M101 in LRGB

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turfpit
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Re: M101 in LRGB

Post by turfpit » Sun Mar 31, 2019 10:29 am

That is a nice M51 Andy.

Dave

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oopfan
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Re: M101 in LRGB

Post by oopfan » Sun Mar 31, 2019 12:58 pm

Hi Andy,

When you are sampling the peak ADU of a star, are you zooming in to make sure that you are picking the right one?

Thanks,
Brian
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oopfan
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Re: M101 in LRGB

Post by oopfan » Sun Mar 31, 2019 2:11 pm

Andy,

One more thing:

In the image screenshot you posted, FITS Liberator set the initial white level at 1800.01 but notice that there are lots of pixels brighter than that in the histogram. One thing you can do is check the box labeled "White clipping (green)" middle right-hand side of the app. Zoom out to show the whole image and then increase the white level by dragging the slider or typing in a new value. Gradually increase it and watch how the green pixels decrease in number. You will eventually reach a point where you've identified the brightest star in the field. I'd like to know what star that is and what is the pixel value?

Thanks again,
Brian

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Re: M101 in LRGB

Post by AndyBooth » Sun Mar 31, 2019 5:17 pm

See private message Brian.
Thx

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Re: M101 in LRGB

Post by AndyBooth » Sun Mar 31, 2019 7:24 pm

Thanks Dave.

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Re: M101 in LRGB

Post by oopfan » Sun Mar 31, 2019 9:19 pm

Hi Andy,

Thanks for the FITS file. Here is what I discovered:

1. The maximum pixel value of 55920 as reported by FITS Liberator are hot pixels, not stars.
2. The brightest star in the frame that I could match against C2A planetarium software is magnitude 11.015 at maximum pixel value of 22800 ADU.

I located three fainter stars having similar color temperature:
1. Magnitude 12.354 at 8125 ADU.
2. Magnitude 13.081 at 4605 ADU.
3. Magnitude 15.068 at 1820 ADU.

I ran curve-fitting algorithms on those data and came to this conclusion:

1. Using the same 300-second exposure you can capture stars as bright as magnitude 8.5 before saturating.
2. Given your sensor's read noise and full well depth of 45,000 electrons you can expect a dynamic range of 10 magnitudes.

Don't feel constrained by the 8.5 magnitude saturation limit. Saturate if necessary to capture the DSO but know that if you can't crop out bright stars you will get "star bloat" when you go to stretch the image.

More importantly this means that if you have dark skies don't be afraid of 15-minute or 30-minute exposures if your guiding is good. Of course if the DSO is bright then those long exposures will do more harm than good. To repeat, if you are looking for the highest signal-to-noise ratio then go for the long exposures. If light pollution is a problem you can still get benefits from longer exposures but only marginally so. As always, experiment. It appears as if 300-second exposures are winners but then again M51 is fairly bright. Try a fainter target like M101.

Good luck!
Brian

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Re: M101 in LRGB

Post by AndyBooth » Mon Apr 01, 2019 7:11 am

Thanks for the analysis Brian, very interesting and valuble to know.
I will take your advice and try M101 next, its a bit higher at start of darkness now UK has moved to summer time, but
Later nights though :roll:

Thanks again.

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