Galaxy filters

[Stephane]

Hi all,
I’ve been reading up on forums about how to image galaxies. As I understand, they are better imaged using broadband filters as they contain a full spectrum of colour. One filter I’ve seen pop up regularly is the Optolong UV/IR cut filter.

Would you recommend me purchasing this filter? I have quite a bit of light pollution in Preston, Melbourne. I understand that this filter does not block light pollution unlike the narrowband filters. Any advice in regards to light pollution when using broadband filters?

Thank you so much for all the help you’ve given me in the last few months. It is really appreciated.

Stéphane

[iborg (Philip)]

Hi Stephane



I am looking into filters for the same purpose as well.


As I understand it, the UV/IR may be required (mainly to filter out IR that you don't want). If you are using a refractor, that will come to focus at a different point. Other filters may also remove the IR.



A UV/IR will not help with light pollution.


I think a broadband filter is more like theOptolong L-Pro. I am not recommending this, as I am currently far too ignorant about filters.


I will be doing some reading soon, and am looking forward to the answers others give.


Philip

[floyd_2 (Dean)]

Hi Stéphane,
depending upon the camera that you're using, it may already have an IR cut window in front of the sensor. Some have an anti reflection filter only (like my ASI533MC) but I'm pretty sure that other cameras already have the IR cut built in, negating the need to use one separately.

I've been reading up on LPR filters on and off and there seems to be polarised opinion regarding whether they're useful or not in terms of producing a better broadband image than having no filter. There will be heaps of experienced imagers here who can shed some light on that.

I have a Baader Neodymium filter from before I started imaging and have tried it out. I'm not sure whether the results are better than using no filter. Perhaps it allows longer subs than using no filter. I'll follow this thread with interest.

[RyanJones]

For galaxies you want a broad band filter. Light pollution filters are still broad band, they just minimize the transmission of light mostly in the sodium range. They often exclude extreme ends of the blue and red ends of the spectrum too. UV/IR.

Take some time to have a look at the visual spectrum and understand the wavelengths and where they reside. Try to develop an understanding of what the filters are aiming to achieve and you’ll be much better places to make these decisions. Narrowband filters by virtue of their narrow band passes make the variety star colours very hard to distinguish. They are intended for use to show a specific wavelength of light from a specific gas. A galaxy really is a compact source of all wavelengths. They are also normally imaged with a surrounding star field ( colour you wouldn’t want to loose ) so a broad band filter is the way to go. Aliens may also have sodium street lamps but it’s unlikely we’d be able to see them so we can cut out that wave length ( light pollution filter ). Yes you can enhance a galaxy image by adding in narrow band data to show pockets of Ha for example but you still want a broadband galaxy to add it too.

If you’ve made it thoigh all my babble and just wants a brand and a model to buy, I’ve recently been working with an Optilong L-pro in Bortle 8 skies and I think it’s the beat broadband filter I’ve used so far.

Cheers

Ryan

[DeWynter (ILYA)]

Petrol is the only one galaxy filter you can buy. Full stop.

[Startrek (Martin)]

I’ve been successfully imaging galaxies under Bortle 8 in Sydney using a Canon 600D DSLR until 2019 and in the last 2 years with my ZWO 2600MC OSC
Both cameras have a UV / IR cut filter ( I used no other filter )
I only imaged galaxies during the “new moon period” and used lucky imaging techniques ( refer Cuiv the lazy geek )
Subs with the DSLR were 60 sec , dithered, with darks ( minimum 3 hours of data ) Point of diminishing return was around 3 to 4 hours
Subs with the OSC were 60 to 90 sec , dithered with Flats and Bias ( minimum 4 to 5 hours of data ) subs file sizes on the 2600MC are 50MB so massive storage required

DSLR galaxy images obviously had a much higher noise floor and less detail than the 2600MC
No issues with colour during processing
Stacking software DSS
Processing software Startools
Cheers

[Stephane]

Thanks everyone, some great advice. “Petrol” is currently not an option for me, but I might be able to create a portable setup in the future. In the mean time a UV/IR cut filter might get me started on galaxies.

Clear skies,
Stéphane

[RyanJones]

Stephane, don’t you have a Nikon D7000 ? And it’s not Astro-modified ? So your camera already has a UV/IR filter built into it.

[Stephane]

Yes Ryan, but I just recently purchased a second-hand zwo asi294mc which I hopefully will be receiving in the coming days.

[RyanJones]

Originally Posted by Stephane

Yes Ryan, but I just recently purchased a second-hand zwo asi294mc which I hopefully will be receiving in the coming days.

Ah I see. Excellent