Laser collimating a Skywatcher 130

[DaveGarland]

Ok, I've bought a laser collimator for my scope only to notice I don't have a centre dot on my primary mirror. The laser is quite noticeably off centre. On a 130 mirror is it enough to just judge the centre?

Advice please!

[Stephen]

Were you supplied with centre spots and instructions with the collimator?

Have you checked the collimation of the laser (assuming it's not a top end one.)

"Judging" defeats the purpose really

[DaveGarland]

No I wasn't supplied with dots. I guess I could easily improvise my own though. But obviously I would have to remove the mirror. Is this easily done? I can see some dust down there so this would be a good opportunity to clean the mirror too.

[Stephen]

No I wasn't supplied with dots. I guess I could easily improvise my own though. But obviously I would have to remove the mirror. Is this easily done? I can see some dust down there so this would be a good opportunity to clean the mirror too.

I didn't have a problem removing my mirror, but careful is the word

If you look at the bottom of the tube, there's screws all round it (6 or so?) Removing these allows the whole mirror cell assembly to come out the bottom (just don't drop it and do this with the OTA horizontal!)

Once out you can centre spot the mirror (donut shaped sticker and tracing paper to accurately centre it.) If you do decide to clean the mirror (bit of dust never hurt no one ,) follow a guide as you can wreck the coatings if done wrong.

Just remember to check the collimators collimation! I made up a jig like the one here and it works really well

[DaveGarland]

Thanks for that. A great help

Cheers!

[fatwoul]

Dave, unless your scope is very old and mucky, a few specks of dust on your mirrors will do nothing to affect your views/images. I'd leave cleaning your primary well alone, because it is easy to wreck them. If it really bothers you, just see if a rubber blower (the photography ones - not the ones with a brush on them! And not canned air!) can shift the specks. If they won't budge, just leave them be. You'll never know they are there.

[DaveGarland]

That guy who made his own collimator, collimator. I hope he checked to see if those nails were properly collimated

[Stephen]

That guy who made his own collimator, collimator. I hope he checked to see if those nails were properly collimated

That's the beauty, they don't have to be!

The collimator is circular, so no matter what angle the V of the nails makes, the collimator rotates perfectly

[DaveGarland]

I read you can use a Sharpie pen to make the dot. Is that ok?

[Stephen]

I read you can use a Sharpie pen to make the dot. Is that ok?

Each to their own, I wouldn't dream of it though

[John]

I read you can use a Sharpie pen to make the dot. Is that ok?

A self-adhesive hole re-inforcer ("donut") is often used. That leaves a mirrored spot in the centre of the primary so that you can clearly see when the secondary is properly aligned (laser dot in the centre of the "donut") and the return beam of the laser is not hampered in it's return to the angled face of the collimator for primary adjustment.

[DaveGarland]

Thanks for your help chaps. It took me a while, and quite a bit of stress, but I got there. To be honest though there's so much slack in the Skywatcher focuser that I can move the laser around the centre dot by quite a large margin by just wobbling it! Is this normal? I've found that I can get relatively acceptable repeatability by loading the collominator and then nudging the focuser in the direction of the primary mirror. Seems far from ideal to me. However I managed to get a good look at the moon tonight and was very happy with my views.

Once again, thanks

[John]

Focusers can be adjusted to remove some of the "wobbles" by adjusting the tension on the 2 small grub screws on the top of the focuser and the 4 underneath, holding the focus wheels / axle / pinion gear in place. I'm assuming it's one of the rack & pinion focusers here.

The best way to find the state of collimation is to star test the scope on Polaris - put the star just out of focus so you get a disk of light and there should be a dark circular spot somewhere in it. The dark spot is the shadow of your secondary mirror and should appear in the centre of the disk of light - so it looks a bit like a Polo Mint. If the "hole" in in the middle of the Polo Mint then your collimation is OK.

[Sam]

Dave, laser collimation is not always the best option for getting your scope well collimated. I still find I get the best results from a collimation cap then using the laser to make sure the primary is reasonably well lined up but the final test is done on a star by using a high powered eyepiece on star and defocusing slightly to observe the star shape then adjusting the collimation screws on the primary to get a nice round and symmetrical image. Lasers are good but often they are don't end up being very accurate because of sloppy focusers etc.

[TheThing]

Lasers are perfect for collimation! I don't understand why people are so anti them! As long as the laser is well centered then it's ok. You'll have the same problem with play in the Focuser with a Cheshire as well.

Lasers are perfect and make the job so much quicker.

Also, far too much fuss is made of collimation. It is important, more so if you're imaging, but the end of the world. If you don't have a donut then judging by eye is fine. Have a look at The Andy's Shot Glass tutorial. He does most of his collimating just by eye - no Cheshires, no donuts - and it works.

DO NOT follow Astro_Baby's guide - ridiculously over complicated! This is the guide to follow only if you prefer looking AT your scope and twiddling with it, rather than actually using it to look through!

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- TheThing.

Sent from my iPhone using Tapatalk

[Stephen]

I started out with the colli cap / cheshire and managed OK with that.

Then I bought a lazer and after ensuring it was perfectly collimated found that it agreed with the cheshire 99% of the time

Granted I don't have a super dooper focuser, but it's a step or two up from the stock ones - not a great deal of play in it.