Collimation headache - help !!!

[Astro_Baby]

Bet no one thought you'd ever see a question from me on collimation (well not for a long time anyway )

I have a headache with a Skywatcher 200 and for the life of me cannot get collimation correct.

Its an oldish tube.

Heres the problem....

Set basic collimation - no problems.  All lined up and easy peasy etc etc

Like a fool I decided to square the focuser so I did the math, worked out where the exact opposite of the focuser centre is and marked it and squared the focuser.  With the focuser squared away no matter how far down the tube I wind the secondary I cant get it far enough to present to the sight tube - its always short by about 2/8 of an inch.

So I thought - well Melzy you probably messed up the math and the focuser is now out of square.  Did the math again and can see no problems but the damned secondary is just short - as if its set to high (ie too close to the front of the scope).  So I thought on the off chance maybe the vanes have been put in the wrong way round or are not right - took the entire front end off and replaced it all so it looks identical to my own scope and its still the same.

If I ignore the focuser collimation screws and just screw it down flat its fine - everything lines up which would seem to suggest to me either;

A) I did the math wrong and I am off by a bit or

the tube itself is slightly warped or

C) the spider attachment holes are set too far forward (ie too close to the front end of the scope)

D) the focuser is set too far back (ie too far down the tube towards the primary)

The only way I can see forward is to buy a laser (but then I will spend time collimating THAT) and using the old threaded rod and laser approach to check the focuser alignment.  I am not keen on that because I don't really want to spend money on a laser and have to mess about finding the threaded rod, cut it to size etc etc all for a scope which I don't even own.

To try and get a resolution on this I have even taken the entire front end of my scope AND the Moonlite focuser off and put the whole lot onto this scope to se if its the same problem and it is !!!!! which suggests it must be B, C or D as the root cause.

With the focuser just screwed flat all is fine - I have tested with an artificial star and it looks perfectly ok but I would like all to be perfect.

Aaaaarrrgghhhhhh  - any takers for trying to guess what the problem may be

[Charic]

Was there an issue before you squared the focuser, or did you just square the focuser for the sake of it? You did call yorself a fool! If there was no issue, remove any correction applied to the focuser assembly and refer back to a standard collimation, as per your guide.

Has there been any modification to the secondary bolt or adjusters, if replaced due to customisation, maybe just not long enough, reducing movement towards the primary.

Go back to basics, forget squaring the focuser, unless there is clearly some deformity, and try again, also as you have stripped out your scope into the Older 200, how do the parts from the older 200  align in your own good scope?

Lastly PM me your address and I can loan you a Laser, the ones you can buy from Skys the Limit, if that helps.

[opticalpath]

I can only think of a complicated combination of circumstances that might explain it, and I've concocted this from reading Nils Olof Carlin's excellent article and diagrams (http://web.telia.com/~u41105032/kolli/kolli.html)

The telescope (before squaring focuser) looks to be well collimated but suppose it is actually suffering 'Error type 3' as described in his article; i.e. the common optical axis is not reflected at 90 degrees.  The optical axis axis is at an angle to the OTA centre line and strikes the secondary at less than 90 degrees.  The secondary has to be tilted slightly and the focuser then also has to be angled at less than 90 degrees to the OTA so that the reflected optical axis meets the centre of the drawtube square on.  The focuser has been adjusted off-square to the OTA in order to meet the requirement of aligning and being orthogonal to the optical axis.  And technically collimation would be OK; the only evidence of this error would be the resulting uneven field illumination and a slightly non-circular view of the outline of the secondary.

So when you squared up the focuser to the tube, it no longer aligned exactly to the optical axis.  And by changing the angle of the drawtube, it's now aiming lower, at a point below the centre of the secondary .... which has unfortunately just run out of longitudinal adjustment!

A complication might be if the drawtube bearings have been adjusted and the drawtube runs in a line that is not at 90 degrees to the focuser base.

All a bit unlikely!

I had a problem something like this when I replaced the focuser on my SW MN190 with a Moonlite.  The Moonlite did not line up exactly with the secondary but unfortunately did not have slotted mounting holes in its base to enable me to move it up the tube a bit.  Instead I had to move the secondary down towards the focuser drawtube (after which collimation of the Mak-Newt - which REQUIRES the optical axis to coincide with the OTA centre-line) was a nightmare!)

Just another thought: when squaring the focuser, are you measuring down from the top of the OTA and is the top rim of the tube definitely at right angles to the tube at all points? 

Adrian

[Charic]

....what a good avatar name considering the subject

[Jason D]

Here is my take:

- Squareness of the focuser is desirable but not critical. If the secondary mirror central bolt is truly short, then it is OK to adjust the focuser to point little higher -- towards the OTA opening -- with hardly any impact on collimation.

- 90 degree deflection of the optical axis is not a requirement and it should not be considered. What is important is following the collimation basic steps without thinking about the angle. In fact, since my personal secondary mirror is mounted centrally on its stalk, the deflection angle is slightly greater than 90 degrees .

- Sometimes, the secondary mirror is tilted forward while attempting to center it under the focuser which gives the illustion it is high. The point is that sometimes an optical illusion gives the impression the secondary mirror is still too high. I am not saying this is the case here but it is a possibility.

Here is what I recommend:

- Ignore the position of the secondary mirror. If you are using a cheshire, adjust the secondary mirror to align the cross-hairs with the primary center spot reflection then adjust the primary mirror accordingly. Now check the reflection of the primary mirror with respect to the secondary mirror edge. Think of the secondary mirror as a window overlooking the primary mirror. The question is: How should the window (secondary mirror) move to show more of (or center) the primary mirror reflection with respect to the secondary edge? Make the move without looking down the cheshire. Then repeat the above until the window (secondary mirror) is well-position with respect to the primary mirror reflection.

[FenlandPaul]

This is like Usain Bolt asking the forum how he might be able to run just a little bit faster!!

[carastro]

Sorry I can't help Mel, I always direct any-one with collimation problems to your website, it's my collimation bible. 

Hope you get it sorted.

Carole 

[Purplehayze104]

so who's gunna add the link to the astro baby website (joke)

[Astroscot2]

Hello Mel, Like yourself I squared my focuser also. can I ask that when you squared your focuser what method did you use to square the focuser lengthwise along the tube? And did your adjustments move the focuser much from the original bolt holes ? did you also use any shims / washers to square the focuser?

Mark

[beatlejuice]

This is like Usain Bolt asking the forum how he might be able to run just a little bit faster!!

And Justin Gatlin gave his answer above.

Eric

[Astro_Baby]

Okies - to answer questions - no the one thing I haven't tried yet is putting the dud scopes optics into my own 200 tube.

The dud scope focuser if left just bolted flat (ie no focuser collimation or squaring) gives a perfect collimation pattern which is easily acheived.  Its when the focuser gets squared (it has a basic old style Skywatcher focuser with collimation bolts around the edge) it all gets messed up.

The method I am using to find the focuser squre is this....remove the secondary assembly.

Measure the circumference of the tube from the exterior - minus the thickness of the tube material.  Produce a paper strip with two marks - one where the focusers central point will be and one at the opposite side of the tube based on the math done by measuring.

Use a Cheshire to align one mark directly at the focuser centre point and run the tape around the tube to the other side of the scope obviously making sure its sure to the front of the tube (which is appears to be and the tube appears to be square.

Mark off distance from the nose of the scope and mark cross point on tape at sme distance as focuser centre point.

I am simplyfying this is a bit but you get the idea - its basiclly done with measures to find the spot exactly oppostite the centre of the focuser (physically) on the tube.

Where that spot is a mark is put on the inside of the tube - its a target marker for the cheshires cross hairs.

I have done this with my own scope and its as near perfect as I can make it - with the wonky scope it should all now align but doesnt.  I am beginning to think in terms of saying 'enough woman' and just accept it as is.  As Jason suggests its probably not critical but I am a bit OCD and cant see why this isn't working and its driving me nuts.

Thanks for the ideas though - I will try some of this at the weekend and see if I cant find out whats wrong.

[opticalpath]

I'm interested to know what you meant about using the cheshire to establish the centre of the focuser .  Do you put the paper strip across the inner end of the drawtube and use the cheshire crosswires as a marker?

Just for info, my usual method for squaring in the up-down direction is to use a carpenter's marking gauge held against the top edge and inside of the ota.  I attach a small cardboard target with a pinhole to the stem of the gauge that I can raise and lower using the sliding adjustment of the gauge. I put a laser in the drawtube and move the target over the inner end of the drawtube, adjusting the gauge till the laser passes through the pinhole, establishing the distance from edge of OTA to centre of drawtube.  Then move the gauge/ target round to the opposite side so the pinhole-to-top-of-OTA distance is transferred to the side opposite the focuser.  If all is well in the up-down direction, the laser spot should hit the pinhole. (Usual caution to verify alignment of the laser and to rotate it in use to check.)

But I prefer the idea of using a more direct physical means of establishing the centre of the (inner) end of the drawtube, rather than relying on the laser being well aligned with the focuser axis.

For the lateral adjustment, I usually have trouble using strips of paper on the inside of the OTA due to screws getting in the way and/or a patch of non reflective material opposite the focuser preventing the strip from lying flat. What works quite well is a long strip of thin card, carefully trimmed to length so it just fits tightly into the inside circumference of the tube when the two ends of the strip  are butted together with no overlap.  The card doesn't buckle as easily as paper strips, and if narrow enough to avoid screws etc, can be positioned so that its edge crosses the centre of the racked-in drawtube. 

Adrian

[Peter Drew]

Blimey!, this seems to be a hobby in its own right. I just use a collicap and adjust till everything looks right and then finally tweak on a star. Any minor anomalies in the image are usually swamped by other factors outside my control. 

[astronymonkey]

Measuring the circumference and then removing the tube thickness doesn't sound right to me but it may just be the way it was explained. Another possible area of error is whether or notmthe secondary is mounted upside down on its stalk.

Cheers

[Astro_Baby]

I'm interested to know what you meant about using the cheshire to establish the centre of the focuser .  Do you put the paper strip across the inner end of the drawtube and use the cheshire crosswires as a marker?

Yes pretty much to establish centre of the focuser I put the card inside the tube and then mark it by using a powerful torch and a peer through the Ches on its crosswires by sticking my hand in the tube and being able to see where the pen is thank to the powerful torch.  You cant do too much of this.  I spent too long messing about with this some time ago and gave myself spots in the eyes for about 2 hours - couldn't see a thing.

For the lateral adjustment, I usually have trouble using strips of paper on the inside of the OTA due to screws getting in the way and/or a patch of non reflective material opposite the focuser preventing the strip from lying flat. What works quite well is a long strip of thin card, carefully trimmed to length so it just fits tightly into the inside circumference of the tube when the two ends of the strip  are butted together with no overlap.  The card doesn't buckle as easily as paper strips, and if narrow enough to avoid screws etc, can be positioned so that its edge crosses the centre of the racked-in drawtube.

Yes - using the same technique I think - like all this stuff is very hard to explain it in text without causing misunderstandings.

Blimey!, this seems to be a hobby in its own right.

Yup - its the joy of a Newtonian - never get out these days but collimation perfection means I can still have a hobby

Measuring the circumference and then removing the tube thickness doesn't sound right to me but it may just be the way it was explained. Another possible area of error is whether or notmthe secondary is mounted upside down on its stalk.

Its probably the way I explained it - basically if you know the circumference you can calculate where the opposing points are.  ie if the circumference is 1000mm you know the opposing points must be 500mm apart to ba at the opposite side of the tube - this is not so different from Adrians card solution - he uses an exact fit with no overlap  -  I have a ribbon of card with two points marked off.  I just did this again using card on the inside and a brilliant idea of folding the card in half which would establish where the exact centre would be then used that to mark a second bit of card.  Same result

Secondary is absolutely not upside down.  The secondary assembly has been stripped and rebuilt to be identical to my own scope.

[Scooot]

I wonder if the hole for the focusser was put in the wrong place when manufactured, which is why the focusser wasn't square in the first place. Sorry, just a though, it doesn't help resolve it.

[opticalpath]

Yup - its the joy of a Newtonian - never get out these days but collimation perfection means I can still have a hobby

If you get tired of collimating Newtonians, try a fast Mak-Newt .... guaranteed to get you tearing hair out  .     Everything is back to front because the optical axis must coincide with the centre-line of the OTA and pass through the centre of the corrector lens, so you have to start with the primary tilt adjustment.  And you can't just reach in through the closed top end of the tube to fiddle with the secondary or insert bits of card etc.  Hours of fun! 

Adrian

[astronymonkey]

Do you have any pictures of the secondary assembly and focusser in place, as there maybe something we can spot which seems to be wrong ?

Cheers

[Astro_Baby]

Nope - I have been busy on other stuff so this has been put to the back burner for a bit.  Best thing to do when you can see no solution is to leave it alone and then come back and maybe see whats wrong with fresh eyes.