Too many diffraction spikes

[David Rayment]

I thought that my C8 Newtonian was collimated OK or are these excessive spikes due to the focus tube inserting itself into the optical tube? All the diffraction patterns of the various stars point symmetrically into the field of view!

David

[Uplooker]

Need for collimation me thinks.

[davew]

Hi David,

It does look like the focus tube. I take it that you have to rack in to focus ?

Have a look down the tube. Does the focus tube line up with one of the secondary arms or as I suspect right in the middle of two ? If the later I think that proves it.

Answer - New focuser. Hack saw. Move the primary up the tube. Or anything else to shift it. It will look worse if you're out of focus.

I think

Dave.

[David Rayment]

Thank you for that. A little light engineering is called for then!

David

[David Rayment]

So after a decent wait I have finally got round to solving this question. I have not been idle, just taking images without bright stars! The diffraction spikes are caused by the bolts that tension the secondary mirror supports. If I make a card iris that obscures the tensioning bolts from the main mirror then I get a normal four spike diffraction pattern.

The question is now - what is the best cure? Flocking? Is there a modification?

The other question is - out of all the C8-Ns to have come from the Synta factory is mine the only one to show this fault? Admittedly I did buy it second hand but it does look original - were the bolt really meant to be chromed (they are mat black now) and should they stick 10mm in from the tube wall?

Any answers gratefully read!

David