Frequently Ask Questions (FAQ)
Secondary mirrors
(last updated Nov. 1, 2007)
1) Does the secondary mirror need to be as good as the primary mirror?
2) Does ProtoStar sell "1/30 wave" flats?
3) What kind of coatings do ProtoStar secondaries have?
4) What is the reflectivity of ProtoStar secondary coatings?
5) Why am I am having trouble stripping the old coating off of a ProtoStar secondary mirror?
6) Can I order a special size secondary mirror?
7) If I want to adhesively mount my mirror to the holder, what glue should I use?
8) What are the differences between the Pyrex and ULS Quartz mirrors (besides the price)?
1) Does the secondary mirror need to be as good as the primary mirror?
Both mirrors are equally important to the final wavefront quality. The secondary's surface has a slight advantage since it is reflecting at a 45º angle of incidence (AOI). This means surface errors on the secondary are only multiplied by 1.4X (instead of 2X for the primary mirror) to get actual wavefront error.
2) Does ProtoStar sell "1/30 wave" flats?
Despite outlandish claims, a 1/30 wave peak-to-valley surface from edge to edge is exceedingly rare. It's impractical to even test a surface accurately to this level. A $65,000 Zygo interferometer (100 mm dia. reference element) has a reference element rated at 1/20th, so measuring to this level of accuracy is difficult in itself.
ProtoStar flats are tested edge-to-edge across the entire surface (i.e., no edge masking, or "trimming"). In many cases, much of the surface error is near the edge, so our method gives the most conservative number. Unlike many optics, a Newtonian secondary mirror is frequently optically active across its full aperture, so this is how the optic should be measured.
3) What kind of coatings do ProtoStar secondaries have?
 All of our secondary mirrors have enhanced aluminum coatings. "Enhanced" aluminum simply means that several layers of a hard, glass-like, material (called dielectrics) are deposited on top of the aluminum layer. In our case, one layer of TiO2 (titanium dioxide), and one layer of SiO2 (quartz) are applied onto the aluminum using an electron beam deposition (EBD) process. These overcoating layers slightly increase the reflectivity of the bare aluminum, but more importantly they protect the aluminum from oxidation. These hard layers also make the coating more scratch resistant.
In addition, the dielectric layers are applied using an ion-assisted deposition (IAD) process. Heavy, inert, ions are accelerated through an electric field, and thrown at the developing dielectric layers. This mechanically pounds, or peens, the layer into a denser structure. This is important because without IAD, the ultra-hard dielectric layers take on a more porous structure, which leads to premature oxidation and failure of the aluminum layer underneath.
4) What is the reflectivity of ProtoStar secondary coatings?
 The average reflectivity across the visible spectrum is 96%. The complete reflectivity curve is shown here.
5) Why am I am having trouble stripping the old coating off of a ProtoStar secondary mirror?
Under the aluminum is a thin layer of chromium. This acts as a bonding agent to promote good adhesion of the aluminum. Chromium can be easily removed with a 50/50 mixture of HCl (hydrochloric acid) and glycerin. Most coating firms can remove this layer for you if you do not have these agents.
6) Can I order a special size secondary mirror?
This is not possible since secondary mirrors are made (and coated) in large batches.
7) If I want to adhesively mount my mirror to the holder, what glue should I use?
 A clear silicone adhesive (also known as "RTV") should be used. These adhesives are commonly available at hardware stores. If you already have some, but are unsure of its age, we recommend buying a new tube. The catalytic ingredient (acetic acid) can evaporate over time, which prevents the glue from curing properly. Hard curing epoxies are to be avoided.
8) What are the differences between the Pyrex and ULS Quartz mirrors (besides the price)?
On average, the flatness of the quartz parts is better than Pyrex for a given size. Quartz is a zero expansion glass, while Pyrex is low expansion, which makes it possible to achieve a better surface before removing the parts from the blocking. (The Pyrex parts will "bounce back" slightly after being allowed to cool to room temperature.) This is why most reference flats are made of quartz (or a derivative of quartz).
Quartz is also a harder glass than Pyrex, which is why it takes a smoother micro polish than Pyrex and other softer glasses. (This property is especially important for laser applications where low laser beam dispersion is desired.)
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