Well, it’s been yet ANOTHER dreadful month, with no clear nights since I started my summer project on the Cygnus Wall; part of NGC7000 – The North America Nebula. However, it has given me time to do some research on my current filter – the Astro Hutech IDAS NGS1 – and other filters that are currently on the market that may be more suited.
With the summer nebula season just starting to begin in the early days of July, comes the start of the large and wonderful emission nebulae once again. This is one of my favourite times of year for astrophotography, especially since swapping to a full spectrum DSLR 12-months ago. I had so much fun last year taking photographs of the Heart Nebula, Bubble Nebula, Crescent Nebula, to name just a few. However, it has made me think if the equipment I’m using, namely the filter, is best suited to the targets that I want to image.
Now, a bit of a recap on a science lesson. The two most common elements associated with emission nebulae are Hydrogen-Alpha and Oxygen-III:
- Hydrogen Alpha – 656.3nm
- Oxygen-III – 496nm and 501nm
Another one being Sulphur-II, at 672nm, but it’s not as common as the other two. Only certain targets have Sulphur emissions, and are best suited to be shot with a mono camera and true narrowband filters.
My NGS1 filter allows these certain wavelengths of light to pass through to the sensor, plus a much wider spectrum, while blocking the wavelengths associated with light pollution. The main culprit being Sodium streetlights; the bright orange street lamps that used to line the streets of Britain. Unfortunately most street lamps have now been changed over to white LED lamps, but the NGS1 still does a excellent job against these new lamps.
However, I have noticed that since the lockdown in the UK has been lifted, the light pollution in the night sky has been steadily increasing. I even suspect that I’m now a Bortle 7! This has been making it more difficult to isolate the nebula itself from the background pollution. To make this easier, I’ve been saving up and finally invested in the IDAS NBZ filter.
As can be seen in the filter response above, the NBZ filter completely isolates the two elements associated with emission nebulae.
The following images show a more detailed representation of each wavelength.
There are many similar filters on the market today, like the Optolong L-eNhance and L-eXtreme. However these filters can struggle with fast optics below F/5, and especially with scopes like the Celestron RASA’s and Hyerpstar EDGEHD’s, that shoot at F/2.
When the light enters an optical train, the wavelength of light is shifted a certain amount, depending on how fast the system is; the faster the system the greater the amount of shift. This is undetectable when shooting in broadband, but when using a filter with such small passbands, that shift can actually take the incoming photons outside of what the filter allows through. This in-turn reduces the efficiency of the filter, and therefore effects how much light is actually captured by the sensor.
Now where the NBZ filter is unique in this regard, is the designers have ‘pre-shifted’ the passbands to negate this problem. This allows for all optical systems to get a very decent response of at least 70%, regardless of their F-ratio. With my Sky-Watcher 130PDS at F/5, I should be getting at least 95% in both O-III and Ha. They also make a UHS (Ultra High Speed) version of the filter, which is capable of F/1.4.
Enough about the physics, what’s it like in use?
Firstly, I had no issues with focusing and plate solving using my DSLR. My usual 10-second exposures at ISO3200 showed plenty of stars, and even some nebulosity! It’s not quite par-focal with my NGS1 as is advertised, but it is still very close. When swapping to the NBZ, I refocused using my Bahtinov mask and only had to adjust it ever so slightly, almost negligible. The image on the live view screen was very dim though when compared with the NGS1, but using a bright star like Vega or Arcturus, it was easy enough to see the diffraction spikes to focus correctly.
When taking an actual exposure though, I had to increase both ISO and exposure from my usual figures. This was to be expected, as the amount of photons hitting the sensor had been significantly reduced by the filter.
Firstly, I had to double the ISO to 1600, and secondly increase the exposure to 360-seconds, or 6-minutes. Even with these values though, the histogram was still showing that the images were underexposed. I would suggest 600-Seconds, or 10-minute exposures, at ISO1600 be required to achieve a decent histogram curve. This brings into account the accuracy of the users mount; it needs to be able to track to a REALLY high accuracy. I’m so glad that I have upgrade to the AZ-EQ6-GT, MY EQ5 Pro certainly wouldn’t of been able to handle it.
Despite underexposing, the detail was certainly there. A single exposure taken with the NBZ filter was almost as good as my previous stacked image! The image below shows one single exposure that has been auto-stretched within N.I.N.A..
What’s more surprising though is the lack of star halos! When comparing the image above to one taken using my NGS1 filter, the bright star on the left-hand side has no halo at all. You can even see my focuser tube!
In my previous image from last month, I had to crop this bright star out, as the star halo just consumed that whole area of the image. Fortunately, there was no such Halo on the NBZ.
There has been two versions of the NBZ filter. The NBX was the first version, which although highly received for its use with fast optics, it suffered greatly with star halos. Therefore Astro Hutech redesigned the filter and released the NBZ in its place.
What else can I say about the filter?
On the few targets that I tried it on, the detail that it revealed was just incredible. When I trained my scope onto the Crescent Nebula, I could see the individual lobes, making it look almost brain-like, and the Veil Nebula just instantly blew me away. It easily replaced my NGS1 filter on the end of the Baader MPCC, as well as being able to be used within an electronic filter wheel, being only 5mm thick optically.
However, I always say the proof is in the final images. Admittedly, I need to add some more exposure to this, but for exactly 2 hours of exposure I’m actually quite amazed at what was able to be captured.
What do you think about the results with the NBZ? Please let me know in the comments below!