Collimateing (AGAIN)

[teztwister]

Hi i hope someone can give me an answer,i have been tyring to collimate my scope here's a picture of the result if you can see the inside ring it like passes over the top of the other lines and goes all the way to the out side,if i get all the out side rings circular the inside rings are of and if i get the inside rings circular the outside ones are of,i hope its not something serious.sorry i should have put it's an sct that i'm trying to collimate

Thanks Tez.

(click to enlarge)

[Paxo]

Tez.

I notice from your info below the post, you have a 8" LX90 and a 114 reflector, which scope is it you are trying to collimate, as it will make a big difference to any answer given.

Steve..

[teztwister]

Sorry i should have said it's an SCT that i'm tring to collimate (8")

[Trevor]

Hi Tez

Looking at the ¾ of the rings that are visible it look’s not too bad , I think you are getting rings looking like that because of tube currents it’s not cooled down sufficiently before use.

It needs to reach thermal equilibrium.

[teztwister]

It can't be thermal because its kept out side,it's like this every time i try and collimate it

This is what it looks like when just out of focus(turn the image anti clockwise 90 degrees top one)bottom one is OK.

[Paxo]

Looks like something to do with the secondary mirror or maybe the internal baffle to me. As Trevor has already said, collimation on 75% of the image looks good. Visually, does the system check out OK, can any obstructions be seen ? Otherwise stumped....

Steve.

[teztwister]

No theres no obstruction,just some dust,i will try and get some different pictures tonight if i can(if its clear)

[Talitha]

Tez, scroll about 1/2 way down this page and look at figure 'D'... it looks very similar to your first image.

The collimation procedure described by Thierry is a lot more involved than I've ever done with my SCTs, but you might find something on the page to use as a guide.

[Trevor]

Tez

Your other pictures show the problem much better, try this chart below.

The movements for the screws are as your looking at the front of the scope.

Trev.

[Trevor]

Just better add read the part in Carols link above called.... Is the collimation difficult or risky …… better safe than sorry.

[teztwister]

Thanks for those links,i tried to get some pictures tonight see what you think.

They should be in order but for the last one thats taken through a 2x barlow.

Figure D is like mine but thats really quite close to focus.

[Talitha]

Just a thought... is there a possibility that the secondary might be slightly 'pinched' due to one screw being a bit too tight, or would the entire collimation be thrown way out of whack if that were the case?

[David Galvin]

Just a thought... is there a possibility that the secondary might be slightly 'pinched' due to one screw being a bit too tight,

I think you may be correct Carol as the anomaly stays in the same position regardless there for ruling out tube currents. The rings do appear to be concentric with the dot at the centre showing reasonable collimation.

Perhaps the over tightened screw could be identified from the above collimation guide (see Trevor) and 'backed' off with the slack be taken up by the other two screws. I know that this means more collimation but it would prove or rule out a screw being tight.

[teztwister]

Hi its defiantly something different to consider ,but what is pinched optics,could you explain in like pinched optics for dummies style,i will have a look on the net for pinched optics and see what i find,i thought it might be something like that,

Tez

[Talitha]

From what I understand, if one (or more) of the collimation screws is a bit too tight, the pressure actually 'pinches' the surface of the mirror and stresses it to the point of disfiguring the reflected image.

[David Galvin]

My main reason for thinking of this is that I had a similair problem years ago with my Newtonian mirror being tight in its cell. I had three screws, 120 degs apart, around the circumference of the cell and these could be adjusted to 'touch' the side of the primary. This would 'hold' the mirror in place to stop it from rolling down the tube. I had a screw that was tight and it produced a similair out of focus image. Now if the 3x screw on your secondary are over tight then you may end up with the same problem. This is just a guess and needs to have someone in the know to confirm that the screws on your secondary actually come into contact with the glass.

If you decide to slacken off the screw, remember to loosen only one of the three screws at any one time and tighten up the slack using one of the other two screw.

DO not undo the central screw as the secondary hangs on that one. :shock:

[Nightjar]

It may help you to know I had a similar image before collimating my LX-90 8",and in fact mine was worse - it really only ever showed a horseshoe shape (with the gap at the bottom, as yours), and never a full circle of concentric rings.

I've just reviewed the notes I wrote at the time, and some of these may be relevant :-

Used an artificial star - a silver bauble with a bright torch shining on it, both about 30 metres away from the scope.

The night was cold ( minus 2 C), and so was the scope

Adjusted the corrector plate so that the manufacturer's marks lined up , and replaced the cork shims.

Turned all three screws anticlockwise by 2 turns and started again

After much fiddling about, I did eventually manage perfect collimation. This might reassure you that there's nothing fundamentally wrong with your optics, and you will be able to find those full concentric rings. Eventually.

Good luck,

Nightjar

[Kaptain Klevtsov]

Used an artificial star - a silver bauble with a bright torch shining on it, both about 30 metres away from the scope.

The night was cold ( minus 2 C), and so was the scope

Adjusted the corrector plate so that the manufacturer's marks lined up , and replaced the cork shims.

Turned all three screws anticlockwise by 2 turns and started again

Nightjar

Turned which screws anticlockwise? I though the collimation procedure needed you to loosen one screw a tiny bit and tighten the other two to compensate or it all goes floppy?

Kaptain Klevtsov

[Nightjar]

Yes, you're supposed to tighten one collimation screw and loosen the other two off by the same amount. All mine seemed tight so I loosened them all in one go, but not to a point that anything went floppy.

NJ