With my passion for lunar sketching, and so being at the mercy of the prevailing atmospheric conditions as to what magnification I can push things to, focusing can become a very tedious affair. The small knob on the back of an SCT is difficult to get really fine control over. Yes, there are focusing motor drives available, but these will just over complicate the simplicity I am looking for with my time at the scope.
Then while playing with my son's Lego one day, a memory of a long ago read article was sparked on how to achieve a fine motion control on the focuser knob of an SCT - make the knob bigger in diameter.
An off-cut from a hole-saw cut gave me the largest diameter dial I could use with the space afforded to me by the knob and the 2" visual back. It was then just a matter of widening the center bore to allow the plywood dial to fit over the knob. The knob is a shade under 20mm in diameter, the 20mm forstner bit made an oversized bore, so a snug and positive fit was achieved with just a little piece of fine sandpaper between the knob and dial - this also allows me to easily remove the dial should I want to and no damage or residue is done to or left on the original focus knob.
PRESTO! A new fine focus control knob!
The amount of force needed to wind the focuser is now just a baby's breath. It is much easier to turn the knob just because of the effect of the larger diameter dial. I can much more carefully adjust the focus, hasn't altered the way I store my old C8, and I've kept the situation for me really, really simple. All that's left to do is give the new focuser dial a lick of varnish. Couldn't get any easier.
This simple mod does not eliminate mirror shift, nor would it, and that is controlled in another way.
Mental.
Last edited by mental4astro; 30-10-2014 at 11:03 AM.
Reason: title change
Ed, et al,
Unfortunately no....
The Feathertouch micro focuser actually replaces the focusing knob and the interface to the screwed rod attached to the rear of the mirror.
It gives a much smoother translation and movement.
Notwithstanding that, the Vegemite type solution is a good step forward from the original. (You can also drive the SCT focusing knob using a cheap SW Accutrack electronic motor and a toothed belt)
Alex, you should go the whole distance and make it look like a ship's wheel!
Graz
You've now got me thinking, Graz. I'm thinking of the lugs I have that I could use for the wheel studs, and ways to sculpt the inside.
I had the C8 out for an hour - conditions were terrible. I had first light with my new 3.5mm LVW with this scope. WAAAYYYY too much magnification for conditions, but the nice thing was focusing was the easiest I've had. As now my smallest finger movements translate to a finer focuser action, frustration had no fun tonight. I'm very happy with this little mod,
Gday Alex
Another very simple and precise mod is to use a bit of spring steel wire to make up a lever that you can adjust using the tip of one finger.
Heres one i made for my ETX125.
Just get yr best focus, then spring open and reset the lever to the position you want. Easy to remove when not needed.
Also, as you dont "grab" it, you get much less vibration in the EP during use.
Gday Alex
Another very simple and precise mod is to use a bit of spring steel wire to make up a lever that you can adjust using the tip of one finger.
Heres one i made for my ETX125.
Just get yr best focus, then spring open and reset the lever to the position you want. Easy to remove when not needed.
Also, as you dont "grab" it, you get much less vibration in the EP during use.
Andrew
bugger me
What a simple yet elegant solution to an age old problem for the GPS 12
Andrew, I love the simplicity of your spring. Ingenious! I guess too that if the fork tyne gets in the way it is a very easy matter to just squeeze the spring open and shift its position on the knob. The amount of leverage you get from such a long spoke would be staggering!
Doug, great machining there to tap the threads. The spokes then just squeeze onto the knob to hold it in place? Yes, the amount of shake that a large diameter wheel provides is bugger all as the amount of effort to move it is so small. I barely need to touch the wheel I've made to achieve sufficient grip to move the focus. Shaking has been greatly reduced due to the focusing process.
My old C8 on its wedge really suffers from vibrations - heck, it shakes when a heavy car rattles past my place on the crappie concrete road in front of my house. The joint lines between the slabs have shifted over the years, and produce a strong 'thunk, thunk' as the wheels crash over the joints. Trucks and buses are even worse. This little mod at least gives some relief to the vibration sources. I had asked for some help with ideas on how to reduce the effects of the external vibrations in the General Chat forum, and I've applied some of these to great effect. Thank you if you had replied to my question there. I've had less agro with your help,
I am a machinist by trade, so I can make these things at my place of work. Yes the 4 threaded rods screw down to the rubberized nob. My next project is making an adapter to join my bino to a prism diagonal directly. Looking for ways to shorten light path on my C8 bino setup. The Badder T2 1.25 prism diagonal with adaptors is a bit out of my pocket range.
Focus Mod
This is what I've come up with, with a lick of varnish too.
