Need help collimting my Dob

[coffee_prince]

I've just set it up and trying to align the secondary mirror. The manual says I should be able to see the 3 primary mirror clips through the focuer, but I only see two.

They say to use the allen wrench or screwdriver on the secondary mirror screws, but the three screws above the secondary mirror (and the main centre screw) are very stiff any trying to unscrew them makes the spider visibly contort/bend.

Am I doing something wrong here, I don't want to break it, or do Ijust have to give it some welly?

[coffee_prince]

P.S is the base of the dob supposed to rattle when you lift it? The central screw at the base doesn't go all the way in

[Mark Porter]

You'll hopefully find this link helpful:

Astro Babys Guide to Collimation

Although having the spider visibly contort/bend doesn't sound good, I'm sure other's will be along soon who are in a better position to advise further.

Can you advise on make and model ?

[coffee_prince]

skyliner 200

[Moonshane]

I've just set it up and trying to align the secondary mirror. The manual says I should be able to see the 3 primary mirror clips through the focuer, but I only see two.

They say to use the allen wrench or screwdriver on the secondary mirror screws, but the three screws above the secondary mirror (and the main centre screw) are very stiff any trying to unscrew them makes the spider visibly contort/bend.

Am I doing something wrong here, I don't want to break it, or do Ijust have to give it some welly?

I lose track a bit now - is this a new scope?

It may just be that they need 'convincing' the first time but they should only ever be 'finger-tip touch' tight not a dying strain variety. maybe hold the spider nearest the bolt you are working on to stop flex and it will probably be OK I reckon. only undo a tiny bit to slacken off each one and then re-tighten (lightly) and you can then collimate per the guide. you should only really loosen one at a time.

you'll get it if you follow the guide.

[Moonshane]

P.S is the base of the dob supposed to rattle when you lift it? The central screw at the base doesn't go all the way in

you might need to make some large washers out of milk bottles (plastic ones!! ) to take the slack up - I think this is a common mod with this scope? others might confirm this.

[coffee_prince]

Thanks for the tip ^

Okay the screws are just EXTREMELY tight, I don't know what to do

[Mark Porter]

The base doesn't sound like it's been assembled correctly to me, you should have a bolt, two washers, a black nylon shim & a nylock nut - Can ¥ou confirm?

p.s. milk bottle shim was the best mod to date :-)

[NGC 1502]

As has been said, hold the spider to prevent it bending while you loosen the

centre bolt. Keep tube horizontal to prevent you dropping anything down the tube.

You should see all 3 mirror clips equidistant, you may need to rack the focuser in

to see them.

The centre bolt at the base of the mount must not be tight, or the azimuth motion

wiil be stiff or unmoveable, so a bit of rattle is ok when you pick up the mount &

shake is normal.

HTH, Ed.

[starstalker]

my secondary adjustment screws were very tight when i first collimated it, u just have to be brave and hold the spider as suggested

[brantuk]

NGC1502 is the first one I see where losening the center screw is suggested. This of course is essential good advice, but don't undo it totally or the secondary will fall off and onto the primary potentially damaging one or both mirrors.

It only needs to be slightly loose so you can adjust the three positioning screws. You may have to grip the secondary tightly to loosen the screws to stop the spiders bending - but don't touch the mirror surface.

Also - ensure the tube is in a horizontal position when you do this so you don't accidentally drop the allen key onto the primary.

Hope that helps

[Astro_Baby]

Loosen the centre screw a tiny bit before doing the tilt screws at the edge. That shoudl help. To loosen the centre screw grasp the back of the secondary mirror holder if needed. Be careful with that - dont wear any watches, rings etc in case you mark the mirror and ideally wear some cotton gloves (you can get them from boots for a bout £1).

Dont put pressure on the join between the secondary mirroir and its holder as its glued on.

Always keep the tube horizontal when doing this kind of stuff - that way oif you drop anything it wont fall onto the main mirror.

Mirror clips - you may need to rack the focuser in a bit to see them and use a collimation cap rather than a Cheshire - on my 200 a Cheshire gives too narrow a view to ever see all three clips which is why I do the fisrt part of the alignment with a collimation cap.

Hope thats some help

[coffee_prince]

okay thanks for the tip astro baby!

I've been trying to collimate the primary but i can't get it perfect and its very time consuming :s

Well I'm going to lug it outside now and see how it goes, will let it cool down a bit too

[watchmakerjohn]

good advice had the same problems myself

[coffee_prince]

First light, aligned finderscope, looked at the moon for a while, and the variable filter helped a lot! Looked at jupiter and the galilean moons, but it was all white bar a red streak through the middle at 120x. Unfortunately it was very cloudy so I had to pack up.

Tomorrow is due to be all wet and cloudy so I hope it clears up in the weekend :s

I have a problem with stray light, the neigbours always has their kitchen light on, let alone my own house lights... There is a street light in view, its very annoying.

Its not very dark either, but I gather its a full moon so thats why!

Verty impressed with the finderscope of all things, some 10x50's seems like a nice addition to have as well

[John]

..I have a problem with stray light, the neigbours always has their kitchen light on, let alone my own house lights... There is a street light in view, its very annoying...

I have this problem as well. What I did was to get a flexible dew shield to fit my tube size (although I could have made one from a dark foam camping mat I guess) something like this:

Dew Prevention - Astrozap Flexible Dew Shield

While dew is not an issue for me, the dew shield does keep stray light out of the top of the tube and off the secondary mirror / focuser area as it projects about 12" above the top of the tube.