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Author Topic: Field of View, actual vs apparent, by JerryW; timing star drift: 4min=1deg  (Read 1742 times)


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On 8/12/2015 1:06 PM, Jerry wrote:
> Hi Tom
> Here's a thought on FOV I wrote for Tim.  you might be interested too.
> Field of view
> Put an eyepiece in your telescope and point at a star.  Turn the drive off and let the star drift.  Note the line the star drifts in.  That drift line defines the motion of the Earth in Right Ascension or RA. Now move the scope so the star moves back long the RA line to the very edge of the field of view. If you moved it even a tiny bit more the star would disappear out of the field of view. Do this and get your stop watch ready.  Most cell phones have a stop watch App.
> Now leave the scope alone and as soon as the star enters at the side of the field of view, start the watch. Let the star drift all the way across the field of view and stop the watch when it disappears.  Record the time the watch shows it took the star to drift from one side of the field of view to the other.
> The Earth rotates once per 24 hours or turns through 360 degrees in 24 hours. Hence it rotates one degree in 24/360 hours. Or one degree in 0.0667 hours.  Times 60 that's one degree in 4 minutes. So to get the field of view in degrees of your eyepiece and telescope combination divide your measured time in minutes by 4.
> By example if it took 1.75 minutes (1 minute and 45 seconds) to cross the field of view, you have a 0.4375 degree field of view or just under half a degree. Since the full moon subtends a half degree this field of view will not quite show the whole disk of the moon in a single view.
> This is the actual or absolute field of view, not the apparent field of view.
> The apparent field of view is a property of the eyepiece alone, without the telescope attached.  If you look in the eyepiece by itself you will not see an image, just a large disk of light.  The angle that disk appears to subtend is the apparent field of view.  Apparent field of view is given by multiplying the true field of view by the magnification of the eyepiece when it is in a telescope with a true field of view. It is also a design feature of the eypiece usually quoted by the eyepiece maker.
> Conversely the true field of view of an eyepiece plus telescope combination is given by dividing the apparent field of view by the magnification.

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