Lunar/Stellar Focus Adjustment

[BlueAstra]

I once thought everything in the sky was far enough away to be effectively at infinity when observing. However, I have noticed that when I go from stars to the moon a small focus re-adjustment is required.

A quick calculation says with the star at infinity, the moon's surface at about 3.76288x10^8 m and my scope focal length at 2 m the focal shift is about 0.01 microns. Surely thats too small to notice a change, or even tweak.

So is my maths wrong, or am I just imagining I need to refocus when I view the moon after looking at the stars?

[johninderby]

Yes you do have to re-focus. In fact when observing just the moon you will need to tweak the focus slightly when looking at different areas as even that small a change in distance will affect the focus.

And the planets of course will also need re-focusing.

John