I have been doing some serious thought and calculations based on how one reaches the focal length in this case 1200mm for the SW880 The theory applies to both the R&P and Crayfords The average 10" dob R&P has a profile of 4" The GSO type Crayford has a profile of 3" and Moonlight's Low Profile a max of 2", based on the following calculations which are copied from an email I sent to Roger a local member. The outcome would necessitate the use of a longer drawtube (2.75") and a 50mm extension for the EP.
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Apparently the problem I am experiencing is common with dobs, the owner of IIS forum has a sticky on the subject and seems that apart from raising the primary as I did on the 8" there is not much one can do. In my case a low profile Moonlite may resolve it with a few mm to spare. The focal length of the 10" with the R&P is as follows (total 1200mm)
Primary to secondary 915mm
Secondary to Tube 140mm
Tube to focal point, 145m = Total 1200mm
The R&P has a 4" (103.0mm) profile The Crayford GSO - has a 3" (76.0mm profile) this from the tube to the face of the 2" socket and does not include 1.25adapter
The CMOS sensor to the camera face is 35mm and the T-Ring and T-Ring adapter 25mm which gives me 60mm with the present setup which puts it 60mm beyond the focal point.
The T-Ring has an 1.25" insert held by 3 set screws which can be removed and by replacing it with a 2" barrel to go directly into the 2" focuser socket.
This would give me a camera profile of 35+15mm = 50mm
A Moonlite low profile 2" focuser has a profile of only 50mm to the 2" face fully in. and 100mm fully extended Thus with the focal point being 145mm from the tube the camera adding 50mm "should" focus around the center. "theoretically" It would mean of course adding an extension for the EP's to compensate for the loss.
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Late Update 6.1.07
I have since been in touch with Steve at JMI in CO USA
http://www.jimsmobile.com/data_ev_preview_frame.htm
and the do an EV Focuser and one can choose the options in the "Build your own" section where you choose which features you want or wish to leave out, like reducers etc.
They make a PWM controller and software is available for around $140US.
But the motors can be be easilly controlled with a home brew 555 IC circuit which I have used for many years to test model servos and speed controllers on my scale model ships. With some good soldering and etching skills, the controller which will give a PWM output from <1RPM to >10,000RPM (a tad fast for the focuser) for around $25 of bits from Jaycar etc Its also possible to add limit switches and preset positioning if you are into electronics.
Cheers
