Have questions about how to use the database? Hopefully, this page will help.
To get started, click on one of the search options on the left-hand side of the screen (either "Basic Search" or "Advanced Search"). If you're just browsing the database, or want to see if a particular item is in the database, Basic Search is the place to start—it allows you to quickly search on a phrase, by accession number, or a few other criteria. If you want to see all of the items currently in the database, just leave all of the search fields blank.
In Advanced Search, you scan specify more specific search criteria, as well as what order the results should be in (example: you can order by date). There is also another option that returns your results in tabular form (you choose the columns—example: date, maker, and objective type), for easy copy-and-paste into a spreadsheet (example: you can request only "date" and "number of eyepiece", and quickly produce a graph of number of eyepieces as a function of year).
- What is the Abbe number (ν)?
In optics, the Abbe number of a lens (represented by the greek letter ν "nu") is ν = (nD - 1)/(nF - nC) where nD, nF, and nC are the refractive indices of the lens at specific wavelengths: the Fraunhofer D-, F-, and C- spectral lines (589.2 nm, 486.1 nm and 656.3 nm respectively). The Abbe number provides a measure of the lens's dispersion: the higher the number, the less dispersion, and conversely. Typically, the crown glass used for telescopes until the 1750s has a value of ν>50, whereas the denser flints introduced around that time have rather lower values.
- Which order are lens elements listed in?
All lens elements are in the order that light passes through them; i.e. starting with the element closest to the sky and ending with the element closest to the eye.
- Why are some dates listed as 1825 (1800-1850) or 1825 c.?
When only a date range or an approximate date is known, a "best-guess" year is listed, followed by the range or a "c." (for "circa"). This makes it easier to sort by date (or, if you copy-and-paste to a spreadsheet, to plot by date). Years should only be taken as authorative if they are not followed by a date range or a "c.".
- What do I do if I notice an error in the database or have further
See the Contact Page for instructions on contacting the curators.
- What does "Private Collection" mean?
Objects which are owned by a private collector, instead of a museum or other cultural institution, are listed under Private Collection. The name of the collector is not disclosed. In most cases, the first letter of accession number indicates which private collection an object is in: A.582 is object 582 in collector A's collection. However, private collectors may elect to remain completely anonymous. Contact the database curators if you have further questions about an item in a private collection.
- How is "curvature" measured?
The curvature C is the reciprocal of the radius of curvature r. It is measured in units of diopters which are 1/meter. Therefore, r (in mm) = 1000/C (in diopters). Similarly, the lens power P is the reciprocal of the focal length. P is also measured in diopters so P (in diopters) = 1000/f (in mm). Light from a star hits surface A before B, and curvature is taken as positive if the incident light meets a convex surface and negative if it meets a concave surface.