Quattro: Spider vanes

[Black Knight]

I'm trying to eradicate the double diffraction spikes on bright stars. See this thread: http://stargazerslounge.com/imaging-discussion/154528-quattro-8-first-light.html. It was suggested that twisted spider vanes may have caused this, and indeed, the spider vanes are a tad twisted.

Do I fix this by loosening off the knobs on the outside of the OTA?

[Astrogeordie]

Hiya,

the double spikes in your picture are parallel to each other. If the vanes would be twisted, you would get spikes that slightly differ in direction.

I reckon you just did not hit the focus right.

On my Mrs Parsons (10 year old Skywatcher 200/1000mm Newtonian with solid cast spider vanes) I get those double spikes when I am not in focus. I even use them to assist me in focusing - when they come together, the focus is reached. A bit like a built-in Bahtinov mask.

[Black Knight]

So these twisted vanes are not a problem then? I don't need to do something with them?

[Astrogeordie]

Hi Andy,

if the vanes are twisted in a way that the opposite ones are not parallel to each other, you would get spikes in different directions. The spike direction is always orthogonal to the extension of the vane - just think about a diffraction slit.

If the vanes are twisted alongside (turning the flat vane into a helical shape), then the projection of this is a vane of variable thickness. This affects the distribution of minima and maxima alongside the spike but it does not double the spikes. However, unwisting them carefully by bending them back creates spikes that look more pleasant and technically they also minimise obscuration albeit this is negligible.

[Black Knight]

OK thanks. I think I understand .

[barkis]

You could tidy them up by loosening one knob at a time, and using a pair of pliers with a piece of cloth in the jaws to hold the vane straight whilst retightening the knob. The cloth is to prevent marking the paint on the vanes. Repeat the process for the four vanes. It would seem the vanes must have rotated a bit when the knobs were tightened initially. The pliers wll prevent the same thing happening.

For aesthetic purposes at least, you may as well do the job .

Ron.

[Black Knight]

Ok, will do. Sounds like a plan

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