GENERAL TELESCOPE TERMINOLOGY
ANNEALING: Please understand that multiple anneals are of no benefit in stress relieving glass as some catalogue companies elude to under the name of a "double annealed blank". Annealing is essentially cooling the glass at a rate that allows the entire blank to solidify at a uniform rate so that internal pressure or stress within the blank itself is minimized. Each time a piece of glass is heated to it's "softening state" (transition point from a solid to a liquid state) , the effects of the previous anneal is destroyed and the quality (residual stress after returning from a liquid to a solid state) of the new anneal is contingent upon how well the fabricator controls the cooling rate through the transition phase from a liquid to a solid during the new annealing cycle. Annealing is important because it has a direct effect on the ability of the polisher to make extremely accurate surfaces. A fine annealed piece of glass should be more than adequate for a surface quality not exceeding a quarter wave. Stress is measured in nanometers (nm) through the use of a polarimeter with smaller numbers reflecting lower internal stress. A commercial anneal is rated as greater than 10nm. Fine annealed is 10nm or less. Precision anneal is 6nm or less.
SAG: In a mathematical sense, the abbreviation for the term "sagitta", the height of a curve measured from the chord. The sag formula is: -
SAG=R -
Ö (R2 - (D/2)2 ) (Clarification: Ö = take the square root of the answer derived from (R2 -(D/2)2)where D = the diameter of the optical surface.
R = radius of curvature of the optical surface.
MIRROR RADIUS: In the general case, the radius of curvature of a mirror is twice its focal length (F.L.).
FOCAL LENGTH: In general, the focal length of a mirror is one-half the radius of curvature of the mirror.
F/D NUMBER: In general, the f/d number is the focal length of a mirror divided by its diameter. A "faster" f/d number is a smaller number than that f/d number to which it is being compared.
PARABOLOIDAL MIRROR: A concave mirror that has the form of a paraboloidal surface of revolution. All axial, parallel light rays are focused at the focal point of the paraboloidal surface without spherical aberration.
SPHERICAL ABERRATION: The failure of a lens or mirror to form a perfect image of a point source axial object.
DIFFRACTION LIMITED: Diffraction limited is a term of art that refers to the phenomenon that occurs when a light wave pass through an aperture, i.e., the light wave at the edge of the aperture diffracts. This phenomenon is more easily dealt with in terms of its impact on the image quality. It is the limiting factor of reaching a prefect optical image. i.e., a perfect point image of a point source such as a star. In reality, diffraction limits the formation of a perfect point image and leads to the creation of the Airy disc (resembles a target). The diameter of the bull’s-eye (containing 84% of incident light) being expressed in general by the formula d1 = 2.44 x wavelength x f/#. The bull’s-eye plus the next concentric ring contains 91% of the incident light and the diameter of this combination is general expressed by the formula d2 = 4.48 x wavelength x f/#.
MATERIAL SAFETY DATA SHEETS - General informational sheets that give an overview as to health and safety issues as they relate to a particular fabrication material. They are made available by the manufacturers solely for your information, consideration and investigation and the manufacturers themselves do not warrant (expressed or implied) their total accuracy. Newport Glass Works, Ltd. provides no warranties, either expressed or implied, and assume no responsibility for the accuracy or completeness of the data contained therein. As a user or potential user, you are expected to review the sheets, do additional investigations on your own and consult with a doctor as to the risks to your health and safety and as to the impact they may have on the health and safety of minor children that you are responsible for. Store material out of reach of children.