9.07.2021 "Skywatcher" DC motor focusing for 2.5" Feather Touch

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 Friday 9th 70-74F, bright start but becoming very cloudy. I had a visual look at the sun in white light and in H-alpha. Apart from some proms in H-a here wasn't anything obvious to see.

The struggle to reach the focuser of my 6" f/10 refractor, from the computer desk, demanded remote focusing. The difference between sharp focus and not-focused, is tiny, but critical to imaging. 

Walking back and forth from the monitor to the focuser is hopeless. The image is constantly changing with the seeing conditions. Which can make the image appear to go in and out of focus.Only by waiting for a really steady moment can focusing be really accurate.

I still had the motor and bracket parts from the 7" [180mm] iStar R35. Which I had made for its 3.5" Feather Touch focuser. The 7" is resting. So I fitted the motor and bracket to the 2.5" focuser instead. 

There was a difference in the dimensions of the tapered, stainless steel ring behind the dual focuser knobs. The 2.5" FT had only a 1" or 25.4mm parallel section. I overcame this problem with a slice of clear 1" hose to make up the difference.

Once the bracket was safely clamped up I could bolt the motor base plate directly to the plastic bracket. The plastic was cut from a kitchen, cutting board. Dirt cheap online and available in black. 

My bracket is ugly. Because I coarse sanded it several times to make it thinner. I could make a new pair of clamping and support brackets if the 7" eventually needs the originals.

A 200t GT2 timing belt does the driving via the black focusing knob. From my own experience it is not worth driving the gold, slow motion knob. It is far too slow even with the motor running flat out. The black knob provides a satisfyingly slow focus movement. 

Note how the motor remains well clear of the focuser's controls. Some very expensive motor drives stick out sideways from the [removed] focuser knob shaft. What, on earth, were they thinking?

Lest ye worry about the " PST Etalon 200mm inside Focus Rule": I roughly focus first using push-pull on one of the 2" extensions after the etalon.  This always leaves the focuser showing 30mm on the graduated scale. Which, from repeated careful measurement, places the PST etalon exactly 200mm inside focus. 

Only fine focusing is carried out by the motor. Which is usually within +/- 1 millimetre of etalon optimum. Not enough to push the etalon off-band. The subtlety of focusing over tiny movements really has to be seen to be appreciated. The focal plane of a quality f/10 or f/12 refractor is not smeared over some indefinable distance. It is sharply in focus. Or it is not.

There is one great advantage of a timing belt drive to a focusing knob: It is the ease with which the belt can be made to slip during manual, focus adjustments. [Coarse or fine] A stepper motor system must, by default, be driven to all points in the focus range.

Much as some Goto mountings must be driven over several minutes of time just to cover 90° of sky. Or even longer to make a Meridian Flip. Where a manual slew would take but a fraction of a second but throw away all hope of the mounting ever finding the sky again. Precision, absolute shaft encoders. Designed to provide constant monitoring of the mountings pointing position. Are still largely unaffordable at this time. Though JMI/Farpoint has a number of add-on systems, I am not familiar with them.

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