The atmospheric issue is more differential refraction, than refraction per say. A zenith pointing camera is likely the best choice. The zenith is the direction of the least atmospheric depth also.
-John =========== >> I can think of two general scenarios here. >> If you planet has air you will need to know how it refracts st >> One is where you "lay the iphone on the table" in a fixed position. One >> could use the internal accelerometers to determine "level", but I don't >> think you could tell orientation, unless, perhaps, you can see >> circumpolar >> stars? That is, by watching the movement of the stars/planets through >> the >> field of view over some hours, could you figure it out? Or is there >> some >> fundamental ambiguity. > > No, you can point to any location and you can (in theory) figure out > where it's pointing given that you have a large enough field of view > to see many stars at the same time. You can make a fixture easy > enough, just some epoxy and a large boulder. I used lag bolts > onto my garage roof and it worked more than good enough. > > If you can choose, straight up is the best aim point. Refraction is > not much of an issue and there is less air to look through. But > looking at the equator means there is less field rotation and the data > is easier to reduce. We looked at the equator because we did not want > to deal with image rotation. Motion blur is minimize down there too. > > But if you want to know "absolute time" then you need more. Looking > at any random but fixed location will get you the period of the > planet's ration to about a mSec with cheap equipment but to get > absolute time you need to measure the aim point relative to the local > meridian. That is not as easy. Star with a protrator and a plumb > bob. That is what I used. But to refine that you need a good > source of time and for the purpose of this exercise we don't have > that. Only the plumb bob which means "a few seconds of error". maybe > an precision level can do 10X better? >> >> (obviously, you can trivially see the moon/sun) >> >> The other scenario is where you get an inexpensive camera (webcam, or >> perhaps some slightly better point and shoot) and build a precision >> mount >> (so you DO have accurate knowledge of sensor orientation and position) >> Could >> you, perhaps over time, do an insitu calibration? >> >> I suppose any of these techniques is going to have issues with the >> uncertainty in when the image is actually captured (e.g. there's >> probably >> 10-100 ms you're not going to get away from). >> >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > > > -- > > Chris Albertson > Redondo Beach, California > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
