Focus issues, help please

[nederman63]

Hope I make sense here! I have a flexi tube 130p, had no trouble focusing, but tonight I find I can only get a good view when the focus wheel is fully out, and when I say out, more or less the eyepiece holder is out of the threaded receiver, i am sure this is not what has been happening before, I hope I have explained that ok.

[Scooot]

I wonder if your secondary mirror has moved

[nederman63]

Woo hope not, don't fancy doing that thing I can't even spell.

[Scooot]

Well if an eyepiece focussed in one position, and that same eyepiece now focuses in a different position (I think this is what you're saying) something must have moved. I just thought the secondary was the most likely. It sounds like you're having to move the focuser much more than before so i thought the secondary moving would have a bigger affect than the primary. But I'm only speculating, someone else might have a better idea. I think checking your collimation would be a good idea.

[nederman63]

How do I check that.

[John]

Are you using a different eyepiece this time ?.

Different eyepieces do sometimes focus in different positions, ie: some need more inwards or outwards focuser movement to reach focus.

If is the same eyepiece as before then another one of the optical components (either the main or the secondary mirror) must have moved. One possibility is that your scope is not fully extended on it's two truss poles - this would cause exactly what you describe.

[Scooot]

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Have a look on this above. There's also been a few threads on here recently about it.

Basically there's 2 stages.

1 aligning the secondary mirror with the focuser so you when you look straight down the focuser you can see all the primary mirror clips.

2 aligning the primary mirror with the secondary mirror, it's easier to do with a cheshire eyepiece.

[John]

I'd not start fiddling around with collimation to be honest. If the eyepiece won't focus at all or is at focus in a markedly different position (as you describe) it's something other than collimation. A scope where the collimation is off will come to focus more or less in the normal place, but the image won't be as sharp or contrasty as it can be.

[cantab]

Truss poles not quite fully extended seems most likely. Followed by the eyepiece previously wasn't fully seated in the focuser (thus adding a bit of outwards travel) and now it is.

[nederman63]

Thanks for all your replies, I will check the truss poles tonight and go from there.

[ollypenrice]

I'll bet it's just the truss poles. It wouldn't take much.

Olly

[garethmob]

My new dobs designed if I have a eyepeice that won't focus I just move the secondary cradle on the poles and then it does:)

Sent from my iPhone using Tapatalk please ignore any spelling typo thingys

[ronin]

If the focal plane has moved out or in then it would most likely be the primary that has moved.

If the secondary has to move up or down then the focal plane will move in/out but also up or down so missing the focuser, or at least the optical axis of the focuser.

If the secondary has to move forward/backwards the focal plane moves in or out but the collimation will need redoing. The secondary having been moved from what was the optical axis of the primary, and as the secondary is supposidly now in the right position then the primary will have to move to get collimation. Same goes for left/right movement of the secondary.

[nederman63]

Thanks for all your replies, it was the trusses, tried it and made sure they were equal, and everything is back to normal, even the clouds over my garden.