Collimation check on 6" Newt with star test and other findings

[Bender]

It has been a while since my last post, as i don’t seem to have the same spare time I used to have.

I decided a while ago on a simple setup, a 10” OO dob with binoviewer for longer sessions and a 6” OO tube on a small photo tripod as grab and go scope. Both with their top spec mirrors.

To help with collimation I got Catseye primary sticker for both, and in both cases found the existing primary spot (which I wasn’t able to remove – but I didn’t try to hard) to be a few millimetres of from the Catseye spots.

It has bugged me since then if I placed the Catseye correctly. Yesterday I all the sudden had the epiphany that a simple star test should show if, when collimated with the Catseye spot, the scope is proper collimated or not.

Infocus showed nice concentric circles when pointing the 6” at Polaris with a 6mm ortho, so I can assume that my Catseye sticker placement was correct after all?

 

Seeing wasn’t perfect with a slightly unstable image and flaring showing but the circles showed clear enough.

Now one thing I noticed was the stark difference between infocus and outfocus. I attached a rather bad drawing. In focus I saw a middle spot and 1-3 ? rings around (depending on focuser travel) all with the same brightness, and a outerring, which was considerably brighter.

Out focus I only saw a bright ring and maybe, a realy dark middle spot and ring inside the bright one.

I only started Suiters book on star testing, but it sounds both infocus and outfocus should look similar?

[alacant]

29 minutes ago, Bender said:

correct after all?

Hi. Not sure if a star test alone will tell you if you have the centre correct. Have a look here.

HTH

[Bender]

Thanks for the link alacant, by the sounds of it, when collimating the scope with your primary spot (which is my Catseye hotspot) and the star image is not asymmetrically skewed than you primary spot should be placed reasonably correct.

[alacant]

12 minutes ago, Bender said:

not asymmetrically skewed than you primary spot should be placed reasonably

Yes. That's how I interpret it.

I always thought that the star test was the ultimate thing to do before -in my case- imaging, but due to atmospheric conditions, never found it reliable. Ever since then, I've only ever used a Cheshire sight tube with cross hairs; a one minute task, even at f4.

Cheers and clear skies.

[Bender]

Thats a good start   i am happy with Cheshire/Catseye collimation, just wanted to confirm that i use the right mark on my primary to do so.

Now i just have to work out what the difference between infocus and outfocus star image means. 

I am visual observer only and generally happy with the views, but interesting to see if my OO mirrors live up to premium you pay.

[AKB]

I have to say that, having recently got one of these...

https://www.firstlightoptics.com/catseye-collimation-tools/catseye-infinity-xlkp-autocollimator.html

... an auto-Collimator is a fantastic tool to aid collimation (albeit one that cost a quarter of the price of my telescope!)