Split Spikes

[Qualia]

I was wondering if anyone could help?

This is a simulation - highlighted in negative - showing the kind of visual image I see when looking at bright objects with the 10" f5 (truss dob). The picture only shows one set of diffraction spikes being split (indicated by red arrows), but in reality, they're also split 180º over on the opposite side.

No matter how attentive I am with collimation I can't seem to better the image. I've even tried tweaking the secondary while observing but it doesn't seem to help. The question is, am I doing something wrong with collimation, or is there some other problem I'm not aware of?

Any insights would be a help.

[michael.h.f.wilkinson]

First thing that springs to mind is two opposing vanes of the spider not being exactly aligned. That would do it.

[Qualia]

Thanks Michael. Is there anything a simpleton can do to align them? Although the poor truss has had many years of tender loving care, it has also been through a fair bit of travelling, and I've probably knocked them on some journey or other

[michael.h.f.wilkinson]

If my idea is correct, adjusting the screws of the vanes at right angles to the ones which are off alignment could sort it. This will mean adjusting collimation again.

[kalasinman]

The opposite spike is longer. I had a similar problem and sighting down the edges of the vanes, some were aligned with the primary spot and others weren't I've carefully loosed the outside tension knob and held them slightly too far the other way and then tightened the knob again. It takes a little fiddling. Unfortunately after I did this , the sky went cloudy for a couple weeks. Hoping to get back to see the results soon. Interested to see how your efforts go.

[glowjet]

Rob this boils down to our old friend Fraunhoffer, who's principles were used in the design of the Bhatinov Mask. Were telescope are concerned, any obstruction to the light path at the opening of an optical tube, be it a strip or slit opening, will cause a diffraction pattern to lie on the centre of the optical path of the instrument, and further this will form at right angles to the original obstruction. Not being a reflector man I have just had a thought, is it possible to shine a strong light down the tube, place a paper disc over the mirror and adjust the vanes till the shadows merge

[Qualia]

Thanks John, Kalasinman and Michael,

If the vanes should be aligned in an exact straight line from one side to the other, then they are not aligned. However, the vanes don't appear to be twisted and adjusting the vane screws doesn't appear to make any difference to correcting the alignment.  

I'm a bit at a loss as to what I should be looking for, or whether this kind of aberration can be corrected.

I've taken the secondary off to make a new washer and to give it a little clean (there's a finger smudge on it) but here's a picture to show what the vanes look like:

Does this look good, or should I try to work towards obtaining a perfect cross shape?

[Macavity]

I am pretty convinced that it (Hey, me too) is non-orthogonality of the spider veins.

Another thing on my lengthening "to-do" list with my GSO F/4 Photo-Newt...

At least yours are symmetrical. My big split is on ONE of four.

But I plan a general revamp - including adding the famous secondary offsets etc.  

[Uranium235]

Definietly one of the vanes being slightly twisted, I had a similar problem with the 130pds after some over-aggressive collimation. First you need to identify which vane it might be, then give it a slight twist to straighten it out as best as you can by eye.

Unfortunately, you wont know if its worked until you get it outside 

However, looking at the picture, they do seem a little off (esp the left one).

[kalasinman]

Wouldn't it be good if the center screw of the secondary were centered in the tube? Measure the ID, take a compass and make a disk of paper the same with a hole in the middle about the size of the center screw. Then you can easily see where it's got to be.

[sharkmelley]

Hi Rob,

Looking at your picture, the left hand vane slopes up to the right and the right hand vane slopes up to the left.  If you place a straight edge against the image, you'll see that they don't form a perfectly straight horizontal line through the secondary.  So the secondary must be moved downwards by tightening the bottom adjuster and loosening the top adjuster until the left and right vanes form a perfect straight line.

I hope that makes sense!

Mark

[steppenwolf]

I don't think this is a twist, to me this is simply a misalignment of the vanes -  a twist will certainly give issues but these double diffraction spikes are crisp and clear and symptomatic of straight but misaligned vanes. What I don't understand is why releasing the 'top' (in your image) vane bolt and tightening the 'bottom' vane bolt doesn't make a correction? As you make this adjustment, you should also expect both the left and right vane adjustment bolts to slacken a fraction automatically.

[John]

I tend to agree with Steve. I think you have two vanes that are not running at 180 degrees to each other - the pair that run horizontally across the photo you posted. That would cause the double diffraction spike I believe. The other pair of vanes do seem to be at 180 degrees - ie: you can put a ruler along them in the photo.

[Qualia]

Thank you for the thoughtful replies.

I've been working a little on the vanes this evening. I've been gently tweaking and loosening the bolts in a clockwise fashion to see if I can get a more or less 180º line on the vanes on all sides. There's very little play and if there is any movement it is extremely subtle. I've left it for now and will see in the morning whether there has been further movement of the vanes before tweaking one last time.

I've also made a couple of new washers from an old soap container to aid with collimation and have even given the secondary a little wash with distilled water.

I'll post up a pic when I think I can't do anything more.

Thanks again for the help, gentlemen. It's much appreciated.

[John]

Hope it works out Rob

[Macavity]

I notice now that one of the veins is indeed twisted...

Maybe it does indeed account for the following:

Interesting you can get just the ONE spike split! 

(Diffraction from both front / rear edges maybe?)

I shall adjust it "carefully" with a pair of pliers...   

[Peter Drew]

My question would be how much does this matter? spikes or no spikes are pretty much an aesthetic issue and shouldn't affect the focused image quality.  If, however it is bothersome, it should be possible to adjust the vanes with care.    

[Qualia]

Peter, the initial desire is probably stemming from aesthetics but on a more practicle note, I also wanted to make sure I wasn't messing up with collimation or whether there was anything untoward happening with the scope.

I also felt it was a good time to give the Moonshane a little 'spring clean'. It's been in almost constant use for the last two years, has done an awful lot of travelling and I really fancied giving it a bit of tender care; a thorough clean (mirrors and wood work), tightening the bolts, new secondary washers etc.

I don't have a problem with clean diffraction spikes and find them gorgeous on bright stars. I'm not that keen on them when viewing planets and it was starting to get on my nerves seeing split vanes as I was trying to tweak detail from Jupiter. We haven't had a clear evening this week, but as soon as there is one, I will see how my tweaking has gone. Hopefully I've done nothing wrong, but I'm not holding my breath

[Charic]

Qualia.....this link http://blog.astrofotky.cz/pavelpech/?p=958 shows similar splitting as you describe!

[kalasinman]

Have you asked the maker?

[Qualia]

Quick update. Took the scope out tonight, tweaked the collimation and everything was spot on, first time . Tight spikes on bright stars and superb quality of detail on Jupiter and the Moon. I guess one can't ask for more. I had had the primary and secondary mirrors out this week and feared on replacing them I might have noticed a bit of mirror astigmatism or misalignment, but again, everything was spot on. I've left the scope for tonight and next clear evening will see how everything has settled in, but for now thumbs up.

Thanks again for all your help

[Dave In Vermont]

There you go, Rob! Just remember these 3 words: Collimation Is Fun! Then the scope will know you enjoy collimating, and it won't bother you again!

Clear & Sharp Skies,

Dave