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'Peanut Butter Lid' fine focus knob for SCT

rwg

By rwg
in DIY Astronomer

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rwg

rwg

Posted
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Been trying to get better focus on webcam captures recently with my 8SE, and having trouble with the focus knob just not giving me fine enough control. While I quite like the idea of a dual speed crayford, another 1kg hanging off the back of the tube doesn't appeal, nor does the cost. Similarly, the dual speed replacement feathertouch knobs cost a bomb (can't imagine how £150+ can be justified), as do JMI motorfocuers...

So, a more cost effective approach was needed. I started thinking about a lever, but couldn't think of how to mount it on the focus knob and it would foul the diagonal when turned. Then I found a mention of a 'coffee cup' mod for making a bigger knob, which sounded the right sort of thing. After a little while casting around for the right material to make the knob from, the Mrs suggested a peanut butter jar lid and I was away...

Here's the finished item in place on the focus knob of the scope :

post-18322-133877488526_thumb.jpg

As you can see, it tucks neatly behind the diagonal (even a 2" one) and allows you to use the end of the focus knob in the normal way. Of course, it's pouring with rain outside right now, so I can't try it in anger, but I can certainly make smaller changes to the focus with the new large knob than the original one.

Construction is fairly simple, first you need to obtain a suitable lid and clean it. I was worried about rigidity, so I decided to reinforce it with 3 sheets of 1mm plasticard glued together then glued inside the lid. Finally I used hot glue round the inside edge to ensure that the reinforcement was firmly bound to the lid.

post-18322-133877488534_thumb.jpg

Next up was making the hole to attach it to the stock focuser. For the SE8 focuser, the stock focuser knob is about 18mm diameter with 8 rubber ridges running down the sides at 45 degree intervals. I drilled a 4mm pilot hole first, then out to 10mm in stages. After that I enlarged the hole with half round files, as I was worried that a larger drill would crack the lid. Obviously the trick is to keep checking that your hole is central as you enlarge it. As you approach the right size, frequently check to see if the hole clears the main body of the knob and is just catching on the ridges. At that point you need to mark 8 evenly spaced points round the hold and cut out small grooves with a triangular file. Don't go to far as you want the lid to fit snugly over the existing knob so that there is no play.

Next, clean up the lid and add the index marks. these are the two little bits of black plastic stuck to the outer rim (opposite each other). The idea is that in the dark you can feel where these are and return the focus to a previous point after adjusting.

Finally, fit the lid to the existing focuser - you could even paint it first!

Robin

Another view :

post-18322-133877488542_thumb.jpg

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Ludd

Ludd

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Eek, I've just forked out for a dual speed crayford for my 8SE (admittedly second hand). It's jolly nice for when I've got the OTA on a GEM but this lid looks the business for using on the original mount.

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rwg

rwg

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Nice job, I'm a clothes peg man myself though...:o

What a brilliant idea - a clothes peg! Coulnd't get much simpler or more effective than that...

I have to admit that the orange colour does clash a bit with the (different) orange of the tube. If I can find a paint that will take on the plastic I might go with black...

cheers,

Robin

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Bizibilder

Bizibilder

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Anyone thought of putting three of these on the collimation screwheads of a newt? On my 200P they are the darndest things to reach when looking through the "eyehole". The big caps would make fumbling easier, I'm sure. Maybe I just have short arms..........

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Astrokev

Astrokev

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Neat idea. Hope you didn't feel too queasy eating all that peanut butter first. In my experience, the perfect lid for the job is always the one on the new jar of 'stuff'.

I had this problem some years ago when trying to get my hands on an aluminium deodorant can that was perfect for what I needed. I was the sweetest smelling man about town for a few days!

Kevin

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rwg

rwg

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I'd like to publicly deny the scurrilous suggestion that it was a jar of smooth peanut butter - crunchy all the way in this household I'll have you all know.

Hadn't spotted the Scopes'n'stuff dials that dweller25 linked to - very much the same idea except they are for fitting to aluminium focus knobs not the rubber covered knobs on the SE series. Also I'm not quite sure if there is much value to the degrees scale - after all you'll be using it in the dark!

cheers all,

Robin

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JamesF

JamesF

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I don't really like peanut butter do you think a jam top would work ????

Any suitably-sized (preferably plastic, I imagine) lid should be fine.  Even Marmite, though there is then the problem of safely disposing of the pollutant in the jar.

James

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r3i

r3i

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I replaced the stock focuser on my C11 with a Feathertouch from FLO (£198) there's no substitute for the real thing a 10/1 fine focus is really needed with the long FL and even then its very tight......if you have the cash get one......

They are indeed superb - I managed to get a 2nd hand one for my LX90, but even then it wasn't cheap.

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mylatestwhim

mylatestwhim

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Awesome idea. I saw it on cloudy nights and a google search brought me here. I'm going to have an insert turned for a spare adapter piece that I have lying around.

Watch this space but it should be neat

post-12512-0-12131600-1390090003_thumb.j

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