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REVIEW OF THE CORONADO PST - SAFETY FIRST
The Coronado Personal Solar Telescope [PST] is a complete and portable instrument for viewing the sun in red hydrogen-alpha [H-a] light at a third the cost of its competitors. The break-through design incorporates a number of innovative features and also a few but acceptable compromises with a 'sweet spot' and hint of ghosting - see below.
The telescope has a modest 40mm aperture f/10 OG of 400mm focal length. There are two wideband blocking filters in the system to reject unwanted and harmful radiation - one within the coating of the front OG lens and the second close to the eyepiece. Additionally the PST view is downwards ensuring the telescope is completely safe and convenient to use. Even a small sun-finder next to the eyepiece removes the need to squint along the telescope tube to aim it at the sun.
THE MAGIC BOX
The sealed 'magic box' at the telescope's rear contains a special filter called a Fabry-Perot etalon that isolates light at 6563A where the solar chromosphere comes to life. Its bandpass is quoted at <1A - essential for viewing both limb prominences and disk detail. Any wider bandpass washes out surface features. To ensure a near parallel beam of light through the etalon it is probably set between weak negative and positive lenses. The resultant PST solar image is about 4mm diameter and the etalon is probably small too and this may explain the modest cost of the PST. Other Coronado solar telescopes have the etalon placed before the OG and are hence much larger and more expensive. However in any form the etalon must have exquisite optical surfaces orders-of-magnitude better than regular lenses to work properly. My scanning spectrohelioscope of old is an alternative to an etalon.
Behind the etalon a small penta-prism redirects the light upwards through 90o to give a correctly orientated view but rotated through 180o with south up. A knob moves the prism to focus the eyepiece. A rotating collar at the base of the telescope 'tunes' the etalon precisely to 6563A. The supplied 12mm fl x33 eyepiece ensures the whole sun is contained in the eyepiece field. I also use my Konig 12mm and 40mm orthoscopic eyepieces. The PST has a 'sweet spot' across a third of the disk where the image is fully tuned and detailed. The PST can be moved slightly so other regions fall on the sweet spot or the tuner can be tweaked to shift the sweet spot up or down. There is also a very slight hint of ghosting on the top limb of the sun [eg a second limb] but for most viewers it passes unnoticed. When the PST is detuned off H-a effectively the view is identical to a white light sun except that it appears bright red.
WHAT CAN BE SEEN?
The H-a sun is guaranteed to impress. There is a dramatic difference in observing the sun through a telescope in white light against the view in hydrogen light. In the former the sun is either safely filtered or projected onto a white card to reveal sunspots, if present, and occasionally light areas near the limb called faculae. In hydrogen light the sun is very dynamic and there are many different phenomena to observe that can sometimes change on a time-scale of minutes so an event may be exclusively seen by a sole observer as my movie adjacent.
The whole solar disk is reminiscent of an orange with a mottled surface like peel. Seen on the disk are dark worm-like features called filaments and light areas, usually around sunspots, called plages. Projecting off the sun's limb are the ghostly beautiful prominences contorted in the sun's strong magnetic field. Prominences dwarf the earth in scale and sometimes extend to a greater distance than the earth to moon above the sun's limb.
As the sun slowly rotates on its axis over a period of a month the prominences traverse the disk in silhouette as dark filaments. Occasionally a plage will brighten briefly to an intense point of light called a flare and these events can cause, a day or so later here on earth, disruption to radio transmissions, surges in power-lines and the beautiful aurora.
The opportunities to observe the sun are much less taxing than observing the nightsky which must be reasonably clear to make setting-up the telescope worthwhile. The sun, being only half-a-degree wide, can just peep through a brief cloud gap to make the view memorable. Even in our UK climate rarely a day passes when the sun is not visible - often the early morning and late afternoon show a clearing of skies during an otherwise cloudy day. Of course being retired does give the solar observer more time!
Observing with the Coronado PST is simplicity itself. Mine is permanently attached to a lightweight photographic tripod with pan-and-tilt head and can be aimed at the sun in seconds. I use a card screen to shield the eyepiece from glare. The PST's modest magnification [typically x16 - x40] and 40mm aperture [smaller than most night-time binoculars] means that the sun can be observed from indoors through modern double glazing - see top picture. This is especially welcome on cold winter days.
I capture images with my Minolta digital camera supported before the
eyepiece often handheld. I'm fascinated by prominences - they are
the easiest to record whilst disk detail proves relatively elusive.
My regularly updated images are posted here.
I also subscribe to PST Internet Forums and view professional sites that
show daily solar progress at SpaceWeather.com.
The technique to safely filter sunlight in these two telescopes is quite different which reflects in their physical appearance [above] . The H-alpha instrument uses a special filter called an etalon to isolate a waveband of less than 1A [0.1nm] to view the upper chromosphere of filaments and plages on the disk and prominences on the limb. The CaK calcium telescope uses a glass multicoated interference filter and is essentially the same as filters used to remove light pollution when viewing the night sky. A bandpass of 20A [2nm] is acceptable for the violet K line because the actual line in the solar spectrum is itself even wider than this. This filter reveals bright areas on the disk called flocculi especially around sunspots - sometimes even before the spot itself develops.
It was common knowledge that not all can observe in the CaK line. This is invariably due to yellowing of the eye’s cornea with age and I was aware, from observing the solar spectrum a decade ago, that it may apply to me also. It did. It set in motion an attempt to defy nature by placing a glass filter in the viewing eyepiece doped with fluorescent pen marker in the hope the far violet light could be transmuted into visible. It actually worked to a degree with the resultant faint green solar image clearly visible [more discussion here and here] but not sufficiently so as to be practical. Fortunately the ubiquitous digital camera has no such problems and the brilliant blue [not violet for some reason] solar image can be seen on the rear LCD viewfinder by just applying the camera to the telescope’s eyepiece.