[Merlin66 (Ken)]

I should explain further....
The newer Canon's have two filters build into them.
The colour compensating filter (#2) and a front (#1) anti-alias/dust filter.
It's usually only #2 filter that is removed to give a "full spectrum" camera mod. This dramatically improves the CCD response to the red region (ie Ha). This #2 filter can be replaced with a clear glass or a Baader UV-IR cut off filter ( 400nm to 700nm)
The claim is made if this filter is removed and NOT replaced that the Auto Focus (AF) is compromised. This is not true.
Also, the addition UV-IR cut filter is NOT required! I found by using the spectroscope that the remaining #1 filter is actually a very good UV-IR cut filter!!! This is never mentioned.

In my case I wanted a FULL, full spectrum coverage (380nm to NIR) for spectroscopy. By removing both the #1 and the #2 filters this can be achieved. Unfortunately by removing these filters, the body can no longer focus standard lenses ( There's 2mm of glass removed from the system). A solution, for EF lenses ONLY, is to add a clear clip in EOS filter (Astronomik MC) - this then allows EF lenses (and my old Zuiko lenses) to focus.

Summary:
Removing #2 filter still allows AF to operate
Removing #2 filter still leaves an effective UV-IR cut off filter
Removing #2 filter significantly improves the red response
Removing #1 & #2 filters gives "FULL, full spectrum" coverage (380nm-NIR)
Removing #1 & #2 filters and adding a clear clip in filter allows the body to focus with EF lenses (or other standard lens)

For "normal" astroimaging, I'd recommend removal of #2 filter.
The attached curves show the response curves of the two filters and clearly shows the UV-IR cut for the #1 filter.
HTH
EDIT: On the Spencer website, they mention the additional removal of the #1 filter - down the bottom of the page....
**** Canon & Nikon SLR users: The Dust Reduction System of some camera models will be removed during the conversion process. Please be sure to disable this function within the settings of your camera. Failure to do so may result in a temporary camera error. If your camera displays a Dust Reduction System error you can still disable the function within the camera menu. ****