Star field photographs, stuff not moving?

[Mr Whippy]

I was taking some long exposures last night, nothing posh just having a play.

But some things were not moving despite the stars leaving long streaks over 30s of exposure.

Now I assume some geostationary satellites will appear still, but won't they tend to be on the horizon and to the south ish?

I saw a few to the NW at about 45deg elevation?!


Just curious what they might be... but I'm guessing it's something common and people who shoot stars will know


Cheers

Dave

[kit80]

Dead pixels?

[jmorgan]

I get this and I think it is a bit more involved. I understand there is a method whereby you take another exposure but with the cap on and a bag over the end to keep all light out then use some program or another to subtract the bagged shot from the timed exposure. Something to do with the ccd and noise on long exposures, I have a setting to reduce this but not really messed with it.

[Mr Whippy]

Hmmm, could be indeed hehe.

Pretty sure it moves around though...

30s exposure here.

Can you spot the non-moving spot?



Bugger it's re-sized it, click through to see it full size...

Cheers

Dave

[MysteryLemon]

Found it!

[Blukoo]

kit80 said:

Dead pixels?

Could be that.

[flat-planedCrank]

Can always test for always bright 'hot pixels' - sometimes called 'stuck pixels' - by putting the lens cap on and taking a similar exposure-length shot



[nerd point]'Dead pixels' generally means they're always black / inactive ) [/np]

[kit80]

flat-planedCrank said:

Can always test for always bright 'hot pixels' - sometimes called 'stuck pixels' - by putting the lens cap on and taking a similar exposure-length shot



[nerd point]'Dead pixels' generally means they're always black / inactive ) [/np]

Nerd! But that does make sense

I have two or three hot pixels, drive me barmy and is the reason I shall be buying a new camera soon.

[flat-planedCrank]

kit80 said:

flat-planedCrank said:

Can always test for always bright 'hot pixels' - sometimes called 'stuck pixels' - by putting the lens cap on and taking a similar exposure-length shot



[nerd point]'Dead pixels' generally means they're always black / inactive ) [/np]

Nerd! But that does make sense

I have two or three hot pixels, drive me barmy and is the reason I shall be buying a new camera soon.

Think my camera has a few more than that Good thing is they are just single pixels so super-easy to edit out. Some RAW processors have tools to spot these 'out of the ordinary' pixels and sort it automatically

[14-7]

My 7D normally has around 5-15+ hot pixels when doing long exposures for night time timelpases.

It does vary though depending on how long the exposures are for and how much time is left between shots.

[kit80]

flat-planedCrank said:

Think my camera has a few more than that Good thing is they are just single pixels so super-easy to edit out. Some RAW processors have tools to spot these 'out of the ordinary' pixels and sort it automatically

Oh crikey. I am quite lucky then, I think I have three but I never seem to remember where they are and obviously don't always show up, just happened I had a print blown up to A3 size and had missed one and now it is all I can see it that picture.

I am still buying a new camera though

[RealSquirrels]

Could be cosmic rays

[flat-planedCrank]

kit80 said:

flat-planedCrank said:

Think my camera has a few more than that Good thing is they are just single pixels so super-easy to edit out. Some RAW processors have tools to spot these 'out of the ordinary' pixels and sort it automatically

Oh crikey. I am quite lucky then, I think I have three but I never seem to remember where they are and obviously don't always show up, just happened I had a print blown up to A3 size and had missed one and now it is all I can see it that picture.

I am still buying a new camera though

Oh I'd never want someone to stop buying new gear on my account!


Think photography is as much about planning and justifying increasingly expensive future purchases as it is about taking pictures

[kit80]

flat-planedCrank said:

Oh I'd never want someone to stop buying new gear on my account!


Think photography is as much about planning and justifying increasingly expensive future purchases as it is about taking pictures

Oh yes! Although as a female I was pretty good at that anyway

[Mr Whippy]

Well I've just done a 30s exposure and I don't see anything that really stands out.

If I pump the exposure a lot then the odd spot (maybe 10-15 clusters) start to get brighter but by that time the whole image is a mid-grey/purple noise field... I'm pushing the 'exposure' in RAW to the far right at import time... when the same settings out that evening shooting stars gave the bright spot without any adjustments right there and then while looking at them on the little view screen.

I'm generally of the idea that it probably is some weird statistical error due to long exposure but doing the lens cap on test doesn't seem ideal for some reason.

I'll give it a shot again tonight in a dark room and see if I can see any specific pattern forming.


Cheers

Dave

[jmorgan]

The business with the dark shot to use to subtract from the good shot was something I came across on astronomy forums etc.

[kit80]

Mr Whippy said:

Well I've just done a 30s exposure and I don't see anything that really stands out.

If I pump the exposure a lot then the odd spot (maybe 10-15 clusters) start to get brighter but by that time the whole image is a mid-grey/purple noise field... I'm pushing the 'exposure' in RAW to the far right at import time... when the same settings out that evening shooting stars gave the bright spot without any adjustments right there and then while looking at them on the little view screen.

I'm generally of the idea that it probably is some weird statistical error due to long exposure but doing the lens cap on test doesn't seem ideal for some reason.

I'll give it a shot again tonight in a dark room and see if I can see any specific pattern forming.


Cheers

Dave

Was probably a spaceship hovering then. Only other logical explination.

[tenohfive]

Showing my inexperience here, but I didn't think you'd get motion from the stars from a 30 second exposure? I thought you'd be looking at 5 minutes plus at least?

[ruddermode]

tenohfive said:

Showing my inexperience here, but I didn't think you'd get motion from the stars from a 30 second exposure? I thought you'd be looking at 5 minutes plus at least?

It depends on the focal length of your lens and how high up you're pointing it.

For example, a Canon 7D with a 24mm lens, pointing upwards at a 45 degree angle, landscape format. Rule of thumb figures are 54 seconds maximum exposure at the top of the frame and 29 seconds at the bottom to minimise trails. So 29 seconds for the image overall.