Flintstone Stargazing

September 24, 2007

What a difference seconds can make

Filed under: astronomy, Astrophotography, Messier Objects, Observation Journal — Tags: — Ed @ 10:10 pm


M57-Ring Nebula Exposure Levels

In responding to a commenter earlier today, I realized it might be a good idea to put together a post discussing how I capture my images and what difference the length of exposure and stacking makes. The above image of M57-Ring Nebula is a composite of 50 different exposures of the nebula that I took this evening. Each row represents a different exposure time and the columns represent the number of images that have been stacked. For example, row 1, column 5 represents 5 1 second images that have been stacked and row 5, column 8 represents 8 15second images that have been stacked.

From this graphic, it’s easy to see why you want to stack the largest number of images that you can when getting images of deep space objects like the Messier Objects. The other thing is that you can also see some things that aren’t really intuitive. For example, a single 8 second image looks better than 10 1 second images or even 10 2 second images. The reason for this is that the fluctuations in image noise cancel each other out over time and within that one frame, they have already cancelled each other out, particularly when the image is leveled – the more light that has been captured the brighter the objects will be (the greater the signal to noise ratio) and so it will also make the noise lower and the image clearer.

Side note: the fact that the 8 second images are brighter than the 15 second ones is not because they are necessarily better, it’s the way the software auto leveled those images. I haven’t actually retouched any of the levels in the above image so that we could compare apples to apples. In pretty much all cases, I pull my final images into Photoshop and then adjust levels to pull out details hidden in the images. (I never paint anything or composite with someone else’s images – that would be cheating)

Below is a final image I got – 115 15 second images for a total of 1725 seconds (28m 45s). It also automatically discarded at least 50 images that weren’t of high enough quality for inclusion in the stack.

M57-Ring Nebula on 9/24/07
M57-Ring Nebula long exposure

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5 Comments »

  1. That’s very interesting. I realize also that the focal ratio for your scope is something like f/4 which is really fast. My Celestron 8 has an f/10 focal ratio which is quite a bit slower so for 30 second exposures, my pictures tend to be fainter. I’m not so sure that Registax helps with exposure so much as it helps with clean up based on what I see above.

    Did you see on Astromart that some folks feel that stacking only helps up to a point. After a certain number of images stacked, the difference isn’t significant. But there was debate as to where that threshold lies.

    Comment by Pierre — September 25, 2007 @ 11:44 pm

  2. Also, out of curiosity, how long can you reliably open your shutter before your shots suffer form star drift on your best polar alignments?

    P

    Comment by Pierre — September 26, 2007 @ 12:18 am

  3. Since I haven’t really used any other scopes, I’m not really clear on the difference that focal ratio makes (though I’ve seen discussions about that), but mine is 4.3, which I understand is pretty fast.

    Regarding a limit on stacking, I think that it’s more of a diminishing return as you get greater. From what I’ve read and experienced the sweet spot with the DSI is somewhere around 100 exposures. At a certain point, the additional exposures will wind up improving the image only a fraction of a percent, but still I suppose if you stacked a huge number it still could make a difference. Regarding maximum length, I normally use 8 seconds because at that length I almost never get movement, but sometimes I’ll do 15 seconds and get good results. The situation where stacking makes the biggest difference (at least in my experience) is in planetary nebula – when I imaged the Dumbbell Nebula I found that by stacking 100 exposures, the amount of detail I could see in the nebula’s structure was way better than even at 50. It allowed me to stretch the histogram further to see more detail without posterization that can occur at with fewer exposures. There are definitely diminishing returns, though.

    In terms of getting the polar alignment right, since I don’t have autoguider capability, I think for me the best I’m ever going to get with this scope is to do stacking of shorter images.

    Comment by Ed — September 26, 2007 @ 8:24 am

  4. Hi Ed,
    I like the grid image in your post, great way of comparing things – final image is cool too.

    On your discussion point:
    1) Increasing the sub-exposure time is good up to a point – what you need to do is lift the level of your subject’s intensity above the level of your noise – ie the spike on your camera’s histogram needs to be about 1/3 of the way from the left. This varies not just by f ratio, ISO and temperature but also by the subject magnitude too. Beyond this point the benefits are little & eventually you may actually be worse off. It isn’t that ‘fluctuations in image noise cancel each other out’ as you put it, it’s that the signal to noise ratio is improving. A longer exposure actually has more noise – it just has even more signal too.

    2) Stacking many subs is indeed governed by diminishing marginal returns. Again you’re improving the SnR. I’m certainly happy that when I’m stacking 70 x 2min subs of various DSO’s there is still more to be gained by further subs.

    3) There is an ideal exposure for each object, beyond this there is no more detail to be gained (with any specific setup that is but this is often many hours of exposure though).

    I guess the process is:
    A] Decide on optimum exposure time
    B] Use subs as long as your mount will allow without going to excess for your camera
    C] Collect sufficient subs to fulfill your total exposure requirement
    D] Process & Enjoy

    Best Wishes
    AnnMarie

    BTW I loved your eclipse sequence.

    Comment by AnnMarie — September 26, 2007 @ 7:43 pm

  5. Thanks – I appreciate your explanation. I think sometimes it’s hard for anybody to get their head around so many variables. That’s especially true when all of them are interdependent.

    Comment by Ed — September 26, 2007 @ 9:12 pm


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