Focusing is the worry of all imaging astronomers: of course the images must be as sharp as possible. Because of turbulence this is often not easy to see and therefore I have made some simple but effective tools to assist me in achieving focus.

Some simple tools

When I am too far out of focus I do not see anything on my screen, even when I have properly aimed at a bright star.
Because my focal plane changes when I use e.g. the Focal Reducer or one of my Barlows, I want to put my focuser in coarse focus, so that I at least see SOMETHING on my screen. I have measured the required distances between the lower end of my focuser and my OTA and I have made small spacers of the proper length and labelled them.
I guess the pictures speak for themselves.
After this coarse focusing I fine tune the focusing with my Hartmann Mask and electric focuser.
An even better approach for fine focusing is Diffraction Spikes Focusing.
I put this - home made from hard plastic - Hartmann Mask at the sky end of my telescope [there are two pins at the back side of the mask]. When I aim at a bright star I will see 4 triangles on my screen. When I improve the focus with the electric focuser [see further on this page] these triangles move to each other. When they have merged completely I have reached focus.
An even better approach for fine focusing is Diffraction Spikes Focusing.

Home made Electric Focuser (Note: Click here for my LX200 electric focuser)

The problem is that during an imaging session the camera has taken the place where normally the eye piece sits.
This has the result that the astronomer is working in the blind: he can only see what the webcam sees, and that is on his computer screen. Because of the very restricted view of the webcam (roughly a pin's head at arm length) already a slight touching of the telescope makes that the telescope has lost its aim ... and the astronomer is in big trouble: he has to start all over again.
Together with my very good friend Peter Katreniak I worked out an idea I had picked up from a German Tal-2M user: Rudy Prinz

Home Made Electric Focuser

Peter Katreniak has an excellent description on his website and I recommend you take a good look there, especially for the sophisticated electronics!



Material used:
1 Motor, 12V, 120 rpm, gearbox reduction: 1:50
1 worm wheel + cogwheel (= with teeth)1:30
1 Polarity switch 12V
2 jubilee clips

Note: Motor has - after gearbox reduction - 2 revolutions per second. This means that the cogwheel makes one full turn around in 15 seconds (30 teeth).

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Experience Report #1
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First the worm wheel needed to be mounted on the axle of the motor. As the axle of the motor was too thick for the worm wheel, the diameter of the axle needed to be reduced. I clamped the motor in my workbench, so that it could not move (but it could run!)
I applied 12V DC (from my car battery charger) to the motor to make it running. Then I used a file to reduce the diameter of the axis. This went rather easy. I had a new (=sharp) file and in 5-10 minutes the job was done. Of course I frequently stopped to check how I was doing. Using a small (rubber) hammer I ticked the worm wheel over the now reduced motor axle. Done.

I made an improvised platform of wood (easier to modify) and mounted the motor on it. I made 2 oblong holes in the wood and threaded the jubilee clip - which I had opened completely - through the oblong holes. Then I closed the jubilee clip again and fastened the motor. Under the motor I had to put some pieces of wood to correct the angle on the cogwheel.

I removed the right focusser wheel and found that there was an axle with winding. The cogwheel fitted over the axle and I had in my goodies box a nut with the proper winding. I fastened the cogwheel as good as I could.

I mounted the wooden platform with motor on my scope.

Applied 12V and presto: it turned! .... but only for a short while.
The worm wheel came loose from the motor axle .... and now the superglue is drying. I hope it will fasten good enough now.

Another small problem: the cogwheel was not fastened good enough on the focuser's axle. I will a my friend to drill a small hole with winding (3mm) and a fitting bolt, so that the cogwheel can be mounted and/or dismounted.

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Experience Report #2
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I made a platform of hard plastic - called TRESPA - (3 mm thick) and gave it the correct shape so that it would nicely fit in place. I took care that this platform was made to make as much contact with the focusser as possible: I had to saw some curves to enable the platform to slide more far over the base of the focusser. I made 2 oblong holes in the plastic and threaded the jubilee clip - which I had opened completely - through the oblong holes.

Then I closed the jubilee clip again and fastened the motor. Underneath the motor I had put the lower (=wall) part of a plastic bracket, which is normally used to mount plastic pipe to a wall. This bracket already had a nice curve in which the motor can rest. By moving this bracket forward and backward under the motor, it is possible to adjust the tilt of the motor and consequently also the pressure between the worm wheel and the cogwheel.

I am thinking of using this mechanism also to detach the motor from the cogwheel by means of a bolt, so that the tilt (=angle) of the motor can be manipulated in normal operation: in contact with the cogwheel or lifted from the cogwheel to enable manual operation of the focusser. This is hard to explain but I hope you will understand ...

I removed the right focusser wheel and found that there was an axle with winding. The cogwheel fitted over the axle and I had in my goodies box a nut with the proper winding. I fastened the cogwheel as good as I could. Alas: it was not fastened good enough and the cogwheel slipped. A friend of mine drilled a 3mm hole in the 'bus' of the cogwheel and I had the proper 3mm bolt in my goodies box. Alas again: the cogwheel still slipped.

I used a grinder to make a flat surface on the axle of the focusser, so that the bolt would have more grip and would not slip anymore. This worked fine!!!

I super glued the worm wheel to the motor axle, and now the worm wheel is rock steady on the motor axle.

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Experience Report #3
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The total travel time from total IN to total OUT is (on my scope) 25 seconds. So averagely 12,5 seconds. I can live with this easily. Normally we have only small adjustments to make: first (Barlow with) low power eyepiece (this also is the biggest adjustment) and after that only minor changes.
So on my scope - for normal usage - I will not make a mechanism to detach the worm wheel from the cogwheel to enable manual operation.

I use 2 jubilee clips now to control the tilt of the motor. I have enclosed some pictures so that you can see what I did. On all pictures you can see how I attached the motor at the plastic base. On Fig. 3 you can see the bracket underneath the motor. It is a bit in de middle of the motor, so that by tightening/loosening the jubilee clips I can adjust the tilt of the motor. This works very nicely.

In the odd case I would like to go to manual operation I would just have to loosen the jubilee clip closest to the worm wheel and tighten the other one, so that worm wheel and cogwheel do not touch anymore.

I am happy that so far it went so well!

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Experience Report #4
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APM Motor corrector (top) and electric focuser control (bottom) station

Now comes the hard part (for me). Electronics …. I know nothing about it, so I need good handholding. And Peter Katreniak held my hand! For your convenience I have written down in layman's terms what I did.

Peter supplied a diagram that I can - more or less - read (but not understand). You can find the diagram(s) on Peter's page about the electric focusser

The idea of the electronics is twofold:
a. To protect the switch, the motor and the relay from burning in and other nasty things. I assume that has to do with little sparks that decrease the lifetime of the components.
b. To reduce the motor speed by 50%, controlled by a switch, so that focussing can be even better!

Although I think Peter will supply a list of what to buy but I also give it here:

6 diodes 1N4148
1 Zener diode BZXB5V006.2 or another Zener diode with 6.2V Zener voltage and power 1.3W
1 condenser 100V
1 on/off switch to control the speed (I have chosen a toggle switch, so that I can feel the position the switch is in)

Total costs (in The Netherlands) approx US$ 4,-- so almost nothing.

PROTECTION
Look at the diodes carefully: they have a dark or black stripe on one end: that stripe is the same one as you can see in Peter's diagram (the diodes are the things with a triangle and a vertical line or stripe). Make sure you mount the diodes pointing into the correct direction: they are polarity sensitive.
When soldering the diodes you also have to take care that they do not get too much heat: best is to use a tool (tweezers) and hold that between the body of the diode and the soldering and: this way you can absorb the heat before it hits the diode itself.

The positive pole (the plus) is on top of the diagram. Make sure you make no mistakes with the polarity!!

The condenser must be soldered over the 2 contacts of the motor itself (no polarity here…)

SPEED CONTROL
The Zener diode must be put 'over' the new switch, thus in effect bypassing the switch. The speed control is based on the principle that the electricity always takes the easy way.
When the switch is in ON position the current takes the easy way: straight through the switch. However, when the switch is OFF, the electricity has only one way to go: via the Zener diode, and only 50% of the electricity can get through the Zener diode, so instead of 12 Volt the motor only gets 6 Volts. Voila: speed control.

PERFORMANCE
The total travel distance (from top to bottom) of the focuser of my scope (a TAL-2M) is 28 millimetres.
At normal speed this is covered in 25 seconds or 1,12 mm per second.
At low speed it takes 44 seconds or 0,64 mm per second.
I wanted to know the smallest 'step-size' I could achieve in low speed operation, so I manually gave 100 short pulses by using the switch and then measured the distance covered by the focuser: 6 millimetres. This means a step-size of 0,06 millimetres, which is quite nice for fine focussing!