Collimation with a Cheshire help

[DAT]

I have recently spent more time trying to collimate my scope (8” dob ) than actually looking up. My mission to get the best possible image has been for want of a less polite word , a total disaster.

i have read numerous guides, including 2 recommended on this forum ( Astro baby and Gary S.) watched various videos and read many discussions , all I know for sure is that I have made the situation worse.

I think I am really struggling to understand how to correctly position the secondary mirror under the focuser and get the tilt right . From what I have read, I know I have not got this right as the shapes and position of things when I look thought the chesire not right.

i have perhaps aligned the primary with the secondary as I can get the black dot in the doughnut but nothing else .

When I follow advised steps, often nothing happens . (Eg turn the central screw to position the secondary up or down the tube. The locking knobs on the dob are also not helping so I am only really touching the tension ones. 
I have a picture if anyone wants to have a laugh. I literally cannot see stuff moment. Opinions and comments are much appreciated as always.

[woldsman]

Sorry you are having a tough time with collimation. First take a look at the secondary mirror by peering down the main tube and then down the focus tube without any eye piece in it to check that it is roughly where it should be - ie pointed to the focus tube. If it isn’t you may have to measure the spider veins to see if they are the same distance from the centre. If they are not, the spider needs adjusting. Secondly, a Cheshire protrudes a fair way into the focus tube. You may not be able to see all three mirror clips. A collimating eye piece will enable you to see the clips. They are very cheap to purchase. When you can see all three clips, the secondary mirror is roughly  aligned. To see the secondary mirror as a disk, with one or two of the clips, you may have to move the secondary by hand or change its position by loosening or tightening the centre screw (it is on a spring so can move the secondary mirror). The three other collimation screws are for fine adjustment to get the three clips visible. Thirdly, switch to the Cheshire and try to align the cross hair with the veins for closer alignment. Fourthly, if you have a centre spot, adjust the primary collimation screws to align the smaller disk with the Cheshire. There are usually locking screws that need loosening to be able to adjust the primary collimation screws. You are right about needing to align the centre spot. Carefully tighten the locking screws and you are done.
 

Collimation is a right pain for most people at first. Over time, you get to know your scope and it gets easier. The good news is that if your Newtonian is f8 or slower and you are chiefly observing, there is a relatively large amount of tolerance on collimation. Faster scopes require a lot more precision.

Edited December 21, 2023 by woldsman

[DAT]

4 hours ago, woldsman said:

Sorry you are having a tough time with collimation. First take a look at the secondary mirror by peering down the main tube and then down the focus tube without any eye piece in it to check that it is roughly where it should be - ie pointed to the focus tube. If it isn’t you may have to measure the spider veins to see if they are the same distance from the centre. If they are not, the spider needs adjusting. Secondly, a Cheshire protrudes a fair way into the focus tube. You may not be able to see all three mirror clips. A collimating eye piece will enable you to see the clips. They are very cheap to purchase. When you can see all three clips, the secondary mirror is roughly  aligned. To see the secondary mirror as a disk, with one or two of the clips, you may have to move the secondary by hand or change its position by loosening or tightening the centre screw (it is on a spring so can move the secondary mirror). The three other collimation screws are for fine adjustment to get the three clips visible. Thirdly, switch to the Cheshire and try to align the cross hair with the veins for closer alignment. Fourthly, if you have a centre spot, adjust the primary collimation screws to align the smaller disk with the Cheshire. There are usually locking screws that need loosening to be able to adjust the primary collimation screws. You are right about needing to align the centre spot. Carefully tighten the locking screws and you are done.
 

Collimation is a right pain for most people at first. Over time, you get to know your scope and it gets easier. The good news is that if your Newtonian is f8 or slower and you are chiefly observing, there is a relatively large amount of tolerance on collimation. Faster scopes require a lot more precision.

Thanks for that quick reply, already I have something I can work with tonight...I have looked inside the tube and the secondary is more or less where it should be (apparently pointing at the focuser).  However, when I look through the focuser, the secondary appears more elliptical than a disc and just by loosening the screw I was not controlling things well. Glad you suggested to use my hands as well, makes sense.

Though the Cheshire, cross hairs are way off from the veins, the only thing I can align is the dot (small hole from the Cheshire) and the doughnut (marking on the primary). The problem I have with the locking screws is that when  I tighten them back up, whatever I have done gets undone. I have read elsewhere some  people do not use them or leave them loose

[wookie1965]

Put a piece of paper opposite the focuser (White) then put a piece of card inside the tube to block the primary mirrors reflection. Then rotate the secondary (back off the three collimation screws a tad) get it so you can see a full circle. Take the card out now using the 3 screws small adjustments get the primary mirror clips all the same.  Thats the hardest part done unless you drop or bang the tube you wont need to touch this again. Now using the three screws on the primary centre the dot in the donut. Once you have done this a few times it gets easier like riding a bike.

Take a look at this site.

https://web.archive.org/web/20200223041214/http://www.schlatter.org:80/Dad/Astronomy/collimate.htm

 

[bosun21]

Hi there. Looks like you are really struggling there. I live in Oxford and not too far from you so if you are ever passing I would be only too happy to collimate your scope and talk you through the steps. Good advice from @wookie1965.

[spike95609]

You can break collimation down into three parts. The first thing to do is make sure that the secondary mirror is in line with the focuser, and turn the big central screw until it is positioned equally within your view, i.e. the direction of travel is from the top of your scope to the bottom, just make sure the distance to the edge of the focuser is equal on of those both sides. Once this is done, the secondary is in line with your focuser.

Next the secondary mirror, and for this I would use a collimation cap rather than a cheshire as the only thing you're trying to do here is to get that mirror tilted so that your view is looking directly down the tube towards the primary mirror. With the collimation cap you will be able to see all of the mirror clips around the edge of the primary mirror but you won't see them with the cheshire. This is important because if you move the secondary mirror about then once you can see all of those mirror clips (three I think) equally around the edge of your view then your secondary is aligned correctly. It's very fiddly to do this with the three screws as they won't change the tilt in an easily understood X-Y 90 degree direction, but at a weird 120 degrees. Make small adjustments until everything starts to come into place, and once you can see all of those mirror clips easily you know that your secondary is aligned. The most important bit though is when you tighten the screws up, if you do them one at a time it will knock the mirror out of alignment again. Instead it's best to think of them as a three-way balancing act, so tighten them up by about an eighth of a turn each in rotation. That will hold the overall view and gradually tighten them up. Keeping checking the view through the collimation cap in case it drifts out again.

Then it's the easy bit and you can use your cheshire at last. All you need do now is adjust the primary mirror until the dot is more or less in the centre of the circle. Everything is now in line.

[DAT]

Thank you for all your advice, I will be spending all of tonight doing this until I either pass out or get it right

 

[PeterStudz]

Don’t worry. It can be confusing at first but you’ll get there. As someone said - it’s a bit like learning to ride a bike - sooner rather than later you’ll get the hang of it and wonder why it was once such a faff. 

I started out collimating less than 3 yrs ago. I had a gifted 8” Dob that needed some work and as a complete beginning I had to completely strip it down to just a tube, fix and modify some things, then put it all back together. Initially very scary but I got there in the end.

If you still have issues then @bosun21 offer of help sounds like something not to be missed! I think that if someone who knows what they are doing shows you the procedure firsthand it’s a lot more obvious than any online guide, text or video. 

[DAT]

Absolutely, I get most of it but there are a few little things that are not clear to me-lets see what happens. I might well take up the offer Bosun!