Oh No! Another Collimation Question!!

[Hoshikomi]

Have been suspicious of the collimation on my 10" Newt for a while, but having been unable to get focus on Jupiter through my webcam (with 2x Barlow), I thought I'd re-collimate.

I felt the issue probably lay with the secondary as when I tried a star test after failing on Jupiter, the ring didn't seem to be a perfect circle (it was a little flat on one side), even though the centres were concentric, so brought it inside and printed off Astro-Baby's guide and set to work this evening.

Having centred and checked the secondary, I got all 6 primary clips in the fov and swapped the film canister for a Cheshire to work on the primary. Whilst I could move the reflection around to get the spiders lined up with the Cheshire's cross-hairs, the donut on the primary remained off-centre so I broke out the Hotech (I've read that lasers aren't too good with secondaries so I tried to avoid using it) and when I turned it on, the red-dot shone proudly on the opposite wall. I re-adjusted the secondary to get the dot on the donut and moved the primary to aligment and replaced the collimation-cap to see that half the primary clips were missing. I repeated the process (got the clips showing) and then used the cross-hairs on the Cheshire centred on the donut and aligned the primary and again, half the clips were missing.

It seems that I can either have the clips visible or the donut centred and somehow, I don't think that's right. I hope to get the rig out again on Friday night and will star-test then, but hopefully one of the SGL Gurus can tell me what I'm doing wrong before then!

Thanks in anticipation.

[nicnac]

hmm, centre the donut with the laser, align the primary and nip out for a star test, then try the other way with cheshire and star test, see which holds the best results.

i start mine with the cheshire and fine tune with the hotech myself

[Moonshane]

the procedure should be this (assuming that your spider vanes are central to the tube axis and the focuser is centralised , and the secondary is centrally positioned vs the focuser drawtube):

1) using the collicap roughly centralise the secondary by doing what you did with the clips. you sound like you achieved stage one - check with an empty focuser that the secondary looks pretty central and that the shiny bit is round.

2)using the Cheshire, ignore the relationship between the vanes and the cross on the Cheshire. This has no relevance to collimating your newt and in a fast newt it's likely they won't be superimposed on top of one another. What you do need to do is get the donut central on the cross hairs. This centralises your secondary mirror. You will hardly ever need to adjust this but always check it.

3) using the Cheshire adjust the primary until the central black dot (which is the hole in the Cheshire) is centralised in the donut.

That's it other than having a quick check again that everything is aligned still.

[NGC 1502]

Hi Geoff, good advice from previous posts.

Briefly, what works for me is to sort the secondary as you have, using a collicap to get the secondary under the focuser, all mirror clips equidistant. Then, using a collicap again, get the primary as close as poss according to the donut.

Then, at night, on a defocused bright star at medium then high power fine tune the primary. If during the day I recheck with a colllicap, the donut is off a bit, then I just ignore it. The donut is handy to get it close, but for me the defocused star check is king.

I know there are lots of methods, but this works for me, and judging by the great views of Jupiter I've had lately, I'm happy with this method.

HTH, Ed.

[Mike Hawtin]

For critical collimation of a newtonian the first step is to check the focuser is correctly positioned on the centreline of the tube and at 90 degrees to the optical axis. If it is not pointing to the centre of the tube and/or not square to the lightpath you will see the effects you describe. You do not say if yours is a commercial scope or home built, with a commercial scope it should be pretty close but with a home built one it could be pointing anywhere, it all depends on the skill & knowledge of the constructor, in either case it would be the first thing I would check.

[Hoshikomi]

It's a Skywatcher.

[Hoshikomi]

OK. Went out tonight and checked the alignment with the Hotech; minor tweak to the secondary, but all the crosses lined up. Pointed at the moon to align the finderscope with the kit 25mm and focussed - it looked really sharp. Did a 2-star alignment and then tried a star-test on Betelgeuse unsing the kit 10mm.

Firstly, the white (orange!) circle seemed to be a bit eliptical, but the inner circle was slightly off-centre so with the aid of my trusty assistant, we moved the primary until the circles appeared to be aligned. Once that was complete, I kept Betelgeuse central and tried to re-align the finder to discover I ran out of travel before I could get Betelgeuse in the cross-hairs. I popped the Hotech back in for a look and the cross was still centralised on the donut, but the reflected cross had disappeared off the target area.

If I had any hair, I would be pulling it out by now! I still have a bad feeling about the secondary, but it looked OK yesterday; I have, however, never had that problem with the finder. Is it the eyepiece I used for the star test? I use the rig for imaging, so my eyepiece collection is fairly limited - I've got an 8mm Hyperion, but the rest is Barlows and the 2 Skywatcher kit eyepieces.

Help!