Secondary mirror needs to be shifted ??

[jarbi]

Hi Everyone,

I would need good advise to fine tune the collimation of my Skywatcher 10" DOB.

When collimate with laser collimator, I see something strange when looking into the focuser. It looks like the upper 2-3 cm of the primary is not visible through the secondary mirror, and I can see the primary mirror clips and the edge of the primary on the bottom. As far as I could measure it the spider is centered correctly and the bottom of the focuser is also symmetrical to the horizontal part of the spider. I made a small drawing to show also what I mean: http://iseestars.net/?pgid=user-image-view&imageid=7113.

Do I need to shift my secondary mirror upwards or is it something else ? Please advise...

[jarbi]

Steve,

Thanks for your advise. I'm afraid the problem is deeper, because I have already rotated the secondary before to make all primary clips visible. After that, I have placed the laser collimator into the focuser and did the standard procedure: first setting the secondary mirror with the 3 Allen screws to get the laser point in the middle mark of the primary, than setting the primary to get the reflection into the hole of the transparent raster of the collimator. After doing all this, I get the image through the focuser as posted. I have also suspected the laser collimator to be out of the focuser axis, but it is not like that - the laser spot is not moving when I rotate the collimator in the focuser.

I think the axis of the focuser points now under the center of the secondary, that is why I have this view. That is why I thought about shifting the secondary or eventually tilt the focuser ( it has also Allen screws to do this ). I really would like to keep it simple but it doesn't seem to be... What do you think ?

By the way, I don't see your reply here, I could just read it arriving to my mailbox. Did you see already something like this?

[FLO]

I deleted my reply soon after posting because I felt this link might be more useful:

http://www.andysshotglass.com/Collimating.html

Hope it helps.

[jarbi]

I deleted my reply soon after posting because I felt this link might be more useful:

http://www.andysshotglass.com/Collimating.html

Hope it helps.

Thank you Steve, I have seen the video, it is really very useful. This evening I will continue collimating, because going outside doesn't promise much today anyway...

[daz]

Jarbi

Make sure your laser is also collimated!

[jarbi]

Jarbi

Make sure your laser is also collimated!

Thanks Daz,

I am about to do that this evening, because the laser collimator shows difference comparing to the "classical" method. I was too optimistic at the beginning believing that the laser is trustworthy "out of box".

Yesterday I have done a reasonably good "classical" collimation I think, using a self-fabricated collimating eyepiece. As I see the laser collimator is a bit out of axe. I will try to reach the situation when the red point is not moving on the primary when rotating the collimator in the focuser. It will take quiet some time i guess...

[daz]

You could make a simple collimating jig for the laser. Something like this is fairly easy

http://tinyurl.com/2pyx4g

[jarbi]

You could make a simple collimating jig for the laser. Something like this is fairly easy

http://tinyurl.com/2pyx4g

What a nice and simple solution ! I will certainly go this way !! Daz, I am learning a lot from you !

[jarbi]

Thank you for all advices, I have now reached the point where I am satisfied with the collimation. This evening I could see fairly good star testing results 8).

But let me share my "discoveries", maybe it helps others in similar situations.

1. I have perfectly aligned my laser collimator, assuring that the laser beam is centered and parallel with the collimator tube ( thanks again for pointing the jig idea Daz ! ).

2. I took my spider apart to see if there is a reason of the misalignment of the secondary. Looking at it closely it turned out that the spider vanes were fastened on a "twisted" position on the central part of the secondary holder. As a consequence the axis of the secondary holder was pointing somewhere deep under the center of the primary . After correcting this and remounting the spider, I could nicely center the secondary as it should be.

3. I have repeated the full collimation procedure.

I am looking forward to have a longer observation session to see Saturn or Jupiter with the new collimation .

I am also planning to get a Barlow for better laser collimation. I have read collimation can be much more precise when I use the laser collimator inside a Barlow. Do you have experience in this area ? What kind of Barlow would be good for this purpose ? Of course I would like to use it for observation as well.

[FLO]

Good to hear you have found and solved the problem

Have no experience of collimating using barlowed lasers but would like to know more?

[jarbi]

Good to hear you have found and solved the problem

Have no experience of collimating using barlowed lasers but would like to know more?

Thanks Steve, I am interested to learn about it for at least two reasons:

1. I would like to get a Barlow anyway, for planet observation

2. When I buy a suitable Barlow for the laser collimator, I will not need a Ceshire probably. I like the easyness of the laser, but I understand also that it may give limited precision when used alone.

So please let me know if you have more info/articles about it !