Monday, February 6, 2017

Comparing Narrower and Wider Narrowband Filters

In my recent narrowband images, I needed very long exposure times to get a decent signal in OIII and SII. And I was wondering if the small window of the Custom Scientific filters (Ha 4.5nm, SII 3nm, OIII 3nm) are maybe too small. And that they remove a lot of the background noise but also leave less of the signal through.

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:

BrightBackgroundContrastSNR
Ha 4.5nm3.50E-047.83E-056.34E-012.099
Ha 8nm4.84E-049.95E-056.59E-014.114
OIII 3nm1.54E-049.62E-052.30E-011.543
OIII2 8.5nm1.67E-041.07E-042.18E-012.481

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:

3nm8.5nm

Using AutomaticBackgroundExtraction, I get the following background images:
3nm8.5nm

Clearly, the wider filter has larger gradients. But after the first background removal the residual background is:
3nm8.5nm

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.

Guiding failures - every night!!!

In the past, I have seen (every couple of months) an error like:

02:01:45.323 00.000 5728 invoke imageready:
(ASCOM.SXCamera) Unable to complete get_ImageReady request due to previous error

during my imaging sessions. After that every image capture fails in PHD2.

Suddenly, I see these errors every night - sometimes after a few minutes after a few hours.

As I didn't make any software changes, I first thought about a hardware issue:
  • Replaced the USB cable
  • Switched the Lodestar X2 for my older Lodestar guider
Without any improvements.

I then
  • Updated PHD2 to the latest version
  • Updated the StarlightXpress driver to the latest version
  • Reverted SGPro back to the last working version
With the same results.

Finally posted in the PHD2 forum for help. I also asked Richard and the others. Everybody points to the USB connection (cable, Hub...) I tried
  • Using a different cable
  • Connecting the guider directly to the computer
  • Using a different hub
... always with the same results.

Somebody on the PHD2 mailing list mentioned that it might have something to do with the temperature. Not that it's REALLY cold here, but it does get below freezing.