So I have it roughly collimated but cant get it to focus.

[White Rabbit]

I wouldnt say that that it's collimated well but roughly but I cant seem to get any focus on my new 12" LB. I'm trying to focus on the light out in the back garden and it's only about 15m away, I'm using a 32mm 1.25" eyepiece.

Am I
A. Not collimated enough?
B. To close to the object?

I'm suspecting a bit of both. However theres not really much to point at out there atm.

Help a brother out.
Thanks

[circumpolar (Matt)]

Originally Posted by White Rabbit

I'm trying to focus on the light out in the back garden and it's only about 15m away, I'm using a 32mm 1.25" eyepiece.

Try to focus on something further away. 15m is way to close.
Go for something at least 500m away like a light pole or some tree branches.

Also, what type of collimator are you using?

[White Rabbit]

Thanks, thats what I figured. I'm using an orion collimating eyepiece.

[astronut (John)]

WR,
The two knobs underneath the focuser are the locking knob closest to the tube, the one further out is the tension knob.
If you undo the lock screw half way out, it will allow a further 15mm of travel.

[Screwdriverone (Chris)]

Hi WR,

Yes you are trying to focus on something WAY too close.

Visual checking with the Collimating eyepiece should show you all the optics lined up and concentric so if this is the case, dont worry about it until you get a clear sky to focus on.

I will explain my fine tuning method of star testing the collimation below:

Once you can see a BRIGHT star (that isnt a DOUBLE or Binary star), fine tune the collimation by focussing on it and then if it doesnt look right (either ghosted image, blob shaped or smeared to one side) bring the focusser OUT of the tube so that the image of the star becomes like a donut with a cross in the middle and a smaller darker circle intersecting the cross. (the cross is the secondary spider vane that holds the secondary mirror - this becomes a black small circle)

With the brightness of the star illuminating the field in the eyepiece (usually about 20-25mm one) you should now be able to see whether the small black circle in the middle of the bright donut of light is actually centred in the image.

NOW: this next bit requires VERY careful and small adjustments to fine tune it. Check the direction you think the centre should go, then make small adjustment to one tilt screw on the primary. Go back to the eyepiece and see if this is the right direction. Repeat this SLOWLY until you get the hang of driving the primary tilt in the direction you want the dark inner circle to travel. Once you think you have the collimation in the spot where the inner dark circle is pretty spot on in the centre of the large light donut, refocus the eyepiece on the star and take a look.

It might take a few goes to get the hang of it, but once you get the idea, it becomes very easy to fine tune the collimation and get super sharp images in the eyepiece.

PLEASE! keep in mind that the adjustments needed are VERY small and dont lock all the primary's lock screws until you are happy with the focus.

See if you can use this method to fine tune if needed and I hope it all works out for you. I was banging my head on the ground until I mastered this and it does help A LOT.

NOTE: because you have a large DOB, you may want someone to help you keep the star centred or the star will "wander off" in the eyepiece.

PS: this also works for issues like colour or chroma where the image of the star or planet has a ghosted red or blue fringe near the edge at or near focus. Slight collimation tweaks can help this problem.

Cheers

Chris