All about Canon Filters.

The Canon Standard Filter Set

Since the introduction of the Canon 450D and the Canon 1000D, Canon have used a standard filter set in all of their cameras. The main components of this set can be seen in the image on the right. The four components are:

The Outer Filter Retaining Spring (top left)

The retaining spring attaches to the plastic filter mounting frame and holds the outer of the two filters in place.

The Anti-aliasing/ UV/IR Cut filter (top right)

This filter cuts UV and IR light and is also anti-aliasing (removes odd effects when shooting patterned surfaces). For an astrophotography mod, this filter normally stays in place and blocks >97% of UV and IR light. This filter sometimes has an integral piezo-vibrator which is used to shake small particles of dust from the filter when the camera is switched off and on (this feature is not included with the base model dSLRs, the 1000D, 1100D, 1200D and 1300D)

The UV/IR Cut and Colour Correcting filter (bottom left)

This filter also cuts UV and IR but crucially also adapts the sensor to work in normal daylight conditions by vastly reducing the camera sensor's response to red light. In the Hydrogen Alpha band, this reduces the amount of light allowed through to approximately 25%. For astrophotography, this filter is removed to increase sensitivity to Hydrogen Alpha to around 97%.

The Sensor (bottom right)

A large CMOS type sensor encased in a steel body, supported by a heavy steel frame and covered by an SiO2 (quartz) coated plain glass heavy cover slip. The sensor is well protected and the quartz covered surface can be carefully cleaned if it is exposed (as in a full spectrum mod), though any damage to this surface will result in the need to replace the whole sensor, so it should only be done with extreme care using the correct materials.

Rear Filter Removal with Sensor Repositioning Modification

For astrophotography, astronomiser recommends a rear filter removal with sensor repositioning.

This mod involves removing the UV/IR Cut and Colour Correction filter and moving the sensor forwards by 0.21mm to correct for the loss of 0.7mm of glass (due to the differences in the refractive indices of air and glass). The front filter remains and blocks the vast majority of the UV and IR light which is not wanted for normal astroimaging because, with refractors, this UV and IR light is not brought to focus at the same point as visible light and causes 'star bloating' where stars appear as larger white discs rather than pinpoints of light.

Please note - red 'haloes' around stars are not bloating and are not due to the passage of IR light. These are usually caused by internal reflections within the imaging train, often by light pollution filters, or in the days when cameras were modded using a replacement filter (i.e., a Baader ACF or BCF filter) by the replacement filter itself.

Full Spectrum Filter Mod

The full spectrum mod involves removing both filters.

If you wish to then use the camera for astrophotography you will need to block UV and IR light to avoid star-bloating. Certain types of light pollution filter, e.g., the astronomik CLS Type II or CCD filter, will block UV and IR light and are a neat solution for both blocking light pollution, UV/IR light and protecting the sensor's quartz coated cover slip from getting dusty and dirty.

The only advantage to having the full spectrum mod carried out on your dSLR is to allow it to be used for daytime IR photography (with an appropriate IR filter in place) and in spectroscopy, when measuring the emission lines of a star or other astronomical object. Astronomiser recommends that if you intend to use a camera for astrophotography, that you get the camera modded with the rear filter removal mod only and keep the camera solely for use in astroimaging. This reduces the chances of getting dust on the front filter which shows up badly in astro-shots, but wouldn't normally be visible in daylight shots.