SCT collimation. It's not going well....

[Jarndyce]

Help!

I'm in the process of collimating my C6S for the first time.

I replaced the silly philips screws with Bob's Knob's at SGL4 (one at a time, by aligning on a distant flagpole) and returned it to approximately it's original collimation.

However, it wasn't quite as good as the other scopes on C6S Alley, so I thought I'd have a go at daylight collimation this weekend.

I'm using the following webpage as a guide, which has been recommended and seems common sense:

Daylight Collimation of SCT or Mak

However, if I position my eye so that the central obstruction shadow is slightly larger and concentric to the central obstruction, the brighter ring on on the outside (2nd ring on the diagrams on the webpage) is not concentric.

I can adjust the knobs to then make the mirror reflection concentric, but the outer ring remains off-centre.

In order to make the outer rings concentric, I have to move my viewpoint, which then no longer places the central obstruction shadow behind the obsruction.

I'm really stuck with this.

Focus is lousy. Compared with my C80ED, there's no comparison.

Any advice very, very welcome. Thanks.

Richard

[barkis]

Do you have anything in the focuser whilst collimating Richard?

Ron.

[Jarndyce]

Thanks Barkis.

WO diagonal and Hyperion eyepiece.

Should I remove them?

I've managed to get it a bit closer by ensuring all the outer rings are concentric and then adjusting the knobs to bring the shadow behind the obstruction. Also using a ball bearing at the end of the garden as an artificial star. Still can't get everything concentric, though.

[John]

When I used to have a C8 SCT I found there was no substitute for using a combination of star testing and tweaking the Bob's Knobs to get the collimation right. I found the daylight collimation method rather confusing and imprecise TBH.

John

[Jarndyce]

I think I agree with you, John.

The ball bearing in the sunlight at the bottom of the garden seems to be yielding better results.

Any other suggestions?

[Merlin66]

Using a webcam to get images from the slighty out of focus star circle can help to check everything is central and concentric..

[Jarndyce]

It would appear that few people seem to use the daylight method. I'll give it a try on a star tonight (clear skies allowing...)

I think I'm making a bit of a meal of this. With only 3 knobs, how hard can it be...?

Thanks for your suggestions.

[moondog]

I've just put Bob's Knobs on my C8 when I'd finished removing and replacing the corrector plate. The daylight method mentioned with Bob's Knobs is only a start. You need to collimate the scope with a star test. I still have to do the star test when I get a decent sky...... I will be using my web cam and K3CCD tools to do this.

MD

[arad85]

It would appear that few people seem to use the daylight method. I'll give it a try on a star tonight (clear skies allowing...)

I think I'm making a bit of a meal of this. With only 3 knobs, how hard can it be...?

Thanks for your suggestions.

I got my scope close using the concentric rings as shown on the Bobs Knobs instructions (and the link in your first post). I then finalised the collimation using the defocused star at high magnification at night.

It does work, but you have to understand how the adjustment of each screw alters the ring pattern to enable you achieve a good collimation by process rather than trial and error.

In your circumstances, I'd to the concentric ring trick and then wait until you have a clear night and spend half an hour on a star just "fiddling" to see how altering the knobs affects the pattern. You will also find that you only need very small adjustments.

[Jarndyce]

In your circumstances, I'd to the concentric ring trick and then wait until you have a clear night and spend half an hour on a star just "fiddling" to see how altering the knobs affects the pattern. You will also find that you only need very small adjustments.

I think that's good advice, thanks, and it's what I intend to do.

I can't get to grips with what's wrong, but my scope certainly doesn't look anything like the pics in the diagram from the site I posted. I can either have the central rings concentric OR the outer rings concentric, but not both.

I just don't get it. This should be easy!

I have to say that after having another look through my C80ED as a comparison, it is streets ahead in terms of focus.

I now begin to understand why so many yearn for good refractors. (and mine's just a cheapo one)

[barkis]

My initial reply, was to ensure that there was nothing in the way of the reflections causing the lack of concentricity. This could be established in a daylight check.

It doesn't apply in the case of the SCT, but it is a fact that in the case of a Newt collimation, the focuser is often found NOT to be 90 degrees to the optical axis. a faulty Star diagonal may cause probs. too.

A star test is always the final act in collimating any scope.

Ron.

[arad85]

I can either have the central rings concentric OR the outer rings concentric, but not both.

That sounds like you are not looking down the tube at the right angle. You need to be dead on centre, at the right sort of distance and using one eye only....

[Psychobilly]

If you have got a webcam download yourself a copy of metaguide and use that...

Peter....

[Jarndyce]

That sounds like you are not looking down the tube at the right angle. You need to be dead on centre, at the right sort of distance and using one eye only....

Thanks. I'm using a tiny hole in piece of card held in a tripod.

It's pretty straightforward to position your eye in the correct place.

Or at least I thought so.

The description on the link I posted is pretty explicit.

Unfortunately I don't have a webcam, as suggested by several posters, so thanks for that.

Perhaps I need to get one?

[Psychobilly]

Webcams make life so much easier you can even set it up in place of your eyeball and use it to provide real time feedback of the pattern if your doing the "visual" collimation method.

I bought one of the 5 pinhole LED torches form HK and apart from getting an e-mail telling me it needs to be modded to stop it melting the reflector it worked a treat.... and only slightly melted...

Peter....

[Jarndyce]

Webcams make life so much easier you can even set it up in place of your eyeball and use it to provide real time feedback of the pattern if your doing the "visual" collimation method.

I bought one of the 5 pinhole LED torches form HK and apart from getting an e-mail telling me it needs to be modded to stop it melting the reflector it worked a treat.... and only slightly melted...

Peter....

Sounds useful, Peter.

Would a budget webcam like the Celestron Neximage work or are they a false economy? I'm still on my very earliest steps with imaging. Need to get myself a better mount and look at guiding etc.

Sorry, being a bit thick. What do you use the LED torch for? An artificial star?

Richard

[Psychobilly]

any cheap webcam that you can fit an nose piece adaptor to will help - and even a cheapo "value" one from tescos will help you with the daytime collimation method as you can watch the pattern change on the pc screen as you tweak the collimation screws at the scopemuch easier than walking back and fore all the time have you got a normal video camera use that if you have ......

Th LED torch has 5 different sized apertures on it and yes you use it as an artificial star...

Discussed here...

Peter...

[Jarndyce]

Cheers Peter.

I'll look into it.

Metaguide also looks great.