Broadband filters - Forums

Topic starter

10 days ago

I thought that it might be useful to post the results a little experiment that I have been running…

I’ve seen a lot of online discussion regarding the use of broadband filters in light polluted skies and whether they are worth it or just a waste of time. The general consensus seems to be that you are simply throwing away photons unnecessarily since modern (post LED) light pollution covers such a large range of wavelengths that filtration becomes near useless. The physics of this argument is undeniable but I very rarely see any real world data to illustrate the effects of using broadband filters. So , here are some comparison images.

There are 4 panels in this plot and, hopefully, the labels are self explanatory. Each panel represents a 3hr stack of 60 sec subs using a Spacecat 51combined wth ZWO ASI 2600MC. I’m on the outer margins of a large city (Melbourne) with a well defined light dome to the SW. Bortle 5, SQM never gets above 20. Moderate light pollution? - I guess that depends on your perspective. lol🙂

The two panels on the left are without any external filters but incorporate the UV/IR cut of the ASI 2600MC, the two on the right include an Antlia Triband filter - the original version, not the mkII. The upper panels are ‘as shot’ with a very basic gradient correction and a simple ‘predefined’ stretch applied in AstroPixel processor. The lower panels are the same but with the addition of a simple star colour correction. All run under clear skies.

📷 filter comparison.jpeg

I’m not sure what I was expecting here but the Antlia website notes the Triband’s ability to ‘increase the luminous flux of the blue channel’ while I would suggest that these data reveal considerable bolstering of the red channel! It also looks like the yellows and blues may be a little subdued in these Triband images…

Anyway, hopefully this may be of use to anyone pondering the purchase of a broadband filter?

Jon

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Tony Gondola

Frequent contributor

Exemplary contributor

9 days ago

I can tell you that I very much prefer the UV/IR images myself.

Jon Woodhead

Topic starter

9 days ago

I tend to agree Tony.

Alex Nicholas

9 days ago

I’ve unfortunately never in 20 years found a light pollution filter that renders the sky perfectly. The L-Quad does a decent enough job if you can deal with Optolong’s apparent requirement for their filters to introduce halos in your images, but aside from that, I feel they all hurt the colour specturm somewhere.. Some attenuate reds too much, others really mute the blues… None of them get it right, and I feel like its both to be expected, but also one of the biggest shames of the Astrophotography world… Broadband images are absolutely gorgeous when done right, but so many of us can’t get to dark skies anywhere near as often as we’d like.

I’m in bortle 5/6 where I am currently, and broadband imaging is still quite possible from my home, but a couple of years ago I was in a bortle 3~4 area and the difference is dramatic…

with colour cameras, I shoot with their in-built UV/IR cut window only or dual narrowband filters, and with my mono camera, I use a standard L filter, rather than a LPR of any sort…. I’d rather process out light pollution, than have a filter reject it along with valid signal.

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Tony Gondola

Frequent contributor

Exemplary contributor

9 days ago

If you shoot mono under high Bortle, one possible use of these filters is to generate deep L frames with controllable gradients to go with RGB frames. The RGB data can withstand a lot of violence to get something usable.

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Astro Hopper

9 days ago

What I would do in this case is extract Ha from Antlia filter and bland it to UVIR photo.

Jon Woodhead

Topic starter

9 days ago

at the end of the day(s) I just threw it all together, which probably had a similar effect:

rho ophiuchi