Increase focus knobs size?

[Lumen Miner]

Hi all!!

Just wondering if I could grab some thoughts on focus knobs.

Basically I find the focus on my 3" refractor very hard to get correct. I am able to focus, but it seems a very fine area to adjust to. Say Jupiter, I go the normal route of putting the object out of focus, bring it slowly back into (noting best focus) focus, then travel out of focus again. I then track back to find the best area. By this time, my focus area could not be more than 5 degrees on the focus knob. Within the focus area where the image is reasonable, it still could get better, yet the focus assembly seems really touchy. I just can't get "perfect" focus. I know it is there and often I can get it, but most times it takes about 20+ secs to hit the sweet spot.... by this time my object is gone... I don't have a guided scope obviously....



So after all that, my question is, the other night I made up a quick replacement focus knob, from a large solid plastic lid. It is about twice the old focus knob. It has made finding that sweet spot really easy now, as the focus area is around 10 degrees.

Why are focus knobs so small? Would focusing not be more precise, the larger knob you used? Or am I getting all confused and missing something? For instance if you had a 30cm focus knob, would the accuracy not be far superior to a 30mm knob.

I am guessing most of your setups have extremely accurate focusing units, and this is just an anomally of cheaper scopes...

I was just interested in your opinions in the matter.

Thanks, Mitchell.

[pjphilli]

Hi Mitchell
I did the same thing for my 80mm Meade refractor using a plastic lid to increase the focus knob size by 2 giving a cheap 2:1 gearing. It works fine and the price was right! You are right about the small size of the original knobs - I sometimes think that astro gear designers have never done any star gazing.
Cheers Peter

[Lumen Miner]

Great minds hey!!? Thanks for conferring what I was thinking. I found the lid worked well and thought I would experiment further. I wanted to find out what the max size/practibility was. I chose to modify a 10cm plastic (so it was lite) trolley wheel. It had a tread cut into the rubber tyre. I wanted a smooth rubber surface, so I used a blade and sand paper to remove the tread. I then fixed the correct sized bolt via a nut and washer sandwich. The wheel did not have bearings.

Took it out for a trial run briefly last night. Seems to make life easier.

[Roger Davis]

You could also buy or make a reasonably cheap motor to fit to the focuser, that would give you very fine control.

[Lumen Miner]

I have a couple of servo's around here somewhere and a controller. Might see what I can rig up. :)