Light pollution filters

[Lawrence]

While I start to learn StarTools I noticed the early paragraph that seems to imply that it is best not to use an LP filter but to use ST to correct the image during processing. Is that a correct interpretation? Until now I have always used an IDAS filter.

TIA

Lawrence Harris

[admin]

Hi Lawrence,

Light pollution filters are a great tool. It's just good to keep in mind that they will irrevocably change your colors in a non-recoverable way. It's obviously up to you whether you deem that to be a problem, but it's something to keep in mind. From the website and manual;

Colour balancing of data that was filtered by a light pollution filter

Colour balancing of data that was filtered by a light pollution filter is fundamentally impossible; narrow (or wider) bands of the spectrum are missing an no amount of colour balancing is going to bring them back and achieve proper colouring. A typical filtered data set will show a distinct lack in yellow and some green when properly colour balanced. It's by no means the end of the world - it's just something to be mindful of.

Correct colouring may be achieved however by shooting deep luminance data with light pollution filter in place, while shooting colour data without filter in place, after which both are processed separately and finally combined. Colour data is much more forgiving in terms of quality of signal and noise; the human eye is much more sensitive to noise in the luminance data that it is in the colour data. By making clever use of that fact and performing some trivial light pollution removal in Wipe, the best of both worlds can be achieved.

[Lawrence]

Thanks Ivo
With my setup, it is not really practicable to swap the LP filter in and out. It is deep within the optical system. I either leave it in or take it out - perhaps for a trial run of a few sessions. I do have some pics taken before it was fitted, though locating them would be time-consuming. I just wondered whether 'starting from scratch', it might be better to leave it to the software - or not.

Thanks.

Lawrence

[admin]

Hi Lawrence,

As quoted above, it's by no means the end of the world, but definitely something to be mindful during color calibration, that's all! It is brought up because knowing that you cannot expect to achieve a correct full-spectrum representation of the data you recorded - no matter how hard you try in post-processing - will help when making processing decisions; this knowledge will prevent frustrations with regards to a perceived inability to get a "good looking color balance".

As far scientific value of the color information goes, there is still much to be distinguished even when looking at LP filtered data (though star color temperature perception definitely suffers some what as stars appear to fall into either a cooler orange or hotter blue/white camp).

Thanks Ivo
With my setup, it is not really practicable to swap the LP filter in and out. It is deep within the optical system. I either leave it in or take it out - perhaps for a trial run of a few sessions. I do have some pics taken before it was fitted, though locating them would be time-consuming. I just wondered whether 'starting from scratch', it might be better to leave it to the software - or not.

Thanks.

Lawrence