Month: December 2019
Tadpole Nebula (IC 410)

Tadpole Nebula (IC 410)

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These past few months I developed an interest in narrowband imaging mostly out of necessity due to the Moon. It turns out to be 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 narrowband imaging. I’d like to share some with you:

Stretch your stacks before combining. You may notice that imaging in Hydrogen Alpha (Ha) is easy due to the strength of the signal. Oxygen III (OIII), on the other hand, is relatively weak. To prevent Ha domination you should stretch your OIII stack before combining them into a single RGB image. By how much? It all depends, so experimentation is the key.

Figure A shows the relative strength of Ha (left) compared to OIII (right). These images were equally “stretched” using Astro Pixel Processor (APP) software. If I were to combine these two into a color image then you would mostly see red with only a few light red regions. You could make the argument that this is how it exists in nature, and you would be correct, but we are striving to show chemical composition, not necessarily quantity:

Figure A

Figure B shows Ha stretched less and OIII stretched more. How much you stretch is a matter of taste. I wanted to come close to Sara Wager’s image.

Figure B

Figure C is the final color image obtained by assigning 85% of Ha to Red, 65% of OIII to Blue, and the remaining amount to Green. This creates a pleasing range of colors:

Figure C

How to interpret the colors:

  • Blue/Cyan on the left side of the image is predominantly oxygen.
  • Scarlet/Orange on the right is almost pure hydrogen.
  • Beige in the center of the nebula is a mix of hydrogen and oxygen.
  • The “tadpoles” have hydrogen tails and oxygen/hydrogen heads.

Here are some other best practices that I learned from Sara Wager:

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.

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. Two years ago I purchased StarTools for $50 for a single license that never expires. It is still available for sale at the same price.

Technical Details:

William Optics 71mm f/5.9
Atik 314E CCD (slightly undersampled at bin1 so bin2 is worse)
Orion 6nm Ha and OIII narrowband filters
Unitron Model 142 German Equatorial Mount.
Tracking: Own design Permanent Periodic Error Correction (PPEC) using stepper motor and Raspberry Pi Model 3B.
Flat-fielder: Own design “The Flatinator”

Image Capture:
Astroberry/INDI/Ekos on Raspberry Pi Model 3B+.
SharpCap for guiding assistance, polar alignment, and PEC learning.
Ha: 18x600s (bin2 to boost signal to keep exposure time to 10 minutes.)
OIII: 23x600s (bin2 also)
Total integration time: 7 hours.

Image Processing:
1. Astro Pixel Processor (APP) for image calibration, integration, stretch, and composition. 2x drizzle to repair square stars and restore image dimensions to bin1.
2. StarTools for star size reduction and additional image processing.

HOO palette:
Ha: Red 85%, Green 15%
OIII: Green 35%, Blue 65% (stretched before combine as shown in Figure B)

High Dynamic Range (HDR) Photography using Photomatix

High Dynamic Range (HDR) Photography using Photomatix

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First Snow 2019
First Snow 2019

While I wait for the weather to improve for astrophotography I discovered High Dynamic Range (HDR) photography through a friend in Great Britain. HDR is commonly used by real estate agents to capture beautiful sun-drenched living spaces. Astrophotographers have used HDR to capture stunning images of the Crescent Moon bathed in Earthshine and images of Total Solar Eclipses. There might be other applications that I want to explore.

Many years ago I was heavily engaged in conventional photography of landscapes and portraiture. This was at a time before digital photography. Portraits were the easiest to capture since they were obtained in a controlled environment of a studio. Shadows that would normally render in black could be filled with flash or flood lights. Highlights that would normally appear washed out on film could be softened with light diffusers.

By far the most difficult was landscape photography. There you didn’t have the option of using flash, flood lights, or light diffusers. You relied more heavily on darkroom techniques. Things changed with the advent of digital photography.

Photomatix is a software product from HDRsoft Ltd, a UK company. They have several versions, some that integrate well with Photoshop, others that are standalone applications. I chose the standalone version for Linux since I find myself increasingly turning away from Windows in favor of Linux. The trial version never expires and is full-featured but they do draw the Photomatix watermark on your final image. The cost of a license is reasonable at $49. For this test I am using the trial version. The software is very easy to use plus there are many videos available on YouTube to learn how to use it to its fullest extent.

The difficult part is capturing the images. Instead of me yammering on attempting to explain what to do, allow me to present the seven photos that I input into Photomatix:

Exposures from 1/1000s to 1/15s
Exposures from 1/1000s to 1/15s

The essential parts of the scene are the sky, the snow, the car, and the snow on the limbs of the trees. The sky and the snow on the ground are the brightest parts. The car and tree limbs are the darkest. The objective is to capture detail in all of them. Notice that there is no single exposure that satisfies us. Perhaps the closest is “exp 60th” but notice how the sky is completely blown out. This scene is a perfect candidate for HDR using Photomatix.

Notice that my exposures range from 1/1000s to 1/15s. I chose 1/1000s because it showed the best detail in the sky and the snow on the ground. I chose 1/15s because it showed the best detail in the car and the tree limbs. Once I determined those endpoints then I proceeded to capture images in full-stop increments: 1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s. It is important to keep the same f/stop. In my case it was f/7.

My camera is rather old so it does not have auto-bracket mode. No worries, I used manual mode instead. My camera has an integrated spot meter. Wherever I point the camera it will read out if it is under-exposed or over-exposed. The meter readout is around the center of the view.

The steps are:

  1. Choose an f/stop.
  2. Adjust the zoom to frame the scene as you like.
  3. Point the camera at the brightest part of the scene, in my case the sky and ground snow.
  4. Adjust the exposure setting so that the meter reads zero (neither under-exposed nor over-exposed). Make a mental note, in my case 1/1000s.
  5. Point the camera at the darkest part of the scene, in my case the car and tree limbs.
  6. Adjust the exposure so that the meter reads zero, in my case 1/15s.
  7. Attach the camera to a tripod.
  8. Double-check the framing.
  9. Click the button to capture the frame. (This should be at our current setting of 1/15s.)
  10. Adjust the exposure one full-stop, in my case 1/30s.
  11. Click the button to capture the frame.
  12. Repeat these steps until you capture the last frame at the terminal exposure, in my case 1/1000s.

That’s it! Download the images to your computer and process in Photomatix. I’ll leave that activity for you to figure out. There are plenty of video resources for that. Good luck!