Urgent collimation question

[Carl]

Retailer awaiting response to purchase a Skywatcher Black Diamond ed120 APO.
How do you collimate this scope if there are no adjustment srews on the front cell housing?

Urgent advise will be greatfully appreciated

Carl Rainer

[traveller (Bo)]

Refractors don't have secondary mirrors (like SCT), so there are no collimation screws. The only "collimation" they require is the alignment of the front objective lens and this is usually done in factory.
But I am not an expert on refractors, so don't know if you can collimate by yourself or have to get them done professionally.
I heard that refractor VERY rarely, if ever, need collimation if they are handled correctly.
Bo

[gregbradley]

Generally speaking collimating your APO at home is not the usual action
others may comment on this but I don't think its a simple action and not normally done.

More expensive lens cells do have collimation screws but that's more for the factory technicians not home use.

I have never collimated an APO scope. I did have to fix up an offcentre focuser rotator that was way off square.

If the scope's lenses needed collimation it would be a huge negative for that scope. It should come from the factory prefect and short of a huge fall should never need adjusting.

Greg.

[Poita (Peter)]

As stated by others, there is no need to collimate, it should be nice and square from the factory and stay that way for ever.

[alocky (Andrew lockwood)]

I have two 4" refractors that have collimatable cells. The easiest way is to put the lens cap on and use a cheshire eyepiece. Centre the reflections from the convex surfaces. NB - first ensure your focuser is square to the tube.
Obviously if you don't have a collimatable lens cell you can only find out if you have a problem this way.
Cheers
Andrew.

[Visionoz (Bill)]

Perhaps a read here might help: http://spacealberta.com/equipment/re.../collimate.htm

HTH
Cheers
Bill

[clive milne]

Originally Posted by Poita

As stated by others, there is no need to collimate, it should be nice and square from the factory and stay that way for ever.

In theory yes, but that is not always the case (even from high end manufacturers like AP) and it does depend on how the lens cell is attached to the tube.

As Andrew suggested, putting a cheshire in the focuser will show you in an instant (obviously the dust cap on the front of the objective should be on when this is done)

Generally speaking though, field aberrations in refractors are well controlled so the collimation has to be a fair bit off for it to be a problem. The caveat to this of course is in designs which have multiple lens groups.

[gregbradley]

Yes it would depend on the type of lens. If its a doublet that would be easier, a triplet harder and of course a TOA well - read exfso's sad collimation tale! Suffice it to say only Tak Japan can handle that!

Collimating a lens is not just tilt/tip its also whether the lens is rotated to the right spot it was figured to be in.

Also a nonsquare focuser could easily be confused for out of collimation lens. Then you could butcher the lens and be worse off.

If you can't have confidence the scopes's collimation is perfect then don't buy it.

Greg.

[Exfso (Peter)]

You are spot on there Greg. Horses for courses I reckon. No one in Australia can collimate these, that is for sure.