Collimation desperation

[mhc]

Sorry folks, but it's another collimation post

I have completely fouled up the collimation on my C8-N. I've watched the video and read the numerous enlightening threads, but still have problems.

To simplify things (remove extra reflections), I've removed the primary cell and pointed the rear of the scope at a blank wall.

Try as I might, I cannot get the secondary looking right.

When looking down the focuser the secondary does not look circular. I know in reality it is ovoid, but surely when the tilt is correct it will look circular? At the moment, I get a slightly clipped oval shape.

Please help. Pretty, pretty, pretty please!

Mark

[Kaptain Klevtsov]

Mark, if its twisted on the OTA axis it will look oval. If you twist it enough you will see it edge on! If it doesn't look circular its VERY likely that its turned a bit. Check that first and then report back.

When its turned correctly you should have the collimation screws adjusted more or less equally to get it looking out of the primary mirror end. Try leveling the collimation screws and, with the collimation screws looseish, twist the whole thing until the colllimation is close. Then recollimate using the screws.

Captain Chaos

[Whippy]

To simplify things (remove extra reflections), I've removed the primary cell and pointed the rear of the scope at a blank wall.

:shock: :shock: :shock: :shock: :shock: :shock: :shock:

Mark, that's a bit drastic!!!!

[mhc]

I guess, but I'm still very new at this collimation lark.

When someone showed me how to collimate my old scope we simply unscrewed the housing so that there were less reflections for my feeble brain to worry about. It worked

I *think* it's sorted now (thanks CC!). The black dot from the Cheshire is inside the primary's centre circle, and the secondary is looking round(ish). Hoping to test it tonight.

What I'm not certain about is how/whether to tighten the grub screws in the primary? Doing so can shift the collimation.

[Kaptain Klevtsov]

What I'm not certain about is how/whether to tighten the grub screws in the primary? Doing so can shift the collimation.

Thats the fiddly bit. You need to have them nipped up or it will flop out of collimation as soon as you tilt the 'scope. You need to tighten the grub screw, check the movement required, loosen the grub, move the other screw a hair then retighten until it all just lines up.

Captain Chaos

[Black Shuck]

When I first tried to collimate I struggled with the depth perception (i.e. what reflection was I looking at, where it was reflecting from, etc). I found it helped putting my hand down the tube so that it was behind the secondary when I looked into the focuser tube.

It just gave me a sense of depth and I found it easier to work out what I was looking at.

HTH

Bill£

[proflight2000]

same here bill, i used my hand at first to identify what exactly I was looking at. MHC put your main mirror back in for a start, once its all back as it was come in and say then we can go from there.

[mhc]

Hi Guys,

I'm still struggling. I've put the main mirror back. The "hand behind the secondary" was a great tip. To make life easier I've wedged a piece of plain white paper there instead.

My current setup is that the secondary looks slightly elongated, stretched outwards in the direction of the bottom right (south east if north is up) edge. It's weird, as the other 3 quadrants look ~ equal!

Any suggstions?

Please tell me I'm not being unduly thick wrt collimation. Feel free to lie

Mark