Eyepieces for the PST Solar Telescope
Submitted: Monday, 28th March 2011 by John Wilkinson
One of the most popular solar telescopes is Coronado’s Personal Solar telescope or PST.
The PST is a refracting type telescope with a focal length of 400 mm and aperture of 40 mm. The eyepieces are a standard 1.25 inches in diameter. The scope comes with a 20 mm Kellner eyepiece, which is a low cost, narrow field eyepiece. It works best on long focal ratio telescopes (f/10 or longer) and suffers some chromatic aberration.
However, since the PST uses a single wavelength of light, that of H-alpha (red), chromatic aberrations do not come into play. The problem with the Kellner is that it has a low field of view (about 40°) and low magnification (X20). The image seen in the PST with the Kellner is rather small and disappointing, although the image is sharp and stays in the field for a good length of time. It was probably chosen by Coronado because it keeps the Sun in the sweet spot region (centre 2/3 of the eyepiece view) longer.
Amateurs with a PST have probably experimented with different eyepieces (this is what we like to do), in order to get an improved image. I have tried a range of eyepieces and thought a review might be useful for others. In this review I compared four eyepieces I thought most amateur would be interested in or have. One of them is the Cemax that Coronado provides especially for the PST. The Cemax is contrast enhanced which basically means it has special multicoated lens for H-alpha light and well blackened inside to reduce light scatter (often a problem in H-alpha scopes). The Cemax series consists of a 25 mm, 18 mm and 12 mm and cost around $115 each in Australia.
In this review I used a 12 mm Cemax. I also had a Meade 15 mm Super Plossl eyepiece (Series 4000), a Celestron 10 mm plossl, and the 20 mm Kellner. All cost under $100 except for the Cemax which was slightly over. The eyepieces were tested a number of times through the PST which was single stacked and un-modified.
The results are summarised in the following table:
Magnification is not really important in a PST since you usually require a view of the whole of the Sun’s disc. With 400 mm aperture, the ability to get sharp images at high magnification is limited. A practical limit appears to be about X100 but this is often reduced by poor sky conditions to below X50. Increasing magnification rarely improves the visual appeal although it can make a difference photographically. Using too high a magnification also put some of the Sun’s disc out of the sweet spot region.
I have found it useful for eyepieces to have a rubber eyecup to reduce ambient light, this is also handy if taking pictures of the Sun through the eyepiece with the camera lens in place (afocal photography).
Also the eye relief of eyepieces seem to vary, the worst found here was the Kellner, the best the Meade 15mm and Cemax. All eyepieces showed some internal scattering of red light (glare).
I have also found that when viewing in H-alpha light, it takes a while for ones eye to adapt, especially when looking for surface detail – a minute at least is required.
After testing all the eyepieces, I found the best was the Meade 15 mm super plossl, but it was almost identical to the Cemax 12 mm (just slightly different in magnification) but I suspect they are almost the same optically. The Cemax provided a slightly better view of surface detail but the Meade provided a sharper image of the limb and prominences. For the extra cost of the Cemax, I could not see it was much better than the Meade. I found the best eyepiece size for the PST is 15 mm (since Cemax eyepieces do not come in 15 mm size, the 12 mm is the best Cemax, giving a slightly larger image than the Meade 15 mm).
Hopefully this review helps people decide on whether to buy a Cemax eyepiece for their PST. I am not disappointed with my Cemax, just that my Meade Super Plossl is a very good alternative.