Skywatcher 130 PDS - Twisted Spider Vane ?

[Spaced Out]

Hi All

I’m a newby just trying to learn about my telescope and imaging so apologies if this sounds daft.

Attached is my first ever image of Andromeda, it’s 9 frames stacked in DSS taken through a SW 130PDS. I was reasonably happy with this as a first attempt until I noticed split diffraction spikes on the brightest stars. I’ve checked each frame and the split spikes are there, so it isn’t something to do with stacking.

Having googled it and searched on here I am guessing the cause is most likely a twisted spider vane ? It is interesting that the split spike is prominent on one side of the stars only and also only appear to be on the left side of the image, just wondering if this means anything ?

If it is a wonky spider vane, is there a hassle free way to identify which vane and straighten it out ? I only ask because I made the mistake of fiddling with secondary mirror a while ago and it took me days to get things good again !

I’ve looked at the vanes and they look OK, if you squint and use a bit of imagination one of them might be just a tiny bit twisted looking, but I’m not really that confident tbh. Could a twisted spider vane be really difficult to spot and still cause this ?

Thanks in advance for any advice you can offer.

[MarsG76]

The image of Andromeda looks awesome... the split diffraction spikes are very strange indeed... Obviously it is caused by the spider vines. First thing I would do is check to make sure then they're all flat toward the mirror and not twisting at all... any twist will refract light in a strange unpredictable way, especially in long exposures.

[Spaced Out]

2 hours ago, MarsG76 said:

The image of Andromeda looks awesome... the split diffraction spikes are very strange indeed... Obviously it is caused by the spider vines. First thing I would do is check to make sure then they're all flat toward the mirror and not twisting at all... any twist will refract light in a strange unpredictable way, especially in long exposures.

Thanks.

All of the spider vanes look pretty straight to me except one that might be just a tiny fraction out but it is really hard to judge by eye, is there a way of determining this without taking photographs of stars through the telescope ?

If I need to twist a spider vane slightly do I adjust by holding it tight and unscrewing the screw on the outside of the tube to loosen then change the vanes position before re-tightening the screw ? If so, will that affect the position of the seconday mirror ? Sorry if these are really daft questions, never touched these before and I don't want to make things any worse !

[wimvb]

To test this, try the following.

Wrap something around the suspected vane and image a star. Repeat with each vane until you find the culprit.

Focus on a bright star (like Deneb in Cygnus), and gradually defocus. Eventually you will see the shadows of the vanes. At that point you can hold a finger near each vane at a time and identify them in the view.

Either of these methods should give you the position of the twisted vane.

Then loosen the screw on the outside of the tube, twist the vane until the diffraction spike looks good, and tighten the screw again. If you tighten as before, this shouldn't throw collimation off. But check collimation of the secondary mirror afterwards anyway.

Easiest way to check collimation is to focus on a bright star, then rock focus in and out. The defocused star should be symmetrical and look the same on inside and outside of focus.

[Spaced Out]

2 hours ago, wimvb said:

To test this, try the following.

Wrap something around the suspected vane and image a star. Repeat with each vane until you find the culprit.

Focus on a bright star (like Deneb in Cygnus), and gradually defocus. Eventually you will see the shadows of the vanes. At that point you can hold a finger near each vane at a time and identify them in the view.

Either of these methods should give you the position of the twisted vane.

Then loosen the screw on the outside of the tube, twist the vane until the diffraction spike looks good, and tighten the screw again. If you tighten as before, this shouldn't throw collimation off. But check collimation of the secondary mirror afterwards anyway.

Easiest way to check collimation is to focus on a bright star, then rock focus in and out. The defocused star should be symmetrical and look the same on inside and outside of focus.

This is brilliant, thank you ! Exactly the info I need to identify and then tackle the dodgy spider vane with some confidence. I've already learnt a bit about collimation and star testing so I understand that bit thanks. 

[Spaced Out]

Hmmmm...... I’m having to revisit this thread because I am still having trouble with this.

I’ve identified the vane that seems to be the issue and I’ve spent over an hour this evening trying to sort it out without any success. I’ve tried everything including being fairly brutal in twisting the vanes in different directions to see what happens, this made zero difference and the split diffraction spike just persists whichever way I turn them.

I’m just wondering if this is definitely a spider vane issue or could it be something else ? What a headache !

[wimvb]

I have the same issue as you, but much less. I've been doing some light diffraction simulations, but can't replicate the proper diffraction pattern. Mainly because any diffraction pattern I get is symmetrical. If you look very closely at the brightest star in your image, you'll notice that opposite of the split spike, the other spike is also split, but much weaker.

The closest I get to this is when I simulate a situation where the vanes are not at right angles, i.e. where the secondary mirror is not exactly centered.  This gives a diffraction pattern similar to that of a Bahtinov mask. Defocused, the position where the spikes cross, moves away from the central bright spot. It may be worthwhile to test this.

I hope this makes sense.

[wimvb]

Here's the simulation I mentioned before. An aperture with an obstruction that is slightly shifted upwards and 4 spidervanes. The vertical vanes are precisely vertical, while the "horizontal" vanes connect  the 3 (9) o'clock position of the aperture to the 3 (9) o'clock position of the obstruction.

The aperture: (Btw, the size of the obstruction shouldn't be a critical parameter here.)

The diffraction pattern it produces at focus:

And at a very sligth defocus (50 microns according to Maskulator, within the critical focus)

(the horizontal spikes are slightly wider, and the vertical X starts to ever so slightly shift upward)

[Spaced Out]

19 hours ago, wimvb said:

Here's the simulation I mentioned before. An aperture with an obstruction that is slightly shifted upwards and 4 spidervanes. The vertical vanes are precisely vertical, while the "horizontal" vanes connect  the 3 (9) o'clock position of the aperture to the 3 (9) o'clock position of the obstruction.

The aperture: (Btw, the size of the obstruction shouldn't be a critical parameter here.)

The diffraction pattern it produces at focus:

And at a very sligth defocus (50 microns according to Maskulator, within the critical focus)

(the horizontal spikes are slightly wider, and the vertical X starts to ever so slightly shift upward)

Thanks so much for taking the time to respond to this, I’ve been twisting and turning the spider vanes about the place for 2 days now without any success, these split diffraction spikes stubbornly remain the same !

Looking at this again I think you could be right with your diagnoses.  Looking at the vanes face on the two that are left and right on my pics seem slightly raised at the secondary holder and curved upwards a little bit, while the others are straighter.

So, how can I remedy this ? Sorry I’m a novice and this is all new to me. I’m guessing I loosen the top screw and tighten the bottom to pull the secondary down slightly ?

[wimvb]

6 hours ago, Spaced Out said:

I’m guessing I loosen the top screw and tighten the bottom to pull the secondary down slightly ?

 

 

That would be the way to do it. Be sure to keep the scope collimated. That's more important than diffraction spikes.

[Spaced Out]

Thanks. I’ve made the adjustments and now the spider vanes in question are a bit more horizontal (see the photo). I’ve got the spider vanes as good as I can get them by eye in terms of being centred and at right angles. I’ve just done a quick test outside on a bright star and the problem persists !

Now I’ve moved the spider vanes the scope is a little out of collimation too (I did a star test to check), so that’s another little job for tomorrow, I've only done collimation once before and it took me 2 days to sort out !

I guess for observing this is just a minor problem but I am trying to get into imaging so I want this scope to be set up as well as I can get it, the split spike thing is really starting to bug me now !

[wimvb]

I've run out of ideas.

[Spaced Out]

These split spikes only seem to appear on the left side of my images, I’m wondering if that means anything ?

[Spaced Out]

Just did a rapid photo test outside. It was a race against the cloud but with a star centred it looks perfect, with a star on the left of the frame it is splitting. I didn’t have time to do one with the star on the right because the thick cloud rolled in.  

[spillage]

Have you tried to cut out a circle the same diameter of the scope, fold in half and half again and just cut of the tip of this. Place it over you scope and make sure the centre screw of the secondary is central. I would also recheck that the secondary is central to the focuser using you collimation cap.  Your last image to me shows some veins (top) seem a bit bent???

[wimvb]

I wonder, is it symmetrical around the centre? Are the spikes split to the other side, on the other side of the centre of the image? Just checked your original image. The right hand side looks normal. It's only in the left half of the image.

Edited October 27, 2017 by wimvb

[Spaced Out]

4 minutes ago, spillage said:

Have you tried to cut out a circle the same diameter of the scope, fold in half and half again and just cut of the tip of this. Place it over you scope and make sure the centre screw of the secondary is central. I would also recheck that the secondary is central to the focuser using you collimation cap.  Your last image to me shows some veins (top) seem a bit bent???

Yeah done that, it does seem central. I'll recheck the secondary tomorrow when I try to collimate it again.

I thought the vanes look a bit bendy in places too ! Is that normal ? Bought the scope 2nd hand and never touched the spider vanes before the last few days so no idea what's happened to them previously. Still doesn't affect the diffraction spikes in the centre of images tho which look fine.

[Spaced Out]

16 minutes ago, wimvb said:

I wonder, is it symmetrical around the centre? Are the spikes split to the other side, on the other side of the centre of the image?

I'll try and collimate it tomorrow and re-test this on the next night with any stars available.

[Spaced Out]

Well I’m feeling that this may have beaten me.

Yesterday I spent all afternoon adjusting the spider vanes and collimating the scope the best I could.

I was meticulous with getting the spider vanes centred and as straight as possible, each vane is now equal in length from the tube to the centre of the centre screw down to around 1/2mm. I spent hours on this and then checked it using the card disc with a centre hole I’ve made previously for collimation, I then made another paper disc just to double check that too !

After fiddling about with the vanes I had moved the secondary about so I then spent ages collimating the scope again. Once that was complete I did a quick star test which showed the scope was pretty well collimated.

I then did some quick tests on a bright star and the problem persists.  

Below are 4 images of the same star moved across the frame. When on the left side the star diffraction spikes split but in the centre and on the right they look OK ? The images are awful quality (1 second exposure sat on a mount with no power at 20,000 ISO) but they show that the problem is not resolved.

I’m left scratching my head now. I don’t know what else to try and I’m wondering if I might just be better finding a telescope shop somewhere to identify the problem and sort it out for me ? Not really a route I wanted to take.

Any further thoughts or ideas would be most appreciated at this stage !  

[wimvb]

It may partly be a focus issue. When I did the simulations, I noticed that the vertical (unsplit) diffraction spike gets wider when defocused. Eventually it will split in two parallel spikes. The vertical spike looks a bit wider in the first image. So my conclusion is that it's slightly defocused as compared to the other images. Did you refocus between exposures? If not, focus seems to shift across the image plane.

If it's not that, I really have no other clue.

[Ricochet]

Where is your focuser in relation to that last set of images? Is it at the top? If the focuser is protruding enough to obscure some off axis rays but not enough to obscure on axis rays then it seems to me that you could get an additional diffraction spike on one side of the image. The additional spike would be orientated at 90° to the focuser tube so the additional horizontal spike suggests a vertical intrusion and your test star not only moves left to right but top to bottom. 

[Spaced Out]

On 29/10/2017 at 11:57, wimvb said:

It may partly be a focus issue. When I did the simulations, I noticed that the vertical (unsplit) diffraction spike gets wider when defocused. Eventually it will split in two parallel spikes. The vertical spike looks a bit wider in the first image. So my conclusion is that it's slightly defocused as compared to the other images. Did you refocus between exposures? If not, focus seems to shift across the image plane.

If it's not that, I really have no other clue.

Thanks

I did not refocus between exposures. For this quick test I just chucked my mount outside and wanged a scope on it, so no laptop or ultra fine tuning of the focus, just got it as good as I could using the back of camera LCD.

Hmmmmm.......

[Spaced Out]

On 29/10/2017 at 12:19, Ricochet said:

Where is your focuser in relation to that last set of images? Is it at the top? If the focuser is protruding enough to obscure some off axis rays but not enough to obscure on axis rays then it seems to me that you could get an additional diffraction spike on one side of the image. The additional spike would be orientated at 90° to the focuser tube so the additional horizontal spike suggests a vertical intrusion and your test star not only moves left to right but top to bottom. 

Hi, thanks for this.

Not quite sure I understand what you mean ? Sorry I'm a beginner and a bit of a duffer when it comes to this, probably why I am struggling ! The focuser was at the bottom in the test shots.

Re- stars moving top to bottom, if you look very carefully at the original picture I put up at the beginning all the bright stars from a little bit left of centre exhibit these split spikes (from top to bottom) and all the others right of centre don't.

This little issue is really messing with my head !  

[wimvb]

16 minutes ago, Spaced Out said:

...

Re- stars moving top to bottom, if you look very carefully at the original picture I put up at the beginning all the bright stars from a little bit left of centre exhibit these split spikes (from top to bottom) and all the others right of centre don't.

This little issue is really messing with my head !  

Any unevenness like this suggests that there is tilt between the sensor and the focal plane. The SW crayford focuser can actually be collimated. There should be three pairs of screws around the focuser base. These can be used to align the focuser.

Here's a video showing how to centre and align the focuser of a SW newtonian (about 25 min in)

https://www.youtube.com/watch?v=3LbR1nIx-jw

Personally I would do such a job either during the summer, between seasons, or during a cloudy weekend.

Btw, my simulations were only for on axis/symmetrical obstructions. I will have a go at simulating off axis effects.

[Ricochet]

1 hour ago, Spaced Out said:

Hi, thanks for this.

Not quite sure I understand what you mean ? Sorry I'm a beginner and a bit of a duffer when it comes to this, probably why I am struggling ! The focuser was at the bottom in the test shots.

Re- stars moving top to bottom, if you look very carefully at the original picture I put up at the beginning all the bright stars from a little bit left of centre exhibit these split spikes (from top to bottom) and all the others right of centre don't.

This little issue is really messing with my head !  

Don't worry about it. My suggestion can't be right looking at the original image again. Go with the suggestion to check your focuser is aligned properly.