Gain and Exposure Time for ASI1600MM & LRGB Imaging

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STEVE333 United States of America
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Re: Gain and Exposure Time for ASI1600MM & LRGB Imaging

#21

Post by STEVE333 »


Hi Greg - I've found a better way to graph the results of the analyses. The first graph below is for your LRGB filters.
ASI1600MM LRGB.JPG
The vertical axis is now showing the "Excess Noise" (%) in your final stacked image, and, the horizontal axis is still showing the Exposure Time used for your images. Excess Noise is the amount of noise above the noise in a single image with an exposure lasting your entire imaging session. Of course such an image would saturate nearly all the stars, but, it would have the lowest noise. The graph shows that Excess Noise increases as the Exposure Time gets shorter. To get the Excess Noise down it looks like an exposure of somewhere around 150 seconds would be a good choice. Hopefully it won't saturate too many stars.

This second graph is for your Ha and SII filters.
ASI1600MM Ha S2.JPG
As you can see the narrow filters require longer exposure times to reduce the Excess Noise. Exposures in the 400 to 600 sec range look appropriate (maybe 400 for Ha and 600 for S2?).

I'm very glad we worked on this. I like this new way of presenting the results better. Much easier to determine how Exposure Time affects Excess Noise.

Hope this helps,

Steve
Steve King: Light Pollution (Bortle 5)
Telescope + Mount + Guiding: W.O. Star71-ii + iOptron CEM40 EC + Orion Magnificent Mini AutoGuider
Camera: ASI 1600MM Pro + EFW Filter Wheel + Chroma 3nm Siii, Ha, Oiii + ZWO LRGB Filters
Software: PHD2; APT; PixInsight ***** My AP website: www.steveking.pictures
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AstroBee United States of America
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Re: Gain and Exposure Time for ASI1600MM & LRGB Imaging

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Post by AstroBee »


Wow! That's amazing. I realize now seeing the graph that I forgot to give you the OIII sample in that zip file. My guess is that it would fall somewhere between the Ha and SII points?

Also, 150seconds in broadband?!?! from my backyard I can't believe that wouldn't be completely washed out with my extreme light pollution but I'm very excited to experiment and try it. Even if I can get a nice round 120seconds that would be incredible.
Greg M.~ "Ad Astra per Aspera"
Scopes: Celestron EdgeHD14", Explore Scientific ED152CF & ED127 APO's, StellarVue SV70T, Classic Orange-Tube C-8, Lunt 80mm Ha double-stack solar scope.
Mounts: Astro-Physics Mach One, iOptron CEM70EC Mount, iOptron ZEQ25 Mount.
Cameras: ZWO ASI2600mm Pro, ZWO 2600MC Pro, ZWO ASI1600mm
Filters: 36mm Chroma LRGB & 3nm Ha, OIII, SII, L-Pro, L-eXtreme
Eyepieces: 27mm TeleVue Panoptic, 4mm TeleVue Radian, Explore Scientific 82° 30mm, 6.7mm , Baader 13mm Hyperion, Explore Scientific 70° 10mm, 15mm, 20mm, Meade 8.8mm UWA
Software: N.I.N.A., SharpCapPro, PixInsight, PhotoShop CC, Phd2, Stellarium
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Re: Gain and Exposure Time for ASI1600MM & LRGB Imaging

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Post by STEVE333 »


AstroBee wrote: Wed Apr 07, 2021 8:24 pm Wow! That's amazing. I realize now seeing the graph that I forgot to give you the OIII sample in that zip file. My guess is that it would fall somewhere between the Ha and SII points?

Also, 150seconds in broadband?!?! from my backyard I can't believe that wouldn't be completely washed out with my extreme light pollution but I'm very excited to experiment and try it. Even if I can get a nice round 120seconds that would be incredible.

One word of caution, the analysis doesn't show when your sensor will saturate. With Gain = 141 you might have some real saturation problems. I ran your analysis again but with Gain = 0 for the LRGB setup. The results are below:
ASI1600MM LRGB Gain=0.JPG
The noise results are almost identical, but, you should have much improved dynamic range. I ran Gain = 0 for my RGB image and it turned out nice. I know you would have to generate a new set of Darks, but, it might be worth it.

Maybe check out an image with Gain = 140 and one with Gain = 0 and look at the stars for saturation?

Steve
Steve King: Light Pollution (Bortle 5)
Telescope + Mount + Guiding: W.O. Star71-ii + iOptron CEM40 EC + Orion Magnificent Mini AutoGuider
Camera: ASI 1600MM Pro + EFW Filter Wheel + Chroma 3nm Siii, Ha, Oiii + ZWO LRGB Filters
Software: PHD2; APT; PixInsight ***** My AP website: www.steveking.pictures
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Re: Gain and Exposure Time for ASI1600MM & LRGB Imaging

#24

Post by AstroBee »


Thanks Steve. Yes, it needs some experimenting.
Curious, did P.I. show my gain as 141? In SharpCap I know it was set to 139. I'm wondering if the metadata actually says otherwise?
Also, how does the Offset factor into these measurements?
Greg M.~ "Ad Astra per Aspera"
Scopes: Celestron EdgeHD14", Explore Scientific ED152CF & ED127 APO's, StellarVue SV70T, Classic Orange-Tube C-8, Lunt 80mm Ha double-stack solar scope.
Mounts: Astro-Physics Mach One, iOptron CEM70EC Mount, iOptron ZEQ25 Mount.
Cameras: ZWO ASI2600mm Pro, ZWO 2600MC Pro, ZWO ASI1600mm
Filters: 36mm Chroma LRGB & 3nm Ha, OIII, SII, L-Pro, L-eXtreme
Eyepieces: 27mm TeleVue Panoptic, 4mm TeleVue Radian, Explore Scientific 82° 30mm, 6.7mm , Baader 13mm Hyperion, Explore Scientific 70° 10mm, 15mm, 20mm, Meade 8.8mm UWA
Software: N.I.N.A., SharpCapPro, PixInsight, PhotoShop CC, Phd2, Stellarium
https://www.nevadadesertskies.com
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STEVE333 United States of America
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Re: Gain and Exposure Time for ASI1600MM & LRGB Imaging

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Post by STEVE333 »


Hi Greg - I checked the FITS Header info with PI and it has Gain = 139. I must have just remembered the Gain incorrectly as 141. Doesn't make a significant difference.

The equation below shows how the output ADU is created:
Equation.JPG
Equation.JPG (10.37 KiB) Viewed 887 times
  • N = number of absorbed photons that are turned into collected e-.
  • g = "gain" as shown in the manual (e-/ADU). In other words, how many e- are required to increase the A/D output by one.
  • Offset = digital offset to be added to measured counts, 10 for your case.
16 = The sensor multiplies the output of it's 12-bit A/D by 16 to convert it to a 16-bit number.

The Offset is user selectable. Sometimes the A/D converters aren't linear at the lowest readings, so, the Offset boosts all data into the linear range of the A/D converter.

Hope this all makes sense.

Steve
Steve King: Light Pollution (Bortle 5)
Telescope + Mount + Guiding: W.O. Star71-ii + iOptron CEM40 EC + Orion Magnificent Mini AutoGuider
Camera: ASI 1600MM Pro + EFW Filter Wheel + Chroma 3nm Siii, Ha, Oiii + ZWO LRGB Filters
Software: PHD2; APT; PixInsight ***** My AP website: www.steveking.pictures
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