Dobs newbie collimation (yeah,i know,sorry)

[JohnR]

I at least understand the principles of the job now,or more accurately,i know where i'm going wrong.

The problem i have with my 10in skywatcher dob is it difficult to line up the secondary properly.You can't get a view of the edge of the primary to centre it (without moving your eye offline and guestimating that each of the 6 clips are roughly the same distance out of shot)

I'm thinking i prob need a laser just to line up the secondary,unless anyone has any suggestions? Ta

I presume this means i'm not getting all the light potential of the mirror either if theres,for arguements sake, a 10mm strip around the edge of the primary that cant be seen.I'll rename it my 230px

[sixela]

I at least understand the principles of the job now,or more accurately,i know where i'm going wrong.

The problem i have with my 10in skywatcher dob is it difficult to line up the secondary properly.

You can't get a view of the edge of the primary to centre it (without moving your eye offline and guestimating that each of the 6 clips are roughly the same distance out of shot)

If you can't see the clips even with a collimation cap and the focuser fully racked in, the secondary is undersized. It must be possible to see the entire primary from the focal point of the mirror! My guess is that you simply need to rack the focuser in a bit more, or that you're using a sight tube that is too long.

If you're using a sight tube and you can't see the whole secondary when you rack in the focuser up to the location where the primary reflection appears just smaller than the secondary, the sight tube doesn't have exactly the correct aspect ratio.

Then it becomes a bit of an iterative process: use the sight tube with the tool set so that you can still just see the edge of the secondary, centre the secondary under the focuser, rotate it so it appears as round as possible, use the sight tube cross-hairs to tilt the secondary correctly (so that the centre spot is under the cross-hairs), and go back a few rounds. That's not the ideal offset, but it's usually very close (usually even enough to skip the following steps, which are for collimation nits only).

If you're a nit about secondary placement, do the rest of the collimation routine, and finally evaluate with a collimation cap (which is simply a sight tube but with the draw tube of the focuser acting as its "tube") whether, when the primary reflection appears just smaller than the secondary (rack in the focuser until that happens), the primary reflection appears centred under the secondary. Tweak position of secondary if not, set tilt again using the sight tube, go back to the collimation cap to judge secondary placement once more, etc.

As you can see, it's not that complicated but it can take long. The good news is that unless you somehow disassemble the scope secondary placement will tend to be good for years, and after that you'll just adjust the tilt of both mirrors, which takes less than a minute once you're used to it.

If you don't have a sight tube, well, then you probably need one (at least once).

I'm thinking i prob need a laser just to line up the secondary,unless anyone has any suggestions? Ta

Laser collimators are good at helping you aim the reflected focuser axis (using the tilt of the secondary), but they won't help you place and rotate the secondary under the focuser. Unless you have a holographic laser collimator, but then you'd know it .

[sixela]

I presume this means i'm not getting all the light potential of the mirror either if theres,for arguements sake, a 10mm strip around the edge of the primary that cant be seen.I'll rename it my 230px

Only if you were looking from the focal plane. If you have the focuser racked out completely, that's normal on many scopes.

By the way, if you have a focal plane that is too far out the tube this can happen on some scopes. That's usually a sign the collimation bolts for the primary are very loose, and tightening all three collimation bolts at the back (after having loosened the locking bolts, obviously) will move the primary backward and the focal plane closer to the secondary.

[JohnR]

Thanks for the replies Sixela.You pointed me in the right direction -groan

When i made my collimation cap i wasn't sure how big to make the hole,so i copied the hole on the cheshire - 2mm.I've now drilled the cap out to 3mm and i can see all the clips.So simple in the end ,yet so frustrating