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Sky Conditions Impact of Light Dew/Condensation on Faint Galaxy Use of Log brightness scaling / False Colour by highlighting detail in faint galaxies >
Sky Condition Summary (Session S108) |
Notice
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Airmass of image. Purple Line
1 is zenith, with higher number indicating positions
progressively towards the horizon
Sky Brightness. Red circles/line.
Sky Magnitude in mag per sq. arc sec. Higher numbers
(towards top of chart) indicate darkest skies.
Elongation. Red circles/line.
Elongation (long axis length/short axis length). 1 = circular star image.
Mag Zero Point. Green
circles/line.
For each Clear Filter image this is the average difference
between the catalog magnitude of stars and their instrumental raw
magnitude normalized to a 1 sec exposure. Only sharp star-like
objects are used in this calculation. Larger numbers (towards top of
chart) indicate more transparent skies . Lower numbers
indicate either a less transparent sky or the formation of
condensation/dew on the front lens of the telescope.
ZP. Small green circles
These are magnitude zero points for individual stars in
Clear Filter images. The range is due to a number of factors including
catalog magnitude error, star colour, impact of star brightness on S/N
FWHM (clear). Blue diamonds
There are the Full-Width Half Maximum measurements of
individual stars in Clear Filter images. Only sharpest star-like
objects above a S/N threshold are shown. Lower number (towards top of
chart) indicate best seeing (most stable atmosphere) and best focusing.
Note that seeing & focusing may drift over an observing
session.
FWHM (RGB). Orange diamonds
There are the Full-Width Half Maximum measurements of
individual stars in Colour Filtered (RGB) images. Only sharpest
star-like objects above a S/N threshold are shown. Lower number (towards
top of chart)
FWHM (RGB). Small white circles
These are objects that fail to meet sharpness & S/N
thresholds. They might include galaxies or low S/N stars
Sky Conditions Chart (S108) |
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The impact of just a light dew / condensation on ability to image a faint galaxy is shown by the following frames of irregular galaxy NGC 1156.
NGC 1156 - Light dew on front lens of scope | NGC 1156 - Front lens cleared of dew | |
CCD Image 120s, clear filter 2006-01-02 00:42h UT (#108109) |
CCD Image 120s, clear filter 2006-01-02 00:49h UT (#108112) |
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Logarithmic scaling and false colour was used to highlight detail in images of several galaxies recorded during the S00108 session. The following example shows the spiral galaxy M109.
M109 - Normal Image | M109 - Log brightness scaling & False Colour | |
CCD Image 6 x 2 min (average combine), clear filter 2006-01-02 01:12h UT (#108113-18) |
CCD image as
left with log brightness scaling & false colour (inverse spectrum) |
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This Web Page: | CCD Image Notes - (2006-01-01) |
Last Updated : | 2015-05-16 |
Site Owner : | David Richards |
Home Page : | David's Astronomy Web Site |