Parafocal rings

[garethmob]

Just a stupid question. Para focal rings only work for one scope don't they? I received 2 from astroboot today (bump it to free delivery:P) and I have a few different scopes. Do i just set them to the most used scope and on the others it will be a little out?

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[Stephen]

I'd expect them to work on any scope : the parfocal ring makes two EPs focus at the same point (drawtube travel.)

Drawtube travel will differ on different scopes, but if one EP / device is focused, then the other should be too.

[garethmob]

thats what i thught thanks kheldar

one more question, is it best to do it at night on stars / planet / moon or day time

[Stephen]

I'm not sure it makes a difference : but I'd go with something you can easily focus correctly on so a bright star + bahtinov (or similar) focus mask.

[garethmob]

thats great thanks kheldar

[Moonshane]

focus on Polaris and then it won't move. I'd expect them to be right for all scopes when set.

[garethmob]

didnt think of polaris thats a good idea, if it works well for this, (30mm and 10mm i might get one for the 25mm too

[cs1cjc]

It does not matter what you focus on as long as the scope is focused. It may be easier to do it indoors with a small refractor on an illuminated pinhole or artificial star. The relative positions will be same for all telescopes.

One way to think about eyepieces is that they are fancy magnifying glasses for examining the image at the primary focus of the scope. So each eyepiece has a fixed distance from that surface.