By entering 'V Cnc' in the target text box, selecting G5 in the system drop-down list and clicking the 'Refresh' button, we get a
nice and tidy filtered image list showing only the images of interest, our three images of V Cnc:
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Figure 2: Filtered File List (click for full size) |
The information shown in the list is system name, target name, date and time of
the exposure, R.A./Dec. of the target, exposure time, and what filter was used during
the exposure. The list is sorted by observation date/time.
Note that the date and time of the exposure is the universal time (UT) of the midpoint
of the exposure.
The first thing to do is to check that the image is properly calibrated. This is
crucial for accurate photometry. Check the value of the 'Cal' column:
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Figure 3: Calibration column |
The meaning of the letters in this column is B = 'Bias corrected', D = 'Dark subtracted'
and F = 'Flat fielded'. A BDF means that everything is OK, which is indicated by
the green color. If F is missing, accurate photometry is unlikely. If D is missing
the case is not that clear-cut. Anyhow, the normal situation is BDF, and anything
less calls for an action from the GRAS team.
Also note the 'WCS' column to the left of the 'Cal' column. Although not important
for the actual result of the analysis, it is mighty nice to have an 'OK' in this
column. It can save you a lot of work. OK means that the image
has been plate solved and the World Coordinate System parameters have been saved
to the image header. If so, identifying stars can be automated. More on that later.
For even more image information, click the icon in the FITS column. This will take you
to a page listing all the content of the FITS header.
Of course, no image inspection without examining the image! To do that, click on
the target name link. In our example this will lead us to an image like this:
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Figure 4: V Cnc Image (click for full size) |
It's a pretty large image, but don't worry. You don't have to download the 6MB FITS file
(other than for your own records). It is ready for analysis in the browser.
A quick visual inspection tells us that this image looks good. No trailing of stars,
well focused, and no apparent clouds.
If you think the image is too bright or too dark you can adjust the display contrast by first clicking the 'Adjust Brightness'
link in the toolbar above the image:
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Figure 5: Go to display brightness |
Next, a preview of the center region of the image is displayed, and you can see the effect of making it brighter or darker. Click 'Use Setting'
when done. This will take you back to the image, now with a new setting for the display brightness.
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Figure 6: Adjust display brightness |
Next: Stacking Images
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