collimation help

taxman (Matt) · #1 06-06-2008, 05:59 PM

Got my new dob today. Am trying to collimate it

From step 1 it all went wrong

Look throught the eyepiece holder and ensure the secondary mirror is round - whoops mine is oval - instructions say to adjust the centre screw if this is the case - tried that, very tight, need to loosen the three secondary screws, did that, rotated centre mirror.

moved primary screws until centre dot in crosshairs - done.

Hang on - secondary mirror is vertically (not at 45 degrees to tube) miss- alligned - moved two of the three center screws to get that back in place.

centre dot not in crosshairs - fiddle fruitlessly - one of the screws is holding the cell tight to the tube, fix that, fiddle again, back in crosshairs

about to take it out, when i realise the secondary is wayyyy off on both axes.

throw screwdriver at wall, write a rant on forum...

mark3d · #2 06-06-2008, 07:03 PM

hi, sorry to hear about the troubles. i think we have all been there! i know mine didnt get right until i had had a couple of attempts, each time with a cooler head.

are you using any collimating tools? my laser collimator was only of little help until i learnt about using it with a barlow lens and now its pretty easy to get the collimation very close to perfect.

it doesnt need to be perfect though... near enough is good enough to at least get it out there to take a look !

wavelandscott (Scott) · #3 06-06-2008, 08:40 PM

Search the forum for threads on how to...

A fellow named Andy Shotglass has a video on the web that is also helpful.

Like many things, the right tools make a world of difference...what tools are you using?

astro_nutt · #4 07-06-2008, 11:28 AM

Hi Taxman!!
Yep!..I know the feeling..
But here's a few helpful hints..
If you've just begun to learn colimation..find a nice quiet, clean spot away from all distractions if possible.
Have adequate lighting.
Have the optical tube secured horizontally whilst doing the work.
Wear a cap to stop any sweat, etc, from falling onto the optics.
Make sure all tools are clean and have a hole in the handle with a strap that can be worn around your wrist.
Be sure to take a break, a cuppa and refresh your hands.
It does get easier each time!
Cheers!

PCH (Paul) · #5 07-06-2008, 03:57 PM

Hi Taxman,

sounds like you're having fun alright

.

It can be a bit tricky when you start, but you will get it right if you keep going.

To collimate, what I did was - when it came time to adjust the primary mirror - I clearly marked the adjustment screws #1, #2 and #3 in black marker. Then I got a willing helper to assist. So, whilst I barked orders like "#1 a half turn clockwise [for example]", I was able to view the effect the adjustment made simply by tracking the red dot of my laser collimator on the face of the primary.

With today's laser collimators, you can line up your mirrors so that the secondary reflects the beam onto the centre spot of the primary, and then the primary reflects the beam back up onto the secondary and into the centre of the EP. In other words, so that the light path into and out of the scope all lines up as it needs to do to acheive focus.

Having said that, some people quite rightly will suggest a laser collimator has limitations, but assuming your secondary is lined up approximately ok firstly, the above procedure works a treat.

Hope it helps.

Cheers

GrahamL · #6 07-06-2008, 07:44 PM

hey congradulations on the new scope

collimation is a little daunting to get your head around for sure even with all the videos and pictures on hand to boot.

http://www.piscescs.com/astro/collimat/notools2.html
Even the first pic in this article dosn't show exactly what you can see
as its a 2d representation of a 3d event

Start with the inside your the focuser draw tube , while peering down it
slowly pull your head back so as to close in the view on the outside edge of your secondary mirror, forget about any mirror reflections , just these two circular outlines , you need adjust your secondary so they both appear almost concentric and merge together.

Primary tilt is likely to be off as well ,again without tools looking down that focuser draw tube you want to be able to see all of your primary in the secondary mirror .. a simple cheshire will do the trick here to align the black dot.. when adjusting the tilt I use only two knobs at a time ,if those two dont work I move on to another two.

theres many great collimation tools on the market and its not a major chore once you get the hang of the basics..

good luck and enjoy your scope

taxman (Matt) · #7 08-06-2008, 08:42 AM

Thanks for the concern, everyone.

I worked out what it was at 5:15 this morning - All my fiddling with the three outside screws and the centre screw on the secondary had pulled the mirror up the tube.

I have got it pretty close now, an even got out to check it with a star test, a little bit flattened, but I can live with it for the moment.

The view was amazing, though. Even with the GSO 26mm 2" eyepiece, the clarity was breathtaking.

Now I need to learn the sky a bit and hopefully see some of the good stuff...

erick (Eric) · #8 08-06-2008, 02:47 PM

Quote:

Originally Posted by taxman

...see some of the good stuff...

Indeed you will!!!

Well done.

White Rabbit · #9 09-06-2008, 11:12 AM

I'm in the exact same boat. I got my 12" light bridge on Wednesday, it's sunday now. I only just figured it out last night. What I found helped me a lot was to cover the primary mirror with the dust cap, this way I new what I was seeing was only the secondary mirror. That might be a bit hard on a non truss style dob though but i sure helped me.

NOW IF ONLY THE BLOODY RAIN WOULD PISS OFF.

Jen · #10 21-06-2008, 04:22 PM

Quote:

Originally Posted by taxman

throw screwdriver at wall, write a rant on forum...

HAHAH hey i thought i was the only one that done that

im having the same trouble and yes ive read all the tips and tricks but dam

nothing looks any different to me

Jen · #11 01-07-2008, 11:46 AM

ok now that i know what im doing thanks to Brett my scope is now collimated and no i will not tell how i was doing it hahahha and no i didnt have a blonde wig on that day

erick (Eric) · #12 01-07-2008, 12:07 PM

Jen, you just gotta tell us so next time someone is having trouble we can ask - are you collimating it Jen's way?

Jen · #13 01-07-2008, 02:34 PM

Quote:

Originally Posted by erick

Jen, you just gotta tell us so next time someone is having trouble we can ask - are you collimating it Jen's way?

@ Jens way

ok i will tell ya only because just incase Brett gets to tell it before me lol (Chris is not gonna let me live this one down for a while

)
Ok i was turning the the big screws on the outside of the scope that hold those spider thingies in place

(collimating jens way

)

(jen runs and hides now) i can hear you all laughing from here

dam it i laughed at myself

erick (Eric) · #14 01-07-2008, 04:18 PM

I'd like anyone on the Forum who hasn't done something STUPID to speak up - then we can all call them a LIAR! Good one Jen, but these things happen. Something we'll watch out for next time we are advising someone new to a scope.

ps. didn't yer read the flamin' instructions and study the pictures

pps. actually, adjusting the secondary mirror position to be central in the tube by adjusting the spider screws is a very early step in collimating a reflector. It's just that you don't have to do it every night.

Jen · #15 01-07-2008, 08:34 PM

Quote:

Originally Posted by erick

ps. didn't yer read the flamin' instructions and study the pictures

um yes well i did read and i did wonder why it always said turn the three screws

and i was like dam i have four wow my scope must just be different to the ones u were talking about

but i did go back and watch the video, cause now i have faster internet speed than i had before which kept stopping and starting so i gave up watching that at the time but that was really good to watch now that i know what im doing hehe

skygawker (Allan) · #16 01-07-2008, 09:29 PM

On the prickly and frustrating subject of collimation - I see in all the how-to diagrams out there on the web that you should see a complete image of your primary in the secondary. I can only see the cell clamps when my drawtube is wound in to the max. Is my secondary possibly undersize or is this normal? (not a freudian slip)

I have a Skywatcher 10" f4.7 Dob so the tolerances for optimal performance are supposed to be very tight (I've read that it's thousandths of an inch!). The manual for the scope suggests you can successfully collimate with a modified 35mm film container - you've gotta be kidding!!! Seriously I'm sorry I didn't do some more research into the problems of fast scopes before I purchased. I just saw aperture and had stars in my eyes (pun intended).

For a start I find I have heaps of slop in the focuser which uses 2 set screws for clamping. These actually move the cheshire off center for collimation but later my ep's are a better fit and locate more central (can't be good for lining up focal planes and maxing performance!). I have seen focusers/adaptors using brass ring clamps. Do these centralise the collimation tool better? They've gotta be easier on the chromework.

So many questions I know but I'm really struggling to crack this black art called 'collimation'.

erick (Eric) · #17 01-07-2008, 10:17 PM

A couple of questions and comments, Skygawker:

Q1 - Are you taking photos or observing? I'll stick my neck out and say that collimation is much more critical for the photographers. I reckon most observers can cope with less than perfect (eg I don't think they need collimation to thousands of an inch)

Q2 - Is it a 2" focusser which uses an adapter down to a 1.25" eyepiece or cheshire? From what I have seen, yes, the brass compression ring version is much better at holding and centring the item being gripped. (And does less damage on the chromework.) Can you get a new adapter with a compression ring? They are a standard accessory. (Or is it the 2" focusser itself that has the screws only. Strange, I thought most came with compression rings these days?)

If your cheshire is still sloppy in a compression ring, stick a layer of contact (yes schoolbook variey) around the barrel and see if it is a snug fit then.

I don't think it is abnormal that you don't see the primary mirror clips all the way through focusser travel. Is this looking through the peep hole of the sight tube or into the empty focusser?

I don't know the telescope, so let's see what others who know of it, can say.

Lastly what do the stars look like? Sharp pinpoints of light at focus? Nice clean concentric circles just inside and outside focus (look up "star test")? If so, what more do you want?

skygawker (Allan) · #18 02-07-2008, 12:34 AM

thanks erick.
Just observing at the moment.
It's a 2" crayford with adaptor and both only have set screws. I've been looking at the GSO and Bintel 2" crayfords and their finish seems a lot better than the SW's. But it does work as a focuser OK - it's just the sloppiness with the cheshire. Maybe your suggestion with the contact padding will work with the set screws.
I can see the whole primary only by looking into the focuser without a peephole and pushing my eye in as far as it can go.
At the moment center field stars do have some coma but it seemed better when I first got the scope and before I tried to 'improve' the collimation (my biggest mistake).
I've never been able to get concentric circles from star testing (using a 4mm EP), only spikey patterns (diffraction?) which I've always put down to the seeing conditions. Jupiter and Saturn don't look too bad in the center. How much of the field should be sharp if the collimation's the best I can get with just a cheshire? 50% too much to ask?

erick (Eric) · #19 02-07-2008, 09:56 AM

OK Perth folks! Who can offer to have a look at skygawker's scope?

50% (linearly) of the field sharp is not too much to ask. This is very eyepiece dependent, however.

The contact idea is good for a compression ring, may not be as good for set screws - but you can try.

Perhaps an investment in a Bintel Crayford focusser? Get the 10:1 fine focus version.

Interesting that you are not seeing all your primary. I'm not sure how much we get from the outer cm of the primary anyway? I read about turned down edges etc., so perhaps the edge of the mirror can be a source of optical problems - best if you don't collect light from it? Still that's just a bit of light lost - won't affect the sharpness of the image.

Yes, seeing is seldom good enough to see the textbook concentric rings. Perhaps only the innermost couple are stable. But are the spiky rings concentric, as best as you can see?

Jone5y (Steve) · #20 07-07-2008, 09:33 PM

Hi Skygawker,
I am having very similar probs with my homemade scope - collimate as best i can with cheshire only to find worsening coma toward the edge of field. I've ruled out the eyepiece as the cause as its a televue plossl which have a very good reputation. Mirrors by Suchting so they're not contributing to my probs. I'll be watching this thread with much interest to see what other advice the gurus have.
I have just replaced my standard bintel 2" crayford with the low profile version because much like you i couldn't see all of the primary in the secondary and it's going cheap. PM me if you would like to upgrade your focuser.

Steve