Narrowband Imaging - Results for M27
Posted: Sun May 09, 2021 5:48 pm
Last night was cold and humid, but I got the results I expected for M27 with only 2 hours integration time.
In preparation for last night's effort, I analyzed the scouting images using Photometry and AstroImageJ. I measured the density of the nebula, and then used it to calculate the Photon Flux Density (PFD) of each filter (units of photons per square-centimeter per second.) Think of PFD as a fundamental constant of M27. It is a measure of the number of Ha photons hitting the Earth's upper atmosphere. Part of the magic is removing the dependencies on the telescope and camera used to measure it. Now that I have the fundamental PFD values for M27, I can run them through another calculator that can provide guidance on exposure and integration times for ANY telescope. At least that is the theory!
In the scouting images, you can see that OIII is much stronger than Ha. There are a couple reasons for that. Planetary nebulae are very strong in OIII, plus my camera's sensor is much more sensitive to green wavelengths (OIII) than red (Ha). Those combined give the impression that M27 is saturated in OIII. The strength of OIII is a fundamental property of M27, but the difference in Quantum Efficiency (QE) was factored out when calculating PFD.
There is something I did that gives last night's image a "balanced" appearance. I used my calculator to adjust each filter's integration time so that the Signal-to-Noise Ratio (SNR) of the two stacks are equal. Here they are:
Ha-7nm: 16x300s bin 1
OIII-7nm: 8x300s bin 1
Notice how I captured half the number of OIII frames. I did that for two reasons:
1. OIII is much stronger than Ha.
2. My sensor is more sensitive to green than red.
Finally, as you can see I'm just starting to pick up the outer shell of M27. The only cure is more integration time. Doubling the exposure would help reduce the total time, but my auto-guiding is not up to it.
Brian
A couple nights earlier, I captured "scouting" images in Ha, OIII, and SII:
You can see the low quality, but that was by design. I was interested in capturing the most nebulosity instead of creating a museum-quality image. I captured one frame in each filter at bin 2 with 600s exposure. It quite clearly shows that SII is very weak compared to Ha and OIII. I took that into consideration when I decided to only capture Ha and OIII last night.In preparation for last night's effort, I analyzed the scouting images using Photometry and AstroImageJ. I measured the density of the nebula, and then used it to calculate the Photon Flux Density (PFD) of each filter (units of photons per square-centimeter per second.) Think of PFD as a fundamental constant of M27. It is a measure of the number of Ha photons hitting the Earth's upper atmosphere. Part of the magic is removing the dependencies on the telescope and camera used to measure it. Now that I have the fundamental PFD values for M27, I can run them through another calculator that can provide guidance on exposure and integration times for ANY telescope. At least that is the theory!
In the scouting images, you can see that OIII is much stronger than Ha. There are a couple reasons for that. Planetary nebulae are very strong in OIII, plus my camera's sensor is much more sensitive to green wavelengths (OIII) than red (Ha). Those combined give the impression that M27 is saturated in OIII. The strength of OIII is a fundamental property of M27, but the difference in Quantum Efficiency (QE) was factored out when calculating PFD.
There is something I did that gives last night's image a "balanced" appearance. I used my calculator to adjust each filter's integration time so that the Signal-to-Noise Ratio (SNR) of the two stacks are equal. Here they are:
Ha-7nm: 16x300s bin 1
OIII-7nm: 8x300s bin 1
Notice how I captured half the number of OIII frames. I did that for two reasons:
1. OIII is much stronger than Ha.
2. My sensor is more sensitive to green than red.
Finally, as you can see I'm just starting to pick up the outer shell of M27. The only cure is more integration time. Doubling the exposure would help reduce the total time, but my auto-guiding is not up to it.
Brian