Help with collimating my 6inch refractor

[John]

Hi,

I have spent the past few nights grabbing gaps in the clouds to startest my newly acquired Helios (ie: Synta) 6 inch F8 refractor. The results are promising but the scope obviously needs collimation. Fortunately this is the later Helios model with the collimatable lens cell so I can do this by adjusting the push/pull screws. What I need to work out is which pair to adjust and in which direction (ie: in or out)

I used Polaris this evening to startest and, after allowing the scope an hour to cool down, this is what I saw though a 5mm plossl (240x) (left image = out of focus, right image = in focus):

I have rotated this image so that the offset diffraction rings are pretty much as I see them through the mirror diagonal.

I tried 2 mirror diagonals (TV Everbright and TAL) and barlowed the scope up to 480x and the results were the same.

Is it possible to work out from this image, taking account the effect of the mirror diagonal (which I think means that visually North is up but East and West are reversed), which direction the lens needs to be adjusted in and how much out of collimation the lens is ?.

Sorry if the answer is obvious and I'm being thick !!.

John,

North Somerset

[FLO]

Have you seen this?

http://www.skywatchertelescope.net/Downloads/AdjustableLens.pdf

[Ambermile]

Hard to say. With an SCT you can stick your finger across the corrector and effectively "point" at a collimation screw - seeing the distortion of your finger in the image. This then tells you which one to adjust. I realise though that a 6" F/8 means you have to be some kind of primate to reach the other end...

Best way to think is to imagine that you are looking into a tunnel with concentric rings going away from you. If the tunnel is over to one side you need to "pull" it over with the collimation screws. So, if the tunnel leans to the right, you need to move the screw opposite such that that part of the objective moves towards you/the diagonal.

Arthur

[John]

Steve, Arthur,

Thanks for the advice. I have seen the Skywatcher pdf and something similar that Orion (USA) have but I found them a little ambiguous in their description of how to decide which side to adjust. Arthurs description is more helpfull - I'll try and use that technique and see how it goes. The worst that can happen is that I have to re-adjust things again.

John,

North Somerset

[John]

I have read that a Cheshire Collimator will help with refractors - has anyone any experience of using one on a synta refractor ?.

John,

North Somerset

[Kaptain Klevtsov]

I'd just adjust one screw a bit and see if it got better or worse. If its better, go a bit more, if not go the other way a bit. Repeat until its good.

Write down what you do and then make a plan based on what actually happened.

HTH

Captain Chaos

[GazOC]

I have read that a Cheshire Collimator will help with refractors - has anyone any experience of using one on a synta refractor ?.

John,

North Somerset

I use one on my TS refractor, its good for getting the scope "there or thereabouts" before you go out. Then you can either leave it or fine tune using a star. TBH I usually leave it.

I've got one for sale in the buy/sell section.....(unashamed plug )

[John]

Hi Gaz,

I've sent you a PM about your collimator.

John.