[Wavytone]

Hi 5ash,

Duncan masks, artificial stars and other gadgets will get the alignment close, but not perfect - for that you really must test at high mag on a bright star.

What I have shown several times to other SCT owners is that while they thought they had it “close enough is good enough” the scope was really underperforming. In excellent seeing an SCT that is just a tad off will appear fairly good, but when the collimation is correct a significant improvement should be noticeable. In poor seeing a star image often breaks down into a smear, yet side by side with my scope, it’s still showing tight pinpoints though they dance around a bit. But not smeared.

For the star test you need an eyepiece atound 5-7mm and choose a bright star close to the zenith, ideally mag 1 or brighter. On a nice bright star when refocused a tad, just enough to show the secondary shadow, you should see the bright Poisson dot inside the secondary shadow. This tiny bright spot in the dead centre of the OTA is caused by diffraction around the secondary shadow - and it dies not move when the secondary collimation is adjusted.

When collimated correctly you should see the Poisson spot concentric with the shadow, with concentric out of focus rings both inside and outside focus. At focus you should be able to see the diffraction disk of the star, plus one or two rings.

However quite a few SCTs do not really produce a diffraction pattern at focus - what you may see is just a jumbled mess of intense light. Over the years I’ve seen quite a few like that.

I see you are in the Hunter Valkey - if you ever come this way PM me - bring your OTA and I’ll do it for you, and maybe show what a really really nice star test looks like in my Santel.