Perquin Optics

Perquin Optics is a four-part tool for visual astronomy. It runs on the device. It declares no permissions.

what it computes

Enter a telescope (aperture, focal length) and an eyepiece (focal length, apparent field). Pick a barlow or reducer factor: 0.5x, 1x, 1.5x, 2x, 3x. The app returns six figures.

1. Magnification. Effective focal length divided by eyepiece focal length.
2. True field of view, in degrees. Apparent field divided by magnification.
3. Exit pupil, in millimetres. Aperture divided by magnification.
4. Focal ratio. Effective focal length divided by aperture.
5. Maximum useful power. Twice the aperture in millimetres.
6. Dawes limit, in arcseconds. 116 divided by the aperture.

One line of reading follows: whether the pairing is a low-power sweeper, an all-round choice, a high-power combination for planets and doubles, or past the point where the aperture can keep up.

gear room

Hold as many telescopes and eyepieces as you own. Each telescope shows its focal ratio; each eyepiece shows its focal length and apparent field. Entries are yours to add and delete.

observing log

One record per session: site, seeing 1 to 5, transparency 1 to 5, number of targets, notes. The header keeps a running count of nights out, your best seeing grade so far, and the total number of targets logged.

sky list

Forty objects. Planets and the Moon, Messier entries, a few NGC objects, five double stars. Each one carries its type, best season, a difficulty grade of 1 to 5, an aperture suggestion and a sentence on what actually shows in the eyepiece. Sorted easiest first. Tick the ones you catch and filter the list by found or still to find.

specifications

- Phone, portrait, four sections, swipe between them.
- Dark interface throughout, meant for use in the field.
- No network permission is declared in the manifest.
- No advertising libraries. No analytics. No account.
- English.

Aperture and focal length are usually printed on the tube or in the manual. Apparent field is on the eyepiece box, and 50 degrees is a safe guess for a plain Plossl.