M31, M45, 180s exposures *unguided*

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turfpit
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M31, M45, 180s exposures *unguided*

Post by turfpit » Mon Oct 15, 2018 1:08 pm

The following were captured on 14/15 October 2018.
  • Altair Astro 66ED refractor, Altair 183C V2 camera, Celestron AVX mount, Planostar 0.8x flattener/reducer.
  • Capture software SharpCap v3.2 beta.
  • 2 alignment stars + 4 calibration stars.
  • All Star Polar Align via hand controller.
  • Focus with Bahtinov Mask, unguided.
  • Bias frame checked with FITS Liberator to ensure balanced histogram, used to decide black level setting.
  • Light frame checked with FITS Liberator to ensure Image Statistics min > 0 and max < 65535.
  • Stack produced using Siril plus colour calibration, SCNR & colour saturation.
  • Stack finalised with GIMP 2.10. About 30 minutes each for calibration and processing.
  • Flats taken with home made EL panel, £25.
  • M31, M45, 20x180s lights, 20 darks, 50 flats, 50 bias.
  • Both images gain=200, black level = 150.
Next time I will keep 180s and try gain=400 & 200.

High-res M45 here https://www.astrobin.com/370439/E/. Note the differences over the revisions.

Images and SharpCap capture settings below.

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turfpit
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Re: M31, M45, 180s exposures *unguided*

Post by turfpit » Mon Oct 15, 2018 1:10 pm

M31, thin cloud drifting in during this capture.
_M31-20x180s-g200-bl150.jpg
_M31-20x180s-g200-bl150.jpg (107.8 KiB) Viewed 704 times
[ALTAIRH183C]
Debayer Preview=On
Output Format=FITS files (*.fits)
Capture Area=5440x3648
Binning=1x1
Pan=0
Tilt=0
Colour Space=RAW12
Fan=On
Temperature=8
Black Level=150
USB Speed=2
Auto Exp Target=120
Frame Rate Limit=Maximum
Analogue Gain=200
Exposure=180000
Timestamp Frames=Off
Banding Threshold=35
Banding Suppression=0
Apply Flat=None
Subtract Dark=None
#Black Point
Display Black Point=0
#MidTone Point
Display MidTone Point=0.5
#White Point
Display White Point=1
TimeStamp=2018-10-14T20:14:57.5555570Z
SharpCapVersion=3.2.5781.0

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turfpit
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Re: M31, M45, 180s exposures *unguided*

Post by turfpit » Mon Oct 15, 2018 1:14 pm

M45, only 11 frames were able to be registered and stacked due to cloud forming. 180s minimum for M45 recommended to me by Gary Palmer at IAS - nice one Gary.
_M45-20x180s-g200-bl150.jpg
_M45-20x180s-g200-bl150.jpg (94.08 KiB) Viewed 703 times
[ALTAIRH183C]
Debayer Preview=On
Output Format=FITS files (*.fits)
Capture Area=5440x3648
Binning=1x1
Pan=0
Tilt=0
Colour Space=RAW12
Fan=On
Temperature=8
Black Level=150
USB Speed=2
Auto Exp Target=120
Frame Rate Limit=Maximum
Analogue Gain=200
Exposure=180000
Timestamp Frames=Off
Banding Threshold=35
Banding Suppression=0
Apply Flat=None
Subtract Dark=None
#Black Point
Display Black Point=0
#MidTone Point
Display MidTone Point=0.5
#White Point
Display White Point=1
TimeStamp=2018-10-14T20:14:57.5555570Z
SharpCapVersion=3.2.5781.0

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oopfan
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Re: M31, M45, 180s exposures *unguided*

Post by oopfan » Mon Oct 15, 2018 4:17 pm

Terrific!

Regarding M45, using my shorthand notation your previous effort used "120s_G400_BL50" compared to this effort using "180s_G200_BL150". Question: is there a reason why you upped the Offset from 50 to 150?

I find it extremely interesting that the total integration time of this image is the same as your previous effort: 60 minutes, that is 20x180s == 30x120s. You can see it here: viewtopic.php?f=16&t=1199&p=5986#p5949 . I know that you worked extensively post-processing that image. Your new image is so much better on many levels: darker background, more nebulosity, etc.

I would like everyone to please note that increasing exposure is almost always a good thing.

Brian

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Re: M31, M45, 180s exposures *unguided*

Post by oopfan » Mon Oct 15, 2018 4:31 pm

Dave,

I need some clarification. Regarding M45 you wrote that you could only capture 11 frames but the name of the file you uploaded is "_M45-20x180s-g200-bl150.jpg" So the total integration time is just 33 minutes? That's really quite impressive considering that your first effort was 60 minutes. My takeaway: long exposures good, short exposures bad!

Brian

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turfpit
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Re: M31, M45, 180s exposures *unguided*

Post by turfpit » Mon Oct 15, 2018 7:50 pm

Thanks Brian. Here you go with answers.

Offset
As you are aware I use the @oopfan FITS Liberator balanced histogram with a bias frame for systematic checking off the offset setting before commencing an imaging run.

On 9/10 Oct I had excellent conditions - clear sky all night, excellent visibility, no wind and no moon. I could see many more lights over Manchester than usual. The offset (black level) setting of 50 gave an optimal bias histogram with my equipment and the 183C. I checked the camera settings file and 50 is correct. the temperature recorded in the file is 13.8.

On 14/15 Oct the conditions were poor. Although I started with clear sky (or so I thought) There was weather coming in from the south east around midnight. Last night I had 3 goes at offset settings (50, 100, 150) before I had an optimum histogram for a bias. I had expected 50 again as with the cap on the scope external conditions should not matter. The temperature recorded in the setting file is 8.

I think this is something I will have to monitor over a period of time unless you have any ideas.

M45 11 frames
I captured 20 frames - when I went back to the camera it had clouded over. When I came to calibrate and stack in Siril, only 11 frames would register. Hence the stack comprises the 11 frames with an integration time of 33 minutes.

The capture settings for M45 came out of a discussion with Gary Palmer at the international Astronomy Show over the weekend.

It gets better - look at this for the M31 image. Using Siril, the 20 frames were calibrated, registered, stacked + cropped, colour calibrated, green noise removal, colour saturation applied + a histogram transform (stretch) applied. Time taken around 15 minutes. The resultant TIFF was loaded into GIMP. Not far from being the finished article. A further 15 minutes in GIMP resulted in the image above.
_straight-out-of-siril.jpg
_straight-out-of-siril.jpg (84.66 KiB) Viewed 688 times


This M31 and M45 are my first longer exposure images. What I have noticed in both cases was the resultant stack has needed less curves/levels manipulation. I am left with very little too do (I like that)! To me, this implies that the longer exposures are resulting in better quality data.

Look at the revision history for M45 at https://www.astrobin.com/370439/E/?nc=user

I have certainly had a quantum leap in my imaging results since experimenting with longer exposures. It will be interesting to see what I can capture with 180s frames in the Orion region. Of course, with longer exposures, the preparation needs to be meticulous - scope balance, polar alignment, mount align, focusing.

For anyone curious about the exposure debate try this simple experiment - M27: take 30x60s and 60x30s - same integration time, compare the results.

To date the tangible benefits I have seen from longer exposures (keeping the same integration time as previously) are, with larger frame cameras (5000x3000 @ RAW12 gives a 40Mb file), reduced disk space for capture and processing, faster calibration and stacking (less frames to process) and a better quality stack which results in reduced post processing.

As I start to use my Atik 314e and delve into narrowband I expect, in order to have decent results, to extend my expsoure times significantly.

Dave

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Re: M31, M45, 180s exposures *unguided*

Post by oopfan » Mon Oct 15, 2018 10:31 pm

Dave,

Thanks for the reply.

Regarding Offset, in my opinion its only function is to ensure that the energy distribution of an image is represented by positive real numbers. Shifting it farther to the right does not add value in my opinion.

I take Bias frames to determine the minimum Offset value. The energy distribution in a Bias frame is just a function of Gain and Exposure (the fastest for your camera), not seeing conditions or filters used in the process of taking Lights.

When taking Lights, Darks, and Flat frames you are using exposures many times longer than the minimum for the camera (mine = 0.125ms). Lengthening the exposure can have only one effect, that is to add energy, never to subtract it, so there is never a reason to increase Offset.

I do have a theory that may explain why you increased Offset above the minimum on 14/15 Oct during poor seeing conditions but first let's look at a histogram during normal seeing conditions. Normally, the blackness of space between stars records at the level of dark current which from experience is around 800 out of 65535 (uncooled camera). From that point onwards you begin capturing faint wisps of nebulosity and very faint stars. But if you have poor seeing conditions the "blackness of space" can be several magnitudes brighter than normal. So under normal seeing conditions you have a rapid rise rate from zero to dark current to faint features, but under poor conditions you experience a gradual rise rate from zero to sky brightness. I believe you think that your Offset vanished due to seeing conditions but in fact it was there.

Brian

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Re: M31, M45, 180s exposures *unguided*

Post by turfpit » Tue Oct 16, 2018 12:36 pm

Brian

Thinking about this, I should able to put the scope/camera on the bench and establish the offset for a balanced histogram in a bias frame i.e. shortest exposure possible in the capture software. Of course this setting would be for a specific gain and may need to change if gain changes? I can test that.

Dave

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Re: M31, M45, 180s exposures *unguided*

Post by oopfan » Tue Oct 16, 2018 12:49 pm

Dave,

Yes, that is correct. That is the method I used here:

viewtopic.php?f=16&t=1163#p5803

Brian

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Re: M31, M45, 180s exposures *unguided*

Post by turfpit » Tue Oct 16, 2018 1:05 pm

Thanks Brian - got that now.
"Set it and forget it" as Craig Stark says!

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