Fisheye lens causing distortion when collimating?

[DJScotty (Scott)]

Hi there all.

This week, I have spent MANY hours collimating my 1150mm f4.5 Newtonian. I am using a video from The Astronomy Shed where you use a camera and Mire de Collimation to firstly perfectly centre the secondary in the draw tube and then align the primary.

I have done this to the best precision I can muster. But I still am getting odd star shapes and the diffraction spikes are tilted.

My question is this...

I am using my ASI120mm and the handy little fisheye lens that came with it. Would the fisheye lens be causing distortion in the image such that what I think is a perfect(ish) circle on the computer monitor is not?

Thanks for your help

Scott

[billdan (Bill)]

Hi Scott,

I don't know the answer to the fisheye lens distortion.

You could try drawing a circle with a compass on some white paper roughly the same size the secondary. Then hold it in position in front of the secondary and see what image looks like on the computer. That should confirm or deny whether the lens is distorting.

However you should be able to see the front edge of the focuser draw tube, does that look like a circle? If it does then the secondary must also be correct.

Cheers
Bill

[julianh72 (Julian)]

The fish-eye lens is intended for stand-alone use as an all-sky camera, without a telescope. To use the camera with your telescope, remove the fish-eye lens, screw in the nose-piece, and fit the camera directly into the focuser, where you would normally use the eyepiece.

Hope this helps!

[billdan (Bill)]

Julian, for collimation purposes the lens is retained to bring the secondary mirror into focus. You are correct for imaging you remove the lens.

Bill

[sil (Steve)]

Fisheye distortion is extreme and using one to symmetrically align precisely is near impossible I can't think how it could be practical for collimation.