Old 12-08-2014, 11:58 AM
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traveller (Bo)
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Filter for full spectrum modded DSLR

Appreciate some input from experienced members on this.
I am currently using a Astronomik CLS filter for my modded DSLR (40D). I have just done a full spectrum mod of a 1100D (removing both the UV/IR filter and the "blue" filter in front of the sensor).
I am told that I will need a L filter to avoid star bloat when using refractors.
Looking at the filter graphs from Astronomik, two things are apparent
1. CLS filter cuts off below 440nm and again between 540-630nm (sodium and mercury for light pollution).
2. L filter cuts off below 380nm, allowing more data bewteen 380-440nm, but also allow for light pollution between 540-630nm.
So I have the following questions
1. What data can I obtain in the 380-440nm range?
2. Is that data a worthwhile trade off for the increased light pollution in the 540-630nm range?
3. Will CLS reduce star bloat like the L filter on a refractor?
I shoot from my suburban backyard, cannot see street lights directly as they are blocked by my house, but they are only 30 meters away (they emit low power white light, not LED and not the orange glow lights like you have on main streets, although those orange lights are a just hundred meters up the road).
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Old 12-08-2014, 12:10 PM
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cometcatcher (Kevin)
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There's two versions of the CLS. You either need the CLS CCD version, or if you already have a standard CLS just add the L filter or other UV / IR blocker.

As for the 540-630nm range, there's often a lot of comet emissions in that part of the spectrum, if you're into comets.

I find the CLS rather heavy handed for mild LP. You might be better off with a milder filter like Baader Moon and Skyglow, Idas LPS or similar broadband filter.
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Old 12-08-2014, 01:11 PM
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traveller (Bo)
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Thanks Kevin,
I have the CLS EOS clip in filter (not the CLS CCD). If I do add a L filter in front of it, the CLS filter will cut off the spectrum bypassed by the L filter, so it defeat the purpose IMO (or is this purely to get rid of star bloat?)
Good point about comets and broadband filters too.
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Old 12-08-2014, 01:53 PM
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Merlin66 (Ken)
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The L filter (or a good UV-IR cut filter) reduces the "bloat" in lens systems by suppressing the usual out of focus wavelengths - the UV (<400nm) and the NIR (>700nm).

In my modded 1000D I just use a clear clip in filter to maintain the focus with standard lenses and add separate filters up stream when needed.
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