M81 Bode's Galaxy in 11.6 hours

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M81 Bode's Galaxy in 11.6 hours

Post by oopfan » Mon Feb 24, 2020 8:12 pm

William Optics ZenithStar 71mm f/5.9 (not APO)
Atik 314E CCD (Read Noise 5.3e-, Full Well Depth 13400e-)
Optolong LRGB filters

Over five sessions:
L: 250x90s bin1, 6.25 hours, SNR 17.7
R: 90x85.3s bin2, 2.13 hours, SNR 10.7
G: 90x50.2s bin2, 1.26 hours, SNR 10.6
B: 90x77.8s bin2, 1.95 hours, SNR 10.5
Total Integration time: 11.59 hours
Total target SNR: 25.6

M81_L250x90s_R90x85.3s_G90x50.2s_B90x77.8s_Rx0.83_Gx0.90_Bx1.00_S15-3-25_SA25-25_G92_SGB2_SH70_2020-02-24.jpg
M81_L250x90s_R90x85.3s_G90x50.2s_B90x77.8s_Rx0.83_Gx0.90_Bx1.00_S15-3-25_SA25-25_G92_SGB2_SH70_2020-02-24.jpg (378.75 KiB) Viewed 173 times
A quick survey of AstroBin revealed a startling variation of colors from members' images of M81. One would think that there would be consensus. Top Picks did converge on a color scheme where the core is yellow and the arms are blue. I was keen on duplicating that result with little post-processing since I meticulously "white balance" my RGB filters and camera. (This explains the strange looking exposure times that I use.)

After capturing 11.6 hours in LRGB I went to work processing the data in AstroPixelProcessor (APP). The result was underwhelming. All regions of the galaxy were predominantly yellow/red with just a hint of the outer arms wanting to go blue. It was a mystery since I've imaged plenty of targets with very good results. What was different this time? Two big differences: I never image facing north to north-north-east, and I never image so soon after astronomical twilight.

What follows is an interesting topic but after some more thought I believe it fails to explain this anomaly. Be sure to read my post at the top of the second page which proposes a better explanation IMHO.

Astronomical twilight ends at 7:15pm this time of year so I started at 7:30pm and ended by 10:30pm before M81 became "meridian-challenged". East of the meridian I always start with the red filter, then the green, and then blue, due to atmospheric extinction. So I imaged in red from 7:30pm to 8:30pm.

What is a common human activity at this time of night? That's right, commuting and/or going out-and-about in your car. My house is above the road but as cars approach their headlights briefly reflect off of a large tree just to the right of where I imaged M81. My guess is that particles in the air, most likely water vapor, reflected some of that light down my telescope tube, onto my sensor.

The next likely culprit is 2 miles away where there is an athletic field illuminated by stadium lights. That time of night I know that the lights are on but after some time the authorities turn off the lights because there are homes (i.e. taxpayers) nearby. The lights were probably shut down shortly after capturing reds.

The bottom line is that my reds contain extra signal from those pollution sources. I could see it. The sky background of a red frame was running around 1100 ADU. Compare that to a blue frame at 900 ADU. Normally they are the same.

I used a trick described by Mabula Haverkamp to discover the color content of an image: turn on saturation, set the saturation amount to maximum, and the saturation threshold to zero. There I could see blue in the arms.

I stepped back a step to the "Combine RGB" tool. My first attempt was to simply boost the blue channel. Unfortunately that result was not pleasing. The second attempt was to keep blue at unity but attenuate red. That was the secret. It was a tad bit green so I attenuated it slightly.

The attached image is the result of that small effort. Normally I don't have to do it since the "white balancing" of my filters takes care of it.

According to a brief survey I took of Top Picks and Image Of The Day (IOTD) winners, winning images have something in common: SNR > 25.

For all intents and purposes my image is competitive with regards to SNR. Where I fail are at the following:
1. The telescope is not an APO. It does not focus blue light very well which explains the blue fringes you see around my stars.
2. My small Full Well Depth easily saturates bright stars in the field. Those are the big fat discs that you see. The only way to combat saturation is to lower exposure time of an individual frame. The unfortunate consequence is that it increases total integration time.

My Bortle 5 skies and f/5.9 scope are the cause of these long integration times. I should make an effort at finding a Bortle 4 travel site.

All comments welcome!

Thanks,
Brian
Last edited by oopfan on Sat Mar 07, 2020 3:56 pm, edited 4 times in total.

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Re: M81 Bode's Galaxy in 11.6 hours

Post by turfpit » Mon Feb 24, 2020 10:15 pm

Brian

Well done with the M81 image and having the stamina to acquire > 10 hours of LRGB data! Thanks for an interesting and informative write-up on your battle with the object. I note Holmberg IX is just appearing centre above M81.

I think your image is comparable with this Astrobin Top Pick https://www.astrobin.com/385501/0/, a 30 hour integration captured at Bortle 4 and Bortle 2 sites.

Dave

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Re: M81 Bode's Galaxy in 11.6 hours

Post by oopfan » Mon Feb 24, 2020 10:26 pm

Thanks, Dave.

Yes, I used that image as a reference. Next galaxy I'll pay greater attention to building up more integration time in the color stacks.
I was fortunate to have four clear nights in a row. All the equipment worked flawlessly. One user error from which I was able to recover :oops:
Holmberg IX, definitely not a Bortle 5 target!

Brian

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Re: M81 Bode's Galaxy in 11.6 hours

Post by mAnKiNd » Mon Feb 24, 2020 10:53 pm

Superb!

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Re: M81 Bode's Galaxy in 11.6 hours

Post by oopfan » Mon Feb 24, 2020 11:26 pm

Thank you, Minos!

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Re: M81 Bode's Galaxy in 11.6 hours

Post by mAnKiNd » Tue Feb 25, 2020 12:09 am

Brian,

Did you acquire the L channel from 730pm to 1030pm?

If so, could you inspect individual L frames and find out if you had an increase in background ADU over time, especially between 730 and 830pm, when you also mention that you acquired the Red channel during light polluting commuters.

If there is an increase during those hours and then a drop after, perhaps you can identify the precise ADU value change in background and then decrease the Red channel accordingly. This might be a way to quantify the change caused by 730 to 830pm light pollution and provide a means for accurate decompensation in the Red channel, perhaps by normalising the values from the two channels and using the % change occuring in the L channel.

Just a thought.
Minos

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Re: M81 Bode's Galaxy in 11.6 hours

Post by BlackWikkett » Tue Feb 25, 2020 12:30 am

Well done Brian!
-Chris

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Re: M81 Bode's Galaxy in 11.6 hours

Post by oopfan » Tue Feb 25, 2020 3:04 am

Thanks, Chris! I appreciate it.

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Re: M81 Bode's Galaxy in 11.6 hours

Post by oopfan » Tue Feb 25, 2020 4:11 am

Hi Minos,

Well this doesn't prove that human activity affected my reds but it doesn't eliminate the possibility. It warrants more number crunching. Here is a graph. I'll explain where I got these numbers in a moment:
Does human activity affect M81 reds.jpg
Does human activity affect M81 reds.jpg (57.57 KiB) Viewed 136 times
The x-axis is the number of minutes past 7:30pm local time. The y-axis is the corrected sky background ADU.

I captured a total of 250x 90-second frames over three sessions: January 29, February 21 and 22. The first and last session I dedicated myself to capturing only luminance frames. The middle session I captured luminance after first capturing reds, greens, and blues.

It appears as if there is greater variance earlier in the evening. This could be due to the following:
1. Human activity (car lights and domes of light pollution near the horizon)
2. Natural atmospheric fluctuations at low altitudes

One thing that is clearly known, I always capture reds first, then greens, blues, and luminance. The reason why is because red light has lower atmospheric extinction than blue light at the same altitude. In others words blue frames will be highly attenuated if captured first but If captured in the order of red, green, then blue, the extinction coefficients are very nearly the same.

I've debated whether to stop the practice of capturing data as soon as it clears the trees. The quality of data clearly improves at higher altitudes. The thing that stops me is not having access west of the meridian.

Raw Data (see notes 1, 2)

Session 1, January 29, 2020
Frame 1, 8:55:57pm, 1195 ADU
Frame 50, 10:34:20pm, 1005 ADU
Frame 100, 11:54:38pm, 915 ADU

Session 2, February 21, 2020
Frame 1, 9:53:38pm, 1063 ADU
Frame 22, 10:28:49pm, 1017 ADU
Frame 44, 11:03:04pm, 978 ADU

Session 3, February 22, 2020
Frame 1, 7:59:54pm, 1190 ADU
Frame 53, 9:22:44pm, 1069 ADU
Frame 106, 10:47:21pm, 972 ADU

Data Reduction - Correct ADU counts for Atmospheric Extinction (see note 3)

Session 1, January 29, 2020
Frame 1, 8:55:57pm, alt 46 56 xtinc 1.06 corr 1267 ADU
Frame 50, 10:34:20pm, alt 55 15 xtinc 1.035 corr 1041 ADU
Frame 100, 11:54:38pm, alt 60 19 xtinc 1.024 corr 937 ADU

Session 2, February 21, 2020
Frame 1, 9:53:38pm, alt 58 40 xtinc 1.028 corr 1092 ADU
Frame 22, 10:28:49pm, alt 60 32 xtinc 1.024 corr 1041 ADU
Frame 44, 11:03:04pm, alt 61 44 xtinc 1.022 corr 999 ADU

Session 3, February 22, 2020
Frame 1, 7:59:54pm, alt 50 19 xtinc 1.049 corr 1248 ADU
Frame 53, 9:22:44pm, alt 56 55 xtinc 1.031 corr 1103 ADU
Frame 106, 10:47:21pm, alt 61 24 xtinc 1.022 corr 994 ADU

Notes:
1. Mean ADU counts obtained using Siril by drawing a same-size rectangle on the same area of sky background devoid of stars.
2. ADU counts include bias (approximately 220 ADU). Frames not calibrated by using darks or flats. Filters and sensor window are clean, no vignetting. CCD was cooled (30 degrees below ambient, no set-point).
3. Atmospheric Extinction is an estimate of the amount of atmosphere between you and the star. It is a function of altitude/elevation of the star above the horizon. By multiplying the raw ADU counts by this coefficient you are essentially transforming each measurement as if it were taken at the zenith.

Closing Statement

Up until today I have had nothing but great success using my white balance ratios that I meticulously measure against a G2V star. The only difference this time around is:
1. 11.6 hours total integration time. (All previous endeavors have amounted to only a couple hours.)
2. Relatively low signal-to-noise ratio of the color stacks (SNR 11) versus the luminance stack (SNR 21).
2. Imaging in the direction of North, NNE.
3. Relatively early in the evening, sometimes just after astronomical twilight.

Brian

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Re: M81 Bode's Galaxy in 11.6 hours

Post by mAnKiNd » Tue Feb 25, 2020 5:01 am

That's great Brian and as always, thank you kindly for the comprehensive answer.

I stratified your corrected background ADU values into two groups: 8-10pm and 10-12pm and applied an unpaired, parametric t-test between the two and found a significant difference between the two groups.

Here is the report:
Column B 10-12pm
vs.
Column A 8-10pm

Unpaired t test
P value 0.0058
P value summary **
Significantly different (P < 0.05)? Yes
One- or two-tailed P value? Two-tailed
t, df t=4.177, df=6

_________________8-10pm____10-12pm
Number of values____3__________5
Median___________1248________999.0
Mean_____________1202_______1002
Std. Deviation____96.02______42.83
Std. Error of Mean__55.44_______19.16

How big is the difference?
Difference between means (B - A) ± SEM -199.9 ± 47.87
oopfan_corrected_adu_mean_SD_early_vs_late_night.jpg
oopfan_corrected_adu_mean_SD_early_vs_late_night.jpg (11.02 KiB) Viewed 130 times
Data presented as mean (standard deviation)

What I find most interesting, is not that the two groups are significantly different, that is to be expected mostly due to reasons you've clearly outlined above, but rather that the standard deviation between the two groups is more than twice as large from 8-10pm vs. 10-12pm. This confirms the greater variance on your data acquisition during those times as a function of "humans commuting and/or going out-and-about in their cars", amidst other nocturnal activities that our species engages in with light bulbs involved.

All that said and done, congrats again on a hard-worked and well earned image borne from a solid methodological and spiritful approach. Keep them coming :)

Minos

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