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[OM] Star trail times [was New Kodak Portra 400]

Subject: [OM] Star trail times [was New Kodak Portra 400]
From: Moose <olymoose@xxxxxxxxx>
Date: Sun, 08 May 2011 21:54:47 -0700
On 5/8/2011 8:53 PM, Fernando Gonzalez Gentile wrote:
> Yes, that's it !, the problem posed in a more logic way.
> I will save this answer so as not to ask it for a third time ...
> Didn't know about the sidereal day, shall investigate this issue further.
> Thanks, Chuck !

I almost posted what Chuck did earlier, but was too lazy at the time to take 
the next step.

1. The horizontal AOV of a 28 mm lens is 65.5° Allow ±5% for sample variation.

2. Then you need to measure the length of the track. Without the film at hand, 
I measured in PS. I had to assume that 
the presented image was full frame and that the actual camera and scanner masks 
were, in fact, 24x36 mm, for the above 
angle to be correct.

3. I measured in pixels, as units don't matter for the calculation. The image 
height I got is 1013. The bright trail 
upper left-center, interpolated into the foliage came out to be 88 and the 
dimmer, shorter ones on the left, about 53 
pixels.

4.  Zo.
     88/1013 = 8.7%
     8.7% of 65.5° = 5.7°
     5.7° times 4 min/° ~= 23 minutes for the long trails.
     About 14 minutes for the short ones.

A. Calculating Moose

> Fernando.
>
> On Sun, May 8, 2011 at 11:35 PM, Chuck Norcutt
> <chucknorcutt@xxxxxxxxxxxxxxxx>  wrote:
>> I assume the question you're trying to ask is:  How do you calculate the
>> exposure time to make a star trail of a given length in degrees?
>>
>> Since the earth rotates 360 degrees in 24 hours a star will leave a
>> trail of 360/24 = 15 degrees per hour or 15/60 = 1/4 degree per minute.
>>
>> To be perfectly precise the 24 hour day only pertains to the sun.  When
>> talking about tracking the stars across the sky one uses what's called
>> the "sidereal" day which at 23 hours, 56 minutes and a few seconds is a
>> bit shorter than the solar day.  But for your purposes the length of the
>> normal solar day will be fine.  You're not talking about building a
>> stellar tracking device.  :-)
>>
>> Chuck Norcutt
>>
>>
>> On 5/8/2011 9:10 PM, Fernando Gonzalez Gentile wrote:
>>> Sorry, forgot the technical data: full moon as only light source,
>>> Zuiko 28mm ƒ/2,8 @ ƒ/5,6, tripod mounted plain Olympus OM 2, +2/3
>>> exposure compensation, KM.
>>>
>>> Another question comes again to my mind (asked this before, some 3
>>> years ago ... and forgot the answer): how does one calculate the
>>> exposure, from the angle of the arc of the star trails?
>>> I'm not _that_ smart in Maths ... ;-)
>>>
>>> Fernando.
>>>
>>> On Sun, May 8, 2011 at 9:42 PM, Fernando Gonzalez Gentile
>>> <fgonzalezgentile@xxxxxxxxx>    wrote:
>>>
>>>> BTW - what do you List, think of this KM, exposed on April 1990?
>> --
-- 
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