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FORUMS Photography Talk by Genre Astronomy & Celestial Talk 
Thread started 13 Mar 2014 (Thursday) 14:00
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What determines exposure time when shooting the stars?

 
Gel
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Mar 13, 2014 14:00 |  #1

As a new Astrotrac user finding their feet is there a particular guide to shooting the stars?

I see 20 x 30 second exposures for one, but have no idea why that is?
I shot the M42 last night at 5.6, ISO 3200 on the 1DX (without tracking) on a 1 second exposure at 400mm and it came out ok.

Sure I like the idea of shooting for 30 seconds tracked at ISO100 but when I've pumped the exposure time the black of the night sky no longer ends up being black, it's not a clean image?

Even so, get a good shot in one take and I'm wondering why people take 20 of the same exposure as they all look the same (unless it's to pull the minute details through like a screen overlay).

I know there will be a decent explanation for this. But if anyone can help that would be great.


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Mar 13, 2014 14:14 |  #2

Heya,

Most of it is trial & error. There isn't a guide for exact exposures that I know of, because you cannot just automatically account for where you are in the world. It would have to account for focal length, temperature, location on the planet, local light pollution(s), sky glow, and on and on and on. So most people take an exposure of a good length and then adjust from there based on trial & error.

When I started using my tracker, I started with 1 minute. Then 2 minutes. Then 4 minutes. I wanted to see what each "stop" of time would result in exposure wise for my location and particular settings (F2, ISO 400). Those three images told me what I needed to know about ISO and shutter speed stops in relation to the aperture wide open (which I was not going to change obviously).

The sky is not black. That's your first assumption mistake. You're shooting through an atmosphere that is far from 100% clear. And the space out there, again, is not black. There is dark space, but there's a lot of stuff out there and surrounding objects you're shooting.

People take the same shot over and over, for the purpose of stacking. Stacking is for reducing noise and hot pixels, keeping only the consistent light captured which results in keeping what you want, and reducing what's not supposed to be there. It's not for enhancing information, usually, it's for cleaning up, reducing noise, etc, which indirectly enhances detail of what you're trying to capture.

If you think M42 at 1 second, ISO 3200, F5.6 came out "ok" wait until you see it at 1~2 minutes, ISO 200~400, F5.6. You'll fall over.

The benefit of the tracker is to increase exposure time without trails. This allows you to take out the ISO, and thus reduce a LOT of noise. It reduces post processing time quite a bit when you don't have as much dramatic noise to deal with. Combine that with some average stacking and you can generate ridiculously clean images of the night sky. Plus, long exposure just lets in a lot more light, and you're usually limited to exposure times based on focal length--but with a tracker, you can just expose for as long as you want based on your tracker's accuracy.

I look at it in stops of light:

Aperture is static, wide open, widest you can use, so that you get as much light no matter what else you're doing.
ISO, each stop is a stop of light capture, but at the cost of noise and loss of dynamic range. Higher ISO gets you more information, but at the cost of noise.
You can reduce ISO, but each stop you lose, is a stop of light lost.
Shutter time, look at it in stops of light as well. 30 seconds... 1 minute... 2 minutes... 4 minutes... 8 minutes... etc.
You get WAY WAY WAY more light from that 4 minute shutter time, than you will from ISO 25600 on a 30 second image. And if tracking properly and accurate, it will be really really clean.

Very best,


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Mar 13, 2014 14:15 |  #3

For what it's worth, onetime when I knew that I was going to have an opportunity to take pictures on a dark clear night, I did a web search and came up with a few articles like this. I'm not sure if this was the article I reference or not, but the info was pretty similar. This is where I started and I did end up with what I was looking for.

https://improvephotogr​aphy.com …photograph-the-milky-way/ (external link)

another thing you can do is check out this thread: many of the shots have readable EXIF info.
https://photography-on-the.net …106101&highligh​t=milkyway

Keep in mind, the amount of time that you can expose for, without tracking, decreases with increasing focal length. Settings that worked on my wide angle failed miserably with my 70-200 (i.e., a lot of little arcs) .

My concentration was the milkyway, I'm not sure what you are looking to do, but that is a great place to start.

Chimping is your friend for this kind of work; so is bracketing (AEB).


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Gel
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Mar 13, 2014 16:25 |  #4

Awesome, thanks guys. It's helped clear some misunderstandings I had. I'm in Tenerife at the moment so don't want to get frustrated with things because I don't understand them.


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mpbowyer
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Mar 13, 2014 21:58 |  #5

Here's my experience.

Untracked, focal length and azimuth are all that goes into exposure time. Just think, if you want to take a picture of Polaris, it doesn't move much. You can take a long exposure even with a long 400mm lens. Probably minutes without tracking.

If you want to image something over the equator, it's going to be RACING across the sky. You'll probably see oval stars even at 1.3 seconds at 400mm. Think the Pleiades or Jupiter.

So without tracking, pick your target and use trial-and-error to get the longest possible exposure before you're not happy with the star shape. I image to a laptop and quickly open the RAW files to zoom in and look for trails. I find that I can get about 1.3 seconds with a 300mm on a crop body shooting something over the equator. Then longer and longer towards Polaris or the South horizon. I get 1.6 seconds on Orion, for example.

Once you have your exposure at the max for your focal length and your target, start messing with ISO. when you are reviewing an image on your camera, hit the "info" button until you see the white histogram. You should see 2 spikes one for bright stars all the way to the right, and another one for darkness more toward the left.

Understanding histograms is the most important part of taking the pictures. With properly exposed frames and stacking you can end up with some wonderful images. With a bad histogram, the data just isn't there and no post-processing can save you.

The spike on the right represents the stars. The high part literally slammed all the way to the right is the collection of pixels that are just plain bright white. No color just bright. This is the few pixels at the middle of each star. Maybe 3-4 pixels per star. You might have 500 stars, so 1500-2000 of your pixels are just white. This is that spike.

Next is the area JUST to the left of the far right. This represents the quantity of non-white, but bright things. Usually in astro, this she's how cool individual stars are going to look (or it tells you that you are out of focus, but that's a different day). If the histogram shows only one LINE vertically at the right, every star is going to be just a few white pixels. It wouldn't even be round. For round stars made up of square pixels, you need at least as many gray or colored pixels as you have white pixels. Take a good picture of Beteljeuce in Orion and the edges should be orange. The middle will be white, but the good color of the star will be orange. This orange is bright, but not completely washed-out white bright, so it is represented just to the left of the far right in the histogram.

The middle of your histogram is basically going to be at the bottom. This is good. There are very few pixels in your image that are perfectly properly exposed colors.

The spike on the left is what you will use to tune your ISO. You do NOT want it to be all the way to the left like the bright spike is all the way to the right. If this is the case, you'll never see good nebulae etc. this spike represents the darkness of the image. The dark sky between stars. You don't want this dark sky to be completely black in the raw image. You will make the background of the final image perfectly black manually later.

The left spike is where all the cool stuff in an astro image lives. Specifically the right half of the left spike. The stuff that is JUST brighter than the plain darkness. The stuff that is such a tiny bit brighter than the plain darkness that you can't even see it in a single frame of your image.

You want to tune your ISO such that the left spike is separated from the far left. You want the spike around 20-15%. This puts the left side of the spike a bit above the far left "black" area. You want to reserve the darkest darky mcdarkeyton pixels for the few spots on your images that literally have zero stars or nebulae or galaxies in them. With the zillions of brewight things out there, these are very few pixels. You'll want to black out the pixels manually later that are darker than a certain amount, but only in the area that is not your subject area.

So the short answer with the kind of explanation is that you make your exposure as long as possible for your rig and your target, then you tune the ISO to get the proper exposure. You have proper exposure when the left spike in the single-color histogram is just right of the far left, at about 20-25 %. If you're shooting with a properly aligned astrotrac, you can take images of several minutes. You will be setting your ISO very low this will result in high quality images. ISO kills image quality. You only use it when you have to because something else in your rig is not up to the task. This is the case for a[most a]ll photography.

When shooting Orion at 1.3s I had to use 6400 ISO to properly expose the image. If I had a tracking rig or a wider lens I might be able to get 2.6 seconds, in which case I would get the same histogram with 3200 ISO. If I had a very good tracking rig, I could get the same histogram with 5.2 seconds and ISO 1600, or 10.4 seconds at ISO 800 or 20.8 seconds at ISO 400 or 41.6 seconds at 200 ISO or 83.2 seconds at 100 ISO.

This would be the limit of my imaging ability with my given amount of light pollution with my aperture and my glass quality.

If I had darkish skies, I could get an even better image by stopping down aperture, and making up the histogram with a longer exposure. If my initial exposure were at F/4. I could stop down to f5.6 (one stop, or half the amount of light) and double my exposure time to get the same histogram again. My particular lens is sharper at f5.6 than it is at f/4, but after that it doesn't get much better. So for my image I would be pretty much optimized at 166.4 seconds, f/5.6, ISO 100.

I could stop down to f/8 and do a 5-minute exposure, but it wouldn't be much better. I'd benefit from double the frames going into my stacking program more at that point.

So if you want a better astro image, go to a darker place so the histogram is Further to the left with all else playing the same, then buy a lens with a bigger aperture so a few stops in it still has good clarity.

I guess there's a reason people spending zillions of dollars on imaging equipment do it in dark places with low f numbers and long exposure times.

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Mar 15, 2014 22:05 as a reply to  @ mpbowyer's post |  #6

I have a Meade 6" f8 reflector telescope that I used to do film photography with in the early 80's. Is it worth dusting it off and dragging out? Or is photo stacking the way to go with a traditional SLR lens? I used magnification not prime focus. Had a few cool pics of Jupiter and Saturn but deep sky objects were a different story. There were issues with the clock drive that I think have been fixed. Finding a good photo lab back then was also an issue.

I was also considering getting an Orion cassegrain telescope (4 or 5 inch) and use it as a telephoto lens for night shooting as well as wildlife. Any opinions?
BTW mpbower That's a great shot of M42


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Mar 24, 2014 21:26 |  #7

Gel: You got some very good advice here.
As said key is "Trial and Errors".
I like to see you 1 sec image but guess its is in the darker side.
then don't forget, M42 is one of the brightest Nebula's up there. Don't expect same good result for the rest, in fact if you need to challenge yourself, just move a bit up and to the east and try "Flame nebular" of same Orion constellation and see if you can get the same result and if you can spot Horse Head?
So you need to explore you ability to gather light in every way possible and easier to achieve by you. Work on that line and you will have the answer to yourself and literary its only the Sky that is the Limit!

aartvark: I would definitely dusting that gem off and would bring the light of life back. Meades are made up of the best out there and I am sure you will be please with what you can achieve in the digital era with it as opposed to what you did before in film time. Its a miracle that there is no more Lab in need but then that is a tricky part left in darkness currently to may be gain use in future specially in Astrophotography. Don't worry about the tracking issues. Start without tracking and go from there. Meade has a facility here near were I live at Irvine California and I know the guys who are working there, are not for money. They are passionate workaholics that love what they do. I am sure you can get any problem with your tool repair for very reasonable price. 6" is a very good size to both give your a high magnification and mobility function to move the glass to a remote lotion with jet black sky.
Go for it and send my loves to guys at Meade if you have to contact them. They will help you out. And if you are to add any other tool, I suggest a wide angel, wide F lens such as Rokinon 14mm F2.8 instead of high mag glass for now.


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Mar 24, 2014 22:19 |  #8

Exposure time for astronomy is no different than daytime exposure. If your data is clipping in the shadows, then you are loosing resolution and it is just empty data, aka noise. Alot of people expose until they have their shadows 1/3 of the way out of the shadows. Sometimes this is impractical due to mount limitations. Alot of times you will see people post stuff which is still completely clipping the shadows, nto proper technique but you can still make out the details and get an understanding of whats going on in the image.

Beyond just an exposure comes stacking, which increases your signal to noise ratio. One 5 minute exposure at iso 1600 is way noisier than 30 iso 1600 exposures stacked up. In astrophotography we preform a linear stretch on data in order to bring out the details. If you have a low signal to noise ratio then you are going to be stretching noise instead of data which accumulates as a median equation by results of stacking up multiple exposures. An example of this can be seen in these two images of the horsehead. The first image is a single 300 second exposure "5 minutes" at iso 800 at 190mm F5. The second images is 18 x 300 second shots stacked up to create an integration time of 1.5 hrs of data. The first image of the horsehead looks purple and oversaturated because my camera is modded and my color profile was on set something rediculously oversaturated and I just saved the profile to the image for a rough preview. Obviously I calibrated out the data in the final image, although it still needs work.

Also, if you save and open the first image you will see that the luminance data clears the histogram completely. This is how to determine a properly exposed AP image.


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Apr 02, 2014 12:58 |  #9

Not sure if this is what you are after but my rule is that I use the 500 Formula to avoid star trail blur if not using a microscope / telescope.

500 divided by the Focal length of your planned shoot. I tend to do my timelapse sequences with a Nikon 14-24mm attached to my EOS5DMkII with a Novoflex Adapter. It gives me at 14mm a shutter of 35 seconds which I round down to 30 and avoid the Bulb setting.

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Apr 02, 2014 13:17 |  #10

^^^
Just to confuse more:
Rule applies to Microscope or telescope as well and identically.
That is if you know how to calculate (Or read) the focal of optical in use. With microscope, it is the objective lenses, for telescope, its the distance from center of mirror or Objective lens to focal point image of object from infinity. This is very different from "Magnification factor" that is ratio of Objective over Subjective (Eyepiece) lens in mm.
Theoretically the problem is (That is if you could hand held microscope or telescope) to match the right shuttle speed as camera may not go that high however in practice, both tools are in build over solid stand and once camera coupled, movements with microscope are None and with telescope, depends on object you observe and its relation to movement of earth.


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Apr 02, 2014 19:13 |  #11
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Also try Deep Sky Stacker to clean up all that noise if your black sky isn't black.


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What determines exposure time when shooting the stars?
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