Maritime zwo did not design it for use with narrow band filters
When I pointed out the SBIG scheme to ZWO a couple of years ago, I had warned them that any filter that is used for the main sensor will also block light to the guide sensor.
While that is good for broadband "light pollution" filters, it will make guiding virtually impossible with the narrow band filters.
It is too bad that they did not include the same warnings for their customers.
Here is the history of the SBIG ST7 (the first of the cameras that had dual main/guide sensor, introduced in October 1994):
http://www.company7.com/sbig/products/st7.html
SBIG was also the first commercial outfit to implement autoguifing, with their ST4 guide camera. The "ST4" interface for controlling the mount has stuck, and we still call that interface "ST4" today.
They are now part of Diffraction Limited (the folks who bring us adaptive optics, where you guide a small prism instead of the mount, at speeds that can freeze "seeing").
Chen
