The vast majority of us pursue astronomy and astrophotography from light polluted skies. If you are one of those with a RASA imaging telescope you may find that the sky background can saturate very quickly as the telescope is “very fast”, and the light pollution sky “noise” overwhelms the “signal” from the celestial object you are imaging, making it more difficult to acquire a detailed image. So what can you do about this? Apart from either imaging from a dark sky site, or “up sticks” and moving somewhere else with darker skies, a light pollution filter will help.
To help combat light pollution, Celestron offer a RASA Light Pollution Imaging Filter for the RASA 8″ and RASA 11″ scopes which has been developed to block the light pollution from some common sources including high- and low-pressure sodium streetlights as well as natural airglow. This filter blocks specific areas of the spectrum instead of wide regions to maintain image brightness and colour balance. The H-α(656nm), H-β (486nm), OIII (496nm and 501nm), and SII (672nm) wavelengths (see below) associated with many emission nebulae are highly transmitted, ensuring minimal loss of image brightness for those particular celestial objects.
The filter is very easy to fit. Simply remove the clear optical window that comes with the RASA and replace it with the Light Pollution Imaging Filter. Full fitting instructions can be found for the RASA 8″ here and for the RASA 11″ here.
Each filter comes with a plastic storage case to safely store the filter or the (removed) optical window that came with your RASA when not in use. You will also receive a spectral transmission graph unique to your specific
Below is an image taken with a RASA 11″ with and without the filters. On the left – without the use of the filter – the sky looks washed out and there is not much detail in the middle and at the edges of the Dumbell nebula. The image on the right – with the filter – shows a darker sky background with the nebula showing more detail and more contrast too.