IDAS Filters

There’s lots on the Internet about how filters work.  SkySlab recently swapped out an IDAS light pollution (LP) filter with a standard IR filter that comes with the QHY.

A lot of QHY8 owners complain that the colour reproduction when producing CCD images is not accurate.  This is true if you use the standard IR filter that comes with the QHY8 CCD camera. So, for a while now, SkySlab acquired an IDAS LPS filter that limits the amount of sodium based light wavelength coming into the CCD camera.  Light pollution is not an issue for SkySlab except that which is made locally on site.  You can see the glow of Canberra and Yass on site but it doesn’t extend to the skies overhead.  That’s one of the great things about living away from a city.

On most clear nights the astronomer can see the Milky Way, Small and Large Magellanic clouds with the unaided eye.  It’s even better with binoculars.  But, notwithstanding the lack of sodium based light pollution,  the experiment was to see what could be done about the red and yellow biased colour shift on the QHY.

The results are excellent. The blue wavelength is brought out significantly, red is not changed and green is reduced.  These results are visible in the differences between SkySlab’s earlier images compared to the latest images.  Add to that, tracking has improved!

SkySlab imaging systems use an IDAS LP4, or 2 from time to time, along with specific CCD filters.  The theory is, with a standard IR filter, you do filter out a lot of the wavelengths that make stars seem bloated, but with an IDAS LPS, on top of getting rid of IR you also get rid of some other troublesome light wavelengths that can make a tracking star bloated.

This is all good and well, but tracking is achieved with an offset prism that collects some light coming out of the objective before it hits the QHY. So, that means no filter on the SBIG tracking camera. How then does it track better when there’s essentially no change to the SBIG image?  No clue, all we can surmise is that somehow the IDAS is reducing whatever is feeding back into the tube and hitting the prism backwards. Or is simply absorbing the wavelengths and less re-emission means less light for the tracking camera to cope with.

It’s a long shot, but then so are most astro-photos taken by SkySlab.

Kazuyuki Tanaka has made some interesting observations on the noise reduction capabilities of the IDAS LPS filters, the results are below (source: also has a lot more on Hutech equipment and IDAS filters.

IDAS LPS Filter Test (Kazuyuki Tanaka)

These are photos taken with the IDAS LPS filter compared with photos taken with no filter. The “m” values given are the limiting visual magnitude at the time the photos were taken. Note the improved noise performance when the LPS filter is used. Test results are by Kazuyuki Tanaka (Arizona).


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