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    Cellphone camera as sextant
    From: Bob Goethe
    Date: 2015 Mar 25, 22:06 -0700

    I just installed the freeware Dioptra application on my Android phone.  It overlays cross hairs on the screen's camera display and, using the onboard inclinometers, indicates the pitch and yaw of the cellphone.  Essentially, it tells you if you are holding the cellphone level or not, and then tells you the angle at which you have tilted the device. I had hoped I would be able to recommend the app as a cellphone analogue to a sextant for a few friends I have here who are interested in celestial nav, but who do not have sextants.

    I enhanced the crosshairs by taping some white thread to the phone (tape on the backside of phone, so I don't get adhesive residue on the glass).  I was able to take a sight of the moon tonight.  My LOP was 24.6' away from my GPS fix.  It was a rough-and-ready sight; the moon showed up as a moderately fuzzy blob.  I took my sight on the center of what I could see rather than aiming for the  lower limb...since I was all-but positive that the on-screen fuzziness extended beyond the bottom of the lower limb.

    I attempted to take a sight of Jupiter so I could get two LOPs to cross each other, but the the results were disappointing.  My camera was unable to "see" the planet, in spite of Jupiter having a magnitude of -2.2 tonight.  All I got was black sky.

    Earlier in the day, I put Dioptra on an old cellphone (so I ran no risk of damaging the camera in my good phone) and tried to observe the sun.  All I got was an enormous fuzzy blob in the view screen.  Taking a solar altitude with the cellphone was impossible.  I could have been 10° out for all I would have known. 

    It did not SEEM to damage the camera in any way...but there is no guarantee that the camera in my old phone is exactly like the one in my new phone...so I am reluctant to experiment further with the sun.

    There was a high haze in the sky tonight, so I may try once more some other evening.  However, I fear that as a sextant analogue, Dioptra is curious but not really useful.


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