Yet more collimation queries

[Jimtheslim]

I have recently made a collimation cap for my newtonian (I already own a Cheshire but find it difficult to line up the donut sticker with all the cross hairs going on). I think I have it well collimated, however when going through this infuriating process I find that one of my collimation screws when I loosen the tightening bolt it will give some resistance going one way, but going the other it seems totally loose, like it is at the very edge of the screw limit (if that makes sense?). The other two screws give resistance when turning and the donut sticker moves around accordingly. Should I consider loosening everything off and starting again?

[rockystar]

Sounds like a similar issue to me the last time I was out. What I did was loosen the locking screws all the way (pretty much just short of taking them out), tightened all of the collimation screws as much as possible (without over tightening them), loosened all the collimation screws off by 1 full turn, then redid the collimation, once it was aligned, re-tighted the locking screws.

[Louis D]

I'm assuming you're talking about your primary's collimation screws.  If so, have a look at the one giving you trouble in the daytime to see what's going on.  My guess would be that you are at the limit of expansion for the compression spring on that screw (it's more of a bolt than a screw, actually).  I'd tighten down all of them equally a full turn or two and then recollimate to see if that solves the problem.

[Jimtheslim]

1 hour ago, Louis D said:

I'm assuming you're talking about your primary's collimation screws.  If so, have a look at the one giving you trouble in the daytime to see what's going on.  My guess would be that you are at the limit of expansion for the compression spring on that screw (it's more of a bolt than a screw, actually).  I'd tighten down all of them equally a full turn or two and then recollimate to see if that solves the problem.

yes it's the primary, will give this a go when I get a chance. Despite the fact it seems well collimated and gives great views I'm the kind of person that just can'tlet this kind of thing lie!

[Louis D]

2 hours ago, Jimtheslim said:

Despite the fact it seems well collimated and gives great views I'm the kind of person that just can'tlet this kind of thing lie!

Collimation is less likely to shift if there is ample tension on the collimation screws, so it's probably worthwhile in the long run.  Lightly tensioned screws or nuts on bolts tend to loosen themselves due to vibrations over time.  That's why you snug things down when tightening screws and nuts.  Obviously, you can't tighten up the primary screws (unless it's like those on a Mak's primary with three more "locking" screws), but the spring should have a good amount of compression in it.

Let us know your results.

[John]

On several newtonians that I've owned, including my current Orion Optics, I've found the locking screws cause more trouble than they solve so I've backed them off and left them unused unless the scope is being transported somewhere (ie: in a car).

Collimation seems to be held better when the collimation springs are under some tension rather than when they are not.

Some scopes seem to have rather weak collimation springs as standard so replacing those with heavier duty ones can bring positivity back to a "tired" collimation system.

 

[spaceboy]

I agree with John regards the so called locking screws. I tend to collimate my scope then just turn the locking screws with a light grip and once they come into contact with the cell let go. May seem pointless and I should just remove them altogether but because my one scope is permanently set up in an obsy it serves as a kind of reference for me as if the lock screw suddenly becomes stiff to turn or overly loose I know something has moved and it's time to check collimation again.

[Jimtheslim]

2 hours ago, John said:

On several newtonians that I've owned, including my current Orion Optics, I've found the locking screws cause more trouble than they solve so I've backed them off and left them unused unless the scope is being transported somewhere (ie: in a car).

Collimation seems to be held better when the collimation springs are under some tension rather than when they are not.

Some scopes seem to have rather weak collimation springs as standard so replacing those with heavier duty ones can bring positivity back to a "tired" collimation system.

 

Yes it makes sense for them to be under tension I suppose, when they're not I guess they are pretty redundant. The other thing that annoys me is the fact that the adjusting screws line everything up perfectly, then doing up the locking screws ever so gently knocks everything out again, so you have to guess where it will be after the locking screws are finished and compensate with the adjusting screws. 

Off on holidays but if I ever get 20 minutes or so to myself when I get back I will try loosening everything off and trying again. I just wanted to make sure that if I did that the primary wouldn't fall out or something daft.

[John]

1 hour ago, Jimtheslim said:

Yes it makes sense for them to be under tension I suppose, when they're not I guess they are pretty redundant. The other thing that annoys me is the fact that the adjusting screws line everything up perfectly, then doing up the locking screws ever so gently knocks everything out again, so you have to guess where it will be after the locking screws are finished and compensate with the adjusting screws....

 

Thats why I don't use the locking screws most of the time. I've heard that they can even be used to make very fine adjustments to collimation which shows that they do alter the position of the mirror cell a little. As you say, annoying if you have got everything in shape using the main adjustment screws