Square Spiders

[Astroscot2]

I was wonderig out of interest how others using newtonians for imaging square the spider vanes to minimise fat diffraction 

spikes on their images ? 

Thanks

[Astrobits]

I assume that you are trying to minimise the thickness of the vanes when viewed from the mirror. The thickness has no effect on the size of diffraction spikes so if they are slightly twisted they will only be reducing the amount of light by a very small amount.

Diffraction is caused by edges -- all edges -- in the optical path. That includes the edge of the mirror, the edge of the secondary and the edges of the spider vanes. Every bit of an edge will cause diffraction at 90 degrees to it's orientation and with a straight edge the contributions of each bit along the edge combine to form the familiar spikes. If the edge is curved then each bit is diffracting at a slightly different angle and in the case of a circular edge the diffraction is spread evenly around every element of the image. If your spider vanes are not straight then the diffraction spike will be spread slightly by the same angular amount as the bend in the vane which would give rise to broader spikes.

So just make sure that your vanes are either straight and in line with each other or go for circular or semicircular ones.

Nigel

[Astroscot2]

Hello Nigel.   Im more concered about the spider vanes being square to each other, the thickness is not an issue . I think Im going to make  a tool that ensure the vanes are 90 deg to each other.

[Steve Ward]

A twisted vane will show up quite noticeably in an image , easiest way to check them is through a collimating cap or Cheshire.

As far as "making" a tool is concerned , don't waste too much time on this task ... folding a piece of paper twice will give a perfect 90 degree angle ... http://thinkmath.edc.org/resource/shape-right-angle

The vanes intersecting angles are going to be determined by the position of the mounting holes on the tube , so don't get too upset if they're not exactly equally spaced ... there's little you can do easily to rectify it.

[Astrobits]

If your vanes are not square to each other then the diffraction spikes will be oriented at the same angle as the vanes. i.e if the vanes are at 80/100 deg then the diffraction spikes will also be at 80/100 deg. They will not be "fatter" as a result so long as the vanes are straight. However, if the opposing vanes are just slightly off straight then one set of diffraction spikes will be displaced by the same offset and if this is small, i.e. 2-3 degrees or so, then it will appear as a "fatter" spike.

Nigel