double spikes....why?

[tlgerdes (Trevor)]

I think you will find it is the adjacency of the each to the other that is causing the double. The fact that you dont have them consistantly all the way around could be centering of your secondary or squareness of your focuser to secondary.

[solissydney (Ken)]

Trevor, double lined spikes indicate an out of focus image. I use a feeler gauge to check the focus on my 12 inch F/5 Newt.. One quarter of one mm makes a visual difference
Ken

[tlgerdes (Trevor)]

On mine it is not, double spike on mine is "in focus", if I dont have them then I notice I am out of focus.

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

Again, like Adam, it is only on really bright stars. You will notice in the attached link that one spike on each side is stronger than the other and the diagonally adjacent spike is weaker than the stronger one

Double parallel spikes like a # are of course out of focus.

Here is an exaggeration of what I mean. It means my spider is slightly out of shape ....... and something I should work on .

[Merlin66 (Ken)]

Have you physically checked the secondary spider vanes yet?????

[tlgerdes (Trevor)]

Out of focus is like this

[tlgerdes (Trevor)]

Originally Posted by Merlin66

Have you physically checked the secondary spider vanes yet?????

My investigations suggest this is the problem...... with both Adams and mine.

Part of this in mine is using an offset secondary ie offset further away from focuser than dead centre

[multiweb (Marc)]

This is because your spider vanes are not square. Meaning the offset of your secondary causes the spider vanes to be slightly bent so your diffraction spikes don't overlap. See my small drawing (dramatisation).

[ZeroID (Brent)]

Interesting. I know my vanes are not perfectly symetrical due the mirror offset but they are very thin ( .5mm) and satin black. Even on brighter stars my spikes are almost unnoticeable, well to me they are. Must be well aligned with the optical path. I'll take a more critical look next time the clouds clear.

[tlgerdes (Trevor)]

I think your .5mm vanes would make a diff. Mine are about 1mm.

Take a 60 sec photo of Canopus.

[adman (Adam)]

I should get some time tonight to check the vanes. I am going to loosen my mirror clips as well, and we'll see what happens...probably just mirror slop!

[solissydney (Ken)]

Since the spikes seems to divide more the further it goes from the center of the star I believe it could be caused by the curvature of the primary mirror.
Ken

[adman (Adam)]

here you go - the possible culprits

2 of the vanes are a little snaky, and I think maybe a couple are offset with respect to the opposite.. what do you reckon?

Adam

EDIT: Looking down the barrel like that with my little catseye red light illuminating things - it could do with a damn good flocking!

[CarlJoseph (Carl)]

Adam,

I'm no expert, but ... could it be because your spider vanes might be bent and potentially not in alignment with it's opposite?

Check the "green" lines painted on this image (your spider vanes look a little twisted). I know it depends on the precise angle of your photograph, but taking a good look at them in real life might help.

Cheers,
Af.

[irwjager (Ivo)]

Marc (multiweb) is correct. Your spider vanes are just a tiny bit wonky
To settle this with some science, I quickly coded a spider vane-wonkifier into StarTools' Synth module (this module synthesizes stars based on a virtual scope model). It clearly shows the effect of the wonky spider vanes on the diffraction pattern - notice how the horizontal diffraction spikes starts to split towards the edges?

[RobF (Rob)]

Originally Posted by irwjager

Marc (multiweb) is correct. Your spider vanes are just a tiny bit wonky
To settle this with some science, I quickly coded a spider vane-wonkifier into StarTools' Synth module (this module synthesizes stars based on a virtual scope model). It clearly shows the effect of the wonky spider vanes on the diffraction pattern - notice how the horizontal diffraction spikes starts to split towards the edges?

How cool is that!
What an awesome thread this has been for Newt owners....

[adman (Adam)]

so i just need a de-wonkifier then?

[irwjager (Ivo)]

Originally Posted by adman

so i just need a de-wonkifier then?

I think they call 'em hammers around these parts

[ZeroID (Brent)]

Originally Posted by irwjager

I think they call 'em hammers around these parts

Anyone got a 'De-Wonkifier' in stock. Someone is going to have to patent that idea...

Actually, going by Afro Boys posting with the green cross superimposed on the picture you can see that the vanes are all twisted around the scope optical axis hence I would say they 'look' wider. Either they could be tighter, pulling them straighter or the attachment points at the hub tends to offset their force because of design or something.

And yes, you need to flock that tube !! Even some good satin black paint would help.
(Wonder if that has any bearing on the matter. ?)

[tlgerdes (Trevor)]

Originally Posted by ZeroID

And yes, you need to flock that tube !! Even some good satin black paint would help.
(Wonder if that has any bearing on the matter. ?)

No bearing, my OTA is flocked with "the good stuff" and I still get them.

[asimov (John)]

I get the same thing in my 14" newt visually. I automatically assumed it was because the vanes where not perfectly 90 Deg to each other.

At perfect focus, I see 4 though, so it's not a problem for me.