star test donuts

anybody no why the donuts on the outer edges have this shadow? is this normal?

It means that the outer edges are vignetted, at the corners very significantly so.

Hey Andrea so this is because of the aps-c sensor and the 2inch corrector?

Jordan Morley:
Hey Andrea so this is because of the aps-c sensor and the 2inch corrector?

I presume they are a signficant factor but I expect the size of the secondary to be contributing too. And so does the length of the CC.

Your collimation is also not good, which can be compounding the effects. Have you also tried using a different backspacing?
Sensor tilt can also be a culprit if it is extreme.

Your collimation is also not good, which can be compounding the effects. Have you also tried using a different backspacing?
Sensor tilt can also be a culprit if it is extreme.

That's quite true in the sense that the optical axis is somewhat off to the right (I'd say about 10mm) so there should be more vignetting on the left side than there is on the right side but I'd discount tilt as a source of clipping of the marginal rays other than for obviously abnormal situations (where till could expressed in terms of several degrees).

Thanks guys! so I need to ajust the secondary mirror? It's 80mm mirror on a sw250pds

What coma corrector are you using?

So looking at the picture the spider is tilted? Left side push or move out and or slide spider to the right, then align mirrors with a camera? Or mabe the mirror is tilted on the spider itself?

I'd recommend a 38% CO against the current 32.5% but some vignetting is inevitable due to the GPU. Looking at the image of the exit pupil there is something bent to the right by I can't tell what it is (only you can). Maybe sending images of the front of the scope?

The bent object to the right is a dew heater cable, 5th line silhouette?

I am attempting to move my sw250pds to a carbon tube however its a struggle I no very little! I will admit its quite enjoyable to work on it after a days work instead of watch tv all night (:

Jordan Morley:
The bent object to the right is a dew heater cable, 5th line silhouette?

OK, that explains it. As for replacing the current tube with one of CF isn't for the faint of heart and most likely would open a Pandora's box of issues when you'll rebuild. I'd focus on recollimating it and maybe replace the secondary with a larger one.

Yep i think iv gotten abit a head of myself I'll post back if I manage it get it going (: thanks for your help

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