Spectroscopic testing of the Canon front filter element

[Merlin66]

Since publishing some tranmission graphs last year which clearly show that the front filter ( the one usually left behind when the colour balance filter is removed for the "mod") acts as a very good UV-IR cut filter, I've been beat up by the guys in the States who say an additional UV-IR "replacement" filter is the ONLY way to go......

Obviously I disagree.

I managed to set up the MG80 spectroscope this morning and test a separate front filter element ( removed from a 1000D).

Surprise, surprise it shows exactly the same...

The attached file gives details.

Feedback and comments welcome.

100_101_comp_small.bmp

Canon front filter test.pdf

[Gina]

Very interesting Quite a number of members find the front filter to be a good IR/UV cut filter while some US members find you need a CCD type filter or separate IR/UV filter to stop star bloating - very strange! I don't think it's that the US versions are different from the UK versions - there would be no point from Canon's point of view. If only we had plenty of clear night skies we might be able to carry out experiments but as things are we just can't afford to spare the imaging time I have both UK and US versions of the 1100D/Rebel T3.

[Gina]

Been having another/closer look at your spectrum for the 1000D front filter. Is the y scale linear? It looks as if it is though there's no zero label.

Assumining a linear y axis, the curve drops by a factor of 2 from the H alpha transmission value (just one stop) then falls off quite gradually up to around 7960. Could this be what the Americans are talking about? Would it be possible to expand this region around and above Ha?

[Merlin66]

Gina,

The base is zero.

I agree there is a "transition" beyond 700nm to zero.....

Is this really a critical area???

[sologuitarist61]

That makes very interesting reading Ken, especially to those of us in the throws of modding, thank you

[Gina]

Gina,

The base is zero.

I agree there is a "transition" beyond 700nm to zero.....

Is this really a critical area???

I don't know - it was just a thought. But I think we need a logarithmic scale to compare things as the brightness of celestial objects covers many orders of magnitude.