Re: Photographic Size of a Star


"Emanuele D'Arrigo" <manu3d@xxxxxxxxx> wrote in message
news:1164719372.616451.163810@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hello everybody,

quoting from wikipedia (Apparent Magnitude article)
"a first magnitude star is about 2.512 times as bright
as a second magnitude star".

Does this effectively means that in a photo, the radius
of a first magnitude star (i.e. in pixels) is 2.512 times
the radius of a second magnitude star?

Can the same be said of the size/lenght of the diffraction
artifacts, those star spikes produced by the struts holding
the secondary mirror?

Thanks for your help!
No.
Stars are effectively 'point' sources (they are so far away, that they
display no measurable 'size'). At the scope, what you see/image, is
stretched by a number of processes. First, diffraction effects in the
scope (the so called 'Airy disk'). Then any movement errors of the mount.
Then before it reaches the scope, by 'wander' from the atmosphere, and
defocus from the same source. All these effects for a given scope/point in
the image field/sky, remain constant, whatever the brightness of the star
involved. The reason they look bigger, is simply that becaue they are
brighter, you see further 'down' the sides of the resulting recorded
image. If you take an image showing a number of stars of different
brightnesses. Say some at 4000 counts 'max', and some others at 24000
counts max, and adjust the viewing intensity range of the image, to run
from the background level to 4000, the brighter stars will appear much
larger. Now make a note of how 'big' the dimmer stars appear, and change
the viewing intensity range to run from the same background level up to
the 24000 count level of the brighter stars. You will find that the images
of the brighter stars, now looks exactly the same size (except for some
experimental error), as the dimmer stars appeared with the other settings.
Imagine that you had a set of cones, all with exactly the same taper on
their sides, but different heights. The diameter of the 'ring' you see if
you cut through them, depends on how far down them you cut. Your 'view' is
like a *** cutting through the 'cones' of information generated by the
stars.
Unfortunately, the shape of the 'cone', is not straight sided. At the
bottom it flares out, and at the top it flattens, hence the exact
relationship to apparent size, is not nicely linear. However it
approximates to the inverse of the magnitude change itself. So for a given
visualisation range, a star that is say Mag4, will appear about 50% larger
than a star that it Mag6.

Best Wishes


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