Focusing on "infinity"

[Roger Fairhurst]

Just a general question about setting up finder scopes really.

Many finder scopes, it seems, have a focus adjustment that needs to be set once and then left - that is to say - doesn't need to be re-adjusted at each outing.

I have seen it stated somewhere that it is sufficient to focus the finder scope on an object at least 300m to 400m away. Is this really the case?

My own feelings are that it should be focused on something much further away than say a church spire at 300m distance. I will focus my finder (SW 9x50) on the moon - the next time I get an opportunity that is! But is the moon far enough away for a fixed focus? Is it close enough to infinity to bring the stars and other sky detail into the sharpest possible focus?

The moon is so attractive to me because with its craters and shadows and abundent detail it should be easy to achieve the sharpest focus that the optics the optice can deliver.

What do other people think/do?

Kind regards

Roger

[Psychobilly]

I think you will be fine if you use the moon...

Whenever I am putting new kit on a scope I always do a daylight test on a distant target -  in my case a pylon about 3 miles away,  so that only small tweaks will be needed at night...

[tdmodels]

Because of the f-ratio of the finderscope, you will find that an object 300m will have the same focus setting as the moon and objects at infinity, so setting the focus on a fixed object during the day and aligning the finder and scope is much easier. 

[michael.h.f.wilkinson]

infinity focus really means that the point of focus is many orders of magnitude longer than the focal length of the scope. Low magnification has the additional advantage small errors are not magnified much.

[Roger Fairhurst]

Thanks for the advice guys - just what I needed to know.

Kind regards

Roger

[umadog]

Also, don't forget that that your eye will accommodate small errors in focusing.