double spikes....why?

[adman (Adam)]

Can anyone shed any light on why I am getting spikes like these?

I have a feeling it may be that my mirror clips are a little tight - would that do this? I don't think that it is focus as I am using a bahtinov mask to get it pretty well spot on, and also the FWHM focusing tool in APT.

Adam

[leon]

Adam, I am no sure but i suggest it may be vibration or movement of some sort during the exposure.

Leon

[Merlin66 (Ken)]

The the vanes on the secondary misaligned or twisted...this can cause that effect

[adman (Adam)]

Originally Posted by leon

Adam, I am no sure but i suggest it may be vibration or movement of some sort during the exposure.

Leon

no - its too consistent. If my focus is bad then the double lines get further apart....

[adman (Adam)]

Originally Posted by Merlin66

The the vanes on the secondary misaligned or twisted...this can cause that effect

how twisted do they have to be?

[Merlin66 (Ken)]

Stand about 6' infront of the tube...do all the vanes look as thin as possible and uniform thickness???

[ZeroID (Brent)]

Possibly your vanes are not aligned directly opposite each other. In a three vane setup you can sometimes see 'ghost' vanes in the view opposite each vanes more obvious spike. One of the advantages of a 4 vanes setup is these are normally hidden in the opposites vanes spike.
You diffraction spikes are also quite colourful and defined. I would guess maybe twisted is another posibbility. Are the vanes blackened and thin?

[solissydney (Ken)]

Only two of the spikes are double, the other spikes are perfect. Collimation perhaps?
Ken

[tlgerdes (Trevor)]

I have an 8" F4 and I get the double spikes as well. I get them though on all 4 points. The fact you get them only on 2 could indicate your collimation is off ie. I used to get them on 2 before I learnt the art.

I never worried about the double spike, I actually use this as my measure of focus, if I dont get them then i am out.

Here is a sample

http://www.iceinspace.com.au/forum/a...se.php?a=87453

[solissydney (Ken)]

I focus with the aid of spikes on my 12 inch F/5. But my spikes are always a single spike for the full length of the spikes, unlike yours that separate going towards the edge. Funny that, I wonder why that should be?
Ken

[solissydney (Ken)]

I better take my words back because I have never taken such a large images of a star
Ken

[adman (Adam)]

Originally Posted by solissydney

I better take my words back because I have never taken such a large images of a star
Ken

yeah - it looks huge doesn't it !

its actually just a close crop of Alnitak - you can see the dust from the flame nebula in the top right. Here's the original sub...

[adman (Adam)]

Originally Posted by solissydney

Only two of the spikes are double, the other spikes are perfect. Collimation perhaps?
Ken

I have a full set of catseye collimation gear, and while I am fully willing to admit I am no expert - I think its close. I haven't spent the time to get all 4 reflections of the spot to line up - but they are all visible. I am not sure how much difference that last 1% makes

I am going to try loosening my mirror clips and repeat the image and see what I get. I will keep you posted.

Thanks for all your suggestions

Adam

[RobF (Rob)]

+1 for what Merlin said Adam as worth trying - they are fairly thin and its not too hard for one or more of them to be twisted - especially if you have been playing with the location of your assembly for the secondary. Tightning the outside screws on the OTA can be enough to twist them a bit. You can it very close by eye looking from the front of the scope.

Just checking - its just a single exposure, not a stack of images?

[adman (Adam)]

thanks Rob - I will check those out too...

Adam

[tlgerdes (Trevor)]

Adam is you newt F4 or F5?

[adman (Adam)]

it's an f5 trevor

[solissydney (Ken)]

Are the double spikes really an issue?
I mean, I doubt that it can be seen on any average size stars on an image.
What do you think?

[tlgerdes (Trevor)]

I think it is the lack of symmetry about them that Adam is possibly concerned about.

I did some reading on diffraction spikes this morning and i believe it is the lack of symmetry in the spider that is causing them. ie the vanes at not 100% adjacent to each other.

If you have a 3 vane spider you normal get 6 spikes (3 actual and 3 reflections for want of a better analogy), I have 8 spikes for a 4 vane which would indicate that 4 of the spikes are reflections from the opposite vane.

[adman (Adam)]

Yep that's about right trevor. They bug me aesthetically, but it also must indicate that something is out of whack.