how to set time and gain

Discussion of using SharpCap for Deep Sky Imaging
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jeruss
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how to set time and gain

#1

Post by jeruss »

Is there a way to know how to set time and gain so that faintest details in an object can be captured without overexposing brightest parts of object. Thinking of andromeda galaxy and orion nebula.
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turfpit
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Re: how to set time and gain

#2

Post by turfpit »

jeruss

For an object with a bright core and faint nebulosity (such as M42), it is not easy to achieve what you are seeking via a single set of exposures. High Dynamic Range (HDR) photography is the technique commonly used to achieve the effect you are looking for. Brian's (@oopfan) land based explanation here https://astrotuna.com/high-dynamic-rang ... hotomatix/ is a good starting point for understanding the technique.

See my M42 https://www.astrobin.com/full/p63izu/0/ as an example. This was created using stacks made up of varying exposures from 120s down to 2s.

If you go to the Astrobin web site and search for M42 and M31 you will find many examples of outstanding images that are created using multiple exposures which are combined with HDR.

Dave
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oopfan
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Re: how to set time and gain

#3

Post by oopfan »

Hi jeruss,

Dave has more experience using HDR for astrophotography. I believe he uses Fotoxx. I've been experimenting with Photomatix but more needs to be done. Like Dave says, I'd recommend looking at AstroBin to see what other people are doing.

In the meantime I use a single exposure. For a CMOS camera use a gain setting that delivers the greatest dynamic range (most often a low gain setting -- refer to your sensor analysis results.) Then discover the exposure that puts M31's galactic core just below saturation (something in the 40,000 to 50,000 ADU range.) Then, believe it or not, stack and stack and stack some more until you are blue in the face. Most cameras have a dynamic range of 8.5 to 10+ magnitudes depending on how much you are willing to pay for it. My camera has 8.5 magnitudes. Right now I am putting the final touches on an image of M35 with companion NGC 2158. I discovered that a 15-second exposure captured the brightest stars just around saturation (they are bright!) With 8.5 magnitudes of dynamic range I can see down to 17th magnitude but in order to get a great signal-to-noise ratio on those faint stars I need a stack consisting of 4 hours. I have a CCD with a high Read Noise compared to your CMOS camera so you can do it in less time. This technique is applicable to all DSO's so you need to treat each one on an individual basis.

Except for star clusters most DSO's are so faint that you can't get the core to saturate no matter what exposure you use. In those cases consider that your field-of-view almost always contains 9th or 10th magnitude stars that you can't crop out. In those cases I would set the exposure so that the brightest of those stars is just below saturation. There is nothing uglier than a fat, bloated star that distracts from the beauty of your DSO.

Brian
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turfpit
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Re: how to set time and gain

#4

Post by turfpit »

Note: Fotoxx runs on Linux (no Windows version), see my Nov 27 post here viewtopic.php?f=16&t=596&start=20 for a (100x4s + 60x60s) example.

Windows software is available or Google Photoshop HDR or Gimp HDR.

Dave
jeruss
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Re: how to set time and gain

#5

Post by jeruss »

Thanks all for the replies. This will give me some things to read and research.
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oopfan
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Re: how to set time and gain

#6

Post by oopfan »

jeruss,

M31 is not an easy target. Be aware that next to M32, one of its satellite galaxies, is a 7th magnitude star. The last time I imaged M31 about five months ago I did not take that into consideration. After all of my effort I have a final image where the star is very nearly the size of M32. Whenever I show the image to someone who is not an astrophotographer they are confused by this when I tell them that M32 is a galaxy but that thing there is a star! Just saying to use caution when determining exposure.

Brian
jeruss
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Re: how to set time and gain

#7

Post by jeruss »

Thanks for the cautions Brian. Hope to try m31 again before it gets away.
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