Tadpole Nebula (IC410)

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oopfan
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Tadpole Nebula (IC410)

Post by oopfan » 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
Attachments
Tadpole_APP-Ha-18x10m-St10-3-0-CON0-R85-G15_OIII-23x10m-St20-3-0-CON0-G35-B65_StarTools-Reduce-Life-Sharp-Color-Denoise.jpg
Tadpole_APP-Ha-18x10m-St10-3-0-CON0-R85-G15_OIII-23x10m-St20-3-0-CON0-G35-B65_StarTools-Reduce-Life-Sharp-Color-Denoise.jpg (269.6 KiB) Viewed 348 times
Last edited by oopfan on Sun Dec 22, 2019 12:44 pm, edited 1 time in total.

nexusjeep
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Re: Tadpole Nebula (IC410)

Post by nexusjeep » Sun Dec 22, 2019 9:24 am

Hi Brian,
Nice image I switched to narrowband because of the moon + light pollution as it cuts out the glow from the local towns and industrial estates just takes more time to acquire the image which is the only downside. I had a go at the Tadpoles a few weeks ago image it is also in the gallery as I had never imaged them before it was the last clear night I had which was the 29th November as the weather really does suck this Autumn / Winter. Which is the big benefit as you say of narrow band as the only clear nights also usually coincide with it being a full moon.

Cheers
Nick

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turfpit
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Re: Tadpole Nebula (IC410)

Post by turfpit » Sun Dec 22, 2019 12:03 pm

Brian

I know that was a hard-earned image. It turned out well in the end, looks particularly good on my iPhone. The image being built from 600s exposures with guiding only on one axis is impressive.

Thanks for the detailed technical write-up, particularly the idea of apportioning colour across the channels rather than a straight 1 to 1 mapping of a filter to a colour.

Dave

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oopfan
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Re: Tadpole Nebula (IC410)

Post by oopfan » Sun Dec 22, 2019 12:14 pm

Hi Nick,

Thanks. I like your wide FOV. The nebula almost looks like the Monkey Head using that particular camera orientation. Nice!

I agree that NB is not for everyone especially if you are into short time frames. Sara Wager routinely uses 30-minute exposures at bin2 but she uses 3nm filters so less light. 10-minute exposures work for me. I've gone longer but found that star saturation with OIII is prohibitive. If my camera had a deeper full well depth (currently 13100 electrons) then I could go longer.

Also, good guiding is a must. One of the great things about having a 50-year old mount designed for lunar and planetary is that I got to replace the old fixed rate induction motor for a stepper motor driven by a Raspberry Pi with Periodic Error Correction that I designed. The mount's peak-to-peak uncorrected error is 60 arc-seconds. After correction it is less than 10 arc-seconds. which for bin2 imaging star elongation is really not that bad. The problem is the atmosphere. Sure you've got atmospheric refraction which is most severe at the horizon, about 0.5 degrees, but that's not the killer. It's the unpredictable inversion layers. Thankfully the effect is relatively slow acting so that guiding software can react to it. Guiding is a must even if you have an expensive mount with sub-arc-second periodic error!

Regarding the lack of clear nights -- same here. Last stretch of good weather was early November. Recently I've read that cloud formation increases during sunspot minima. We are currently at an historic low. Hopefully next year we'll be singing a different tune!

Brian

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oopfan
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Re: Tadpole Nebula (IC410)

Post by oopfan » Sun Dec 22, 2019 12:24 pm

Dave,

Thanks. Except for the weather the easy part was image acquisition. It was the processing that was the difficult nut to crack. The last key to success was the star size reduction software.

One of these days I'll part with the big bucks for a large format giga-pixel CCD but for now I'm happy with my 10 year-old, $400 CCD camera!

Brian

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oopfan
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Re: Tadpole Nebula (IC410)

Post by oopfan » Thu Dec 26, 2019 1:45 pm


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