Spaced Out

19 hours ago, Adam J said:

Honestly I am now out of ideas, spikes are almost universally caused by protrusions into the light path. It could still be a twisted spider but you have checked that. 

So...strand of spider silk in the light path?

Perhaps a primary mirror clip? 

Its an interesting one, i doubt flocking will help. 

Hope that you get it resolved. 

 

Hey don’t worry and thanks for the help !

My gut feeling is that it is probably focuser tube intrusion, it does stick quite a way in ! However I’m going to follow the advice here, I will centre the focuser and re-collimate first. If that doesn’t fix it then I’ll chop the focuser tube down.

I’m sure the flocking won’t resolve this issue but I was thinking of flocking/darkening the tube and mirror cleaning anyway so I figured why not do it all while I’m fiddling about with everything !  

23 hours ago, spillage said:

I would strip it down flock it, put it back together and get everything right and then look at doing the focus tube after re-testing.

Good advice, thanks !

Right I’ve fumbled about with the primary and moved it down the tube to the point where it fell off one of the screws ! I managed to get this screw back in and tighten it up a little, so the mirror is now as far forward as I can safely get it, still I have the split spikes on the left side of the image !

At this point I’ve decided to take the nuclear option and strip the scope down. I will lop off a section of the focuser tube (i’ve marked on the tube where the focus point is for the DSLR & MPCC) and centre the focuser. While I am at it I shall clean the mirrors, flock the scope and get the blackboard paint out too.

Once rebuilt and collimated if I still have this issue I may just chuck it in the bin !

Any advice on this little project ? I’ve just watched the astronomyshed YT vid on mods which has explained the dismantle/re-assemble process quite well, it’s spurred me on a bit.

With regard to chopping the focusser tube, I’m only going to use this scope for imaging so I am happy to try it, how much should I be lopping off ? Is it still OK to have a bit protruding into the scope at the point of focus ?

Ok.... so I’ve pushed the primary forward by (I’m guessing) 8mm or so on the adjusters, a quick test on a bright star and the problem is still there.

The lock screws still have a bit of thread visible now (about 5mm), not sure if I should push it any further or if it would make much difference if I did ?

I’ve tried using an eyepiece on a bright star to see how that looks but I’m finding it really difficult to make the spikes out clearly. Probably my tired old eyes !

Is there an easy way to assess if the focuser is tilted somehow ?

Before I have a fiddle I am just wondering how far forward the primary will move on the rear adjusters ? I've just moved it forward a few turns now (maybe a few mm) and the lock screws still have a bit of thread visible (about 5mm). If I go too far will the mirror disengage ?!

2 minutes ago, Ricochet said:

I think it is possible. Consider the simplified diagram of a Newtonian tube below (secondary/spider not shown). Light enters from the top and strikes the primary mirror at the bottom. The red lines signify the parallel light rays from a point at the left hand edge of the field of view and the blue lines those from the right hand edge. 

At a certain distance of drawtube intrusion it must be possible for some of the light from one side of the field of view to be diffracted by the drawtube but not the light from the other side. However, note that the real angles will be smaller and should not be constrained by the width of the telescope tube. Judging by your first image I'm going to estimate your field of view at about 3° and I'm going to estimate the distance between the primary mirror position and the focuser at 500mm. We can then estimate the "distance" of the edge of field rays compared to the on axis rays when passing the focuser as 500 * tan (3°/2) = 13mm, let's call that half an inch. The distance between the right and left hand edge rays is therefore an inch and the gap between the tube and mirror is also usually in the region of an inch. Based on that we can estimate that:

• If the focuser intrudes by less than 0.5" it will not affect the image
• If the focuser intrudes by more than 0.5" but less than 1.5" it will affect part of the image (at 1" it will affect half of the image)
• If the focuser intrudes by more than 1.5" is will affect the entire image

Of course, the question of whether the focuser intrudes into the light path is not necessarily the same as the question of whether it doing so is responsible for the effect that you are seeing. There was a small clear-ish patch of sky last night so as a test I tried varying the amount by which my drawtube intrudes into my Dob. At times I thought that I could see dual spikes but I could never be sure that it was definitely there and not caused by something like a slight unfocusing due to eyepiece field curvature or the like. Perhaps the hazy conditions did not allow a good test, or I would need a camera to test it. As such I would suggest it is worth exhausting every other possibility (i.e. possible focuser/sensor tilt) before cutting into the drawtube.

Thank you for taking the time to write this response, it makes sense.

I think I'll try bringing the primary forward first and see how we go with that idea. I'm a bit scared of moving it too much in case if falls off or something ! I'll take a look at it this afternoon and see how easy it looks.

I've only got the two standard thumb screws on my focuser but the MPCC and camera seems to lock nice and flush/tight with those so not convinced that it is moving/tilting, but I'll take another look.

If the moving the primary doesn't help then I'll look at the focuser tilt. If that doesn't work then I'll probably get the hacksaw out. I'm now thinking that if I have to do that I might just strip the whole thing down, flock the tube and clean the mirrors while I'm at it ! Could help me learn more about how it all works, or, could be a nightmare getting it back together again !   

Looking at my issue with the split stars, would this focuser intrusion account for the split stars appearing on one side of the images only then ? At this point I am willing to try anything but it would be nice to be fairly confident before I get the hacksaw out and butcher it !

1 hour ago, spillage said:

Have a go at this mod:

Thank you for this. That looks quite scary initially, but it's actually quite straightforward I guess. Did this mod solve a similar issue ? Do the remaining longer parts of the tube cause any problems ?

12 hours ago, Adam J said:

I am going to cut the the big revelation here...its not your spider vanes. Its not tilt. Its not colimation.

It is the focusing tube interrupting the light path due to the large amount of back focus required by your DSLR camera.

This will only show up on a DSLR because of the back focus required and it also requires that the primary mirror is set as for away from the secondary mirror as is possible on its adjustment screws when skywatcher put it together.

You can move the primary up the tube on the adjustment screws (dont go too far) and it will move the focal point out and hence the focuser tube with it. The alternative is to lop about 1.5cm off the focuser tube. Either way this will move it out of the light path.

Adam

 

Wow ! Thanks for this explanation. Would this account for the split stars appearing on one side of the images only ?

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 !  

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.......

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 !  

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.

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.

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.  

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

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 !

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 ?

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 !

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. 

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 !

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.

18 hours ago, Adam J said:

If this is your first ever image that is really very very good going.

Thanks very much !

Yeah it was my first attempt. Read up a bit of background beforehand and have done a bit of landscape stylee astrophotography before (milky way etc) so not a complete novice with a camera. However, I am a complete novice with a telescope and mount and DSS software.

Really enjoyed getting this first image, I learnt a lot from it and it has spurred me on to read up more and try to improve at this (and probably get better equipped too)

Some of my landscapey astro stuff is here....

https://www.flickr.com/photos/132427272@N04/

I hope to be adding some more DSO stuff to this soon !

On 31/12/2016 at 15:34, wimvb said:

Great first image. Just keep doing what you are doing, and things will get easier after a while. Once you have the data, you can always reprocess as you learn the software.

To get focus right, I recommend a Bahtinov mask. It's cheap, easy to use and speeds up this part a lot.

For polar alignment you can use the routines in the HEQ5 Pro hand controller (Synscan). Roughly polar align your mount (level, set alt to your latitude and use compass to point az to north)

Then do a 2-star alignment. Use a barlow and a short fl eyepiece for accuracy. Then do the synscan polar alignment routine. (Just watch carefully in what direction the stars move when the mount wanders off.) Then repeat the 2-star alignment. Repeat until satisfied. I find this routine gets me close enough for photography.

Good luck

Thanks for the advice, was thinking about getting a Bahtinov mask next, they seem well recommended.

Still finding my way around the Skyscan bizness, tbh honest I find it harder to align that than polar aligning the mount itself. Just getting the scope locked onto the correct star takes me ages ! Sure it'll come with more practice and learning some more stars.

Next job is to read up more about methods and processing, got a very basic grasp on things, enough to chuck my dslr on there and get some frames, but feel I need to have a much better understanding in order to improve my images.

Hi all

This is the thread that encouraged me to buy a 130 PDS as my first telescope. I used it for the first time a couple of nights ago and this is my first ever image from it.

I had a real hassle setting up because I’m a total newbie and didn’t have a clue what I was doing. Took me hours to polar align my mount but got there in the end, then had hassles getting anywhere near focused, then tried using DSS which has a lot of settings to give you a headache !

Anyway, I got something in the end. It is far from perfect but I am reasonably happy with this as a first time effort, it has given me enough encouragement to start learning more about astrophotography.

Thanks to everyone on here who’s 130 PDS images have inspired me to get involved !

21 hours ago, Adam J said:

And now with some OIII

8 x 900s ISO 1600 OIII - Blue

16 x 600s ISO 800 Ha - Red

Did not bother with any calibration frames.

Posted in imaging section too but I like to share with my fellow 130PDS users

WOW !!