Sorry, I missed the S/H bit. I thought you were talking about A$18k worth of kit. S/H would have been a bit cheaper, but alas limited options such as astrosital glass etc. Still kick butt none the less.
I prefer on-axis than off-axis, but there are merits in both methods. I use on-axis, even with the narrow band Ha filter (which is typically darker than SII and OIII filters). Simply bin the guide chip and increase exposure times between 5-10 sec. Works a treat. Helps to have an accurate tracking mount (PEC enabled) and good polar alignment. I don’t have problems in taking 30min subs on-axis guided with the Titan. If the Titan can do it, the ME will do it easily. Finding guide stars isn’t an issue with an instrument rotator. However, for robotic setups such as yours you need to consider usability – don’t want the system to be difficult/complex to use hence the beam splitter would make it attractive. The acp3 interface does simplify things.
I don’t think I can explain the native configuration better than Russ. It has nothing to do with camera type or size, but how the optician has figured the hyperbolic mirror sets and the optimal distance between the primary and secondary mirrors of the RC.
“A consequence of the Richey-Chrétien design is that the spacing between the primary and secondary mirror must be kept very near the value intended by the optician who figured the mirrors. If this spacing is allowed to deviate much more than a few millimeters, significant deterioration of performance will result due to spherical aberration.
The method of focusing by moving the secondary mirror is thus at odds with maintaining the performance of the system. The solution is to locate the instrument, be it a CCD camera, an eyepiece or what have you, close to the focal point of the telescope with the secondary mirror at its proper distance from the primary. The secondary focuser can then be moved only for fine adjustment of the focus.
Adding a focal reducer into the mix complicates matters. With the reducer in place, where is the new focal point of the system? How does one determine the correct position of the reducer such that the system will be nearly in focus without having to move the secondary mirror from its optimum position?” – Reference -
http://www.rc-astro.com/resources/reducer.html
I suggest you read the above link.
Feel certain Brad will know about the spacing requirements etc.