The Coronado PST™ Solar telescope

It was during our visit to Astrofest 2005 that Emma suggested that astronomy should be carried out on warm summer afternoons sitting by a pool, sipping chilled wine rather than freezing your socks off in the dead of night! Emma was also quite taken by the photographs of the Sun in H-alpha and so it came that we ordered our PST from the Telescope House stand that day.

Basic design - In broad outline the PST consists of a small refractor with a built in narrow band solar filter centred on the H-alpha spectral line at 656.28nm. In practice filtering is carried out in two main stages - the tuneable Fabrey-Perot filter and the blocking filter into which the eyepiece fits.

Review - After our PST arrived it was it was four days before we even saw the sun – hence my first chance to try out our PST. Just to set the scene conditions weren't great to say the least. We have had gale force winds, accompanied by occasional horizontal rain showers and an endless stream of clouds crossing the sky at the speed of a military jet. On top of this the only time the Sun did come out it was nested neatly among the flailing branches of a large tree – just great!

In the end we decided there was absolutely no point setting up outside, so the scope was set up on the Min-EQ and we did what observing we could through the open kitchen door. Not good for seeing I agree, but better than being hit by a falling branch.

To get my gripes such as they are out of the way first:-

I found the packing that our PST came in incredibly tight, so tight in fact that I was concerned that the scope would be damaged by trying to remove it.

As this packing is also used for permanent storage I trimmed it all the way round so that the scope is now a snug but not tight fit.

The second complaint is that the instruction sheet that came with our PST is incorrect. It quite clearly states that the focus control "is the only external portion of the PST you will ever need to adjust". It fails to mention the filter tuner at all.

Another minor complaint is that on our PST I found that the clever little finder is slightly out. Aligning the Sun precisely in the centre as described in the instructions places the Sun right at the edge of the FOV using the eyepiece provided - however I quickly learnt how far to offset it by.

I'm not really sure why but I also found the focuser less intuitive than anything I've used on other scopes – for some reason every time I had to focus I found myself initially turning it the wrong way. That said once I did turn it the right way I found fine adjustments to the focus easy and smooth.

All of these minor issues however become completely irrelevant the second you bring the Sun into sharp focus for the first time, only then do you realise what a great piece of equipment this is.

Within minutes of setting the PST up I had seen all manner of prominences, despite the adverse conditions - there was a small but perfectly formed arch, a huge prominence that looked like a mountain range, one that looked for all the world like a drip coming out of a tap, a few light wispy ones which I missed completely at first and finally a little flying saucer hovering just above the surface – all very scientific descriptions I know!

I found that all the eyepieces I tried could be brought to focus, including the 10mm Speers-Waler, which just looks ridiculous on this scope! I think a 20mm and a 15mm are a must with the PST to compliment the 12.5mm provided. I found myself wishing that I hadn't given all those old Kellners away years ago because the one that comes with the PST works well.

Adjusting the filter to bring out various details is a cinch; it's a bit like having your own brightness control for prominences. On some of the prominences fine adjustment could make different parts of them appear more prominent – presumably due to differences in velocity of the ejected materiel.

Not having used a PST before I have no idea if this is normal, but on ours the prominences showed best when the tuner was turned almost fully towards one end of the adjustment and only faded slightly before reaching the end stop - not a problem, more of an observation.

I like the fact that you can de-tune far enough to obtain an almost white light view, albeit through rose coloured spectacles! For such a small scope it shows an amazing amount of detail in the sunspots, but then there are huge advantages for a refractor working in monochromatic light. From a purely aesthetic point of view I loved the colour of the Sun's disk – it just reminded me of a tropical sunset.

I found that by far the best views of surface details were obtained at the higher power using my 7mm Ortho - I found them really quite difficult at lower powers but they snapped into view instantly using the Ortho.

The scope does have a distinct sweet spot - allowing the Sun to slowly drift across the FOV rather than tracking it clearly demonstrated this. I did not find this a major problem, but it does make it easy to miss features that fall outside this sweet spot.

In terms of magnification I think the scope pretty much follows the 50x per inch rule - at 80x the images are still sharp but signs of softening start to show. At low power (16x) there is some vingnetting caused by the blocking filter, but if the image was centred this remained unaffected by it – if anything it may actually have been helping the contrast slightly, at least as far as the eye is concerned.

I have read reports of that the PST produces ghost images yet I saw absolutely no evidence of this at all; if any exist then they must be extremely faint.

Despite the frustrations of having tried it on such an atrocious day; it is obvious Coronado have produced a superb instrument. It's not often I say this, but the PST has far exceeded my expectations and I can't wait to use it again under better conditions. In particular I want to push the magnification and also try some imaging – so far I'm blown away by it!

Yes the PST has its quirks, annoyances even, but these are far outweighed by some of the views it provides.

Note:- the H-alpha image on this page was taken by us using our PST on somewhat better day than that described in the article.

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