Jump to content


- - - - -

Coronado SolarMax 90 H-alpha system

SolarMax 90 plus BF10 Blocking filter on TeleVue TV101


I recently decided to buy a SolarMax 90 filter from Coronado. Before I launch into the review proper, you may be interested to hear about the process which lead me to the purchase of this filter, and my current ‘observing lifestyle’ for want of a better phrase.

I live in Dublin, Ireland and am a very keen deep sky observer. My main scope is a TeleVue TV101, a 4 inch apochromatic refractor, on a Gibralter alt-az mount with Digital Setting Circles. The aperture of the TV101 is small for deep sky, however, I have had a 10 inch Dobsonian in the past and found that I didn’t use it on account of it’s weight and bulkiness. I have reviewed this telescope before and don’t intend to repeat it here, suffice it to say that with astronomical equipment, I like it to be portable, very easy to use, hence the computerised alt-az mount and very quick to set up.

Ireland is not blessed with a good climate and I found that I was not getting as much observing done as I would like. This is what triggered my original interest in Solar observing. It struck me that if I developed an interest in Solar work that I would double the amount of potential observing available to me. In fact, a bit of consideration reveals that I would more than double the amount of observing available to me. Before observing, I look out the window periodically over the evening and if by bedtime the sky hasn’t cleared I go to bed. As soon as I go to bed, I lose out on any observing opportunities that may present themselves if the sky clears overnight. This is not the case with Solar observing. With Solar observing, you are awake all day so in principle you can take advantage of any sky clearance. Other advantages of developing an interest in the Sun include the fact that the Sun is actually the majority of the observable Solar System, it’s surface occupying more of the sky than all the planets put together. You don’t need a large or expensive telescope in order to observe in H-alpha and in addition, the Sun is completely unaffected by light pollution, in fact it is the cause of most daytime ‘light pollution’!

I have tried white light observing in the past and it just did not interest me. Prominence filters, such as those made by Lumicon or Thousand Oaks, which have a wide bandwidth, are considerably cheaper than the true narrowband H-alpha systems. However, narrowband systems, such as those manufactured by DayStar or Coronado, reveal the fascinating disk detail in the chromosphere, which I definitely wanted to see.

After considering both companies offerings, I went with Coronado in the end because I wanted a ‘no fuss’ system and didn’t want the hassle of having to configure my optics carefully in order to get the system to work. The wisdom of doing this was borne out when I read the half page ‘instruction manual’ I got with my system, which basically said put one element on the front, another on the back and look!! Just what I wanted.

Originally, I intended going for the 40mm version, but after corresponding with several kind individuals online and chatting to the people at Coronado, I was talked into going for the SolarMax 90. Like many of you reading this who have bought equipment in the past, I wanted to be talked into buying the bigger one, but had to justify the increased expenditure to myself….a well worn path!!!. Although Coronado do make a larger 140mm version, my understanding is that you would only get the benefit of the extra aperture for a few days a year, due to atmospheric turbulence. You are also looking at serious money for the larger version.

I felt that if I went for a smaller filter than the SolarMax 90, I would constantly wonder what the view through the larger aperture was like. If I went larger, I would constantly wonder what the impact of a bread and water diet for an extended period on my life expectancy would be!!

When I called Coronado and ordered the SolarMax 90 I was told that there was a 3 month waiting list and was given an expected delivery date. The filter arrived on time as promised. I have to say that all the people at Coronado I spoke to offered an exceptionally good service. They were always willing to answer my questions, either by email or on the phone and were unfailingly helpful and polite. Even now, long after my purchase, I have had occasion to call them for various reasons and they remain a pleasure to deal with. I mention this because, in my experience, you rarely read of good purchasing experiences online since those who have a hassle-free buy rarely feel obliged to tell others.

Out of the box

The instrument was exceptionally well packaged for transit. It arrived in a sturdy cardboard box. Inside the box was a plastic case suspended within the outer box in a foam construction for protection. Nestling inside the foam filled plastic case was the front element, consisting of the Fabry Perot etalon plus Energy Rejection Filter, and the BF10 blocking filter encased inside a custom made 1.25 inch star diagonal.

The front element is very well built, with an almost ‘overbuilt’ quality reminiscent of an Astrophysics or TeleVue refractor. It is satisfyingly heavy and comes complete with two thick metal dust caps which screw in both sides of the solid metal cell. Coronado also include a tilting mechanism on the front element, allowing you to detune the filter slightly off the H-alpha bandpass to view Doppler shifted features. This consists of a wheel on the front element which tilts the element slightly. The front element is gold coated, presumably to reflect heat and prevent the element from getting too hot.

SolarMax 90 Front element, note the tilting mechanism for viewing Doppler shifted events

The BF10 blocking filter comes within a star diagonal and again is gold coated. In order to use the system you pop the BF10 into the back of your scope and attach the Front element in front of the objective. In my case, it screws directly into the lens cell of my TV101 although I am in the process of getting a Takahashi Sky 90 for it, which requires a simple adapter, available from Coronado, to mate the front element to the telescope.

First light

First light was from my back garden, on Dublin’s south coast. I screwed the front element into my TV101, popped the BF10 into the focuser, added a low power 20mm Plossl giving 27X and looked at the Sun.

Everyone in this hobby remembers their first views, probably through a shaky 60mm refractor of the night sky. The visual impact of those images of Saturn’s rings or the first quarter Moon are only rarely equalled afterwards. My first view through my SolarMax 90 was one of those moments. A world I never knew existed was suddenly revealed to me in glorious detail. I actually started laughing out loud I was so excited.

The first thing you notice is the vivid orange-red colour. The H-alpha light has a quality that is absent from everyday experience, presumably because the narrow bandwidth generates light with a purity of colour unmatched in Nature. The Sun looks like it has been painted with a laser. At low powers the sun has a 3D quality I have never seen elsewhere, even on the Moon. The overall impression, particularly at low power, is that the Sun is transformed from a harmless outdoor light into a seething ‘Death Star’. You really do get an appreciation of the fact that what you are witnessing is a contained thermonuclear explosion.

All around the solar limb is what looks like orange ‘grass’. Superimposed on this are prominences with an incredible level of detail. Big ones, small ones and all showing fascinating changes as you watch for an hour or two.

Then, there is the chromosphere. The disc has an incredible level of detail on it, particularly around sunspots. Bright areas or plages are obvious. In addition, the Sun appears as if it had split in several areas as dark filaments crossed the disk. The whole disk is covered in a dense intricate network of very fine dark lines.

The SolarMax 90 allowed plenty of light through, and a beautiful image was obtained at 135X giving a nice balance of detail and brightness. A very low magnification does give a striking 3D quality to the image, you do lose some detail and the image can be a bit too bright. I tried 180X but the sky was a bit turbulent at mid-morning and the view was more satisfying at lower powers. However, I will try 180X again at some time nearer sunrise, since I felt that there was plenty of light at this magnification and I was merely being limited by atmospheric turbulence, which is lower soon after sunrise, before the air heats up.

Deteuning the filter using the tilting mechanism allows you to view Doppler shifted events. These are flares that are moving so rapidly that their wavelength is effectively compressed or shifted to a higher frequency. Since the H-alpha filter has a very narrow bandpass, the light from these events can be shifted right out of the bandpass, rendering them invisible. Tilting the front element detunes the filter allowing you to see these features. Although this tilting mechanism was optional in the past, they are now an integral part of the package. To my mind, this is a big advantage, as they offer the ability to see features that were invisible otherwise. In addition, they allow various features to be made more prominent, depending on the tilt. At one position, you see the prominences mainly, while at another, the chromospheric network is more obvious while the prominences are somewhat less visible.

As I watched a plane flew across the H-alpha disk. As it crossed the flaming limb, racked by prominences, the exhaust caused the solar limb to shimmer momentarily. I wished I had a camera to capture the almost uniquely 21st century moment, a stunningly beautiful conjunction of two pieces of advanced technology which I felt truly privileged to witness.


I highly recommend this system, The views have actually exceeded my expectations. As I explained, I bought this system to increase the amount of observing I do, and it has definitely done this. My frustration with the vagaries of the Irish weather has diminished considerably, no mean feat!! Before I got the filter, I thought that Solar observing would be a nice diversion, but that it would always remain ‘second best’ when compared to the pleasures of viewing the night sky. I have to say that I have completely revised this opinion. The views through this system are nothing short of incredible. Viewing the Sun in H-alpha is now right up there with the deep sky in terms of sheer observing pleasure. This system has totally re-energised my interest in observing, opening up a whole new and exciting area of interest to me.

As I mentioned earlier, I could spend $10,000 on a new scope tomorrow and I’d still have to wait for a clear night to use it. In addition, many people travel half way round the globe to view prominences for a couple of minutes during a total eclipse. With a H-alpha system, you can view these features every day, for a fraction of the cost of a couple of trips chasing totality. Although the system is expensive, you have to compare like with like. Reading the technical details on www.coronadofilters.com I would imagine that fabricating one of these filters requires a similar, if not greater, level of effort than making an apochromatic refractor, which many people will wait several years for and pay a lot of money. It is almost certain that worsening light pollution will make solar observing one of the big growth areas in amateur astronomy over the next few decades. Therefore, the cost of H-alpha systems will only go up as more people realise the fun to be had in this area of astronomy. In addition, I can see waiting lists for these systems growing.

My only regret is that I have so long to go before the next Solar Maximum, I should have bought years ago. Don’t make the same mistake. Buy one while you can.


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics