferometric test to arrive at a very good approximation of the mirrors wavefront. Both interferometry and Foucault result in an approximation of the mirrors wavefront. Both can be used to make a fine mirror but interferometry has better test resolution.

How do I clean my mirror?

Cleaning your mirror is a fairly straight forward process. The interval between cleanings will vary depending on your local conditions, that is; air pollution, amount of moisture and salt in the air. Directions for cleaning a mirror along with photos showing the process can be found here.

How often do I need to recoat my mirror?

Every mirror will need to be recoated at some point. The time between coatings will vary depending on your local conditions, that is; air pollution, amount of moisture and salt in the air. As coatings age they begin to lose reflectivity and as a result performance. The final decision on recoating will need to be determined by the owner as the amount of performance loss acceptable to each individual varies. Typical times between coatings are 3-5 years.

Do you offer re-figuring?

Optical Mechanics, Inc. can examine your mirror and provide test results to let you decide if you should have your mirror re-figured. Please contact us for more information.

Do you offer re-coating?

Optical Mechanics, Inc. offer re-coating services. Please contact us for price and delivery schedule.

What size mirrors do you make?

Optical Mechanics, Inc. manufactures a wide range of mirror sizes. A list of our standard sizes and focal lengths can be found here.

What focal length?

Optical Mechanics, Inc. manufactures a wide range of focal lengths. A list of our standard sizes and focal lengths can be found here.

Can I get other sizes/focal lengths?

Optical Mechanics, Inc. can produce mirrors in many sizes and focal lengths. Please contact us to discuss your needs or to obtain a quote.

Can you test my mirror?

Optical Mechanics, Inc. offers interferometric testing of mirrors up to 40" in size.