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Minor planet 24 Themis


 Minor planet 24 Themis

Canon EOS 5DMkII 254mm Newtonian @2400mm (2x Barlow) f/9.6 ISO6400 16x30s 2011-9-21 23:51:18-00:01:48UT
From Rookhope 54.8N 2.1W 330m asl. Rural, almost no light pollution (3 Bortles)

This may seem a rather boring photo. Themis is known to have water ice on its surface so measuring its brightness is of interest.

The area shown by my photo is 0.47° x 0.64°. The stacking was done so that Themis remains stationary and the stars trail but the motion over 10 minutes is almost undetectable. The 3 stars marked with their magnitudes (x 100) in red are:

Tycho catalogueMagnitude (V)RA (J2000.0)Dec (J2000.0)
5252 247 110.7123h49m09.86s-01°53'23.5"
5252 289 110.3923h48m15.36s-01°48'03.1"
5252 918 110.9023h48m21.70s-02°09'21.3"

How do I know that the star-like object at the intersection of the white lines is 24 Themis? Well, I aimed my telescope according to its predicted position to take the photo. After processing I checked against the Aladin star atlas. That shows the other stars but nothing of similar brightness at the object's position. Here is Aladin's plot, in which the cross hairs mark my estimated position for Themis: 23h48m50.7s -1°56'16" (look closely at my photo and you can see 4 fainter stars around Themis, shown by Aladin).

What about the magnitude then? Well it cannot be determined from the JPEG version of my photo, shown above, because I have enhanced the contrast and the brightest objects have saturated. But going back to my original stack of exposures Themis (like the other bright stars) does have a smooth unclipped profile:

So I used GRIP again to estimate the magnitude by extrapolation from the 3 known stars tabulated above. Here is the result:

Only the green magnitude should be used because I do not know the R and B magnitudes for the reference stars.

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