Filter and CCD frequency response curves.

[[1ponders] (Paul)]

As an addendum to Dennis' great thread on LED Circuit Advice Required and the subsequent off topic discussion of filter responses, I felt that a dedicated thread in relation to camera and filter frequency responses may be of use to members.

So..... if you have a particular CCD camera (mono or otherwise) and a brand of filters (r, g, b, Ha, OIII etc) please post an image of their frequency responses or a link to a chart on the net somewhere. I for one would certainly be interested in see them.

[Dennis]

Okay Paul - let's kick off with my ST7E QE and SBIG Filter Response curves.

Cheers

Dennis

[jase (Jason)]

I'm using the same LRGB filters as Dennis, these are the Custom Scientific set made for SBIG.
There are more filter spectra responses on the Custom Scientific website.
I would also recommend reading Alan Holmes (PH. D, optical science) article on the SBIG/CS filters - http://www.sbig.com/pdffiles/SBIGFilterSet.pdf
I use a starting ratio of 1.35:1.0:1.65 (R:G:B) to compensate for the green wavelength sensitivity.

[[1ponders] (Paul)]

Thanks guys, nice kickoff.

[MadMonk]

Hello -

I'm completely ignorant when it comes to imaging, but I have this question:

whereas there is an overlap between Blue and Green in the RGB Transmission curves there is a gap between Green and Red, around 600nm wavelength.
If this is on purpose, why? If not, does it mean we never capture anything orange-ish in colour?

thanks - T

[jase (Jason)]

The gap between the green and red wavelengths is intentional and is a characteristic of the SBIG/CS RGB filters. If you read the .pdf document I provided (http://www.sbig.com/pdffiles/SBIGFilterSet.pdf) it explains this well.

“There is a “hole” in the new filter set between 570 and 610 nM. While this may offend the purist, there are no significant astronomical emission lines in this hole. Stars have very wide spectral distributions, so no loss of information results from this hole – you do not miss “yellow” stars (which are yellowwhite). However, sodium vapor streetlights terribly pollute this region, …” page 5 of above document.

“There is a “gap” between the red and green filters -The gap in intentional. It reduces the effect of light pollution and misses no important emission lines. The gap was necessary to balance the counts from a sunlike star.” Page 6 of above document.

The SBIG/CS LRGB filter set purposely minimises colour cross over, which provides excellent contrast with clean emission lines. I’d like to note that this gap is only present on the RGB set - the CS UVBRI filters include this emission line for photometry data collection.

I'm happy to also elaborate on colour cross over techniques used by Astrodon, but will leave this for another day.

[Terry B]

So how do you measure the frequency response through each filter and produce a graph?

[jase (Jason)]

There are a few ways of measuring the spectral response, but the most accurate is to use a spectrograph. If you're technically inclined you can build and calibrate one. An excellent resource for building your own can be found at http://www.astrosurf.org/buil/ and http://www.astrosurf.org/buil/lisa/lisa.htm.
Once you've completed the calibration process you can download Visual Spec - free spectral analysis software. This will produce the spectral response graphs through each filter.

Once you're done, you can complete a comprehensive filter spectral response library such as this - http://astrosurf.com/~buil/filters/curves.htm

Off topic; but for those DSLR users that are using filters - you may find some information presented here interesting - http://astrosurf.com/~buil/us/test/test.htm