Why is my star, well, star shaped?

[cwis]

Hi all, you're about to see the glory that is my very first astrophotograph (compressed quite a lot for bandwidth) !  It's of a famous not binary star and there are many things wrong with it that I can see, so there are probably loads I can't - underexposed background stars is the one that stands out for me...

 

My question however - why is the brighter star "star" shaped?  I see six points! My telescope (Sky-watcher 130ps) has four (quite chunky) secondary mirror supports so I'd expect four points? I think I see the same effect with a 4mm eyepiece if I get the focus bang on...

Camera is a SvBony sv305 if that makes a difference?

[cwis]

Quick thought - could it be the focuser tube? I've not checked where it sits with the camera on it when it's focussed... If it was sticking into the light path, could I expect another two points? 

[cwis]

I'm an idiot. I just pulled the cover off the front to have a look how much the focuser projected into the tube to be confronted by three mirror supports.  I'll let myself out....

[andrew s]

1 hour ago, cwis said:

I'm an idiot. I just pulled the cover off the front to have a look how much the focuser projected into the tube to be confronted by three mirror supports.  I'll let myself out....

We all live and learn.  Regards Andrew 

[vlaiv]

1 hour ago, cwis said:

I'm an idiot. I just pulled the cover off the front to have a look how much the focuser projected into the tube to be confronted by three mirror supports.  I'll let myself out....

Star spikes are actually perpendicular to original spider supports - this is why you have 6 of them on 3 prong spider support.

[cwis]

Just now, vlaiv said:

Star spikes are actually perpendicular to original spider supports - this is why you have 6 of them on 3 prong spider support.

I am going to have to sit down and understand that at some point. Not on a hot Friday afternoon though....

[andrew s]

7 minutes ago, cwis said:

I am going to have to sit down and understand that at some point. Not on a hot Friday afternoon though....

Think about shinning a light through a slit. You get a pattern on both sides as well as the central peak. The spider gives an inverse "pattern" - bright be comes dark etc.

Regards Andrew 

PS I have glossed over the technical details.

[vlaiv]

2 minutes ago, cwis said:

I am going to have to sit down and understand that at some point. Not on a hot Friday afternoon though....

Maybe easiest way to understand it (but also not very true technically) - is that it is quantum phenomena and related to Uncertainty principle. Once photon passes very close to straight edge - it is defined in position (closer it is to edge - well edge is not fuzzy so photon must be at certain distance to it) with respect to edge - but it does not have well defined position "along the edge" - only at direction towards the edge.

Uncertainty principle says - you can't have both speed or rather momentum and position known at the same time. Photon moves at speed of light - so speed is fixed, it also has known energy, so only component of momentum that can change is direction - closer to the edge - more defined position with respect to edge - more deviation in direction with respect to edge (or in that direction - not direction parallel to the edge).

Very similar thing happens when you have water running from the tap and you put your finger - it will start bending perpendicular to your finger. Mind you - this happens for completely different reason - but effect is the same.

Here is illustration with comb - this time, third reason - electric charge - but again same effect water jet bends perpendicular to comb edge:

Another example would be dual slit experiment / diffraction grating - where light spreads "horizontally" if slits are vertical - in any case at 90 degrees to slit orientation.

[sploo]

6 minutes ago, vlaiv said:

Once photon passes very close to straight edge

I thought that light was made up of waves.

I mean photons.

Waves.

Photons.

I'll get my coat.

[vlaiv]

2 minutes ago, sploo said:

I thought that light was made up of waves.

I mean photons.

Waves.

Photons.

I'll get my coat.

We can do waves as well

Here is ripple tank simulation of this phenomena - how waves "bend" behind edge.

This is why I said above picture is not very precise technically - but to my mind - it is easier to grasp - you can't have both things at the same time - "closeness" to the edge and "direction". Much easier than trying to explain why waves bend behind edge

 

[cwis]

I think it's the 3D aspect to it. I'm fine with a diffraction grate, or  the waves coming into a harbour. Conceptually I don't have an issue - I'm fine wiv der maffs too. It when I switch it into 3D and try and visualize it something inside my head goes Fzzzt.

[andrew s]

1 hour ago, cwis said:

I think it's the 3D aspect to it. I'm fine with a diffraction grate, or  the waves coming into a harbour. Conceptually I don't have an issue - I'm fine wiv der maffs too. It when I switch it into 3D and try and visualize it something inside my head goes Fzzzt.

Drink more think less. Regards Andrew 

[Paul M]

6 hours ago, cwis said:

It's of a famous not binary star

Albireo? Hmm. I thought the jury was still out on the binary or not issue? 

From: https://en.wikipedia.org/wiki/Albireo

It is not known whether the two components β Cygni A and B are orbiting around each other in a physical binary system, or if they are merely an optical double. If they are a physical binary, their orbital period is probably at least 100,000 years.^[32] Some experts, however, support the optical double argument, based on observations that suggest different proper motions for the components, which implies that they are unrelated

I actually thought it's a nice image of my favorite double. I was going to image it myself the other night but had, errr, technical problems getting Astroberry to understand me!   And in that vein, I offer no technical analysis of your star shaped stars!