What has caused this please?

[MARS1960]

I knew it was going to cloud over in about an hour so i just put the 100-400 on the 80D and did about an hour of unguided 2min subs.

What on earth has caused this to happen?

[JOC]

Err, for the novice - what's wrong with it?

[Yearofthegoat]

Am guessing he means the star bursts?

 

"A starbust from a light source is the result of light diffraction. Diffraction is the slight bending of (light) waves around small obstacles and the spreading out of (light) waves past small openings. As light passes into your camera through a small opening, i.e. a small aperture at a low focal length, it bends around the edges of the blades and creates the “star” look. The number of rays from each starburst is related to the number of aperture blades in your lens. The more blades your lens has, the more “starburst” is possible. "

 from: https://www.slrlounge.com/diffraction-aperture-and-starburst-effects/

[MARS1960]

All stars are full of diffraction spikes, seeing was okish and zero moisture.

[Yearofthegoat]

Lens blades?

[JOC]

They may not be desirable, but they sure are pretty!   TBH I am surprised that you didn't get any tails with a 2 min exposure unguided.  I've been sticking to 10 and 15 seconds which is far less time to avoid any rotation effects. 

[MARS1960]

2 minutes ago, Yearofthegoat said:

Lens blades?

Not thought of that but i don't ever recall it happening before, though it is a long time since imaging with the 100-400 and my memory is getting worse by the day lol.

[MARS1960]

1 minute ago, JOC said:

They may not be desirable, but they sure are pretty!   TBH I am surprised that you didn't get any tails with a 2 min exposure unguided.  I've been sticking to 10 and 15 seconds which is far less time to avoid any rotation effects. 

There is some slight trailing but you have to look close :).

[Yearofthegoat]

Trying to think of what could be 'in the way' - lens blades are definitely one thing, but maybe atmospheric stuff also: ice crystals? high faint cloud?

[Yearofthegoat]

Maybe try shooting something terrestrial, like a far-off street light say, with the same camera settings and check for similar bursts?

[MARS1960]

2 minutes ago, Yearofthegoat said:

Maybe try shooting something terrestrial, like a far-off street light say, with the same camera settings and check for similar bursts?

Good idea, i'll try it tonight. but at £1600 it better not be the lens ?.

[Yearofthegoat]

Well all, most (I think) variable aperture lenses have blades... Could be there's no solution it's just an attribute of the lens system.

[wxsatuser]

Diffraction spikes from the blades as mentioned, stacking makes them worse.

There are 2 per blade, 9 blades 18 spikes.

[MARS1960]

5 minutes ago, Yearofthegoat said:

Well all, most (I think) variable aperture lenses have blades... Could be there's no solution it's just an attribute of the lens system.

Could be i guess, i just dont recall seeing it before. I am imaging with a new camera though, i wonder if it's a sensor thing, don't remember seeing it with my 70D, strange.

 

[MARS1960]

Just now, wxsatuser said:

Diffraction spikes from the blades as mentioned, stacking makes them worse.

There are 2 per blade, 9 blades 18 spikes.

That's it then, verified.

Such a shame, guess i'll have to not shoot M45 with a camera lens, thank goodness i didn't spend longer than an hour on it.

[wxsatuser]

Bright stars make it worse but it's to be expected with most lenses, the more stopped down the worse it gets.

[JamesF]

My first guess would be the aperture blades in the lens, too.  It's pretty much unavoidable because any "edge" in the lens system will cause diffraction, just like the spider vanes in a newt.  If you haven't noticed them in the past perhaps you had the lens wide open at the time?

What you could try doing if you don't like the spikes is to make a circular aperture mask from card or (probably better) black plastic sheet for the aperture you want and then leave the lens aperture wide open.  Diffraction still happens, but because there are no straight edges acting to cause the effects to reinforce itself in specific directions, the spikes shouldn't appear.

James

[R26 oldtimer]

Next time you can try wide open, using an aperture mask.

[R26 oldtimer]

Beaten by James!

[JamesF]

1 minute ago, R26 oldtimer said:

Beaten by James!

:D

Always good to have a suggestion confirmed, nonetheless.

James

[michael.h.f.wilkinson]

I have seen similar diffraction spikes when stopping down a lens. At full aperture these don't appear. An aperture mask is indeed the solution

[steppenwolf]

1 hour ago, MARS1960 said:

I am imaging with a new camera though, i wonder if it's a sensor thing, don't remember seeing it with my 70D, strange.

It is the aperture blades, not the camera. I always use an aperture mask on my camera lens images and leave the aperture wide open to obviate this issue. Filter reducer adaptors make great aperture masks.

[happy-kat]

1 hour ago, MARS1960 said:

Good idea, i'll try it tonight. but at £1600 it better not be the lens ?.

Astro imaging is way more demanding on a lens then day time use. Don't be surprised if when used fully wide open that you don't see other things like lens aberrations.

[RichLD]

+1 for step down rings for an aperture mask

[ollypenrice]

From The Boy's Own Book of Making Jolly Good Aperture Masks With Everyday Household Materials.

Black card, barbecue or stove paint (not ordinary matt black which uses partially reflective dyes rather then pigments), compass cutter, draft excluder foam.

Olly