Help needed....kinda cocked up collimation!!!!

[JonM]

Hi all...

I haven't posted for ages,since i got my scope in fact! So,i thought i had the collimation process sussed...oh how wrong was i! Spent the last few hours trying to correct my balls up,but i just can't seem to get it done right. I think i'd be better off with someone who knows how to do it showing me,i'm better with visual demos than i am reading up on it. So,my question is thus...i live in Folkestone,Kent. Is there anyone fairly local who'd be willing to collimate my scope and teach me in the process. Obviously i'll travel to you! I have an 8 inch Skywatcher dob,in case you need to know. Even if there is no one local,maybe someone can talk me through it...Thanks either way.

I have followed Asto Babys guide,but i'm still not 'getting' it(even though once before i thought i had 'got' it! Duh.

Jon M.

[Doc]

What part of the process are you hving difficulty with?

[rowan46]

this may help Collimating a Newtonian

[JonM]

Hi,

Doc,the bit i'm really struggling with is getting the secondary lined up properly. It was out slightly,however,i just can't seem to get it all lined up again....rowan46, i shall watch the video shortly,hopefully it'll help..i'll be sure to let you know.

JonM

[brantuk]

Squaring up the secondary can be confusing. But - looking in the focuser tube - if you can see the three primary mirror clips equally spaced around the circumference then you know it's square on to the focuser tube - and that's it apart from the tilt of the secondary.

The tilt will be sorted when you put the laser in and adjust the beam to hit the centre spot of the primary by moving the secondary tilt, using a hex key on the 3 bolts at the top. You'll have to slacken the centre bolt slightly to do this.

Finally adjust the primary tilt till the laser hits the black centre spot of the target - Done!

[JonM]

Hi all..

It's done,at least it all seems to be...clips lining up,primary centred. Rowan46,that video helped a lot,reading about it is very confusing for a new guy. The scope is now in the garden cooling down..Thanks all.

JonM

[sixela]

Squaring up the secondary can be confusing. But - looking in the focuser tube - if you can see the three primary mirror clips equally spaced around the circumference then you know it's square on to the focuser tube - and that's it apart from the tilt of the secondary.

Nope. The secondary tilt moves the primary reflection.

What you should be doing is to make the secondary itself appear as circular as possible (if it's a skewed ellipse there's a rotational error) and concentric with the far end of the focuser (or better sight tube) from a centred vantage point that makes the secondary only slightly larger than the primary reflection.

If you have that vantage point it may be difficult to se the secondary edge (as the contrast is black on black), so it's best to insert a coloured piece of pape between the secondary and the primary, or if that's too difficult, on the tube opposite the focuser (but then you see only a crescent as the secondary holder occludes part of the coloured piece of paper).

Then you set tilt correctly, which would (from the same vantage point) centre the primary reflection under the focuser and thus in the secondary itself (it's best to use a sight tube's cross hairs or a laser collimator for this, so that you don't depend on secondary placement to be correct --indeed, you can evaluate whether secondary placement is correct after setting tilt if you use these tools).

But setting tilt adjusts position a bit (especially if you don't make balanced adjustments --unscrew on one side, screw on the other to make balanced adjustments--), so you may have to go back a few times.

Fortunately, positioning the secondary properly is something you normally do only once (unless the secondary rotates or moves or you remove it or tweak the spider vane tension).

[brantuk]

Yep - when you look in the focuser tube (eg through a center hole of a tube cap) it should appear circular with the primary clips equally spaced around it. Astro Babys guide will have explained that.

[JonM]

Yeah,that was my problem,i somehow made the secondary go elliptical...and i had a bit of a mare getting it sorted and then getting it all lined up. To be fair,astro babys guide is great,but because of my haste/stupidity/lack of experience i got into a bit of a pickle,and every time i tried to correct it,i made it worse. However,i will say that having got into the mess and,thanks to you guys,back out of it,i feel much more confident for next time!

JonM