Click to Enlarge Coronado PST

Disclaimer

I just wanted to say up front that I am not an experienced solar observer. My solar observing experience is really limited to what I’ve done since I’ve had my PST! So I can’t offer much by way of comparison with other solar observation methods or scopes. What I offer here are my impressions and experiences with this great little scope.

First Impressions

I ordered my PST from Sirius Optics just a couple of months ago (June 2006) and it was delivered a couple of days later. The box it arrived in was a sturdy cardboard box that leaves you in no doubt that it comes from Coronado, and the scope was well protected in firm fitting solid foam.

Initial impressions were that this is a quality instrument – even the packaging is well thought out. The solid foam around the scope and supplied eyepiece (a 20mm Kellner), is pre-scored to allow for the optional MALTA table top mount and tripod, and also to allow fitting of the foam directly into the optional PST case. There is also provision for up to 3 more eyepieces.

The build quality and appearance of the PST is great – it looks and feels like quality with a capital Q. The objective lens cap clips securely into place and sports a distinctive “Danger” warning sign. Focusing is achieved with a small knob at the lower back of the scope body, and the focussing action is smooth and light.

The H alpha filter is adjustable, to slightly change the bandwidth to highlight either limb detail or surface detail. The action of the filter adjustment ring (at the front of the black scope body) is stiff but smooth. This helps to make sure the adjustment ring stays in place once adjustments are made.

There are two ¼” tapped holes in the bottom of the scope body for attachment to a scope dovetail or camera tripod.

The one thing that I felt was a bit of let down in quality was the plastic eyepiece clamp screw. A small point I know, but for the quality that the rest of the scope demonstrates I had expected a metal screw.

First Light

Aligning with the sun is a snack. I found watching the shadow of the objective lens cell on the tube of the scope a good indicator of rough solar alignment. Once close, the built in solar rangefinder comes into play. Light enters a “pinhole” at the front of the scope body and is projected onto a small screen on the top of the scope body. Just adjust the scope till the little image of the sun is near the centre and you are ready to view.

After reading the brochure that comes with the scope which suggests that first time users should refrain from adjusting the filter initially, I was a little disappointed by my first viewing. Having achieved focus on the sun’s limb, there was little if any detail visible on the disc. The brochure does mention, however, that altitude can affect the performance of the filter, so, being at an altitude of 1100m, there was nothing for it – adjustments had to be made!

It’s no big deal to adjust the filter. Steady pressure on the adjustment ring and the filter is slowly adjusted to suit your needs. Once adjusted appropriately, surface detail such as sunspots, plague and orange peel become evident. At a different setting, limb features such as flares and prominences become more distinct. Having successfully adjusted my PST and viewed my first solar surface features, I was very happy with the scope and it’s first time performance.

Later impressions

From time to time I do find it difficult to keep the detail on the solar disc sharply in focus. This may be due to poor focussing on my part, the seeing conditions, poor adjustment of the Ha filter or transient temperature effects. I often use the PST late in the afternoon after work on a camera tripod just to get a quick half an hour to an hour of observing in before sunset. Often the best results seem to be at the end of that time, whether that is due to transient temperature effects in the scope settling down, changes in air movement in the atmosphere, or me simply “getting my eye in” I’m not too sure yet!

The 20mm Kellner eyepiece supplied with the PST works well, and gives a sharp image. For observing from a fixed tripod, an eyepiece with a wider field of view might be in order to keep the full disc in view longer. Using a fixed tripod and a 13mm Nagler type 6 EP gives very satisfactory results to me – plenty of image scale and field of view.

Maximum theoretical magnification with the PST is about 80x – achievable with a 5mm EP. I have briefly tried a number of high magnification combinations, such as a 6mm Plossl, and the 13mm Nagler with a 2.5x Powermate but they appeared a bit too soft for my liking, with the 6mm Plossl also suffering from lack of eye relief and the need to frequently move the scope (when used with a fixed tripod). Choosing a less aggressive combination around 50 to 60 x magnification seemed to give better results to me.

There have been some quality issues relating to the objective lens on some PSTs. This manifests itself as an orange discolouration behind the objective lens (inside the scope). Fortunately, my scope has not yet shown this problem, however, reports are that some of the replacement objective lenses/scopes have a better quality coating and improved performance with a slightly narrower bandwidth output making details more distinct. Reports from those that the author has heard about, are that Coronado are very good to deal with about this problem and have achieved a quick turn around time for the scope – considering it has to go back to USA.

Improving Performance

As is normal for most scope owners, once you have a scope you start to look at ways to improve performance. One way to improve the PST’s performance is to use an optional SolarMax 40 to “double stack” the scope. At roughly $700 to $800 dollars for a SolarMax 40, that option is out of the question for the author at the moment, but reports are that this can reduce the bandwidth of the PST to under 0.5 nanometres (currently a PST is about 1 nm bandwidth) improving contrast and making detail more distinct.

Another way reported to improve PST performance is to use a polarising EP filter. This is a much less expensive way to increase contrast in the detail as the output of the PST is polarised. Careful rotation of the eyepiece and filter can then be used to enhance contrast in the image. The author has no experience with this technique but at the time of writing is considering purchasing a filter to test this out.

Why did I buy a PST?

Well… it was a whole new facet of astronomy to me. My entire solar observing experience prior to buying the PST was projecting sunspots on to a screen with my old 8” newt 20 years ago! Finding out about the PST just told me there was a whole area of astronomy I knew very little about!

I considered buying a solar filter for my C8 but as I have to set up and align the whole scope each time I figured it probably wouldn’t get used often. The PST is easy to set up and use on a basic camera tripod, so I figured I would be more likely to use it after work and between activities on the weekend, and that’s certainly proven to be the case.

Conclusion

The Coronado PST is an impressive instrument priced within reach of the amateur astronomer. It can provide detailed whole disc views of the sun, revealing prominences, flares, sunspots, plague, filaments and “orange peel” surface detail. The PST is well built, easy to use, extremely portable and versatile. It won’t give you the super close ups of sunspots that a larger scope will, but it is surprising just how much detail can be clearly seen with this scope. It is great value for money and to be recommended.

PST Reference Data

	PST_reference.pdf	21.06 KB
	PST Reference Data