M101 in LRGB

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oopfan
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Re: M101 in LRGB

Post by oopfan » Fri Mar 29, 2019 4:08 pm

I just completed an analysis of my sub-frames. I wanted to get to the bottom of why my image doesn't show more blue in the outer galactic arms. This is what I found:

My 36-second exposures with the blue filter is just not enough to raise the signal out of the noise. Sure, on close examination you can see blue in some of the "hot spots" but the pervasive "blue glow" is not to be seen; instead it is replaced by a muddy brownish color.

If my camera was cooled then I would see more blue. If my camera was cooled and I doubled the exposure then I would see even more blue. But remember to get the color balance right I would need to double the exposure of the red and green channels too.

My recommendation to you or anyone considering imaging M101 -- save yourself the grief and double your exposure if at all possible.

Brian

AndyBooth
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Re: M101 in LRGB

Post by AndyBooth » Fri Mar 29, 2019 8:31 pm

Interesting Brian.
I have taken 50 frames of m51, and m81, its the most frames i have managed so far with it.
All at 300secs through the QHY10, cooled to -20.
Havent had time to process yet though, hopefully this weekend.
From my location, the spiral arms arms are only about 10% brighter than sky background.

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oopfan
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Re: M101 in LRGB

Post by oopfan » Fri Mar 29, 2019 11:02 pm

Andy,

You are in very good shape. Your camera has a full well depth of 45,000 electrons. It can handle long exposures. My camera on the other hand is 7,000 electrons, so I can easily put stars into saturation which leads to star bloat and incorrect star color rendering. Keep going with those long exposures! How is your light pollution?

Brian

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Re: M101 in LRGB

Post by AndyBooth » Sat Mar 30, 2019 8:35 am

Brian,
I have at best Bortle 5, on the darkest nights.
A lot of new LED lights are now the norm in the area.

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Re: M101 in LRGB

Post by oopfan » Sat Mar 30, 2019 12:53 pm

I reworked the image after having discovered that the blue stack was not contributing enough to the final image due to low signal-to-noise. Next time around I will double the exposure on all channels.

Difference between this image and the original on page one:
1. Boosted blue channel by 15%.
2. Boosted luminance channel by 100%. (I had systems issues the second night that thwarted my attempt to capture more.)

Unfortunately all of this boosting also boosts noise.

Here it is:
Attachments
M101_151m_LRGB_version_4.jpg
M101_151m_LRGB_version_4.jpg (323.89 KiB) Viewed 624 times

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oopfan
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Re: M101 in LRGB

Post by oopfan » Sat Mar 30, 2019 1:08 pm

Hi Andy,

I am at Bortle 5 consistently but some nights push Bortle 4. No street lights or anything but I am in a valley.

Brian

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Re: M101 in LRGB

Post by AndyBooth » Sat Mar 30, 2019 11:37 pm

Nice M101 result Brian.

Here is my first pass at m51 with QHY10 , had problems with guiding and my calibration frames so only 40 frames at 300secs, and i binned it down a notch in processing to help with colour mottling noise.

Still, my longest exposure total for any object, and i like the detail i got in the arms and ha regions .
No filters were used, straight OSC , and there was high cirrus cloud.
This was actually at f8 , 0.8” per pixel.
Attachments
D8D27F44-1723-46D7-9B66-17D826A69773.jpeg
D8D27F44-1723-46D7-9B66-17D826A69773.jpeg (126.32 KiB) Viewed 611 times
Last edited by AndyBooth on Sun Mar 31, 2019 6:50 am, edited 1 time in total.

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oopfan
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Re: M101 in LRGB

Post by oopfan » Sun Mar 31, 2019 12:57 am

Andy,

Wow! That's 200 minutes total integration time in one night, right? I can't wait for my Atik!
Did you save the light frames as FITS? If you did can you do me a favor? Can you bring up one of the light frames in FITS Liberator, take a screenshot, choose a few of the brightest stars and tell me the peak pixel value of each? Sort of like the attached:

Thanks,
Brian.
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NGC_4565_star_peak_adu.jpg
NGC_4565_star_peak_adu.jpg (224.68 KiB) Viewed 610 times

AndyBooth
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Re: M101 in LRGB

Post by AndyBooth » Sun Mar 31, 2019 8:30 am

Thanks Brian,
Yes 200 mins total.

Sorry for my amateur attempt at screen shot!
This is un bayered raw frame f8 300 secs, 200mm OD

Sky adu = 1120
Spiral arm adu = 1300

Star 1= 1152
Star 2= 1158
Star 3= 1143
Star 4= 1229

Galaxy core 5 = 1397

Brightest star in whole image is 1309

So alot of slack to go!
Attachments
19EDF5E0-8AC0-45D9-8C53-B73EB977287E.jpeg
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oopfan
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Re: M101 in LRGB

Post by oopfan » Sun Mar 31, 2019 10:12 am

Thanks, Andy. I am trying to measure the effects of the lower QE of CCD vs CMOS but there are too many variables. CCD has lower quantum efficiency than CMOS but I've read that it roughly makes up for it in larger pixel size. I'll just have to wait for my camera to arrive. I want to take advantage of the deeper well but I am concerned that the lower QE will require longer exposures than my mount can handle. We will just have to see won't we :-) Thanks for taking the time to run this experiment!

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