Meade 1209 Focuser Upgrade (Again)

Modified Meade 1209 Zero Image Shift MicroFocuser.
"Original" Modified Meade 1209 Zero Image Shift Microfocuser

It's been about 2 years since I modified my Meade 1209 focuser with a stepper and controller. It has worked well, when it comes to moving the payload, but it just has too much backlash for reliable unattended auto-focusing. As an attempt to solve that problem I picked up a Pegasus Astro FocusCube 2 motor kit. It came with a temperature probe and a pair of mounting brackets. There is a hand controller available for manual focusing, but at $73 I can live without it.

The FocusCube 2, unlike other Pegasus Astro focus motors, has the controller built in, so only a 12V power supply and a USB cable are needed. Both were donated by my old focus controller. The temperature probe cable is a 24" cable with a metal probe on the end. I'm going to use it and see how well it works for temperature compensation. The FocusCube 2 does not inherently support temperature compensation, but there is a way using Ekos to refocus each time the temperature changes by n°.

Mount to fit Meade 1209 focuser
Support for the Motor

The slick plastic box around the existing motor and gearbox had to be retired. I built a little frame for the new mounting bracket to grab. Everything is round back there - no flat surfaces, except on the front and back of the roller housing, where there are four #4-40 screws. I used the screws to hold two pieces of 0.22" thick acrylic, with a 1" x 3" piece on top, to which the FocusCube 2's mounting bracket fastened. The acrylic parts were made by Ponoko, using modified SVG source files from the old housing. The svg file is actual size.

The FC2 came with a set of rigid motor shaft couplers. 5mm to 5mm is the closest size. The focuser shaft is actually 3/16" (4.76mm), and the Pegasus shaft is 5mm. The couplers turned out to be 13.8mm in diameter. The roller housing is 15.2mm wide. The net result is the coupler fit with clearance in between the acrylic mounts.

Pegasus FocusCube2 and Meade 1209 focuser.
FocusCube 2 and Meade 1209

The Pegasus Astro FocusCube 2 Universal cost me $253 delivered from Agena Astro. It was $303.84 everywhere else, leading me to think there might be some collusion between retailers. The laser cut acrylic parts cost $36 delivered. One might put something together for less, like maybe 3D printing a complete mount (2 sides and a top) in one piece.

The newly modified focuser.
The Other Side

Specifications

  1. The total range, stopping just shy of the hard stops, is 0 to 4950 steps.
  2. The total for one revolution is 5755 steps, so it isn't half-stepping.
  3. The total travel distance is 11.4mm.
  4. The resolution is 2.3μm per step.
  5. Time from 0 to 4950 steps is 25 seconds.
  6. The distance the motor sticks up above the top of the focuser at 0 steps is 6.9mm.
  7. The range of the focuser is 86% of a revolution, or 309.64°

#3 is arrived at by subtracting the 0 position (+1mm) from the full out 4950 position (+12.4mm).
#6 is the approximate amount the motor interferes with the filter wheel. I added two 2" parfocalizing rings to the front of the filter wheel to prevent a collision between the filter wheel body and the focus motor.

The Indi Pegasus FocusCube driver supports different speeds via the maximum speed. The actual average step rate is 190 steps per second, on a long slew, when set for 400. It is probably 200 before the acceleration and deceleration. It can be changed to a value higher than the 400 stated maximum, but skips at values over 600. That is with no load on the focuser. Setting the maximum to values lower than 400 causes the motor to vibrate more and make a little more noise. At 400, it is nearly silent. Under load, it makes a little more noise.

Does It Work?

I ran it many times, getting the focus as good as I could manually, then autofocusing. Each time, the autofocusing was as good as the manual focus, and the manual focus was better than manual on the old geared stepper. There is very little, if any, backlash in the FocusCube 2. The autofocus app always found best focus in two or three passes, and I couldn't improve it manually. That is exactly what I had hoped for. I was using a step size of 25 for both manual and autofocusing. Reducing the step size might improve focus a tiny bit for manual focusing. The autofocusing code will calculate down to a single step when it determines the best focus, so it doesn't need smaller steps.

So, the total cost of the project, if I were to build it from scratch:

ItemCost
Meade 1209$239.00
Pegasus FocusCube 2$253.00
Plastic parts$36.00
TOTAL$528.00

That's not cheap, but it is a really nice setup. I don't know that I would do this as opposed to buying a new focuser, but if you already have a 1209, this is a good way to make it useful.

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