double spikes....why?

[adman]

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]

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

[adman]

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

[adman]

how twisted do they have to be?

[Merlin66]

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

[ZeroID]

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]

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

[tlgerdes]

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 :lol: 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/attachment_browse.php?a=87453

[solissydney]

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]

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

[adman]

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]

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 :shrug:

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 :thumbsup:

Adam

[RobF]

+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]

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

Adam

[tlgerdes]

Adam is you newt F4 or F5?

[adman]

it's an f5 trevor

[solissydney]

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]

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]

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

[tlgerdes]

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]

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]

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/attachment_browse.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 :lol:.

[Merlin66]

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

[tlgerdes]

Out of focus is like this

[tlgerdes]

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]

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]

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]

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

Take a 60 sec photo of Canopus.:lol:

[adman]

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]

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]

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]

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? :shrug:

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. :shrug:

Cheers,
Af.

[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?

[RobF]

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

[adman]

so i just need a de-wonkifier then?

[irwjager]

I think they call 'em hammers around these parts :lol:

[ZeroID]

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

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]

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

[asimov]

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.

[adman]

I know its not part of the problem this thread relates to - but I have just ordered some prostar flocking....should take care of that!

Adam

[multiweb]

Flocking is good for stray light but any object in a light path, regardless of shape, color, thickness or reflective index will cause diffraction.

One or two vanes in line will give you one line through your star. (the opposite sides overlap - like cables of an hyperstar)

Three vanes at 120deg will give you 6 lines (the opposite sides bisect the angle created by the two other vanes in the middle)

Four vanes at 90deg will give you 8 lines (the opposite sides overlap unless there is a slight angle offset and you then see the 8 distinct lines - problem discussed above)

[pmrid]

Adam, I have a good length of that Spotlight black felt with adhesive backing. I have no use for it now so it's yours if you want it.
Peter

[adman]

Doh! Too late! I ordered some prostar flocking yesterday! Thanks for the offer though. Hold onto it I'm sure there will be a Ten Chainer who could use it.

Adam

[peter_4059]

Interesting thread. I turned up a rod that fits tight in the secondary centre bolt hole (with the secondary removed) and then measured from the centre of the rod to the inner edge of the tube all around with a micrometer to ensure the secondary was centred. I didn't bother with offset.

[adman]

well - a bit of bad news.

I decided that I would loosen my mirror clips yesterday, so off came the end of the scope. I thought that while I was there, I would shorten my collimation bolts to better suit the moonlite focuser - it reached focus near the end of its outward drawtube travel - a legacy from the old skywatcher focuser.

Anyhoo - a few hacksawing minutes later, I had shorter bolts, and then it took me about a hour to reattach the mirror cell because the springs are now longer relative to the bolts - must be the tightest collimation springs around now!

So, I'm thinking - this is going to be great, we'll get to the bottom of these double spikes now....but of course after you muck around with the mirror, the scope needs collimating again. Out comes the catseye gear, and while I was checking the alignment of the secondary, I thought (well actually I didn't think) - cool - i'll just straighten out those wonky vanes while I'm here....

So I have corrected both problems, and now I will never know which one caused it. Does anyone have a re-wonkifier? :sadeyes:

Adam

[irwjager]

Love to see an 'after' shot of Alnitak now :)

Oh, and if you ever get nostalgic for double spikes in your images, let me know and I'll re-wonkify ST's Synth module for you. ;)

[adman]

Yep - don't worry - I'll still post an "after" shot. Knowing my luck it won't have changed a bit!:lol:

[RobF]

How's that bad news if you've got decent star shapes now Adam!?
I thought you were going to say you dropped the 2ndary on the primary or something at first ;)
Now THAT would be bad news.....

[adman]

Yep that would be bad news! Although good excuse to buy a new scope....?

Clearing nicely here tonight - should have an "after" shot to show you all later...

[adman]

...and then the clouds rolled in again for a few hours. Cleared up around 9:30 I think - but too late by then to set up :sadeyes:

[ZeroID]

The suspense is killing us all ... do we need a De-Wonkifier ? Do we need a Re-Wonkifier ? Will the vanes EVER be straight ? Will the clouds eventually roll away ?
Will Mary-Jane ever find her long lost little brother ..... :question:
sorry, wrong program there ... :D :rofl:

[adman]

I'm not convinced they will ever go away :sadeyes:

[adman]

still waiting for a clear night to test it out........

[adman]

well tonight wasn't a complete waste of time. I did a re-shoot of sirius with mirror cell loosened and vanes de-wonkified and it looks like I have pretty much sorted out the double diffraction spikes. They are still there to a small extent - but much less obvious.

From this I would imagine that the cause was the vanes not being straight / aligned.

EDIT: there is a nasty reflection in there also - would that be off the MPCC??