CLS filter issues

[8472]

Hi

I've been imaging for a few months with a Canon compatible clip filter in my Baader modded 1100D, but I've lately been noticing my images all seem to have an overly red tinge to them, regardless of how I care to colour balance them.

Prime example being the Flame Nebula coming out red, as opposed to orange/yellow in other folk's images.

A quick search for other users with similar kit yields images with a similar hue to mine.

I've used the align RGB channel option in DSS to limited success, but the colours are still a little off.

I'm primarily processing with Startools and have been told once a CLS filter is in the optical chain it will block LP along with some light wavelengths you want, and no amount of colour balancing will bring them back.

If that is true, am I better off without said filter? Downside is my subs look washed out far quicker, so sub length has to be curtailed to keep the histogram in check, therefore total integration time needs to be increased.

Thoughts, anyone?

[Scooot]

I've used a CLS clip filter for some time with my Canon 450 and I constantly had problems with star colours. I've recently stopped using it and its now become much easier to process stars. However, as you say, I can't expose for so long without it, which is OK for stars but when I target nebula I might have to put up with it and use it again. Or perhaps buy a different filter?

[serbiadarksky]

Yes the filter is a star color killer for sure.

Sometimes I use it, sometimes no..

Its totally up to you will you use it or no

this is with the filter and still have star color

[vlaiv]

CLS filter is pretty basic LPS filter - and it cuts too much into spectrum.

There are color balancing methods to try to recover some of the color, but it will not work 100% since indeed some of frequencies are completely cut off. Best you can try is star colorimetric calibration.

There are other filters out there that help with LP that are far less aggressive. Hutech IDAS LPS range is considered to be the best by many people - so have a look at it.

[8472]

Yes, after further research by bringing my images to GIMP, and then attempting to adjust the Yellow saturation channel, I've concluded the adjustments have zero effect. 

So it looks like it obliterates anything yellow, regardless of it being light pollution, stars or whatever.

Ah well, back to the drawing board.

Could I get away with forgoing the filter and lowering my ISO one stop to compensate (keeping the same sub length), or would that be to the detriment of loosing valuable imaging data?

[vlaiv]

21 minutes ago, 8472 said:

Yes, after further research by bringing my images to GIMP, and then attempting to adjust the Yellow saturation channel, I've concluded the adjustments have zero effect. 

So it looks like it obliterates anything yellow, regardless of it being light pollution, stars or whatever.

Ah well, back to the drawing board.

Could I get away with forgoing the filter and lowering my ISO one stop to compensate (keeping the same sub length), or would that be to the detriment of loosing valuable imaging data?

Don't think LP should saturate your sensor that fast. Do you actually have clipping if you shoot without filters?

Only problem with LP if there is no saturation is lower SNR and gradients. First one you can overcome by more total imaging time (but be aware, it really needs silly number of subs / total imaging time to reach SNR that can otherwise be easily achieved in dark skies). Gradients can be removed in post processing to some extent. It is better if you have smaller FOV as gradients over small surface tend to be pretty much linear. Greatest problem with gradients is when using large FOV - then you get "curved" gradients - much harder to process out.

Don't worry if your subs look washed out without filter - simple setting of black point in processing will compensate for this.

[8472]

6 minutes ago, vlaiv said:

Don't think LP should saturate your sensor that fast. Do you actually have clipping if you shoot without filters?

Only problem with LP if there is no saturation is lower SNR and gradients. First one you can overcome by more total imaging time (but be aware, it really needs silly number of subs / total imaging time to reach SNR that can otherwise be easily achieved in dark skies). Gradients can be removed in post processing to some extent. It is better if you have smaller FOV as gradients over small surface tend to be pretty much linear. Greatest problem with gradients is when using large FOV - then you get "curved" gradients - much harder to process out.

Don't worry if your subs look washed out without filter - simple setting of black point in processing will compensate for this.

Thanks for that.

You are right about the FOV having an effect.

I've no issues with 300s subs at 1000mm FL, but at nearer 300mm the histogram peak is about 2/3rds to the right. It's just about processable, but far from ideal.

Guessing I should just put up with it providing it doesn't get any worse?

[vlaiv]

There are couple of things you can do:

1. If you use ISO higher than 800, bring it back down to 800 - that will give you more "room"

2. If you are going for emission nebula type of target - shoot nebula with CLS and shoot stars for color without CLS filter. You can use significantly shorter exposure for stars, and less total imaging time. Blend in star color in processing. This will not work well for broad band targets like reflection nebulae and galaxies.

3. Shoot without filter and if you get uncomfortably close to saturation - lower sub exposure time. Going long exposure times makes sense when read noise has significant contribution. In heavy LP, LP associated shot noise will swamp read noise very fast, so you will not benefit from going long exposures - there is little to loose by going shorter subs - but more of them - same total integration time. This also has benefit of less problems with guiding (tighter stars), and anything that might lead you to reject particular sub will lead to less percentage of total imaging time lost (better to loose one 1 minute sub then one 10 minute sub)

4. Depending on the type of your LP and budget - consider Hutech IDAS LPS filters?

[serbiadarksky]

1 hour ago, 8472 said:

it looks like it obliterates anything yellow, regardless of it being light pollution, stars or whatever.

Not true.

For example, Betelgeuse will still have the orange color, Shedir, Aldebaran..all of them will keep the star color

[vlaiv]

2 minutes ago, serbiadarksky said:

Not true.

For example, Betelgeuse will still have the orange color, Shedir, Aldebaran..all of them will keep the star color

That really depends on sensor used. Observe following graph:

In trichromatic RGB system, yellow is represented by mix of green and red (so we are not talking about spectral "yellow", or part of rainbow that our eye perceives as yellow, but any yellow color - stimulus that our eye/brain sees as yellow, one that stimulates both green and red receptors in equal / almost equal measure). In above graph we have sensor RGB sensitivity superimposed on CLS "blocking" regions (marked in grey). As you can see right grey strip is cutting massively into R and G response of sensor (in almost equal measure). Anything that is rendering as yellow due to having energy in that part of spectrum will be simply cut of by CLS - so yes, statement that it obliterates anything yellow is pretty much true.