Collimation headache

[parkway]

Headache is an understatement. I am at my wits end with what should of been a relatively straight forward task! It's a wonder I have any hair left or the scope hasn't been smashed into several pieces.

I had some time off work so I decided to collimate my Skywatcher 200 tube. I had tweaked the primary in the past but this time I thought I'd learn to do it all from scratch and found AstroBaby's excellent Collimation Tutorial.

First thing I noticed, the spider vanes weren't centred. So I centred them perfectly. I followed the rest of the tutorial and soon enough I appeared to have a collimated scope. I used a Colicap and Cheshire.. and my eye saw exactly what all the tutorials show as to what a collimated scope should look like.

I took the scope out the next evening as the clouds had departed and I put in the 25mm EP and everything looked fine. Although the seeing wasn't particularly great, stars appeared to be sharp. I dropped in my 10mm and I could not achieve focus at all.. infact, no where near. if I lifted the EP out of the focuser about 1/2 cm or so, it comes to focus.. I've since purchased some TV plossl's and they're even worse. The TV 15mm almost reaches focus with the focuser completely drawn out..

I have spend the best part of a week reading everything about collimation and learning about secondary offsetting etc. and I'm still none the wiser as to whats wrong.

Although I don't believe the secondary needs off setting on my Skywatcher, since I've read its built into the mirror, I have tried it.. and I think it may of made a little difference. I'm perplexed since I THINK the vanes were offset before I started because I was surprised at how much they were off centre (this was before I knew about offsetting).

I've also reset the primary mirror incase it'd somehow sneaked up the tube from my minor tweaks in the past... that improved it ever so slightly, but it still ain't right.. will I ever be able to focus using my higher powered EP's or am I stuck at low powers forever more..

Why oh why did I touch the secondary.

Please somebody have the answer as to why this is happening, I've exhausted the search button and google and I cannot seem to find the answer.

Incidentally, I managed to get focus using the 11mm EP pointing the scope at a distant Pylon holding the secondary in my hand below the focuser, but as soon as I reattach it to the holder, collimate it.. it's back to how it was before...

I've used graph paper to centre the secondary exactly in the middle of the focuser view, the primary mirror clips sit perfectly in the middle when I come to adjusting the tilt.. and the primary is a doddle after that.

I repeat.. why oh why did I touch the secondary....

*groan*

Cheers,

Peter

[Astro_Baby]

Okies - that doesnt sound like collimation to me. Does the scope have the standard Sky-Watcher focuser in it ?

The most likley cause is that some Eyepieces wont come to focus with the standard tube. My Baader Hypersions dont when they are fitted into the scope using their two inch adapters. I had to get an extension tube to cover that.

I also had grief with a Barlow doing the same thing. My Celestron Optima Varlow wouldnt come to focus with ANY eyepiece unless the Barlow was slightly pulled back from the focus tube.

Do any of your EPs come to focus ? If so its osunds like a focus tube length issue.

The fact that your EPs come to focus when you jack them out a bit sounds like you either need an extension OR there may be another way round this.

When you collimated was the primary mirror wound out (towards the focuser by any great degree ). The first thing to check would be if the primary has been allowed to come too far forward. A Simple telet would be to wind the primary back to the stops - DONT GO MAD and overtighten it. Finger tight will be enough. Take a look then - dont worry about collimation just check if the focus is OK. Ummm possibly scratch that cos I just re-read your post.

My bet is it either wont be OR it will be slightly better because I had the same headaches with my SW200 with the Hyperions initially.

Check out the primary because the amount of adjustment availabel in the secondary will be marginal at best.

Secondary offset is also about the secondary coming further up the tube (or indeed down the tube).

There are two ways to consider secondary offset in a fast scope (and I am talking here about offset up and down the tube). The first is to offset so you get perfectly concentric circles like a slow F ratio scope would show. This would mean the secondary moves up the tube until the collimation circles are concentric - at this point the secondary will not be centred to the focuser. It will be 'offset' somewhat.

The alternate way is generally considered best (which is how my guide shows it) which is to centre the secondary to the focuser because it provides;

a) an easier way of doing things and

better field illumination (in theory) field illumination is at its heart just the fact that the secondary gets equal light all over it from the primary.

The secondary offset further up the tube might solve your problems and field illumination isnt generally an issue. I was using my scope for a while offset up the tube and I really couldnt tell the difference. I just did it out of curiosity.

Finally hang in there - collimation can be a complete nightmare at times - when I rebuilt mine after flocking the tube it took me three nights on the trot to get collimation down pat again - sometimes I can do it quick - other times not.

[mik]

Hi Peter,

My TV Plossle's wont come to focus in my XLT 150 with the primary wound back, I needed an extension with the 8 & 11mm the others where fine.

My veiw is refractors all the way here, less messing around.

Mels Tutorial is very good though,

Mick.

[parkway]

Hi AB, thanks for replying.

Yes, I have the standard Skywatcher focuser. I've not made any modifications to the scope. It's the original 10mm EP that came with the scope that won't reach focus. It used to be fine until I meddled with it.

The original SW 25mm is fine, the TV 25mm, TV 20mm are both fine.. the TV 15mm almost gets there, and obviously the TV 11mm does not. All the Tele Vues are plossls I recently purchased after I messed up my scope.. I am hoping they'll be ok once I sort out the problem, I'm just using the original 10mm to get it back to how it was.. I am sure the TV's will be fine..

If I stick in the SW Barlow x2, then it all works fine as well.. obviously this increases the length..

Yeah, I had already took the primary all the way back.. and only used 2 of the bolts to align it as someone suggested in another post, and left one bolt alone completely.

I am going to try offsetting it down towards the primary, see if that makes any difference to the focusing. I originally thought it was better with it closer to the vanes.. but I'm not so sure any more..

*sigh*

Cannot seem to find your tutorial any more, adur-astronomical.com : Parked Domain no longer works.. do you have a new one AstroBaby? Thanks.

If anyone has any ideas, then I'll be very grateful.

Peter

[johninderby]

Have you looked at Collimating a Newtonian for the animated tutorial on colimating?

Is the secondary centered when you look through the focuser ?

John

[parkway]

The SW 10mm is now coming to focus, and even the TV 15mm is reaching focus just as the focuser comes to an end.. so I am gonna need an extension for the 11,8mm (and maybe the 15mm as its pretty close!) TV's then.. no clue what I've done to get it working, apart from fiddling endlessly with the secondary.

What would I need to use my 11, 8mm TV's then? Would this suffice:

Baader T2 Extension Tube - Adaptors - Baader T2 Extension Tube And what size would do? Comes in 7.5mm, 15mm and 40mm flavours.

Thanks for your help guys.. I'm gonna go look to see exactly what I've done to get it working.. but I'm feeling very relieved now.. and perhaps a little wiser.. I think.. I think its all slowly clicking into place now.

Cheers,

Peter

[Astro_Baby]

The guides changed address to Astro Babys Guide to Collimation

Its strange your 10 was ok before. When I collimate my scope it amkes almost no difference to focusing at all. I know because when I was having gyp with the Hyperions I tried everything under the sun. I could improve it very slighlt but not enough to make any difference. Hence the extension tube.

Do keep us updated - I'd be curious to know what the issue is.

By the way = a collimation guru read my guide and agrteed with it - I'd not have posted it up without peer review.

[albedo0.39]

Hi Peter - don't worry. Unless something is broken, you are never far away from trouble.

Primary

As you have already mentioned, I'd wind the primary all the way in (i.e. tight on all screws) then back each one off 360-720 degress - one to two turns. Then from that point on, only use 2 to collimate.

Secondary - 1

Looking through the focuser tube and covering the mirror to avoid confusing the images, ensure the secondary is aligned to show you all the image - i.e. you are viewing angle is perpendicular to the flat plane of the mirror. When I did this on mine, I loosened all the secondary screws and then twisted the mirror to make sure it was "obviously" misaligned. I then progressively re-adjusted to make it "obviously" clear that the image was correct. Practise makes perfect.

Secondary - 2

You then you need to ensure the secondary is correctly positioned in the telescope tube. There should be a centre screw to adjust:-

Loosen and the 2ndary moves towards the primary.

Tighten it, and it moves away from the primary

Again, when I did this, I spent a good half an hour deliberately getting it totally wrong so that I got used to the image I saw. Once you are convinced it's correct then (just for rough adjustment) ensure all 3 2ndary collimation screws are roughly aligned

Collimation - Secondary

Insert cheshire collimator and ensure the images align as per Astrobaby's excellent guide

Collimation - Primary

Adjust mirror with the 2 screws to ensure the cross-hairs, reflection and centre dot are all aligned

Top tip...

Stand as far back from the peep-hole in the collimator as you can whilst still observing all the reflections. In this way you are very effectively making it harder to align (you have to concentrate and shift your eye's focus from the x-hairs to the reflection) but a side effect of standing well back, is that you can really see if the cross-hairs actually DO align. When looked at close up, you might get a view that suggests they are, when in fact they are somewhat mis-aligned

Since I've collimated in this way, I've obtained a 7 to 9 ring airy disk in my 'scope ( I wondered what it was at first !) and images are sparkling

Again - I wouldn't worry. If you have correctly aligned the spider in the tube, then perfect collimation is only a few moments away

Good luck

Steve

[parkway]

AstroBaby,

I am baffled as you. I've noticed it comes to focus a little sooner if the secondary is offset away from the focuser. I've just tried my TV 25mm and 20mm and they're identical to the 15mm, they just come to focus as the focuser comes to the end.. Hadn't thought about them being parfocal.

I am thinking my collimation woes have been confusing due to purchasing the TV's AFTER I started screwing around with the collimation. Figuring the 11mm be roughly the same as my SW 10mm.. I'd been using that today while targetting a distant Pylon...and not been able to focus.... the 10mm SW seems fine. I'm going to have to get an extension tube...

But I originally did have problems focusing with the 10mm SW the first time round.. and probably may of had it licked had I not switch to the TV... if that makes sense... hopefully I am collimating correctly.. I'll go over your guide again and read over Steve's post a few times a well.. make sure its all clicked in my wooden head.

Thank you all again, will post again when I finally got it licked.

Any suggestions on what extender I need?

Cheers all,

Peter

[Astro_Baby]

Well ites easy when collimation goes wrong to start chasing red herrings.

I'd suggest doing the scope as per my guide and see if the 10mm and 25mm supplied with the cope work ok. My bet is they will,

Then try the TV plossls and see what they do. To be honest I would be a bit surpised if they wont work because theres not much in a plossly design as far as I know and I would have thought they would focus at more or less the same point. Perhaps a Televue Plossl owner could comment. With that said its entirely possible that they dont in fact reach focus till your past the end of the focuser.

As to what extension to get I cant really say - measure the pint at which the TV plossls focus and get an extension tube thats a bit longer than that would be my guess. If its any help my extension tube is about 3.5"

[parkway]

Thank you AB.. you've been a big help.. I've printed a copy of your guide off.. I almost cried when I couldn't find it after the old page went offline.

After searching the board on what extender to get, I found a gem of info I never knew... you can unscrew the lens from the SW 2x Barlow supplied with the scope.. and ta-da.. extender.. it's a little dark to be looking at the Pylon clearly now but I got all the TV's to focus no problem!

I guess I'll make do with that! Collimation seems spot on anyway, need to star test it now.. I just took receipt of a Hubble Optics torch today too, but I'm too tired to give it a go.. maybe a task for tomorrow. I got to work out how I'm going to get the distance between the torch and the scope.. hadn't thought that through either.

I've certainly learn't a lot about my scope over the last few days.. lets hope future lessons are a bit less painful!

Thank you again for all your help.

Peter

[profees]

i used this one How To Collimate a Newtonian Telescope - The Galactic Fool

hope it helps

[Astro_Baby]

No worries - I learnt how to collimate the hard way myself but I learnt an awful lot in the process