Introduction: Iphone Sextant Project

This project uses the iphone orientation sensors (magnetic compass and gyroscope) to work as a sextant for celestial navigation. With this device, you can take altitudes of a couple stars and calculate your astronomical position.

At first I tried to use the bare phone camera as pointing device. But phone camera is too wide angle. Even with the phone at maximum zoom, the celestial objects appear small and difficult to center. The camera cannot focus on the faint stars. The phone also lacks inertia to be a sextant. Didn't work.

I went on to attach a telescope viewfinder to the phone and got a much better instrument.

I suppose one could use one of those little phone tele objectives. I didn't try that.

You may ask: why not just use the phone GPS to get the position ?

Well.. It is not nearly as fun as celestial navigation.

Step 1: Materials

This project uses the following materials:
1- A plastic triangle ruler for the instrument frame.

2- A telescope viewfinder. These are small eyepieces that are mounted on the side of telescopes, to help point the main instrument. This viewfinder has a cross hair that is good to point objects in the sky. It shows an upside down image, but there are non-inverting viewfinders.

3- A phone case - Use a hard case ( not rubber cases ).

4- A couple Lego blocks

5- Dark welder helmet glass, to make the Sun filter. Can be found in construction supply stores. Ask for shade #14.

6- Epoxy putty and epoxy glue (two components).

7- Hand saw

8- iphone 5 or latter

Step 2: Assembly

Using a hand saw, cut off the point of the triangle ruler (you don't want that pointing to your eye).
Make it suit the viewfinder length more or less.

Bond the pieces together

* Handle epoxy glue and putty with gloves.
* Avoid skin contact with epoxy.
* Wash hands well in the end

The idea is to glue the pieces in a rigid and solid setup. The angle between the eyepiece and the phone must be fixed. Try as much as possible to keep the right angle between the eyepiece and the phone case, so that both the phone and the eyepiece point in the same direction ( I.E. the viewfinder axis is parallel to the phone Z axis). This is very difficult to do perfectly and a small axis error is acceptable. ( known by navigators as index error ). It can be corrected later in calculations.

The Lego blocks (red pieces in the image) were used to support the phone case. The case was glued to the frame and Legos with epoxy glue. Use sand paper in the contact points between the ruler, Legos and phone case. I like to scratch the contact surfaces with a metal point before gluing. The epoxy glue in the scratches holds really strong.

Use epoxy putty to glue the eyepiece to the frame. Make sure the angles are correct.

Step 3: Taking Altitudes

Warning: If you plan to observe the Sun, you need a strong Sun filter for the eyepiece. Sun light can burn the eye, with nasty consequences. Read more about this in this link.

The Sun filter I made, shown above, uses dark welder helmet glass ( shade #14 ).
Welder glass is normally rectangular. Take it to a glass shop to be cut to suit your eyepiece. Take care with the recently cut glass, as the border is really sharp. A fine sand paper in the glass borders helps.

The shade adapter to the eyepiece tip was made with paper ring and packaging tape ( lame solution ).

Still, it is a bit uncomfortable to observe the Sun with this sextant, even with the filter, for it has to be pointed directly to the Sun (unlike the conventional sextants, which are pointed to the sea horizon). A 90 degree prism would be useful, if my eyepiece had one..

Again, take care with the Sun, even with the best filter. If you start to see circular ghosts around, give up observation.

The filter must be removable, to allow star observations.

Celestial navigation

Now that you have your iphone sextant, you need to understand a few facts about celestial navigation.

Celestial navigation uses the altitudes of stars, planets, Sun and Moon. To those not familiar, the altitude is the vertical angle between an object - say a star - and the horizontal. Each pair time/altitude, for one celestial object, gives a line of position. Collect two or three lines and you can find your position: the point where those lines meet.

With conventional sextants, the altitude is measured between the star and the sea horizon. The iphone sextant uses the phone's electronic gyroscope as a reference. Point the eyepiece and center the cross hair in the target object. Make sure the image is stable and mark the altitude (using Navigator app, the altitude is captured by tapping the attitude panel)

In case of the Sun and Moon, it is more precise to read the altitude of the lower part of circle instead of the center, where there is nothing to point to. The radius of the object can be accounted for later.

The phone gyroscope - actually an electronic microchip - has a precision of about 1 degree, which is not enough to compete with conventional sextants (marine sextants are precise to a couple minutes). Still pretty amazing.

Another advantage of the iphone sextant in relation to marine sextants is it can be used the 24 hours of the day. Marine sextants cannot be used during the night, for one cannot see the sea horizon in the dark ( most of the time ). Star and planet observations are restricted to the two short twilight times. BTW, you also don't need the luxury of the sea horizon in sight: it can be used anywhere, like a bubble sextant.

This device was designed to work along with Navigator HD app, which I developed (see link below).

Update jan/17: Now you can also use free CamSextant app. See link below.

Navigator offers altitude capture and index error correction. The observations are passed to the app's astronomical calculator up to the final position calculation, paperless.

This sextant can be used with the standard iOSCompass app ( the one that comes installed with the iphone ). Unfortunately it only works with the phone in a different position, pointing to the side, not with the phone facing the observer in portrait orientation, like I prefer. Position the phone in your design to suit your sensor app of choice. There are many apps available ...


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