M51 Two Images Two Nights

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oopfan
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M51 Two Images Two Nights

Post by oopfan »

Here is a good example of how atmospheric turbulence can affect LRGB astrophotography. In the first image, the atmosphere was steady from beginning to end. In the second image, the atmosphere was turbulent in the beginning, but quickly steadied. The first filter I use is Red, so it was most affected by the turbulence. As a consequence, red photons in the second image were spread over a greater number of pixels, and therefore lowered its signal-to-noise ratio.

Steady atmosphere:
m51_w12-83x60s-b1_r-48x60s-b1_g-20x60s-b1_AFF.jpg
m51_w12-83x60s-b1_r-48x60s-b1_g-20x60s-b1_AFF.jpg (540.37 KiB) Viewed 206 times
Turbulent atmosphere during red capture, followed by calm:
M51_w12-26x150s-b1_r-6x300s-b1_g-3x300s-b1_AFF.jpg
M51_w12-26x150s-b1_r-6x300s-b1_g-3x300s-b1_AFF.jpg (580.16 KiB) Viewed 206 times
Technicals:
William Optics 71mm f/5.9
Atik 314E

First image:
Wratten #12: 83x60s
Red: 48x60s
Green: 20x60s
Integration Time: 151 minutes
Total SNR: 6.64

Second image:
Wratten #12: 26x150s
Red: 6x300s
Green: 3x300s
Integration Time: 110 minutes
Total SNR: 7.40

Processing:
Astro Pixel Processor (APP): calibration, registration, integration, color combine.
Affinity Photo: de-noise, black level, curves.

Brian
timh
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Re: M51 Two Images Two Nights

Post by timh »

Hi Brian, Pretty good despite the turbulence. I was wondering if you would just try stacking them all together but I see that you have done that in your follow up post.

Clearly galaxies should be the sort of objects where longer focus and image scale should deliver an advantage in terms of resolution. I thought long and hard before getting a f = 1000 mm Newt because it meant getting a bigger mount but decided in the end longer focus stuff covered most of the type of objects that I like to view and so went ahead.

Nevertheless I have never been quite sure whether, in practice, shorter focus instruments like yours might do about as well in terms of resolution? Although my image scale is 1 arc sec/ pixel , in practice, even with steady skies and good guiding (within 0.8 arsec RMS) I have never gone below an overall FWHM resolution of about 2.8 arc sec. For an 8 inch scope Dawes limit doesn't come into it but, presumably, your 71 m refractor is inherently Dawes limited to about 2 arc sec - but that number is still probably below the sky + mount plus optics practical limit to resolution. Further, even if your image scale is up above 2 arc sec/ pixel or whatever there is also the possibility of reducing that by dithering and drizzling? Is it a myth that longer focus instruments resolve finer detail when used for deepsky as opposed to lucky imaging type astrophotography? I am not sure - my understanding may be flawed - and I do really like the large telescope because it is fast as well as long focus and a lot of astrophotography seems to be about taking advantage of short windows of opportunity.

Tim
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Re: M51 Two Images Two Nights

Post by oopfan »

Hi Tim,

I have only anecdotal evidence to support the assertion that no matter the aperture, one can only expect to do no better than what the current seeing conditions allow. One exception is lunar and planetary imaging. My yearning for more aperture stems from a desire to capture more photons per unit time. With my current kit, it takes nearly 10 hours of integration time to achieve a decent image of bright galaxies like M51 and M81. Still, one can choose to sacrifice image quality for the benefit of seeing deeper by using binning. My next mini-project is an effort to capture the Integrated Flux Nebula (IFN) around M81 using bin3. From my research I believe it can be done. Image quality is in the toilet, but who can say that they captured the IFN with a 71mm scope under Bortle 5 skies? I'd like to be that person.

Brian
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Re: M51 Two Images Two Nights

Post by Menno555 »

oopfan wrote: Tue Apr 06, 2021 1:24 am ....
My next mini-project is an effort to capture the Integrated Flux Nebula (IFN) around M81 using bin3. From my research I believe it can be done. Image quality is in the toilet, but who can say that they captured the IFN with a 71mm scope under Bortle 5 skies? I'd like to be that person.

Brian
Thanks for the turbulence comparison. Over here I don't take that into account anymore. There are so few clear nights, that I take every opportunity. Having said that: I have started with IC 2574 Coddington's Nebula and that's SO faint, that I can only capture it with the best, most steady, nights.

On the IFN: two weeks ago I did decided that too as a project :) I know that with my focal length and Bortle skies are totally not in my favor, but with reducer (around f7 for me then) and only a IR/UV Cut filter and integration time (and like you said: likely with a lot of noise) it should be possible.
Decided for the dense part in Ursa Major for that: just above the line from M82 to the star Althiba VII (HIP 46977). And (maybe) also use it for my first mosaic. But will be a long during project.

Menno
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Re: M51 Two Images Two Nights

Post by oopfan »

Hi Menno,

In this photo of M81/M82/IFN, I indicated the position of HIP 46977. Is the dense part of the IFN above the top boundary of the image? I was planning on the area within the red rectangle.

A few days ago I searched the InnerWeb for discussions of the IFN, and particularly surface brightness. I found an interesting exchange at CloudyNights where someone claimed to have seen it visually with a 36-inch scope. Of course, everyone piled on and said it was impossible. There was one person who seemed to know what he was talking about, saying that the IFN was 26 mags per square-arcsec. However, even he injected some doubt about the veracity of that number. I plugged it into my calculator. 26 mags is beyond my camera's ability. LP will saturate the sensor. However, if it is 25 mags, then I have a shot at it. A 600-second exposure at bin 3 should cause LP to consume 30,000 ADU (about 7,000 electrons). The IFN should be at the limits of detection in a single frame. Of course, the key to success is your camera's Full Well Depth. Mine is only 13,400 electrons. It would be great if I had a deeper well, like 25,000 or 50,000 or 100,000 electrons would be ideal.

Menno, you've got a lot more aperture than I do, so don't use my 600-second exposure. Yours will differ. In my opinion, increase the exposure until LP consumes half of your dynamic range (about 30,000 ADU). Take a single test frame. Stretch it. Avert your vision. If you can't see the IFN, then your chances are low that you will capture it in a stack, but give it a try. If you've got a CMOS camera, select the lowest gain setting. You want the deepest well you can get. If tracking/guiding is a problem with long exposures, then switch to bin 2. The key is to find the exposure that brings LP up to 30,000 ADU. Of course if the IFN is clearly visible in a single frame, then lower the exposure.

Good luck,
Brian
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Project IFN.jpg
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Last edited by oopfan on Tue Apr 06, 2021 5:53 pm, edited 1 time in total.
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Re: M51 Two Images Two Nights

Post by Menno555 »

Thanks for the tips Brian! Especially the ADU and Gain value. My record for tracking only is now 535 seconds, so that should/could work really as maximum but preferably less yes. I'll have to wait and see, also when it comes to the light pollution here.
I do know that my camera has an advertised 46.000e full well.
I just give it a try and see what happens. That's my way of working anyway, so it's totally for me this :) If it works, it works and otherwise it's just not for me :)
And no, the dense part I think of, is visible better here: http://www.astrosurf.com/jguimond/engli ... 82_en.html, the long stretch on the right.
But maybe going for M81/M82 now that I think of it. The pair just fits in my FOV with the reducer. Ah well, time enough. First I want to capture more data of M106: have 11 hours of that now and aiming for around 16 hours :)
In FOV it's around the screenshot below, which is with the reducer in mind.

Menno
ifn.jpg
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oopfan
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Re: M51 Two Images Two Nights

Post by oopfan »

Menno,

Ah yes, I see the IFN is much brighter up there. Maybe I'll give that a try. The trick is to find a FOV that has a filament structure with light and dark areas. If you choose a region that is just solid nebula, then you won't be able to distinguish LP from the IFN.

The author of that image at the link is from Portneuf, Quebec. If he lives in Portneuf proper, then he is at Bortle 6. If is in the suburbs, he is at Bortle 5. If he is rural, then it is Bortle 3 or 4.

I looked at his specs. He says the camera is a SBIG STL11Km. A search turned up empty. However I believe he is playing loose with the naming. Really, it is an SBIG STL11000m. He replaced "000" with "K". Talk about a geek! The specs for that camera say the pixel size is 9 microns. It looks to me like he is seriously under-sampling given his short focal length of only 381mm. However, under-sampling works to his advantage when imaging faint nebulae. It is equivalent to me running at bin2. He used 360-second exposures in luminance. He's got a 4-inch aperture. Mine is 3 inches. His camera has got a 50,000 electron Full Well Depth. Mine is 13,400 electrons. I think that it is quite likely that I can image it at bin3!

Thanks,
Brian
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Re: M51 Two Images Two Nights

Post by oopfan »

Menno,

I think I am going to go for this:

Brian
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Re: M51 Two Images Two Nights

Post by Menno555 »

Brian

Again thanks. To make it more interesting, I found a Cloudynight post that says the opposite, to go as high as possible with Gain, see https://www.cloudynights.com/topic/6899 ... try9904308.
For me this makes more sense too. I had longer exposures of 400 seconds with Gain 0 and the ADU was around 300 (what I can remember). But I also had a low Offset/Brightness of 4. Maybe the Offset/Brightness also has to be raised?

Edit
Just found this one: https://www.flickr.com/photos/terryhancock/33892130255
Not comparable at all with setup and location but a very high gain and high offset.
And of course I will come nowhere close to that ;) But even if I can capture a whiff of the IFN, I'll be satisfied.
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oopfan
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Re: M51 Two Images Two Nights

Post by oopfan »

Menno,

This is how I see the problem:
IFN formula.jpg
IFN formula.jpg (32.33 KiB) Viewed 112 times
At cell B1, I am giving you the benefit of the doubt that your Bortle class is halfway between Bortle 7 and 6.
At cell B2, I am giving us the benefit of the doubt that the IFN's surface brightness is 25 mags per square-arcsec.

At cell B4, we use one of the most fundamental equations in Astronomy:
m1 - m2 = -2.5 * LOG10(L1 / L2)
therefore L1 / L2 = POWER(10, (m2 - m1) / 2.5)
In other words, you suffer 263 electrons of LP for each electron of IFN.

At cell B6, this is your camera's Full Well Depth (FWD).
At cell B7, this is half your FWD.

At cell B9, this is the total number of IFN electrons captured per frame.
In other words, Dark Space is 25000 electrons, and IFN plus Dark Space is 25095 electrons.

Now, to compute Total Noise:
At cell B11, this is Sky Noise, the square root of cell B7.
At cell B12, this is Shot Noise, the square root of cell B9.
At cell B13, this is your Read Noise.
At cell B14, this is the Total Noise (all three noise sources added in quadrature, ignoring Dark Noise.)

Now for the Signal-to-Noise Ratio:
At cell B15, the Signal (95 electrons) divided by Total Noise (158 electrons) equals 0.60.
This is borderline. With plenty of stacking you should see it.

The problem with the CloudyNights analysis is if you raise the Gain, then it lowers your FWD (cell B6).
If you raise the Gain such that FWD is 10000 electrons, then the signal-to-noise ratio plunges to 0.27.
Raising your Gain even more continues to lower your SNR.

Brian

PS: By all means, follow CloudyNights. I would love to be wrong.
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