Collimating 10" GSO Dob..

[barees63]

Hi there,

I'm a newbie from NZ with a new GSO 10" DOB and I'm having some fun with collimation, I bought a laser collimator with my scope (but have since ordered the CATSEYE collimation kit and a Techron sight-tube).. in the meantime I have a question about collimating with the laser (I have asked this on another forum but I'm still confused ), I have centre-spotted the mirror using a paper template (ie. removed primary and traced around it, folded template into cone, snipped off end placed over mirror and stuck a ring in the middle as accurately as possible), I have collimated the laser using a V-jig and a wall about 20 feet away. I put the collimator in the focuser (a bit sloppy, added one layer of cellotape to collimator to try and tighten it up a bit), tightened the focusor setscrew as I would with an eyepiece then adjusted the secondary tilt until the dot was right in the centre of the primary mark. I then adjust the primary until the return beam goes back thru the hole in the collimator target. so far so good, now I take my home-made collimation cap (film canister) and look thru the focusor with it and the Primary center mark is off center by about 4-5mm vertically, if I adjust the primary so the mark appears dead center thru the collimation cap then the laser beam misses the target hole. Can anyone suggest the most likely problem? I know I'll be able to get a better collimation with the tools I've ordered but in the meantime would like to try and get it as good as possible with what I have.. Thanks for any advice! Bruce.

[RAJAH235]

Hi, Bruce. to the 'mad house'. MERRY CHRISTMAS.
Sounds like you are O.K. with the laser side of collimation. This is probably more accurate than 'mechanical centre', which is what you get with the film can cap.(correct here please guys?).
All dobs require a bit of 'offset' towards the primary & away from the 2ndary. 4~5 mm sounds right. (if you look down the front of the t'scope, it doesn't look right, but it is). You are lining up the optical centres.
Q. If you look thru the focuser without anything in it, (move your eye around & back, to line everything up), is the 2ndary in the centre at the bottom?
I've found it's easier to start with a sheet of white paper, bluetacked to the inside of the tube, opposite the focuser, to line up the 2ndary FIRST.
Should see an white circle outside/around the 2ndary. Adjust it till it's in the centre. Adjust primary, then do a laser check.......
Do a 'STAR TEST' after the laser collimation to check & make final small adjustments.
HTH. L.
see here >http://skyandtelescope.com/printable...rticle_790.asp
& here >http://www.andysshotglass.com/Collimating.html

ps. You shouldn't need/use sticky tape or any kind of 'gap filler' when collimating.
You don't with all your E/Pc's, do you?

[acropolite (Phil)]

Welcome to IIS Bruce, I can't answer your question, I'm not one of the dobber hoard, but sure as eggs there will be someone here to answer your question. We've got a few NZ members here at IIS. Enjoy your stay...

[barees63]

Thanks Guys, (and Merry Xmas!) this looks like a great forum.. wish I'd found it sooner.. it's nice (and reassuring) to see so much discussion about the GSO scopes, I live in a rural area so its difficult for me to get hands on help out here, I think IIS will be a great resource.. since I got the scope the weather has been pretty appalling (of course) so I've only been out 3 times, last night (well, 2am this morning) I was able to view Saturn for the first time which was a thrill (little detail at 130x but exciting nevertheless). I'm still finding my way around the sky but have managed to find a few nebulae and clusters, saw colour and a 5th star in the Orion Nebula the other night which was great, I think I starting to get the hang of "looking".. Anyway, it's all been good but the whole collimation thing has been a bit trying, I ws pretty naive and thought the laser was all I needed, taking out the primary to mark (and clean, it was a shop model and loaded with dust and film) was a bit more excitement than I bargained for ;-) but I managed (in case you're cringing at this point I cleaned VERY carefully with distilled water and sterile cotton per Sky & Telescope website). Anyway, I'll try the paper thing and see what I find.. thanks again! Bruce

[ausastronomer (John Bambury)]

Hi Bruce,

Thats an easy 1.

You have just discovered 1 of the major shortcomings of a laser collimator. Fact is, your scope isn't even close to collimated.

If you do a star test you will find that the secondary shadow (the black blob in the centre) is not centered within the diffraction pattern on both sides of focus. Also when you defocus on a bright star you should get a small spot of light "the poison" appear in the centre of the secondary shadow. This should appear at the same point on either side of focus, ie. with the same amount of defocusing in both directions. If it doesn't your not perfectly collimated.

Now that I have explained all that cr_p and probably confused you further let me explain the problem. The laser does not take into account the positioning of the secondary in relation to its distance from the primary. ie how close the secondary sits in relation to the primary. This is compounded by the fact that notwithstanding the secondary positioning is incorrect, you can adjust the tilt of both mirrors to have the laser return beam strike back perfectly on its point of origin, making the scope "appear" to be perfectly collimated when its not.

The only way to correctly position the secondary mirror in regards to its distance from the primary is in 1 of 2 ways:-

With a sight tube, which you have just ordered and the other is to use a laser collimator and center spot the secondary. The latter method however is only applicable with a truss dob where you can see the laser hitting the face of the secondary from underneath it.

IMO a laser is only useful for quick adjustments of the tilt of the primary mirror in the field, once the telescope has been properly collimated in the 1st place by other means.

CS-John B

[barees63]

Thanks John, looks like I'll have to wait for the sight tube to arrive from the states then (xmas mail aargh!).. oh well, I'm enjoying the scope as it is and it sounds like "I aint seen nothin yet"

Bruce

[ausastronomer (John Bambury)]

Bruce,

Sorry I forget to mention something else.

Rajah, correctly refers to the offset of the secondary mirror. He has a guess and estimates it to be 4 to 5mm for your scope which was a pretty good guess. In the case of the 10"/F5 GS dobs its actually 3mm. Here is a post I made on the talking telescopes Yahoo Group 4 days ago. This explains a bit about secondary offset and how to calculate it accurately for your telescope.

http://groups.yahoo.com/group/telescopes/message/119450

However, I will conclude by saying that its not necessary with a 10"/F5 dob for visual use, however you can offset the secondary if you wish. At this stage of your collimation learning curve its only likely to confuse you so I wouldnt worry about it.

CS-John B

[RAJAH235]

I then adjust the primary until the return beam goes back thru the hole in the collimator target. so far so good, now I take my home-made collimation cap (film canister) and look thru the focusor with it and the Primary center mark is off center by about 4-5mm vertically, if I adjust the primary so the mark appears dead center thru the collimation cap then the laser beam misses the target hole.

CS-John B, fwiw, I used these figures... Maybe mis-interpreted his meaning. L.
ps. My Meade DOB has it 'factory built in' to the 2ndary holder. ~4 mm...

pps. Damn hard over the net...

[barees63]

Maybe mis-interpreted his meaning

Just to be clear and FWIW, what I meant was when I look thru the small hole in the film canister I can see the primary mark (ring sticker) is shifted vertically (from my perspective) about 5mm from the center of the secondary rather than being nicely centered.

[RAJAH235]

Bruce.
L.

[barees63]

Sorry, here comes another long post - I got my Techron sight tube today and had a go at setting up the secondary, however there is one point about which I'm still confused, I have conflicting instructions.. following the procedure on http://www.schlatter.org/Dad/Astronomy/collimate-2.htm I taped a piece of red paper opposite the focuser and a piece of white paper blocking the tube, sure enough this made it easy to see the seconday mirror without distracting refrections and I then fiddled around until the secondary appeared perfectly centered (and round as far as I could tell) with an even ring of red (paper) around the outside.. so far so good, now at this point the instructions on this web-site say the secondary is now done and you don't touch it again, removing the white paper that was blocking the tube revealed the Primary mirror way off center (half in half out of the secondary) and I'm supposed to tilt the primary to line it all up.. I started playing with the Primary tilt but it seems so far out that there is not sufficient adjustment to get it centered. The other instructions I've read (including the Catseye collimation tool instructions) say that I should tilt the secondary to line up the primary image (and center the center-spot under the crosshair of the sight tube) and just only adjust the Primary while using the cheshire to line up the center spot, but when I start tilting the secondary it now appears off center with regard to the red background paper..

So, what should I try next?

[Starkler (Geoff)]

Originally Posted by barees63

The other instructions I've read (including the Catseye collimation tool instructions) say that I should tilt the secondary to line up the primary image (and center the center-spot under the crosshair of the sight tube) and just only adjust the Primary while using the cheshire to line up the center spot, but when I start tilting the secondary it now appears off center with regard to the red background paper..

So, what should I try next?

I had similar experiences with mine until I discovered that my spider was off centre, ie the secondary was offset sideways and not in the centre of the tube. I had to place a washer under the spider retaining nut on the outside of the tube to get mine centred.

Step 1: Get the secondary centred under the focuser.

Step 2: Adjust secondary tilt so that ALL of the primary is visible and centred in the secondary viewing through the site tube. Your laser can help here by aiming the dot at the centre of the primary. Rotate the laser in the focuser to check how the dots tracks on the primary, this will show how accurate your laser is. The laser will ensure that the optical axis are aligned. Go back to the site tube and check again the mechanical alignment. Any error here will only effect the shape of the fully illuminated field and a small error can be lived with.

Step 3: Adjust primary mirror to get its centre spot lined up with the cheshire.

Getting the secondary lined up is the most time consuming part but you wont need to do it again unless you pull it apart for cleaning etc.
Take your time and keep the screws loose until close to being aligned then nip them up carefully. The secondary holder is made of plastic and can be damaged easily by overtightening, not to mention that the screws can grind a pit in the plastic, making small adjustments difficult as the screws will want to fall into these pits.

[barees63]

Thanks Geoff, I think maybe I have it good now, I used the sight tube to center the secondary to the point where if I slide the sight-tube down then the secondary fills it perfectly.. I then tilted the secondary so the primary was fully visible and centered and the center spot was under the sight-tube crosshair, although the crosshair is so out of focus (?) it's hard to be precise - I then used the laser to fine tune it so the spot was exactly in the center of the primary spot. The I put the cheshire in and adjusted the primary so the center spot was centered in the cheshire ring, I then tried the laser and it agreed almost perfectly (return spot just on edge of hole in secondary target). End result is the reflection of the spider is very slightly offset (and I think in fact it is offset very slightly away from the focuser - hard to measure exactly but seeems about 1 mm or so, but I'm told this is correct for an F5 scope) everything else appears lined up.

(The thing that really threw me was the instructions I got off the web which did not tilt the secondary to line up the primary image)

[Starkler (Geoff)]

Sounds pretty good to me

Ill just add that the cheshire should be your final reference for collimation of the primary, not the laser. The laser is however useful for aligning the secondary.

[barees63]

Ill just add that the cheshire should be your final reference

Yes, I don't trust the laser.. just thought it was interesting as a cross check..

[davidpretorius]

well done bruce,

now onto the star tests.

after 7 months, i am feeling alot better about star testing, it takes a while to know what each knob does when you adjust it

http://legault.club.fr/collim.html

[barees63]

now onto the star tests.

I hate you!

[davidpretorius]

it is the fine tuning for sharp stars and details on planets. warning, once you start collimating and fine tuning, you will never be 100% happy - "perfectionist" springs to mind!

[Starkler (Geoff)]

Align carefully with the tools and the star test will look good guarenteed

This will just confirm that your centre spot has been placed accurately (or otherwise).

[janoskiss (Steve H)]

Originally Posted by Starkler

Align carefully with the tools and the star test will look good guarenteed

Until you crank the mag ...

My sighttube collimated f6 8" just starts failing the star test at around 400x, on the rare occasions that conditions allow star testing at such high magnifications. It is very fiddly to get it any better, if not impossible, without tracking. But if you're not imaging there is little point.