Dichroic prism for image data collection

[nytecam]

One aspect of the dire UK weather this summer is the brevity of clear night skies in which to image - especially so for those using separate RGB filters on monochrome cameras which effectively triple the total exposure time.

BWolf has started using twin scopes+cams simultaneously to speed-up data collection but I'm wondering if there's any mileage using a dichroic prism as my sketch below. Unlike RGB filters which kill 66% of the incoming light, a dichroic filter can transmit one colour whilst reflecting a complementary colour with low overall light loss.

Some 3-CCD videocams use the complex Philips style RGB prism [below left] and these cams are available on the s/h market although the sensors are now very dated and 'slow' and unsuitable for astro use. I've no idea as to the QE through the complex prism. Perhaps they could be replaced with modern sensors under computer control.

My simpler cubic prism version [below right] collects R and B data via two CCDs and synthasise G as R+B all in one hit. Any thoughts?

[pete_l]

That's an interesting idea.

I've not seen any 3-way splitting prisms, but you can get a range of dichroic mirrors. Each mirror will split a beam so that light below a given wavelength is reflected and light above is passed through. If you arranged for 2 mirrors, one to reflect blue and pass everything else and a second that reflected green and passed the red, you might get something to work - once you can compensate for the losses in the mirrors. They're about £100 a pop, but it's cheaper that 3 scopes - though you'd still need 3 cameras.

The only problem I can see is that they're designed to work with non-diverging beams, so maybe only long focal length scopes?

[nytecam]

Thanks Pete for that - not sure that operating under a convergent beam would be problem IF all lightpaths were equal as shown. Below a couple more sketches using dichroic mirrors and common [but dichroic coated] prisms.

As an aside I recent used a couple of stacked plane parallel photo filters immediately before my webcam on my Coronado PST scope which shifted the focus point outwards just a few mm so that focus was achieved ! The normal 'trick' is to use a weak Barlow lens to do this but image scale is changed and enlarged so less of the sun's area is recorded per shot.