diffraction spikes

[chrisseal42]

Hi

I have a sky watcher 8inch SKY Watcher on brighter stars I get a Double diffraction spikes on the top and bottom why is this.

[Qualia]

As far as I gather, diffraction is caused by the secondary spider. The incoming bundle of photons, raining into your telescope in parallel light rays from the star, encounter the spider vane and diffract before arrving to the primary mirror. I think the diffraction noted is related to the length, pattern (curved or straight spider) and thickness of material used in the secondary support. I have read that curved spiders don't so much as rid one of diffraction, as much as they smear it over 360 degrees, so although diffraction is still happneing, spikes do not appear and if this is so, then I imagine the optimum secondary for a newt would be a single, very thin, curved spider.

I'm not too sure what is meant by double diffraction spike but if you mean something like the image below, then I too suffer from the same problem:

The thing which was going on here wasn't twisted vanes but that two vanes were not running at 180 degrees across the secondary cage (I have a truss dob), that is, two opposing vanes of the spider were not exactly aligned and to correct this I needed to gently adjust the screws of the vanes at right angles to the ones which were off alignment; you couldn't put a ruler along them in the photo and muster a straight line:

After a little help from SGL, a little fiddling and re-collimating the scope, everything was back to normal. Some six months latter and after much regular use, I've noticed the split diffraction just slightly beginning to happen again, so around the end of June when I get some time off work, I'll be back out sorting them out

Hope this helped a little

[JamesF]

To add to what Rob has written, there's an important point here that each vane of the spider creates a diffraction spike on both sides of the star, thus a three-vaned spider will create six diffraction spikes.  A four-vane spider hides this because the spike created by one vane is overlaid by the spike created by it's opposite vane, as long as they're perfectly in line with each other.

Of course we should not ignore the possibility that the additional spikes are created by something completely different that is interfering with the light path.  One of the "favourite" reasons for multiple diffraction spikes with Skywatcher newt OTAs is that the secondary mirror is not completely silvered and has a sliver of clear glass on one of the long sides.  Blackening the non-reflective surfaces may help there.

James

[chrisseal42]

As far as I gather, diffraction is caused by the secondary spider. The incoming bundle of photons, raining into your telescope in parallel light rays from the star, encounter the spider vane and diffract before arrving to the primary mirror. I think the diffraction noted is related to the length, pattern (curved or straight spider) and thickness of material used in the secondary support. I have read that curved spiders don't so much as rid one of diffraction, as much as they smear it over 360 degrees, so although diffraction is still happneing, spikes do not appear and if this is so, then I imagine the optimum secondary for a newt would be a single, very thin, curved spider.

I'm not too sure what is meant by double diffraction spike but if you mean something like the image below, then I too suffer from the same problem:

The thing which was going on here wasn't twisted vanes but that two vanes were not running at 180 degrees across the secondary cage (I have a truss dob), that is, two opposing vanes of the spider were not exactly aligned and to correct this I needed to gently adjust the screws of the vanes at right angles to the ones which were off alignment; you couldn't put a ruler along them in the photo and muster a straight line:

After a little help from SGL, a little fiddling and re-collimating the scope, everything was back to normal. Some six months latter and after much regular use, I've noticed the split diffraction just slightly beginning to happen again, so around the end of June when I get some time off work, I'll be back out sorting them out

Hope this helped a little

A little but mine are split right down to the star.

[JamesF]

A little but mine are split right down to the star.

That perhaps suggests that it's something else causing the additional spike.  Do you have an example image?

James

[chrisseal42]

no double image could it be collamation wish never clearned mirror for second time. the secondary fell out and had to put it back.