Double diffraction spikes on reflector

[Manav]

Hi guys,

I took a couple of test shoots with my reflector and noticed double diffraction spikes on a couple of them. Just wondering if you identify the possible culprits.

Updated here (http://s564.photobucket.com/albums/ss84/andronicus_/?action=view&current=IMG_0323.jpg)

Regards
Yugant

[DavidU]

Yugant, that image is a tad small to see any spikes, do you have a bigger one?

[Kevnool]

Yep way to small of an image.
Cheers Kev.

[Manav]

Lol Oops! Let me fix that...

[multiweb]

Hi Yugant, that's an easy one. If you get 8 instead of 4 then:

1_ they're rotated & offset (i.e. X shape). Your secondary is twisted on the primary optical axis.

2_ They're just offset (like your shot). You need to focus better until they merge.

[Manav]

Focus....Sorry about the silly question I'm just starting up :P

I was worried its a collimation issue or something wrong with the mirror.

[multiweb]

Doesn't look like a collimation issue at this stage. Once you're focused though you still can fish for other issues if you have any remaining diffraction or field curvature, coma, etc...

[Garyh]

yes, just have to focus more carefully. The diffraction spikes can really help in getting a good sharp focus!

[RobF]

Yes - Liveview on a bright star can give you very sharp focus. I now find a Bhatinov a little less fiddley, but I'm not sure my focus is necessarily any better than it used to be using diffraction spikes and Liveview + a bit of care.

Its good practice anyway to see what "just off" focus looks like so you can check for it by blowing up images on the camera screen or laptop. Nothing worse than wasting shots at a dark sky site from bad focus.

[scopemankit]

Pushing the image shows similar double diffraction spikes on many of the stars. Are your spider's legs exactly opposite each other or are the slightly displaced? If they are, that could be your problem.

[pmrid]

Hi Yugant. Looking at the photo, it seems to me that the stars are elongated (running across the diagonal of the image more or less) and this seems to me to coincide with the displacement of the diffraction spikes.
May I suggest a little experiment? Find a good bright star, Canopus, Sirius etc and take a short exposure of it - no more than a few seconds. Use live-view to get the focus the best you can first of course.
I woldn't be surprised if there was only a single diffraxtion spike on a short exposre - sugesting to me that the issue may be more about tracking/guiding than about defects in your optical path.
Peter.

[Manav]

Thanks for the advice guys will try this out next time I get a chance to image. I do have a feeling it's focus issue cause the first couple of pics are ok. I may have forgotton to lock the focuser in place. Once again thank you all will update you if it's still not resolved.

[Geoff45]

I agree that this is the most likely cause. each spider leg causes a diffraction spike on both sides of the star. If these don't line up, you'll get double spikes.
BTW this is why a spider with three legs shows six diffraction spikes.
Geoff

[Manav]

Hi Geoff - I'll take a short exposure shot tonight!

If the issue is the spider legs not matching I wouldnt know how to fix it since the assembly that hold the secondary togather is one unit.

Fingers crossed