What has caused this?

[Black Knight]

So last night was a good opportunity run off a few test shots using my new Quattro. This pic is a test shot from around Vega, just 10x15 second subs, no calibration frames.

The question is: What has caused the donut visible just below Vega?

Quattro 8"

10x15 second subs, 800 ISO, no calibration frames

Skywatcher coma corrector

Canon 500D (modded)

[Starblazer]

Looks like a reflection in the light path somewhere.

[Black Knight]

I wondered that ... the coma corrector?

[Black Knight]

Actually could it be caused by collimation? Perhaps an incorrectly aligned secondary? I had a helluva job to align the secondary ....

[Budgie]

Are you sure it's not just "Lens Flare"?

Not sure about the physics of it but you get the same effect when taking a photo with a bright light source in the background, it's also a nice effect in PhotoShop.

[lukebl]

I think it's pretty inevitable when you have something REALLY bright in the frame, like Vega.

[Black Knight]

I think it's pretty inevitable when you have something REALLY bright in the frame, like Vega.

Really? I didn't know that. I shall have to remember that!

[FraserClarke]

Yeah. Just a 'ghost' -- a reflection in the optics. Totally normal

Even with the best anti-reflection coatings, lenses still reflect ~1% of the light (filters/CCDs are even worse often at 10-20%). So 1% bounces of a lens (or filter, CCD window, CCD itself, etc), then 1% of that bounces back off a lens in front of it and heads back toward the CCD. Because it has travelled further than the other 99% of light, it is an out-of-focus doughnut when it gets to the CCD

Happens for all objects, but you only notice it on really bright ones because 1% x 1% is 0.01%, which is not very much!

[Black Knight]

Thanks guys, I shan't worry too much about it then .

[ollypenrice]

You could select an area just larger than the ghost in Ps, feather it, then use the pretty standard halo reduction technique in curves to reduce it. Begin by Alt-clicking with the cursor on the background sky to pin it on the curve. In the second image below, this is the lowest point on the curve. Then experiment by dropping the curve above that point. To finish you might need to give colour balance a nudge.

Olly

[Black Knight]

Thanks Olly, that's very useful. Actually, I hadn't bothered with any post processing, but I'll definitely give your suggestion a go .