Astrophotography with digital cameras
Since there are always questions about camera modifications and confusion about the various terms, we have compiled some information here.
How is a digital camera constructed?
All digital cameras, whether DSLR, system, compact or mobile phone cameras, have the following components in their optical path starting from the lens: ① UV and IR blocking filter ② colour correction filter ③ low-pass filter ④ CMOS sensor. The numbers refer to the illustration above.
The function of the UV and IR blocking filter ①
The UV and IR blocking filter blocks light outside the spectrum visible to humans, as otherwise the CMOS sensor would capture an image that differs greatly from the image perceived by humans. In addition, optical errors in lenses are much more pronounced outside the visible spectrum, which is why UV and IR light should not reach the sensor. This filter appears colourless to the eye and allows almost 100% of H-alpha light to pass through.
The function of the colour correction filter ②
The colour correction filter changes the spectral sensitivity of the camera so that it better matches the sensitivity of the human eye. To achieve this, the red portion of the light is reduced, which is why the filter has a blue-green colour. However, the typical colour correction filter only allows 25% of the incoming H-alpha light from hydrogen nebulae, which is so interesting for astrophotography, to pass through.
The function of the low-pass filter ③
The low-pass filter ensures minimal blurring of the image so that the RGB Bayer matrix of the CMOS sensor does not cause image errors. Without such a filter, coloured patterns would be visible at sharply defined, large differences in brightness. These unwanted patterns are called Moiré patterns.
The function of the sensor ④
The CMOS sensor built into modern digital cameras converts the light captured by the lens, which has been modified by the filters, into the desired digital image.
Differences between manufacturers
Depending on the brand and model of camera, some or all of the built-in filters are combined into filter packages. For example, in current Canon cameras, colour correction and low-pass filters are combined into a single component. The exact design can have a significant impact on how a camera can be modified. The order of the components may also be changed.
What needs to be changed for astro-modification?
For astro-modification, only the colour correction filter actually needs to be removed. However, removing the filter increases the flange focal distance, changing the optical distance between the lens and the sensor.
Significance of the flange focal distance
The flange focal distance refers to the distance between the sensor and the mounting surface of the lens.
The accuracy with which the flange focal distance must be maintained in a camera depends on the intended use.
If the colour correction filter is simply removed, the flange focal distance usually increases by about 0.2 mm. If a package of all filters is removed, the flange focal distance can increase by up to 0.7 mm. The flange focal distance increases because the mechanical length of optical glass elements is greater than their optical length.
With a flange focal distance that is increased by more than 0.2 mm, it is usually not possible to use lenses on the camera for astrophotography. The lenses can no longer focus to infinity. However, it is possible to use the camera on a telescope without any problems.
With a flange focal distance that is increased by less than 0.1 mm, normal and telephoto lenses can usually be used. For wide-angle lenses, however, the flange focal distance is usually still too large.
With a flange focal distance that is increased by a maximum of 0.05 mm, all types of lenses can be used. However, this accuracy is not sufficient for the autofocus to work.
For the autofocus to function properly, the flange focal distance must be maintained at a fraction of the pixel size. For example, the pixels on a Canon EOS 800D are 3.7 µm in size. The necessary accuracy is not achieved during the production of the camera by adhering to extremely low tolerances, but by measuring the camera and storing the measured value in the camera. This is how we modify our cameras for astrophotography.
Suitability of other modification variants for astrophotography
The following list only refers to spectral sensitivity; the explanations above apply to the flange focal distance.
MC-modification for IR-photography: Astrophotography is possible with an additional good UV-IR blocking filter.
UV-modification: Since uncoated special glass is used to achieve maximum UV-sensitivity, disturbing reflections are to be expected in astrophotography.
An IR-modification, in which the filter stack is replaced by an IR pass filter, is unsuitable for astrophotography.
If all filters are removed, the camera achieves maximum sensitivity, but the susceptibility to visible dust on the sensor increases dramatically. With an additional good UV-IR blocking filter, astrophotography is possible with telescopes.
