Is a 3nm Ha Astrodon vs a 7nm Ha worth it?

[John K]

Hi guys,

I am looking to become more efficient in the way that I do my LRGB mono deep sky imaging with my rig which is a 12.5" f/5 telescope and a Cooled Zwo 174mm camera. I live just 5.5 kms North of the Melbourne CBD in severely polluted skies, and was hoping to be able to gather a substitute of my Luminance data in Ha and then by RGB data from a dark sky.

So questions are:

1. Can I fully substitute Luminance data with Ha data?

2. Will a 7nm Ha filter cut it in severely light polluted skies? The 3nm Astrodon (1.25") is astronomical in price!

My rig is low cost, and I want to keep it that way, but at the same time I don't want to be buying stuff I cannot use.

Look forward to advice/experiences.

Clear skies,

John K.

[Slawomir]

Hi John,

IMO you can very successfully use Ha for Lum, but for emission nebulae only. It would not work very well with galaxies.

Money permitting I suggest getting a 5nm Ha Astrodon - you will love it when imaging emission nebulae and won't be thinking in the future: should have I gotten something else?

My second choice would be a 6nm Astronomik. In a heavily light polluted site, wider bandwidth will result in a lower SNR, so I would not recommend a wider bandwidth, if you want the best possible data.

Hope it helps a bit :)

[Atmos]

As Suavi says, the narrower the bandwidth the better contrast you'll get (better at rejecting everything other than the Ha signal). In some cases the 5nm will give a better signal than the 3nm as the 5nm contains both the Ha and NII signal; the 3nm is narrow enough to split the Ha and NII.

If you already own LRGB Astrodons I would suggest keeping with Astrodon in an attempt to remain par focal. Astrodon filters are 3mm thick which I think is thicker than most others.

[John K]

Thanks for the info guys.

Looks like the Astronomiks could be good value and I do use the Astronomiks LRGB filters.

Clear skies.

John K.

[Slawomir]

No worries John, looking forward to seeing your new images with the 6nm Astronomik.
I forgot to mention, that for most of my images I used Ha only for Luminance, and these turned out alright IMO. Unfortunately, I need to collect sky data from about 2km from Brisbane's CBD.

[John K]

Thanks! That's encouraging!

[rustigsmed]

I believe peter ward uses the 7nm very successfully from the middle of Sydney. of course narrower is better but they are really expensive.
7nm is fine for me but I am further from the city than you john!