Am I collimating right?

[ElijahB]

Hi All,

I have a Celestron CPC 800 SCT and I'm just curious whether I'm collimating it right.

After reading the various material there are 2 ways to do this:

1. Indoors - position yourself several feet away and look directly in to the front of the scope. What I should see is a nice set of concentric circles. If not then play around with the screws on the front until you do.

2. Outdoors - find a nice brights star in the eye piece, de-focus until you see the rings in the unfocussed image and then move the screws on the secondary mirror again until they are all centred.

When I upgraded to the good ol' Bobs Knobs I went through the 1st option, when I took the scope outside to try the night time collimating I got confused. I centred the scope on Sirius and then defocussed ..... In order to see the concentric rings I had to keep the defocussed image really small and it was quite hard to differentiate the rings apart, let alone see if they were all centred. If I took the focussing even further then it just because a white haze. If I looked hard, when the image was small, they looked all centred, but I'm wondering if this is right or not. Certainly reading the manual, it doesn't mention that I'd have to almost use a magnifying glass to tell the rings apart.

Cheers All,

Elijah

[Kai]

What size eyepiece are you using ?? .I use a 9mm Ortho on my C8

Also i find Sirius too bright.

[ElijahB]

Hi Kai,

Can't remember what size I used .... which is better, high or low power?

Also, percentage wise ... how much of the view in the eyepiece should I expect the concentric circles to be taking up?

Cheers again,

Elijah

[MartinB]

Elijah, good seeing is needed for accurate collimation. If the seeing is poor the star will be 2 wobbly to show up good diffraction rings. Sirius isn't a great choice since it is quite low lying which will always give you poor seeing.

If you want the best planetary views or are imaging bang on collimation pays big dividends. If this is the case don't use a diagnonal. Put the EP directly into the visual back. Get a mat because you are going to be kneeling down in an uncomfortable position. Start with a fairly bright star. Any star you can see with the naked eye will be ok. Use a low power eye piece. This should show some rings even if the seeing isn't that good. Adjust collimation until central. Now up the magnification to around x200. You might find it helps to move to a slightly dimmer star. Look carefully at the rings and the gap between them on either side. Make very small adjustments to the screws to get it right. Be patient, you get slicker with practice.

Balance loosening adjustments with tightening ones. Increase the magnification as much as the seeing will allow. When the seeing permits you should aim to go all the way up to x1000. using your shortest EP and a barlow. Certainly you should be aiming for x600 as a minimum. The adjustments you need to be making at this point are tiny but they do make a difference to with high power planetary work.

[Nightjar]

I had the same questions as you when collimating for the first time. My problem was finding a dim star on a night of good seeing - I never did get to see the concentric rings. My solution was to use the 'artificial star' method. What worked ofr me was putting a small silver Christmas bauble at the bottom of the garden (about 35m distant) and placing a bright white torch about 3 metres in front of it. Bingo ! A star that doesn't depend on weather or seeing, and tends not to drift out of the FOV before you touch the collimation knobs. Perfect little concentric rings at 500X. Might be worth trying if you're struggling to rack up your magnification on a dim star.

Good luck and persevere, it's worth it !

Nightjar

[thing]

I sa the concentric rings for the first time the other night while I was fine tuning the focusing at x200. 'They'll be the concentric rings everyone talks about' I thought. My collimation was spot on as well which was nice.

[ElijahB]

Thanks for all the info everyone ... all I need now is a clear calm night or a nice large garden (mine unfortunately is ony 7 metres long). Personally I think it would be quicker to move house than wait for a clear calm night at the moment.

[Rob]

A 10mm ep or larger ep barlowed will be fine. You will get there I'm sure, don't panic...take you time. I collimate twice a night (I'm a perfectionist). It takes me around 35 seconds to colimate...yes 35 seconds i jest you not.

Once there it will all drop into place!.

All the best

Rob