| review: SLR Canon EOS 300D (digital rebel) for long time exposures and astrophotography (appr. € 850,- body only) |
Introduction
The Canon EOS 300D is an affordable Single Lens Reflex digital camera with a true bulb mode.
The capabilty for changing the lens, i.e. by a telescope, and the exposure times up to 10 minutes and more is making it very interesting for astrophotography.
Many excellent reviews have been written about the fantastic day light capabilities of the 300D.
This review will focus on the long time exposure capabilities, the necessary camera settings for that purpose and the usage of the 300D in astrophotography.
Technical specification
- 3072 x 2048 pixel
- Pixel size: 7.1 x 7.1 uM
- Chip size: 22.7 x 15 mm
- Read out noise / Dark current: very low for an uncooled CMOS chip. Built in software noise reduction.
- About 25% overall quantum efficiency as measured by Christian Buill
- One-shot colour with a filter relation of RGB of 1/2/1
The CMOS chip and the imaging electronics
The pixel size of 7.1 uM is demanding for a spot size of the imaging optics of about 20 uM when used at highest resolution.
This is slightly more than the typical 30 uM spot size of most lenses constructed for 35 mm film cameras.
In addition the chip's diagonal of appr. 28 mm is asking for a fairly large aberration free field of 100% illumination.
While a fast Newtonian with a Baader MPCC coma corrector is able to meat the spot size criterion the huge diameter of the chip is leading to big obstructions due to the big secondary mirror needed.
On the other hand the good SLR lenses like the ones from Zeiss are able to fit the 300D at least when stopped down by 2 or 3 f-stops.
It was sad to see that my Takahashi FS-60C was not able to give good result across the whole field of view.
The focal 'plane' of that 2 fluorite apochromatic lens design is just not flat enough.
Severe come-like aberration at the corners is the result.
The imaging electronics of the EOS has a built-in and undocumented software noise reduction.
While it works fine for long exposures the unknown effect to the photon count is rendering the 300D almost useless for photometry, of course.
On the other hand the results on pritty pictures awith low noise are very promising.
The EOS 300D has a one-shot-colour chip, of course.
Canon opted for best day time usage by chosing 2 times more green than red and blue filters to reflect the eye's high sensitivity to green.
That is very unfortunate for astro photos of nebula whch are often dominated by red (h alpha nebula) and blue (reflection nebula).
But this is the case for several good CCD cameras like the HX916 from Starlight Xpress as well (though not by filters but by it's sensitivity for certain wavelengths peeking at green)!
Please refer to one of my first nebula shots with the 300D.
After 50 minutes at f/4 the eagle nebula was nicely color saturated and noise free as well.
The setup for astrophotography
My setup of the 300D is allowing for very convient usage and has the features as follows:
- automatic sequences of frames with unlimited exposure time for each frame
- frame names are name of object and optics followed by frame number for easy data reduction
- automatic transfer of raw frames to the laptop without storage in the limited camera's memory
- software focusing aid with FWHM plots, peek values and more
- all camera settings (resolution, ISO and so on) are controlled by the laptop
Setup with Canon accesories, tailored cables and software
- (1) Canon EOS 300D body
- (2) Canon EF-T-ring / Canon EF - M42 adapter
- (3) Telescopes / M42 lenses / EF lenses
- (4) Canon Remote Controller TC-80N3
- (5) Tailored cable to fit EOS 300D, see below
- (6) USB extension lead 3m
- (7) Canon Power Supply ACK-E2 for all night operation
- (8) DSLR Focus V2 from www.dslrfocus.com
- (9) Canon Remote Capture software
The 300D must be set to 'M' and bulb.
ISO setting of 800 or 1600 for faint objects is recommended.
Set the 'Parameter Set' to maximum contrast and saturation when working with jpg compression to extract as much information of the rwa file to the 8 bit jpg.
However any noise sensitive sharpening should be done after data reduction so set this value to neutral.
The sequencing and exposure timing is done by the TC-80N3.
Set number of exposures to zero.
The controller will now continue to take exposures until you later press stop.
Also set the exposure time here.
Set the intervall to 30 seconds to allow for the downloading process of the single frames.
The Canon Remote Capture software shall be connected to the camera during exposing.
Setup the desired frame name and directory here before starting the exposures.
Remote Capture will download and stote the frames automatically.
Connect to DSLR focus and fine tune your focus.
A bad focus is not allowing for good astro photos.
Invest enough time into that process to have the best possible focus.
The cabling of the TC-80N3
Though the TC-80N3 is perfectly compatible with the 300D the plug on its cable is not.
In my case I did not want to lose warranty so I hesitated to cut or unsolder anything.
My solution was, to buy a 2.5 mm stereo jack ready made with a cable.
I connected the cable by directly inserting it into the plug of the TX-80N3.
After a functional test I taped everything with gaffa.
Because the leads of the cable nicely snapped into the plug like the real pins that worked like a charme in my case.
Conclusion
The operation of the 300D when capturing raw frames with my setup is as convient as with astronomical CCD cameras.
The quick adaption of the camera to any telescope or lens by the EF bayonet connection is as fast as my Baader T2 quick connector before.
The fact that a SLR body can be used as an eyepiece for rough centerning the object or rough focusing is a big plus.
Also I like the fact to have a one-shot colour camera now.
Shooting, reducing and aligning seperate LRGB frames are far more demanding and time consuming.
My first tests took place at an ambient temperature of 15 to 20 degrees Celsius, a typical german summer night.
The noise level at that tempereature was fairly low and will further decrease in winter times, of course.
But be aware that the data reduction of raw frames with only 8 bit of luminance per pixel is slightly different from the 16 bit of astronomical CCD cameras.
An article about that subject can be found here: Thoughts about Astronomical Image Processing for Digital Cameras.
Search photos taken with the Canon EOS 300D
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