Collimation help

[boomstix]

Hi guys,

Been trying to get my head around collimation these last few days its kind of doing my head in. I did a star test last night and noticed that when it is out of focus I dont get a complete circle, one of the edges has been cut. When I use my laser collimator the red dot shines into the middle of the circle on the primary. However when i move the telescope up or down the dot the comes back up to the target moves a bit. It might be lined up perfectly when I have the dob at say 45 degrees but if i move it up or down from there the laser dot will move slightly out of the centre. (I mean centre of the target, the dot stays in the centre of the primarys circle the whole time)

After getting everything lined up i have taken a look into the focuser and notice that the primary mirror isnt completely showing - kinda like my star example. I have attached a picture but its real dark, hopefully you can see what I mean. When I look down it the circle from the primary lines up with the middle of the secondary but all of the primary mirror is not in view. Hopefully I have explained myself well enough so that someone here can point me in the direction of how to fix this.

I never even noticed until last night and for the most part I can get everything into focus quite well. But I have no reference to go by as to whether what i am seeing is as good as it could be as have never looked through another telecope before. And to be honest would never have known if i didnt see an article about collimating with a star.

Cheers in advance,

Pat

[bmitchell82]

Just a quick look off that image your secondary mirror isn't square to the focuser.

Also laser collimators are generally not collimated themself and the red dot is also a sausage makes it a bit hard. So first of all pull it apart and put a mask over the end of the laser so you get a tiny pin prick for a laser dot, then collimate the laser itself by rolling it round and round untill the dot doesn't move over say 6m distance from the laser.

Next get a ruler lay it across the spider vanes and make sure that the spider vanes are straight.

Open up the primary mirror cell and replace any springs with far bigger and stronger springs (this is the most probable source of your change in collimation).

Next. open up your focuser eg have it as a 2" straight though, and look at your secondary. it should present a circle that is centered within the focuser up down left right.

Now is when you put your laser in and do its job. get the dot to land on the middle of the primary using the secondary to get it there. then use the primary to return the red dot back to the collimator. This makes sure that the Secondary, Primary and Focuser axis are in line.

Hopefully that gets you in the ball park.

For better collimation you need different tools and my preference is no lasers :) Auto collimator, Cheshire and Sight tube will give you virtually perfect collimation No guessing and no star testing, as star testing newts is a very tricky and hard task with every mirror floating! :D

[boomstix]

Thanks mate. I changed my secondary screws to bobs knobs on the weekend so probably nudged the secondary out of place when doing that. Ill let you know how I go :)

[Suzy]

Hi Pat :hi:
I hope this will help.

http://www.youtube.com/watch?v=YAVGcGEBmCE

[coldlegs]

Pat
Here's the latest way of collimating a newt. It shows how a laser can lie.
http://www.youtube.com/watch?v=8ROvNH5uwDo
Cheers
Stephen

[boomstix]

Thank you Suzy and Stephen, great videos.

[Jason D]

That video is doing more damage than good in the astro communities. It just shows you how anyone can upload a video to youtube, present themselves as experts, then make ridiculous statements which come across as believable.

lasers are used to align the focuser axis wrt the primary mirror then align the optical axis of the primary mirror wrt to the focuser. That is called axial alignment. They are not meant to center/round the secondary mirror under the focuser.

Single beam lasers (or barlowed lasers) can’t be used to center/round the secondary mirror under the focuser. After all, the laser never hits (or interact with) the edge of the secondary mirror so how in the world should we expect the laser to center/round the secondary mirror under the focuser.

Lasers do not lie. That video is blaming the lasers for an alignment it was never meant to perform.

Jason

[Jason D]

Pat, your secondary mirror is too low. You need to bring it up closer to the OTA opening.

See attached animation


Read the first page of the following thread to get an idea about the 3 collimation alignments. Lasers can handle only 2 of the 3.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/reflectors/Number/3033065/page/0/view/collapsed/sb/7/o/all/fpart/1

Jason

[boomstix]

Thanks Jason

[floyd_2]

I'm with Brendan. I've been using a Tectron Sight Tube, Cheshire and Autocollimator for about 20 years. Great gear. The book that comes with the Tectron set (Perspectives, by Vic Menard) is worth the price of admission alone. It's a no nonsense guide on how to collimate a newt and is really worth a read.

I would be keen to get the Catseye colimation tools one day as they look quite nice, but really, the Tectron tools do the job admirably.

Dean

[boomstix]

Ok so trying to eyeball it doesnt work for me. Spent several hours cursing myself for changing to bob's knobs and not taking more care. Couldnt get it properly collimated so eventually gave up. I ordered a skywatcher collimating tool this morning so hopefully that will improve my chances of getting things right.

[mplanet62]

I would think there is astronomy shop in Brisbane. Go there and talk to the guys - for moderate pay (50-100$) they may do the collimation for you - first time. From there, having a proper reference point and with use of multiple manuals found across the net you will do it yourself. Worked for me. Also, there's strong astro community in Brisbane. They may give a hand I reckon - if asked.

[PeterM]

Pat one of the things most people don't do when trying to centre the secondary is to remove unwanted and confusing refelections from the primary mirror.
If your tube is a collapsible one then simply leave the primary cover on. If it is a solid tube then very carefully tape a piece of A4 paper to the inside tube wall behind the secondary (towards the primary) so that it cuts the reflections from the primary. Have the tube on an angle and not straight up and down so nothing can fall on the primary. Be very careful not to let the tape touch the secondary.
Now with this in place you will need to move the 2ndry along its centre bolt towards the front of the scope until it looks centered as much toward the front and back of the focuser. You will probably have to rotate it also along this centre bolt so it is equally centred top and bottom in the focuser. A sight tube (or 35mm film canister with tiny hole in centre) helps very much here as trying to only eyeball centre is not that easy. Once it is centered you can remove the paper (and tape) and go about aligning the 2ndry to the primary centre dot with the laser and 3 adjusting screws on the 2ndry - very small movements. Once this is done you can then adjust the primary by looking at the return beam on the 2ndry and adjusting it until the return beam disappears in the centre of the laser.
If you have any issues bring the scope to Sirius Optics about 10am tomorrow and I will go through it with you. And no we won't charge you - we are about helping as much as possible.
Regards
Peter

[Allan_L]

Hi Pat,
I know your pain.

Barx1963 (Malcolm) posted this link in another thread.

Honestly. it is the best guide to collimating I have read.

There is a bit to do, and it is not quick, but if you work through it slowly it covers all the issues (as far as I know),

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide .htm

[boomstix]

Thanks Allan and Peter. Ill give it another shot once I get my new collimating tool and if I still cant get things aligned Ill hit you up on that offer Peter.

[PeterM]

No worries Pat.
I work there every second Saturday so I will be in there this Sat (not tomorrow - lost track of days there) and then Greg is there on other Saturdays. Either of us will be happy to assist. If you do come in do so around 10ish any earlier is a problem because we are presenting astro info to customers and any later we are usually pretty busy.

[alistairsam]

Hi
Will any of the steps mentioned correct the distance of the secondary from the ota edge or in other words centre it along the primary mirrors axis
This is in case the secondary hasn't been glued correctly to the holder
You could use a ruler to centre the holder but just wondering if the AC will correct it
Pat, I don't mean to detract

[boomstix]

Well I still havent been able to get it 100% to my liking though it is almost there and will be sufficient to view Venus tomorrow. Have spent hours and hours trying to get it perfect but its always just slightly not there. Here is a pic I took tonight of the moon through my scope. Happy viewing tomorrow all!

[Jason D]

Precise centering of the secondary mirror inside the OTA is not required for good collimation.

Jason

[Peter.M]

Jason is correct, infact if your scope is fast enough the secondary needs to be offset away from the focuser and so will not be centered inside the OTA

[fauxpas]

Was laser collimating my 4" desktop dob today (the collimator came in the mail) and ran into trouble when I realised after I adjusted the secondary mirror, it was pointing in the centre but the mirror's centre is not reflective...

Bugger!

Took me ages to work out what I was doing wrong... I'd seen all the youtube videos and thought it'd be easy but no matter how I adjusted the primary, I couldn't get a nice red dot onto the collimator target...

I ended up getting there but had to work off the feint laser reflection... Anyhow, it's better than it was before I started...

[Marcus]

I easily found the centre of my primary mirror and centred the laser.
Then I need to face the laser to the back of the telescope and check it is in the middle of the target, where the hole is. I get no laser spot, and a little bit of "spray" as it suggests around the hole I guess, but can't turn the mirror to move the spot out to check I am really there. I do occasionally get a laser line across the target...does this even matter?