Collimating Skywatcher 130P

[Unicronicus]

We decided to try our hand at collimating the other day using the astro baby guide on the inter web but we are having trouble, or so we think...

The guide said you can make your own collimating tool out of an old camera film pot, which we did, so we set about the collimating process as per the guide. Everything seemed to be going ok until it got to the alignment of the primary mirror to the secondary mirror. It said to use a chesire or laser collimating tool, which we didn't have at the time, but you could also shine a light into the peep hole of the film pot and look through it (how exactly if a torch is in the way?) to see if the light beam is in the centre of the donut on the primary mirror. This last step seemed impossible because you cannot shine a light into a tiny 2mm peep hole AND look through it at the same time can you! At this point we ordered a Bresser laser collimater.

When the laser collimater turned up, we collimater that to a fine degree of accuracy at circa 20 feet and proceeded with the collimation of our scope again but using the video from youtube (the astro and nature place with the English goatee guy and his mate). Now, we can get the laser spot on with the centre of the primary mirror and back to the exit hole of the laser itself so in essence it is perfectly aligned. But when we remove the collimater and peer down the focused using our homemade peephole tool, the secondary mirror doesn't look central to the focuser. It also seems to cut out part of the primary mirror's reflection. But if you look down the focuser tube with nothing in it, you can see your own eye in the centre of the secondary mirror, which is more or less centre with the primary mirror and you can see all three clips. It just doesn't seem as central as opposed to before we started playing around with the collimation of the scope...

Are we doing everything right? We can go in and out of focus to a star and it does display concentric rings around it but it feels like the eyepiece isn't aligned properly with the secondary mirror so eye relief also feels affected. But if a collimater laser is aligned with the primary and secondary mirror surely everything is aligned right? The video makes it seem to easy but to us it seems far from it!

Any help is much appreciated as always!

[m37]

How big is the hole in your diy collimation cap?

[Owmuchonomy]

I believe you have to align the secondary before attempting anything else? I have started following these threads only because I am going to buy a Dob so apologies if I'm incorrect!

[Cornelius Varley]

If you shine a torch through the hole in the collimation cap you can look down the open end of the telescope tube to see the reflection in the primary.

[jkwhinfrey]

I've always used a Cheshire eye piece rather than a laser collimator, but the order should be the same... I think.

Use a ruler to make sure the secondary holder is in the middle of the telescope tube. Adjust the spider vanes if it's not.

Then the secondary needs to be aligned with the focuser. Use you're home made sight tube to make sure the secondary mirror is nice and round and central in the view through the eye hole. It could be an ellipse if it needs rotating - loosen the central screw on the secondary holder very slightly to do this.

[jkwhinfrey]

I was just about to add... don't touch the secondary if it looks Ok, getting the secondary positioned is the hardest part... to the beginning of my response when I accidentally hit Reply :-) The collimating on the small 130P tends to be very robust.

When centralising the secondary, the best hint I ever had was to block he reflection of the primary with a piece of bent cardboard and a bulldog clip to attach it to one of the spider vanes. Much less confusing.

The primary mirror needs to be centralised in the secondary by small adjustments to the three screws around the edge of the secondary holder. These tilt the mirror until you can see all of the primary through your sight tube. I aim to be able to see all three of the primary mirror clips just peeking into view.

This is when you want to centralise the primary dot, and I assume that's a matter of adjusting the collimation screws on the base of the telescope until the laser collimator says everything's inline.

I don't know if that helps.

James

[Unicronicus]

If you shine a torch through the hole in the collimation cap you can look down the open end of the telescope tube to see the reflection in the primary.

The collimation cap has a 2mm hole slap bang in the centre. When I shine the torch into it, the light on the primary mirror, when I look down the end of the tube, seems to move depending which angle I am looking at it. A laser on the other hand projects a dot onto the mirror and can be viewed at whatever angle you look down the tube. If you get what I mean?

We measured the vanes and made sure the secondary mirror was central in the tube. We even started off with the secondary mirror perfectly circular and aligned with the focuser (quick side question, should the focus tube be in or out?) before using our laser collimator. Maybe we should get the primary mirror into full view (clips on view) before using the laser collimator? But then even if the secondary mirror is perfectly aligned by eye, what if the laser is pointing way off the central donut on the primary mirror? Do we re-adjust the secondary mirror again to centralise the laser dot or just adjust the primary? It all looked so simple when watching the videos...

[m37]

I am new to collimation too and I have had a good result using a combination of various sites and videos.

The method I used was with a Cheshire but may be of use to you.

Measure vanes and centralise secondary using screw as a centre marker

Block the reflection of the primary by securely wedging/taping some card in the tube a bit further down then the secondary then put a piece of white paper behind T the secondary opposite the focuser

Now use a collimation cap to look at the secondary. It should be perfectly round and centred.

Remove all paper/card carefully and look through the collimation cap. I wound the focuser all the way in then backed off until the focus tube was not visible

Make sure all clips are visible equally

Using a Cheshire inserted fully into the focuser, adjust the primary bolts to align the dot and donut

Tested this with a star at high mag and it looks bob on

As far as I can see you could do the same for the collimation cap bits then use your laser collimator for the primary. This video looked pretty straightforward

Good luck!

[wookie1965]

I use this in conjunction with astrobaby makes things a lot easier i think.

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

[Uranium235]

Also, take your time - its not a race. When adjusting the secondary, to make sure no rotation occurs its best to just grab hold of it when making bold adjustments (a bit fiddly). Try not to overtighen the secondary screws too, as excessive force can twist your spider vanes.

[m37]

Also, take your time - its not a race. When adjusting the secondary, to make sure no rotation occurs its best to just grab hold of it when making bold adjustments (a bit fiddly). Try not to overtighen the secondary screws too, as excessive force can twist your spider vanes.

We should totally have an online collimation race

[AndyWB]

Use a ruler to make sure the secondary holder is in the middle of the telescope tube. Adjust the spider vanes if it's not.

This assumes that the tube is circular, mind. When Moonshane had a go at collimating my 250px, we discovered it's kinda oval.

[jkwhinfrey]

Good point AndyWB :-)

[Unicronicus]

We believe the scope is collimated now, properly... The secondary mirror is now perfectly inline and circular with the focuser and you can see all of the clips on the primary mirror. The collimating laser is even spot on with very minor adjustments needed on the primary. I think the videos that I found on collimating with a laser need a bit more detail in the process. Cheers for the help guys!