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Kullat Nunu
2005-Feb-17, 11:02 PM
According to this Sky & Telescope article (http://skyandtelescope.com/news/article_1463_1.asp).


NASA's Spitzer infrared telescope produces triangular star images at short wavelengths.

Fortunately it is not a real problem.

The Bad Astronomer
2005-Feb-18, 12:05 AM
Cripes, I've known this since shortly after launch. The first image released had triangular stars; I even emailed some friends at Spitzer and made fun of them about it. 8)

I also figured that this was before they had finished focusing the mirror, though, and it would eventually be fixed (or at least minimized). I don't think this is hurting them much, though.

Jpax2003
2005-Feb-18, 04:32 AM
Just wait 'til Hoagland gets a hold of this one. Next thing ya know he'll claim that it's evidence of either hyperdimensional energy or a dyson pyramid.

Madcat
2005-Feb-18, 04:44 AM
BA, do you have any idea what would cause that sort of distortion? That sounds odd.

Evan
2005-Feb-18, 07:25 AM
I would guess a three point mount during figuring.

A Thousand Pardons
2005-Feb-18, 10:22 AM
From the article at Kullat Nanu's link: "trefoil aberration caused by a slight distortion of the 30-centimeter secondary mirror in its mounting," and in one of the captions: "This trefoil aberration arises in the telescope's secondary mirror, which is pinched at the edge by its three-blade support structure."

badprof
2005-Feb-18, 03:34 PM
Hi all,

This sounds like the problem that any observer can see by overtightening the mirror mounting clamps on their telescope.

I have found it to be a very common problem of people trying to use their first telescope.

Cheers

Evan
2005-Feb-18, 03:42 PM
It was an artifact of the figuring process. As they say
The trefoil aberration from the secondary was evident in prelaunch testing but left uncorrected since it didn't render the telescope out of specification.

A Thousand Pardons
2005-Feb-18, 08:01 PM
It was an artifact of the figuring process.
Wouldn't the figuring process just involve the grinding/shaping/polishing of a mirror, and not the effects of the mount that holds it? I just found a link that says you can avoid some of the figuring process by adjusting the mirror with the mount.

PS: Glossary at the ATM faq webpage (http://www.atmsite.org/contrib/Poulson/faq/atmsect6.html)

Evan
2005-Feb-18, 09:53 PM
Based upon their description I expect the figuring had to be done with the mirror in the mount. In order to test the figure the entire assembly had to be cooled to the cryogenic temperature that it would operate at. Difficult, time consuming and expensive. Trying to figure it out of the mount and then remounting it each time would introduce additional variables.

A Thousand Pardons
2005-Feb-20, 06:52 PM
Based upon their description I expect the figuring had to be done with the mirror in the mount.
I doubt it. It's the secondary.

carter
2005-Feb-21, 01:37 PM
In reading the S&T article, it sounded like the scope is performing within specifications. True, it could be better, but it sounds like they made tradeoffs to limit costs which is true for all equipment. Comments?

Evan
2005-Feb-21, 04:43 PM
The secondary has a mount too.