Skywatcher 130 PDS - Twisted Spider Vane ?

[Adam J]

On 10/7/2017 at 19:28, Spaced Out said:

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.

Here is my first ever astro photo from over two years ago. Ah the memories, but take a look at the brightest star. Looks kinda like what you have? Apologies for my poor focus.

I spent about 6 months obsessing over my spiders....

Yet still I had additional spikes! Here is M45, but what you will notice is what you have noticed. See the star ringed in red. My collimation is off and its the closest to the center of the optical field. to the left the extra spikes lean one way to the right the other way. 

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.

Below is after the fix with a mono modified DSLR I was playing about with at the time. Nothing else changed just the primary adjustment screws moving it further up the tube. 

 

Adam

 

[Adam J]

2 hours ago, Ricochet said:

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. 

Actually, I think you got it. If I had a penny for every time this issue comes up with the 130pds when coupled with a DSLR...I would have something like 10p....

[spillage]

Please do not chop of a bit from the end of the focus tube as if you change your camera this can cause issue. You can remove a square front and back leaving the sides of the tube intact to still enable you to get full movement from the focuser. I will post up an image tomorrow when I find it.

[Ricochet]

10 hours ago, Adam J said:

Actually, I think you got it. If I had a penny for every time this issue comes up with the 130pds when coupled with a DSLR...I would have something like 10p....

Possibly, if the diffraction is different to how I was thinking it would be. I was assuming it would act like a thick spider vane and create a spike perpendicular to the drawtube axis, but given the thickness perhaps it acts as three independent edges as shown in the image below where we have the following diffraction causes:

• Red - Spider vanes
• Blue - Secondary obstruction
• Green - Sides of drawtube
• Yellow - End of drawtube

If the drawtube is only just catching the lightpath then possibly we only have the yellow drawtube diffraction spike and not the geeen side spikes, which makes sense in the original image if from the perspective of the camera, the drawtube was on the left.

[spillage]

Have a go at this mod:

[Spaced Out]

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 ?

 

[Spaced Out]

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 ?

[Adam J]

13 hours ago, spillage said:

Please do not chop of a bit from the end of the focus tube as if you change your camera this can cause issue. You can remove a square front and back leaving the sides of the tube intact to still enable you to get full movement from the focuser. I will post up an image tomorrow when I find it.

It will not cause an issue, the back focus from the coma corrector remains exactly the same at 55mm so you need spacers between the coma corrector and the camera anyway.

The only time it would be an issue is if you did not want to use a coma corrector and even then you could use an extension tube to allow you to reach focus. Removing only a square will not solve his issue he will just be left with additional surfaces at different angles in the light path and more diffraction lines.  I know many people who have cut the focuser straight across and had no problem with using a CCD. You just need to add a stop to prevent it from being able to move out of the read bearings at maximum adjustment. 

 

Edited October 31, 2017 by Adam J

[Adam J]

1 hour ago, Spaced Out said:

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 ?

 

2 hours ago, spillage said:

Have a go at this mod:

That still leaves two sections of the focuser in the light path. Also you really don't need to remove that much only 1.5cm that looks like much more. Once you add spacers between the coma corrector and the CCD the focus tube will end up in exactly the same location when using a CCD as it does with a DSLR it has no choice but to. 

The main issue is when using it for observing...but you can still use an eyepiece extension. 

Edited October 31, 2017 by Adam J

[Adam J]

2 hours ago, Ricochet said:

Possibly, if the diffraction is different to how I was thinking it would be. I was assuming it would act like a thick spider vane and create a spike perpendicular to the drawtube axis, but given the thickness perhaps it acts as three independent edges as shown in the image below where we have the following diffraction causes:

• Red - Spider vanes
• Blue - Secondary obstruction
• Green - Sides of drawtube
• Yellow - End of drawtube

If the drawtube is only just catching the lightpath then possibly we only have the yellow drawtube diffraction spike and not the geeen side spikes, which makes sense in the original image if from the perspective of the camera, the drawtube was on the left.

The issue is more that it does not go across the entire tube diameter and so the angle changes as you move across the image plane. 

[spillage]

8 minutes ago, Adam J said:

It will not cause an issue, the back focus from the coma corrector remains exactly the same at 55mm so you need spacers between the coma corrector and the camera anyway.

Sorry not really sure how using a comma corrector has anything to do with the focus tube entering the light path.

[Adam J]

4 minutes ago, spillage said:

Sorry not really sure how using a comma corrector has anything to do with the focus tube entering the light path.

Its because when using the coma corrector you need to place it as a set spacing from the image plane (55mm). As such the camera moves backwards 55mm relative to the front edge of the focus tube (where the cc sits) and so to get focus you need to move the image plane (sensor) in by 55mm. In a DSLR the image plane is 55mm back from the coma corrector due to the depth of the DSLR mirror box so no spacers (apart from the t-ring) are required to gain the required spacing. If you use a CCD there is no mirro box and so you need to make up the spacing using spacers between the camera and the CC. Hence the focus tube position does not change when using a coma corrector irrespective of the camera being used. 

If you still don't understand I would be happy to provide a diagram.

Edited October 31, 2017 by Adam J

[Adam J]

55 minutes ago, spillage said:

Sorry not really sure how using a comma corrector has anything to do with the focus tube entering the light path.

Here is a diagram mate. 

Yellow = coma corrector

Blue = Draw Tube

Green = spacers / t-ring

Pink = Camera body. 

 

In order: 

1) A DSLR with a coma corrector, spacing from the back of the coma corrector to sensor = 55mm, this is made up of the DLSR mirror box 45mm and the T-ring 10mm.

2) A CCD using a coma corrector, spacing from the back of the coma corrector to sensor = 55mm, this is made up of a M48 to M42 spacer. 

3) A CCD with no coma corrector, the spacer is not required so the tube is moved out to reach focus...although you could still use an extension to keep the tube in the same position. (Tube now not in light path). 

So the point is that the camera type has no effect what so ever, even in case 3 you can use an extension. 

You just need to mark the tube at focus to ensure that you don't take too much off it when cutting. Or as I said before, just move the primary mirror up the tube. This whole problem is why you only use a 35mm draw tube when you use a Moonlight with a 130PDS. 

Edited October 31, 2017 by Adam J

[spillage]

May I am being stupid. My cc connects to the T2 ring, the T2 ring has not being moved doing this as the cc sits inside the T2 ring and this sits on  the focus tube. I am sure that I have not need to drastically move the focus by using the cc.

I guess I have the baader so maybe this is why?

 

I would like to add that yes my was is more difficulty then just taking a bit from the bottom of the tube but you do ensure that if you do remove too much then you still have a fully functional tube. I have a second hand 150pds where the previous owner butchered the tube.

Edited October 31, 2017 by spillage

[Adam J]

10 minutes ago, spillage said:

May I am being stupid. My cc connects to the T2 ring, the T2 ring has not being moved doing this as the cc sits inside the T2 ring and this sits on  the focus tube. I am sure that I have not need to drastically move the focus by using the cc.

I guess I have the baader so maybe this is why?

Your correct, you will not have had to move the focus tube on a DSLR due to using a coma corrector, well maybe a 1 or 2mm as the badder MPCCMkIII does move it very very slightly. 

Its with a CCD that the position of the focus tube will move if not using the coma corrector (unless you space it anyway). Its because while the DSLR naturally places the sensor at 55mm from the coma corrector, you cant just screw a CCD into the coma correctors as it will be too close to the sensor and not correct correctly.  

NOTE: The position of the focus does not change, but that is not the same as the position of the focus tube changing. Different camera back focuses will result in different tube positions to place the sensor at focus. The use of the CC just forces you to modify the camera back focus to match the CC back focus requirement and so when using a CC all cameras result in the same focus tube position, just so long as the CC back focus is correct. 

See the diagram above. 

Edited October 31, 2017 by Adam J

[spillage]

I think maybe this got a bit mixed up as I do not think I ever mentioned anything about back focus to the sensor, only retaining the full movement of the original length of the focus tube. I guess if I ever come to sell it then I have not made any alterations that will effect its use from as new. If that makes sense.

 

[Adam J]

7 minutes ago, spillage said:

I think maybe this got a bit mixed up as I do not think I ever mentioned anything about back focus to the sensor, only retaining the full movement of the original length of the focus tube. I guess if I ever come to sell it then I have not made any alterations that will effect its use from as new. If that makes sense.

 

Yes, the only thing that will be effected is its use with eye peices.

Got to be honest though, I only use the 130PDS for imaging as do many others, as its not the best for visual so I would have a preference for one optimized for imaging. Beyond that, I consider it to be almost disposable at that price, just so long as I get a couple of years of use from it then I got my money worth, same with my DSLR. I cant sell it as a normal DSLR with its modifications but there are plenty of imagers who would take it from me. 

[wimvb]

Regarding the baader cc and its distance to the sensor:

When using the cc with my pentax dslr (cc + t2 + dslr), the focus tube went quite far into the light path. When I replaced the pentax with an ASI174 and spacer, the cc distance was initially 1.5 - 2 mm too far out. This affected focus remarkably, and the focus tube wasn't in the light path anymore. Because of the much smaller chip, I didn't have problems with coma. But when I corrected the distance, focus moved back in again.

Numberwise:

Cc + dslr, focus with tube at about 10 -12 mm from all the way in, no visible coma on aps-c

Cc + ASI174 + too long spacer, focus almost all the way out (about 80 %), no visible coma on small chip

Cc + ASI174 + better spacer, focus about midway of its travel, no visible coma on small chip.

Since the baader cc is supposed to have a best distance of 55 mm ± 1 mm, and focus seems to change substantially with distance, you could just try adding a 1 mm spacer before taking out the hack saw.

 

Edited October 31, 2017 by wimvb

[Spaced Out]

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 !

 

[wimvb]

Imo, the asymmetry strongly syggests some kind of tilt. But Adam's advice to put the primary mirror as far into the tube as the screws will allow, makes perfect sense. As this change doesn't involve any butchering, I think it's worth pursuing.

If your mount is stable enough, you should also be able to SEE the diffraction spikes in an eyepiece. Especially when viewing a very bright star, such as Vega. Moving the fov such that the star is viewed on one side, centre, and finally on the other side (without refocusing?), may reveal possible tilt.

[Waldemar]

In the pictures of the inside of your scope, I see some shiny bolts protrude. Those may very well be the cause of the diffraction.

Maybe you can paint them mat black and see if that makes a difference.

[Ricochet]

15 hours ago, Spaced Out said:

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 !

 

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.

[Spaced Out]

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 !   

[Spaced Out]

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 ?!

[Adam J]

This was a while back so it's difficult to remember. I'll measure it for you later.