This needs collimation, right?

[szymon]

View through my new Cheshire collimation eyepiece. Just an iPhone photo I'm afraid. Looks like it's off, correct?

Sent from my iPhone using Tapatalk

[RT65CB-SWL]

Hi 'szymon', it looks it is out of collimation to me!

[FunkyKoval35]

Szymon, you can use CCDInspector. It is a grate tool to check if colimation is needed.

[RobH]

Szymon, you can use CCDInspector. It is a grate tool to check if colimation is needed.

Only if you're imaging......you need to run starfield images through it.

[RobH]

It is a bit out of collimation. Have you seen Astro-baby's collimation guide? Very easy to understand.....do a google search, it comes up straight away.

[Martin Meredith]

I suspect you need to rack the focuser out more so we can see if the secondary is presenting as a circle or an ellipse. That will help diagnose which of several adjustments are needed.

Martin

[szymon]

Yup, I'm going to play with the Astro-Baby later.  I have a lot to learn ;-).  I have two tools (purchased via SGL), a Cheshire collimator and a Laser collimator -- my theory is, do it with the Cheshire and then use the laser as an extra "double check"...

[Martin Meredith]

Yup, I'm going to play with the Astro-Baby later.  I have a lot to learn ;-).  I have two tools (purchased via SGL), a Cheshire collimator and a Laser collimator -- my theory is, do it with the Cheshire and then use the laser as an extra "double check"...

This is also a great meta-resource: http://www.physiol.ox.ac.uk/%7Eraac/collimationLinks.shtml

[Uplooker]

Yup, I'm going to play with the Astro-Baby later.  I have a lot to learn ;-).  I have two tools (purchased via SGL), a Cheshire collimator and a Laser collimator -- my theory is, do it with the Cheshire and then use the laser as an extra "double check"...

Do it with the Cheshire and then confirm /fine tune it with a star test. The term laser implies to people that it is scientific and exceptionally accurate - not so. More often than not a laser collimator requires collimation itself.

Ian

[AndyWB]

Yup, I'm going to play with the Astro-Baby later.  I have a lot to learn ;-).  I have two tools (purchased via SGL), a Cheshire collimator and a Laser collimator -- my theory is, do it with the Cheshire and then use the laser as an extra "double check"...

It's rare that I can get both to agree. I tend to trust the Cheshire more.

[greglloyd]

Yeah, I would definitely focus (pardon the pun) on getting the collimation right with a Cheshire. It's surprisingly easy to do. I usually complete the process of fine tuning the collimation within about 1 to 2 minutes most times. Get the secondary sorted first (positioning and tilt angle) so that it is centred under the focuser and that all the primary clips are visible and the primary is exactly round in shape. You should not have to do this often at all once you've done it good the first time. Then fine tune the primary so that the view hole of the Cheshire is exactly in the middle of the centre dot on the primary. This is the very minor tweak that you may have to do more often from moving the scope around. All very easy to do after the first few times.

Then, finish up with a star collimation - but only on a very steady night. If any turbulence in the view, simply forget about this step and just enjoy the views as the collimation will already be close to perfect for DSO viewing and medium power planetary.

[szymon]

Ok, first question. Where should the focuser be when collimating?

I made me a collimation cap (film canister with hole in the middle). The first image shows the focuser all the way in. You can clearly see the mirror clips nicely around the edges, just like in astro baby's photos -- but you can also see the tube of the focuser which are not visible in hers.

The second image shows the focuser pulled out so that it doesn't Intrude on the image -- however you can no longer see all three clips.

So, do I need to realign the secondary mirror or not? :-)

Sent from my iPhone using Tapatalk

[szymon]

Here is the current view through my Cheshire - the focus is on the reflection.

I've rotated the Cheshire so that the reflected veins are obvious. Does this look better?

Sent from my iPhone using Tapatalk

[dannybgoode]

Secondary looks pretty good.  Igore the vanes - it is the Cheshire's crosshairs you want centred.  A bit of fiddling with the collimation screws on the primary should do it.

I am not sure I agree with others re: laser collimators and think it very much depends on the collimator in question.  Thing with Cheshire's is that they are pretty simple and therefore cheap to make accurately.  A laser collimator is far more complex to get right and therefore more expensive.

I agree that a cheap laser collimator isn't worth the paper that its printed on but a good one can be very useful.  I have recently bought the Baader LaserColli mk III and it is excellent.  I have collimated using just that and then checked with my Cheshire and both agree with each other.  That said the Baader is nearly £70 so expensive as collimators go but (is apparently) engineered to very tight tolerances and it shows.

[greglloyd]

This still looks off in more than just one way.

The secondary is not correctly aligned with the primary. You can tell this because only one primary mirror clip is visible. All three should be equally visible. That is the first part of collimation you should sort out before touching anything else. It can be helpful to focus out your focuser a little so that it doesn't show in your view (as it can be seen at the top of the photo. 

Once you get the secondary sorted so that the whole primary mirror is view with all three clips equally visible, you can adjust the collimation of the primary.

The photo shows the primary is also out of collimation. I would rotate your cheshire so that the cross hairs are correctly aligned. I know it's harder to see behind the big cross hairs right in front of you, but you simply need to learn to look past / ignore the ones in the foreground and just focus on the cheshire crosshairs shown in the primary.

You need to get the cheshire crosshairs that are reflected in the primary to square up exactly with the spider vanes (to perfectly complete the + shape of the spider vanes.

Once you do this, there is a final step which is to perform a final collimation on the primary where you should adjust the primary screws to make the peep hole of the cheshire (which is reflected on the primary) go exactly in the middle of the centre spot / circle of your primary mirror.

Once you have this "primary mirror centred with all three clips equally visible", "reflected cross hairs of cheshire completing a perfect + shape of the spider vanes reflection in the primary", "peep hole of cheshire exactly in the middle of the primary mirror centre spot" - you are done!! :-)

[greglloyd]

Secondary looks pretty good.  Igore the vanes - it is the Cheshire's crosshairs you want centred.  A bit of fiddling with the collimation screws on the primary should do it.

I think it still needs a little tweak. I think he should rotate the cheshire to align with the spider vanes. It still looks slightly out. The reflected Cheshire cross hairs should be made to create an exact plus shape with the reflected image of the vanes and I think they are just a tad left of centre and ever so slightly too high. A slight down and right tilt of the secondary is required. This is why the primary mirror clips at the top and left are not visible in the view as well. Rotating the cheshire will help to make the final adjustment of the secondary.

[szymon]

Ok, now I am *really* confused.  (Money for nothing that collimation is the thing that confuses more novice astronomers than anything else, hell I'm usually good at anything technical and it's confusing the hell out of me and the more I read the more I get confused).

The secondary is not correctly aligned with the primary. You can tell this because only one primary mirror clip is visible. All three should be equally visible. That is the first part of collimation you should sort out before touching anything else. It can be helpful to focus out your focuser a little so that it doesn't show in your view (as it can be seen at the top of the photo.

Could you do me a favour and read post number 12?  In there I show how if I have the focusser all the way in, all three clips are visible, but if I move it out then they're not.  I think if I could understand why that is, I'd understand how you're inferring that the secondary is incorrect...

[greglloyd]

Ok, I read post 12. 

In my own scopes, if I start with the focuser fully in I will see all three clips. As I focus out they will all eventually leave the field of view - equally.

I would suggest putting your focuser all the way in. Adjust the secondary to get the clips all even. Then focus out slowly and watch to see if any of the clips go out of view first. If so, make tiny adjustments to the secondary so that they are all in view again. Continue this process until they all leave the field of view at the same time as you focus slowly out more.

Take heart that once this is done it is not something you will have to do very often again. Only after giving the scope a good knock :-)

[szymon]

Ok, that's logical and makes a lot of sense.  I'll start with that.  Thank you!

[szymon]

(As an aside, the reason it's out of kilter is probably because it was shipped to me by courier, I'm sure the previous owner had it perfect!).

[greglloyd]

It's simply because a very minor off axis error in the secondary will not show up so much when the focuser is fully in. This is because you are closer to the secondary so the error doesn't show itself so much. But will show as you focus the tube out. It's just a minor error, but should be sorted first before finishing the rest of the collimation to ensure the very best collimation you can get with the cheshire. 

[The BFJ]

I tried for a while to colli with a cheshire and a colli-cap and couldn't quite get it right. I bought a Baader laser-colli and found that much easier. It may be possible to get it more exactly accurate with a cheshire but I couldn't get the hang of it. Using the laser colli seems to work more than well enough to give me clear, sharp images. The Hercules cluster comes in beautifully anyway.

[szymon]

Ok, here are some images with a white card blocking off the primary and a coloured brown card behind the secondary (as per astrobaby). This is before I have done any changes, with the focus tube all the way out. First via collimation cap:

And with Cheshire:

(Close up):

It looks quite round to me. So, what do I change now?

Sent from my iPhone using Tapatalk

[Martin Meredith]

Using the secondary collimation bolts try to get the two circles concentric (that is, the white circle and the edge of the Cheshire), but be careful not to lose the circularity when doing this adjustment -- it will take some iteration. It will be easier to do this if you rack the focuser in so you've only got a narrow gap between the sight tube and the white circle.

Martin

[szymon]

It's not easy at all ;-)

I'm trying to figure out the mechanics underneath.  There are three screws for adjustment, and one that pulls the mirror back onto them...so why are they defying the laws of physics and moving the mirror in the wrong damned way each time (and in a random direction each time at that!)?