Stepper focuser for lenses

avandonk · #1 23-07-2006, 08:08 AM

Here is a simple stepper controlled focuser for lenses. It consists of a geared stepper motor 7.5 deg per step (48/rev) and a gearbox with a ratio of 1350:1.
Available from
http://www.oatleyelectronics.com/stepper.html
for only $16. I glued a small 10mm gear from a dead printer to the shaft.The belt is also from a dead printer. I used a stepper motor controller Cat no AA0352 from Jaycar. But any suitable one would do.

Below are a series of sequential crops of images taken 1000 steps apart. 1000 steps corresponds to 0.2 mm linear movement at the lens focus ring.

This will work for any lens and get very accurate focus as you can go back to a known number of steps. It is also better if you go past the focus point and approach from the same direction as the test exposures to eliminate any backlash in the gearbox.

Bert

avandonk · #2 23-07-2006, 08:11 AM

Here is a picture of the focuser. Note the tension spring to eliminate any slack in the belt.

Here is an image taken with a Canon 85mm F1.8 at F2.8 focussed with the stepper motor.
1.0MB
http://members.optusnet.com.au/~cheekyfish/SAG.jpg

and with the Canon 300mm F2.8L at F2.8 focussed with the stepper M11 Wild Duck
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http://members.optusnet.com.au/~trlee8/WDS2.jpg

I think you will agree that very accurate focus is noticeable in any final astrophotograph no matter what focal length.
It certainly surprised me.
Bert

Astroman (Andrew Wall) · #3 23-07-2006, 01:14 PM

Thats a similar setup to what they can do with the robofocus. Nice work there Bert, beautiful lens too..

avandonk · #4 23-07-2006, 01:42 PM

Quote:

Originally Posted by Astroman

Thats a similar setup to what they can do with the robofocus. Nice work there Bert, beautiful lens too..

Not only is she beautiful, she performs beautifully as well!

Bert

iceman (Mike) · #5 23-07-2006, 04:07 PM

That's excellent Bert, you're such a tinkerer! A great idea and very well constructed.

Mighty_oz (Marcus) · #6 25-07-2006, 04:39 PM

Hi, amazing ! Been looking for a cheap way to focus my telescope. What software are u using to control the stepper with ?

Marcus

Striker (Tony) · #7 25-07-2006, 04:53 PM

I think you should make them and call them BertsFocus...you may give Robofocus a run for their money.

Looks very practical...could do with some cosmetic enhancements but well done Berty Beetle.

avandonk · #8 26-07-2006, 09:59 PM

Test animated gif

avandonk · #9 30-07-2006, 11:54 AM

Thought it may be of interest how the lens temperature is monitored. These thermistor temperature indicators are the sort used in PC's to indicate temps.
These use 5V and I just mounted them using velcro. The thermistor on the lens is covered by a bit of polystyrene again held with velcro so it measures the lens body temperature at it's thickest part the other measures ambient air temperature.
It turns out by coincidence that for every degree drop in temperature of the lens the focus needs to be moved by 1000 steps. I have found this so far to be linear from 1 to 12 degrees C.

Bert

h0ughy (David) · #10 30-07-2006, 12:15 PM

excellent work and resut Bert. I too have noticed that the focus changes with temperature, especially noticed it at duckadang, I originally focussed the ed80 about 1 hours after sunset and it was 15 degrees then had to do it again 2 hours later when it was 7 degrees and the focus was out heaps. I like what you have done to monitor this change and the stepper motor. I am not that clever as to make one of these, but full credit to you as you are truely a resourceful man!