collimation of a f4.5 12 inch DOB -Cheshire vs Laser

[michelbechara]

Hello,

I am having some trouble collimating my 12inch f4.5 truss dobsonian and getting different results between the Cheshire and the laser method.

Everything looks very aligned through the Cheshire key (picture attached)...however if I then insert the laser, everything is not aligned, the reflection of the secondary is not hitting the centre of the circle of the Primary and the reflection of the Laser is way off....(pictures attached) 

I have been told that 'the reflection of the secondary in the primary will not be centered with the other reflections, but that ok, as the secondary itself is not exactly centered on the optical path (there is a tilt, which is due to the focal ratio ( fd 4,5). If it where fd 5 or more, it would be centered, and there would not be any tilt'

If this is the case, then I guess it is normal for the laser not to be aligned or am i missing something ? Thanks a lot for the help :)'

 

[johninderby]

Get a Concentre. The perfect tool for getting your secondary set up perfectly. I now consider it an essential for setting up a newt (dob) as I’ve found once you've got the secondary right the primary will only need minor tweeks.

After the Concentre then I use the laser.to adjust the primary. I use the Hotech laser which is propely collimated to start with. There are good lasers and cheap lasers but no good cheap lasers.

https://www.firstlightoptics.com/other-collimation-tools/ts-concenter-2-adjustment-eyepiece-for-newtonian-telescopes.html

Edited February 11, 2022 by johninderby

[Spile]

A Cheshire eyepiece and sight tube combination tool will confirm or otherwise that the secondary is centred under the focuser as per https://astro.catshill.com/collimation-guide/

[michelbechara]

@johninderby Thanks for the suggestion! I actually got a concenter and have been trying to try most positions of the secondary but the circles never seem to be concentric with the secondary....it feels like the secondary needs to be pulled away from the primary even more but it is the highest it could reach 

 

 

 

How much would this actually affect the visual performance of the telescope?  I understood that it could impact the field illumination but the primary collimation would affect the sharpness..

Any tips would be greatly appreciated 

 

[johninderby]

That would indicate a problem with the focuser not being squarely mounted.. The wooden part the focuser is mounted to may be slightly out.So perhaps a bit of shimming is called for. 

Edited February 16, 2022 by johninderby

[Guest]

Lasers need to be tweeked usually.  They are almost never quite right when you get them.  The reality is you have to collimate the laser first.  You will need a jig to securely hold the laser.  Stick a target up say 15 to 20 feet away and put a x on it then turn the laser on and hit the x.  With the laser dot on the x spin the laser and I bet the dot goes around the x.  Use the set screws on the laser to adjust this.  One screw at a time until the laser beam just sits and spins on the x.  Now your laser will work properly.

[johninderby]

Get  a decent laser like a Hotech and it will be spot on when it arrives. Cheap lasers do indeed normally need colimating themselves. You get what you pay for.