I know it's a cheek but...

[Frankenstein]

CAN SOMEONE HELP ME PLEASE?

I've just bought my Sky Watcher 10" Dob and I've really screwed it up. I decided to 'fine tune' the collimation and now it's all wrong. I know now that I should have left the secondary mirror alone but I couldn't resist tweaking it just a bit, then a bit more, and just a little bit more... you can see where this is going!

Now I can't get the secondary mirror properly re-aligned. I've read all the theory and there are some fantastic tutorials out there, such as:

Astro Babys Guide to Collimation

I just can't do it! Is there anyone in the UK, Marchwood (Southampton area) who can show me how to collimate my Dob - I WILL DRIVE TO YOU. I have a Cheshire Collimating Eyepiece and a homemade collimating cap (the hole is perfectly centered). I know that I will get there in the end but it's my kids that can't wait, and I've promised them Saturn's rings at the next clear sky:eek:

[GB-UK]

Is there a local astronomical society/club you could go along to? They can give you a hand if all else fails

[yeti monster]

Oh dear! Any photos (of the mess, not the kids sulking) which might help someone offer advice from a distance.

Is there supposed to be an offset on your secondary? I know that some scope require this, I'm assuming that you have over adjusted the position of the secondary. Did you "chase" the adjustment with the secondary collimation screws?

I've no experience of anything greater than my 114 reflector, as yet, but the larger ones appear to be little more complicated in their collimation.

Optically, what kind of scope is it? I've been swatting up on Smidtt-Newtonians of late, as I'm in the process of purchasing, so I realise that each design has its own foibles.

Good luck, and sorry I can't really be of any help.

[Frankenstein]

Hi Yeti Monster,

Before I started fiddling with the secondary mirror, I measured the length of the four vanes which support the mirror. They were not even and this is why I started the adjustments - I just assumed that they should be equal. You mention an 'offset'. Perhaps they should have been unequal - I don't know. And yes, I did chase the adjustment with the secondary collimation screws - for about 6 hours!

[yeti monster]

Ah! What was the apparent collimation like before you "sorted" it? Have you reset the vanes to their original lengths?

I think you may need to give FLO a call, if there's no one here who can give you the correct spec.

[Frankenstein]

Apparent collimation - oops! This is the first time that I've ever heard of it. I think that I will have to give FLO a call. Thanks for the advice.

[yeti monster]

What I mean by "apparent collimation" is, how well could you see things through it, was it "apparently" in collimation at the start of the excercise. Had you done a star test? I'm wondering why you felt the need to recollimate the scope, that's all. I don't think that there's a specific term apparent collimation (I hope I'm not making things worse).

[kev 102 42]

Glad its not my kids who are going to be damaged for the rest of their lives:(............kev

[Doc]

I done this as well, just a little turn and then Oh No what have I done.

Don't panic..

Adjust your spider vanes until all four read the same, don't worry what the measurement was just make sure they are all the same now.

Then place your collicap into your focuser and undo only slightly the centre screw on your secondary mirror.

Move your secondary around until you can see all three primary mirror clips and the primary mirror is centered in your view through the collicap.

When it is centered and the mirror clips on view tighten the centre screw.

Then collimate as normal with either a cheshire or a laser.

[crepitis]

Please make sure that when you are adjusting your secondary mirror that the OTA is horizontal or pointing slightly downwards..but Im sure you already knew that.

[Astro_Baby]

The spider vanes should be equal lengths. Then just follow my guide....but to quickly recap the guide.

1/ Adjust the spider vanes until all are equal.

2/ Centre the secondary mirror under the focuser making sure its its concentric to the focuser tube - ie it should look like a perfect circle under the focuser. To do this you will need to loosen the centre screw and adjust the small tilt screws.

As a tip if your really stuck - loosen the tilt screws completely. Now just use the centre screw to get the mirror centred. Grab 2dary mirror holder and twist it about until its looking good and have someone else VERY GENTLY tughten the tilt screws.

If you do this make sure you are weariong cotton gloves so that you dont get sweat on the secondary mirror and make sure you have taken your watch and any rings off - clean soft hands only please for this.

Once the sceondary is perfectly centred and looking like a circle under the focuser move on to the next step.

3/ Now adjust the secondary so that you can see all three mirror clips in the sceondary. Be careful not to cause secondary mirror twist while doing this.

At the end of step 3 the 2dary should look circular and you should see all three mirror clips from the primary in it. IGNORE ANY OTHER VISUAL INFO. ALl you want is a clear circlke of the secondary with the three clips showing.

4/ Final stage - adjust the primary. Look at my guide for the how this should look cos its too hard to explain.

Follow my guide but use what I have put here as the basic steps and all will be alright.

READING FOR TODAY....

Dont mess with collimation unless you have to for the Astro Baby sayeth he that messes with collimation shall repent and tear his clothes and gnah his teeth.

Neither see the trials of collimation as something fearful as that shalt certainly lernest much.

When collimation is wearing you down seek solace in prayer and a cup of tea for verily shall a 20 minute break and switching off thy mins reap you greater benefits in collimation heaven.

Here endeth the lesson

[Astro_Baby]

ps ignore the offset red herring - if ytou follow my guide the offset is taken care of automatically. The offset refers to the lack of concentric patterns at the end of the process. If the 2dary is under the focuser and true and the colimation corss hairs match on the black circle at the end than all is alright.

[Frankenstein]

NEARLY THERE

THANK YOU ALL FOR TAKING THE TIME TO HELP.

Here's where I am:

1. Secondary mirror looks good. All 4 vanes are equal length (about 42mm each).

2. Secondary mirror is a perfect circle when viewed through collimator cap and Cheshire Eyepiece. I believe that I have devised a simple trick to get a perfect circle - James at FLO seems to think that it could work. When I've got my dob sorted I will post the technique.

3. I have never touched the centre screw of the secondary mirror mount, so this is still at the factory settings. I have only adjusted the 3 'tilt' screws (allen key).

OK, now here is where it's not quite right;

4. When I pop in the low mag eyepiece (25mm) I can reach a good sharp focus. HOWEVER...

When I first popped in a high mag eyepiece (10mm and 8mm) I could not reach a sharp focus - the focuser was racked out to its max. I had to move the eyepieces out a further quarter inch. So (and I have chatted with James at FLO about this) my primary mirror was too far forward. So here is what I did...

I gradually pulled the primary mirror back millimetre by millimetre with the 3 primary mirror adjustment screws - I can now reach a sharp focus with my high mag eyepieces (in their normal position i.e not poking out an extra bit) - BUT ONLY JUST!

I've reached the maximum possible adjustment on one of the 3 primary mirror adjustment screws. So my question now is - is it normal to only just reach the focus point of a high mag eyepiece?

[mdstuart]

No I have the skywatcher 10 inch and my mirror is pretty much mid travel and all my eyepeices focus with spare travel so something is not right..

I would say "just maybe" that secondry is still not right. Ignoring how it looks through the eyepeice does the secondry support look like it is physically central or is it at an obvious angle.

Take a pic down the tube at an angle so we can all see and one through the eyepiece..

By the way noo clear skies until at least Monday night and probably much later so take your time.

Mark

Mark

[Tim]

Is the secondary mirror centred with the centre of the focus tube? The large central screw on it, although you havent touched it, makes it possible to rack the secondary up and down, that is, closer or further from the primary, at least, it does on my newt.

You learn more from doing things the wrong way, than by following instructions sometimes. This way you have learnt how everything works and what difference it makes, so no real harm done

[TopHouse]

Re the focussing thing, merlin66 might be the man to answer that if he's about.

[Frankenstein]

I'm not sure about exactly which shots will be useful, but here are some photos.

Down focus tube

http://i818.photobucket.com/albums/zz102/frankystein/Scope/001.jpg

Down focus tube - tube move away - I can see 6 retaining clips, is this wrong because I thought that I was only supposed to see 3

http://i818.photobucket.com/albums/zz102/frankystein/Scope/003.jpg

Looking up at secondary

http://i818.photobucket.com/albums/zz102/frankystein/Scope/009.jpg

Scope levelled horizontal with spirit level, camera on tripod. Primary looks completely wrong but I suspect that this may be a parallax thing.

http://i818.photobucket.com/albums/zz102/frankystein/Scope/012.jpg

3 shots from top of scope at an angle.

http://i818.photobucket.com/albums/zz102/frankystein/Scope/013.jpg

http://i818.photobucket.com/albums/zz102/frankystein/Scope/014.jpg

http://i818.photobucket.com/albums/zz102/frankystein/Scope/015.jpg

Very interesting comments from mdstuart who has the same dob. His eyepieces are at mid-travel, So something ise definately wrong. At the moment I'm very reluctant to go anywhere near the central screw on the secondary mount.

Thanks again to all of you for your time.

[room16]

I am wondering if that is a lower profile focuser, so in that case you should have been supplied with a 2" extension tube which you would put your eyepieces into. This would then allow sufficient back-focus. Without it there, you would not get focus due to not enough outward travel.

The reason I say this is that I have recently upgraded my focuser on my 250px due to lack of inward travel for a DSLR to focus properly. But in doing so I also noticed that I was no longer able to focus with me eyepieces, so had to rack them out a little or use a barlow without it's lense.

[Orion]

Daft question! Are you sure the focuser/secondary housing tube is in the fully extended/out position? It would cause the lack of back focus you describe!

[Frankenstein]

I do have a 2" adapter but it can't be used to provided extra travel for my 1.25 eyepieces.

[room16]

Sorry, i meant a 2" - 1.25" adapter. Then realised that you would need a 2" extender to go inbetween.

If your finding there is not enough outward travel on the focuser tube then and extension is what is needed

[Frankenstein]

Here is a pic with the focuser fully racked out with the 8mm eyepiece in its correct position:

http://i818.photobucket.com/albums/zz102/frankystein/Scope/017.jpg

And here is a pic with the focuser fully racked out with the 8mm eyepiece pulled out a bit to allow me to reach focus:

http://i818.photobucket.com/albums/zz102/frankystein/Scope/018.jpg

Also, the scope came with a 10mm plossl and it did reach focus within the normal limits of the focuser's position. It's certainly something that I've done.

[Astro_Baby]

OK - couple of questions for you.

Does the scope still reach focus at a mid position of the focuser with the 10mm plossl that it was supplied with ?

Reason for asking is my 200P when suplied new wouldnt come to focus with a Barlow unless I edged the Barlow out a bit. Knowing if it will stiull come to focus with something basic like a Plossl will address whether the focusing issue is related in anyway to the eyepiece. I havent used an X-Cel so I dont know if they exhibit any 'odd' behaviour.

If a 10mm wont fopcus then its either something you did OR and I wonder looking at your pics of the focuser really is racked out all the way. Try pulling it gently and see if it will move any further out at all. Its been sometime since my scope had the SW CRayford but I think mind came out further which makes me wonder of perhaps the Crayford is stuck in some way.

2nd question - is this scope brand new or is it 2nd hand - it could be someone else has messed with it.

Your pics down the focus etube above would suggest collimation is ok. the 6 mirror clips are a bit different but the fact you can see all of them shows theres nothing to wporry about. Its hard to know from pics if the collimation is spot on but it looks like its pretty close.

The focus issue - its could be (because its is hard to say from the pics) that the secondary is too close to the primary. It could have been badly set up at the factory or by a previous owner. Its very hard to say from the pis but when you look down the focus tube with a collicap does the secondary seem centred to the focus tube. Could it be displaced a small amount towards the primary ? If so it needs to come up the tube a bit.

[pvaz]

Everything seams right on the photos. As AstroBaby said you should try the original EPs to see if you can reach focus with them.

1 of my new EPs needs a few extra mm to reach focus.

[yeti monster]

Sorry to raise the "offset" again, but...

Was the "original" offset such that the secondary was held further away from the focuser?

My thinking being, that if it is now central then the focal point of the reflected light will have moved up the focuser, thus making it difficult to gain focus with some EPs.

Just a thought, don't hang me or nuffink...........