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Coronado Instruments ASP-60 Hydrogen-Alpha Solar Filter

The Coronado Instrument ASP-60 Solar Filter is a Hydrogen-Alpha solar filter set designed for use with the Tele Vue Pronto. (It will work with other telescopes "with a suitable adaptor.") The ASP-60 offers performance similar to the dedicated solar scope offered by the same manufacturer (Coronado), the Helios 1, but costs about $300 less. Since I already had a Pronto, I opted for the ASP-60 set rather than the Helios. I ordered my set through Jim Kendrick at Kendrick Astro. He was quite helpful in assisting me in wading through the ins and outs of the various solar viewing possiblities, and helped me decide on this particular setup.

Physical Description:

The literature that accompanies the ASP-60 describes it as a "fixed wavelength, thermally stable filter for observation of the Sun in the light of H-alpha. It will mount directly to the [Pronto]. The filter comprises two unit: the narrowband element [the larger, round lens shown in the photos] and the blocking filter [the diagonal shown in the photos.] The narrowband element includes the energy rejection filter (ERF) which serves to elimnate the infra-red heat from entering the instrument as well as cutting off the UV side of the blocking filter's rejection band."

The technical specs state: Central Wavelength 656.28 nm. Full Filter Bandwidth: <0.7 A Temp. Coefficient: ,1A/2,000 deg. C Aperture: 60mm.

The ERF screws onto the end of the Pronto. Just unscrew the lens cap. Leave the dew shield retracted, and unscrew the metal cove on the back of the ERF. The ERF then screws to the thread on the Pronto. (The ERF has its own metal lens cap.) The blocking filter then goes into the focuser of the Pronto. It is an 1.25 diagonal. If you have the 2" back on the Pronto, simply take the 1.25 adaptor out of the 2" diag and then put that into the focuser back. The blocking filter fits neatly into place at that point. The filters should be mounted - and then dismounted- indoors, as the literature warns against removing the ERF when the scope is pointed at the Sun since the incoming light and heat will crack the blocking filter with the ERF out of the system.

Setup and Observing:

Set up takes only a minute or so, and the only real change in the way the Pronto feels is evident when you mount it on the normal Pronto balance aid - it is quite 'nose heavy' given the extra weight of the ERF. I cured that problem by reversing the normal position of the mounting plate and mounting the Pronto well back on the Gibraltar mount. As you never need to go 'heavier' in your eps than a 26 Plossl, your back end is quite light and the scope still needs to be pulled as far back as it will go to balance. Once there, however, it is very comfortable and easy to use.

The photos show the Pronto with the ERF mounted on the front element and the blocking filter in place with a 7.5 Takahashi LE in the blocking filter. The Pronto is mounted on my Gibraltar mount, no doubt overkill for this light a system, but the 140 normally sits on this mount as my 'grab and go' scope, so it was easy just to mount the Pronto/ASP in the normal Gibraltar head, lift the 140 and its special head off the Gibraltar, drop (but not literally, thank God) the Pronto/ASP and head into the tripod, and take the mount and scope outside

Solar observing was a whole new experience for me and, now that I have tried it several times, it is very addictive. The number of days that you can observe is proportionally greater than 'regular' star gazing as this particular star is accessible on partly cloudy days and the times you can observe are much more flexible. Indeed, I now leave the Pronto/ASP setup fully assembled and mounted on the Gibraltar, ready to go especially on weekends or in the mornings, when the sky is steady and I have a few minutes before leaving for work. It is, to coin a phrase, a blast!

Using the Pronto/ASP is simplicity itself. First rule - NEVER USE A FINDER ! IF ONE IS MOUNTED, REMOVE IT OR COVER IT. IT IS DANGEROUS TO USE AND A MOMENTARY LAPSE IN CAUTION COULD BE A DISASTER. Besides, you really don't need. it.

Once you have the scope mounted, you don't need a wide angle ep. Finding the Sun is easy. The entire Sun will fit neatly in the field of a 20 Plossl. I followed the recommend procedure, and put a Meade 26 Plossl into the blocking filter and did a rough solar alignment. With my eye well away from the ep, I moved the scope up and down slowly and right and left until, looking into the ep, the red ball appeared. I focused the ep until the prominences and surface details (granularity and sunspots) became sharply focused. That is all it took, about two minutes of careful 'tuning' and you are in business.

What do you see through this combination? The classic 'red ball' with the 26 or 20 Plossl or a 24 Takahashi LE.. (The Pronto comes with a 20.) At this magnification, say during a late afternoon summer session (4pm), the entire disk of the Sun can be seen in the ep. No 'finder' is needed..

Even at this relatively low mag, sunspots and the granular surface features of the Sun are startlingly clear. The prominences are easily seen and very distinct, coming off the edge of the solar disk and standing up like frozen geysers. As you can see from the photos, I normally start with a Takahashi 24, then move down through the 18, the 12.5, the 7.5 and the 5 mm LE to hone in details on the surface and at the edge of the solar disk. The 7.5 gives a nice combination of magnification and FOV. The 5 gives sharp detail but is pushing the edge of magnification, conditions permitting. The amount of detail through the relatively modest aperture of the Pronto is all you really need for full observing - particularly since this system is quite expensive (about $2600) and the larger H-a filter sets several 'magnitudes' more pricey. Magnification is, just as at night, a function of sky conditions. The literature notes that a barlow can be used either before or after the blocking filter/diag. The surface of the Sun is different every day - or even slightly different the same day. The surface details are anything but 'routine' and the pattern of sunspots and clusters of spots much like watching a ballet. I find the prominences at the edge of the disk especially fascinating.


It is not the function of this review to delve into the ins and outs of Solar Observing. I am a rank novice and am currently working my way through several excellent texts and spending as much time as I can at the eyepiece. I can say that observing the Sun through an H-a filter is far more interesting and 'alive' than through the more 'mundane' (but far less expensive) systems. The surface of the Sun looks, here goes the cliches again, almost alive and moving as you watch. The prominences look like the aftermath of what you know them to be - gigantic thermonuclear explosions. As I said, addictive viewing of the nearest star, and in detail you can't get on any other.

As a system, the ASP-60 delivers quite well. It is simplicity itself to use and well matched to the Pronto. The color of the finish matches the Tele Vue scope and the fit and finish is superb - again a match for the mechanical excellence and finish of the Pronto.

Pricey, but worth (IMHO) the price of admission for a unique astronomical experience, and one often overlooked by many of use.

As always, thank you for letting me share my thoughts with you. Dave

UPDATE (6/22/00):

After a few conversations with some more experienced solar observers, it was suggested to me that additional focal length would be helpful. Since the Pronto is a 'fast' scope, my friends suggested a barlow or a Powermate to "lengthen" the Pronto at the next observing session.

Last night (well, this morning, actually) I got up at 4:00 to do some lunar observing. Afer breakfast, and as I didn't have to leave for work as early as normal, I got the chance to spend an hour working with the newly-risen Sun in a remarkably clear and steady sky. I started with a 2.5 Powermate in the top of the blocking filter. Using a 24mm Takahashi LE and the Powermate, I was rewarded with the full solar disk, much better-defined than my last efforts, and with a series of prominences at the 'top' of the disk very crisply-defined.

Working down from the 24 through the 12.5 and 7.5 Taks, the Pronto/ASP combination appears to run into a 'wall' of magnification at about 5mm, using either a 5 LE without the Powermate or with 12.5 and the 2.5 Powermate in the blocking filter for the same 'real world' magnification. The 12.5/Powermate combination gave better resolution in the H-a view through the eyepiece than the 5mm without 'amplification', confirming the suggestion that the extra focal length would give a better sight picture. (I have been told that the ideal focal length for H-alpha viewing is f/30!) At this magnification, you can hone in on the prominences and still have a substantial portion of the disk showing to give some perspective to the show.

Other eyepieces were not as useful as the Takahashis. For example, the 7 Nagler gave a relatively poor view with the Powermate, simply too much magnification and perhaps too many elements for proper viewing through 60mm of aperture and what is still not an 'optimum' focal length. Removing the Powermate made the Nagler more useful, but still not as nice as the Takahashi 7.5, also sans Powermate. (Since nearly the full disk was visible with the Tak, the extra FOV of the Nagler was not needed, and the extra elements a decided drawback.)

The Takahashi LE eyepieces would appear to be the best choice for use in the Pronto/ASP combination, with the Powermate 2.5 stretching the focal length nicely. The next step will be to borrow a 5X Powermate to see if that additional focal length will improve the view with these and with some of my other eps.

More to follow. And, getting more and more addictive. Time for more aperture? Perhaps the 90mm AS-1? We will see. Dave


In further usage, it became apparent that what I thought was a system improvement using the 2.5Powermate and the ASP-60 was simply improved seeing having nothing to do with this combination. There is really no "improvement" in the focal length by this method. Sorry for the confusion. Dave


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