Review of the Askar Dual Narrowband Filter

10-15 minutes read time

Disclaimer: this is not a paid promotion of any filter or equipment in any way, shape or form. Nobody has approached me for my views with an offer of monetary or other incentives. The views expressed here are my own without coercion or compensation. Research into the different equipment has been carried out via publicly available online sources.

Imaging with a one shot colour (OSC) camera and dual narrowband filter is a cost effective way of introducing yourself to the joys of capturing specific bandwidths without the expense of going down the more traditional route of using a mono camera and dedicated filters for individual bandwidths. Granted, the mono route with dedicated filters for the specific bandwidths will more often than not yield richer and more detailed images, but this can be prohibitively expensive, in a hobby where the costs are already pretty eye-watering, which is why a lot of us go the OSC and dual narrowband route.

There are a number of reasons for imaging in narrowband, the main ones being; counteracting local light pollution, mitigating the washout effects of a bright moon, capturing those specific bandwidths (usually hydrogen alpha (Ha) and oxygen (Oiii) associated with emission nebulae such as the Cygnus Wall or the Heart Nebula.

Prior to using this particular filter I was using the ZWO Duo Narrowband Filter. This however came with inherent issues, mainly that the bandpass in both Ha and Oiii was so wide (15nm and 35nm respectively) that the effects of the moon and extremely bad light pollution weren’t entirely mitigated. Lessened yes, but not entirely blocked. The other issue was that in the Oiii channel the stars developed quite severe halos which can be difficult to remove in post processing (see cropped image of Cygnus Wall below.) That being said, the ZWO has served me well, and it’s certainly upped my imaging game.

Cropped version of Cygnus Wall taken with the ASI183MC and ZWO Dual NB filter. Pretty bad halos.

From what I’ve read online, even the L-Enhance has a bandpass of 10nm in Ha and 24nm in Oiii (cited by AstroFarsography and Astrobackyard), which puts it closer to the ZWO Duo Narrowband Filter, although with slightly tighter bandpasses.

With the Askar Dual Narrowband Filter the bandpass is 13nm and 30nm in Ha and Oiii respectively, which is the around the midpoint of the difference between the Optolong L-Enhance and ZWO Dual NB filter. Interestingly, the Askar is around the same price point as the L-Enhance, but on paper the L-Enhance appears to have the edge.

Transmission lines of the Askar duo narrowband filter, taken directly from the Askar website

A lot of astrophotographers swear by the L-Enhance, and I’ve seen some excellent results from users. So it seems Askar has some stiff competition with this particular filter. But at an equivalent price point, how does it hold up against its rivals?

The Capture

With it being the start of the new year (2023), I’m hoping it’s not starting as it means to go on. With the only forecast clear night for several weeks, and none in the foreseeable future, typically we’re approaching a full moon tonight. That said, it should be a good test of the blocking abilities of the Askar filter. I’ve gone after two targets, after spending an age wondering what I was going to try for; the Heart Nebula in Cassiopeia, followed by some time on the Jellyfish Nebula in Gemini. Both had clouds at times. I would’ve stayed on the first target, however the meridian flip failed on auto and trying it manually only invoked “Exception occurred” even after a full disconnect and restart of APT.

The setup was my usual DSO rig; the SkyWatcher EQ5 Pro, carrying the 72ED, Altair Hypercam 183C and Altair Lightwave reducer/flattener, going with 5 minute subs at unity gain. Guiding was done with the 9×50 guidescope and ZWO ASI120MM in PHD2. Capture software was APT.

Processing

Each one was stacked in Astropixel Processor (APP) firstly as a more traditional RGB-style image, and then with the individual bandwidths extracted, with only the data-stretching abilities of APP itself used, ie crop for edges, background neutralisation and calibration, light pollution removal, and star colour calibration, plus the default saturation addition. Zero star reduction was carried out.

The Results

Jellyfish Nebula

50 minutes at 5 min subs.

The initial RGB-style extraction looks good. The stars are nice and tight, with no discernible bloating, even out to the edges.

No detectable halos in the Ha extraction and good blocking with an 80% moon out at the time.

Oiii is showing decent blocking with the 80% moon. No discernible halos. Obviously need more time on target for a decent amount of data in this channel, but that’s pretty standard for Oiii anyway.

The Heart Nebula

1hr 20mins at 5 min subs

Same night as the Jellyfish Nebula data, so an 80% moon. Again no discernible halos after pixel-peeping around the image.

Ha extraction. Good strong signal, this was closer to that moon than the Jellyfish Nebula in Gemini. No real need to try and remove any gradients. No halos.

Oiii channel. Good blocking of that nearby 80% moon but probably wouldn’t want the moon much brighter or closer. Still no halos.

Summary

It almost seems unfair to compare this to my old ZWO filter because it’s like comparing chalk and cheese. Whilst the ZWO filter is cheaper still than the Askar one, it’s very much a case of getting what you pay for. But does the Askar compare to the L-Enhance?

Not having an L-Ehance to test it’s impossible for me to do a direct comparison, so I can merely go by “on paper” and compare my own results with the Askar with that of others with the L-Enhance. But knowing that I’ve seen reports of halos with the L-Enhance, and not seeing any with the Askar, at least not the particular one I have, then I have to say that, in my opinion, the Askar more than compares.

It has good blocking in both bandwidths, and I would be happy in Oiii up to an 80% moon reasonably close by, although you’ll likely have to post process some gradient out. Ha is usually a better bandwidth and if you have a full moon, I’d probably still be happy to extract just the Ha data using this filter as well. Halos on my test targets were non-existent. And lastly, it’s about the same cost as the L-Enhance (at the time of writing.) To me, in a hobby that already costs major organ donation to fund, this is a no-brainer. Just because everyone else uses a particular thing, doesn’t follow that if a cheaper, just as good alternative is out there, we shouldn’t use that or at least consider it.

Update

Interestingly, the Askar website itself uses data that implicitly says that the FWHM for the two bandwidths is 13nm for Ha and 30nm for Oiii, even though the actual packaging for the filter itself specifically states 7nm duo band. Whether this is a packaging issue or Askar have used incorrect data on their own website remains to be seen and I am in contact with Askar/Sharpstar directly for clarification.

I’ll keep you updated on Sharpstar/Askar’s response to my queries regarding the above discrepancy.

Further Update

Sharpstar Optics have confirmed that it is the website information that is correct, NOT the packaging.

More Testing

I’ve had the opportunity to grab some more data on different targets, namely IC410 (Tadpoles Nebula) and Caldwell 49 (Rosette Nebula.) Both of these targets for my field of view (335mm with the reducer/flattener) have several quite bright stars that the ZWO filter have previously struggled with. I’m more than happy to say that there are still no halos using the Askar filter, as you can see in the images below.