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Old 26-08-2007, 06:23 PM
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Digital Focuser

I have a problem as Astronomik filters and Hutech filters have a slightly different focus position with the same optical train. To get a set of narrowband and wideband images is difficult as you need a bright star to focus visually.
Here is a solution stick a digital vernier caliper on the focuser. I have established there is 1.4mm difference between the two types of filters and if you stack two Hutech filters 2.8mm difference.
Below are two pictures of the caliper and method of mounting (hose clip and superglue!). An animated gif with focus changing by 0.2mm between each frame. And a pic of Terra.

Bert
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Old 26-08-2007, 06:48 PM
Dennis
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Nice job Bert, and very nicely illustrated via the animation.

Cheers

Dennis
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  #3  
Old 26-08-2007, 08:20 PM
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Thanks Dennis. What is more evil I reckon I can get a 150mm F5 Achro to perform like an APO by using filters and knowing where focus is for a given wavelength. The only complicating factor is does field curvature vary with wavelength? As far as I know it should not.
Next set of experiments!

Bert
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Old 27-08-2007, 08:35 AM
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Quote:
Originally Posted by avandonk View Post
Thanks Dennis. What is more evil I reckon I can get a 150mm F5 Achro to perform like an APO by using filters and knowing where focus is for a given wavelength. The only complicating factor is does field curvature vary with wavelength? As far as I know it should not.
Next set of experiments!

Bert
It does... this depends on the optical design, though. And the effects are more prominent away from the optical axis.
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  #5  
Old 27-08-2007, 09:50 AM
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h0ughy (David)
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Fantastic Bert, you need a design award for that effort. do the calipers deflect if the worm is overtightened?
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Old 28-08-2007, 04:26 PM
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No its fine h0ughy the 100mm calipers are solid. I think these from Jaycar should be easier to mount as they are made of carbon fibre composite and can be drilled easily. I think that 0.1mm accuracy is all that is needed.
I tested more fully last night and the focus positions are easily repeatable and far better than focussing visually.
Below is a 100% crop at focus of a 1 min exposure at 1600ISO taken with a O3 filter.

Bert
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Old 29-08-2007, 04:19 PM
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I was using printed setting circles program for this...
http://www.startrak.co.uk/MakingSettingCircles.htm
To make the vernier, I printed one set of short tracks (Setcirc.exe, altitude, 70mm) with resolution of 600dpi. as main scale.
Then I printed another set, with resolution of 660dpi, as vernier.
After a bit of cutting with scalpel and gluing with sticky tape and I have good vernier with resolution of 0.1mm for the cost of two sheets of printer paper (or plastic).
It is not digital, but still accurate enough :-)

Edit: picture added for clarity...
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Last edited by bojan; 29-08-2007 at 06:46 PM. Reason: Picture added
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