Collimation Problem?

[dbuzz]

Help would be much appreciated! I'm out of ideas. No matter what I play with...spider vanes, secondary in-out, rotate, center and primary adjustment, the final image, as seen through the Cheshire piece looks like this. 

The green circle (Cheshire shiny disk) is always off center, slightly away from the primary. Any ideas what can fix this? Many thanks!

Edited March 27, 2021 by dbuzz

[faulksy]

that looks like your secondary offset, is it around f4 ?

[dbuzz]

Thanks, Mike!  No, it's an f6. I've tried to push it way in towards the primary, as well as tighten it out quite a bit. I've even bought 3 longer adjustment screws for the secondary to allow me to push it further towards the primary and still be able to adjust focus. No matter, when I center everything else, the green circle is still off-center...

[Stardaze]

I ended up buying a concentre which was invaluable for setting up the secondary : https://www.teleskop-express.de/shop/product_info.php/info/p5507_TS-Optics-Concenter-1-25--Collimation-Eyepiece-for-Newtonian-Telescopes.html

It doesn't look like you are far away. It doesn't sound right that you need longer screws though. If you follow Astro Baby's instructions with paper blocking out the primary and a different colour paper behind the focuser, you should be able to get the distance set right. The concentre comes into its own for perfecting the tilt of the secondary. I did get quite fed up at the time, but it came together in the end, go again with a fresh head.

[Pixies]

The outer edge of the secondary (orientated to show as a circle) is what you need to be looking to get aligned under the focuser - shown in yellow:

Once you have that right, you just need to adjust the tilt of the secondary (using the 3 adjusters) and this will cause everything reflected in the mirror to move. It's easier to make the initial secondary orientation by inserting coloured paper behind the mirror and between the secondary and primary. You get a view like this:

Then adjust its position under the focuser using the cheshire as a sight-tube. Get it perfectly concentric: 

Now you can remove the paper and get on with the collimation part. You shouldn't have to move the secondary up/down the tube now. 

 

[dbuzz]

Ok, thanks, all! I'll give it another go with the colored paper. It's what I've done, thus far. I get it right in the center, but then I adjust the three screws to get the collimation correctly and I'm back to the weird off-center look. Let me try again...

Much appreciated!

[Stardaze]

Another good tip someone said at the time, was to level up your primary best you can to give you a good starting point there too. I took a steel rule to the base I think from memory. I got very frustrated with it all at the time, but it came together in one of those eureka moments. 

Edited March 27, 2021 by Stardaze

[Ricochet]

3 hours ago, dbuzz said:

 

If your collimation looks like this then your collimation is perfect. The shadow of the secondary must be offset towards the primary mirror, which can make the reflection of the cap look off centre when it isn't. 

This is a picture I took of the collimation of my 8" f6 dob. Yours should look similar. 

[dbuzz]

THAT’s what I thought I had read! But wanted to make sure I understood it well. Unfortunately it’s been cloudy, else I would have just tried it. Thanks a lot!

[JOC]

This definitive work on collimation and the standard reference on SGL explains in the text why it looks like that.  http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/