Collimating advice

[tripleped]

I use a laser collimator on my 10” skywatcher dob and have gotten better at it. My question is how perfect does it have to be? Lately when collimating secondary I can get the laser dot on the white “donut” on primary mirror  close to the inside rim of it but can’t seem to get it right smack in the “donut hole” any advice ? Am I trying to be too perfect ?

[rwilkey]

Hi, I have always found Astro-babies guide the most help.  Note in adjusting the secondary: About 90% of collimation errors are down solely to the primary mirror so don’t jump in and start meddling with the secondary unless you are sure that there is a problem.

Here is the link:

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

Generally, for visual, I find near enough is good enough, in my opinion.

[kev100]

Hi there, it definitely should be possible to get the dot in the centre of the doughnut given the three way adjustment ...

A couple of questions: have you changed the secondary adjustment screws? How old is the scope? Have you been tightening the screws really tightly?

The reason I ask is that I had a similar issue. My 10 inch is about 8 years old, I replaced the adjustment screws to make them easy to tweak, but found that they were digging into the metal base at the back of the secondary, and I couldn’t get the dot centred (it would get close, but then just ‘slip’ to the side, like there was a groove worn in the metal  ....).

 I found a set of replacement screws which had a domed end to the threaded part, and also put a couple of thin pieces of plastic between the screws and the back of the secondary, and it’s easy now to centre the dot ...

Kev

[tripleped]

20 minutes ago, kev100 said:

Hi there, it definitely should be possible to get the dot in the centre of the doughnut given the three way adjustment ...

A couple of questions: have you changed the secondary adjustment screws? How old is the scope? Have you been tightening the screws really tightly?

The reason I ask is that I had a similar issue. My 10 inch is about 8 years old, I replaced the adjustment screws to make them easy to tweak, but found that they were digging into the metal base at the back of the secondary, and I couldn’t get the dot centred (it would get close, but then just ‘slip’ to the side, like there was a groove worn in the metal  ....).

 I found a set of replacement screws which had a domed end to the threaded part, and also put a couple of thin pieces of plastic between the screws and the back of the secondary, and it’s easy now to centre the dot ...

Kev

Thanks for your input. Scope is new. Got it 2 months ago. Also I’m very careful to turn the screws very gingerly. I’ll keep working on it. Thanks again 

[tripleped]

1 hour ago, rwilkey said:

Hi, I have always found Astro-babies guide the most help.  Note in adjusting the secondary: About 90% of collimation errors are down solely to the primary mirror so don’t jump in and start meddling with the secondary unless you are sure that there is a problem.

Here is the link:

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

Generally, for visual, I find near enough is good enough, in my opinion.

Thanks a lot! I’ll check out that link. 

[Matt61]

Have you checked the laser is accurate? A laser told me my 5 inch was spot on .A Cheshire told a different  story. Sure enough on checking the laser was out. Not by much but enough to have me chasing my tail collimating

[Per_S]

Tripleped, since you already have a laser collimator, try the barlowed laser collimation method developed by the late Nils-Olof Carlin:  https://www.obsessiontelescopes.com/learning_center/collimation/index.php

/Per

[tripleped]

Did a little collimation experiment tonight . Collimated the primary spot on.  Then I manually slewed the scope from level position to almost 90 degrees and noticed that the collimation drifted off slightly, but noticeably. Seemed to improve when I tightened the adjuster screws on the primary a bit more.  So I’m guessing the adjuster screws were not tight enough ? Does that make sense? I’ve always been real nervous about over tightening the screws but now  thinking I’ve been a bit too careful about it.  Thoughts? I’d appreciate any input.  Thanks!

[johninderby]

You mean the locking screws? They should be pretty tight. After adjusting the collimation with the adjustment screws tighten the locking screws. Note that this will throw off the colliation a bit so a bit of final collimation using just the locking screws will be needed. Colimation can also change if the mirror shifts in it’s cell. The holding clips on the mirror should almost touch the surface of the mirror but leave enough space to slip a piece of paper under the clips.

Do your collimation with the scope at about a 45 degree angle.not level as this is closer to the angle you will be observing at.

Edited November 11, 2020 by johninderby

[tripleped]

Ok. I’ll try that.  Yes I was taking about the locking screws.   Sorry I didn’t specify that. I am 100% sure I had the locking screws much too loose.   Thanks for the input.  It helps! 

[AlexK]

To verify if your laser collimator is collimated well enough simply rotate it in the focuser. To avoid false positives make sure you hold it approximately at one wall (usually the one dictated by the gravity) and tight to the rim of the focuser to define the orthogonality. If it shifts less than 3mm around the donut target - that's OK. However the final check must be done with the star-test.

The acceptable collimation accuracy depends on your targets. E.g. for planets you want an ideally concentric airy disk at least within 1/2 of your planetary eyepiece FOV.

I'm second to the Barlowed Laser collimator advice. It removes a lot of uncertainties and deficiencies of cheap stock laser collimators. For the starter you can try the simplified Barlowed laser method: simply stick your laser into the Barlow, and observe the shadow of the donut right on the laser's screen, as you would do with the dot. That's exactly what I'm doing for almost a decade checking the collimation of my z12.

Edited November 11, 2020 by AlexK

[tripleped]

Problem solved! My DOB is the collapsible truss type. I discovered the finger screws for tightening down the truss rods were loose so I’m sure there was vibration because of that while slewing the scope. Once I fixed that and made sure the primary mirror locking screws were finger tight collimation was great and held while slewing. The result were clear, sharp views of Mars- best planetary viewing I’ve had since I got my scope 2 months ago. I’m sure I had the locking screws too loose up until now. Lesson learned!

[iapa]

On 13/10/2020 at 19:27, kev100 said:

Hi there, it definitely should be possible to get the dot in the centre of the doughnut given the three way adjustment ...

A couple of questions: have you changed the secondary adjustment screws? How old is the scope? Have you been tightening the screws really tightly?

The reason I ask is that I had a similar issue. My 10 inch is about 8 years old, I replaced the adjustment screws to make them easy to tweak, but found that they were digging into the metal base at the back of the secondary, and I couldn’t get the dot centred (it would get close, but then just ‘slip’ to the side, like there was a groove worn in the metal  ....).

 I found a set of replacement screws which had a domed end to the threaded part, and also put a couple of thin pieces of plastic between the screws and the back of the secondary, and it’s easy now to centre the dot ...

Kev

Add a metal washer between plastic and

screws will mark, and hold washer, silicon will allow secondary to be rotated without catching 

if that makes sense?

 

think it was a video from astronomyshed where I saw this

[AlexK]

That hack is called "Milk Jug Ring" for like 20+ years. Just cut a ring or a pair from the side of the milk jug with scissors.