overcorrection undercorrection: what is it?

[janoskiss (Steve H)]

In many scope reviews and here on the forums too, I often read people talk about a scope having a bit of under or overcorrection. What are they talking about?? What is the thing being corrected? What is the measure of how far under or over the scope is? Why is it never right? No one ever talks about "correct"-correction; it's always under or over.

[slice of heaven]

Hopefully someone else might give a more precise technical explanation but its basically the way the mirror is figured. If it was 'correct' the light would focus in a sharp point. Undershoot the mark and its undercorrected, overshoot the mark and its overcorrected.
The startest link below shows how to check for it.
http://www.astunit.com/tutorials/startest.htm

Be nice to have a perfectly 'correct' mirror

[janoskiss (Steve H)]

Originally Posted by slice of heaven

If it was 'correct' the light would focus in a sharp point. Undershoot the mark and its undercorrected, overshoot the mark and its overcorrected.

Wouldn't that just mean that the focal length is slightly shorter or longer than intended? This would not affect the image quality though.

[slice of heaven]

The problem of whether the mirror is under/over corrected is not which one is better but how much. Too much of either and youll have a soft image and a wider range of focusing at the EP instead of one definite position where the focus is crisp. I think a warm mirror is always overcorrected?? due to the mirror not being the correct shape.
For a comparison my Parks mirrors practically snap into focus whilst the the gs has a very small range of travel in the focuser where its acceptible. The gs is fractionally undercorrected but still gives me extremely satisfying views.

[Starkler (Geoff)]

To answer the original question, these terms have their roots in the grinding process of newtonian mirrors. Firstly, mirrors are ground to a spherical figure, and then later figured into a parabolic shape.
An undercorrected mirror is on the spherical side of the perfect parabola, and an overcorrected one has taken the parabolising process too far.
An under or overcorrected refractor would exhibit a star test with characteristics of a newtonian with the corresponding error.

[slice of heaven]

Thanks Geoff, now I know how they get that way. I new the effect but not the cause.

[janoskiss (Steve H)]

Yes, thank you Geoff! Makes sense now. Two more terms to add to the Glossary, Ken!

[asimov (John)]

And I guess to alter or fix the fault, would be to have the mirror or len's sent away to be re-figured.....or to mask the mirror/lens down to a smaller aperture?

[slice of heaven]

Originally Posted by asimov

And I guess to alter or fix the fault, would be to have the mirror or len's sent away to be re-figured.....or to mask the mirror/lens down to a smaller aperture?

For us, Yes

For the Hubble, NO

That was the problem with Hubbles 90 odd inch mirror and they somehow fitted a pair of mirrors/lenses in between each camera and mirror.

Wonder what they did with the spare mirror they made, that was perfect.

Hey, BD the spare was made by Kodak.