Diffraction Spike Issues

[Spacehead]

Hi All,

My Skywatcher 200p is getting on a bit now - and at some point over the last five years it developed these diffraction spike "issues" as in my pic.
So there on the left - that spike goes on "forever" whilst the other 3 are the same length.
And the top spike is two spikes not one.

This is on all photos of bright stars and both problems are really bugging me.

Can anyone advise?  What can I do to test which spider is wrong (if thats the prob).
I collimate it with a cap, the hole is dead center in the donut on the mirror (jfi).



 

[Merlin66]

Check your secondary spider arms... could be that one is slightly twisted and causing excessive diffraction....

[JRWASTRO]

Interesting picture.

Judging by  " ... over the last five years it developed these ... " Sounds like a dirt/dust problem.

1. Diffraction patterns should be symmetrical so that "little spike" presents a problem.

Notice that the star has multiple diffraction patterns - there is an aperture and flare issue here.

** Do you see the same patterns in an eyepiece ? If not then the camera sensor may need cleaning.

** Check the cleanliness of the primary and secondary mirrors.

Jeremy.

 

 

 

Edited May 13, 2019 by JRWASTRO
Add the word issue

[wimvb]

Diffraction spikes are at right angles to the diffraction causing obstruction. But this doesn't exactly make it easier to find the culprit. If you add an obstruction to each spider vane in turn, and take an exposure of a bright star, you can probably find out which vane needs adjustment. The fact that you have well degined spikes, suggests that both focus and collimation are spot on.

[Spacehead]

Cheers all - I have just finished resetting everything.  Took the primary right back down to base by tightening all the screws - lifted it up again with 2 turns on each - so hopefully perpendicular to the tube.  Unscrewed the 3 secondary tighteners/positioning screws and pulled the mount on the mirror right back against the support mount - and then reset from there.  Re-collimated.
So all being well - I will go out tonight and see what I can do to cause some obstructions and see whats what.  Cant see even a fleck of dirt on pri or sec.

 

[Spacehead]

Oh the gods - what a mess I made.  6 hours of messing. Got rid of the stretched left side spike - pretty much all the same length now, will have to re-address the extra spike tomorrow as despite a practical complete re-build - its still there.
Arcturus and an extra spike

[andrew s]

Does the focuser tube extend into the light path? It could be adding the extra spike, as could the iris from the camera.

Regards Andrew 

[happy-kat]

On 13/05/2019 at 00:44, Merlin66 said:

Check your secondary spider arms... could be that one is slightly twisted and causing excessive diffraction....

Did you check the secondary vanes

[CraigT82]

Looking at the secondary from the front of the scope, is there anything at all protruding into the light path? I'm thinking screw heads or secondary heater or something like that? 

[Spacehead]

3 hours ago, andrew s said:

Does the focuser tube extend into the light path? It could be adding the extra spike, as could the iris from the camera.

Regards Andrew 

Hi Andrew, I can state with confidence that yes, the focus tube goes into the mirror reflection - I can see it looking through the pinhole in the collimation cap once the focusser is in beyond a certain point.
It is in view at the focus used for the star above.

Would not all 200p scopes suffer this problem though if the spike was from that?

RE the camera, I have tried two different cameras and the effect is the same - the extra spike is present on both.
 

[Spacehead]

3 hours ago, happy-kat said:

Did you check the secondary vanes

To my eye they are straight, I actually twisted one though to see what effect it would have - I couldn't detect any effect so I put it back to how it was.

[Spacehead]

1 hour ago, CraigT82 said:

Looking at the secondary from the front of the scope, is there anything at all protruding into the light path? I'm thinking screw heads or secondary heater or something like that? 

The focus tube does - but I cant test this as its out of focus on a star by the time its retracted far enough - so no spikes anyway.
There are 3 mirror clamps (black) holding the primary in place - these do show up in the secondary when looking through the collimation cap - I cant see it being these though - they have been there forever.

 

[Spacehead]

If its nice tonight I will do some more testing - lost a lot of time on collimation yesterday.

[Ben the Ignorant]

Try attaching a round disk of black paper or cardboard on the secondary, make it larger than any protrusion the secondary housing might have, and do a test shot.

Then remove it, and make an aperture mask, a ring smaller than the mirror so the mirror's retaining clips and the screws inside the tube are in its shadow. Do some test shots, you'll know where the spike is caused. 

[FaDG]

I'm pretty sure that the additional small spike is due to reflection. 

I found it in only ONE of my images, namely M45, I acquired with my 150pds.

It was on ALL subs in that sessione and NO other. Neither different targets, nor the same target taken with a different FOV. And I'm sure that in my newt the focuser doesn't protrude in the light path, so it must have had a different root cause. 

Could you try to take a longer exposure of a rich star field, in order to cause spikes around multiple stars around the image? It could help debugging... 

Fabio

Edited May 14, 2019 by FaDG

[Spacehead]

3 hours ago, FaDG said:

I'm pretty sure that the additional small spike is due to reflection. 

I found it in only ONE of my images, namely M45, I acquired with my 150pds.

It was on ALL subs in that sessione and NO other. Neither different targets, nor the same target taken with a different FOV. And I'm sure that in my newt the focuser doesn't protrude in the light path, so it must have had a different root cause. 

Could you try to take a longer exposure of a rich star field, in order to cause spikes around multiple stars around the image? It could help debugging... 

Fabio

This is in all images I have where a given star is bright enough to show the spikes.  There are very few constellations which have multiple stars bright enough - Pleiades is one - but its not there atm.  I will take a picture of a star in each corner instead.

 

[Spacehead]

4 hours ago, Ben the Ignorant said:

Try attaching a round disk of black paper or cardboard on the secondary, make it larger than any protrusion the secondary housing might have, and do a test shot.

Then remove it, and make an aperture mask, a ring smaller than the mirror so the mirror's retaining clips and the screws inside the tube are in its shadow. Do some test shots, you'll know where the spike is caused. 

I may have to give this a go.  Cheers.

[Spacehead]

Ive done some pics tonight - moving a star around the mirror - interestingly, the spurious spike is in some pics and not others.
 

[Spacehead]

Ive consolidated these into one image - pasting in the position of each shot.  Im pleased with the collimation - the flaring is center in the center - and "directionally" towards the center in those images with the star around the edges.
The green dots show the stars position when the extra spike does not show - the red dots are showing the spike.
Strange or what?
I will be honest - I dont know what this means or whether it helps at all?  Just hoping someone will know where next to look.  

Edited May 14, 2019 by Spacehead

[Spacehead]

This is a close up of the star (just lower iso) - in the center of the frame.  I think the flat left side of the circle is the intrusion from the focus tube maybe.
(just for info really)

[FaDG]

No, not strange at all, actually!

If you needed a confirmation that it was a reflection, now you have it. 

The fan like flare is a reflection of the light cone off the focuser drawtube. To solve it I took a hacksaw and cut about 3 cm off it. Issue solved, but it won't focus with eyepieces anymore: I don't care as i never use it visually. 

The single stray spike is a reflection off something depending on the specific position of the point light source (star).  It seems to be a characteristic of the Skywatcher newts design, as I have the same in my 150 f5

[Spacehead]

45 minutes ago, FaDG said:

No, not strange at all, actually!

If you needed a confirmation that it was a reflection, now you have it. 

The fan like flare is a reflection of the light cone off the focuser drawtube. To solve it I took a hacksaw and cut about 3 cm off it. Issue solved, but it won't focus with eyepieces anymore: I don't care as i never use it visually. 

The single stray spike is a reflection off something depending on the specific position of the point light source (star).  It seems to be a characteristic of the Skywatcher newts design, as I have the same in my 150 f5

Ok - regarding the general flaring - I am not too worried about that, as I see it all the time on photo samples from the 200p where there are many bright stars in one shot.  I admit - I don't like it - and may address it by cutting the focus tube down.

The single stray spike is my pet hate though - I mean its wrecking my life (lol) - are you saying that is also characteristic of sw newts?  Can you show me a pic displaying this problem from your 150?

 

[andrew s]

You could try flocking the inside of the focused tube first.

Regards Andrew 

[Spacehead]

8 minutes ago, andrew s said:

You could try flocking the inside of the focused tube first.

Regards Andrew 

Will do that later today and test tonight if clear - yup.  The rest is flocked.

 

[andrew s]

Are the edges of the secondary blackened? These can cause reflection

Regards Andrew