|My current location and equipment
I live in Perth, Western Australia (-31° 58' S, 115° 47' E), where I image
from the backyard of my house. Before this we lived in Singapore (01°
22' N, 103° 50' E), where the DSLR images were taken, and Miri, Sarawak (04°
20' N, 114° E) where the SAC7 images were taken.
I use an Orion 80ED apochromatic refractor, a Celestron C9.25 with a x0.63 focal reducer, and a Vixen GPD mount with
SkySensor 2000 PC. I recently (Feb 2010) bought an SBIG
ST-8XME CCD camera which I use for photometry. The picture shows the C9.25 set up on my
driveway in Singapore.
The guidescope you see in the picture
is a 700mm focal length Skywatcher refractor, mounted onto the C9.25 with a
couple of home-made tube rings made from steel gutter pipe fixing rings.
Each ring has 3 thumb screws for adjusting the direction of the guidescope.
The SAC7 is now used as a guide camera. In long exposure mode, it
enables guiding on some quite faint stars (but I haven't measured the
The imaging camera in the picture is a Canon EOS 300D Digital SLR.
This DSLR was modified following a procedure
described by Terry
Lovejoy. The modification involves removing the internal IR-cut filter which severely reduces
the camera's sensitivity to the important
Some words of warning - making
this modification obviously invalidates your camera warranty. There
is also a fair bit of fine work involved and you run a real risk of
ruining your camera! I had to open up the camera and readjust some
connections 3 times, before the camera would work properly again. I
did not replace the original filter with a less aggressive IR cut filter,
nor another piece of glass, so the camera now does not auto-focus properly
and, because of it's response to IR, its colour rendition is severely
compromised for normal photographs. You have been warned...
Artemis 285 CCD
This is a camera designed by Steve Chambers,
Jon Grove and Arthur Edwards, which I bought in kit form and assembled at
home. It is a 16 bit camera based on the 1.4 Megapixel Sony ICX285AL
sensor. Peltier cooling enables the camera to operate about 15 - 17 deg C
below ambient. The low dark current of this chip means that a
temperature of operation of about 3 to 5 deg C is good enough for dark
frames not to be needed.
The camera runs off a power supply providing 12V and 5V
which I built from a PC power supply. The power supply box also
provides test points for monitoring of CCD and case temperature.
The completed camera and power supply are in the picture near right.
I have run linearity and other tests on my completed
camera and the result is summarised in a
calibration sheet. The results show very low read noise and dark
current. Linearity is also shown, but I haven't done enough work to
quantify the linearity and any effect or otherwise on photometry.
|At right are the parts as they arrived out of the box.
Below, the camera partially assembled, with the circuit boards showing.
At right, the 5V peltier (thermo-electric cooler) mounted
on the case, and an LM35 temperature sensor, which was not in the original
|Telescope Optical Quality
I've been very
pleased with the optical quality of the C9.25. For an example of the
resolution achievable, have a look at this image and detail of the
Lagoon Nebula taken in H-alpha.
Yes there is some distortion of stars at the corners - see the top right of
that image for example, and the corners are vignetted when run with a 0.63
focal reducer, but most of the image is very usable.
At right are a couple of out of focus (one on each side
of focus) star images taken when collimating. I'm no expert in reading
star tests, but a comparison with simulations produced by
Aberrator shows no
obvious pinching or coma. Since I wasn't too careful about having the
two images the same distance away from focus at either side, I can't tell if
the inside focus and outside focus rings are the same, so I don't know if
there is spherical aberration.
guide scope and the SAC7 camera described below (working in long exposure
mode), I have been able to get the tracking performance shown on the graphs
to the left.
Performance in Declination is very good. Guide
response is set to be very un-aggressive, and together with a slight error
in polar alignment, the corrections in Dec are always in the same direction.
The result is tracking accuracy in the order of +- 1 arcsec, or about 1
pixel in my setup.
In R.A. the guiding performance could be better.
The error is usually in the order of +- 2 arcsec, but with some excursions
out. Here settings are more aggressive, but perhaps could be made more
so. Note that the periodic error (period of approx. 10 mins) of the
mount is superimposed on higher frequency errors.
On the whole, with single exposures of max 3 mins these
errors are acceptable, but some processing out of oval stars is usually
necessary in the R.A. axis.
For autoguiding, I
have built a parallel port driven relay box that works with the
This box sends guiding signals to the GP-D mount through
the Autoguider port of the SkySensor 2000 hand controller. The picture
on the right shows the tangle of wires at the base of the telescope during
an imaging session.
The circuit is based on one published by
Astronomy, The circuit diagram is at the back of the User Manual
of the guide port, and is based on an opto-coupler (a relay that uses
infra-red LEDs to switch the output) chip. I have modified the circuit
to include 4 LEDs that show which directional guiding signal is
active. Since I use a SAC7 in long exposure mode as a guider, and this
is controlled via the PC parallel port as well, I have combined all these
signals into one cable that runs to the PC. More pictures of the box
below - yes I've very proud of the work that went into this!
One of the challenges
with imaging with a large sensor such as the 300D has is that there will be
significant vignetting at the corners of the frame.
In order to correct for this, as well as the occasional
dust particles on the sensor, flat frames are taken and then used in image
processing. One way of taking flat frames is to take images of the
twilight sky, or as I used to do, of a wall that is evenly illuminated.
There are problems with both. I normally set up well after dark in
order to take advantage of darker skies later at night, so the first option
is out. As for the 2nd option, my 'evenly' illuminated wall turned out
to be not so even after all, resulting in difficult to remove gradients in
the final images.
So I built a light box. This is a box made of
plastic board with two layers of tracing paper acting as diffusers.
Light is provided by 4 white LED's at the top corners of the box (as viewed
at left) that shine toward the bottom (in fact there are 3 stages of
diffusion - the bottom of the box, lined with white paper, then the 2 layers
of tracing paper spaced about 2" apart). The inside of the box is
shown below, left. Note how the 4 LED's are held at the corners.
Subsequent to this picture, I lined all the inside with white paper to
improve evenness of illumination.
The LED's are powered with a 9V battery through resistors
and a potentiometer that provides about 3-3.5V to the LED's. Normal
exposure needed with the light box is about 6s (ISO800).
So how well does the light box work? Below is a
picture taken of the illuminated box, showing how even (or un-even) the
illumination is. The green colour is because this is from the original
RAW file conversion, without any colour correction - the actual colour is
very nearly white.
The graph below right show measurements of the light
level across from left to right. The effective part of the
illumination has levels from about 1600 to 1720, i.e. about 8% unevenness.
I don't know if this is good enough, but the flats that are produced result
in images that are fairly even - nothing that can't be removed in Iris.
A right is a typical flat frame (contrast
increased to show vignetting) taken with my typical imaging setup, i.e.
C9.25, LPR Filter, 0.63 focal reducer and the Canon 300D. There are a
few specks of dust, but not too many. Vignetting is obvious, but more
of concern is the slight red fringe at the left of the flat. My
optical system apparently introduces this colour shift. I don't why
this happens, but it's probably the LPR filter.
It implies that to adequately correct a colour image I
have to do flat fielding separately in the 3 colour channels. Applying
one monochrome flat leaves me with the same colour fringe on my images.
So it's a bit more work.
|For information on my previous equipment, please see
since 20th Sept 2008