Filter advice

[Celestron4]

I am looking at filters to be able to improve detail and contrast in my images and have my eye on the Optolong L-eXtreme/L-eNhance. I am using a Canon EOS600D DSLR attached to my telescope. I have Bortle 4 skies if it is any help. Which would be the best to go for? 

[TheMan]

Not sure if it's of any worth. But I've bought a 7nm svbony Ha-filter from ebay for around 170AUD (!). contrast and detail improvements are significant and works well for long exposures in my Bortle 8 backyard. The coatings are decent and haven't seen any reflection on really bright objects

Edited March 31, 2021 by TheMan

[Clarkey]

Do you need filters for Bortle 4? I am in B6 and I don't use filters. I would rather remove gradients in processing than lose the detail with unnecessary filtering.

My local 'dark' site is B4. 😁

[KP82]

I'd only use these dual/tri-band filters if I desparately want to take images of some emission nebulae with a 50%+ waxing or wanning moon in the sky. Generally speaking I don't like these filters and usually take a break when the moonlight is washing out everything. BTW I'm in Bortle 5.

 

[mackiedlm]

I am in bortle 4 and I use an l-enhance. I have used it successfully on both a DSLR (1100d and 700d both astro modded) and recently on a cooled dedicated astro-cam. In my opinion the L-enhance is a fantastic tool. With it imaging nebulae during bright moon becomes possible and that has been really important for me this winter when clear night have been almost exclusively around the full moon. Alsoby separating out the channels it is possible to produce very pleasing false colour (HOO) images.

 

I have not used an l-extreme so cannot speak from direct experience, but I have seen references to it being difficult to use with a DSLR.

 

Good luck.

[Celestron4]

Thanks very much for the advice! The filter would mainly be to deal with the moon rather than light pollution. Could you not also produced HaRGB images by separating out the Ha from the OIII?

[Thorney]

I use a L extreme with a 600D. It’s a great filter and works very well. Sometimes I think the filter is a bit much for the camera though, not sure if it’s just down to me, sensor temperature being high and seeing conditions  I’m still well in the early stages of post processing so haven’t figured out how to separate the channels for a false HOO yet. 
 

Hopefully I’ll be getting a OSC camera soon so looking forward to using it with that. 

[newbie alert]

For detail and contrast,  nothing will beat a ha filter...

[ollypenrice]

On 02/04/2021 at 10:32, Celestron4 said:

Thanks very much for the advice! The filter would mainly be to deal with the moon rather than light pollution. Could you not also produced HaRGB images by separating out the Ha from the OIII?

No, because the filter is largely opaque to blue if I'm not mistaken. What it gives you is an approximation to the HOO palette.

The first thing to ask is, how much Ha does your camera pass at the moment? This depends on how recent it is and/or on whether its modded. If its Bayer Matrix is blocking Ha it won't see the Ha passed by the filter anyway.

Then it's important to understand what filters do and don't do. They do not let you capture more Ha or OIII (or whatever) in a given time. What they do is isolate those wavelengths and block the rest of the light, allowing you to expose for longer without letting other light sources (either from the object itself or from polluting sources) swamp the signal from gas emission. This means that we use them for three reasons:

- To block LP including lunar LP.

- To block non-emission signal from emission nebulae so as to obtain higher contrasts which reveal structure in the gasses.

- To exclude LP and non-emission object signal so as to allow us to expose for long enough to capture very faint emission signal which we then stretch above the level of the background sky. (In reality it is not above the level of the background sky even at a dark site but, in processing filtered data, we can stretch it to make it brighter.)

The second two of these reasons make the use of the dual band filter valid even from a dark site but, in order to replicate an HaLRGB or HaOIIILRGB image you would need to shoot both filtered and unfiltered OSC and blend the two in the way that we do with conventional filters.

Olly

Edited May 3, 2021 by ollypenrice
typo

[Celestron4]

Thank you very much for the detailed reply! My DSLR is modified and does pick up a lot of the red is this helps. My skies are a bortle 4 (according to the clear outside app but shows as nearer to a 5 on the light pollution maps). Couldn't you use the filtered images as luminance data - isolate the red and green channels in photoshop and apply them to an unfiltered RGB image?

 

[ollypenrice]

3 hours ago, Celestron4 said:

Thank you very much for the detailed reply! My DSLR is modified and does pick up a lot of the red is this helps. My skies are a bortle 4 (according to the clear outside app but shows as nearer to a 5 on the light pollution maps). Couldn't you use the filtered images as luminance data - isolate the red and green channels in photoshop and apply them to an unfiltered RGB image?

 

You can use absolutely anything as a luminance layer, but what does that mean? It means that what you are using as luminance will define the brightnesses across the image. To take an absurd example, I could take a picture of St Paul's Cathedral with my DSLR and apply it as luminance over my RGB data of the Heart Nebula. The nebula would then be illuminated as if it were St Paul's cathedral. (I ought to try something like this by way of example but I wouldn't expect it to win any prizes! )

But my serious point is to identify what a luminance layer does when applied over a colour image. To quote Nik Szymanek, it gives that colour layer its light. The reason why we use a luminance filter to make a luminance layer is that it is truthful. This filter passes all colours of light and so gives an accurate illumination of the target. If we use, say, an Ha filter as luminance we are lighting the entire image in the light of deep red - which is less dishonest than lighting it in the light of St Paul's cathedral...  but you see my point?

What many of us prefer to do is to create an honest LRGB image, illuminated sincerely in its own light, and then enhance it with narrowband data. So we shoot a deep narrowband image, stretch it hard because the NB filter holds down the LP/skyglow, and use that to brighten the colour channel in which it appears. (So Ha brightens red, OIII brightens green and blue because OIII lies on the G/B border.) The highly stretched NB data is not used as luminance but in its own colour channel.

If you use Ha or L-enhance data as luminance it will suppress the blue channel, Ha totally and L-enhance significantly. If the target has little or no blue reflection nebulosity you'll probably get a presentable or fixable result but it will never be a terribly honest one.

Olly