There are a few reasons why filter replacement has generally been a better option than a simple filter removal:

• The sensor surface remains protected.
• UV and IR light are blocked by the replacement filter
• The camera's autofocus is retained.

Even though filters for the 400D, 450D, 500D, 550D, 40D, 50D and 1000D models are now once again freely available, it's worth going in to some detail on the differences between the filter removal vs the filter replacement for these models.

Firstly, it's important to note that this does not effect astronomisers stand point on the 20D, 30D, 300D, 350D and D60. astronomiser recommends that you have the filters for these cameras replaced to protect the camera's sensor, block unwanted UV and IR light which are not brought to the same focus point as visible light when using a refractor and can cause bloating, and to retain auto-focus.

It may now be better for you to consider a filter removal rather than a replacement if you own a dual filter camera, i.e., 400D, 450D, 500D, 550D, 40D, 50D or 1000D.

The following looks at each of the above points in detail with respect to the dual filter cameras.

Protecting the sensor surface.

In a dual filter camera, the rear of the two filters blocks UV and IR and visible light well in to the red end of the spectrum, only transmitting an average of around 25% of the available red light which is why this filter is removed. This leaves the front filter in place and the sensor remains sealed and protected and the piezo vibrator which removes small dust particles from the front filter remains fully operational.

Blocking UV and IR light.

In a dual filter camera,the front filter blocks UV and IR light with a very sharp cut off at the edges of the visible spectrum. This is information that has come to light recently when a well known filter manufacturer tested these filters after noting that they blocked IR light. The filter blocks UV and IR very effectively and is of very high quality. This means that no second filter is required to block UV and IR and the Baader filter would not do a better job.

Autofocus.

A majority of people who have their cameras modified for astronomy only use them for this purpose, so retaining the autofocus is not important as focussing is done manually either through a lens or telescope, so removing the rear filter is generally the better option for these people when modifying a dual filter camera. As it is, the filter that is removed is only 0.8mm thick so the difference in the focus point when considering the refractive index of glass vs the refractive index of air is very slight. In practice, this means that the difference is practically unnoticeable when taking everyday snaps. When using very 'fast' lenses or ones with a very short focal length, i.e., for macros and fish-eye shots, this slight mis-focus becomes more noticeable.

In conclusion, because of the above factors, astronomiser now equally recommends filter removal or replacement for dual filter cameras, depending on how the camera will be used. The cost of removal is £150, £40 cheaper than replacement so we feel that this may better suit the majority of our customers.

Andy Ellis, 14 April 2010.