Help with collimation

[Jason D]

that's a good suggestion with the milk bottle washers. I use a steel washer myself for the same purpose. it prevents gouges in the soft monkey metal secondary stalk which do cause re-rotation back to an original point (which is usually the wrong place). it also allows more easy turning of the stalk.

Hello Shane,

I found the milk bottle washers to work much better than steel washers because the milk bottle washers have some elasticity which helps make that final tweak with one set screw without the need to touch the other two set screws.

Jason

[Moonshane]

great point Jason. I have some 1mm teflon sheet which I might use in that case and see what the difference is.

[Tim]

Also, I notice that the laser spot isn't hitting the secondary remotely near it's middle:

I presume it should?

If visually the secondary mirror is centred, you would expect that the laser dot would point at the centre of the secondary on its first 'hit', at least along its long axis.

Your focuser may be pushing it out of line, what happens if you rack the focuser all the way in and out with the laser in place? If the focuser is not 'collimated' the laser dot will move on the secondary as the laser moves closer/further from it.

If it appears that the focuser itself is not aligned, you might be able to insert a couple of shims under one side to bring it into line.

It's usually a process of finer and finer iterations with these things

[Moonshane]

this is generally good advice of course but I am pretty sure the scope is a SW heritage dob with the focuser part of the moulded housing and cannot be adjusted as such?

[Tim]

Hmm, in that case perhaps a bit of tape on one side of the laser or something to hold it true. Maybe it's ok anyway???

[AndyWB]

this is generally good advice of course but I am pretty sure the scope is a SW heritage dob with the focuser part of the moulded housing and cannot be adjusted as such?

Yup, that's correct - it's moulded in, and can't be adjusted/changed. And it isn't a great focuser - there is a certain amount of movement. It's not too bad close in, but when it's further out it can wobble a bit.

Right then - I've done a few things:

• I added a little plastic ring like Jason suggested
  
• I got a Cheshire - it does seem to be the tool of choice. (Kudos to FLO for fast delivery!)
  
• I carefully tried to get everything aligned again.
  

I ended up with the following (though the image in the camera lens is a little different to my eyeball - see below):

I note the following:

• When I look through the collimation cap the bulge of the secondary is more towards the primary than in the image, and the centre cap hole perfectly aligned with the primary mirror ring. It's still slightly 'low', but so little I'm not going to worry about it. I'm guessing this difference between eye and camera is due to incorrect positioning of my iPhone when I took the picture.
  
• I think the plastic ring did help me rotate the mirror to this much better position. Certainly, next time I tried to align the dark bulge pointed much more toward the primary - it didn't return to the previous location.
  
• The Cheshire and laser both show things as being in alignment.
  
• The Cheshire shows the very edges of the 3 clips on the primary.
  
• Screwing the focuser out does not change either of these results (at least through while the focuser is still pretty close in - it does get loose further out.)
  
• Looking through the collimation cap you do see the image of the primary is not centred on the secondary, but...
  
• ... the Heritage 130p doesn't have 4 secondary vanes. It's got just one support. I wondered if it was central. It's kinda hard to measure, but I tried with a tape measure and steel rule, I reckon that the axis of the centre bolt is a one or two mm off the centre - so it's perhaps not surprising the reflection of the primary isn't centred on the secondary, 'cos the secondary probably isn't centred - as Tim suggests.
  

So, upshot of it - I'm happy that it seems to be aligned, the tilt/rotate error pretty much removed, and that I now have an idea why reflection of the primary isn't central on the secondary - but I do wonder if it ever was.

Anyway, I reckon it's pretty good - centralisation of the secondary accepted - and as I think Moonshane was alluding too, there are plenty of other factors including that this is a little cheap starter scope that are more likely to be limiting factors.

Of course, I now have about one third the value of the telescope in collimation tools! Still, I'm young, I hope to be at this a while, and they can be used with future scopes, if I ever live somewhere with a bit more space and a garden. (Lord I'd love to have a garage. Or a decent shed.)

Thank you very much for your help everyone. Sorry my learning process was so laborious for everyone (including me!)

Job done.

[AlexB67]

A handy thread this. seeing I just got one of these http://www.firstlightoptics.com/collimation/cheshire-collimating-eyepiece.html but second hand as a bargain from someone quite local here through the classifieds and looked as new. Thanks Anthony.

So ... I had to have a little play of course and see how the collimation was. Of course in the process, seeing the Heritage has the secondary suspended on one strut/rod. I accidently moved/rotated the secondary a lot on this rod as I coud see it needed a minor tweak but had to force it a little bit to get it to move. Now the scope is way out of course. The problem I had when I got the scope anyway, the secondary was completely out of position.

Let the collimation fun begin while the clouds are out but first impressions with the sight tube is focuser slop is not helping my cause, plus I cannot see the outline of the secondary without raising the sight tube quite a bit in the focuser, which is not helping either. I don't think my secondary has ever been near perfect position since I got it, but with the cap only found it to hard to judge so lived with it, the star test did always look good though.

Let the fun or frustration begin