So, I bought Baader narrowband filters that are significantly wider (Ha 7nm, SII 8nm, OIII 8.5nm).
In order to compare them, I shot images of the jellyfish nebula. It has a very strong signal but also dark areas nearby. In order to minimize the influence of changing seeing conditions, I shot one hour with one filter, then one hour with the other and then again with the first and so forth.
I'm still struggling with my unreliable lodestar guider, so I had to throw away a number of images. But ended up with enough data to compare them. I decided to compare them by contrast and signal to noise ratio.
I calculated contrast by first measuring average signal in a bright area and in a dark area
and then calculating it Contrast = (Bright - Dark) / (Bright + Dark)
For Signal to Noise ration I measured the images with Pixinsight's subselector script. It doesn't report absolute values but relative values.
Here are the results:
Bright | Background | Contrast | SNR | |
Ha 4.5nm | 3.50E-04 | 7.83E-05 | 6.34E-01 | 2.099 |
Ha 8nm | 4.84E-04 | 9.95E-05 | 6.59E-01 | 4.114 |
OIII 3nm | 1.54E-04 | 9.62E-05 | 2.30E-01 | 1.543 |
OIII2 8.5nm | 1.67E-04 | 1.07E-04 | 2.18E-01 | 2.481 |
The contrast is very similar between both filters. But the Signal-to-Noise ratio is significantly better with the wider filters.
The other question is how bad the background gradients are that get created with both filters. I used the OIII images to compare as the Jellyfish nebula is very weak in OIII:
3nm | 8.5nm |
Using AutomaticBackgroundExtraction, I get the following background images:
3nm | 8.5nm |
Clearly, the wider filter has larger gradients. But after the first background removal the residual background is:
3nm | 8.5nm |
Looks like we can remove the stronger gradient fairly easily.
So, this does seem to support my suspicion, that the smaller narrowband filter extend the integration time. Although I am not sure what the reason is - the Ha/SII/OIII signal should be the same with the smaller and the wider filters.
Anyway: I'll probably stick with the wider narrowband filters.
No comments:
Post a Comment