Thinking About a New Scope? Think Big!
18" f/4.2 on polishing machine at Ostahowski Optics.
Ostahowski coating chamber
40 inch test flat
Chief optician James Mulherin working 1 meter mirror
25" mirror fine grinding
OMI test rig with 25" optic
OMI test rig with 25" optic
- Obsessions are built with premium diffraction limited optics from OOI (Ostahowski Optics Inc) and OMI (Optical Mechanics Inc.).
- We use only 2" thick, low expansion glass
- An Obsession extra: all primary mirrors are center marked for precise collimation.
About our mirror suppliers
Obsession Telescopes feature primary mirrors made by two of the best established and most reputable optical companies serving the astronomy community today.
Optical Mechanics Inc. (OMI) supplies Obsession with 12.5" to 25" aperture mirrors.
Ostahowski Optics supplies Obsession with 15" to 22" aperture mirrors.
Ostahowski Optics is well known as the original supplier of large aperture mirrors to the amateur astronomy community, and their long standing reputation for high optical quality is well deserved.
OMI's mirrors have developed a reputation for consistent high optical quality coupled with the most responsive customer service in the industry. OMI's founder and chief optician, James Mulherin, is a frequent participant in various on-line discussions groups, providing support and innovative ideas to the amateur community
Obsession Telescopes chooses to work with Optical Mechanics and Ostahowski Optics - as these two companies have proven to be reliable and responsive to the needs of our customers. We do this to insure that every Obsession Telescope customer receives the finest optics and customer support available.
Quality Control with Interferometry
The Key to Consistency
OMI and Ostahowski Optics mirrors have the highest optical specs on the market because they continuously invest in improving their quality control process. In the professional optics market, interferometry is the last word in quantifying the quality of an optic. By developing the capability to use interferometry as the final go/no go step in their QC process, OMI and Ostahowski Optics have raised the bar by providing customers with professional level quality certification. They are so confident of their quality that they post test data and interferograms for every mirror they make on their web site.
All OMI and Ostahowski Optics mirrors are tested and certified with laser interferometry to be diffraction limited over the entire face of the mirror. It is important to note the distinction between measuring the entire surface with interferometry and measuring only a cross section of the mirror, as with the zonal Foucault test. An interferogram measures the shape of the mirror's entire wavefront. The P-V, RMS and Strehl values obtained from the interferometric test provide an accurate representation of the mirror's figure. The zonal Foucault test measures only along one cross section of the mirror's wavefront. From the data acquired in the zonal Foucault test you can estimate the Peak to Valley wavefront error. RMS and Strehl values must be "inferred" by assuming that the mirror is rotationally symmetric, but for lack of data, these inferred values can never be as accurate as the values obtained with interferometry. Inferring the RMS and Strehl from Foucault test data consistently overstates the quality of a mirror.
Producing an accurate interferogram is a time consuming and expensive process. It requires costly equipment and technical expertise. The results of this effort are accurate RMS wavefront error and Strehl ratio values. These are the most important values to the customer as they are the best predictor of performance at the eyepiece. RMS and Strehl tell you much more about mirror quality than the simple P-V measurement. This is one reason we offer OMI and Ostahowski Optics mirrors. With a Ostahowski Optics or OMI mirror you can be assured that the P-V, RMS and Strehl values are accurate because they are determined by interferometry.
How Interferometry Works
We start with precision annealed PYREX 7740 low expansion glass from United Lens. During the figuring process, the surface of the mirror is polished from a sphere to a paraboloid using the appearance of fringes in the Ronchi test and shadows in the Foucault test. These tests are qualitative and serve only to guide the figuring process. After the optic appears to pass the Ronchi and Foucault test, it must then pass the scrutiny of the interferometer. In an interferometric test, the shape of the wavefront produced by the optic under test is determined by combining its wavefront with a highly accurate reference wavefront. Constructive and destructive interference between the combined wavefronts produces interference fringes. These interference fringes are analogous to contour lines on an elevation map and they represent deviations of the wavefront under test from the optimal shape. The interference fringes are captured using a camera and image capture board, and then displayed on a computer monitor. Fringe analysis software then picks hundreds of points along the fringes over the entire wavefront to accurately quantify the deviations. By averaging the results of several interferograms, thousands of digitized data points from the interferograms are utilized in determining the accuracy of an optic.
The output of the fringe analysis software describes the quality of the optic under test by reporting its P-V and RMS wavefront error, and Strehl ratio. Due to the qualitative nature of the Ronchi and Foucault tests, and to the subjectivity in their interpretation (using human eye and brain only), it is not infrequent that the interferometer will reject a mirror, sending it back for more figuring. The interferometer is the final impartial go/no-go point in the quality control chain. It is a completely objective, quantitative, and accurate assessment of the quality of the optic under test.
P-V (Peak to Valley) Wavefront Error
This is simply the distance in waves from the lowest point to the highest point on the wavefront. P-V is not the best measure of optical performance. To illustrate this point, consider two mirrors both with the same P-V wavefront error. The error on mirror A consists of a small hill at the center of the mirror. Because this error is at the center of the mirror, it comprises a small fraction of the mirror's surface and hence contributes a smaller fraction of light to the image. Mirror B on the other hand has its error near the edge of the mirror. Zonal errors near the edge of the mirror cover a larger surface area and hence contribute more light to the image. Even though mirrors A and B have the same P-V error, mirror A will outperform mirror B on the sky. This is why we prefer to focus on RMS and Strehl as the measure of a mirror's optical quality.
RMS (Root Mean Square) Wavefront Error
RMS wavefront error is calculated from all of the measured data and gives the best indication of a mirror's overall performance. To obtain the RMS, interferometric data points numbering in the hundreds are placed uniformly on all the fringe lines over the entire area of the mirror's wavefront. Fringe analysis software takes all these points and measures them precisely to determine the error between the point positions on a perfect wavefront and the actual point positions of the wavefront under test. The errors between point location from a perfect mirror to the actual mirror are then squared and averaged, and the square root is extracted. In simpler terms, the RMS wavefront error is a statistical measure of the deviation of the mirror's wavefront from the ideal. The literature states and physical tests prove that an optic with a RMS wavefront value of 0.076 or less is diffraction limited.
A telescope mirror is designed to form a diffraction limited image of a point source. Ideally, a certain percentage of the light in the image will fall within the diffraction disk diameter. The Strehl ratio compares the mirror's actual performance in terms of its ability to focus light into the diffraction disk, as predicted by interferometry, to the performance of a theoretically perfect mirror. In other words, the use of the Strehl ratio is a fundamental description of the amount of intensity reduction due to wavefront errors. The common convention is to consider an optic with a Strehl ratio of 0.8 or higher to be diffraction limited. The Strehl ratio and RMS wavefront error are mathematically related. It can be shown that a Strehl ratio of 0.8 corresponds to an RMS wavefront error of 0.075 or approximately 1/14 wave. OMI and Ostahowski Optics mirrors are certified to meet or exceed the following wavefront specifications at 550 nm:
P-V < 0.25 wave - RMS < 0.05 wave - Strehl ratio > 0.80.
This is a very strict quality criterion and there is no better way to measure it than with interferometry. Although it is possible to produce diffraction limited mirrors using methods such as the Ronchi and Foucault test, it is impossible to accurately verify the quality of the entire wavefront to a fraction of a wave without interferometry. The Ronchi and Foucault tests are excellent evaluation tools during mirror fabrication as they show the general shape of the wavefront as well as localized and high frequency errors. This makes them indispensable tools during mirror figuring. However, unlike interferometry, they do not provide a means of accurately quantifying the whole wavefront because they only measure a cross section of the mirror. As a result, a bad mirror can escape detection using these tests. Interferometry on the other hand, is an extremely strict statistical analysis that assures the customer has a truly diffraction limited optic over its entire wavefront. After all, you want the entire surface of the mirror to be accurate to catch those precious photons.
"Assumed" Strehl Ratio
There's a big difference in the Strehl ratios calculated only by zonal Foucault testing and interferometric testing. The Foucault test measures only a few points across one diameter of the mirror to generate a 2 dimensional cross section of the mirror's wavefront. There isn't enough data in this representation to accurately calculate an RMS or Strehl value. Some opticians take that cross section and assume that the mirror has perfect rotational symmetry. Because of this false assumption a Strehl based on Foucault test data is always over optimistic.
The interferometer takes data points over the entire wavefront (entire face of the mirror) and uses them to generate a 3-dimensional representation of the mirror's wavefront. Each interferogram contains roughly 300 data points from which the RMS and Strehl are calculated. At least five interferograms are taken and averaged to produce the final results for a mirror. There are roughly 1500 data points in this calculation, which goes a long way toward generating statistically valid results.
A good mirror should be well corrected, smooth and have an excellent edge (no TDE). If a mirror meets these basic criteria and has a true Strehl above .80 it will be an excellent performer.
Primary mirror coatings are 96% enhanced aluminum which are more durable and more reflective than standard 88% aluminum. Coatings are placed under planetary rotation providing excellent coating uniformity. A state of the art electron beam gun is used to provide excellent control of material evaporation rates and layer thickness. Then Ion Bombardment Assisted Deposition (IBAD) to produce dense coating layers. You cannot get a better coating at any price anywhere. Secondary mirrors are 1/10 wave or better and coated with super enhanced 96% Brilliant-Diamond. This premium aluminum coating has 96% reflectivity at 550 nm. These highly reflective coatings are standard on both mirrors at no extra charge. No one else offers this level of performance. Compared to scopes with plain aluminum, it's like getting an extra two inches of aperture for free! Faint galaxies and nebulae are brighter and easier to see. When it comes to deep sky observing, there is nothing better. For more coating info please see: http://opticalmechanics.com/?page_id=449