How far off collimation is this?

[Zermelo]

150mm F/5 Newt, as received new, view down eyecap collimator (no adjustments attempted by me):

I've read a couple of intro guides, so I'm led to believe that the off centre image of the secondary in the primary is normal for a fast reflector.
The whole image of the primary in the secondary seems to be reasonably central too.
My concern is obviously that the black dot from the collimation cap is not within the "doughnut" centre marking from the primary.

I've tried here to superimpose a circle with centre lines, but I'm not sure if I've got the circle itself correctly positioned (I'm not sure what I should be lining it up against):

Now, the interesting thing is that it's one of those jobbies with a sealed, non-collimatable primary. But I believe the secondary has all the usual degrees of movement.

Does it look likely to be far out?

I can try a star test tonight (first time for that too).

Edited June 24, 2020 by Zermelo
typo "refractor">"reflector"

[John]

It's really not far off. If you can get the black dot of the cheshire in the centre of the "donut" on the primary, I'd be happy with that.

 

[Mr niall]

I’ve just bought the same scope and it was identical. It’s 99 percent fine. Don’t overthink it - just adjust the primary mirror so the hole in the collimation  cap is in the centre of the hole on the mirror and you’re good to go

edit; ignore that sorry didn’t realise it was a glued primary!

Edited June 24, 2020 by Mr niall

[Pixies]

The secondary mirror is quite large, with respect to the image of the primary. That will make the initial positioning of the secondary much more forgiving. So it's just a case of a quick alignment with the doughnut.

Perhaps this is all part of the design?

[Pixies]

4 minutes ago, Mr niall said:

I’ve just bought the same scope and it was identical. It’s 99 percent fine. Don’t overthink it - just adjust the primary mirror so the hole in the collimation  cap is in the centre of the hole on the mirror and you’re good to go

Ah! That's the issue. This is a scope where you can't adjust the primary.

[johninderby]

On this scope you can only adjust the secondary as the primary is fixed. However not a problem as with a fixed primary still possible to collimate it just with the secondary.

Edited June 24, 2020 by johninderby

[Mr niall]

5 minutes ago, Pixies said:

Ah! That's the issue. This is a scope where you can't adjust the primary.

Yep sorry my bad! You can do it through the secondary mirror adjustment screws too. Just be really ginger, back off each of the three screws a quarter turn then you’ll have enough room to tighten it into shape. Try to avoid twisting the mirror if you can. The trick is to make sure you can always see your three primary mirror clips. The good news is once you’ve got it it’ll stay like that for months/years.

[Zermelo]

15 minutes ago, John said:

It's really not far off. If you can get the black dot of the cheshire in the centre of the "donut" on the primary, I'd be happy with that.

 

Thanks John, but is that a secondary adjustment, or primary (which I can't do on this scope - at least, not unless I try to emulate the factory process)?

[John]

16 minutes ago, Zermelo said:

Thanks John, but is that a secondary adjustment, or primary (which I can't do on this scope - at least, not unless I try to emulate the factory process)?

What ever you can adjust, adjust in very small steps. You might have to experiment a bit - looking through the cheshire, make a slight adjustment to one screw and note which direction the dot moves in. If it's the wrong way, adjust back then try another screw.

As long as you only make small adjustments, it's easy to undo as needed.

 

 

 

[Zermelo]

OK, I was a bit prematurely worried then.
I'll try to do a star test tonight, and depending on the result (and how brave I'm feeling) I'll give it a tweak.

[John]

49 minutes ago, Zermelo said:

OK, I was a bit prematurely worried then.
I'll try to do a star test tonight, and depending on the result (and how brave I'm feeling) I'll give it a tweak.

Good idea. Remember to use a high magnification for the star test - 200x or more ideally and make sure that the scope is fully cooled.

 

[Zermelo]

Update.
I managed to get a test on Arcturus last night at just over 200x with my shortest EP + barlow.
Unfortunately my phone camera refused at those light levels so I'll have to describe the view.
When defocused, I saw approx. 6 concentric rings, inner ones dimmer and outer ones brighter. It was shimmering about a bit, but I wasn't surprised at that magnification. At any particular instant there was some random distortion in one direction or other, so I looked over a period of time at (i) the "average" shape of the rings and (ii) their "average" distribution relative to the centre. As far as I could tell, (i) they were circular, and (ii) showed no directional preference. I take this to be good news.

As a more real-life test, I included the Lyra double double in the observing schedule later on. I thought it might be a good test of the mirror and alignment, as each pair would be on opposite sides of the FOV, when blown up enough to split them. (I've attempted the double double before with a small refractor, but never succeeded).
Impressive. Both pairs split as perfect dots, no tails, orientations at right angles to each other.

Based on these tests, I'm not going to bother adjusting the secondary. I'll just check it now and again against my original photo.