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Thread: How do I work out my exposure times?

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    Default How do I work out my exposure times?



    I am just getting started with image processing and need a little advice. I have an ASI183mc pro (one shot colour, cooled). Capturing with SGPro and processing with Pix in Sight. What I'm not sure about is how to work out either in Pix or SGPro what level of exposure I am getting and there for how long my subs need to be. How do I check if I'm over exposing stars or galaxy centers, and how much exposure is a good guide to work from?
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    Default Re: How do I work out my exposure times?

    The only 'real' practical measure unfortunately is trial and error. There are so many variables between your sky conditions, your imaging train, filters if any, etc. It also varies from one target for another.

    For instance a cluster may only take 60 to 120 second subs, a faint nebula might need 300 to 600 second subs. Some targets, for instance the Orion Nebula can require a mixture of shorter and longer subs to bring out details of the bright core.

    It is pretty easy to tell when stars become saturated, they will become un naturally large and 'bloated'. You will soon get to recognize it at first stretch.

    On the subject of saturated stars one thing that helped me a lot with my OSC was to put a clear IR "Luminance" filter in the imaging train.

    As far as stretching the image PixInsights Screen Transfer function usually does a fairly decent job of 'first guess'. It tends to over do it a tad but that can be adjusted easly.
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    Default Re: How do I work out my exposure times?

    Thanks for your reply, does using a luminance filter mean you don't have to extract a luminance frame in LRGB tool in pix? I think I may have found what I was looking for in that if I click on a bright star in a debayered sub as long as the RGB values don't= 1 then its not over exposed is that right?
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    Default Re: How do I work out my exposure times?

    No, frankly I dont even bother with channel extraction for the most part. I just debayer the subs and align and stack them. Acutally batch pre processing does that for me. The Lum filter only filters out Infrared spectrum and helps with reducing star 'bloat'.

    Honestly I dont even know how to answer your second question. I have never really gone by values, rather I go by appearances. Perhaps someone more educated on that can answer your question.

    When it comes to AP especially for faint subjects it is rarely, if ever over exposed.... unless you have a bright star close to the subject, Altinak in the Horsehead is a good example. No your real limiting factors will be your equipment (quality of guiding) and the amount of sky glow, and or light pollution you have to deal with.

    For me using an ASI1600mc OSC I have found that subs of about 300 seconds at unity gain work out best... for me... Only trial, error, and experience can help you there.

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    Default Re: How do I work out my exposure times?

    Thanks again, I think my problem being new to this is not knowing what I'm looking at yet and how to interpret what I see on the screen.
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    Default Re: How do I work out my exposure times?

    The simplest way to see if you are over-exposing is to look at the histogram. Most image editors have one (those are the ones you should use). For me, the exposure time is determined by blown-out stars. If the histogram extends all the way to the highest value, that's how you can tell.
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    Default Re: How do I work out my exposure times?

    There are many factors to consider in exposure time. My starting point is to calculate the histogram value needed to make the subframes shot noise dominant aka swamping or burying the read noise. You first calculate this value or use one of the calculation spreadsheets floating around. Then take a sub, examine the mean or median of the histogram and adjust exposure till it matches the target value, give or take. The rationale is that read noise limits the effectiveness of stacking at improving SNR. There’s a bit of maths involved so I’ll leave it there for now to see if you want more detail.
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    Default Re: How do I work out my exposure times?

    For the faint fuzzy stuff, basically longer is better. If one or two bright stars are saturated (= over-exposed), I wouldn't worry about it. If a lot of stars are saturated, then it is time to consider shorter exposures, with more frames to preserve the signal-to-noise ratio. If the bright parts of the fuzzy object are saturated, then the exposures are way too long.

    How do you tell if stars or galaxy cores are saturated? You can check the histogram, but it is unlikely that a handful of pixels will show up there. You likely need at least a few hundred pixels to register on the histogram.

    In PixInsight, you can hover the cursor over any pixel and it will tell you, at the bottom of the screen, the brightness of the pixel. There are several scales you can choose, but the 0.0-1.0 scale works well. On that scale, 0 indicates saturated black, and 1 indicates saturated white. You don't want more than a handful of either.

    When I am stretching an image using the Histogram tool, I move the white level down until the number of clipped (i.e. saturated) pixels is 0.001%. (The Histogram tool reports the number of clipped pixels as you adjust it.) That is something like 100 pixels out of 8 million, which is not bad. I accept a few clipped pixels as the price for stretching the levels of my target object.

    I don't generally accept any clipped pixels in the core of my fuzzy target. There are exceptions. Some targets, notably M42, have such an extreme dynamic range that any exposure that captures the oter edges will over-expose the core, and any exposure that gets the core right under-exposes the edges. In that case, you have to combine exposures of different times using HDR or similar techniques to give an acceptable image.
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    Default Re: How do I work out my exposure times?

    Thanks for the replies everyone, just what I was looking for. Will read up on noise swamping next haha.
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    Default Re: How do I work out my exposure times?

    Quote Originally Posted by Wolf359 View Post
    Thanks for the replies everyone, just what I was looking for. Will read up on noise swamping next haha.
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