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Coronado Instruments AS1-90 Hydrogen-Alpha Solar Filter


After my recent purchase of a Coronado Instruments ASP-60 Hydrogen-Alpha solar filter system I found myself 'hooked' on H-alpha solar observing. Before going any farther, after I posted the last review, I got more than a few notes saying, "okay, what the hell is H-alpha, and why is it so much more expensive than white light observing." The short answer is as follows, cribbed from the Coronado website (www.coronadofilters.com):

"The red light of hydrogen alpha with a wavelength of 656.3 nm (1 nm = 1 billionth of a meter) is a very prominent feature in the solar light spectrum since hydrogen makes up 75% by weight of the outer layers of the sun. This light shows us layers up to 1700 km above the sun's visible surface and is especially important for imaging the lower chromosphere. The lower chromosphere is the coolest layer in the sun's atmosphere. The H alpha picture of the sun is very useful in predicting eruptions. The bright regions around sunspots, called plages, and brilliant solar flares are easily seen at this wavelength. Filaments, vivid string-like regions, and sunspots, large blotches on the solar surface, appear dark. Filaments are common sources of eruptions. Filaments on the solar limb appear bright against the blackness of space and are called prominences"

In dummy talk (and I am a neophyte astronomer and a newbie at solar, so I sure qualify as a classic dummy) when you look through the ep with a H-alpha solar setup, you see a reddish-orange star closeup and VERY active. Solar prominences, flares, granulation are all easily seen, in detail that shows the inner working of this nearest of all stars. Sunspots, usually in the classic double grouping, move around the surface each day, and sometimes shift slightly from early morning to late afternoon views, changing as do the activities at the edge of the solar disk.

Another thing to consider is usable observing time. I get up in the morning, and with the scope left up and the filter in place, 15 minutes or a half hour at the earliest part of the day is available, with a partly cloudy sky not a problem, as the Sun usually ducks out long enough to get at least some observing time in. Today was a perfect example: The clouds rolled in last night. This morning, the sky was partly cloudy but I still got in about ten minutes of quality viewing before the sky closed up. Early this afternoon, the wind opened up about 70% of the sky, and I spent several hours enjoying myself.

How has my 'normal' observing time been? Total toast. The skies here in Chicagoland have been overcast. I came home Friday to clear skies, and the clouds rolled in while the AP was cooling down. Last night? Socked in. What happened to today's mostly sunny afternoon? It is about 60% clouds overhead now. Solar? Sure adds to your observing time, and this star is a whole lot more fun to watch than some of its more distant cousins.

Physical Description:

The Coronado Instruments AS1-90 is a larger (90mm aperture) version of the ASP-60 H-alpha filter I acquired a few months ago. Like the smaller ASP-60, the system consist of the H-alpha element that fits at the front of the telescope in front of the objective lens, and a blocking filter before the eyepiece. Coronado offers two choices of blocking filter. The Prom 15-T described in the ASP-60 test is a 1.25" diagonal with an adjustable element to 'tune' the image. The second choice of blocking filter is the BF30, a 2" 'straight through' filter that can go before or after the diagonal. Cribbing once again from the Coronado site, the filter and system are described as follows:

"A thermally stable H-alpha filter for installation on the front of the user's telescope provided with either a Prom 15T or BF30. Its 90mm aperture and <0.6A bandwidth provide very high resolution and result in unsurpassed views of the finest surface details and prominences when mounted on a good quality telescope. The AS1-90 is threaded to fit directly to the TV 101 and Genesis SdF instruments, but will mount on most other telescopes using a suitable adaptor plate. This filter is available with Prom 15T (see above)blocking filter as standard equipment. An optional 30 mm blocking filter , BF30, (2" straight through)is available for use with 2" accessories or where full disc viewing is required on telescopes with a focal length greater than 1500mm" (The Prom-T is recommended for focal lengths of <1500.)

At this point, you might ask how this type of filter differs from other forms of H-alpha filters that require a fairly elaborate setup, including extending the focal length of the telescope with focal extenders, etc? Borrowing once again from the information at the Coronado website:

"The most significant difference between the models offered is the mode of mounting on the telescope system. The two basic forms are the AS1 Series, which are installed at the front end of the telescope and the SMn Series, which mount on the rear of the telescope. The AS1 Series make set-up very simple. The filter is simply threaded into an optional adaptor plate, specific to the particular telescope,- and the telescope is used normally. This type of filter does not normally require any further modification to the telescope optical configuration such as focal ratio and can, therefore, be used on any instrument. This is particularly useful if a short focal length is desired to optimize the image scale for CCD imaging. These are also the optimum models to use with the popular Schmidt Cassegrain telescopes,- a design for which it is normally very difficult to properly configure narrowband filters. A further advantage of the AS1 Series filters is that they are highly thermally stable and can be used under all normal operating conditions without temperature control."

Enough for the techno-babble. The AS1-90 is a big brother to the ASP-60 as you can see from the side-by-side pictures taken of the two 'brothers' shown below. These pictures also illustrate far better than words that, while the 'move ' up from 60mm to 90mm doesn't sound like much when you simply compare the numbers, the huge jump in light-gathering power is self-evident in these photos:

(The 'central obstruction" shown in these photos on both filters is the spacing that lets this filter work properly. Rather than attempt an explanation, the Coronado website contains detailed information on all of this and more, so the more technically minded can check out that site - which also contains a "Solar Chat" section.)

These photos also show why I decided to make this move in the first place. In using the AS1 happy as I was with the ASP-60 system (and as you can see from the review I posted, I was very happy with it), the limitations of a 60mm filter aperture and the Pronto left something to be desired. I was reluctant to move up, as the cost for the filter alone is more than double, but some conversations with 'the pros' (Gordon and Jack) convinced me that sooner or later, I would need more aperture, and that the 90mm size was ideal. (And, it is the largest Coronado makes - although a 140 is rumored.) When I saw a picture of the larger filter, and as I began to run into the limitations inherent in working with 60mm, I decided to buy the larger filter. The problem was what to mount it on.

What Scope?:

Like the ASP-60, the AS1-90 is designed for ease of mounting on the Tele Vue refractors, screwing directly onto the end of an SDF or TV-101 as noted in the quote set out above. I don't own either, and really saw no need to purchase another refractor given my already over-ample stable. And, there seemed to be an in focus problem or other 'dance' necessary to the use of this system with the TVs that was disquieting to a rank novice such as myself. Cribbing again from the Coronado site and some notes at Jim Kendrick's site:

"It is important to know that the BF 30 adds 26mm to the focal length of your telescope. Consequently, many telescopes do not have enough "in" focus to come to focus when using a 2" star diagonal. The best configuration for achieving proper focus would be: focuser - BF 30 - *2" to 1.25" adapter - 1.25" star diagonal - eyepiece. *The 2" to 1.25" adapter must have a thin top lip also. Adapters with a thick lip or a long extension will still prevent focus from being achieved. When using the AS1 90 / BF 30 on a Televue 101, you must use the following configuration exactly in order to achieve focus: focuser - 2" star diagonal - BF 30 - 4x powermate - eyepiece. "

On the "users'" section of the Coronado site, this note appears:

"Today was the first day that I was able to fully use the AS-1 90mm. Several issues have been resolved concerning the attachment to and use of the filter with the Televue 101 F5.4. The final optimum rear train setup seems to be in order as follows: (1) 2" Televue star diagonal, (2) 2" 4x Televue Powermate Barlow, CIG AS-1 blocking filter, 2" Televue 35mm Panoptic or 55rrm Televue Plossl eyepiece."

This all seemed like a lot of work, and seemed to limit the potential eyepiece selection. Since the filter was designed to work with a 4" refractor, and as I was quite happy with my 100 f/8 TMB, I inquired if an adapter was available for that scope. The answer was that one could be made for it by Coronado. Another plus in using this scope was that the focus travel on the TMB was excellent. In focus and focus travel had never been an issue on that scope as you normally use the extender for everything from a 31 Nagler to a zoom. Hell, you could work with a binoviewer without a barlow by just removing the extender (thank you Tom and Markus.) I also hoped the longer focal length of the TMB would be better suited for this type of observing as well. With that in mind, I ordered the adapter and hoped for the best.

The adapter is made from aluminum and allows the AS1-90 to screw into the front while the inside of the adapter fits over the end of the TMB dewcap. The adapter and the actual mounting is pictured below. The workmanship was excellent, as is the workmanship on the filter itself. The scope fits well and is quite secure. The one thing I don't like about it is that it uses a velcro strip on the inside, which leaves a smudge on the outside of the dew cap. While it fits snug and secure, a better solution would have been a set of teflon-tipped mounting screws, such as are found on the inside of many finder brackets. A minor point, but one that is easy to correct - and one I will, as soon as I measure the screws and make a trip to my local hardware store.

As should be apparent from the photos, the filter screws firmly onto the adapter, and the whole assembly fits should then be placed on the end of the scope and carefully secured over the end of the retracted dew cap. (Screwing the filter onto the adapter once the adapter is on the scope is, I suppose, also possible, but clumsy and potentially a source of grief - as in dropping it!) I found that the mounted filter would stay a tad up from the lens with the natural tension of the dew cap, but since the end of the lens cell projects out from the actual objective, there is no danger of scratching the objective or damaging the back of the filter as there is a fair amount of separation even with the two up snug against one another. (The same is true if you screw it onto a TV, as apparent when I screw the ASP-60 on the front of the Pronto.)

One it is on, the OTA is a bit nose heavy - a bunch in fact. If you look at the photos, however, sliding it back as far a possible in the mounting, and using a mount that is not balance sensitive is helpful. I had no problem with the Vixen Custom D shown nor with my Giro-2 or Giro-2 Deluxe. In actual use, it wasn't a problem.

The next question was the choice of blocking filter. Since I thought I would be able to get the full disk in by staying with 1.25" eps (and as I already had the Prom 15-T blocking filter that came with my ASP-60) I decided to skip the 2" blocking filter and stay with the diagonal rather than the 2" straight through (and more expensive) filter.

How Does It Work?:

Just great. Setup consisted of taking out the Vixen Custom D AltAz, mounting the tube rings for the TMB, put the OTA in the rings, put a 1.25" adapter into the back of the focuser, put the Prom 15-T into the adapter, and then go inside and mount the AS1-90 to the adapter. Come out and put the filter and adapter on the end of the TMB and tighten the set screws. Remove the cover from the filter and go around and pull up the Starbound chair and put an 18mm Takahashi in the Prom 15-T. Tune the filter four turns counter-clockwise (or 'anti-clockwise' as the chaps at Coronado say in the manual) and point it toward the Sun, sighting along the tube to get close and then moving it over a tad and look into the ep.

My first glance showed a full Solar disk in the ep, without the need for 2" eps. A second or two of 'tuning' on the Prom 15-T and then rack out a touch on the focus knob of the scope and I am rewarded with a deep red, perfectly focused full disk image, with about four double groups of sunspots picked out on the surface and only a tiny and isolated prominence at the 11 and 1 positions. The surface detail was excellent.

That was it. No fuss, no magic, just a simple, sharp image. The 'instructions' that come with both of the Coronado filters are so simple as to be laughable -- consisting of a single, inch wide laminated card. Do you need more? Not at all! While it will take time for me to learn what I am doing and looking at, and my skills will need to develop beyond these basic step? Of course! However, this system is incredibly easy to use -- and use right the first time (quite literally) out of the box! No need to be intimidated by this venture into a whole new area of observing.

How are the image scale and definition compared to the ASP-60? Much better than the smaller filter system. Much more detail showing in the 18 than when I used it on the Pronto. The prominences are apparent in both scopes but, although they were almost entirely absent from the view this afternoon except as noted because of the Solar conditions, those few that were there to be seen were more readily apparent in the 90 rather than in the 60. (I had both systems out, and switched off between them for a short while today.)

There was simply no contest on surface detail and definition, either on the sunspots of the 'texture' of the Sun's 'surface.' The difference between these systems is similar to the difference between the view offered by a Pronto and a TV-101, aperture is, after all, aperture at night or when viewing the Sun in H-alpha. The details may be there in the smaller system, but the difference in capabilities of the larger light gathering lens cannot be overlooked.

I switched off between several types of eps, working down from my 'longest' 1.25" (the 40 TV Plossl) through all of my Takahashis, the 30 Ultima, etc. Since the entire Solar disk was visible in the 18 Tak, there was really no need to go to the 'longer' eps. The 24, 18, 12.5, 7.5 and 5 Taks all performed well, although the 5 was surely overkill in the late afternoon sky conditions. Again, these Taks seem to be ideal for Solar use.

One pleasant surprise was the ease of use of the higher power Naglers in this system. The Naglers were harder to bring into sharp focus and then to 'tune' with the smaller filter. With the AS1-90, however, the 9 Nagler worked very comfortably, and the 7 Nagler gave outstanding contrast and deep views of the edge of the disk and the 'granular' surface. (The 4.8 Nagler 'held' today, but like the 5 Tak, was not really 'comfortable' due to sky conditions. Later in the day, the sky settled a bit, and it become obvious that either of these two eps should do fine in the future.) Whether this was a function of the AS1-90 or the longer focal length and larger aperture (or more 'forgiving' focusing system) of the TMB, I could not tell.

Since I had tried (with only partial success) to use a barlow and/or Powermate to boost the magnification and improve the image on the Pronto, I tried the 2.5X Powermate with the larger system, putting it int the top of the Prom 15-T. Worked just fine, with the Coronado/TMB coming to focus well with all the eps, even with the 7 Nagler and 7.5 Tak! It was happier with the 12.5 Tak, yielding 160X or about the same as the 'naked' 4.8 Nagler. Again, whether this was a function of the AS1-90 or the longer focal length and larger aperture (or more 'forgiving' focusing system) of the TMB, I could not tell. I will have to leave that to the more experienced solar types and/or Coronado to answer.

The surface and edge detail under higher powers were all you could ask for. I see no need to use the 2" blocking filter with my TMB and these eyepieces, although the use of the straight through blocking filter may be desired for photography, which I know less than nothing about despite the efforts of 'Focus' and others to goad me into it.


As happy as I was with the smaller ASP-60 system, and as fine as it is for most use, if you can stand the pain of more than double the price (including the blocking filter) then the larger AS1-90 and appropriate blocking filter deserves serious consideration for what is an experience not to be missed. While this is not the time nor the place to wax eloquent about my new found and growing interest in H-alpha observing, what is clear is that the AS1-90, particularly on a scope like the TMB 100 f/8 or another moderately long 4" refractor (such as the FS-102 or the TV-102 (?)) with a decent focusing system, will give you a simple to use, powerful, and very satisfying system and observing experience. Solar observing is, as I said before, addictive. It does add a whole new dimension to, and expand the available time for, the whole observing experience. This is an expensive 'fix' for that itch, but I am sorry to report, it is a substantial enough of a 'move up' that I highly recommend it.

UPDATE 8/19/00 and 8/20/00:

I thought it might be appropriate to 'update' with two observing reports from consecutive mornings using this system. This will illustrate the changes in the Sun each day and discuss further the actual system usage as conditions change:

Date: 8/19/00
Time: 5:57a.m.
Location: Local beach, Lake Forest, Illinois (a suburb of Chicago)
Conditions: 3/4 cloud cover, but horizon at water line is partly clear, extending up about 28 to 35 degs.)
Temp: 65 F.
Equip: TMB 100 f/8 APO, Coronado Instrument Group 90 mm Hydrogen-Alpha Solar Filter (AS1-90) and Prom-15T blocking filter. Various Takahashi 1.25" eps, 18mm down to 5mm. APM/TeleOptic Giro-2 Deluxe AltAz mount. adapter for Meade Field Tripod. Starbound observing chair.
Portable CD Player and several Mozart discs.

(Attached are several new pictures of what is now my 'standard' solar observing setup, as described above - CD player and great music not pictured:)


Up at 3:30 and the cloud cover is solid. No planetary tonight. Come down to work on a case I am prepping for trial next week as I can't fall back asleep. At about 5:20 am the urge for a cup of coffee kills any hope of going back to sleep, so load up what is listed above which I now consider my "solar kit." Drive 5 minutes to the local beach even though the skies appear to be cloudy. Worse thing that may happen is that I will sit there, finish my coffee, listen to a little Mozart, and then go home to wake up the wife and go out on the lake. Heavy cloud cover on the short drive. Nice surprise when the Sun appears below a band of heavy clouds just beginning to peak over the horizon, due up 'officially' in 8 minutes. Setup takes about that long.


Several conversations with David Lunt, the principal of Coronado, convinced me that while an f/16 ratio may have been 'ideal' for use with the Prom-15T blocking filter, the trade offs I was getting with the barlow wasn't worth it. Today, no barlow, just the regular eps. Seeing allows for a crisp view of the entire solar disk in the 18 Tak. The view is sharp enough that the edge prominences are visible even at this relatively low magnification. In the 12.5 Tak, a large group of massive prominences appear at the 10:30 to 11:15 position. Just below them, also in the upper left quadrant of the Sun and much closer to the edge of the solar disk than my last encounter, is a zig zag black line of a filament. That feature is sharply picked out against the background, which appears more red than its more usual red orange. (I am not an experienced solar observer, although I am well and truly hooked and hope to work up to that state of knowledge as I progress. So, unfortunately, I don't know if this is a normal solar change -- the deeper, 'redder' color this morning -- or a trick of the seeing conditions. I suspect the latter, but I will try to get that issue figured out asap.)

By contrast to the sharply increase activity at the edges of the Sun, sunspot activity appears to be relatively small this morning. The seeing is fine, and I can push the magnification and the views of the prominences and the solar surface structure just fine with the 5LE. I am getting excellent surface (or, I suppose, subsurface because of the inherent nature of H-alpha observing) this morning. Return to the prominences at the upper left once again, and it appears that even in this brief period of time, they have 'grown' slightly. Their structure has clearly changed, and they are now very distinct in the 5mm ep. I spend about ten minutes carefully charting this group -- and the clouds move in (actually, the Sun has now 'moved up' into the cloud bank) and it is time to pack up and go home.

General comments:

I am finding H-alpha solar observing increasingly addictive. Given our lousy weather and poor evening seeing conditions the last few evenings (for at least two weeks, to be frank), my morning 'solar fix' keeps me in a better mood. I am also getting more and more comfortable, and more and more pleased with this particular setup.

The Coronado system is easy to setup and use, the best proof being the fact that a rank novice like myself had no trouble setting it up in a few minutes, and getting excellent views with very little effort. I am also finding that the eps of choice here -- and many times at night -- are the light, but marvelous, Takahashi eps.

Date: 8/20/00
Time: 5:50a.m.
Location: Local beach, Lake Forest, Illinois (a suburb of Chicago)
Conditions: Only light and scattered clouds. Horizon at water line is
Temp: 69 F.

Equip: TMB 100 f/8 APO, Coronado Instrument Group 90 mm Hydrogen-Alpha Solar Filter (AS1-90) and Prom-15T blocking filter. Various Takahashi 1.25" eps, 18mm down to 5mm plus Tele Vue 2.5X Powermate. APM/TeleOptic Giro-2 Deluxe AltAz mount. with adapter for a Meade Field Tripod, OTA and filters mounted on same. Starbound observing chair. Portable CD Player, Renaissance music this morning, no Mozart.


Set up and ready when the Sun begins to clear the horizon. First ep in is the 12.5 Tak since the entire Sun is not yet available. The group of distinct prominences at the 10:30 to 11:15 position noted yesterday are even more pronounced this morning. Again, and just below them in the upper left quadrant of the Sun and now with its 'top' portion much closer to the edge of the solar disk than yesterday is a 'zig zag' black line of a filament, except it is not as dark today, but noticeably more elongated. As the full solar disk emerges from the horizon as we near 6:15, I am able to switch to the 18 Tak and take in the entire disk.

The prominences from yesterday are joined, or, perhaps more properly, the seeing and lack of clouds give me the chance to study the whole Sun in detail and without needing to hurry before conditions shut me down. The image I am getting today is the best I have ever experienced, either with the 90 or the 60mm Coronado setup. There is a lengthy and almost perfectly vertical prominence at the 12:10 position, the type described by Volker in the "Solar Astronomy Handbook". pp 337-338 large bar-shaped or SB. To its left and just above the group of promincences in the upper left quadrant described earlier is another, shorter and less distinct prominence of the same general type, classified as an SA. At the 3:00 position is a large, 'area prominence' or Volker type FB. with a scalloped top facing out from the Sun. Indeed, there are a number of new or previously (by me) unnoticed prominences in a Sun that is quite active this morning.

The nearly perfect viewing allows me to study the group of prominences and the nearby filament I noticed for the first time yesterday in more detail. The 7.5 Tak gives me the most comfortable view, and by tuning the Prom-15T carefully, I can now classify the group, switching to the 5mm Tak for extreme magnification and, in one instance, using the 2.5 Powermate with the 12.5 and the 7.5 The result of this examination is as follows:

The prominence farthest 'down' or in the 10:50 position is what some would classify as a 'tree' prominence but Volker would call a large area prominence or FB. It does look for all the world like a large oak tree, with the 'trunk 'quite wide, but with the upper portion at least twice as large and starting to spray out -- again, the tree prominence being a more descriptive term.

Moving up and to the 'right' along the edge, the next prominence is what is traditionally called a medium to large 'loop' prominence, looking again as described by its label, an inverted bowl-shaped arch, with the center portion detached from the edge of the Solar disk, and the ends still attached. Volker prefers the term arch-shaped prominence, and would classify this as a medium or small, so note it as an "BA" by his standards (there is no 'medium' listed and his next class is large or BB. This is not as large as his 'large' shown on the diagram in the book, nor as small as the "BA", so I will have to err on the side of understatement.)

The final prominence in the group is a large bar-shaped or SB.

The filament I had been observing has "blurred" but also moved closer to the edge. It now has what my friend Jack describes as an almost "3-D" appearance. Very interesting.

Date: 8/20/00
Time: 11:54a.m.
Location: Parking lot, near Lake Michigan, Pleasant Prairie, Wis. (About 20 miles north of Lake Forest, Illinois
Conditions: Very light and very scattered. Very little humidity and generally good seeing conditions.
Temp: 78 F.

A few hours later and the Sun (obviously in light of the time) near its zenith. Set up the same rig as before (minus the CD player) in a parking lot in a harbor.

The seeing is perfect. The comments above are now changed only to the extent that the Sun looks so crisp as to look like a photograph in an slick magazine. With the 7.5 Tak, the 'cellular' areas surrounding the sun spot groups look are perfectly defined. The prominences are more sharply defined than before.

Changes on the Sun noted as follows: Whether because of better - no, **** near perfect - seeing, there are substantially more prominences noted all around the edge than several hours ago. The grouping at the upper left has become either larger (by a tad) or the seeing has made them more readily apparent. In any event, my 'classification' of the prominences seems accurate. The filament I noted before has changed ever so slightly, and now does indeed look like it is getting closer to the edge. Will it shift over and become visible as a prominence, or will it breakup. Again, I am not experienced enough to hazard a guess, but this should bear watching.

Technical comments:

The Takahashi eyepieces are clearly the best for this usage. I haven't tried the binoviewer yet, and suspect that these eps will prove just as good in that application, if the orthos are not any better. Today, I was able to use very high power and get very distinct views of the Solar features described above with very little 'fussing'. The seeing allowed me to work the most comfortably between 64X (the 12.5 in the 800mm fl of the TMB) and 106X, using the 7.5. The 5mm was just a little less 'comfortable', but still allowed to examine features in detail at 160X.

Conclusions and the Future:

I continue to be amazed at the ease of use of this system combined with the simply incredible performance. The TMB/AS1-90 combo has given me wonderful views and seems to soak up magnification that I would not have thought possible for this type of viewing. However, 160X appears to be a practical limit. Use of a binoviewer, particularly since I have one of the larger, 2" Coronado blocking filters on order (the BF-30) which should make the use of the AP/Baader viewer feasible. Markus has told me that he get great "3-D views" with his own Coronado setup. Time to try that next.

Clear Skies -- night and day. Dave Novoselsky


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