Figure 7a.
120 second “raw” image of the Veil with no histogram
stretch. Note the faint details that are barely visible.
7b.
After
adjusting the Range bars to just wider than the histogram hump,
more detail appears.
need to be adjusted for best the best possible result. For
longer nebulae exposures, trigger mode is recommended.
2. For a nebula or star cluster, choose something bright
for your first target, and ideally an autoguider is already
locked on and tracking a star, so your resulting images are
well guided. Pick an object like the Ring Nebula (M57),
Dumbbell (M27), or Orion Nebula (M42) since they are all
very bright and easy to see in short exposures to make
sure the framing is correct. Center the object and proceed
to set the exposure.
3. Set the analog gain slider to around 25% from the left
(around 40), and a trigger mode exposure of 30 seconds,
and make sure the bit depth is set to the highest bit depth
possible. This is just a starting point, these numbers may
have to be adjusted depending on your resulting image.
4. Click Single and wait for the image to download. You
should see a rather grainy looking image of your target in
the preview window on the right. Now is the time to look
at the histogram window, and determine what needs to
change in order to get the best exposure. Scroll down the
left side panel to find the histogram window.
5. The histogram shows a graphical representation of the
number and brightness of the pixels in the image. The far
left of the histogram is black, or no light on a given pixel,
and the far right is full white, or fully maxed brightness for
those pixels. A normal astrophoto has a lot of black in it, so
the histogram bump should be left of center (approximately
near the left 1/4 to 1/3 of the histogram for the hump) for
a properly exposed photo of a nebulae with a lot of black
sky around it
(Figure 6a)
. If the hump is too far to the right,
either the exposure or the gain is set too high, and you’re
recording too much background light pollution.
6. Adjust the exposure to somewhere between 30 seconds
and 180 seconds. For a CMOS chip like the G Series,
individual exposures probably do not need to be longer
than 180 seconds, especially when you’ll be stacking
many shots together for the best results. The G16 color
and G21 color camera are not quite as sensitive as the
G10 color, so exposures longer than 180 seconds may
be possible and advantageous to gather more light.
The G16 mono is the most sensitive of all -- but since
it is a monochromatic camera, in order to obtain a full
color image you’ll need to shoot through red, green, and
blue filters, and process the resulting frames together to
produce a color image. If the histogram still doesn’t fall into
the proper area, the gain can also be adjusted. Higher gain
setting means more noise in the image, but it will provide
more sensitivity for a given exposure. Finding the right
exposure vs. gain combination is a matter of trial and error,
and will vary depending on the telescope focal ratio and
sky conditions.
7. If you want to “screen stretch” the histogram, meaning
change the black and white points in the image in order to
see what kind of faint detail you have captured, grab the
left and right edges of the histogram window, and slide the
vertical bars closer to the histogram bump. Moving the
right edge bar closer to the end of the hump will raise the
brightness of the entire image, and moving the left edge
bar closer to the start of the hump will darken down the
black point, boosting the contrast.
(Figure 6b)
. Note that
the live histogram stretching will NOT change a trigger
image that is displayed on the right. It will affect the NEXT
image that downloads from the camera, so take another
exposure and check the results.
8. Once you’ve dialed in the right histogram, it’s time to start
a sequence so you can stack several images together.
Remember that each individual image will have lots of
noise, and only by stacking many images together will you
get a smooth low noise image.
9. In the Capture and Resolution window, click “Options”
in the lower right corner of the window and choose a
file location to save your resulting images. Make sure to
save them as .fit files so 3rd party astro-imaging software
can read them (.fit is a standard in the astro-imaging
community). Verify that the resolution you wish to image
at is set, and also make sure RAW is set for the file type.
RAW fits files are saved before color converting so other
programs can read the raw files and provide the best data
from the camera. Also, make sure “Light Frame” is selected
in the window across the top, so the correct meta-data is
7
7a
7b
7b.
After adjusting the Range bars to just wider than the histogram
hump, more detail appears.
Figure 7a.
120 second “raw” image of the Veil with no histogram
stretch. Note the faint details that are barely visible.
