Yet more cloud forces a new project - autofocusing lenses

[steppenwolf]

Cloud, cloud and more cloud – but looking on the positive side, this presents an opportunity for a ‘project’. Now, I’ve already done the observatory ‘spring clean’ – I even have the photos to prove it – so I thought I’d tackle a project that has been on the back burner for years; auto-focusing my camera lens(s) for wide-field imaging.

Despite two attempts at putting the TS Telefokus 105 microfocuser on my Christmas list so that I could accurately focus my Canon 200L telephoto lens on the stars, there were no takers so the project stalled. However, having bought a second motor for my existing autofocus controller to install on a new telescope got me thinking that maybe this would actually be a better way of controlling focus and might even allow autofocus during a long imaging session to compensate for temperature changes.

So, I put a third motor on my birthday list and my two sons clubbed together to buy it for me moving the project forwards! Yesterday, I completed it with the addition of a finder-guider that I had in my ‘goodies’ box that is normally used for guiding short focal length review telescopes.

Now, in fairness, it is untested on the night sky (cloud, remember, hence the project!) but what I can tell you is that the focuser moves in invisible increments or quickly as required, is ultra smooth and shows no sign of slipping!

Unfortunately, much as I’d love to, I can’t install my mono camera as, with its built in filter wheel and OAG, there is insufficient back-focus. This means that the system will be used with my OSC SXVF-M25C coupled to the Canon 200 L lens using a custom adaptor I had made when Adam was a lad.

What could possibly go wrong?

Front View

The two pictures above show the 72mm lens has been stooped down to 52mm using a step-down ring. This reduces the aperture from f2.8 to f3.8 to improve star shapes at the periphery of the FOV but without adding the diffraction spikes that would be a feature of stopping the lens down using the iris.

Belt Drive

Finder-Guider Connection

 

[Stu]

It looks like a work of art Steve, lovely stuff! I'm sure it will work just as well, look forward to seeing the results.

[andyboy1970]

Hope you've got it on the focus ring rather than the zoom!!!!

Only joking, looks the business.

[steppenwolf]

17 minutes ago, andyboy1970 said:

Hope you've got it on the focus ring rather than the zoom!!!!

Luckily, I am saved from the embarrassment of doing that as this is a fixed telephoto lens - but it would have been hilarious to get this wrong if it had been a zoom!

1 hour ago, Stu said:

It looks like a work of art Steve, lovely stuff! I'm sure it will work just as well, look forward to seeing the results.

Thanks, Stu, I'm looking forward to hitting the band of the Milky Way with this in the summer.

[Alien 13]

Very nice work, its funny that someone mentioned "zoom" because that is the only bit of the puzzle missing on my DSLR/Lens remote shooter which I use over wifi for both wildlife and astro shots. The solution you have does look like it would be equally happy on the zoom ring (focus is taken care of with remote live view and micro-stepping with the DSLR) so I might do some more investigation.

Alan

[steppenwolf]

18 minutes ago, Alien 13 said:

The solution you have does look like it would be equally happy on the zoom ring

I can think of no reason why this would not work on the zoom although this is a greater load than that of the focuser - that said, this motor produces a high torque with its 60:1 reduction and it happily pulls my FeatherTouch R&P focuser loaded with a 1.6Kg weight!

[ollypenrice]

Lenses are great but are sooo critical on focus that I'm sure this is the only way. Very nicely done indeed.

Olly

[steppenwolf]

34 minutes ago, ollypenrice said:

Lenses are great but are sooo critical on focus that I'm sure this is the only way

That is rather what I am hoping, Olly, as I can confirm that trying to focus one of these by hand on the focus ring is very difficult - I developed a way of doing it by 'brushing' the focus ring but even that was a bit hit and miss. The motor/gearbox I am using here produces 2,880 steps/revolution at the small gear end and this is, of course, stepped down further dramatically via the belt drive ratio. I can now adjust the focus using a 'jog' control in single or multiple steps so am confident that it will work really well manually - what I don't know yet is whether or not it will work well under autofocus control but in theory, it should work fine, it is just a matter of choosing the right magnitude star for the software to work on. For an astro-photographer like you @ollypenrice who prefers manual operation, this should be a great way of doing it.

[Mr Spock]

A terrific piece of engineering. I bet you can't wait for the sky to clear!

[ollypenrice]

1 hour ago, steppenwolf said:

That is rather what I am hoping, Olly, as I can confirm that trying to focus one of these by hand on the focus ring is very difficult - I developed a way of doing it by 'brushing' the focus ring but even that was a bit hit and miss. The motor/gearbox I am using here produces 2,880 steps/revolution at the small gear end and this is, of course, stepped down further dramatically via the belt drive ratio. I can now adjust the focus using a 'jog' control in single or multiple steps so am confident that it will work really well manually - what I don't know yet is whether or not it will work well under autofocus control but in theory, it should work fine, it is just a matter of choosing the right magnitude star for the software to work on. For an astro-photographer like you @ollypenrice who prefers manual operation, this should be a great way of doing it.

Heheh, yes, I'm a bit 'manual.' Tis true. My home made tangent arm microfocuser (which I love and Tom hates) needs, as its smallest useful adjustment, a 180 degree turn on the knob. This just about registers on the FWHM. What's not to like? Don't answer that! 

Olly

 

[steppenwolf]

20 minutes ago, ollypenrice said:

Heheh, yes, I'm a bit 'manual.' Tis true. My home made tangent arm microfocuser (which I love and Tom hates) needs, as its smallest useful adjustment, a 180 degree turn on the knob. This just about registers on the FWHM. What's not to like? Don't answer that! 

Actually, Olly, I'm with you here, the tangent arm is an amazingly simple piece of engineering that provides an unbelievably infinitesimal amount of movement of the focuser knob - just what we often need for accurate focus! What I REALLY love about the tangent arm is that you can further increase its resolution by increasing the distance of the arm from the centre of rotation and the sky is the limit here! Digital? Pah!

[johnfosteruk]

Great job Steve, looking forward to some images once the cloud clears.

[JOC]

This looks impressive, can I please ask the dimwit question of how does it know how much to adjust the focus by, do you have something that knows when a lens is in focus (which sounds a clever bit of kit if it exists) as part of the setup which sends instructions to the motor?

[steppenwolf]

Not a dimwit question at all. The motor is connected to an autofocuser controller that in turn is connected to my laptop PC which runs an autofocusing program. The software commands the camera to take a photograph of a star and the star is analysed for Half Flux Diameter (HFD)  - the focus motor is commanded to move and another image is captured and analysed. This is repeated until a measure of HFD is found at a reading of approx 12.0 either side of focus and a table of results is produced (and represented by a 'V' curve) - the actual focus position is calculated as being at the 'bottom of the curve'.

 

[JOC]

5 hours ago, steppenwolf said:

Not a dimwit question at all. The motor is connected to an autofocuser controller that in turn is connected to my laptop PC which runs an autofocusing program. The software commands the camera to take a photograph of a star and the star is analysed for Half Flux Diameter (HFD)  - the focus motor is commanded to move and another image is captured and analysed. This is repeated until a measure of HFD is found at a reading of approx 12.0 either side of focus and a table of results is produced (and represented by a 'V' curve) - the actual focus position is calculated as being at the 'bottom of the curve'

Many thanks for that explanation - that's a mighty clever and powerful bit of kit, the plot really helps the explanation and I can easily see how it works it all out.  I had no idea that sort of thing was possible so it forms today's 'thing learned'  :-)

[steppenwolf]

We imagers are indeed fortunate to live in a time where this sort of technology is readily available. A little fettling and it can be put to use on all sorts of instruments as I hope to have demonstrated here.

[iapa]

On 23/03/2017 at 14:33, steppenwolf said:

What could possibly go wrong?

Having given this some thought ... 

The moon becomes geostationary, directly above the UK.

[John78]

12 minutes ago, iapa said:

Having given this some thought ... 

The moon becomes geostationary, directly above the UK.

Still wouldn't be able to see it through the perma cloud!

[iapa]

3 minutes ago, John78 said:

Still wouldn't be able to see it through the perma cloud!

damn - I thought I had it cracked LOL

Still, will be book marking this as I've some DSLR lenses somewhere that I could press into use.

[John78]

Yeah me too - but I think I like the idea of using Olly's tangent arm contraption attached to a 3d printed focus ring collar on that 200mm SMC takumar I have ...