PF Photography Thread

Andy Resnick

Quote by fluidistic

Nice photos Andy! I'm curious, can you take such photos from a big city? I ask this because I'd think it's hard to see that many stars in the sky in a big city due to "light pollution".
Also for the second photo, you made a composite of "1/100s ISO100 and 1/4s ISO400". I am curious... shouldn't 1/100s ISO 100 be almost a fully dark photo?!

Thanks!

To the first question (light pollution): it's all about increasing the signal to noise ratio. What I did here was simply subtract the background- the slowly-varying intensity. Any location with a rapidly-varying intensity (starts, tree branches, etc) is left alone. It's easy to do- duplicate the image, blur one of them (I used a Gaussian blur radius of 100 pixels), and then subtract the blurred image from the original. Another method is 'image stacking', where multiple images are added together. There are lots of subtle details that go into optimizing a particular strategy, and there are folks here with a lot of experience who can help you.

As a point of reference, I have been able to consistently pull out magnitude +15 objects from the background. I think I can do even better once the weather warms up and I can spend more time outside.

To the second question, the answer is that I am using a lens with a large aperture; the lens is a 400/2.8 (with an optional 2x extension tube) which means for astrophotography it's a 6" refractor. This lets me use short shutter speeds, but more practically it lets me aim the camera by eye; I can 'star hop' to a particular faint object fairly quickly.

Drakkith

Quote by Andy Resnick

To the first question (light pollution): it's all about increasing the signal to noise ratio. What I did here was simply subtract the background- the slowly-varying intensity. Any location with a rapidly-varying intensity (starts, tree branches, etc) is left alone. It's easy to do- duplicate the image, blur one of them (I used a Gaussian blur radius of 100 pixels), and then subtract the blurred image from the original. Another method is 'image stacking', where multiple images are added together. There are lots of subtle details that go into optimizing a particular strategy, and there are folks here with a lot of experience who can help you.

I'm assuming that the background subtraction doesn't actually increase the SN ratio, it just removes the background light from the sky without affecting the abrupt changes where something blocks out the background light, such as the tree limbs you mentioned, along with the sudden increase in signal from stars? That way you can still see the tree limbs and such, thanks to the "glow" around them, and at the same time you can see the stars as well.

fluidistic

Thanks Andy for the explanation.

Andy Resnick

Quote by Drakkith

I'm assuming that the background subtraction doesn't actually increase the SN ratio, it just removes the background light from the sky without affecting the abrupt changes where something blocks out the background light, such as the tree limbs you mentioned, along with the sudden increase in signal from stars? That way you can still see the tree limbs and such, thanks to the "glow" around them, and at the same time you can see the stars as well.

That's a good point. My intention was to describe ways to stretch the contrast between star and sky.

Drakkith

Quote by Andy Resnick

That's a good point. My intention was to describe ways to stretch the contrast between star and sky.

Got it.

Borek

Spring is in my backyard. Definitely.

fluidistic

Have you seen this video borek? http://www.youtube.com/watch?v=sfSJP8avHWI

By the way, nice photo. Do you know whether it's a toad or a frog? Probably the former...

Borek

Quote by fluidistic

Have you seen this video borek? http://www.youtube.com/watch?v=sfSJP8avHWI

Sigh. I thought I already know what it is about and then... Now I have coffee everywhere.

By the way, nice photo. Do you know whether it's a toad or a frog? Probably the former...

Check the file name.

fluidistic

Quote by Borek

Sigh. I thought I already know what it is about and then... Now I have coffee everywhere.

lol.

Quote by Borek

Check the file name.

Ah, I didn't know that was possible. Nice... According to wiki:

Quote by Wiki the Great

When alarmed, it emits a very loud call (alarm call) and it can exude a noxious secretion which smells like garlic, hence the common name "garlic toad".

. So your specimen was around 6 to 8 cm?!

Borek

Quote by fluidistic

So your specimen was around 6 to 8 cm?!

Perhaps even larger.

fluidistic

Quote by Borek

Perhaps even larger.

Nice :)

Andre

Wow, that must be Touchwood from Catweazle

source

Andy Resnick

This is a chip from the paleolithic era: a circa-1968 RCA CD4012 (dual 4-input NAND gate) device.

I've found a stash of TTL devices (and a lot more) dating from 1962 (!), I will be posting images of these on my lab blog as they become available.

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fluidistic Recognitions: Gold Member	Thanks Andy for the explanation.

Andre Blog Entries: 2 Recognitions: Gold Member	Wow, that must be Touchwood from Catweazle source

Andy Resnick Recognitions: Science Advisor	This is a chip from the paleolithic era: a circa-1968 RCA CD4012 (dual 4-input NAND gate) device. I've found a stash of TTL devices (and a lot more) dating from 1962 (!), I will be posting images of these on my lab blog as they become available.