Focal Length: 800mm (f/8.8)
Supplier: Telescope House
Telephone: 01342 837610
The Coronado SolarMax II 90 is a hydrogen-alpha solar telescope that gives you a substantial 90mm of aperture with which to view our nearest star.
As hydrogen-alpha scopes go, this is a relatively large one and, despite what may seem at first quite a high price-tag, is actually rather well priced.
H-alpha solar scopes are precision-built, specialised instruments.
The main filter, known as an etalon, needs to be made to extremely high tolerances if it’s to work at all.
Constructed properly, the scope’s filter system blocks all wavelengths of light coming from the Sun except for a very narrow range centred on 656.28nm (1nm = one billionth of a metre) – a precise wavelength known as the hydrogen-alpha (h-alpha) line.
The tiny range of wavelengths let through either side of the h-alpha line is known as the scope’s bandwidth and this defines the amount of detail delivered when you look through the eyepiece.
A smaller bandwidth brings out finer surface detail while dimming features like prominences.
The SolarMax II 90 has a quoted bandwidth of less than 0.7 Ångströms (1 Ångström = 0.1nm).
This translates into a view that should give fine, high-contrast surface detail while keeping solar prominences detailed and bright.
At the eyepiece, the SolarMax II 90 lived up to expectations with a view that was excellent. Using the supplied 25mm Cemax eyepiece, the whole of the Sun’s disc could be seen in one go.
On the days we tested, there were many different h-alpha features visible, including leaping prominences, contorted active regions and dark, snaking filaments.
Each one was brought out superbly by this scope.
We tested the SolarMax II 90 with optional 18mm and 12mm Cemax eyepieces, our favourite view beingthat provided by the 18mm, which put the entire solar disc on show.
The Sun had an almost 3D-like quality to it and there was so much fine detail on offer that we revelled in the view for quite some time.
The SolarMax II 90 has excellent resolving power and this really stood out when looking at the Sun’s edge.
Here, careful examination revealed the presence of spicules – thousands of small (by solar standards) columns of hydrogen plasma, seen in profile at the edge of the Sun.
These short-lived features, as long as the diameter of the Earth, merge together to give the Sun’s edge a ‘furry’ appearance.
It’s not uncommon to find that h-alpha scopes have a ‘sweet spot’ in their field of view where the filter works at its best.
The large sweet spot of the SolarMax II 90 gives a great uniformity of view, delivering good prominence and surface detail from one side of the Sun’s disc to the other.
A rich view
The etalon can be manually tuned so that it’s centred on the h-alpha line by looking through the eyepiece and moving a small lever on the side of the optical tube.
This ‘RichView’ tuning system is new to the Coronado II series of scopes and is very easy to use.
The focuser end of the scope is excellent.
The review model was equipped with a 1.25-inch, 90° diagonal that contains a blocking filter – a necessary component that complements the main etalon.
The diagonal slides and locks into the main focuser’s 1.25-inch drawtube, the first of two extending tubes at the rear of the scope.
Together, these provide almost 20cm of focus travel, giving a range suitable for virtually any camera or eyepiece.
The draw-tubes are manually extended for rough focus, then locked, and then finely-tuned with a rubber grip helical focuser.
This has a very fine thread and really enabled us to sharpen things up well.
The focuser is designed so that an eyepiece or camera won’t rotate as you turn the focus ring.
We really liked the SolarMax II 90.
It’s a solid performer, beautifully engineered and produces great views of the dramatic h-alpha Sun.
Whether the coming solar maximum is weak or strong, one thing’s for sure – the SolarMax II 90 is a great setup to catch the best of it.
Superbly constructed as the SolarMax II 90 is, it’s the etalon at its heart that dictates how good the view will be.
In this case, the etalon is excellent and delivers an impressively uniform h-alpha field of view.
It’s not uncommon to find that tunable etalons can produce a ‘sweet-spot’ area, which gives the best view of the Sun with the most prominent h-alpha detail.
Areas outside the sweet spot show nowhere near as much detail.
In the case of the SolarMax, the sweet spot is large, with a view that is pretty evenly detailed across the field.
The etalon is visible through the objective end of the scope.
It’s housed in a solid-looking ring that rotates when the RichView tuning lever, which sticks out of the side of the tube, is moved.
The 60º movement range of this lever was enough to deliver some excellent views of the Sun’s disc and prominences in h-alpha.
This review was first published in the October 2011 issue of Sky at Night Magazine.