Solar Observing: PST's rock!
... and no, PST doesn't mean "Pacific Standard Time" here! Rather, PST stands for Coronado's Personal Solar Telescope (http://www.coronadofilters.com/cgi/display_catalog.cgi?w=1024.)
The PST is a wonderful way to look at the sun in Hydrogen-alpha, and has produced a legion of new "solar addicts" in the short time since its release. Hydrogen Alpha (or Ha) is the wavelength at 6562.8A (A=~atom size). Interesting things occur ~1 A. on either side of this peak, and there are pronounced Doppler shifts at work (Coronal Mass Ejections can move at or away from the observer at ~3 million mph). The PST has a tunable filter to accommodate these Doppler shifts. Virtually all users tune their PST to the same spot, which is nearly at the end of the tuner's travel range
Coronado made a price and technology breakthrough using SDI ("star wars") research based on the Fabry-Perot Interference effect in a device called etalon, which is 2 semi-reflective air spaced surfaces that reject all but a wavelength specified by the gap between them. This gap typically changes w/temp and pressure, so Coronado invented a central spacer (central obstruction), which is internal in the PST and external in the Solarmax filters. This grating is one of the most precisely machined items amateur astronomers will use, since the gratings must be made and positioned to ~1/100th wave accuracy. This allow for a relatively temperature independent filter that doesn't require active heating (ovens and thermostats).
The PST is relatively cheap--$500; the next cheapest Ha scope, Coronado's Maxscope 40, is $1600. In both cases this is the price is for a “bare scope”-- you'll need to add in a tripod, at minimum.
I previously owned a SolarMax 40 filter mounted on a Maxscope 40 --Coronado's dedicated 40mm diameter solar scope, with superlative baffling and general design optimized for solar viewing. I bought the PST to be able to stack the Maxscope's filter on it. In A/B side-by-side testing of the two prior to stacking, I was shocked to discover that the PST was rendering the better image resolution, so I ended up selling the Maxscope/SolarMax duo.
The mythical (?) "Super PST"
To say that my particular PST performs better than its nominal specifications is something that the manufacturer's marketing department rejects. I came to the conclusion on this particular unit's resolution with some reluctance, and only after:a) discovering that it yielded appreciably better resolution than the Maxscope I also owned when I did head to head testing between the pair
b) seeing that the image got dimmer but not finer in resolution when stacked with a second SolarMax filter-- and I've tried with several SM40's, not just one
c) comparing it side-by-side to multiple 'stacked' PST's and again noticing no definitive edge between the two in terms of resolution-- although the unstacked image again was brighter
d) comparing it head to head against other unstacked PST's, both directly and via other observers who had looked through a combined total of three dozen+ PST's. All of the observers agreed this one rendered more detail than the others, and many felt that it had less of the "ghost" image artifacts around the solar disc which PST's are prone to because of their design. For what it's worth, I approached this like designing an experimental study, and didn't prejudice any of the observers by mentioning resolution at all-- much less trying to convince them before hand that there was anything superior about this scope's image. Bias and user expectations therefore shouldn't have been a factor.
Again, I went into my PST viewing experience with the strong expectation that it would be a lesser performer than my Maxscope; it took consistent empirical evidence to convince me otherwise. I later heard reports from other solar observers of a handful of other PST's manufactured in this same period (4th Quarter '04) that also seemed to outperform the rest when, for example, all of the units in a store's inventory were tested against each other. This was not reported when the same inventory testing was done in 2005.
UPDATE: At the Winter Star Party in Feb '06, I stopped in at the Coronado booth. After describing my triple scope setup and superlative quality H-a PST, I invited a Coronado technical rep (who shall remain nameless to protect his candor) to look through my setup. He confirmed that my H-a PST DID have an arc of outstanding resolution (he thought 0.5A was not out of the question...). He said that there certainly was variability in optical performance from unit to unit, but opined that this was not unexpected given the number of sophisticated components (each of which could meet-- or exceed-- spec) and hand-assembly. If a unit that happened to have a number of particularly "good" components was assembled to better than the required tolerances, the resultant PST could outperform the normal unit. Here's confirmation from Coronado that these "super PST" units are not mythical after all!
Comparison to Maxscope 40:
Having owned the PST and Maxscope 40-- now called SolarMax 40-- simultaneously, I can make the following points from having tested them head-to-head.
The Maxscope has a better focuser, better nominal resolution (0.7A vs. "0.8+"), more focuser travel (which allows for easier binoviewing and imaging), better baffling (darker contrast with the surrounding sky), and a more Uniform FOV (due to the external etalon). You can stack filters to reach 0.5A resolution, although Coronado recommends you send your scope back to the factory so that they can insure you receive a compatable add-on filter.
The PST offers: greater cost effectiveness (~90% of the performance for 1/3rd the cost, and one can get lucky and get better than Maxscope level performance). The PST is user stackable (vice factory), and in this configuration exceeds the resolution of the Maxscope MS40 for ~75% of its price. On the downside, the PST's field of view (FOV) is plagued by Sweet Spots -- an artifact of the internal etalon which causes features to appear unevenly across the FOV. In practice, this means that as you move the scope around the sun, features will "pop" into and out of view depending on where they are in the FOV. PST's are also prone to varying degrees of ghost images, ranging from highly objectionable to virtually non-existent.
Imaging. The PST can be used for afocal digital camera work, prime focus imaging with a webcam or digital camera (consumer grade, "prosumer" or dedicated astronomy CCD). Reaching focus in prime focus imaging may be problematic, due to the relatively short travel range of the PST.
Mounting Options The PST can be mounted on:
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a photo tripod (basic or upscale) |
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alt-az, either manual (Dob-like) or computerized (ETX-style, with tracking) |
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Equatorial mount with slow-motion controls (don't need to polar align!). Can piggyback on a white light scope |
I have mine "piggyback" mounted on an Orion Telescopes "Maxblast" 102mm f/12 Maksutov-Cassegrain compound scope, which I use for white light solar observing with Baader safety film. The Maxblast is mounted on a Chinese clone of the "CG-5" tripod. Both the PST and Maxblast have had their mounting hardware modified, the PST's to avoid "scope sag" under the weight of binoviewers, and the MaxBlast's to allow for alignment thumbscrews on the mounting rings.
Solar features
Please follow this link to a labeled PDF image taken by NOVAC member and solar guru Greg Piepol. http://www.novac.com/solar/NOVAC-Solar-Definitions.pdf
The solar disc is ~100X the diameter of the earth's, so virtually any feature you can see is at least as large as the Earth. Not bad for 93 million miles away!
Magnetism is the big creator and regulator of solar features, including:
· Prominences (ionized gas in/above coronosphere). These are among the most dynamic solar features, often showing discernable movement (growth/shrinkage) in mere tens of minutes.
· Filaments (prominences facing us-- Dopplered)
· Sunspots, (paired due to magnetism)
· whitish features know as plage (French word for "beach")
· spicules --small jets of gas, which are also dynamic, but hard to track change in since there are so many of them
· sunspots (harder to see in H-A than visible light)
This image shows roughly the degree of detail that I can see through my PST. "Stacking" it with a second Coronado Solarmax filter renders an image that is dimmer but arguably no better in resolution, and the standard for stacked PST's is ~0.6A. This PST consistently bests Coronado's higher end "Maxscope 40" units in resolution, though not in uniformity of field of view or darkness of surrounding sky. While Coronado claims there is no product variability between PST's, too many other experienced solar observers have used my scope and come away impressed for this just to be "owners bias".
Ambient light at the eyepiece. One of the biggest and often overlooked factors in solar resolution is the impact of environmental light flooding in at the eyepiece. (This is also a factor in nocturnal viewing, especially from light polluted areas!) I have found that bellows-style eyepiece cups work phenomenally well in single eye/mono mode viewing on the PST, especially if you don't wear glasses and can press your eye socket down into the rubber. This can produce a virtually perfect light seal.
I found that for the best views (highest contrast, darkest background)
during my solar observing sessions, I reverted to the old tried and true
method of covering my head and the eyepiece holders with a dark cloth or
towel. This is tolerable in December, but decidedly less appealing in
summer. I'll keep experimenting with "on the eyepiece" solutions-- if you've
got something that works, please drop me a note!
Optical cleanliness: Eyepiece glass tends to accumulate lots of
debris-- "eyelash oils", flakes of skin, bits of hair, etc. Eyepieces can
hold a considerable amount of this junk without affecting their nighttime
performance, and I know some observers who prefer to put up with this
cosmetic debris rather than taking *any* chance of adversely affecting the
coatings of their eyepieces by cleaning them. Because of the comparatively
much greater illumination of the field of view in solar viewing, however,
any bit of foreign matter shows up as a defocused blur or spotting if on the
image of the solar disc.
Hartmann Mask: this
old imagers' trick of ensuring super-accurate focus doesn't work well with
the PST. I suspect it is because of the small sizes for the openings
in the mask and the baseline between them. I find it easier to just
manually hunt around for the sharpest view.
PST on its simplest mount-- a photo tripod
PST piggybacked via homemade mount onto ETX-60 "Go To" style scope
My "super PST" piggybacked on an Orion Maxblast 102mm Mak-Cass with Baader white light solar filter
My solar observing session. Note the 9-21mm eyepieces in the binoviewers and the Starbound observing chair.
