Friday, August 30, 2013

First narrowband image

I tried to capture the veil nebula from our backyard, but the noise level was just very bad. So, I tried to take the same images with my Ha, SII and OIII narrowband filters. There are various formulas out there how to combine these, e.g:

R: 0.8*Ha + 0.2*SII
B: 0.85*OIII + 0.15*Ha

another formula that I found:

R: 0.4*Ha + 0.6*SII
G: 0.4*OIII + 0.3*Ha + 0.3*SII

Guess, I'll have to play with these.

I wonder if I should create the 3 color channels in CCDStack with the respective weights and combine them there. Or if I should just stack the individual images and then combine them in Photoshop. I'll try CCDStack first. An email to the CCDStack support mailing list pointed me to File Math or Combine with Pixel Math or Weights.

First, I calibrated all frames. Because I took them over several nights, I used a different strategy:

  1. Calibrate all images per night and store the calibrated frames
  2. Rotate (if necessary), align, correct and stack all subframes
  3. Align all 3 resulting frames and store the aligned frames
  4. Now, I can load Individual aligned frames into CCDStack to combine them. E.g. for the first hubble palette, I load the aligned Ha frame and the aligned SII frame. 
  5. With pixel math, I multiply the Ha frame by 0.8 and the SII frame by 0.2
  6. Combine-Sum the resulting frames for my red image
  7. Repeat for the other colors
  8. Finally combine the 3 frames to a color image.
Here are the resulting images:

I also tried to just use the individual images for individual colors: OIII=Red, Ha=Green, SII=Blue:

I like the last one the best, it shows lots of fine details in the nebula.

I processed the other image in the same way:

So far so good...

Now, I have to do some more processing:
  • there is still a lot of background noise
  • need to calibrate both images to make them blend better together
  • and then I need to stitch them together