Star test questions

[cwis]

Hi all,

I was looking at Mars last night and I thought I'd have a go at a star test of collimation - never done one before.

I was led to do this because although the seeing looked pretty good I was still low on contrast and had better views (more contrast, sharper) with most of the aperture of the telescope obscured by its cap - with just the small hole in the cap uncovered. The planet seemed a bit fuzzy edged, indistinct. I could see the polar ice cap, but as a  blurred splodge.  So far I've not been thrilled with the planetary views from this scope - I can just make out the GRS and just about make out the Cassini Division (I think) but as this is my first season of observing I thought it was just sky conditions as the planets have always been low.

But Mars was pretty high in the sky last night so doubt crept in.

I pointed at Polaris...

I've badly drawn what I saw just inside focus...

So the first image is the diffraction pattern looking at Polaris when centered in the view - it told me collimation was a bit out . Hey ho - I can improvise:

I moved the pattern around the view until the pattern was concentric - I ended up near the top of the view as on the second image.

I remembered this position and pointed at Mars, and put the planet in the "sharp spot". My word! Proper sharp edged planet with occasional glimpses of razor sharp detail on it's surface, the edges of the ice cap sharply defined, as sky conditions allowed - amazing!

So - the questions!

Why is my pattern oval (I've exaggerated it a little). I expected a round one? Interesting that it is in line with gravity - is it the focusser bending?

How do I get my sharp bit in the middle? I have the Skywatcher Explorer 130ps - the main mirror is not adjustable.

Using a collimation cap the secondary looks to be centered on the main - do I need to be more precise in this or is that the limit of adjustment for this scope?  Or is the focusser bending when I fit an eyepiece?

I've seen the occasional 650mm collimatable mirror cell pop up on Astroboot that looks a direct replacement for my non-collimatable one (assuming from a Heritage 130 et al)  - should I fit one of those?

Am I just being too picky for an entry level scope? I was using a Nirvana 4mm eyepiece by the way so 162x  - not excessive? 

[Nik271]

Collimation is always checked in the center of the field of view. Yours is definitely off. A collimation cap on its own is not enough to verify it, in fact the star test you've performed is the best. And yes collimation is best checked at highest power. For 130mm mirror 162 seems reasonable, a well adjusted scope should easily handle that.

I'll be very surprised if the 130ps is not collimatable, Skywatcher is a very reliable brand. Do you have 6 screw at the back of the main mirror? In some scopes they are hidden behind a thin removable panel. The small screws are locking, and the big screws adjust the tilt of the mirror. 

Edited September 22, 2020 by Nik271

[Nik271]

PS: I stand corrected: the Explorer 130ps has a fixed main mirror and not collimatable. What a shame!

But maybe you can adjust the secondary? You will need to do this in daylight, so you can use a cheshire collimating piece or and artificial star.

I can't believe they are selling a 130mm Newtonian with a fixed mirror!

But then Celestron are selling Astromaster 130 with a spherical mirror at F5 so everything is possible...

Edited September 22, 2020 by Nik271

[cwis]

6 minutes ago, Nik271 said:

PS: I stand corrected: the Explorer 130ps has a fixed main mirror and not collimatable. What a shame!

But maybe you can adjust the secondary? You will need to do this in daylight, so you can use a cheshire collimating piece or and artificial star.

I can't believe they are selling a 130mm Newtonian with a fixed mirror!

But then Celestron are selling Astromaster 130 with a spherical mirror at F5 so everything is possible...

Synta ARE naughty scamps! 

I can adjust the secondary, yes - maybe I need a cheshire then - I've got a Dob on order so it won't be wasted. Hopefully it arrives before the heat death of the Universe....

I adjusted the secondary using my c-cap as best I could. I guess that's not precise enough?

I have to say though - when I got Mars in the sharp spot last night I was amazed. The optics are VERY good. All I need to do is get that spot in the middle!

[Pixies]

Does the primary mirror have a doughnut marked at the centre?

 

[cwis]

Just now, Pixies said:

Does the primary mirror have a doughnut marked at the centre?

 

Hello! Yes it does. When looking through my collimation cap I see my three mirror clips, a centered secondary mirror, the reflection of the back of the cap in the main mirror and the reflection of my pupil as close to the middle of the doughnut as I can make out.

[vlaiv]

Adjusting secondary won't cut it in this case - you need to adjust primary mirror as it is parabolic.

If the scope does not have adjustable primary mirror - send it back.

Fact that they did not include collimation screws in primary mirror cell points to their belief that scope should stay in collimation for end user. Since your scope isn't - it's not working as expected and this is not your fault - reasonable grounds for returning the scope I would say.

[Pixies]

1 minute ago, vlaiv said:

Adjusting secondary won't cut it in this case - you need to adjust primary mirror as it is parabolic.

If the scope does not have adjustable primary mirror - send it back.

Fact that they did not include collimation screws in primary mirror cell points to their belief that scope should stay in collimation for end user. Since your scope isn't - it's not working as expected and this is not your fault - reasonable grounds for returning the scope I would say.

I agree. Contact the seller.

Unless SW can suggest how you should adjust it - I can't see how any adjustment of the secondary will help: your eyehole appears to be centered within the doughnut. Perhaps there has been some movement in the focuser alignment?

[cwis]

31 minutes ago, Pixies said:

I agree. Contact the seller.

Unless SW can suggest how you should adjust it - I can't see how any adjustment of the secondary will help: your eyehole appears to be centered within the doughnut. Perhaps there has been some movement in the focuser alignment?

Hmmm. Issue there is everyone is out of stock of everything at the moment. Currently I have a telescope - it's a wonky one by the sound of it, but at lower powers of magnification seems fine. If I sent it back I'd just have my eyeballs currently. I've got a Dob on the way but who knows when it will turn up. I'm completely prepared to look sideways at Mars for the next month or so!

When this scope turned up the secondary was WAY out - seagull shapes in the 10mm eyepiece.  It's loads better now but obviously not perfect. It's been good enough to get me totally hooked on the night sky though!

The focusser is quite stiff and has a fair bit of slop in it - I think I need to do some more investigation - it could be the culprit?

 My "sharp bit" was directly above the centre of the view.  Is that really a coincidence?  If my focusser "droops" would that move the sharp spot up in my field of view (as well as tilting the focal plane and all sorts of other stuff)? I'm finding it hard to visualise...

Next clear night I think I'll star test on stars at different inclinations and see if the sharp bit moves about. 

 

 

[John]

I wonder if the primary mirror could be shimmed slightly to improve the collimation ?

 

[Pixies]

7 minutes ago, John said:

I wonder if the primary mirror could be shimmed slightly to improve the collimation ?

 

If you watch this vid - at about 9:50 it shows how there is a way to make a slight adjustment to the primary. It's a slightly different scope, but I assume the mirror cell would be similar.:

 

I would assume this would void the warranty.

[Pixies]

10 minutes ago, cwis said:

Hmmm. Issue there is everyone is out of stock of everything at the moment. Currently I have a telescope - it's a wonky one by the sound of it, but at lower powers of magnification seems fine. If I sent it back I'd just have my eyeballs currently. I've got a Dob on the way but who knows when it will turn up. I'm completely prepared to look sideways at Mars for the next month or so!

When this scope turned up the secondary was WAY out - seagull shapes in the 10mm eyepiece.  It's loads better now but obviously not perfect. It's been good enough to get me totally hooked on the night sky though!

The focusser is quite stiff and has a fair bit of slop in it - I think I need to do some more investigation - it could be the culprit?

 My "sharp bit" was directly above the centre of the view.  Is that really a coincidence?  If my focusser "droops" would that move the sharp spot up in my field of view (as well as tilting the focal plane and all sorts of other stuff)? I'm finding it hard to visualise...

Next clear night I think I'll star test on stars at different inclinations and see if the sharp bit moves about. 

 

 

Still worth noting the issue with the seller. They might have some advice, or be able to contact SW for support. Was it one of the major stores?

[cwis]

9 minutes ago, John said:

I wonder if the primary mirror could be shimmed slightly to improve the collimation ?

 

I'm wondering whether it already is? Maybe during manufacture? Surely processes aren't so precise that they can throw them together without checking and assume they are collimated?

[cwis]

5 minutes ago, Pixies said:

If you watch this vid - at about 9:50 it shows how there is a way to make a slight adjustment to the primary. It's a slightly different scope, but I assume the mirror cell would be similar.:

<Deleted vid>

I would assume this would void the warranty.

Interesting!  I wonder how they could tell you attempted this...

[cwis]

5 minutes ago, Pixies said:

Still worth noting the issue with the seller. They might have some advice, or be able to contact SW for support. Was it one of the major stores?

Yup - FLO!

[Pixies]

1 minute ago, cwis said:

Interesting!  I wonder how they could tell you attempted this...

By reading this.... 🕵️‍♂️

[cwis]

1 minute ago, Pixies said:

By reading this.... 🕵️‍♂️

So does flocking a scope void the warranty? You normally take the mirror cell out for that....

[vlaiv]

2 minutes ago, cwis said:

Yup - FLO!

That should be easy then ...

@FLO

What is supposed to be done with SW Scope that has no collimation screws (by design) yet goes out of collimation?

[Pixies]

2 minutes ago, cwis said:

So does flocking a scope void the warranty? You normally take the mirror cell out for that....

I would assume it does.

[markse68]

21 minutes ago, cwis said:

The focusser is quite stiff and has a fair bit of slop in it - I think I need to do some more investigation - it could be the culprit?

 My "sharp bit" was directly above the centre of the view.  Is that really a coincidence?  If my focusser "droops" would that move the sharp spot up in my field of view (as well as tilting the focal plane and all sorts of other stuff)? I'm finding it hard to visualise...

Was it damaged in transit? Maybe the focuser got knocked and isn't perpendicular to the tube throwing the secondary into apparent wrong alignment which you then adjusted but now everything's off? No idea if that would cause the ovality but maybe? I was going to suggest seeing what happens if you lift the focuser tube in it's slop but if the secondary isn't where it was supposed to be then it'll still be bad

[cwis]

2 minutes ago, Pixies said:

I would assume it does.

Cripes. Glad you said that before my Dob turned up... 

[Pixies]

6 minutes ago, vlaiv said:

That should be easy then ...

@FLO

What is supposed to be done with SW Scope that has no collimation screws (by design) yet goes out of collimation?

How do you collimate a scope that can't be collimated because it can't go out of collimation (but it has)

2 terms of Mathematical Logic at university never prepared me for that!

[cwis]

3 minutes ago, markse68 said:

Was it damaged in transit? Maybe the focuser got knocked and isn't perpendicular to the tube throwing the secondary into apparent wrong alignment which you then adjusted but now everything's off? No idea if that would cause the ovality but maybe? I was going to suggest seeing what happens if you lift the focuser tube in it's slop but if the secondary isn't where it was supposed to be then it'll still be bad

Good question! Box was fine though...

I'll do more investigation next time it's clear. It was near the end of my session on Mars that I had the thought of doing a star test, so time was fairly short. Mars appears round the side of my shed above lovely cool woodland for an hour or two before hiding behind next door's house.

I found the sharp bit and started waving it at Mars straight away.

[Ricochet]

If you think the focuser might be the issue have you tried:

1. Applying some downward pressure/weight to the focuser while looking through the collimation cap to see if it throws the collimation off
2. Applying some upward pressure to the focuser while star testing to counteract the eyepiece weight to see if the "sweet spot" moves to the centre?

Without getting the collimation centred I don't think the shape of the diffraction rings can be investigated as eyepiece aberrations will be seen at he edge of the field of view.

[cwis]

52 minutes ago, Ricochet said:

If you think the focuser might be the issue have you tried:

1. Applying some downward pressure/weight to the focuser while looking through the collimation cap to see if it throws the collimation off
2. Applying some upward pressure to the focuser while star testing to counteract the eyepiece weight to see if the "sweet spot" moves to the centre?

Without getting the collimation centred I don't think the shape of the diffraction rings can be investigated as eyepiece aberrations will be seen at he edge of the field of view.

All good questions. I'll see next clear night.

As I recall when I received the telescope it looked like the secondary mirror had rotated. While pondering the seagull shaped stars under higher magnification during the day,  I looked down the focusser and could see the mirror quite clearly was pointing in a direction!