B33 Horse Head Nebula
Posted 08 January 2013 - 08:17 PM
A friend and I imaged the Horse Head Nebula last night using my Meade Series 5000 80mm APO refractor, Televue .8x field flattener, Hutech IDAS LPS2 filter and Canon unmodded 60D DSLR.
We used ISO 1600 and took (20) 300 second light frames. The transparency was only average. Combined in DSS.
Vignetting is apparent in this image. We did not have the opportunity to create a master flat frame. No darks were used in this image.
After stacking and processing the image, I was disappointed in the way some of the bright stars look. Most notibly Alnitak. It has a blue ring around the star. Does anyone have any ideas why the stars look like they do? Could it be chromatic aberation from my Meade refractor? If so, should I be using a filter of some sort to rid this blue halo?
Thanks for any ideas to help me make my image better.
Posted 08 January 2013 - 08:26 PM
After a couple of days of heavy processing, I manage to limitate this side effect in PI. The first processed images were nearly like yours.
You should investigate your case changing the optical configuration and shooting.
Posted 08 January 2013 - 11:45 PM
As far as the blue halo around Alnitak - if you do a Google search for "horsehead and flame" and then click on Images, you'll find that its quite common to have a pretty ugly Alnitak. Its so bright, that it takes a fair amount of manipulation or HDR combination to tame it. Alnitak IS a blue star. Your blue ring is mostly reflecting that almost all of the center of the star is saturated (255,255,255), and the halo around it is not saturated in red and green (the inner ring typically has values of 200, 240, 252, while the more blue outer ring is typically 140, 170, 252).
You may want to look at your stretching to make sure you're not aggravating the star bloat (by stretching parts of the star and making them that much brighter). One of the techniques which may help minimize blowing out the star too much is to use a stretching curve that has the white point brought down from 255 to something like 245 and is "flat to the finish" (meaning that you stretch the faint parts and then flatten out the top of the curve - typically starting around 128 and pulling it down until the upper part of the curve is fairly flat). This will tend to take the 200 range of brightness and keep it around 200, rather than brightening it, and will take the brightest parts and tone them down a little. When you're all done processing your image, you can do a final white point adjustment to ensure you're using the whole dynamic range in your image.
You do have a little bit of chromatic abberration. If you zoom in very tight and look at each of the 4 corners, you can see that your blue channel is shifted towards the center of the frame, and the red channel is shifted away from the center (towards the edge of the frame). This CA is easily fixed in Photoshop using the Lens Correction filter.
Hope this is helpful.
Posted 09 January 2013 - 04:55 PM
I am going to apply a gradient and reduction in the blue color saturation. I will try the curve suggestion. I may see if I can find a good chromatic aberration filter to attach to my camera.
My friends camera is a Canon 60D and our club recently purchased the 60Da and I am looking forward to using it on the Horsehead nebula. I've read somewhere that unmodded camera's tend to result in noisier and slightly bloated stars due to the blocking filter.
Thanks to the both of you for your suggestions.
Posted 09 January 2013 - 07:20 PM
For your image, I was able to pretty much eliminate the CA by using the Lens correction filter and setting "Fix Red/Cyan" (under the custom tab) to +30, and "Fix Blue/Yellow" to -70. You'd probably want to spend a little more time than I did to nail down more perfect settings, but they'll probably be pretty close to these.
Once you establish the correct settings, any image you take with the same imaging train should end up with the same results. So, its not a complicated process you have to repeat many times.
To give you an idea, here's a tight crop of the lower left corner of your image:
And here's the same corner after applying the Lens Correction filter with the above settings:
You can see the Blue/Red fringes in the bright stars in the "before" version, but they're mostly gone after applying the Lens Correction filter.
Hope this helps.
Posted 18 January 2013 - 07:09 PM
The result is dramatic. Thanks for the info and illustrations.
Posted 19 January 2013 - 12:43 AM