Ultimate Image Newtonian Telescope Mirrors
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Diameter
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f/Ratio
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Thickness
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Price
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Availability
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Lead Time
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18"
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f 4.2
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2.0"
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$ 3395
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In Stock
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18"
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f 4.5
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2.0"
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$ 3195
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In Stock
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20"
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f 4.0
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2.0"
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$ 5995
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6 months
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20"
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f 5.0
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2.0"
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$ 4095
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In Stock
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22"
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f 4.2
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2.0"
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$ 6495
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In Stock
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25"
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f 4.0
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2.0"
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$ 11995
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6 months
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25"
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f 5.0
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2.0"
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$ 8195
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In Stock
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C-1 Coating on all primary mirrors
Galaxy Optics Quality Comparison Checklist
Substrate Materials
Galaxy Optics uses only the finest quality precision annealed Schott Supremax 33 and Pyrex 7740 low expansion borosilicate glass. Each substrate is custom manufactured to our exact specifications including testing under polarized light for internal strain and striae. Precision annealed borosilicate glass exhibits higher thermal stability over fine and coarse annealed materials. The glass cools and heats evenly without developing localized hot spots. The greater stability allows for a higher quality optical figuring in the shop and improved performance in direct use. All substrates are free of any chipped edges.
Multiple Platform Testing
At Galaxy Optics each mirror receives several hours of testing in a atmospherically controlled environment using five precise optical tests. The results from each test are cross-checked against each other in order to provide an accurate qualitative and quantitative assessment of the overall optical quality. Please read on to learn more about the methods in which we use each test.
Foucault Test: Used to obtain a reading of the overall center to edge correction and inspect the optical surface for smoothness. The overall correction reading provided by this test is verified against results obtained from the Dall-Null test and Interferometry.
Figure of Revolution Test: Used for detecting stress induced flexure errors and astigmatism.
The primary mirrors manufactured at Galaxy Optics are tested in the horizontal optical axis using an optical mounting sling. The sling contacts the mirror on the lower 180-degree circumference. Horizontal axis testing of thin large diameter optics may induce stress flexure errors. The most common and severe stress flexure aberrations are astigmatism and trefoil. Each optic receives several hours of thorough testing to determine if the optical surface is exhibiting stress flexure or true astigmatism. The astigmatic error that is not induced by mounting stress is always removed during optical figuring. The personnel at Galaxy Optics have studied astigmatism and stress induced flexure errors for many years. We are highly proficient in detecting true astigmatism and removing it from optical surfaces.
Dall-Null Test: The primary test method used at Galaxy Optics during figuring of optical surfaces. The advantage of the Dall-Null test lies in its ability to provide a quick and accurate assessment of zonal defects on the surface of the mirror. When a uniform null is achieved using Dall-Null test the optic is finished and ready for interferometric testing.
Image Test: The image forming characteristics of each mirror manufactured at Galaxy Optics are closely inspected using our proprietary high magnification imaging test. By image testing each mirror in an atmospherically controlled environment we are able to effectively use magnifications much higher than what is typically used in the field to examine the image produced by the mirror. This enables us to ensure each mirror will produce tack sharp images with minimal light scatter.
Interferometric Testing: Interferometric testing is the gold standard that aerospace industry, professional optical labs, government labs, and well informed consumers use for a quantitative assessment of optical wavefront quality. Interferometry is a time consuming and expensive process requiring a high degree of technical expertise. It produces accurate test results enabling us to consistently produce outstanding mirrors. This is the final test that all Galaxy Optics mirrors undergo during manufacturing.
During interferometric testing the shape of an optic's wavefront is determined by superimposing its wavefront over a reference wavefront that is certified to be highly accurate. Constructive and destructive interference created between the superimposed wavefronts generate the interference fringes. A CCD camera and image capture board are used to transmit an image of the interference fringes to a computer. Interferometric fringe analysis software is then used to analyze thousands of data points along the fringes and accurately determine the deviation between the two wavefronts. After at least six or more separate inteferograms have been captured and analyzed the results of each are averaged together. Interferogram averaging minimizes the effects of any air currents that may be in the light path during testing providing accurate information about the true wavefront quality. The fringe analysis software then generates a detailed report containing the peak to valley wavefront error, RMS wavefront error, Strehl ratio, synthetic interferogram and wavefront map.
Foucault Testing Vs. Interferometry for Quantitative Wavefront Analysis
By its nature the Foucault test is not suitable for obtaining a true quantitative analysis of an optics wavefront quality. The data samples that are measured come from only a few large selected zonal areas that are read from a cross section of the mirror's surface. When using the data collected from the Foucault test the values for peak to valley wavefront error, RMS, and Strehl can only be inferred by assuming the mirror is rotationally symmetrical. In comparison, interferometry measures the shape of the entire wavefront using several thousand data points that have been compared against a known high quality certified reference wavefront. A substantially larger data sample area clearly makes interferometry the only choice for true quantitative wavefront analysis. Mirrors certified by zonal Foucault test rarely attain the same quality of results when tested by an interferometer and almost always overstate the quality of the mirror being tested.
Test Data
Galaxy Optics test data reports contain the following information:
- Interferogram
- Strehl Ratio
- RMS Wavefront Error
- Peak to Valley Wavefront Error
- Wavefront Map
The Interferogram:
The interferogram is the most important component of the mirror test report. An interferogram can be readily dissected by someone with a limited knowledge of interferometry. Even without a computer generated plot and data report, one can make a reasonable assessment as to the general quality of the optic under test.
Astigmatism and Test Stand Stress Flexure Errors:
At Galaxy Optics any astigmatism that is not stress induced is always removed via optical figuring, however, even the best mirror will still have a trace amount of astigmatism that is not by detectable by the human eye. Interferometry can detect even the smallest amount of this error and report on its severity. If it is determined that a mirror is exhibiting test stand stress induced flexure errors a portion of the error may be removed from the test data. It is always noted on the test data report if any stress induced flexure errors were removed during the interferometric data reduction.
Strehl Ratio:
The Strehl ratio value is very important in the evaluation of an optics quality. It compares the mirror's actual performance in terms of its ability to focus light to a theoretically perfect mirror. Put simply, the Strehl ratio is a fundamental description of the amount of focused light intensity reduction due to wavefront errors. It is the simplest meaningful way to express the effect wavefront aberrations have on the mirrors ability to produce an image. Mirrors with a strehl ratio above 0.80 are considered to be diffraction limited and will perform exceptionally well in a telescope.
RMS Wavefront Error:
RMS wavefront error is a statistical measure of how much a mirror's wavefront deviates from the ideal theoretical wavefront. The RMS wavefront error is calculated from all of the measured interferometric data points and is the best indication of a mirror's overall performance. To obtain the RMS value the fringe analysis software measures all of the data points to determine the error between the point positions on a theoretically perfect wavefront and their actual positions on the wavefront under test. The deviation between points is squared then averaged and the square root is extracted. The literature states and physical tests prove that an optic with a RMS wavefront value of 0.076 or less is diffraction limited.
P-V Wavefront Error:
P-V only measures the error in waves between the highest and lowest point on the optical surface. It does not account for how many zones there may be, how wide the zones are, or their location on the optical surface. All of these factors are tremendously important in determining the overall quality of an optical surface.
Optical Coatings
"ARC" REFLECTIVE OPTICAL COATINGS
High quality reflective coatings are critically important to the performance of your Newtonian telescope mirror. In order to meet this requirement, Galaxy Optic's thin film optical coating system was specifically designed for large diameter mirrors. The thin film coating system produces finished optical coatings that are better than 98% uniform. This is less than a 1/100 wave center to edge thickness variation for an optical coating 270nM thick, based on 2% of 550nM wavelength. The ARC process employed for applying Galaxy Optics coatings guarantees near 100% oxidation of all refractory materials and extinction coefficients less than 0.001. The ARC coating process produces reflective coatings that are brilliant in their appearance and have no coloration. They are exceptionally durable and well suited for use in the harsh environments associated with observing.
How the "ARC" Coating Process Works:
"ARC" is a trademark of Galaxy Optics and is an acronym for "Activated Reactive Coating". This process operates on the same principals as IAD, Ion Assisted Deposition. We have developed a proprietary method of implementing this technology that allows us to ionize reactive gases closer to the surface of the mirror. Activated reactive coating is the process of ionizing pure oxygen or other reactive gases at very low pressure into plasma. The large increase in the molecular energy is used to force a chemical reaction to near 100% completion at a lower temperature. The coating process for telescope optics involves the use of metal monoxide compounds as starting materials. The two most common for telescope optics are silicon monoxide (SiO) and titanium monoxide (TiO). The metal monoxides must be fully oxidized during the coating process to form silicon dioxide (SiO2) the low refractive index material and titanium dioxide (TiO2) the high refractive index material. Chemical reaction equations are: 2SiO + O2 > 2 SiO2 and 2 TiO + O2 > 2 TiO2. If the starting materials are not fully oxidized the coating will be dark in color and have very low reflectivity. The best way to obtain 100% oxidation is ARC. ARC has two very important qualities: (1) the high-energy ionized oxygen plasma drives the reaction to completion assuring a completely transparent film with an extremely low extinction coefficient. (2) The increased molecular energy is sufficient to force the SiO2 or TiO2 molecules to form a densely packed micro-crystalline structure that is extremely hard. Depositing thin films of silicon dioxide (SiO2), titanium dioxide (TiO2) and other metal oxides using ARC yields very low scatter coatings. This is the same optical coating technology used in the manufacture of ultra low light scatter optics for use in super high-energy laser systems.
Galaxy Optics primary mirrors are coated with our proprietary C-1 coating.
C-1: 96% average visual reflectivity, enhanced aluminum.
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Galaxy Optics Diagonal Mirrors are Coated with our Proprietary C-2 coating.
C-2: 97% average visual reflectivity at 45 degrees, Enhanced Aluminum.
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