Tadpole Nebula (IC410)
Posted: Sun Dec 22, 2019 3:08 am
It's been a long time since I last posted to the SharpCap forum. I still use SharpCap but for purposes other than image capture. I do use it for polar alignment, and for periodic error correction determination of my 50 year old mount, but also recently for guiding assistance.
Within the past couple months I developed an interest in narrowband imaging mostly out of necessity due to the Moon. It turned out to be a lot more challenging than I thought. One of my heroines in the field is Sara Wager. I recommend her website for anyone seeking to discover the secrets to good NB imaging. I'd like to share some with you:
1. Stretch your stacks before combining them. You may have noticed that imaging in Hydrogen Alpha (Ha) is easy due to the strength of the signal. Oxygen III (OIII) and Sulphur II (SII) are relatively weak. To prevent Ha domination you should stretch your OIII and SII stacks before combining them into a single RGB image. By how much? It all depends on the target so experimentation is the key to success.
2. When combining stacks to create a color image try not to assign a stack to a single channel. For example the "HOO" palette says to assign Ha to Red but if you do that it will render as brilliant red. A more appealing color is scarlet to orange. You can accomplish this by assigning, say, 85% to red and 15% to green.
3. You need star size reduction software. There are lots of hot blue stars in the sky that strongly emit at the wavelength of OIII. You may have noticed that stars saturate easily in your OIII frames and therefore are fatter than stars in your Ha and SII frames. As a consequence your image will suffer from what I call "oxygen halo". Also fat stars detract from your subject. Photoshop and PixInsight have tools for reducing star size but they are expensive. StarTools also has a tool. Three years ago I purchased StarTools for $50 for a single license that never expires. It is still available for sale at the same price.
4. Exposure is your friend. Astrophotography is all about achieving the highest signal-to-noise ratio (SNR) in the least amount of time. Buying a faster scope isn't always an option given your budget so how about buying a new camera with a higher Quantum Efficiency (QE). Again it's costly which leaves only two choices: exposure and stack size. You will always achieve a higher SNR in less time by increasing exposure instead of increasing stack size. SNR doubles when you double the exposure but when you double the stack size SNR increases by only 41% (i.e. square root of 2).
Without further ado here is my take on the Tadpole Nebula (IC410).
6nm Ha 18x600s
6nm OIII 23x600s
7 hours total integration time
William Optics D71mm f/5.9
Atik 314E bin2
Astro Pixel Processor (APP) drizzle 2x
HOO palette
Ha stack: Red 85%, Green 15%
OIII stack: Green 35%, Blue 65% (stretched before combine)
Major complaint: Fuzzy details are due to a combination of bad seeing and bin2 capture. (Camera slightly undersamples at bin1 so bin2 is even worse)
How to interpret the colors:
1. Blue/Cyan on the left-most portion of the image is predominantly oxygen.
2. Scarlet/Orange near the right border is almost pure hydrogen.
3. Beige in the center of the nebula is a mix of hydrogen and oxygen.
4. The "tadpoles" have hydrogen tails and oxygen/hydrogen heads.
Brian
Within the past couple months I developed an interest in narrowband imaging mostly out of necessity due to the Moon. It turned out to be a lot more challenging than I thought. One of my heroines in the field is Sara Wager. I recommend her website for anyone seeking to discover the secrets to good NB imaging. I'd like to share some with you:
1. Stretch your stacks before combining them. You may have noticed that imaging in Hydrogen Alpha (Ha) is easy due to the strength of the signal. Oxygen III (OIII) and Sulphur II (SII) are relatively weak. To prevent Ha domination you should stretch your OIII and SII stacks before combining them into a single RGB image. By how much? It all depends on the target so experimentation is the key to success.
2. When combining stacks to create a color image try not to assign a stack to a single channel. For example the "HOO" palette says to assign Ha to Red but if you do that it will render as brilliant red. A more appealing color is scarlet to orange. You can accomplish this by assigning, say, 85% to red and 15% to green.
3. You need star size reduction software. There are lots of hot blue stars in the sky that strongly emit at the wavelength of OIII. You may have noticed that stars saturate easily in your OIII frames and therefore are fatter than stars in your Ha and SII frames. As a consequence your image will suffer from what I call "oxygen halo". Also fat stars detract from your subject. Photoshop and PixInsight have tools for reducing star size but they are expensive. StarTools also has a tool. Three years ago I purchased StarTools for $50 for a single license that never expires. It is still available for sale at the same price.
4. Exposure is your friend. Astrophotography is all about achieving the highest signal-to-noise ratio (SNR) in the least amount of time. Buying a faster scope isn't always an option given your budget so how about buying a new camera with a higher Quantum Efficiency (QE). Again it's costly which leaves only two choices: exposure and stack size. You will always achieve a higher SNR in less time by increasing exposure instead of increasing stack size. SNR doubles when you double the exposure but when you double the stack size SNR increases by only 41% (i.e. square root of 2).
Without further ado here is my take on the Tadpole Nebula (IC410).
6nm Ha 18x600s
6nm OIII 23x600s
7 hours total integration time
William Optics D71mm f/5.9
Atik 314E bin2
Astro Pixel Processor (APP) drizzle 2x
HOO palette
Ha stack: Red 85%, Green 15%
OIII stack: Green 35%, Blue 65% (stretched before combine)
Major complaint: Fuzzy details are due to a combination of bad seeing and bin2 capture. (Camera slightly undersamples at bin1 so bin2 is even worse)
How to interpret the colors:
1. Blue/Cyan on the left-most portion of the image is predominantly oxygen.
2. Scarlet/Orange near the right border is almost pure hydrogen.
3. Beige in the center of the nebula is a mix of hydrogen and oxygen.
4. The "tadpoles" have hydrogen tails and oxygen/hydrogen heads.
Brian