Auto Exposure to avoid star-trailing

[nedex]

Hi guys,

Today i saw this tool:
http://news.alpine-photography.com/2013/06/get-your-stars-right-tool-long-exposure.html

That calculates the maximum exposure time when shooting at stars at night to avoid the trails.
Since this could be useful to avoid shooting tests, a function or a script can be made to automatically set or simply tip the best time with the current focal length.
I know it isn't exactly for everyone but magic lantern should be, for everyone.

Thanks guys :D

EDIT:
I've quickly "fiddled" the tool for better understanding, you can find it there: http://jsfiddle.net/NedEX/z6ep4/

[electriclemon]

I'm not sure about auto exposure but I would find an overlay useful (perhaps it could only show if eg the camera is set to manual mode and the shutter time is at least 5 seconds).

[a1ex]

This can be a hint in the intervalometer menu (where it already gives advice on how to avoid flicker, how many shots you will get and so on).

[calypsob]

As you increase declination from 0 degrees your exposure time becomes maximized. 

The "get your stars right " link is using what is called "the rule of 600" to solve for maximum exposure possible based on lens focal length.  The rule of 600 however was designed for 35mm sensors, if you are using APS-C then you need to divide 600 by 1.6 to get 375.  For crop sensor bodies you use the rule of 375.  To find your exposure time divide 375 by your focal lenght, for instance 375 seconds / 11 mm = 34.09 seconds.  This is the maximum amount of time you can expose stars before getting star trails.  I believe that the calculation on the link will give you 31 seconds at 11mm because they used the actual value of the aps-c sensor which is a little less than 1.6x smaller than a full frame 35mm sensor.  If you are in the northern hemisphere, USA, then North is going to be 0 degrees declination, the north star is actually at 89 degrees so dont rely on polaris to find zero degrees declination , use a compass instead. 

The farther you move away from polaris, the tighter your stars will be.  I am not 100% sure but I think as you aim towards the south the trails will come back, so east and west are going to allow you to attain maximum exposure times with a fixed tripod. 

If you look at this guys work it is easy to see that the trails get bigger the farther away you get from polaris http://www.lincolnharrison.com/startrails/

[mageye]

@calypsob

How can it be that the farthest away from polaris the tighter the stars will be?

If you mean less trails then I would say that the oppersite would be true (further away from centre of rotation (Polaris) = more trails). Maybe I am misunderstanding what you are saying?

EDIT: OK I am sorry that I missed the bit at the very bottom but that completely contradicts what you say above. Sorry to point that out but it really doesn't make sense?

[calypsob]

ok you are right sorry about that, I got things a little backwards http://backyard.8m.net/startrail.html  I was looking at that diagram and thinking exposure time not arcseconds.  So exposing at 0 degrees will give the least amount of star trails.  This stuff gets confusing sometimes.

[mageye]

Yes mate. Sorry to come across as being so damn pedantic!

It's just I have also enjoyed star lapse photography and this is something that has been especially poignant to myself. There are lots of things to take into account to get the perfect star exposures. I am far from perfection myself.