LP filter and CCD's

[swag72]

I am interested to know whether people use LP filters when they are imaging with their mono CCD's.

I am currently using an Atik 314L mono with Baader LRGB filters. When I used my DSLR the LP filter was a permanent fixture. So, is there a benefit to using a LP filter with my setup? How do I know whether I would gain anything from one?

[Psychobilly]

If the transmission band of the filter falls outside the emmision band of the light pollution source then theres no need...

Some of the LRGB sets are designed that istead of overlapping there are gaps at where the main Na and Hg emission lines occur...

like this set

http://www.firstlightoptics.com/rgb-filters-filter-sets/baader-lrgbc-ccd-filter-set.html

(hopefully Steve FLO ) wont mind me Including the pic of their site

For narrowband then there won't be an ovelap between the LP and the Emission lines...

Peter...

[swag72]

Thanks Peter for using the graph for my exact filters!!! So, I would gain nothing from a LP filter with these LRGB's?

[Psychobilly]

What are the predominant light sources causing your LP?

Is it the typical Orange Na glow?

Peter...

[swag72]

Yes

[Psychobilly]

Then it should fall into the gap between the R and G filters...

Peter...

[swag72]

Excellent - Many thank you's kind Sir!!

[AndyUK]

Hi Peter - What would you suggest when taking Luminance though, as the Luminance includes that dreaded "LP" gap between R&G...?

I've been "umming and arring" about this for quite a while - My initial thought (on suggestion from Dennis) was to always take unbinned RGB and then throw all of them together to create a false Luminance (which would therefore exclude that "LP" gap), but if you wanted to 2x2 bin the RGB, then that would seem to defeat the object as you'd then either get the Luminance effected by the LP or need to take unbinned RGB (which would take 3 times as long!).

Anna very kindly performed a test for me on an image, one processed as "true" LRGB and the other (RGB)RGB - The result is attached. I need to attach her covering text though:

Considering my Lum had 1200s vs 900s on the RGB filters it might not be a fair comparison, but here ya go. I processed the RGB section the same, but for the Luminance layer of the comparison one I stacked all the RGB and used it instead of the Luminance filter.

With the longer subs on the Lum filter it definitely goes deeper both star-wise and nebula-wise. Also, without the Luminance to block it out, the halos on my blue filter (soon to be replaced hopefully) show more. But its not TOO bad in spite of that.

I could maybe stretch the RGB as Lum stack a bit farther, but the blue bloated stars were getting a bit much for my liking

If possible, I (probably like Sara!) would probably prefer NOT to splash out £180+ on an IDAS or something, but I also understand that they're also great at removing gradients and also increase contrast (Hutech Astronomical Products - LPS Filters: CCD Examples) , so I'm still completely undecided... I can't really decide whether I really need to spend the cash... or not!

[Psychobilly]

I only use DSLR's which are OSC I have a range of LP filters from Orion, Astronomik and Hutech my most used filters are the Hutech IDAS onrs which I have in 2" and MFA inbody format..

The IDAS works well with my Low Pressure Sodium LP and gives the best colour reproduction needing less tweaking in the post processing

Peter...

[swag72]

So luminance is the problem then - Light pollution affects only the luminance channel.

Do people use a LP filter then just for luminance or just create the false channel as Andy shows above?

Also Andy - What filter would be needed? A 1.25" that screws into the Atik nosepiece? I'd rather purchase a filter than mess about with even more PP!!!

[swag72]

Any other thoughts out there on LP filters and Luminance?

[AndyUK]

Also Andy - What filter would be needed? A 1.25" that screws into the Atik nosepiece?

Sorry - I missed this bit... I was thinking of buying a 1.25" and putting it on the end of the nosepiece, but I did consider getting a 2" (£10 more) and putting it on the 2" side of the 1.25-2" adapter. However, that wouldn't fit my Geoptik adapter and, as any "hoped-for" CCD upgrade would be to an Atik 4000 (please?!), the 1.25" would be the one for me.

If you don't foresee something like Geoptik in your kit box, then I'd probably suggest the 2" might be a better bet for you...

(EDIT; For more info on the IDAS LP - http://www.sciencecenter.net/hutech/idas/lps.htm)

[Spikey]

Sara I've been pondering this myself as I'm about to try out my Atik16ic for imaging LRGB. I've yet to purchase a manual filter wheel or filters but will probably go for the Baader LRGB set also.

I was wondering if it's possible to use a 1.25" LP filter in place of the luminance?

So, in effect the Lum channel is taken through the LP filter ? Saves having an extra filter in the chain (the Baader Lum) for no gain?

[AndyUK]

Hi Spikey - You obviously could just use the LP filter in place of L (removing the L filter and replacing with the LP in the carousel), but apparently even the IDAS LP will gives a slight colour cast so it's recommended that you also use the LP filter with R, G and B as well to make it all consistent...

If you look at the spectrum graph for the IDAS (Hutech Astronomical Products - Light Pollution Suppression (LPS) Filters), and compare it to the spectrum graph of the Baader filters as posted by Peter, you'll see that it also takes some little slices out of R, G and B to remove Mercury LP, which I guess is where this "shift" might occur. Having said that, I have no idea how much of a difference those "little slices" would actually make to an image (or how much extra effort might be required to take it out in post-processing... )

Of course if you'd bought the LP filter anyway you could try both ways - using the LP on its own instead of L and then putting it in series with the RGB filters... (I wonder if anyone's ever done a test with a mono CCD to see exactly what the impact of ONLY using an LP filter for Luminence is...? )