Jump to content


- - - - -

H-Alpha Showdown! CORONADO versus DAYSTAR

I have been a solar enthusiast for 12 years now, and have owned a Lumicon, and several DayStar H-alpha filters. I recently took delivery of a new Daystar 0.45a ATM system, and a Coronado AS1 90 / BF 30 filter. I thought it would be an interesting comparison, and being familiar with the design and function of the Daystar, was eager to compare the new Coronado.

I just returned from a week in Fort Davis, Texas where I observe several times a year. I had expected Del Woods of Daystar to deliver the filter system before Christmas, and after several frustrating delays received the filter the day before I left for West Texas. The Coronado was sent directly to my observing site from the Isle of Man, and Gerry Hogan was extremely helpful and accommodating. More on that later.

First some detail on the order and delivery process for both filters. I ordered the Daystar last August, and Del Woods said that he would have the filter before Christmas. I specified the ATM filter with the narrowest bandpass possible because I want to see the disc detail at maximum contrast and highest resolution. Del advised that a 0.5a would perform well.

For those not familiar with the Daystar system, it requires a nominal f 30 beam, which is usually accomplished by stopping down the aperture with a red "energy rejection filter", or ERF, and a barlow combination to reach f 30. The ERF prefilters the infared and UV, and keeps the filter from overheating. The incident light angles are critical to filter performance, and the use of a barlow can cause the off axis light to be magnified causing low performance, and spreading the bandpass of the filter.

The Daystar will be used on my AstroPhysics 155 EDF refractor, and since I wanted maximum resolution I went with the Astrophysics telecentric system. The telecentric system consists of the excellent AP 2x convertible barlow, 3" extension, telecentric lens mounted in a tube that screws into the front of the filter, and a 2" eyepiece tube with the cool brass locking ring for the rear of the filter. This system lets me use a large ERF, 5" to achieve max resolution, at f 27. The telecentric lens makes the light rays parallel before reaching the filter to preserve the narrow bandpass. The configuration is as follows:

Focuser extension-barlow-extension tube-diagonal-telecentric lens-filter-eyepiece.

Daystar is a one-man operation, and Del Woods is the man. From remarks gleaned from Del over the years the fabrication of the filter is more of an art than science, and because of the nature of the beast delivery is long and erratic. Del is a good guy who has bent over backwards over the years to help me, and yes, he is hard to reach, and forgets to turn on the fax machine etc. As he told me once, if he has decent weather to test the etalon, he can produce a filter a day, but the telephone is a distraction, so it is best to fax him a note and he usually returns the call. Del told me that he only provided ERFs up to 3.5" and I would have to order a custom 5" ERF from JR Cumberland, who happens to make Questar's optics along with a lot of military stuff. I contacted Cumberland, and was advised that the special red glass comes from the Schott glass works in Germany, and because of the expense is purchased in batches. So, I placed my order and received in three months, with a Zygo interferogram showing 1/21 wave. Pricey but cool! Now all I need is a filter!

My patience paid off when Del called two days before I was to leave for Texas, and said he had a filter for me. ATM 0.45 angstrom! Needless to say I was thrilled. Is this a wonderful country or what?

Now for the Coronado. I ordered from Mike Bieler at Astronomics in Norman, OK. I have to praise Mike and Fred for the excellent service provided in this transaction. After browsing the Coronado webpage and talking with Mike I decided on the AS1 90 filter with the BF 30 blocking filter. Gerry Hogan at Coronado advised me that delivery would be 2- 3 weeks, which would cut it close for the trip. I provided the diameter of my AP 155 lens cell for the filter adapter.
I have to commend the excellent customer service provided by Gerry Hogan and David Lunt at Coronado. I received prompt replies to my email inquiries, and Gerry was very patient and understanding with all of my questions. The filter was delayed a week, and Gerry offered to ship to me directly at Texas because of the short time line. Coronado is an American company based in Tucson, AZ, and the filters are fabricated on the Isle of Man in the UK. I gave the address of my neighbor, Wayne Gindrup in Fort Davis and asked her to FedEx the filter to me there. Of course Wayne toyed with me saying that the FedEx for west Texas goes through El Paso, and they take extra time to deliver to Fort Davis. Long story short, the filter left the UK on Thursday, and arrived in Fort Davis on Monday. Christmas in March!

The first impression of the AS1 90 was very positive. Coronado has switched to a gold anodized finish for better thermal stability, and the filter has a first rate fit and finish. Machined anodized covers, hard case, and the custom adapter for my AP 155 was gorgeous. The adapter has six screws pressing against felt so it does not scratch the lens cell, and has a very positive lock. I have seen the filter bandpass listed at .6 and .7 angstrom, so I am not exactly sure.

The Coronado could not be simpler to use. Thread on the AS1 90 to the adapter, lock adapter to lens cell, and place the BF 30 blocking filter in focuser, diagonal, and eyepiece. Unlike the Daystar ATM no electricity is required, and no f 30 focal ratio required. As I understand it, because the etalon is in front of the objective, the focal ratio is not important, and the filter is thermally stable. To bring on band simply turn the knurled ring on the blocking filter to optimize the view.

This is an apples to oranges comparison, and to make it as equal as possible

I used the Daystar on my 80mm f9 guidescope with the telecentric system piggybacked on the AP 155 with the Coronado AS1 90 / BF 30. I just happened
to have an 80mm ERF for the Daystar J

The conditions for the comparison were great. Altitude of my site is 5400 feet, temperature in the 70s, and wind from the south 5-15 mph. I had my neighbor Wayne Gindrup, who is an experienced observer and CCD imager, and my observing buddy John Brandon who is also an experienced deep sky observer to assist in this comparison. I asked for there impressions of each filter. Ease of use, disc detail, prominances, etc.

The Daystar ATM filter requires electricity for the oven to heat the etalon to the on band temperature, about 45 degrees Celsius. Since I do not have power to my site I used a 12v-dc power inverter. Del warned me that there have been problems with some inverters not producing a square wave causing the oven to overload damaging the filter. He puts a thermal "breaker" in the currant production filters to prevent problems. I recommend AC power, but that was not an option for me so I used my trusty 300-watt inverter for my computer. I plugged her in, and set the knob pot adjustment to 8.0, and started my clock to see how long it would take the filter come on band.

Meanwhile I was eager to see first light through the Coronado. I placed on the AP in the above configuration, and wah-la! H-alpha view showed proms before I turned the BF 30. Just turning the knob 5 or 6 clicks brought out the solar chromasphere in all its glory. Proms were bright, sunspots showed great detail, filaments were dark, and the overall view was very crisp. I moved from the 32mm plossel to the 12mm Nagler type 4, and the entire solar disc fit nicely in the field. Detail was fabulous! Very contrasty, and I was taken back by the detail in the prominances.

I was worried about the central obstruction in the Coronado, and also had heard that there was considerable "ghosting" on the AS series filters. Well, the obstruction was not at all noticeable, and while there was very slight ghosting at very low power, it did not detract from the detail, and when the mag was increased it disappeared completely. I was pleasantly pleased with the view through the AS1 90. It was every bit as good is I remembered my old Daystar .7 ATM.

After twenty minuets the Daystar came on band and showed outstanding detail on the disc. The filaments were black, plage and sunspots were great, and spicules were visible on the solar limb. Eyepiece used was a Tele vue 32mm plossel which barley got the entire disc in view with the 80mm ERF. The overall view in the Daystar was dimmer, causing me to use my black felt headcloth for the best view. This would make sense considering the narrower bandpass. The proms were much dimmer in the ATM compared to the AS1 90, but visible. I highly recommend the use of some kind of cover to block the stray light. Makes all the difference.

Now for the head to head comparison by Wayne and John. They first looked through the AS1 90 and were blown away by the view. Wayne commented on the clarity of the prominances on the limb, and John liked the plage and magnetic lines near a monster sunspot group. After a leisurely view they switched to the Daystar and I heard similar exclamatives. John saw more disc detail as expected with the narrower bandwidth, Wayne said he liked them both. When John switched back and forth between the eyepieces he observed that the Coronado showed all the features as the Daystar with the narrower bandpass, you just had to look a little closer. Wayne concurred, so I took a look, and sure enough the detail was there. I made several comparisons and the Coronado kept up very well.

I switched to my Teleview binoviewer and had a time getting the As1 90 to focus. After trying several different barlow configurations I found a winner. I removed the barlow element from the AP 2" convertible barlow and threaded it in front of my maxbright diagonal. Using two 32 plossles I was rewarded with one awesome view. I was now seeing about 50 % of the disc, and the resolution was excellent. There was so much detail resolved in the proms that it is hard to describe. The overall darkening made the fine chromospheric detail leap out. We all oohhed and ahhed and then switched to the Daystar. A pleasant surprise was that because the telecentric lens already was barlowed, I did not have to use another to bring the binoviewer into focus. This resulted in a FULL DISC view that was so good I almost feel off the stool. The black cloth was a must because of the reduced brightness. All I can tell you is it was one of the best H alpha views I have ever seen! We were all hooping and carrying on over all the incredible detail. I cannot forget the three dimensional view of the spicules of the limb.

Over the years I have found that about 3-4 inches of aperture is optimum for most conditions when viewing the sun. Rarely will the daytime convection currants allow more aperture and resolution. But… when they do it is something to see. The conditions were great in West Texas, so I tried the Daystar with the 5" ERF and AP telecentric system, and was treated to the "mother of all views". It looked so good that we were all delirious. With this setup I estimate 50-60% of the disc with the binoviewer and the detail was scary. It looked like pictures from the BBSO! The sunspot detail and magnetic lines were so easy to see, the structure resolved in the proms, and the overall look of the disc was "hairy". The only negative would be the seeing factor. We had to be patient and wait for the steady view. Not nearly as obvious on the 80 and 90mm versions.

Our collective conclusions:

  1. Ease of use goes to the Coronado. So well made and easy to use. The instructions are on a single laminated card.
  2. Prominances were a little better in the AS1 90. Big and bright, single eyepiece or through the binoviewer.
  3. Disc detail was significantly better on the Daystar as it should be with the narrower bandpass. Again, the detail was visible through the Coronado, just not quite as obvious.

The Daystar with the 5" ERF was not a fair comparison, and I included the observation because it was so unbelievably great.

Coronado Pros:

  • Availability. Short delivery time.
  • Customer service. Outstanding. Pleasure to deal with.
  • Ease of use (no electricity required)
  • Durability. The coatings are said to last indefinitely. No degradation, time will tell.
  • Good performer for all h-alpha viewing. Nice balance of detail.


  • AS design has a central obstruction of about 27%
  • Expensive for respective aperture. 140mm AS is 11k!
  • Warranty. One year. The thing is built like a tank! Why not longer?

Daystar Pros:

  • Unobstructed system
  • More flexible aperture with different ERFs
  • Narrower bandpasses available
  • Warranty. Five years


  • Very hard to get. Very long wait. PATIENCE!
  • Electricity required on narrowband ATM filter
  • Requires f 30 focal ratio. Slightly more complex setup
  • Blocking filters wear out. Average life is 10 years. Cost $400

These are my personal opinions based on my experience. I am very impressed with both systems, and each performs well in their respective niche. Replacing the blockers is an issue on the Daystar filters, and I have had extensive conversations with Del about this. He says that the etalons will last from now on, but the blockers do wear out. He said some after five years, and some are still going strong after twenty. $400 spread over ten years comes out to about three bucks a month. No biggie.

According to the Coronado webpage the AS filters use a new ultra-hard coating that does not wear out. They spared no expense in the construction of their product. Davis Lunt of Coronado holds the patent for the etalon, and has been making them for over thirty years, and Del Woods almost as long. I regard the etalon fabrication as a "black art" and David and Del are the masters.

If you have never had the pleasure to view the sun through a narrowband Hydrogen alpha filter then you are in for a real treat. It is one of the most beautiful and rewarding areas of amateur astronomy. Be warned it is very addicting!

Clear, sunny days, and best regards,
Doug Finch

***UPDATE*** 4-3-2001

I need to make a correction regarding the 12v-dc inverter for the Daystar filter. Most inverters DO produce a square wave and the square wave can cause the heater to overheat and potentially damage the electronics. If at all possible use AC power.

Dave Groski regarding this issue has contacted me, and he advised that he has built custom 12v inverters that are safe to use with the Daystar. For more information you can contact Dave at:


I also heard from Gerry Hogan at Coronado Instrument Group. I will quote her email regarding the AS1 90 bandpass and the CIG warranty:

"Doug: just read your review in Cloudy Nights .com and offer the following comments;

  1. all of our filters are less than 0.7A We have to have a tolerance range and it is <0.7A The reality is that by far the majority of our filters are at 0.6 A
  2. 1 year warrranty. Our theory is as follows. If it doesn't go wrong within the first year it isn't ever going to go wrong! Truth is, we are 100% behind our products and, if several years down the line someone has a problem we will most certainly look at it."

I will soon have an update on the Coronado As1 90 and the Questar 3.5. Stay tuned!

***UPDATE*** 4-10-2001

I have received a response from David Lunt of Coronado regarding my comparative review of the Coronado AS1 90 and Daystar ATM:

"I am obviously gratified to see that you are enjoying the filter and thank you for your gracious comments. May I make a couple of points that may be of interest to you when observing and comparing.

Firstly, the AS1 series filters are classified as having a bandwidth of <0.7A. The reasons are that I believe this bandwidth gives the best overall view of the Sun for the average observer when it is configured correctly and that it gives some specification tolerance for the manufacturing process. By this I mean that the actual design, if all tolerances were zero would result in a bandwidth of 0.55A,- but tolerances are never zero.

A further reason for choosing this bandwidth for our standard filters is that it compares very closely with the effective bandwidth of a

Incidentally, the AS1 series filters can be configured in this mode on a larger telescope if you wish. So as not to compromise the performance characteristics of the filter we have a 'rule of thumb' which dictates that this can be accomplished on a telescope up to approximately twice the aperture of the filter. As an example, we are currently constructing an afocalised system for a 300mm aperture Solar telescope using the AS1-140 filter. Used this way, your AS1-90 could possibly be fitted to a 180mm aperture telescope with suitable re-engineering.

Incidentally, referring back to the bandwidth of these filters,- they are available to custom order in other bandwidths. We have, in the past provided this type of filter with bandwidths a small as 0.1A although, in this case, one has to utilise two etalons so the price is somewhat prohibitive!

I hope you find the above of interest. Let me know if there is anything else we can do.

David Lunt "

I found the above fascinating and decided to contact Del Woods of Daystar to clarify my understanding of the bandwidth measurement of the ATM filter, and give him an opportunity to respond. Del is not much on email and the Internet, but was very gracious and patient in his explanation to me regarding this matter. Quoting Del:

"All Daystar filters are measured in the solar beam at f 30 for MAXIMUM bandwidth."

He explained that he fabricates each etalon at a narrower bandwidth to compensate for the "spreading of the bandpass" because of the placement of the etalon at the rear of the optical train. He told me that my ATM filter's maximum bandwidth is 0.45a at f 30.

Del said that because he tests each etalon / blocker / oven for proper bandwidth through a scope @ f30 the weather is the main factor in complicating the delivery schedule.

My take on this is a difference in philosophy between the two respective systems. Both work extremely well, and provide wonderful views. Coronado's front mounted etalon allows extreme ease of use, and very good overall balanced H-a views at .6 angstrom. Daystar by having the rear etalon allows the use of a larger ERF, thus high solar resolution at nominal cost. I have had several opportunities to use both systems since my initial review and continue to be impressed. My reason for this comparison is not to say one system is better, but to give the pros and cons of each respective system and I have tried to be as straightforward and objective as possible

I have been experimenting with a 4x neutral density filter on the Coronado to dim the disc just a tad, and it seems to help. It is interesting to compare the view with and without sunglasses. The jury is still out, but it seems to boost the disc contrast slightly at the expense of the prominances. NOTE: single eyepiece, NOT binoviewer. The AS1 90 is a joy to use, and I highly recommend.

I have to say the Daystar 0.45a with 5'' ERF, Astrophysics telecentric system and binoviewer is simply the most incredible view I have ever seen! Amazing resolution and detail on the disc AND prominances. Probably the most awesome view I have ever seen through a telescope! Another interesting capability of the ATM is to change the bandpass by adjusting the oven temperature. The knob pot "on band" setting is 8.0. By dropping to 7.0 the bandpass will shift 1/3 angstrom to the blue wing of H-alpha. Likewise, a 9.0 setting shifts the bandpass 1/3 angstrom to the red wing of h-alpha. The observations are very interesting and worthwhile. I personally like the view in the blue wing. Del can also build the filter (add additional expense) for + - one full angstrom tuning for Doppler work.

Lastly, I have received numerous requests about the "headcloth" used in this review. I use black felt with another piece of white felt sewed together. I found that viewing under a black felt cloth in the summer is not very pleasant. With the white layer on the outside it is more bearable. The cloth is indispensable for critical views. Highly recommended.

I will soon post some filtergrams through the respective systems. Stay tuned!



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics