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Hydrogen-Alpha Solar Filters: Day Star And Coronado

AS1-90 on a TMB 4" f/8 APO on left, Day Star on a AP 155 f/7 on right


There are currently several Hydrogen-Alpha solar filter systems available on the market. The best known system, and the one that has been the most sought after by amateurs, is the Day Star system. (One of the Day Star products was reviewed elsewhere on this site.) Another company has developed a system that first became available only in the last few years, a system that works quite differently from and makes an interesting contrast to the existing Day Star system - the filters now sold through many US and Canadian dealers and manufactured on the Isle of Man in the United Kingdom by Coronado Instruments. Their ASP-60 (a 60mm, <0.7 A system) and the AS1-90 (a 90mm, <0.7 A system) were reviewed by yours truly also here at Cloudy Nights. (I purchased the ASP-60 first, got myself well and truly hooked on Solar, and now use the AS1-90 exclusively and am selling the smaller unit.)

How do these systems compare? That is what we are about to find out, and also spend some time going through the ins and outs of Solar observing.

I am fortunate, as a Solar newbie, to live close to two of the foremost amateur solar observers in the United States, Gordon Garcia and Jack Mosevich. Both Jack and Gordon have Day Star systems, and both have been gracious enough to share their expertise with me as I begin to struggle through the first phases of Solar observing.

Since none of these systems are commonly seen, I thought that a detailed comparison, as well as a discussion of the ins and outs of H-alpha solar might be of interest to those of you who frequent Cloudy Nights. For that reason, Jack and Gordon and I will be getting together in the near future and, over the next few months, hope to have spend some time observing together and report to you. The first 'chapter' in this extended review is as follows:

8/26/00 6:00a.m. CDT
Jack and I speak, and decide that the nearly 100% morning cloud cover is not going to let us get together today. We plan on calling each other the following morning.

8/27/00 8:45 a.m. CDT
Location: Lake Forest, Illinois (a suburb North of Chicago) Public Beach
Sky Conditions: About 75% cloud cover, some breaks

Jack drives up to see if we can salvage at least a little time observing. We drive down to the beach, set up, and end up with about 15 to 25 minutes of observing, and about an hour and a half of pleasant conversation.

There is, in the brief time we can see it, a series of very distinct and very active prominences, with a very large arch-shaped (BC) or the beginning of a detached arch-shaped (BD) in the lower quadrant of the Sun, from my orientation. Jack gets the same view in his scope, with the different orientation before the clouds close in.

8/27/00 4:45 p.m. CDT
Location: Lake Forest, Illinois (a suburb North of Chicago) on the deck behind the second story of our house
Sky Conditions: About 25% cloud cover, many large breaks

Going out late in the afternoon, the Sun has changed its appearance in the intervening few hours. The very large arch-shaped (BC) or the beginning of a detached arch-shaped (BD) in the lower quadrant of the Sun, from my orientation, has grown, is now in my 3:00 position with the Sun oriented in this position, and has become almost completely detached at the 'lower' end from my viewpoint. It is so large, and I can now see enough detail - the seeing is **** near perfect - that I dither between classifying it as BC, BD or is it so broad and almost 'tree shaped' (looking all the world like a large tree bent over almost double by the wind that its branches are touching the 'ground') that it could be called a large area prominence (FC) almost ready to classify as a detached area prominence (FD) although still strongly attached at its 'top' end.

There are many other prominences picked out in the almost perfect seeing, including some that are detached like tiny, spikes jetting off out from the Solar disk, and a truly incredible 'triple loop' or double arch prominence at the 11:00 position.

Surface detail is excellent as well, with several filaments noted and only a few groups of sunspots, but all well defined.

Preliminary Physical Comparison Of The Systems:

Jack has a .7A T-2 Day Star system that he uses with his AP 155 f/7. Pictures of the system, set up on his AP 600E mount and AP Tripod are shown below, set up next to my 'standard' observing kit as discussed in my review of the Coronado AS1-90, the TMB 100 f/8, on a Meade Field Tripod and using the APM Giro-2 Deluxe AltAz mount:

The first thing I noticed is that the Day Star requires a more elaborate, but not necessarily difficult setup. With the Coronado, you simply screw the Etalon unit on the end of the objective or put it on an adapter, put the blocking filter at the other end, and you are done. The Day Star puts the T-Scanner unit in the focuser just in front of the eyepiece, and its Energy Rejection Filter", or "ERF", which goes in front of the telescope objective. (The photos show the two systems set up side by side. The obvious difference in size had nothing to do with the systems themselves. Both filters are roughly the same 'diameter', Jack's using a 4" while the Coronado is a 3.5" or 90mm. Jack has his mounted on the 155f/7 AP and the 600E AP EQ mount. My Coronado is mounted on my TMB 100f/8 and the Giro-2 Deluxe AltAz, a much smaller package. Both systems would work equally well swapped to the other OTA and mount. Indeed, I have taken to using the AS1-90 with my AP 400 GOTO on the portable AP pier when the sky is clear enough that I anticipate a longer session and want to use the Solar tracking speed to avoid the bother of nudging the tube from time to time to remain properly orientated.)

As you can see from the photos, Jack's T-2 uses a Barlow behind the focuser, then a AP 'Telecentric' unit, and then the heated T-Scanner. Since the system requires at least an f/30 light cone to work, this stacking of barlows and the Telecentric unit required Jack to use a 55 Plossl for his full disk views. The T-scanner requires a battery for heating. Jack had the system up and running in a very short time. It is not as complex as it looks. The ERF is mounted on the objective end of Jack's 155 AP using an adapter of his own manufacture. (Photos of the entire system are included below.)

Set up on the Coronado does not require stretching out the light cone, since the Etalon unit mounts before the objective. The ASP-60, screwing onto the Pronto's dewcap, works fine at what is about f/10. (The AS1-90 is optimized for f/16 using the Prom-15T 1.25" diagonal blocking filter. The system is quite happy at f/8 or even less. Going to a barlow tends to jack the focal ratio on my f/8 TMB up well beyond f/16 since barlow would have to go between the filter and the focuser. David Lunt, the principal of Coronado advised that going beyond f/16 tends to cut down on contrast, and suggested that I switch to the BF-30, 2" straight through blocking filter, much less sensitive to focal length than the Prom-15-T. That should be here tomorrow, and I will try that out in my next session.)

Real world setup time on the two systems was not that far apart. A newbie might struggle at first with the Day Star but Jack or any other experienced user would be able to get it up and running with little effort. Of course, one advantage of Solar is that you do your setup when it is daylight or just before dawn, when it is relatively easy to see what you are doing. However, given the amount of gear needed to get the Day Star up and running in contrast to the Coronado, the nod for ease of setup clearly goes to the latter.

Preliminary Observing Comparison Of The Systems:

Jack and I had only a few minutes to compare the two systems. His first comments - completely preliminary and subject to some changes in equipment by both of us, are as follows:

Both units are .7A so they are roughly comparable. The Coronado provides a 'blacker' background than the Day Star T-2, making viewing the prominences easier. At the same time, Jack found the surface detail better in the Day Star, a function perhaps of the longer focal ratio of that system.

I agree with Jack, with a few caveats: I found the differences subtle at best. However, as noted above, I am using the Prom-15T, intended to be used with the ASP-60 but normally perfectly acceptable for use with the larger AS1-90. In speaking to David Lunt, he believed that the 2" BF-30, with its different bandpass, might provide slightly better contrast than the Prom-15-T. Unfortunately, the BF-30 wasn't available today as it is still in transit. I also found that 'tuning' the Prom-15-T would allow you to increase contrast on the surface for some loss of detail on the prominences.

As an experiment, Jack and I tried pulling the dew cap of the TMB forward and away from the objective. While this did not alter the focal ratio, it appeared to alter the way in which the system dealt with contrast, increasing it to a certain degree and cutting down on the detail on the prominences.

My initial reactions to the two systems were as follows: I prefer the ability to use the smaller and 'sharper' eyepieces that will work with the Coronado system but not the Day Star. Jack needed to use the massive, 55 TV Plossl to get full disk because of the f/30 light cone demanded by his system. The Coronado will let me do that with the 18 Takahashi and (just barely) with the 12.5 Tak under some conditions. The Coronado should therefore allow use of lighter and 'sharper' eps, and may allow for more 'contrasty' views in that manner. Time will tell. Of course, it is a lot less load on a binoviewer to put a pair of Tak LE or some orthos in it (which you can do with the Coronado).

If Jack is correct, and the Coronado is superior on the prominences and a tad behind on surface detail, then it is a question of what part of the Sun one finds more interesting. I am, as I confessed before, a novice, but I did not find the differences on either that great, and want to wait until the BF-30 arrives and we all have more time to spend observing with the systems side by side.

So, Jack and I decided that we should 1) do more work when we had more time to observe; 2) get Gordon in on the study, with his better-developed knowledge and his slightly more sophisticated Day Star unit; and 3) see how the Coronado AS1-90 works with its intended partner, the BF-30.

One thing Jack and I agreed on without any question today - the Sun viewed through a binoviewer is incredible!!!!!!

More - hopefully much more - to follow in the next few weeks. Dave Novoeslsky

UPDATE 8/29/00:

Well, I am now thoroughly embarrassed. When we first did a 'side by side' between Jack Mosevich's DayStar T-2 and my Coronado AS1-90, Jack and I thought that while the Coronado seemed to do better on the prominences, the surface contrast and general detail on the Solar surface was superior in the DayStar. When I did the comparison this weekend, I was using the AS1-90 Etalon with the Prom-15T blocking filter- a filter set into an 1.25" diagonal. David Lunt, the principal and designer at Coronado said that this filter was optimized for use on the smaller ASP-60 and that while it worked just fine with the larger, 90mm AS1, that the BF-30 2" straight-through blocking filter would provide superior contrast and surface detail with my 100 f/8 TMB because of its greater ability for mirror tilt and its superior bandpass. (Feel free to jump in here at any time if I am getting the technical details wrong, David.) The drawbacks? Greater cost and, because it adds 26mm to the length of the focuser, potential in focus problems with some scopes.

The BF-30 arrived yesterday. This morning at 6:30 CDT I was at my usual, beach-side observing site waiting for the Sun to clear a patch of low-lying clouds on the horizon. Same 'standard' AS1-90 setup as before (TMB 100f/8, Tak 18 LE) but this time with the BF-30 in the focuser, a shallow 1.25" adapter in the BF-30, and a Takahashi 1.25" prism diagonal - the 'shortest' group I could attach to the back of the focuser. When the Sun cleared the clouds, I noticed that I needed all but about a quarter inch of the TMB's normally generous in focus travel to come to focus. The view, however, was simply stunning - far better than anything I had previously seen in the AS1-90/Prom-15T or Jack's DayStar. The surface detail (in the 25 minutes I had before I had to close shop and go home to shower and head to the office - I am writing this on the train on the way in this morning) was as if a veil had been lifted from the Sun's surface. The prominences were still outstanding, nothing in the excellent definition and sharpness Jack and I had noted being given up by the BF-30. On the other hand, using the larger range of adjustment in the BF-30, the surface detail matched and, as I recall from Sunday, exceeded that I had seen through the DayStar. This became apparent as I followed the instructions that came with the BF-30, turned the adjusting knob all the way counter-clockwise, and then moved it back, until (as the instructions said) the optimum position became apparent. Wow!!! The 'granulation' (yes, I know that is the wrong term) was crisply defined. The sunspots and the area surrounding them were picked out almost in 3-D relief. Several filaments were now more crisply defined and the general surface 'fuzziness' that I thought was normal was missing entirely - replaced by a view that was very much like the detailed surface photos taken by Gordon and others with their DayStar systems. A marked improvement indeed.

Drawbacks? Coronado wasn't kidding when they said the BF-30 needs a fair amount of in focus due to its size. The TMB using the Yang focuser has an incredible focus range, even with the focus extender that is used for most situations. With that extension in place, the BF-30/18 (and 12.5) Tak LE combo came to focus with only about a quarter inch of in focus travel left. I tried removing the extender, and it worked, albeit now racked almost all the way out! Since I am going to use the AP/Baader binoviewer with this system next, I am sure that either with the extender on, or with it off and using a Tele Vue extender tube if needed, all should work well. Those of you with more limited focus travel should check before buying.

More to follow. Dave Novoselsky


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