I see a lot of posts where someone has less than perfect stars in a corner. The usual answer is that it's tilt so I'm trying to understand this better. In thinking it through, I would expect that in a simple optical system, meaning no reducers or correctors, anything that demands exact back spacing, the effect would be stars gradually going out of focus in the direction of the tilt, both positive and negative. It seems that the tilt would have to be really extreme to cause stars to be elongated to noticeable degree. Is that a reasonable assumption?
Tilt, field centering or collimation.
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I don't think so. Unless you have a flat image plane which is rather hard to come by.
So what would the appearance be? I'd really like to understand this.
They are all going to be different depending of the specific of the "simple" optical system, assuming "simple" being one without field correctors. E.g. newtonians would show coma in addition to tilt, with coma being the more preponderant the further out in the FOV you look and nil at the optical center, since tilt is constant throughout and newtonians are reasonably flat field instruments, at least for small angles.
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I shoot with a Newtonian and of course, coma is expected the further off axis you go. I suppose if collimation is confirmed and I see a difference in the corners then tilt would be the reason?
In the real world you can't reliably diagnose tilt from a single image. Differing optical designs display out of focus stars off axis in different ways.
The easiest way to figure out if your sensor is tilted is to frame two stars in opposite corners of the field and manually identify if they have differing focus positions.
The easiest way to figure out if your sensor is tilted is to frame two stars in opposite corners of the field and manually identify if they have differing focus positions.
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Tony Gondola:
I shoot with a Newtonian and of course, coma is expected the further off axis you go. I suppose if collimation is confirmed and I see a difference in the corners then tilt would be the reason?
*Most likely, assuming an otherwise perfect optical system.
Seems that it really is a process of elimination and it might very well be caused by a combination of factors depending on the complexity of the system and the state of the optics.