DIY Remote focusing devices

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[whrudey]
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DIY Remote focusing devices

Post by [whrudey] » Mon Jan 04, 2016 7:08 pm

Often masterful digital solar images are compromised by poor focus. Whilst post processing may salvage some, ultimately the best course is to start with a properly focused image. I think the solution is to use a remote focusing system. Although a number are available commercially, many are quite expensive and, cheaper generic versions are at best a compromise. Ultimately, one retires to the drafting board, or CAD system and, then to one’s shop to craft a custom and functional design.
Over the years, I’ve had the opportunity to do so for a variety of scopes (5 to date) and would like to share some of these results. Though several YouTube videos are available on this topic, little can be found on searching the SolarChat site.

Our Observatory is an 18 foot by 40 foot concrete block building with a powered roll-off roof over the elevated observing deck. The classroom, to the left, comfortably seats 18 students with desks and, is equipped with a ceiling mounted hi-res LCD projector which can project real time images from any of the several available scopes. The roof rolls over the classroom and, for solar observing, all the solar scopes are close to the end such that the roof need only open partially, providing shade for the operator.
Obs bldg.jpg
In my setup, the scopes may be as far away as 8 feet from the Desktop PC Monitor in the Observation deck. Even with using a laptop adjacent to the scope, manually fine focusing the scope results in vibrations that require time to dampen. The remote system solves these issues.

A key requirement in the design is that no irreversible changes be made to the scope or its focusing mechanism. Thus, when the remote focuser is removed, the scope focusing mechanism should remain unscathed.

Generally, one will need a small yet powerful (around 70 rpm) 12 volt DC gearhead motor (http://www.hobbyengineering.com/H2532.html), a pair of small and larger plastic spur gears (https://sdp-si.com/eStore/Catalog/Group/207), a Pulse Width Modulated speed control (http://www.hobbyengineering.com/H1742.html), a variety of aluminum extrusions and stock (cuttings from your local machine shop) and, the appropriate tools and skills – or, a friend with such.
An alternative for motors is to use small stepper motors. They would afford finer adjustment but, are a little more complex in terms of obtaining a suitable controller card. Those with skill in electronics would likely have few problems.
The following images show the small, yet powerful, 12 volt DC 72 rpm gearhead motor and the PWM kit which, can be assembled in less than an hour with a little soldering skill.
GHmotor PWM.jpg
GHmotor PWM.jpg (31.19 KiB) Viewed 1600 times
The basic design involves mounting the gearhead motor in a short length of 9/16” ID aluminum tube which in turn attaches to the base via a swinging and spring loaded arm. The larger spur gear is fitted to the rear of a thick plastic disk which has been bored out to have a friction fit over the existing fine focus knob. An elastic band provides tension on the swing arm to engage the two spur gears. Adjustable spring systems can be used but, the simplest is still the elastic band. The cable from the gearhead motor then attaches with a suitable length extension cable to the PWM controller/power supply which should be equipped with a momentary ON switch (a doorbell switch works fine) as well as the bidirectional speed control. Thus, whilst comfortably observing the monitor screen with the capture software ready to go, one can focus and quickly click Capture Start when focus is achieved.

The following image shows both the desk controller as well as a portable controller for field use. Each has a momentary ON switch for activation and, both speed and direction control via a knob. Indicator lights confirm the motor is being energized.
PWM control.jpg
The first example is on my Meade LS-8 equipped with the very essential Feather Touch fine focuser. The base clamps around the focuser tube (not too tight though, as the tubing is thin walled). The motor tube swings off the base and tension is provided by an elastic band. Rather than use spur gears, I chose to machine two “pulleys” and fit rubber “O” rings – 1 on the motor shaft and, 2 on the focuser knob. These easily engage and have sufficient friction to function well. The original Feather Touch fine knob was replaced with the custom one shown – it was placed in a “safe place” but, I’ve since forgotten where that was.
LS8 focus.jpg
The Lunt 60 PTDS remote focuser was a little more challenging as fitting the unit without drilling holes in the focuser was necessary. The following image shows the unit in place – this time utilizing a pair of plastic spur gears.
Lunt focus.jpg
The remote focuser for my TS 115mm APO triplet was relatively easy as there were a number of available threaded holes in the 2” focuser. This unit uses a pair of plastic spur gears but now, tension is provided by a steel spring adjustable with a brass thumb screw. The portable remote controller is also shown:
TS focus.jpg
Both our 12.5” Newtonian mounted on a split-ring equatorial mount (https://www.youtube.com/watch?v=55j-x5mXJ68) and, my new 8” Solar Newton utilize 2” Crayford style focusers from GSO. Each use a pair of plastic spur gears with tension provided by elastic bands.

8” Solar Newton:
SN focus.jpg
12.5” Newtonian:
PT focus2.jpg
Overall, these units have made digital astro imaging so much easier – and, at a fraction of the cost of store bought equivalents. I regard them as essential and, any new scope I acquire, will automatically be provided with one.
The above was intended as a general discussion however, I am willing to discuss more specific design and construction details with anyone interested in pursuing this route.

Do hope this was of interest and perhaps helpful to someone.

Bill

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Re: DIY Remote focusing devices

Post by marktownley » Sun Jan 10, 2016 9:10 am

Nice engineering there! I like it :)
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Re: DIY Remote focusing devices

Post by Derek Klepp » Mon Jan 11, 2016 5:16 am

An excellent article Bill.Hopefulley Mark can put it in the library section for reference

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Re: DIY Remote focusing devices

Post by marktownley » Mon Jan 11, 2016 5:59 am

Derek Klepp wrote:Hopefulley Mark can put it in the library section for reference
Good plan!
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Re: DIY Remote focusing devices

Post by 04Stefan07 » Wed Jan 20, 2016 3:40 am

Looks great.

Are you looking to make/sell some in the future?
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