There is no doubt that commercial digital cameras cannot match the performance of dedicated astronomical CCD cameras. Custom astrocameras use active cooling to significantly reduce CCD dark current and allow much longer exposures. Even though they are not ideal for the purpose there are a few things which you can hope to do with a digital camera with or without a telescope. If you want to do good astrophotography the best advice is buy a proper CCD camera designed for the purpose. On the other hand if you want to see what can be done with a good commercial digital camera then read on. One of the best features here is that no film is wasted by poor shots and feedback is immediate. The worst feature is fighting with peculiar little buttons in the dark.
The Kodak DC-120 is unusual in that it allows a wide range of exposure control from 1/500s to 16s duration and has a relatively good fast f2.5 zoom lens. It also allows readback of the entire CCD array without compression which is helpful when looking at high contrast sharp stars on a black background. I have found in most cases the highest quality compression does not produce seriously annoying artefacts (the other settings are hopeless for astro work).
The simplest test of the camera is to try a star trail time exposure of a familiar constellation and then compare the image with a star chart. This test shows that at 16s exposure the camera is sensitive enough to capture images of stars down to 5.5 magnitude. Unfortunately, the camera also picks up a distracting ghostly glow in one corner of the CCD array. There are also a number of "warm" pixels which are always bright on each and every long time exposure taken with a particular camera. These could easily be mistaken for stars.
Note that both these problems are peculiar to long time exposures made using an uncooled CCD chip and this in no way affects or interferes with normal use of the camera for photography in normal lighting conditions.
Constellation of Orion DC-120 wide angle f2.5 16s The dark object at
bottom left is the silhouette of a tree. A large scale version of the
images showing more detail at 640x480 resolution is also available (approx
80kb).
Orion pictures
Star map produced using Chris Marriott's SkyMap Pro v4.0, details
available from
SkyMap
Stars are shown down to 6th magnitude
Warm pixels are caused by leakage of charge from a few individual CCD pixels and this effect occurs faster at more elevated temperatures. This is one of the main reasons why low noise astro cameras are actively cooled. Most pixels are affected to a much lesser extent and it gives rise to a coloured mottling of the background noise.
The radially striped bright corner effect was explained to me by a CCD specialist as being due to low levels of infra red emission from the CCD's analogue amplifier circuitry. CCD's are rather sensitive to IR and so on a long exposure this effect will eventually show up even on top astro CCD cameras (after an hour or so). Some CCD's chips have better low leakage and long exposure performance than others, but most digital camera manufacturers are not brave enough to allow exposures which would allow these effects to be seen. I think this is a pity.
Fortunately the majority of the dark current glow and "warm" pixels signal is very reproducible on successive shots, and it is quite practical to take a dark frame to subtract from the sky image in post processing. This allows quite reasonable images of constellations to be obtained to about the same depth as an ordinary observer can see in a moderately dark suburban sky.
This is the final image after dark frame subtraction. It is significantly more pleasing.
It is not possible to use the DC-120 at prime focus as there is no easy way to remove the standard lens. This means you need to use a rather clumsy procedure of photographing through the eyepiece and telescope. There is some risk of telescope eyepiece and camera lens colliding whilst doing this sort of photography as the scope tracks the object it may move the eyepiece towards the camera if you are not careful.
It is not helped by the lack of a cable release input on the camera so that many otherwise good manual shots can be spoiled by vibration. You can control the camera from a PC but I have not used that for these tests.
This is a shot of Jupiter taken in twilight at 16:56 on 16th January 1998 which just about shows two of the Galilean satellites at about 4 o'clock from the main disk. It was taken through my 1000mm f10 Maksutov lens using a 10mm eyepiece. Four belts were clearly visible through the scope, but unfortunately the camera exposure did not capture them at all. The problem is primarily due to camera shake and atmospheric turbulence on an exposure of 1s. Shorter exposures failed to capture sensible colour information and were an odd shade of green.
I plan to get some better shots of Saturn with the 10" next time it is clear.
This is a shot of the moon taken at 18:18UT on 2nd February 1998 with my Meade LX200 10" f6.3 with a 35mm eyepiece which gave a wide field and a bright enough image to get a one decent exposure at 1/15s before weather intervened and I had to beat a hasty retreat. The image is over exposed, and the automatic exposure proved to be useless at these light levels. And again the level of detail visible in the image is rather disappointing compared to the crisp visual image in the telescope. A shorter exposure would doubtless give better resolution and more detail away from the terminator. Unfortunately, big fat rain clouds intervened.
The manual shutter control also proved to be something of a pain to use as the exposure shift only allows easy movement to ever longer exposures (so that changing from 1/8s to 1/15s exposure requires 20 fiddly keystrokes). It seems from the legends on the back panel that this was perhaps not the original intention, but only the cursor down key works! Perhaps it is a defect with my camera - does anyone else have this problem with manual exposures?
The best way to get started is using a long focal length eyepiece with good eye relief so that alignment is less critical and there is less chance of accidental collisions between camera lens and telescope. Unfortunately, the cameras preview function does not work very well at low light levels and so you have to take shots blind after making the trying to align the optic axes of camera and telescope. This is harder to do than it sounds and a high proportion of my initial attempts were entirely blank! I am sure it will be possible to improve on my initial efforts using this camera for astro shots given enough time and patience.
I think the conclusion is that the DC-120 is not ideal as an astronomical camera, but if you happen to have one there is a possibility to take simple shots of constellation star fields and telescopic shots of the moon and planets. Do not buy one solely for the purpose of low light astrophotography. I am well pleased with mine for general use and put this page together with a few sample images to show what can be done at low light levels. Deep sky photography is basically not practical because of the maximum 16s exposure, but I am sure that better planetary and lunar images would be possible with more practice. The limitation at present is one of technique.
More DC-120 low light images showing the from the late twilight sky with the planets at a range of exposures from 1 to 16 seconds. Note how the underexposed images show extra noise and pronounced colour casts to green. Also a more conventional night shot of my southern outlook by available light at 16s exposure.
Broadly speaking the DC-120 can see roughly as well as a partially dark adapted human eye in suburban skies.
Other astronomical info -
Nonad filters for eliminating low
pressure Sodium light pollution.
Nacreous cloud observations
