“The seeing is terrible”

[Brettstar6754]

How many times have you heard?
“The seeing is terrible”


But is it the seeing or is it your telescope?

Attached are two images of an outdoor Ronchi test. It was a warm day and a cool night.

In the first image the mirror has been on the stand for about 3 Hours! (9.30pm)

In the second image the mirror has been on the stand for 7+ Hours! (after 1.30am)

An organic (Eyeball) look at the mirror shows a beautiful smooth figure with lots of diffraction fringes.
The bands do wobble with the heat but it is easy to see they indicate a lovely figure.

[multiweb (Marc)]

Aren't the bands supposed to be vertical and parallel?

[Starkler (Geoff)]

Easiest way to see this effect is on a very defocused star. If the mirror isnt thermally acclimatised you'll see ripples like you do rising from a hot road in summer.

[Brettstar6754]

Originally Posted by multiweb

Aren't the bands supposed to be vertical and parallel?

Hi there, I should have mentioned the test was done at radius of curvature and not at focus. They would be straight at focus, thanks.

[Brettstar6754]

Originally Posted by Starkler

Easiest way to see this effect is on a very defocused star. If the mirror isn't thermally acclimatised you'll see ripples like you do rising from a hot road in summer.

Yes an excellent field test.
I wonder how well the de focused star thermal effects will show up in an image?
Something to try one day.
Thanks.