Diffraction spikes...question

[wulfrun]

I'm a bit puzzled! I have 3 scopes, all Newtonians. Two of them have three-vane spiders and one (the 150PL) has a four-vane. Now, on the three-vane ones I expect and get 6-spike bright stars. However, I have been observing Jupiter several times lately and the 150PL is the tool of choice. I get an unexpected 6 spikes! Four are as expected, being at roughly 45, 135, 225 and 315 degrees, due to the tube/spider orientation. The other two appear about 0 and 180 degrees, straight through Jupiter's poles or close to that...coincidence? On a bright star there are the expected 4 spikes, not 6.

Over to the experts to satisfy my curiosity as to what's going on please. No, it doesn't bother me beyond idle curiosity.

EDIT: mods feel free to move if this isn't in the best place, I wasn't sure.

Edited October 31, 2021 by wulfrun

[rl]

Is the focussing tube racked in far enough to intrude on the light path?

How about the clips on the main mirror?

Are the 2 extra spikes of the same intensity as the other 4?

[wulfrun]

16 minutes ago, rl said:

Is the focussing tube racked in far enough to intrude on the light path?

How about the clips on the main mirror?

Are the 2 extra spikes of the same intensity as the other 4?

1) no, almost certainly not anyway.

2) don't know since I'd not considered that.

3) yes, as best I can judge by eye.

[Stu]

I’ve popped this into the equipment forum, which I think suits it better 👍

[Andrew_B]

Are you wearing glasses and does the spike position change as you rotate your head?

[wulfrun]

11 hours ago, Andrew_B said:

Are you wearing glasses and does the spike position change as you rotate your head?

No and no, respectively. I do wear glasses but not at the eyepiece. I am pretty sure I have little/no astigmatism, just presbyopia (!). I'm due an eye test shortly so I'll be sure to ask.

[Pixies]

Has the scope cooled down properly? I het 5 or 6 spikes (4 vanes) when the scope is warm. That or a telephone line is in the way!

[wulfrun]

3 minutes ago, Pixies said:

Has the scope cooled down properly? I het 5 or 6 spikes (4 vanes) when the scope is warm. That or a telephone line is in the way!

The scope is kept in the shed, so certainly cooled by the time I've set up. However, you might have hit on something with the "telephone line" idea...or more specifically, washing line! It might just be in the line of sight, I'd have to re-check that one.

Thanks for all the suggestions so far...