Dob collimation change with Altitude

[Scosmico]

I will appreciate if someone can give me a tip in how to maintain the collimation of my truss Dob during my obs sessions.

I have noticed the collimattion varies a lot depending on the altitude where the object is in the sky.

Tightening the screws on the trusses has helped. But I think someone with more experience on these can help me to get better results.

Any tips?

Many thanks

[Charic]

I hope its a one off situation?
I've often wondered how  collapsible scopes maintain collimation at all ? Sliding out the top half of a scope just seems wrong to me ( just my opinion, as I don't own one)

[Moonshane]

could you upload a photo so we can see the design?

[Moonshane]

I found once that it was my secondary spider that was changing as it was not quite tight enough. Tightening them worked well.

[Ricochet]

Aside from tightening the screws or making some sort of modification to strengthen it I would suggest collimating at 45 degrees to reduce the maximum altidude change from the correctly collimated position. 

[Singlin]

I collimate my Dob with the Cats Eye collimator as soon as I pull my Dob out of it's shed. I then check collimation just before I start using it.

If I suspect that something is wrong I just check it again, It only takes 3-4 minuts to precisly collimate once the first collimation is done

[Charic]

Ricochet there first, and yes,  I have seen mention in the past of collimating the scope in the same/similar angle that matches the viewing angle most often used, so for me,  that's all over the place?, but worth a try. There is also a worry of over tightening everything to the stage where it hurts your fingers, or damages/compresses the actual fittings, so try not to overdo the torque.

[Scosmico]

I have got a Explore Scientific Dob 12". I think it can be caused by the eyepiece, RACI etc weight.

 

[Scosmico]

2 hours ago, Singlin said:

I collimate my Dob with the Cats Eye collimator as soon as I pull my Dob out of it's shed. I then check collimation just before I start using it.

If I suspect that something is wrong I just check it again, It only takes 3-4 minuts to precisly collimate once the first collimation is done

True but I am just checking how often other people do this. Sometimes I am in a sort of flow and collimation seems to get in the way.

[Louis D]

My 15" truss primary mirror sits in a sling and tends to tip forward against its clips below a certain point (maybe 20 or so?).  Have you checked that your primary isn't shifting about?

[Scosmico]

2 hours ago, Louis D said:

My 15" truss primary mirror sits in a sling and tends to tip forward against its clips below a certain point (maybe 20 or so?).  Have you checked that your primary isn't shifting about?

I will check that. But I am sure it wasn't. 

[brantuk]

Optical gear is dead sensitive - it's not uncommon to find small collimation changes even just down to temperature fluctuations. I do a collimation check 2 or 3 times every session. Other than that make sure everything is snicked up tight enough to hold - and a tipping mirror deffo wants sorting out. Hth