Collimation. Where have I gone wrong?

[long_arms]

Hello,

I got my 200p last October, Its had a LOT of use mainly observing then imaging as well (http://www.flickr.co...s/90652641@N04/)

But I had Never collimated it.

So I thought it was about time and got a cheshire eyepiece from First Light optics (great service as always btw, couldn't believe how quickly it came)

The secondary looked pretty central to me and circular. However I don't have a collimation cap and even with the focuser racked right out and with an extension I couldn't see any mirror clips through the cheshire....not 1.

Because I couldn't see any I assumed (stupidly) that with any adjustments to the secondary I wouldn't be able to see all 3.

Anyway I moved to the primary with the cheshire and adjusted until the primarys centre spot and the cheshires reflection were good and central in the cheshires crosshairs.

It looked perfect (obviously it isnt)

Anyway last night I did a star test and when I defocused the star inward it looked perfect, concentric circle. When I defocused the other way however it looked terrible, the secondary obstruction wasn't in the middle and the primary wasn't circular whatsoever.

I am about to try again but I don't understand how I can sort this?

Thanks,

Dan

[Derick]

Hello,

I got my 200p last October, Its had a LOT of use mainly observing then imaging as well (http://www.flickr.co...s/90652641@N04/)

But I had Never collimated it.

So I thought it was about time and got a cheshire eyepiece from First Light optics (great service as always btw, couldn't believe how quickly it came)

The secondary looked pretty central to me and circular. However I don't have a collimation cap and even with the focuser racked right out and with an extension I couldn't see any mirror clips through the cheshire....not 1.

Because I couldn't see any I assumed (stupidly) that with any adjustments to the secondary I wouldn't be able to see all 3.

Anyway I moved to the primary with the cheshire and adjusted until the primarys centre spot and the cheshires reflection were good and central in the cheshires crosshairs.

It looked perfect (obviously it isnt)

Anyway last night I did a star test and when I defocused the star inward it looked perfect, concentric circle. When I defocused the other way however it looked terrible, the secondary obstruction wasn't in the middle and the primary wasn't circular whatsoever.

I am about to try again but I don't understand how I can sort this?

Thanks,

Dan

So what your saying is that the laser is centered on the primary miror (in the black cricle on the miror) and on the crosshaire the laser disapeared inside of it? Adn despite those facts when you defocuse on a star the circles seems awfull?

[Scott]

So what your saying is that the laser is centered on the primary miror (in the black cricle on the miror) and on the crosshaire the laser disapeared inside of it? Adn despite those facts when you defocuse on a star the circles seems awfull?

I don't think he's using a laser. cheshire I think.

[Moonshane]

you don't need to see mirror clips to collimate. some mirrors don't even have them (although yours will have them).

to see the secondary clearly and to centralise and round it in the focuser, you are better (at this point) blocking off the primary anyway so it would not be possible to see the mirror clips for the first stage as you cannot see the primary.

clip a piece of white paper to your vanes so it forms an L and blocks the view between primary and secondary

use whatever extension you can to see the whole secondary (white reflection if using white paper) down the Cheshire - pull the Cheshire out a bit if needed, too

by rotating the secondary / holder and/or tightening/loosening the central bolt (you may have to slacken the three adjusters) on the secondary holder, make sure the secondary provides a round and central place in the Cheshire view.

when happy with the above, remove the paper and ensure the three adjusters are 'finger tight'.

then using the fuzzy cross hairs on the Cheshire centralise where they cross with the centre of the reflected donut using the three adjusters

when done tighten the three adjusters gradually and evenly ensuring nothing moves. not too tight but tight enough to hold everything

finally using the three adjusters on the primary, get the black dot central in the donut.

the last stage is the only one that will need to be done regularly and takes seconds.

the more times you climate, the easier it gets and the more accurate you'll become. ironically, doing collimation m ore often means you are better at it and it needs to be adjusted less and less as you get better and better.

eventually you'll have a eureka moments and wonder why you were worrying. Polaris is a good star to test on as it does not move and is about the right brightness.

[long_arms]

So what your saying is that the laser is centered on the primary miror (in the black cricle on the miror) and on the crosshaire the laser disapeared inside of it? Adn despite those facts when you defocuse on a star the circles seems awfull?

I wasn't using a laser, nor do I own one to try sorry

I'm saying when I look through my cheshire sight tube eyepiece, the centre spot of the primary and the reflection are aligned and are intersected by the cheshires crosshairs. I think the secondary must have not been central in the first place.

[Moonshane]

this is a great thread and explains a lot of what I said above a lot more clearly. http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/3033065/page/0/view/collapsed/sb/5/o/all/fpart/1

it's possible of course to have a badly aligned and rotated secondary and to align the secondary and primary correctly. what this will do is reduce the light entering the eyepiece and effectively reduce your aperture.

[long_arms]

Thanks so much moonshane, I think I understand completely where I went wrong now.

When I first looked through, the cheshires reflection was one side of the crosshairs and the primarys centre spot was the other side.

By just adjusting the primary I got both aligned and central but clearly my secondary was off to start with.

My issue now is...it looks aligned, even though the secondary and primary are clearly off where they should be for good collimation. How can I now correct it?

Also with the secondary adjustment allen screws, do I have to undo the central screw even if I just wish to adjust the tilt? And do I have to slacken off all 3 or just one at a time?

Thanks again,

Dan

[Moonshane]

hi Dan

if I were you (I'm serious here), turn the primary adjusters by 1/2, 1 and 1 1/2 turns (you choose which gets turned by what) and loosen the three secondary adjusters a little and turn the secondary so you cannot see the primary. this puts almost every aspect of your collimation out. then start again and follow the process above.

I bet by the time you have finished you will be happy that everything is aligned and you'll also probably understand it a bit more.

after you have completely put the collimation out :

- step one is centralise and round the secondary (use only rotation of the secondary (to make it round in view) and tightening / loosening of the central bolt (to move the secondary up or down the scope)). what you are doing here is aligning the secondary to the eyepiece. this must come first.

- step two is to centralise the primary / secondary. you do this with the three adjusters on the secondary to put the crosshairs in the donut. what this does is ensure that the secondary (which is already aligned to the drawtube) collects and reflects the light properly and completely from the primary. this cannot be done until step one is correct.

- step three is the most critical but the easiest. this is to align the primary and secondary mirrors to the centre of the drawtube / eyepiece. this is done by centralising the dot in the donut via the three primary adjusters. again this can only be done after step one and then step two has been done otherwise it's a waste of time.

- step three has the most impact on the image and is also easiest. step one and two rarely need doing once correctly set (if you knock your scope this may be a reason). I'd recommend you check step two (usually no adjustment) and step three (usually needs a tweak) before each session (or if imaging before each run).

[LukeSkywatcher]

I really dont worry or bother about collimation because my smallest scope that would require it is a 5" Heritage Dob and my biggest scope is an 8" SCT. None of my scopes (i have a couple of refractors also), dont travel more than 60ft from where i store them. At most i will do a simple "star test" on them.

200mm newts or Dobs do require the collimation to be checked more often.................but certainly dont lose any sleep over it. If the views are pleasing to you and you are enjoying astronomy.............leave it be.

I am thinking of buying a Cheshire just to check what is really going on in my scopes though.