TS Focus contraption.

[ollypenrice]

To say I'm impressed by the Canon EF200L lens would be an understatement. It is an incredibly fast, flat, 71mm astrograph capable, quite possibly, of working wide open on my 15mm square chips... if you can focus it. But, believe me, touch the focuser and the FWHM reading leaps wildly! A CN post discussing the focal depth in microns convinced me that shivering fingers were not the tool of choice so I ordered this Dickensian contraption from Telescope Service. Fifty euros. It comes in three diameters and this one is on the big side so the nylon bolts are well exetended. I may see if steel ones stiffen it up if necessary. I'm concerned that any springiness in the bolts might translate into slight movement during exposures. Another option would be to put locknuts under the bolts to tighten against the underside of the rings.

Its function should be self explanatory from the picture. Fine focus is achieved by twiddling the knobs. I think it looks very promising and will update you when it stops raining. (Yes, we do see the wet stuff sometimes!)

Olly

[A McEwan]

Looks ingenious! Simple and exactly what's required! Hope it works well in practise.

Ant

[barkis]

There is quite a distance those bolts have to reach the lens focus ring Olly, so any play in them will be amplified by their length perhaps? It might have been better to have a clamp around the lens focus ring, with an extended finger, on to which the devices focus adjusting knobs could bear. They could have used finer threaded bolts too, those look pretty course in the picture.

Ron.

[ollypenrice]

There is quite a distance those bolts have to reach the lens focus ring Olly, so any play in them will be amplified by their length perhaps? It might have been better to have a clamp around the lens focus ring, with an extended finger, on to which the devices focus adjusting knobs could bear. They could have used finer threaded bolts too, those look pretty course in the picture.

Ron.

I agree, but I guess the problem is that there are so many lens diameters. It is a shame that the smaller diameter rings were just too small for my lens. I don't think the thread pitch will be an issue because the final tweak will done by tightening the pushing side against the fixed side for a minute precision.

We'll see how it goes in practice.

Olly

[Merlin66]

For those who may be interested:

The critical focus zone(CFZ) (ie to maintain 1/4 wave accuracy)

is:

CFZ= 4 Lambda x F^2

where lambda is the wavelength of the light

and F the focal ratio

so for green light (550nm) and focal ratio f10, CFZ = 0.22mm

and for a f4 system, CFZ =0.035mm ( 35 micron)

So you can understand the need for some pretty capable focusers on fast telescopes/ lenses.

[ollypenrice]

For those who may be interested:

The critical focus zone(CFZ) (ie to maintain 1/4 wave accuracy)

is:

CFZ= 4 Lambda x F^2

where lambda is the wavelength of the light

and F the focal ratio

so for green light (550nm) and focal ratio f10, CFZ = 0.22mm

and for a f4 system, CFZ =0.035mm ( 35 micron)

So you can understand the need for some pretty capable focusers on fast telescopes/ lenses.

Excellent info Ken. Thanks. So for f2.8 it all gets a bit scary...

Olly

[RyanParle]

I've had the same problem with focusing DSLR lenses. this looks like a handy tool, as Ron said it looks like it has lots of potential for flexing, but its got to be a big improvement on trying to focus by hand.

I've often wondered about a computer interface for controlling the focus motor & aperture diaphragm, unfortunately I don't have the technical knowledge to develop such a device and my Google searches have been unsuccessful so far. I'm sure it would be a popular bit of hardware if it was built.

[ollypenrice]

A camera lens robofocus? Yes please!

Since the pic I have found I can move the rings much closer together, which is better.

Olly

[blinky]

Looks an ingenious solution to the problem - as Ron said though, I would have liked the bolts a little shorter.

[ollypenrice]

Looks an ingenious solution to the problem - as Ron said though, I would have liked the bolts a little shorter.

You could buy the version with 70mm rings if you were sure they'd fit over your lenses. I wasn't sure so went for the 100mm versions.

Olly

[Psychobilly]

This thread finaly convinced me to do something with the rings I had made for this last year... needs a clean and some chemical metal (or a few mins with the Benz-o-matic but I hate "welding" alloy...)

Billy...

[barkis]

That looks good Billy.

Were you aware of TS's device when you made this? Do you use a lens like Olly's, and if so, have you had focusing issues?

[Psychobilly]

Yes I remember Dangerous Dave showing th TS one last year used on Pentax 6x7 lenses...

I had some material left over from a set of guide rings I was making... and had roughed the rings out but hadnt done anything else with them...

So after another weekend in the garden and seeign olleys post and the "Maths" I decided it was time to finish them off...

I do use Lenses but on DSLR's the latest two beign the 70-200 f4L and the 24-105 f4L although the 200 f2.8L may be added to the mix fairly soon...

Wth canon lenses on Canon DSLR's you can remote manual focus them from a computer whilst monitoring the image on screen and with a Magnified "ROI" or you can use APT to do the same with FWHM readings or leave the program AF for you......

I wanted something that didnt need a computer for when I am away from the obs with the EQ3 Pro... and was too tight to buy something that I should be able to "make" so all it cost me was some time, some electric and the screws..

Looking at the witness marks on the TS ones they are laser cut out of alloy sheet... (you can see the "piercing" mark on the part where the bar is attached) ...

[ollypenrice]

That looks great, Peter. More chunky than mine and with less extension in the bolts. It is a shame I was at the limit of the smaller rings. However, fiddling with mine has convinced me that it will probably work pretty well. The maths on the focal depth says it all. I saw a 200L lens image on CN, processed without star reduction using a different and very fancy temp compensated device, home made. The stars were clearly smaller than I have yet acheived. I'm dying to get out there.

Olly

[barkis]

Shouldn't be too difficult to add stepper motor aid to focusing, to give a hands off assist. A man of your calibre should have no difficulty with that. Or would that be overkill do you reckon?

Ron.

[Psychobilly]

overkill probably for DSLR use.. you might as well use the USM motors in the lens Remote manual from a "PC" is a neat trick and I am sure that IVO could implement triggering a refocus in APT......

Just had a quick chance to try it before the clouds started rolling in . it suprising how fine the contol you actually have is...

I gave it a go on Betelguese. with the Multi-Y mask I have been playing about with on the 70-200 f4L @ 200mm

I Ran of 20x30s... of Orions Belt and Sword so that the field can be checked...

I then changed over to the filter replaced 1000D and it clearly showed the slight difference in the focus...hopefully get soem more subs off but the clouds are rolling in

Managed another 10 test subs then it started to rain

Grabbed a few flats with the EL Panel...

12x60s ISO200 70-200 f4L at f4.5 Unguided on EQ3-Pro... (my dark site setup)