Collimation Help.

[Spacehead]

Hi All,

I just collimated my SW 200p, it was in a real state - I dont know why.

I have ended up with something looking a bit odd to me.

Here is the image, you can see that circle 1 is the main mirror outer edge, circle 2 is not in the center of the spider leg crosshair reflected in the main mirror.  Whereas circle 3 is, and (circle 4 which is the donut sticker on the main mirror) has my (green) dot in it - which is the hole in my collimation cap.

My worry is that circle 2 isnt centered on the cross hairs - should I be worried?
I thought I had done well - but am confused now.

 

[wookie1965]

http://www.schlatter.org/Dad/Astronomy/collimate.htm

This might help.

[ronin]

What is Circle 2 ?

If it is a collimation cap hole then what guarantee is there that is is central to everything? Just thinking that that is likely the least accurate bit in the chain and maybe should be considered more as a window then an accurate collimating feature. Nice if it were centered but as to being necessary I am not sure.

I can only think of there being Col cap hole, main, secondary and centre spot. If I interpret it right then main, secondary and centre appear all fairly well centered.

[bottletopburly]

circle 2 is offset slightly on a sw 200p, it won`t be a true circle that your secondary mirror

[Spacehead]

Ok - cheers all - I will do some more poking round with it tomorrow to see what info I can get to the questions, plus check out whats been said already.
I couldnt seem to get circle 2 to move in relation to the cross hairs - I think to do that, I would have to either move the position of the primary or secondary mirrors left or right within the tube (as opposed to tilting them or moving them forward or backwards - I dont think the scope is designed to even be able to do that though - so feeling more confident).

Will spend more time on it tomorrow.

Thanks again.

[Spacehead]

1 hour ago, ronin said:

What is Circle 2 ?

If it is a collimation cap hole then what guarantee is there that is is central to everything? Just thinking that that is likely the least accurate bit in the chain and maybe should be considered more as a window then an accurate collimating feature. Nice if it were centered but as to being necessary I am not sure.

I can only think of there being Col cap hole, main, secondary and centre spot. If I interpret it right then main, secondary and centre appear all fairly well centered.

Im not sure what exactly circle 2 is TBH - its hard to try and discern what is a relection of what looking down the focus tube!!  I will do some more work on it tomorrow.  Cheers.

[Dave In Vermont]

The offset to one side like that is perfectly normal in a fast Newtonian-Reflector (F4, F5, etc.). This is clearly illustrated in Astro_Baby's Collimation Guide, which is now out-of-print unfortunately. But more fortunate - I'll give you (and anyone else) a copy.

Note: There is one of these going around that is missing some pages/information. But this one I'm leaving below is the complete & unabridged edition. Keep it somewhere safe.

Starry Skies -

Dave

 

Astro Baby's Collimation Guide.pdf

 

p.s. - The images showing offset begin on Page 11. But do read the full text, don't be a kid and just look at the pictures!

 

[Charic]

Ive just re-collimated my scope, though I  find  the process easy, that's not to say it is for those just starting out!

The guide I have always related too is available here  http://www.forumskylive.it/Public/data/serastrof/201281510358_Astro Babys Guide to Collimation.pdf

During this last week whilst observing Jupiter, the image should have appeared a little sharper, as I felt the conditions were good enough, not only that,  my Airy disk was not as sharp as I normally see it, so with my Cheshire installed, there was some misalignment, and I have  not adjusted collimation  in probably over a year, but the Cheshire showed some misalignment. Not bothered whether the secondary or primary, I went to work, first  loosening the secondary tilt/lock adjusters on the secondary, then tightened  the central screw, in order to start the secondary adjustment from the flat surface of the holder. A couple of turns on the screw, and the secondary mirror was central to the inner side of the Cheshire tool. I then adjusted the lock/adjusters in order to see the three primary mirror clips when viewed through the Cheshire, then confirmed that the secondary was still central to the focuser tube and circular in appearance. Once happy with the settings, I just tweaked the locknuts to their full travel, the secondary mirror is perfect, to my eyes.

I then loosed the grub screws on the primary, and asked Mrs Charic to adjust the primary, starting with any screw, in any direction, with me looking through the Cheshire. Once everything central, aligned with my cross hair, I tweaked everything yet again, with a final look down the Cheshire. It all looks good, but the rain has just started here, its just past 2100 BST. 

lastly, I also checked through the pdf, just to confirm I had missed nothing out? Just need a gap in the clouds now. 
Hope you get your scope collimated, just follow whichever guide you fancy, there are a few to choose from, but they all do the same thing.

Ooops, almost forgot? I also re-measured the central point of the secondary mirror screw/bolt, to ensure it was equidistant to the edge of the scope.
Mine is around 11cm from centre to a point on the leading edge of the end cap +/- 1mm, 

[bottletopburly]

1 hour ago, Spacehead said:

Im not sure what exactly circle 2 is TBH - its hard to try and discern what is a relection of what looking down the focus tube!!  I will do some more work on it tomorrow.  Cheers.

if you place a piece of coloured card behind the secondary so when you look through the cheshire you will be able to see the edge of the secondary mirror and the flat of the mirror you will see why circle2 doesnt look a true circle 

[davehawkins]

2 hours ago, Dave In Vermont said:

The offset to one side like that is perfectly normal in a fast Newtonian-Reflector (F4, F5, etc.). This is clearly illustrated in Astro_Baby's Collimation Guide, which is now out-of-print unfortunately. But more fortunate - I'll give you (and anyone else) a copy.

Note: There is one of these going around that is missing some pages/information. But this one I'm leaving below is the complete & unabridged edition. Keep it somewhere safe.

Starry Skies -

Dave

 

Astro Baby's Collimation Guide.pdf

 

p.s. - The images showing offset begin on Page 11. But do read the full text, don't be a kid and just look at the pictures!

 

Dave has posted this link a couple of times, I downloaded it and used it to collimate my 200p and never looked back. He's right that the offset is perfectly normal and nothing to worry about. you will need a cheshire as well as a collimating cap though.

[Jason D]

On 5/27/2017 at 09:31, Spacehead said:

My worry is that circle 2 isnt centered on the cross hairs - should I be worried?
I thought I had done well - but am confused now

You are fine.

The cross-hairs do not need to be centered with your "circle 2". It depends on how the secondary mirror was mounted. If it was mounted with an offset then the cross-hairs will not be centered -- just like your illustration. If the secondary mirror was mounted without an offset then the cross hairs will be centered.

The general rule is to ignore the cross hairs reflections when collimating.

As far as "circle 2" being shifted, it is normal. I am including few illustrations to explain the reason for the shift:

 

Edit: "Cross hairs reflection" term used in this post should have been replaced by "spider veins reflection". It should not be confused with "cross hairs" of the cheshire/sight-tube collimation tool.

 

[Moonshane]

What Jason said.  assuming the fat bit of circle 2 points toward the primary your drawing shows a well collimated fastish newt.

[Jason D]

To emphasize, when the secondary mirror is mounted without an offset then the spider veins reflection will be centered with respect to the "shifted" secondary shadow as shown in figure 2.

When the secondary mirror is mounted with an offset then the spider veins reflection will be centered with respect to the collimation cap reflection and the center spot reflection as shown in figure 1.

 

[Moonshane]

Spot on of course Jason although for the benefit of the op most commercially available newts do have offset built in. You'd only generally get no offset if you do it on purpose or by mistake when literally sticking your secondary on a stalk during a build/ rebuild.

[Jason D]

4 hours ago, Moonshane said:

most commercially available newts do have offset built in.

Not mine

[Spacehead]

Can't thank you guys enough - i've checked over/read each and every document.  I left it as was (after I made my initial changes - the ones which lead me to worry I had done it wrong) and managed to get out last night to experiment.  Here are the results (first pic is pre-collimation).

This first image was taken with the scope as it was - mis-aligned.  The top left spike goes off all the way to the edge.  There is some motion, but I felt it didn't account for the graded shading across the whole image - which you cant see here because I cropped it out - but basically, the images were coming in brownish on the left, and black on the right - consistently.

 

This one was taken after I collimated.  Lesser movement - but pretty much bob on for my level.

But this was realistically the main give away that my collimation was out massively.  This shading business.  Getting darker to the right - it spoiled everything I was doing.

And now a star field how it should be (again - at my level)


This is exactly what I would expect - slight over concentration of light in the middle of the frame, with coma issues evenly spread around the edges - getting worse towards the edges - I will get a coma corrector at some point.
At least now I know how to get sorted with that.

Once again - thanks very much for all the advice.

More questions from me to follow