Introduction: CamSextant Iphone Sextant

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Recently I published the iphone Sextant project ( iSextant ). That device combined a telescope viewfinder with the phone and uses the phone gyro to read the altitudes.

The device described here, named CamSextant, is also an iphone sextant and is even easier to build. Instead of a separate eyepiece, I used the phone camera as a pointer device for observations.

An special app - also called CamSextant - was developed to work with it, capturing altitude readings.
It is available on the Apple App Store ( links in the end of this text )

Warnings before you start

  1. While the phone gyro is a nice piece of equipment, it is not nearly as precise as a marine sextant. In a rocking boat, the gyro response is even worse. So - if you want to practice celestial navigation - get yourself a marine sextant. See link in the end of the page.
  2. You don't need the eyepiece thing described below to use CamSextant app. The app works even without it. It just feels more like a real sextant image with the eyepiece. The phone camera has a wide view. Looks impressive too.
  3. You do need the proper filter to observe the Sun. Failing to use the proper filter may result in damage to your eye and to your phone camera. Take care. Read more ahead.

Step 1: Glue the Objective Adapter to the Phone Case

You don't need any adaption to the phone to use CamSextant app. A plain iphone will do. But I wanted to try something different. The original phone camera has wide field angle. This is not so good for a sextant viewfinder. Marine sextant eyepieces are more like tele objectives. The small field angle is important for precise pointing of the instrument. To remedy that I bought one of those little phone lenses. It is a 8X magnification objective ( no, I'm not associated with the lens manufacturer in any way). There are a many options in the market. This one costs some 10 bucks.

This is what I did:

Materials:

  • phone case. Use a rigid aluminum case (epoxy will not hold to a rubber case)
  • 8x magnification lens set for mobile (tele)
  • welder helmet dark glass (shade #14) - Cut the rectangular glass in two square parts. Go to a glass shop for that.
  • epoxy glue ( two components fast cure - 10 minutes )
  • brown packaging tape
  • iphone 5 or newer

This particular objective is supplied with a clip adapter, to attach the lens set to the phone (see below). This was not practical for my sextant design. I wanted the phone case and tele objective to form a single, rigid unit. I cut off the clip part and kept the objective adapter screw. It was bond to the phone case using epoxy glue ( two components fast 10 min cure ).

  • Clean and sand the contact surfaces.
  • Make sure the camera and the objective axis are well aligned and the camera image is good.
  • Mark the adapter position on the case, with a fine point marker pen.
  • Remove the phone from the case before bonding the objective adapter ( while liquid, a epoxy glue drop may ruin the phone camera ).
  • Glue the adapter to the case with epoxy glue. Watch position and alignment while the glue is curing.
  • Avoid skin contact with epoxy (some people are allergic to epoxy). Wash your hands well in the end.

This lens set has a manual focus adjustment, which must be set to infinity.

I found the gyro sensor error can be as much as one degree, not a very good sextant. It may improve in the future. A decent sextant has an error of a couple minutes. Still, it is pretty amazing.

Step 2: Altitude Capture Apps

Actually, I made two apps to work with CamSextant:

Navigator - Complete celestial navigation app, with altitude capture, perpetual almanac, celestial calculator and star finder. Allows capturing readings and calculating your position using celestial navigation, in one go. USD 20

CamSextant - Simple sextant altitude capture using the phone camera. Free

Apps are available on the Apple App Store. Links in the end of the text.



Step 3: Reading Celestial Object Altitude

Both apps allow calibration and capturing of altitude readings.

Differently from marine sextants, which compensate instrument shaking naturally by design, the phone version must be hold pointing sharp to the object and steady while capturing the altitude. Difficult sometimes. Hold your breath. Tap gently.

It is also hard to see faint stars ( a camera limitation I guess ).

CamSextant App

CamSextant app is as simple as can be. By default, the camera view is a little bit zoomed in, to allow more precise pointing. The angle of view is smaller than the regular camera app.

Calibration is done by pointing the phone to the sea horizon (if you are sailing) and carefully tapping the [Calibrate] button. This sets the "zero" of your device. Instrumental altitude reading is automatically corrected after calibration. Use the X button to clear the calibration, setting it back to 0.

Once calibrated, to read the altitude Hs, point the device to the celestial object (a star, planet, Sun or Moon) and tap [--Mark--] ( or tap anywhere on the camera view ). You'll hear a click. No, the app did not take a picture
of the star. Instead it saves the local time and instrumental altitude Hs, as shown in the image. The camera image is frozen for a couple seconds, so you can inspect the quality of the altitude capture. Tap again to release (or wait the 5 seconds)

note: Magnetic azimuth indicator in the app does not work on some early phones.

CamSextant can be used in landscape and portrait orientations.

Step 4: Sun Filter

Warning: Don't use any optical instrument to observe the Sun directly, unless you have a proper Sun filter installed. The UV rays can burn the eye, causing permanent loss of vision and other problems. Lenses in the instrument amplify the light power. Don't even look to the Sun without a filter.

Best materials for Sun filter are:

  • Aluminized Mylar film - this is a material specifically developed for solar observation. Can be found in science supply stores. Best material available.

  • Welder's glass - strong filter used in welder helmets (used in this project ). Can be found in construction supply stores. Ask for shade number #14

The filter used in this project uses welder glass. Most welder lenses are rectangular. I had mine cut square in a glass shop and made the eyepiece adapter with paper and brown packaging tape.

The shade must be removable, to take off when observing stars, planets and Moon.

I found it difficult to point faint stars with the app. The phone camera is not so good when it is dark. Bright stars and planets can be focussed. Moon and Sun are easy.

Related links: