collimation issues

Hi,
I have linked a microsoft paint picture i have done for the final result of collimation.

www.flickr.com/photos/59002426@N04/8140105170/

I have been through all the initial setup and squaring procedures and I was pretty confident that everything was aligned very well for an initial alignment.
I use the Catseye Sighttube with crosshair, Cheshire and 2"inifinity Autocollimator for final collimation. I also follow the step by step procedures provided with these tools. I also upgraded the Spider assembly to a Protostar for easier and more accurate adjustments.

I apply the secondary offset adjustment (approx 3mm).

Basically as far I see it everything aligns nicely, when I come to the autocollimator I have all the reflections of the primary spot that is expected when there is a closed light path. At this stage I expect that all the previous steps must have been done correctly to acheive this. I proceed with the final autocollimation step. Unfortunatly the final result just isn't correct.

From the picture 2 things stand out:

1. The sides of the telescope tube walls are visible in the primary mirror reflection
2. The reflection of the focuser tube in the secondary mirror is not centered on the secondary mirror.

I intend to restart the entire procedure, that is loosen all bolts etc and relax the mirrors. Item 1 bothers me the most at the moment as I didn't see this in the initial steps and actually only noticed it as I was sketching down the attached picture. I have no idea how this crept it, perhaps while using the Cheshire.

What errors can cause this? Is it initial alignment of the secondary?

What errors would cause the focuser reflection to be offset from the center of the secondary mirror? Is the secondary mirror adjusted too low down the tube axis?

Is it possible to get to the final step with the Autocollimator with decent results when the mirrors and focuser alignment are so skewed?


Regards,
Mike

Hi Mike,
I had simillar isues and at the end used laser and barlow method witch worked a treat!
www.irishastronomy.org/index.php?option=...2&id=90762&Itemid=40
Once i alligned the secondary everything just fell in place.

Have you watched this video?

at the risk of been completely wrong,...
it looks like the primary mirror is not tilted correctly,
I have found in the past that odd collimation issues are often caused by the focuser not been square with the optical system, its the one thing people dont think about.
maybe your collimation technique is tilting it to compensate for something else been wrong, like a non-square focuser.

Could be completely wrong of course where it might be a secondary issue.
but something in there is definitely not at a right angle..

like yourself, I have my secondary offset aswell.

I had heard of the barlowed laser method, looks good and I am thinking that giving the tools I have at the moment, which should be enough to get the job done the laser collimator would be real nice to confirm some steps during the initial alignment.

I have removed the mirror so that I could rotate the center spot (triangle) as instructed in the video. While doing this I took a look at the centering of the main mirror again. It is difficult to get a measure from the edge of the mirror to the tube becuase the tube is a rolled sheet of aluminium which is not a perfect circle. What I have found is the most that the mirror is off center relative to the tube is about 1mm. Due to the type of cell there isn't really any lateral adjustment and I am guessing that 1mm is just two small to worry about?
One interesting thing from the video is the final result shows that the focuser reflection isn't centered on the secondary mirror.

The primary is definately tilted two far, I am just not sure why it is but I agree that something during the initial alignment is off. I spent a lot of time looking at the squarness of the focuser, again not easy using standard methods because of the way the tube if made. The methods I have seen all assume a perfect round tube. How is this done on Truss tube scopes, maybe there is a different method used here?

Cheers for the replies!!

Mike

squareness of the focuser is done the same way in both truss and tube scopes,
a 1mm shift in the primary mirror is nothing to worry about.
The thing about a laser collimator is that it is NOT a guaranteed way of collimating, infact, if you have something wrong with the alignment such as an off square focuser, or a off square secondary, then a laser collimator will always mis-collimate your scope.
You need both mirror cells and the focuser to be square for a laser collimator to work, otherwise it will always throw off your collimation.

This is what I did to collimate my own dob when I got it many Moons ago,

1, Take out the secondary mirror cell, (not the spider).

2, Put the laser in the focuser and turn it on , now its beam is hitting the far side of the inner tube.

3, Measure the distance from the top of the tube to where the laser beam hits the inside of the tube, opposite the focuser.

4, Measure the distance between the center of the focuser to the top of the tube.

(These 2 distances should be the same, If this is ok, then you know the focuser is not pointing towards or away from the primary mirror.)

5, Point the scope towards the horizon, Now you can use your good judgement or a spirit level to make sure the focuser is not pointing too far up or down, this is a harder one to measure. the laser point should be exactly opposite the focuser center.

6, Get a good measuring tape, make sure that the center of the spider is exactly centered in the tube, (I can adjust this as I can change the length of the spider arms, not all dobs can do this)

7, Make sure that each of the spider arms are exactly (as possible) the same distance from the top of the tube. this is to ensure that the spider itself is not tilted, this is not critical but does help make final collimation abit easier

8, Make sure that the primary mirror cell is as centered as possible.

9, Put the secondary mirror back into the scope.

If your able to get all this done, then you should be in a good position to perform a good collimation procedure by tilting both mirrors, using a cheshire or laser. I do it by eye first and then fine tweak it using a laser.



Hope this helps.