Collimating an F4.5 scope - offset?

[BrendanC]

Hi all,

I'm having difficulty collimating my 130PDS which is an F5, brought down to F4.5 with the addition of a 0.9x coma corrector.

I've collimated it plenty of times in the past, with my Cheshire and Concenter, but for some reason I'm not quite getting the star shapes I feel I should be.

So, I want to do a star test, but I'm not sure what I should be looking at when doing it.

My understanding is that faster scopes have an offset, and I can see this clearly when I look down the focuser tube.

So, should I see this offset when I defocus to do a star test? Or should everything be fully concentric?

I've looked around online and opinions differ!

Thanks, Brendan

Edited June 8, 2022 by BrendanC

[Pixies]

My understanding is a star test is done just very slightly out-of-focus on both sides of focus and you'll be looking at the diffraction rings, rather than the gross alignment you see (along with the silhouette of the secondary and offset) if you take it far out-of-focus. See: https://www.astroasheville.org/wp-content/uploads/2013/09/No-Tools-Collimation.pdf

[BrendanC]

Sure, I know what to do, I just don't know exactly what I should be seeing. Basically, should there still be the offset, or concentric circles? It's definitely concentric circles for slower scopes, I just cannot find a defnitive answer to this fairly simple question for faster scopes. 

Edited June 9, 2022 by BrendanC

[Stu]

5 hours ago, BrendanC said:

Sure, I know what to do, I just don't know exactly what I should be seeing. Basically, should there still be the offset, or concentric circles? It's definitely concentric circles for slower scopes, I just cannot find a defnitive answer to this fairly simple question for faster scopes. 

The rings should be concentric regardless of the scope type being tested.

[Dan_Paris]

9 hours ago, BrendanC said:

So, I want to do a star test, but I'm not sure what I should be looking at when doing it.

 

Collimating with a star test ensures that the optical axis and the mechanical axis of the focuser intersect at the field stop of the eyepiece, but does not ensure that the two axes are coincident. So if you have issues with tilt, it would not solve them.

 

9 hours ago, BrendanC said:

My understanding is that faster scopes have an offset, and I can see this clearly when I look down the focuser tube.

As far as collimation and offset are concerned, your telescope is  f/5 and not f/4.5, the corrector does not play a role here.

 

9 hours ago, BrendanC said:

So, should I see this offset when I defocus to do a star test? Or should everything be fully concentric?

 

Far from focus you will see the secondary shadow with its offset, but  close to focus you'll see a perfectly concentric diffraction pattern.

 

[Captain Scarlet]

As others have intimated, the Star Test for a newt, particularly a fast one, is only useful or valid at or very close to focus at high magnification, where the diffraction rings should be concentric.

Away from focus, far enough that the secondary shadow shows clearly, the offset will and should be apparent and what you're looking at should _not_ be symmetrical. Obviously, the faster the newt, the greater the offset and the more obvious it will be. Under no circumstances should you try to "collimate away" the offset in rough alignment! (such as you might do for a truly symmetrical scope like a mak or SCT).

Cheers, Magnus

[Spile]

Forget offset when it comes to checking the primary alignment with a star test. Offset is intrinsic to the secondary mount. As long as a=b=c=d and e=f* you are good to go in terms of the secondary. As for the primary what is your cap or Cheshire telling you? 
 

* https://astro.catshill.com/collimation-guide/

[BrendanC]

My Concenter and Cheshire look ok but I'm still getting egg-shaped stars. So, I thought I'd give a star test a whirl, as a precursor to checking for tilt.

[bottletopburly]

Check the focuser is square to the ota Brendan first put the concentre in focus tube rack fully out then using a long metal straight edge line it up with the spider vane parallel to the focuser and eye the ruler edge with concentre body see if it’s parallel, my 200pds was out a lot which affected collimating, since then collomation has been a breeze .

[BrendanC]

This sounds like an interesting approach but I'm having difficulty visualising what you mean about using a straight metal edge, aligning that with the spider vane, and then looking at the Concenter.

Do you have a diagram?

[barbulo]

I am interested too. Collimating my 130PDS is driving me crazy.

[bottletopburly]

Will update