My focuser is a "rack-and onion" job. Its a pretty crude device, and the amount of shaking I get in the image when trying to focus makes it even more difficult. I've seen mention of some devices called focusing aids that I think you put on the end of your scope, with 3 holes drilled in it. Not exactly sure how it works. I would be tempted to try and make one if it's worth the effort.

But here is what I have been wondering about, how to achieve really good focus? Let us say for example, that I want to observe or image a planet. I can look at the disk and try and guess where the focus is, by the size or brightness perhaps but its done by guess-work. It seems to me that what we are doing when looking at the sky, is focusing on infinity, regardless of the distance of any object. If thats the case, could I not then focus on a bright star instead, which is a point source that would be easier to dial in, and then swing my scope over to the planet with this setting? Ie. would that still be the right focus for other objects? Also how about the moon, would this approach work with it?

I appreciate hearing from anyone who has tips on how they achieve proper focus. Thanks in advance for any replies!

 The device you're referring to is a Hartman Mask. Here's a link to a review that will help you in making one. They are very inexpensive to make, and they do help in achieving focus.

I find that fucusing to the stars is good. If the object then needs addional focus then so be it.
With the planets it's quite easy... Just focus untill the disk is sharp and no colours bleed. (I find that a slightly out of focus planet has a blue ring.)

Couple more focusing tips:

• Tighten the focuser lock (if yours has one) slightly, to put a little "drag" in the mechanism. This will also help offset focuser shift to some degree.
• Increase the diameter of the focuser knob to provide a better "feel" when you focus. Scopestuff, for example, has replacement focuser knobs. Or you can wrap one of the knobs with a couple of folded paper towels, then cover it with plastic cling wrap and a rubber band or two.
• Another option is to attach something like a JMI MotoFocus unit in place of one of the knobs. This will provide you a lot finer focus ability, and will greatly minimize shaking. By the way, it's probably the mount/tripod, not the focuser, that are responsible for the shaking. Make sure the bolts are tight where the legs join the mount.
• For rack-and-pinion focusers, always go past focus in one direction, then reverse to get fine focus. This takes up the slack in the gearing due to backlash (even high-quality R&Ps have backlash) and provides better focus adjustment. Pay attention to which direction "lifts" the focuser and eyepiece against gravity. Move the focuser in that direction as your final adjustment.
• Another fine-focus option is a helical microfocuser. If you have a 2" focuser, this replaces the 1.25" adapter and then you put your eyepiece in the microfocuser. Focus with your focuser knob to get close, then rotate the microfocuser's upper part to achieve fine focus. This also helps to eliminate shake due to focusing. But it only works if you have a 2" focuser, and it prevents you from using 2" eyepieces.

Thanks for these excellent tips. I like the idea of increasing the wheel diameter. Of course, it makes sense! Will have to give that a try, prefer to make one in this case. I'm thinking some kind of a rubber wheel from a toy, or a plastic gear attached to the side. Lets see what the hobby shop has got...

On another similar question- What does it mean if I can't focus all three colors, R,G,B simultaneously in a newtonian?

I find that my images are focused well in one colour, but not in the other two, when using a webcam to take color images.

Does it mean my collimation is bad? Or is this some sort of inherent problem with the newtonian scope design?

Thanks,

Thomas

A mirror shouldn't cause this. It is more likely there is a glass element somewhere in the optical train that is causing chromatic aberration.

Is your scope one of those designs which incorporates a corrector lens at the bottom of the focuser tube?

Otherwise, it might be something related to the webcam. Color webcams have a matrix of color filters built onto the surface of their chips. This could be misregistered. Can you see these separated colors visually, or only through the webcam?

Thanks for the tips again... I should have mentioned that I used a 3x barlow. Its a cheaper brand too. Based on your info I suspect it could be the problem.