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Coronado PST

Tahir Saban's

Personal view on

Coronado PST

Last update: 28.11.2004

I am very happy with this little gizmo despite some optical shortcommings (see below). I regularly take it to my office in a packpack. Because h-alpha filters are delicate equipment I have placed the original foam into the packpack. It is a wonderful light package, I often use it with a table mount. I guess everything together (scope, mount, eyepieces, backpack) weighs less than 3 kgs, very managable! Thus the PST allows me to view the sun from my office every clear day, which was impossible with my SolarMax 40 setup.
The PST has just a 5mm block filter (integrated in the eyepiece holder) which is IMHO a little on the small side. Coronado warned me not to exchange the 10mm block filter of the SM40 for the PST for safety reasons. Indeed the prism in the light path between the etalon and the block filter is narrow and only about half the size of the prism in the BF10.
The PST has a build in finder. A small hole in the lower right side of the body reflects the light of the sun through a small glued prism onto a screen on the upper side. Genuine idea and poor engineering! The hole is misplaced, the lens of the refractor vignets the view of the finder in the upper left corner!

I don't see too much difference in bandwidh compared to my SM40 (which is slightly better but has an obstructed etalon). On a side-by-side comparison at the 2004 ITT meeting in Carynthia more than a dozen bypassers compared the PST to the SM40 and all but one found the PST was providing a better image!

PST and SM40

Usually I get the best view at the end of the tuner position. However, when the sky is covered with haze I have found that other positions give better views (remember: cirrus clouds are the biggest enemy of Coronado filters!).

Optical deficiencies in my model:
1) Has 2 dim ghosts of the sun that move slightly during tuning. There is no optimal position, in any case some part of the sun loses contrast.

2) With some eyepieces the center of the sun is so bright that details get lost.

3) Prominences are more pronounced on the right side of the image. Because of this I have opened my PST to see if there is a simple way of adjustment/collimation (I am on the far side of the Atlantic and sending it back is not worth it). The short answer: No, don't open your scope. The etalon is placed in its housing with spanner wrenches, far too delicate for me to touch.

Inside the PST

 
PST and the Philips ToUCam:

To reach focus I removed the extension tube attached between the PST body and the block filter. However I hear from others that the block filter is often glued to the extension tube. Coronado claims that the position is important, so be careful and don't damage your scope.

 
Below is a mosaic I took with the Philips ToUCam 740K.
 
 

PST and SolarMax40:


The SOlarMax 40 can be screwed before the lens of the PST. This produces a smaller bandwidth, so surface details get more contrasty. The effect is most pronounced at low power and less evident at higher power. Of course the sun also gets a lot dimmer because of the reduced bandwidth. Prominances loose some meat and look as if they were too much contrast enhanced in an image editing software. Some of the subtle details get lost.

On the downside the image is not as sharp as with the PST alone and I see a ghost image of the sun (my SM40 is a very early model). I have to use the TMax-Tuner to move the ghost image out of the field. The TMax-PST combination is IMHO also an "engineering marvel", because when screwed on the PST the adjustment wheel of the TMax blocks exactly the pinhole finder of the PST!

I have found, that I most often use the PST without the SM-40 because I want to get the most out of prominences.



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