Wednesday, December 26, 2007

Inside the C5/PST setup

The first component is the ERF, usually, people will use the C-ERF from Baader but it's pretty expensive. Instead, I used a big red aerial lens filter which I call it a Cheapy-ERF for its cost is something like $20-30 USD shipped! Mine has 6.5" clear aperture, and I've made an adapter to mount it on my C5 with the help of a local folk.

The ERF is used to filter out unwanted light, in this specific case, only red light will be passing through it. There is no guarantee that there will be no signal leakage, i.e. it could contain green and blue light, UV/IR could pass through too. This is just a very preliminary filter to reject some unwanted energy. It's particularly important for my C5 based setup, since if you let unfiltered sunlight into it, the baffle tube can get burnt, the secondary can be damaged as well.

I found that the sun at prime focus after the ERF is still quite hot, if a C-ERF is used, the image stay cool, and that's the C of C-ERF means. However, with the Cheapy-ERF, the image at prime focus is good enough.

I tried to use a CaK after the Cheapy-ERF and I got nothing, that means the Cheapy-ERF is quite effective to remove other light than red.

Since the light after Cheapy-ERF is still strong, it could cause internal reflection, so I added some soft padding to tilt it up slightly, most of the internal reflection issue is solved.

Next is the PST etalon, basically, the heart of the whole system. An etalon will filter light to enable ultra-narrow bandwidth, like in this case, at around 1A to show prominences and surface detail in H-alpha. The PST etalon can be tuned like the rest of the solarmax filter, there is a collar ring to let you adjust. And the PST etalon has a setup of afocal lens to make light parallel before going into the real etalon, and then to converge light again for viewing/imaging. H-alpha, and its harmonics will pass through the etalon.

A 1.25" nosepiece is made to insert into the rear port of the C5, and a 1.25" visual back is made to let us use a blocking filter there.

Finally, after the cheapy-ERF and the etalon, only H-alpha light and its harmonics will pass through. So to filter out the harmonics, we use a dielectric H-alpha filter and it's the blocking filter. Its bandwidth has to be narrower than the distant of the H-alpha light and its first harmonics.

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