streaks in the sky?

I recently made my first attempt at photographing the night sky.

I used a Canon powershot A630, and the CHDK software,
to take 19 frames or 8 seconds each, using
a 35mm lens.

I then used Registax under Wine on Linux to align and stack the
frames.

Sadly I didn't take dark frames, so the contrast is not
so great - next time...

I'm fairly happy with the result - amazed at the numbers
of stars my little P&S has picked out.

Here's a subsampled piece of my image:

http://s48.photobucket.com/albums/f2...ent=streak.png

But what are the streaks? Since I got registax to "lock"
to the stars, the streaks are moving relative to the stars.

There's too many to be planets, so I'm guessing satellites,
but would welcome confirmation.

BugBear

bugbear wrote:
I recently made my first attempt at photographing the night sky.

I used a Canon powershot A630, and the CHDK software,
to take 19 frames or 8 seconds each, using
a 35mm lens.

I then used Registax under Wine on Linux to align and stack the
frames.

Sadly I didn't take dark frames, so the contrast is not
so great - next time...

I'm fairly happy with the result - amazed at the numbers
of stars my little P&S has picked out.

Here's a subsampled piece of my image:

http://s48.photobucket.com/albums/f2...ent=streak.png

But what are the streaks? Since I got registax to "lock"
to the stars, the streaks are moving relative to the stars.

There's too many to be planets, so I'm guessing satellites,
but would welcome confirmation.

BugBear
Nice satellite trails.

"bugbear" wrote in message
et...
I recently made my first attempt at photographing the night sky.

I used a Canon powershot A630, and the CHDK software,
to take 19 frames or 8 seconds each, using
a 35mm lens.

I then used Registax under Wine on Linux to align and stack the
frames.

Sadly I didn't take dark frames, so the contrast is not
so great - next time...

I'm fairly happy with the result - amazed at the numbers
of stars my little P&S has picked out.

Here's a subsampled piece of my image:

http://s48.photobucket.com/albums/f2...ent=streak.png

But what are the streaks? Since I got registax to "lock"
to the stars, the streaks are moving relative to the stars.

There's too many to be planets, so I'm guessing satellites,
but would welcome confirmation.

They look like the effect of the 'hot pixels' on your camera's sensor.
These exist on all cameras, expect some pro cameras which cost
many thousands of pounds, where the sensors are hand-selected.
However, as we all have them, someone figured a long time ago
that using (subtracting) "dark frames" (ie equal length exposures,
but with the lens cap on) from the images will eliminate most of
them. If you camera has a "noise reduction" feature, it does
this automatically, but each shot takes twice as long, so most
do this in software.
http://en.wikipedia.org/wiki/Dark_frame
http://www.users.zetnet.co.uk/pete/darkframe.htm

Most stacking software can do this, if you take and add dark frames,
including Registax.

HTH & clear skies,
--
Rob (~52N, ~1W)

Rob wrote:
"bugbear" wrote in message
et...
I recently made my first attempt at photographing the night sky.

I used a Canon powershot A630, and the CHDK software,
to take 19 frames or 8 seconds each, using
a 35mm lens.

I then used Registax under Wine on Linux to align and stack the
frames.

Sadly I didn't take dark frames, so the contrast is not
so great - next time...

I'm fairly happy with the result - amazed at the numbers
of stars my little P&S has picked out.

Here's a subsampled piece of my image:

http://s48.photobucket.com/albums/f2...ent=streak.png

But what are the streaks? Since I got registax to "lock"
to the stars, the streaks are moving relative to the stars.

There's too many to be planets, so I'm guessing satellites,
but would welcome confirmation.

They look like the effect of the 'hot pixels' on your camera's sensor.
These exist on all cameras, expect some pro cameras which cost
many thousands of pounds, where the sensors are hand-selected.
However, as we all have them, someone figured a long time ago
that using (subtracting) "dark frames" (ie equal length exposures,
but with the lens cap on) from the images will eliminate most of
them. If you camera has a "noise reduction" feature, it does
this automatically, but each shot takes twice as long, so most
do this in software.

Yes - CHDK allows me to disable this feature,
which (in my camera) is enable for any exposure 1 sec
or longer.

http://en.wikipedia.org/wiki/Dark_frame
http://www.users.zetnet.co.uk/pete/darkframe.htm

Most stacking software can do this, if you take and add dark frames,
including Registax.

I've now done a "dumb" stack, using simple netpbm scripts,
and there do appear to be some hot pixels,
along with a couple of spots a little larger than a pixel.

http://s48.photobucket.com/albums/f2...urrent=hot.png

Well, this *was* a first attempt. I guess if I'm going to disable
camera-resident dark frame subtraction, I REALLY ought
to do a software dark frame subtraction...

BugBear

On Fri, 06 Feb 2009 11:27:04 +0000, bugbear wrote:
I recently made my first attempt at photographing the night sky.

I used a Canon powershot A630, and the CHDK software,
to take 19 frames or 8 seconds each, using
a 35mm lens.

I then used Registax under Wine on Linux to align and stack the
frames.

Sadly I didn't take dark frames, so the contrast is not
so great - next time...

I'm fairly happy with the result - amazed at the numbers
of stars my little P&S has picked out.

Here's a subsampled piece of my image:

http://s48.photobucket.com/albums/f2...ent=streak.png

But what are the streaks? Since I got registax to "lock"
to the stars, the streaks are moving relative to the stars.

There's too many to be planets, so I'm guessing satellites,
but would welcome confirmation.

BugBear

They could be (high flying) aircraft. Depending on your location they'd be
much more common than satellite trails.

bugbear wrote:
rrent=hot.png


Well, this *was* a first attempt. I guess if I'm going to disable
camera-resident dark frame subtraction, I REALLY ought
to do a software dark frame subtraction...

Not had a good night for stars yet, but did some trial
to make sure my ideas were "ready".

Having discovered that on maxmimum zoom (4x or 140 mm)
I was getting star trials on a 8 second exposure,
I've cut down to 2 second exposure (I have no means
of tracking).

I've improvised a lens cap, by sawing of a section
of nicely opaque bean tin, which I can fit
over my lens mate (the lens on a Canon Powershot
is famously weak and unreliable, so a lensmate
protects it, as well as providing filtering).

http://www.lensmateonline.com/newsite/A620A610.html

So, being set up for dark frames (I took
5 before the main exposure, and 5 after),
I took 168 2 second exposures, of a tree in my garden.

I have run into a problem with registax; since the exposures
are so dim, I would prefer registax to "add" them rather
than "average". I am aware that average is a simply
"add then divide by n".

Is this possible - a brief reading of the manual
has not revealed anything "obvious", but perhaps
it's under a label (or piece of jargon) I'm not
expecting.

BugBear