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Thread started 07 May 2010 (Friday) 15:15
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DRH - Southern Cross

 
David ­ Ransley
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May 07, 2010 15:15 |  #1

Hi

Southern Cross or Crux

My first post here. I always wanted to take photographs of the stars and now I have. Not much right now, but it is a start. 15 seconds, F 4.0 and did the stars move :-) At 100% crop, it is possible to see them move. Rigt now I have no mount to stop that and had to use a normal tripod. No stacking either, just one shot at it.

Two pointers Left and the cross itself on the right. I stay in Johannesburg and this is from my house on the city edge.

ISO 400 and 53mm

In the foot of the cross you will find Acrux, the brightest star of this constellation. Acrux is really a double-star system. Despite its small area, Crux contains at least ten open clusters visible with small telescopes.

Because it is not visible from most latitudes in the Northern hemisphere, Crux is a modern constellation and has no Greek or Roman myths associated with it. Crux was used by explorers of the southern hemisphere to point south since, unlike the north celestial pole, the south celestial pole is not marked by any bright star.


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David ­ Ransley
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May 07, 2010 15:28 |  #2

This is a crop of the top part of the cross. Just look at the stars move. 30 seconds later and we have a line.

Greetings


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David ­ Ransley
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May 07, 2010 15:44 |  #3

Some information:

The two pointer stars, alpha Centauri and beta Centauri, are again very bright and consequently almost impossible to miss. Beta Centauri is the 11th brightest star in the sky while alpha Centauri is the 3rd brightest star of all.

Historically the Southern Cross has had an interesting life. Although the stars in the Cross were known to the ancient Greeks, they regarded them as the hind legs of the constellation Centaurus, the centaur, who today surrounds the Cross.

It wasn’t until the year 1516 that it was first described as a cross and not until later that century that it became adopted as a separate constellation by astronomers.


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zeldaboy101
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May 07, 2010 17:09 |  #4

Nice, this is one constellation I plan on seeing sometime but I just have to make my way to that side of the planet. :-)

Focus is very good, you've got a nasty dust mote though at the bottom middle. I would also try to push the saturation more, you definitely can get more out of that.

If you're using that 28-70, set it at f/2.8. Don't worry so much about super pinpoint stars when you can get more signal, it's all about bringing in that extra light, they'll be pretty sharp at f/2.8 if you can nail your focus.




  
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SteveInNZ
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May 07, 2010 17:32 |  #5

David Ransley wrote in post #10140308 (external link)
My first post here.
Rigt now I have no mount to stop that and had to use a normal tripod. No stacking either, just one shot at it.

Good stuff David. Don't let the lack of a mount stop you. You can take multiple exposures on the tripod and stack them in DSS to simulate a tracked exposure and use the same set of frames for startrails.

Steve.


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David ­ Ransley
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May 08, 2010 00:50 |  #6

zeldaboy101 wrote in post #10140916 (external link)
Nice, this is one constellation I plan on seeing sometime but I just have to make my way to that side of the planet. :-)

Focus is very good, you've got a nasty dust mote though at the bottom middle. I would also try to push the saturation more, you definitely can get more out of that.

If you're using that 28-70, set it at f/2.8. Don't worry so much about super pinpoint stars when you can get more signal, it's all about bringing in that extra light, they'll be pretty sharp at f/2.8 if you can nail your focus.

Thanks, I am planning to stack and took this shot as a sample. The bottom middle is me. I had a two second brain wave and removed a hot pixel. Wasn't too worried about the net result as I wanted to see whether the stars will show up. I am reading up on using DSS and will stack the CRUX for sure after I saw what I could achive with my tripod.

Used Live fiew on PC to focus and triggered the shot from there. Will post my first stack of this constellation as soon as possible. I noticed you need a full battery to start with on the camera.


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David ­ Ransley
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May 08, 2010 05:09 |  #7

Planning another shot at CRUX. This time with stacking in mind. On the attached pic the Jewel Box is just visible (centre left on the edge) as a few small\dots lines (about 4).

It is on the edge of the Coal Sack and near the second brightest star in the Cross. This is one of the finest open star clusters in the entire sky, the Jewel Box. It gets its name from the various colours visible when the stars are viewed through a telescope.

The Jewel Box contains about 50 bright members, the brightest of which are supergiants with average luminosities equal to 80,000 times that of the sun. The cluster is only about 7 million years old, about 50 light years across and about 6,440 light years distant.

This was just one shot 15 seconds, F4.0, ISO 400 at 53 mm (image cropped). Right now, doing the planning to get sufficient photographs for the stack.


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David ­ Ransley
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May 08, 2010 08:42 |  #8

Still a long way to go I see. I didn't have all the required files for a stack - dark frames, flat frames, bias frames and so on, but tried to make some. Used these and a few shots of last night and shoved it through DSS. The net result is that you can see the constellation, but all the other small stars are missing. :)

Will plan to have the correct files tonight and see what happens tomorrow.

This is exactly what the naked eye will see in a city :-)


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zeldaboy101
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May 08, 2010 09:36 |  #9

You don't NEED darks, lights, etc to do this, but they do help. In fact I didn't use any of them for quite a while and still got very good results. That milky way shot over the lake I posted didn't have any darks etc with it.

You definitely need several batteries if you plan on doing this all night, keeping the shutter open is what really drains it quick. If it's on the cool side the batteries will go even quicker.




  
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David ­ Ransley
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May 08, 2010 15:38 |  #10

Hi, I gave it another run at F2.8, 20 seconds and ISO 1000 at 70mm. First impressions of the photograph was that it was Red. Looked over exposed and I had to bring it back in RAW, plus reduce the red channel. Slight up in contrast and this is it.

I also have a load full of photographs to try out stacking.

Greetings


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lazer-jock
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May 08, 2010 17:24 |  #11

Much nicer...you learn quickly. (I want to play around with stacked exposures this summer, and this thread gives me some hope. :) )


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David ­ Ransley
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May 08, 2010 17:38 |  #12

My first official stack :-)

Took 5 photographs and I must say. I am impressed with the software's ability to align and stack. Had to adjust the mid-tones and reduce the Red, but all in all it lives up to a first attempt.


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David ­ Ransley
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May 08, 2010 17:55 |  #13

lazer-jock wrote in post #10145778 (external link)
Much nicer...you learn quickly. (I want to play around with stacked exposures this summer, and this thread gives me some hope. :) )

A few tips from a beginner:

1. On camera noise reduction - switched off
2. ISO 800 looks like a good starting point
3. If you don't track on a mount, then 20 seconds looks like balance between movement of stars and good results at 70mm.
4. Each focal length has its own cut-off duration.
5. The 10 MegaPixel RAW image looks bad at 100%, but when processed and reduced in size you smile. Problem is the stars move fast.
6. The stacking software is something to get used to. Very technical and requires lots of reading to start to understand the settings and the result.
7. Due to the movement, F2.8 delivers sharp results. I tried F4, but the reduction in light is more of a problem.
8. Use live view to focus from a PC. This works great and also use the remote shooting capabilities from there. You don't need to use live view the whole time. Only for focusing. The files download to the PC after each shot as well. You need a small table and chair and obviously a portable laptop.

Have fun.


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zeldaboy101
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May 09, 2010 10:32 |  #14

Not bad!

A few more tips:

NEVER use the "fake" ISO, for astronomy and noise related purposes, use ISO 200, 400, 800 (very good one), or 1600. Never use 3200, 1250, 1000, or any of those in betweeners because they add a lot of additional noise for little signal gain. You'd get better signal/noise ratio with 800 and 1600. Before anyone debates all of this...i'm just quoting information provided by the author of a very very good astrophotography book, so this guy knows what he's talking about when it comes to ISOs and astrophotography.

It looks red and overexposed because of light pollution. Light pollution tends to put a lot of signal in the red channel, you'll need to bring the reds down and really boost the blue channel a lot for the right looking background color.

99% of astrophotos look TERRIBLY before you stack them, because they're very long noisy exposures, but when you stack them you get a very good result for 25-50% of the original size. When you want to sink like $10,000 or $20,000 into equipment you'll have a mount that can track well enough, a scope with good enough and flat enough optics, and a camera that will all come together to let you show your images at 100% when you stack a whole bunch of them.




  
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David ­ Ransley
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May 09, 2010 13:46 |  #15

Thanks, Zeldaboy

I studied my technique and the results when the sun was up and hiding the stars. I came to the conclusion that the stacking software is really there to remove the noise, and if something steaks across the skies, the lines will stay lines. Look at "1" and "3" in the pic below. This is from a 20 second exposure. Compare that to "2" and "4" of the same region, which is a five second exposure. More noise in the latter one, but less movement and to an extent better defined stars.

My feeling was that one should look at 60 second and divide it into manageble buckets of light to stack. 12x5 seconds is better that 3x20 seconds in other words. Further below you see the 12x5 seconds stacked and the noise is missing. What you see is the Jewelbox with more definition and even the stars around there look better.

The longer exposures seem to work better at first, but the shorter ones stack as well and provide detail. The only problem now is that I see many more stars in DSS and as soon as I save that to a file a whole lot of them fade away to nothing.

Thanks for the ISO 1000 advice, I will guard against doing that again.


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DRH - Southern Cross
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