Filter comparisons In light polluted areas it is useful to use a filter with the
camera.
The following are pictures comparing
the Astronomik CLS filter and the Hutech IDAS
filter. |
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Daylight image taken with a
Canon 350D.
The
Canon has an IR filter built in.
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Image taken with the SXV-M25C
- no filters.
No IR
filter causes a weird colour balance.
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Image taken with the SXV-M25C
- Astronomik CLS filter in place.
Very little green is
left.
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Image taken with the SXV-M25C
- Hutech IDAS filter in place.
The white car is properly white.
Green is showing -
note the yew hedges in the distance.
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Overcast and dull day.
Canon 350D, no filters.
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Astronomik filter held in
front of camera by hand.
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Hutech IDAS
filter.
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This chart shows the
transmission curves for the Astronomik and Hutech filters.
Most emission lines for the Mercury spectrum fall in the regions
where the Hutech transmits virtually no light.
The double Sodium orange line is
removed by both filters.
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To use the Baader coma
corrector with the camera some extension tubes are needed.
The rear of
the coma corrector needs to be exactly 55mm from the CCD
sensor.
The sensor is set back 18mm from the front of the
camera.
To get the required 37mm I used a Baader Varilock extension
which can be adjusted between 20 and 29mm, and a 15mm extension. |
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Off-axis
guider
In the eternal quest for long, perfectly guided
exposures I have bought an off-axis guider and a Starlight Xpress Lodestar
camera. The only problem is that the Baader coma corrector needs to be
55±1 mm from the imaging CCD. With the 20mm spacer the distance is
more like 59mm, so I need to shave off 3 or 4mm. |
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Starlight Xpress say that the
distance from the CCD to the front of the camera is around
18mm.
Before I attempted getting the
parts machined I wanted to verify this distance.
This is difficult to measure because
there is a glass window over the chip.
I
borrowed a travelling microscope from the school where I used to
teach. |
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A hair stretched across the
camera aperture. |
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The view when the microscope
is focused on the hair.
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Microscope focused on the
CCD.
The microscope has an extremely short depth of focus, and repeat
readings did not vary by more than ±0.1mm
Result: The distance is
17.6mm
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The T-adapter to which the
coma corrector attaches has a flange which is about 2mm thick. This does not
need to be there. |
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The 20mm spacer can be reduced
in width by a small amount. If too much is taken off then it will be difficult
to get both the guide camera and imaging camera in focus at the same
time.
Terry Platt at
Starlight
Xpress did the necessary machining and removed 1.92mm from the
T-adapter and 1.98mm from the spacer. All parts went together perfectly
and now the distance from coma corrector to ccd is
54.71mm.
This should be perfect for
pinpoint stars across the whole frame.
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The image below shows small round stars over the whole frame, so
it looks like the coma corrector is doing its job well.
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