The principal test used during manufacturing is the EZ Hartmann test from Lonnie Robinson, and the finalization test is the At-the-Focus Hartmann test from Jeff Baldwin.

These flats are tested on a contact interferometer using a highly precise reference flat, a double-pass bath interferometer using a highly precise reference sphere, a double pass false star test and double pass Ronchi and knife-edge tests.

LAFFRTs

1 The mirror blank is milled round, flat on back, minimalized wedge.

2 A stress-released mirror blank is confirmed by cross polarization.

3 The mirror’s back is ground flat and polished clear to slide on cell contact points.

4 The mirror’s optical surface is fully polished and minimal pitting confirmed by laser examination.

5 The mirror is smooth visually on knife-edge examination, no roughness, no zones.

6 The wave-front P-V error less than 1/10 wave at 550 nm using the EZH/FigXP. Shop arithmetic usually shows much better.

7 The wave-front RMS error less than 1/26 wave at 550 nm using the EZH/FigXP. Shop arithmetic usually shows much better.

8 The Strehl ratio computes more than 0.94 EZH/FigXP. Shop arithmetic usually show much better.

9 Astigmatism measured and controlled by a feature in the EZ Hartmann and ATF Hartmann tests.

10 TDE is controlled.

11 There are minimal/no scratches.

12 Choice of Coatings.

The mirrors are processed in the shop at the ROC using Lonnie Robinson’s EZ Hartmann test. This test figures the mirror on 8 axes as well as calculates an astigmatism value for each zone. When the mirror is finalized, the 8 axes have Millies LaCroix graphs very nearly down the middle, and are registered together by the Zone 7 astigmatism value that is so small it confirms a surface of revolution with that outer zone as the datum, and the other zones measured in respect to the outer zones. The Hartmann photograph shows the spots and measures them to 1/1000 of a pixel, and there are 5905 pixels per inch, so they are measured to incredible accuracy. The Hartmann mask itself has been manufactured to 1/1000”. Together they make two landmarks of high precision. The spread of the spots is also measured on 4 diameters and compared to give an astigmatism value in pixel and percentage difference on 4 diameters. The largest minus the smallest gives the difference in pixels for each zone. Since this is roughly 2 inches [varies] from the crossings of the rays, and since 6 pixels is about 1/1000”, then at the crossings the difference is literally a non-issue, confirming the lack of astigmatism. This is done for all zones giving 7 astigmatism values, all of which are restricted in the shop to 6 pixels. The test is extremely repeatable, extremely accurate, and by itself is enough to finalize a mirror.

But after the EZ Hartmann test is completed, a Null Hartmann test is performed at the focus of the optic using photographic measurement similar to the EZ Hartmann test, only this time linear equations are computed and a plane is mathematically inserted into the region of focus, and moving this plane shows the spot convergence. The spots must all pass inside a pre-designated circle, and it is easy to see zone issues, astigmatism issues, collimation issues, anything. It is the end-all test.

Other reasons for the Null-Hartmann test are fairly logical. It mathematically shows the mirror is either focusing within specs or not, and if not, how not, which allows it to be corrected. Also, mirrors tested horizontally in a shop may perform differently when at observing angles. Gravity can bend a mirror. So testing it at 0^o, then using it in the sky may be different than testing it at 37^o and then using it in the sky. If you generally observe most of the time between 15^o and 75^o, testing it at 37^o to a null is a lot closer than testing it at 0^o to a not null. Testing at the focus is using all the components of the scope, testing it nearest the angle it will be will used at and under natural conditions, shows the rays converging, and as a result, there will be no surprises. Literally what you see is what you get. Why 37^o? BAO’s latitude in Lathrop, CA is roughly 37^o and the shop uses Polaris for Null Hartmann testing!

It is conceivable that a mirror can be perfected horizontally, and then when put in the telescope with another secondary mirror and a cell and whatever else, that it will not perform to the degree it ought to. The Null Hartmann test is one of the reasons BAO suggests that you buy a complete three-component set, the primary and secondary mirrors, and even the primary mirror cell, from BAO so that they can be Null Hartmann tested together.

LMASMs

1 The mirror blank is water jet cut out, flat on back, minimalized wedge.

2 The mirror is a stress-released blank confirmed by cross polarization.

3 The mirror’s back is ground flat and polished clear to slide on cell contact points.

4 The mirror is fully polished and minimal pitting confirmed by laser examination.

5 The mirror is smooth visually on a double-pass knife-edge and Ronchi examination against a reference sphere, no roughness, no zones.

6 Contact Interferometer shows half-wave spaced fringes to be straight, parallel and evenly spaced.

7 A double-pass Bath Interferometer or Williams Fizeau interferometer against a reference sphere confirms Item 6 [half-wave fringes due to double pass].

8 A double pass false star examination shows no astigmatism [cylinder], zones or edge rolling.

9 Fringe Analysis Software confirms flatness and produces error information and offers a record of the mirror’s perfection.

10 Minimal/no scratches.

LMASMS start out as a glass disk, and the glass can vary from plate glass, to borosilicate glass, to fused quartz. The glass is cross-polarization checked for stress, then taken to the water jet to cut out into the elliptical shape, then once again checked for stress. The glass and the surrounding disk remnant are then blocked to a backer disk and are ground flat, then they are put into a lapper to be polished. Periodically they are interferometrically tested, the machine adjusted, polishing continues, and eventually the mirror is completed. The optically finished mirror is then released from the backer and retested [see criteria 4 – 9 above]. At this point the mirror is sent out to get a standard aluminum and silicon monoxide coating applied. Aluminum and SiO offer a long-lasting coating that is roughly 89% reflective. Alternate options include spray silver, a 99% coating that lasts roughly 1 to 2 years before tarnishing and needing respraying, or another alternative is to send the mirror out to to get higher reflective coatings that last as long as the aluminum coatings. Each of these options have a different costs involved.

Due to the fact that our lapper conditioners have an inside diameter of 12.5”, the largest minor axis achievable is 8.5” if the mirror is the standard shaped oval of a secondary mirror set in the telescope at 45^o. If the secondary mirror of your telescope is at another angle, such as focusers that are not perpendicular to the optical axis, then larger secondary mirrors can be made. Contact us for dimensional limitations.

Mirrors can be made extremely near perfect only to be optically ruined by lousy mirror cells, collimation, lack of coma-corrector, bad secondary mirror, a tube that doesn’t release heat well, etc. BAO can control some of that by selling the primary mirror, secondary mirror, and the primary mirror cell as a set. This controls many of the issues associated with good mirrors looking bad in the telescopes they end up in. The other items to control in your telescope will be up to the customer, but if you spend thousands of dollars on optics, you are probably going to go to the effort of spending time and money taking care of all the other items involved. At f/3, you will absolutely have to use a coma-corrector. BAO recommends the TeleVue – Starlight SIPS for visual use, and the Baader for photographic use.

Mirror cells can destroy optics, especially for LAFFRTs. BAO invites you to check out the mirror cell pages from both Mike Lockwood and JP Astrocraft. Their 18-pt cells are awesome. BAO can make you a mirror cell that will control the warpage and flexure that gravity can exert on your mirror. Again, getting the primary mirror, secondary mirror and the primary mirror cell as a package deal and having the optics tested together on a Null Hartmann test is the safest ticket.