Collimation-so close!!

[popeye85]

Just when I thought I had this collimation thing nailed reality decided to come and kick me up the OTA!!!

Following astrobababys guide as I always have done I started work on my 12"dob (f 4.9)

I managed to get the secondary mirror done ok (pic 1)so moved onto the primary and although got most things lined up the Cheshire crosshairs where still refusing to line up on the centre spot(pic 2) Any help would be greatly appreciated! 

P.s sorry for the poor picture quality-my phone armed obsessed with focusing on the crosshairs!!

 

[Ricochet]

Aligning the doughnut on the primary mirror with the crosshairs is part of the secondary collimation. When you move on to the primary collimation the goal is to put the shadow of the hole in your collimation cap in the centre of the doughnut. At this stage it is useful to not have cross hairs obscuring the view. I use a long cheshire/sight tube (the red FLO one) for all secondary collimation steps and a short cheshire with the cross hairs removed for primary collimation. 

[Pixies]

Yeah - Astro Baby doesn't really mention Cheshires. Don't worry about trying to align the outer-edge of the primary (clips, etc.) just adjust the secondary tilt so that the cross-hairs hit the doughnut. Sometimes this can be hard to confirm, as the crosshairs are so close to the eye.

Once you've done that, you can adjust the primary so that the black eye-hole dot is exactly inside the doughnut.

What size Cheshire are you using?

Edited October 8, 2020 by Pixies

[Captain Scarlet]

A thing that often confuses is the fact that there seem to be lots of crosshairs around! ... actual ones and reflected ones, and the reflected ones are clearer because you can actually focus on them.

In this case what you're trying to do is align the focuser axis to bounce off the secondary and "hit" the doughnut. (the "outbound ray").

Your eye is at the centre of the focus tube. The actual crosshairs are also at the centre of the focus tube, at the other end of it, but they'll likely be a bit out of focus as they're so close. If you line up those crosshairs with the doughnut, you're guaranteed that your (centred) eye, the centre of the far end of the focus tube, and the doughnut are all in a straight line (albeit bounced off the secondary). You make these adjustments by finely turning the secondary's tilt screws.

THEN, you adjust the primary tilt to get the primary's axis (the "return ray") to coincide with the outbound, by getting the reflected (smaller and not fuzzy) crosshairs to coincide with doughnut.

I hope that makes sense

[popeye85]

@Pixies that's interesting-i was always under the impression that the primary clips where the objective when doing the secondary. I will have another go later.

@Captain Magenta yes I have fallen fowl of the shadow monster in the past!! I'm sure there must be less stressful hobbies!! (P.s liking your profile picture-i spend my days running trips round the Bass Roc so I'm well aquinted with me Gannet.

[Captain Scarlet]

The most important thing in Newt collimation is getting the 2 independent axes lined up and coincident: the focuser => doughnut axis; and the focuser <= doughnut axis .

The lining up of the edge of the primary with the edge of the secondary, while worthwhile, is far less serious if you don't get it quite right. By lining up the edges you're placing the secondary's edge on the edge of the primary light-cone. If you don't line the edges up, all it means is that one part of the secondary is outside the primary's converging light-cone and hence "doing no work", and the other side of the secondary is a bit too far inside the light-cone and hence missing some available converging light. It just has the effect of slightly effectively reducing your aperture and giving you slightly uneven illumination which for visual you wouldn't notice.

Summary: Fail to line up the axes properly, you get actual aberrations. Fail to line up the edges, you just miss a bit of aperture and illumination.

BTW that Gannet was about to plunge into Lough Ine in Ireland, there was one that frequented the Lough last year, a young adult I think. Lovely creatures.

Cheers, Magnus

Edited October 8, 2020 by Captain Magenta

[Pixies]

@popeye85 - what sized Cheshire do you have? I found the shorty ones to be much harder to use as the difference in focus distance between the cross-hairs and the doughnut to be too much for my eyes. Yours are much younger though.

[popeye85]

1 hour ago, Pixies said:

@popeye85 - what sized Cheshire do you have? I found the shorty ones to be much harder to use as the difference in focus distance between the cross-hairs and the doughnut to be too much for my eyes. Yours are much younger though.

I use the standard length Cheshire (branded premium because it's red 😁😁). You can however take the top of to use it just as a collimating cap which I was using to see the clips of the primary mirror-guys I can put that back together again now 🙄🙄🙄

 

@Captain Magenta I think I understood that!! In simpletons terms It is far more important to "hit the doughnut" rather than getting the secondary into a perfect circle?

[Pixies]

2 hours ago, popeye85 said:

It is far more important to "hit the doughnut" rather than getting the secondary into a perfect circle?

No. He means that it's more important to 'hit the doughnut' than have the image of the primary perfectly contained within the secondary mirror.

You most definitely want the secondary to appear as a perfect circle (concentric within the Cheshire) - that means it is aligned under the focuser.

[Matt61]

Just done my 150P f5 skywatcher. Discovered the laser that came with it was way out. I trust my eyes and a Cheshire. To be honest I didn't find it difficult. ( will build a jig for the laser and get it aligned)