Trouble collimating secondary mirror on a Newtonian

[Riv39]

Hi, help please. I am having no end of trouble collimating my new Sky-Watcher 8" Newtonian (which was not collimated out of the box in fact the primary mirror locking screws were all loose). I am trying to bring my secondary in line using my GSO laser collimator but I just can't seem to get it right in the sweet spot no matter what combination of screw adjustments I try. I can align the primary so that the laser returns back on itself but the secondary is not right as stated above.

I will confess that this is the first time I have tried to collimate any scope but from my research doing the secondary first is correct.
Any suggestions would be appreciated.

Thanks Chris

[multiweb]

First thing you'll need to do without the aid of any laser is center your secondary visually under the focuser. I'd start by squaring the secondary with the spider vanes, also center its axis in the tube, then center it under the focuser so you see a centered circular shape (outline of the secondary mirror against the focuser and tube background - ignore any other reflections). Then tweak its tilt until you get the laser spot in the center of your primary. That's ll get you started.

[Riv39]

Thanks Marc, as we speak I think I managed to get it spot on (finally). I don't understand why there appears to be so little adjustment for the secondary. In the end I had to do up the screws quite tight to pull it in the last couple of mm's but all seems good now as the laser is right in the sweet spot of the primary and is returning back on itself perfectly. I hope that I do not have to realign the secondary again.

[multiweb]

Assuming the primary is centered in the tube and the center spot is indeed in its center then having the secondary mechanically centered in the tube by measuring the 4 spider vanes length should get you pretty close to start with. If you find you have to tilt the secondary to its limit to hit the center spot on the primary, I think you might have it offset by too much to start with and you don't have enough room to compensate with the 3 collimation screws.

[Riv39]

Well can't comment on the primary except to assume (as you say) that it is centred after all it is new out of the box and one can only hope Sky-Watcher got it right. I checked the spider vain lengths and they appear equal so hopefully that is that.

I must say though that the very small amount of play around the collimator where it fits into the 2" to 1.25" adaptor (and probably the the adaptor to focuser as well) does not make this a precise science.

[multiweb]

There is always a little assumption to get started but you can iterate down to an extremely precise collimation only compromised by the mechanical limitations of your rig. Have a chat with JasonD on these forums. He'll be able to answer anything you'll throw at him in a more concise explanation.

[Jason D]

I will add few things:

1- There are infinite positions for the secondary mirror which will give perfect laser reflections for both the forward and return beams but only one of these positions is optimal. In the first animation, the laser beam reflections will look perfect for the setup in each frame.

2- The optimal secondary mirror position is when the secondary mirror edge coincides with the primary mirror reflection edge. You might need to move your eye further away to line up both edges. See the second attachment.

3- If one of the secondary mirror 3 set screws becomes tight yet you need to tighten it even more to move the laser beam, then you will need to loosen the set screws and rotate the whole secondary mirror by a small amount. There is a relationship between rotation and tilt. You can compensate for some rotation by tilt and vice versa. Therefore, when you run out of tilt then you will have to rotate to continue your secondary alignment.

4- When you start with the laser collimator, it is recommended to rotate the laser collimator back and forth while watching the laser on the primary. You will notice the reflection dot on the primary mirror will trace an arc. Stop when the laser dot is closest to the primary center. See attachment.

Jason

[Riv39]

Thanks Jason, it appears to be ok now even though two of the screws are tighter than the other however from what you say I may need to start again :eyepop:. I really expected this to be right out of the box (silly me). Given the time I have invested thus far I really didn't want to start again and loose where I am. If I rotate the laser now it appears to stay centred so is all ok or not?

[Jason D]

The secondary mirror placement under the focuser is not critical for visual observation especially if the secondary mirror is somewhat oversized which is the case for many mass produced scopes.
If you are happy with the secondary mirror placement, leave it as is.

When we align the secondary mirror, there are two different alignments we have to take care of that are independent of each other. One alignment has to do with the secondary mirror placement under the focuser to center what is referred to as the 100% illumination field. The other alignment has to do with eliminating the tilt between the eyepiece and primary mirror focal planes.

Check out the following post

http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/4242330/page/0/view/collapsed/sb/5/o/all/vc/1

[Riv39]

Thanks again Jason I will have a read of the post.

[multiweb]

Jason, just checking I got this right - quick question. You can exhibit case 1 and still have the scope collimated but case 2 would show up with the A/C correct? So there's no way the scope can be accurately collimated and show tilt right? :question:

[Brian W]

If all else fails Google 'Andy's shot glass'. He has a good video on collimation. Arguably he has all the circles perfect which is rarely the case but he does take you through the procedure rather painlessly.
Brian

[Jason D]

Hello Marc,

The A/C XLK offset pupil will clearly flag case # 2 since P+2 stack is directly related to the tilt between the eyepiece vs. primary focal planes.

Case # 1 can also be flagged by the A/C. Well, it can be flagged by any tool with a central pupil whether it is an A/C, a cheshire, a collimation cap, ...etc. The idea is to move the eye back and compare the secondary mirror edge to the primary mirror reflection edge as I have shown in an earlier post.

Interestingly, case #1 and case #2 use the same 3 secondary mirror set screws for adjustment. Many question how can you adjust for both cases using the same set screws? Well, case # 1 takes coarse adjustments and case # 2 takes fine adjustments. Therefore, case # 2 adjustments are small and should have little to no impact on case #1.

Unfortunately, many are not aware that the secondary mirror requires two kinds of adjustments. Even many of those who are aware of the two adjustments do not know the significance/impact of each.

Jason

[multiweb]

Thanks for the clarification. :thumbsup:

[Riv39]

Jason, I aligned the secondary edge with the primary using a collimation cap. I assume it was correct as I could just see all three primary clips. The problem is that when I then use the laser to complete the process only one clip can then be seen when aligned.

Is this correct or have I a different problem?

Edit: The collimation cap is hand made from a 35mm film canister.

[jjjnettie]

Chris,
I thought I'd let you know, that collimation is only painful for the first few times. It quickly becomes second nature. :)

[Riv39]

Thanks, it's driving me crazy at the moment but at least I feel that I am really getting to know the scope. It is difficult to discuss these issues using words, if somone coiuld actually show me it would be so much easier. I'll get there in the end somehow even if I have to go back to the shop where I bought it.

[jjjnettie]

I only do small adjustments each time.
adjust the primary, then the secondary, back to the primary, back to the secondary.
Little tweaks each time.
Steve Massey has a guide here.
http://www.myastroshop.com.au/guides/collimating.asp

[Brian W]

Something to realize is that lasers can need collimating. Might I suggest just using the collimation cap. I gave up on a laser years ago and the cap works just fine.
Brian

[jjjnettie]

I just use an old film cannister to collimate. Works fine for me too. :thumbsup:

[Screwdriverone]

Hi Chris,

I too have recently bought an 8 inch Skywatcher reflector and had the same problem with the mirror reflections not lining up like Jason D's avatar pic says they should look.

After having three different reflectors and not having this problem before, (the secondary circle was not centred inside the primary circle) I found the problem was the PRIMARY. It was too high in the tube, what I did was back the locking screws right off and then screw the collimation screws right down so the primary moved towards the bottom of the tube. I had spent over an hour fiddling with the secondary adjusters as well as squaring up the focuser (mine had an extra washer on one screw which skewed it off to one side), until I thought to try the primary screws....

Problem solved, the secondary then lined up concentrically with the primary mirror and all LOOKED fine, then all I had to do is laser collimate slightly to finesse it a bit.

I think the dudes in the factory wind it up and away from the bottom of the tube to avoid vibration damage or chipping during transit. Once you wind it ALL the way down to the bottom, start all over again with the secondary and you should find it will easily all line up and look like Jason's avatar pic like it should.

Hope this helps.

Cheers

Chris

[Jason D]

It is important to maintain the eye axis along the focuser axis when assessing the alignment of the secondary mirror. It is possible to look slightly off-center to align the secondary mirror and primary mirror reflection edges but that is improper. A sight-tube can help to maintain the eye axis along the focuser axis by referencing the sight-tube edge in addition to the secondary edge and the primary mirror reflection edge.
It helps to ensure the focuser is squared and the secondary mirror is centered.
Finally, ensure the laser collimator is aligned by monitoring the laser dot on the primary mirror while rotating it in the focuser.

Here is a useful link – just the first few pages.
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/reflectors/Number/3033065/page/0/view/collapsed/sb/7/o/all/fpart/1

Jason

[floyd_2]

Hi Chris,
there is a book called New Perspectives on Newtonian Collimation (5th Edition) that's a must have for your book collection. I have the 3rd edition and it's an excellent resource that taught me how to collimate a newtonian telescope to perfection. The 5th edition also has discussions on using laser collimators, as well as the older sight tube / cheshire / auto collimator that I use.

Dean

[Riv39]

Thanks Chris, I will follow your suggestion and try again! Glad you suggested this as I would never have worked this one out.

[Riv39]

Thanks Dean, I will track a copy down and have a look.

[Jason D]

I 2nd Dean's book recommendation. It was written by Vic Menard who is an expert on collimation.
Besides, I have contributed some of the graphics in that book ;)
Jason

[floyd_2]

I thought your avatar looked familiar Jason :D