silhouette
Posted: Thu Sep 21, 2006 5:31 am
Does anybody know what technique is used to take the photography?
The glare from the sun must have been massive and the two subjects barely dots on the sun.
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Silhoutte (APOD 21 Sep 2006)
Posted: Thu Sep 21, 2006 7:41 am
Posted: Thu Sep 21, 2006 4:04 pm
I would guess that they simply used a telecope with a filter to cut out most of the light.
The objects themselves are actually a reasonable size in astronomical terms, if we take the space station to be 100m across and located 550km away then it subtends an angle of 0.01 degrees (or a 50th of the solar diameter). In astronomy numbers that is a very reasonable 36 arcseconds across, very simple with to resolve with a big telescope.
In fact from the picture we can estimate the size of the telescope used, the resolving power of a telescope is given by 1.22 x wavelength / Diameter of the telescope. This all equals the separation of two objects in radians that can just be resolved.
So rearranging and converting the formular into arcseconds from radians gives
diameter of telescope = (252000 * wavelength) / theta
Now actually we can resolve things in that image that are about 20m across on the ISS, the wings for instance, which therefore are about 7.5 arcseconds across. If we assume the light is about 5000nm in the visible and a nice round number.
We get that the telescope is
(252000 * (5000 *10^(-9))) / 7.5 = 0.17m or 17cm across, pretty standard for an amateur scope.
The objects themselves are actually a reasonable size in astronomical terms, if we take the space station to be 100m across and located 550km away then it subtends an angle of 0.01 degrees (or a 50th of the solar diameter). In astronomy numbers that is a very reasonable 36 arcseconds across, very simple with to resolve with a big telescope.
In fact from the picture we can estimate the size of the telescope used, the resolving power of a telescope is given by 1.22 x wavelength / Diameter of the telescope. This all equals the separation of two objects in radians that can just be resolved.
So rearranging and converting the formular into arcseconds from radians gives
diameter of telescope = (252000 * wavelength) / theta
Now actually we can resolve things in that image that are about 20m across on the ISS, the wings for instance, which therefore are about 7.5 arcseconds across. If we assume the light is about 5000nm in the visible and a nice round number.
We get that the telescope is
(252000 * (5000 *10^(-9))) / 7.5 = 0.17m or 17cm across, pretty standard for an amateur scope.
Posted: Thu Sep 21, 2006 4:06 pm
Or we could always save time and look at the damn site. 
http://www.astrophoto.fr/iss_atlantis_transit.html
so the telescope is actually 15cm in diameter, not too bad a guess.
http://www.astrophoto.fr/iss_atlantis_transit.html
so the telescope is actually 15cm in diameter, not too bad a guess.
Posted: Thu Sep 21, 2006 4:30 pm
you know your math indeed 
considering how simple image resizing could screw it up.
considering how simple image resizing could screw it up.Posted: Thu Sep 21, 2006 7:05 pm
5000nm? visible? Wish I had your eyes 
Posted: Thu Sep 21, 2006 7:49 pm
Good point, should be Angstroms 
Perfect example of why peer review is essential.
That'll teach me to try and work in anything but astronomys backwards nonsensical units. Should have tried to work out the magnitude of the station.
Of course the minimum resolved size is actually about 2m, as you can see the width of the solar cells which are wider than that guess. So all the working still arives at the same point.
Perfect example of why peer review is essential.
That'll teach me to try and work in anything but astronomys backwards nonsensical units. Should have tried to work out the magnitude of the station.
Of course the minimum resolved size is actually about 2m, as you can see the width of the solar cells which are wider than that guess. So all the working still arives at the same point.
Posted: Thu Sep 21, 2006 9:19 pm
http://www.space.com/scienceastronomy/m ... 30507.html
It seems to me that the apparent size of the space station against the solar disk is about the same as the apparent size of Mercury.
It seems to me that the apparent size of the space station against the solar disk is about the same as the apparent size of Mercury.
Posted: Mon Sep 25, 2006 10:26 am
Posted: Mon Sep 25, 2006 6:33 pm
In the video of Mercury crossing the solar path to Earth, why is the focus constantly changed, causing such a jumpy video??
Posted: Mon Sep 25, 2006 7:12 pm
Per the write up at the bottom of the images, this is why things appeared as they did in the movies
"MDI took a series of full disk continuum images at varying focus positions during the entire transit, in order to better determine absolute spacecraft roll and MDI absolute plate scale, and to better understand MDI image distortion. The varying focus positions are what causes the Sun to "breathe", or "pulsate" in size, in the movie. "
"MDI took a series of full disk continuum images at varying focus positions during the entire transit, in order to better determine absolute spacecraft roll and MDI absolute plate scale, and to better understand MDI image distortion. The varying focus positions are what causes the Sun to "breathe", or "pulsate" in size, in the movie. "