7.9 Saturation, Blooming, and Other Effects
99
observed star. Some cameras have an IR flash that can help prevent RBI by prefilling
the pixels. For cameras without this feature, removal of RBI is troublesome, as it
persists for some time (the author has seen RBI last 20 min). One way to fix this is
to warm up the camera and allow thermal electrons to displace the RBI electrons,
although the best way to fix RBI is, of course, to not let it happen in the first place
by not saturating.
On occasion, when taking an image you may see a strange, almost ray-like feature
vignetted across the image. A very bright star, planet, or the Moon located just
outside the field of view of the telescope but close enough that light is still entering
the telescope is the usual cause. In general, there is no alternative solution for this
other than not imaging under those conditions. You should not confuse this problem
with dome clipping, whereby the edge of the dome is very near, or actually in, the
field of view of the telescope and stray light from inside the dome is reflecting into
the aperture. Nor should you confuse these with very straight single or double lines
crossing your image. These are satellite and aircraft trails.
You might also see fractal-like structures appearing at the edges of an image.
These are ice crystals forming on the window of the CCD. Within your astronomical
camera, the CCD is sealed within an airtight windowed box. The box is normally
filled with an inert and very dry gas such as argon or nitrogen. Over time, water
vapour gets into the window and can freeze on the inside. If this is happening, warm
the camera up again, and then cool it slowly in steps. This should stop the icing, but
inform the observatory staff, as they will want to address this issue, as it might lead
to CCD failure.
7.10 DLSR Astrophotography
If you do not have access to an astronomical camera, then it is likely you will be
using a digital single lens reflex (DSLR) camera. These cameras are the standard
for both professional and serious amateur photographers for terrestrial photography.
They will have good quality CMOS sensors, good sensitivity to light, removable
lenses, and relatively low noise compared to compact cameras and phones. Some
models now have a live view mode that displays what the sensor is seeing on the
screen. There is a new generation of mirrorless DLSRs coming onto the market that
allow the user to effectively see the sensor’s output. As they are so new, it is difficult
to gauge how mirrorless DLSRs will perform in astronomy. However, they have a
reputation for having short battery lives, which given the long exposure times used
in astronomical imaging is disheartening.
A DSLR camera provides excellent low-cost astronomical images. You need to
mount it to your telescope using a T-piece, which replaces the lens of the camera.
Just as a camera lens is specific to a camera manufacturer, so are T-pieces. So if you
have a Nikon DLSR, you need a Nikon T-piece. You will also need a remote shutter
release, most of which are now infrared or wireless. Some DLSRs have camera
control software bundled with them (or available as an add-on). If your camera has
99
observed star. Some cameras have an IR flash that can help prevent RBI by prefilling
the pixels. For cameras without this feature, removal of RBI is troublesome, as it
persists for some time (the author has seen RBI last 20 min). One way to fix this is
to warm up the camera and allow thermal electrons to displace the RBI electrons,
although the best way to fix RBI is, of course, to not let it happen in the first place
by not saturating.
On occasion, when taking an image you may see a strange, almost ray-like feature
vignetted across the image. A very bright star, planet, or the Moon located just
outside the field of view of the telescope but close enough that light is still entering
the telescope is the usual cause. In general, there is no alternative solution for this
other than not imaging under those conditions. You should not confuse this problem
with dome clipping, whereby the edge of the dome is very near, or actually in, the
field of view of the telescope and stray light from inside the dome is reflecting into
the aperture. Nor should you confuse these with very straight single or double lines
crossing your image. These are satellite and aircraft trails.
You might also see fractal-like structures appearing at the edges of an image.
These are ice crystals forming on the window of the CCD. Within your astronomical
camera, the CCD is sealed within an airtight windowed box. The box is normally
filled with an inert and very dry gas such as argon or nitrogen. Over time, water
vapour gets into the window and can freeze on the inside. If this is happening, warm
the camera up again, and then cool it slowly in steps. This should stop the icing, but
inform the observatory staff, as they will want to address this issue, as it might lead
to CCD failure.
7.10 DLSR Astrophotography
If you do not have access to an astronomical camera, then it is likely you will be
using a digital single lens reflex (DSLR) camera. These cameras are the standard
for both professional and serious amateur photographers for terrestrial photography.
They will have good quality CMOS sensors, good sensitivity to light, removable
lenses, and relatively low noise compared to compact cameras and phones. Some
models now have a live view mode that displays what the sensor is seeing on the
screen. There is a new generation of mirrorless DLSRs coming onto the market that
allow the user to effectively see the sensor’s output. As they are so new, it is difficult
to gauge how mirrorless DLSRs will perform in astronomy. However, they have a
reputation for having short battery lives, which given the long exposure times used
in astronomical imaging is disheartening.
A DSLR camera provides excellent low-cost astronomical images. You need to
mount it to your telescope using a T-piece, which replaces the lens of the camera.
Just as a camera lens is specific to a camera manufacturer, so are T-pieces. So if you
have a Nikon DLSR, you need a Nikon T-piece. You will also need a remote shutter
release, most of which are now infrared or wireless. Some DLSRs have camera
control software bundled with them (or available as an add-on). If your camera has