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Coronado Instruments AS1-90 Hydrogen-Alpha Solar Filter/Takahashi Sky 90 Apochromatic Refractor: The Ideal Compact Solar Observing Package?


I have become a confirmed Hydrogen-Alpha addict since my purchase of a Coronado Instruments ASP-60/Prom-15 T combination last year to use on my Pronto, an addiction that reached chronic levels when I sold the 60mm filter and purchased the half-again as large AS1-90 a few months later and added that and the BF-30 2" blocking filter as a combination to be used with my TMB 100 f/8 in the second generation William Yang OTA.

As I have bored you with the original review of that combination and followed it up with several updates, things have been quiet on the solar reviews from Dave for the past few months. The explanation for this silence from my otherwise verbose keyboard is easy:

Beginning in late Spring, and through early Fall, the Sun came up early enough where I could get up early, catch a little early a.m. deep sky and or lunar, and drive down to the beach about ten minutes from the old house to get the Sun coming up over the Eastern horizon of Lake Michigan and do some observing before it was time to head home, shower, shave and dress for another day of cheating widows and orphans, pulling the wings off flies, and other things we lawyers find to fill our day time hours.

With the change in seasons and the time change off of daylight savings time, that was no longer possible. Solar observing was now a weekend pursuit here at the new house, although the easy of doing so was marvelous, as the scope can be left set up on the porch and hauled out on the deck at a moment's notice.

However, the weather (as I am sure I don't need to tell anyone) has been horrendous this Winter. So even the solace of Solar has been denied me except for an occasional snatch here and there.

One bright spot this Winter has been the new Takahashi Sky 90, a sub 14", sub 7 pound, 90mm apo. This is the easiest scope I have ever set up, other than my Tak FS-60C, a far more limited instrument because of its diminutive aperture rather than any failings in its optics. No, the Sky 90 is quite a package of size and performance.

In looking at the front end of the Sky 90 just after I got it, I noticed that the lens cell and the inside lip of the sliding dew shield were both threaded. I recalled that the AS1-90 was designed to thread onto the lens cell of a Tele Vue 101. Hmmmm, the wheels started to turn. Would the AS1-90 work with the Sky90? Not that my TMB 100 f/8 was a particular problem to set up and use with the Coronado filter, but it was double the size, and the adapter I had gotten from Coronado was an aluminum sleeve that fit around the outside of the TMB's dew shield, and it had always made be nervous that it might slip off. (Indeed, one day when I left the Dec knob of the Giro-2 too loose, and turned to get another ep, the OTA with the heavy filter on the end went down towards the ground, and while the AS1-90 stayed on - thank you Coronado - I had visions of $5,000.00 of Etalon going bye-bye!) Threading the AS1-90 to the end of the Tak would be a lighter, more secure package.

Of course, the question was how would it work with an f/5.6 rather than an f/8? Would there be enough focus travel. How would it compare to the TMB?

I called Coronado's Arizona office and was fortunate enough to reach David Lunt over here with Gerry Hogan. Putting them together with Art Ciampi, "Mr. Takahashi" at TNR, he agreed to have a Sky 90 and an Extender Q sent to Coronado on the Isle of Man for measurements and a determination of whether the whole package would work, and then how well.

A few weeks later, I received a note from Coronado that not only did the Sky90/AS1-90 combination work, but it worked very well. The AS1-90 would thread onto the inside of the dew shield rather than the lens cell, as the only way to reach the lens threads with the large solar filter would be to remove the dew shield. I was told that the adapter held the filter quite securely with the dew shield slid all the way to the rear.

Mr. Lunt also said that the scope/filter combination worked at the Sky 90's 'normal' focal ratio of f/5.6. (I then recalled that the TV-101 is an f/5.4!) He also informed me that the Extender Q (which turns the Sky 90 into an f/8.8) worked very well with the AS1-90 and would come to focus using the BF-30 2" straight-through blocking filter that I preferred to the 1.25" Prom 15T diagonal blocking filter. In fact, the folks at Coronado were so impressed with the Tak, that I have since been told they intend to keep it.

The note concluded by asking if I wanted and adapter set made up for my own Sky 90. I think my response saying 'yes' was back to them in about 10 seconds!

Physical Description:

The AS1-90 and its companion blocking filter have already been described in detail in the review I did last year and as still available elsewhere on this site. The new adapter to let you use this combination with the new Tak apo is even simpler to use.

As you can see from the photos, the adapter itself is a neat, silver and black two sided ring, with the male threads at the back of the AS1-90 threading into the female thread of the adapter. The male threads on the other side of the adapter then thread into the female threads on the inside of the Sky 90 dew shield.

After a couple of tries, the preferred method is to thread the adapter onto the Sky 90 with the dew shield pulled up a hair so you don't need to worry about putting finger prints on the lens when you are threading. Once the adapter is secure, you thread the AS1-90 onto the adapter, and then gently slide the dew shield into its fully retracted position and secure it with the fastening knob.

I then recommend putting the BF-30 and the diagonal into the focuser (or the Extender Q and the Adapter Q into the focuser and then put the BF-30 into the Adapter Q as the AS1-90 makes the tiny Tak just a TAD!!! nose-heavy. I then load the whole assembly into the Giro-2 or it can be used in whatever mount you wish. It simply balances a lot easier with the back end of the OTA pre-loaded with the necessary hardware.

How does it work?: Quite well. Beginning with the scope in its 'bare' or 'normal' state without the Extender Q, I used the 'zero length' 1.25" adapter made by Coronado that slipped neatly into the back of the BF-30 and then put a Tak 1.25" prism diagonal in and began with an 18 Tak LE. (Since H-Alpha solar work with the Coronado system allows the use of fast focal ratios you do not need 2" eps or large, 'slow' eps such as the 40 TV Plossl's, 50 mm Taks, etc. to come to focus or get a marvelous view of the Sun. As with the TMB 100 f/8, the AS1-90 will give you a full solar disk with the 18 and (usually) the 12.5 Takahashi LE eps. Maximum magnification in all but the most rare of seeing conditions will require no greater magnification than a Tak 5 LE.)

The 18 gave me a full solar disk this morning, showing several well-defined prominences and excellent surface details. Working down to the 7.5 and (for a second) the 5 LEs, the view was as good as I had seen under like conditions with the AS1-90/TMB combination. In focus travel was good, and I had some remaining even with the Sun low on the horizon where focus is more demanding.

Switching to the Extender Q, the in focus remained pretty much the same, which frankly surprised me. What also surprised me was that the image and the detail was improved, not be leaps and bounds, but a clear improvement! To see if I was imaging the difference, I switched up and back between the Extender Q and the 'bare' scope several times. The improvement, primarily in what seemed to be the scale of the image and a hair more detail, was there to be seen.

The 'normal' or f/5.6 version of the Sky90 and AS1-90 were superb. This combination works without compromise and, if you have one of these scopes, the adapter should be your next purchase -- assuming of course you have the AS1-90. However, if you have the Extender Q (and I find this accessory hard to resist for this scope) the view will only get better.

One thing I was not able to do was get the AP/Baader Binoviewer to come to focus using the diagonal prism that I normally use with this product. Again as you can see from the photos, it will work by simply unscrewing the collar from the supplied prism, slipping that on the nose of the viewer, and then threading that onto the 2" collar that is normally used (and supplied with) the binoviewer. That slipped neatly into the back of the BF-30 and cane to focus easily. Since I was observing as I usually do, from a Starbound chair, I used it in this fashion throughout the day when using the Extender Q. The views were still superb, and the annoyance only minor.

Since I had to work to come up with creative solutions to using this particular binoviewer with the Sky90 even under 'normal' conditions, I am not surprised that I had this problem today. Given time, I am sure I will come up with a solution.


My enthusiasm for H-alpha in general, and the AS1-90/BF30 has not diminished one whit since my initial review and my update. I look forward to mornings here on the Eastern Shore of Lake Michigan during the week beginning with observing the Sun, with this filter combination now semi-permanently attached to the Sky 90. For travel purposes, there is simply no comparison. Whether you need a quick setup, or the smallest possible package to take with you, with no compromises despite its incredibly compact dimensions, the AS1-90/Sky 90 is the most 'bang for the pound/inch' in H-alpha - and by a substantial margin. Indeed, given the fact that the Sky 90 is a hair smaller than the Pronto, and as that scope normally functions as a base for the smaller ASP-60/Prom-15T H-alpha combination, the astonishingly-compact but high-performance nature of the AS1-90/Sky90 combination needs no further comment from me to be self-evident.

Update: 3/00/01-3/18/01: The Last Word? Yep!

A small slice of solar observing heaven

I have been working with the Coronado filters now for almost a year. I started out with an ASP-60 with a Prom 15T mounted on a Tele Vue Pronto, and enjoyed that combination so much that I moved up to the AS1-90/BF-30.

The AS1-90/BF-30 was initially mounted on my TMB 100 f/8, a telescope that worked well with it and provided me with a lot of solar viewing pleasure, as I reported in my reviews and in several posts on Egroups (now YahooGroups), on Solar Chat, and on SAA. More recently, I replaced my TMB as the base for the Coronado with the new Takahashi Sky 90.

The Sky 90 provided the minimum aperture needed for the 90 mm solar filter, and did so in a package that was the smallest possible in terms of length and weight (a hair under 14" and only 7 pounds.) Using the Sky 90 in its 'normal' or f/5.6 version with the AS1-90 provided excellent solar views, as David Lunt advised me when he used the same scope to build the adapter and confirm that it would work with the AS1-90/BF-30 combination at this fast focal ratio. My own tests confirmed that as well.

As good as these views were, screwing the Extender Q and its Adapter Q on the back of the Tak focuser gave me a f/8.8 base for the AS1, and turned excellent views into superb images. Comparing the views to the larger, TMB, the views were so close as to be the same. (The benefits of color correction etc of a fine triplet are not needed in h-alpha, where a good doublet fluorite or ED from Tak or Tele Vue will do the job just fine.)

One thing I was not able to do initially was get the AP/Baader Binoviewer to come to focus using the diagonal prism that I normally use with this product. Not enough in focus with for this binoviewer, something I experienced in night time observing as well. However, I recently had the 'lightbulb' go on over my head and found out that you can solve this problem by simply unscrewing the sleeve that normally lets you fit the Extender Q on the end of the Adapter Q and, instead, simply screw the optical portion of the Extender Q into the body of the Adapter Q. The optical portion of the Extender then comes back almost to the outer edge of the Adapter Q and you end up picking up almost an inch or more of additional in focus travel.

For our purposes, this lets you insert the 2" barrel of the BF-30 into the Adapter Q and over the barrel of the Extender Q and then put the AP Binoviewer 2" coupling into the BF-30. (See the photo below, which shows a close up of the Tak focuser - ExtenderQ - AdapterQ-BF-30-AP binoviewer.) This results in more than enough in focus with the Tak LEs that I use in the binoviewer and several other combinations that I have tried -- and with in focus travel still not at its limits! The resulting 'train' is also shorter and easier to attach and move.

As I have found, using the binoviewer provides you with far better images, particularly surface detail and contrast which are substantially enhanced when using the same eps in the binoviewer rather than singularly in a diagonal. Why? Can't say, but David Lunt, the designer and owner of Coronado agrees that the view is better, as do several others who I have posed this question to and all of whom have said the same thing -- we are not sure why, but the view in the binoviewer is much, much better than when using a single ep in a diagonal.

So, I feel I have finally found what I started out to find last year, the ideal, portable or easy to use solar observing package. If you look at the attached photos, you will see my standard 'rig.' The Tak Sky 90 with the AS1-90 etalon firmly attached to the adapter built by Coronado that threads to the filter threads on the inside of the Sky 90 dew shield. Then the Sky90 with its dew shield retracted (normal h-alpha configuration.) Then the Extender Q threaded to the inside of the Adapter Q, with the BF-30 sliding into the outer end of the Adapter Q. The final piece of the optical train is the AP/Baader binoviewer attached using the standard 2" coupling and diagonal. (The eyepieces shown are a pair of Tak 12.5 LEs, providing a sharp, and clear view of about 35 to 40% of the solar disk today, and spectacular views of several flares and sunspot groups that have been developing as I have been observing in perfect skies since 6:45 this morning and at half hour intervals all day. The case under the tripod holds pairs from 5mm through the 7.5, the 12.5, the 18s, the 24s and the 30s. Since the 18s will provide a view of the full solar disk, the last two groups are used for evening viewing only.)

The scope is on the TeleOptik Giro-2 Deluxe. The whole thing is mounted on a 9 pound Bogen/Manfrotto 3036 tripod. Views are rock steady.

While this combination is not driven and you have to track the Sun manually (the scope works well on almost any mount, my smallest being the GP-DX on which it works well and tracks smoothly) the whole package will go into one airline portable case, with the eyepieces in the 'briefcase' as shown (the handiwork of Dan Folz) and the tripod going into its own travel bag. An incredibly easy and light package to travel with, and one that gives superb views. Or, if you don't want to travel, the combination as you see it sits out on an enclosed porch and is small and light when you go out first thing on a weekday morning to do some observing before you leave for work.


The ideal Hydrogen-Alpha viewing combination. Great views, small, quick to set up or move, and truly a 'no fuss' package. My highest recommendation for those of you looking for the best solar 'bang' for the least size and fuss.

Dave Novoselsky


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