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70mm NearStar Hydrogen-Alpha Solar Telescope Tube Assembly
When most of us think of solar-system observing, we tend to imagine nighttime observing, generally consisting of planetary views of the cloud bands of Jupiter, looking to see if we can pick up any divisions in the outer rings of Saturn (or pastel ovals on its disk), unsatisfactory glimpses of the smaller planets, and occasional, too brief, squints at Mars as it whizzes past the Earth. Of course, Venus is big but it doesn't offer much change except its phases, and the Moon is an awe-inspiring spectacle, albeit a fixed one except for changes in illumination angle. If you are looking for impermanent phenomena, things that won't be there a month or year from now, most of the nighttime changes consistently visible take place on Jupiter, which offers us less than 2000 square arcseconds of varying sky area. If you add the rest of the rapidly changing solar-system sky area, I bet it doesn't add up to 20,000 square arcseconds on the average.
Two solar instruments, high-tech and low
Then you include the Sun. The Sun has an angular area of over two million square arcseconds, and it's changing nearly everywhere, all of the time. If we discount observers' special interests and just weight the observing by different things to see, then the Sun wins the same way it wins the mass contest. The solar system IS the Sun and assorted trash.
At least that's the argument I use when people ask me why I would pay nearly $3000 for a tiny telescope that can view only one object. It is because I can see so large a fraction of the day-to-day changes in the solar system.
The problem is that the Sun is very bright and the most interesting detail does not happen in all colors simultaneously. Hence you require sophisticated narrowband filters to see these changing patterns safely. Coronado has specialized in making suitable narrowband filters, but seems to be branching out to offer complete tube assemblies to those wishing to avoid the troubles of mounting the filter on an existing general-purpose telescope.
Where the NearStar is Coming From
What is the purpose of the NearStar? It is basically an elementary hydrogen-alpha telescope that cannot easily be disassembled or otherwise made unsafe without completely breaking it. It is a small, portable, instrument in a compact case. It is made to be a portable resource, perhaps belonging to a school system, that can be easily set up and used by persons unfamiliar with its operation with a high chance of success. One can't see Doppler-shifted gas using a tilt adjustment, but at the same time teachers who have checked it out can't bungle a class session if they haven't been informed that such an adjustment should be made. The H-alpha image looks best right in the center of the field, so you don't have to fish around for the best view. Coronado has a 60-mm MaxScope as an alternative to the NearStar. If you are a more technically-minded amateur astronomer, you may prefer the tunability of the MaxScope 60. The difference of aperture is minor.
How the Filter Works
Although the precise way it is constructed is no doubt proprietary, the operation is based on the Fabry-Perot etalon. This is basically a narrow gap held apart by thin spacers. Such a gap would offer no more of an effect than a soap bubble if it were just glass, but it is lined with special coatings that force the light to bounce around inside a number of times before it exits. The spacing determines the ultimate bandwidth of the device and the multiple bounces act to suppress a certain wavelength range on either side of the allowed band. Anyway, the quoted bandwidth is 0.08 nanometers or, in the old way of measuring, 0.8 angstroms. This is about 1/3500th the visible spectrum, and at one end of the eye's range. When viewing this wavelength at such a limited bandpass, the eye is comfortable. To prevent aberrations from accumulating, the inner surfaces of the etalon must be exceptionally smooth and parallel.
Such a filter must be aided by more conventional filters to block out non-H-alpha side bands and, indeed, to keep heat out to begin with. It seems to have a red power filter on the front and has a 10-mm diameter sideband blocking filter near the eyepiece. If my estimates are correct, this blocking filter is no slouch itself, allowing only a few nanometers bandpass. Of course, that is as wide as a barn door compared with the etalon, but necessary because the etalon has many transmitted sidebands. We want only hydrogen-alpha at 656.3 nm, because that's where the most interesting visible stuff is happening, so the blocking filter eliminates all other transmitted bands of the etalon.
There is also a little circle of sticky foil centered down inside the telescope. I don't know the precise function of this little circle, but it is apparently needed to shade a component of the etalon. This gives the little NearStar the distinction of being a rare refractor with a designed-in obstruction! Don't worry, however. The obstruction doesn't significantly harm the images. The natural air turbulence is a much bigger source of degradation. I also don't see how the apparent rearward etalon positioning allows the etalon to be in an afocal beam with only the advertised doublet, but it seems to work fine.
One good feature of this filter is its invariance with temperature. It does not require warming in an oven or a temperature control like the filters made by Daystar. Yet other etalon makers space the elements with piezoelectric material and then do tuning for the maximum wavelength electrically, but Coronado avoids any reliance on power, with its cumbersome auxiliary equipment or stabilization time. The etalon here seems to be spaced with materials either immune to temperature changes or automatically compensated, similar to the bi-metallic elements of a regulated mechanical clock.
The view down the tube shows that this is a very unusual instrument
Arrow points to foil obstruction.
The Telescope Arrives
The NearStar arrived in a reinforced-corner foam-lined storage case, which itself was in a well-packed box. The shippers could have thrown this package overhand without damaging the contents. The telescope itself was in perfect shape. It is basically the size and shape of a large finder telescope, made with gold-colored trim. It has a non-removable right-angle diagonal, which contains the blocking filter. It is short, only having a 400-mm focal length, and it operates at f/5.7. Don't worry about the performance being degraded by the speed; the usual rules governing refractor aperture ratio are not valid for one-color instruments.
There is no method of mounting it for a quick check before you send in the warranty slip. There appears to be plenty of metal at the base of the focuser mechanism, and they easily could have machined a flat area and mounted a 1/4-20 hole-plate to mount it on a substantial photo tripod. Instead they sell an optional clamshell 1/4-20 mounting ring for a surprisingly high price for such a simple thing. Anyway, I quickly mounted the scope on a wooden rail held with nylon ties, and screwed my tripod into little brass inserts I had mounted into the wood.
If you order a NearStar, be sure to order the mounting ring from Coronado or get rings from somebody else, and have the mounting on hand when the telescope arrives. Sending in the warranty information implies that you have inspected the telescope and state that it is in good working order. You need to be able to examine this telescope within 30 days of their ship date. The rings should be a little over 3.3 inches (~ 84 mm) in diameter, which is the diameter of the rear focuser tube. Rings for an inexpensive conventional 80-mm refractor, assuming they are a bit oversize to accommodate the thickness of the nighttime telescope's tube, should work, but be sure to measure first. Do not plan on separating two mounting rings longitudinally much over 6 inches, because the uniform cylindrical section of the NearStar's tube isn't much longer than this.
The eye's pupil doesn't usually expand much larger than 3 mm in the daytime, so the maximum focal length eyepiece that uses the full aperture is about 3 mm x 5.7 = 17 mm (f/5.7 is the aperture ratio). Of course, longer eyepieces are not forbidden, but the obstruction causes an out-of-focus spot to dance before your eyes with such monstrosities as a 32-mm eyepiece. Besides, you don't need them. A 16 to 19-mm eyepiece sees a field out to a radius of about a degree and at that angle the bandpass of the filter has moved off the H-alpha wavelength. More to the point, the field begins to do some serious vignetting beyond a radius of about a degree because of the limitation of the blocking filter. A 10-mm blocking filter at a focal length of 400 mm allows a field with radius a little over 3/4 degree. The only reason you can see it out to a degree or so is because the blocking filter is well inside focus and the light cone is only partially occluded.
At the other end, the performance will degrade seriously below about 0.5 mm exit pupil due to atmospheric turbulence and diffraction. Furthermore, high magnification darkens the image so much that I wouldn't want to push the exit pupil smaller than 0.5 mm even if these degradations did not exist. A 0.5-mm exit pupil occurs at an effective focal length of the eyepiece of 0.5 mm x 5.7 = 2.85 mm, giving a magnification of 140. There are 3-mm eyepieces available, but you could probably make do with a short Barlow and a 6 mm. Unfortunately, you cannot use just any long-barrel Barlow lens because of the limitation of in-tube travel caused by the integrated diagonal mirror/blocking filter combination (a middling-short Dakin 2.4x Barlow worked). For my astigmatic eyes, where glasses are unavoidable, a range of Televue Radian eyepieces would be ideal because of the long eye relief (so far I have only the 6 mm). Any person that has uncorked $3K for the basic telescope would be criminally remiss if he or she didn't peel forth some additional dollars for dedicated eyepieces. There are even two slots already cut into the carrying case.
You will use 9 or 10 mm more often than any other. I have noticed that with high-power eyepieces, finding the exit pupil is often a problem, so the shorter eyepieces should be purchased only if you are always reaching for more power. I make do with a conventional Orthoscopic on the low-power end because it already has enough eye relief. Please note that these comments do not apply to eyepieces used in eyepiece-projection photography. The best eyepiece in that case depends on details of your camera.
One accessory I found useful was a dark observing hood to shade my eyes and face. Light bouncing from the vicinity of your eyes into the eyepiece and then back out again competes with the true image. (You wear the hood, not the telescope.) A good way of doing this is to put a dark washcloth on your head so that it hangs about to your chin, and then put a ball cap on backward to hold the cloth in place. Fold the cloth back when you want to see and move around. Hold the dangling cloth cupped around the eyepiece when you are viewing.
There isn't a finder. The first glance at such a tiny scope might make you believe that one isn't needed, but recall that the blocking filter limits the field of view more severely than the typical 80-mm finder scope that the NearStar superficially resembles. A 2-degree field is hard to hit when shooting from the hip. I found (more by luck than design) that one of the nylon ties used to hold my telescope to its homemade 1/4-20 block had a buckle that cast a shadow on the 90-degree tube of the diagonal. By placing this shadow just so, the solar image was always in the range of a low-power eyepiece. The first thing you should add is a finder, but you should wait until you have it fully mounted and know the spacing or thickness of the rings and other details.
Of course, it is dangerous to use a telescopic finder for a solar telescope. One finder that works well is related to a pinhole camera. The front element is a perforated shade for the rear white screen. You can either buy a fancy commercial one or make one that works fine but is less pretty. My first one, for a white-light solar telescope I made in the early 1980s, had a rear screen on which I stuck white adhesive labels. It wasn't even capable of adjustment; I just marked the pinhole image of the aligned Sun with a pencil. After a number of uses marked up the screen (this was a reflector, so I had to align the finder every time), I put on a fresh label and started over. If you make the rear screen out of plain steel, your alignment method can even be a small refrigerator magnet that you move around.
The arrow indicates a buckle shadow that I use as a finder in this configuration
Shadows or pinhole images are the safest finders.
The Instrument in Use
Unlike some other narrowband H-alpha filters, this one requires no power source to run a temperature- control oven. You just put in an eyepiece, point it, and start observing. This is a significant advantage. A convenient telescope gets more use than an inconvenient one.
Views of the Sun are amazing. I was lucky in my delivery date. In the first week of operation (June 30 to July 6, 2002) the Sun offered good prominence details and a long dark filament that stretched over a fifth of its diameter. There were spots and faculae all over the disk. However, this article is not a pulpit to orate about the joys of solar observing. We all know the Sun is glorious. The purpose here is to review the instrument.
The first thing I looked for were "sweet spots," areas offset from the center in which the prominences were more evident than in the center or opposite side. This would happen with a built-in tilt or an improperly mounted filter. I found none. The best image was centered. I checked this by using a low-power eyepiece and looking for the offset where the image of the prominence disappears. It was a centered circle about a half-degree in radius. I had pre-computed that a 0.08 nm bandwidth should begin to move off the H-alpha wavelength at a radius a little less than 1/2 degree, so the observation agreed with my calculation nicely.
This match also confirmed that the bandwidth met specifications. It seems to actually be at or slightly less than the nominal 0.08 nm. Again, this agreed with a previous experience with a 2.5-angstrom prominence filter I saw at a meeting in the 1980s. The disk was so bright in the other filter that you could not view the prominences comfortably without a shading circle (mounted in the field stop of the eyepiece) to occlude the main part of the disk. Here, I could tolerate the solar disk in the same scene as the prominences without any shading. Bandwidth had to be a lot less in the NearStar.
I fitted my Televue binocular eyepiece-holder on the instrument with the click-stop zoom eyepieces (and with the internal Barlow) and found that it did just barely reach focus with almost all of the in-travel used up. I was somewhat concerned about the diagonal's ability to carry all that weight, though. It seemed to survive okay that time. The binocular viewer caused the system to get completely out-of-balance, but that is going to happen with heavy accessories on such a little telescope. With some configurations the weight after the diagonal's bend exceeds the weight in front of it!
So far the atmosphere has not permitted optical testing beyond 25 power per inch, but I have no reason to believe that the performance is not excellent. The background is dark if the eyepiece is properly baffled from backlight and focus is crisp. To date, I've found morning views are steadier than evening.
The diagonal, because it contains the blocking filter, does not come out of the telescope, but it does back up 1 inch. This distance is coupled with about a 3/4 inch focuser travel. The NearStar focuses normal parfocal eyepieces when the focus and extension tube is nearly all the way out. This is the proper location to put the focus, because more back-focus is a simple matter of getting a tube extender. If, on the other hand, it had insufficient in-travel the only solution would be to modify the tube, a dangerous move to say the least, and one that voids the warranty.
Because of this backward lean of the focus, one might expect that even 35-mm SLR prime-focus photography is enabled. However, it is not. I checked with my Olympus OM-1 and a standard T-thread bayonet adapter coupled to a 1 1/4-inch tube. With the focus ring turned completely in and the diagonal also clear in, it was beginning to focus but did not quite reach. I then removed the T-ring adapter and focused by hovering the camera at the end of the tube and found focus when the end of the diagonal tube was just inside the bayonet. It missed the required in-focus by about 1/4 to 3/8 inch. That was so achingly close, that I wonder why Coronado didn't just redesign the scope slightly and enable 35-mm prime focus too.
Complaints Unrelated to the Filter
Let's talk about the telescope operation without bringing the filter into the discussion. The filter is excellent and accurately aligned, and I'm sure that everyone will agree that the filter is 90 percent of the instrument (as well as 90 percent of its cost). Now look at the other 10 percent, and please keep in mind that anything I say here is more than completely counterbalanced by my favorable impression of the filter.
First of all, the telescope is hard to focus. The knurled focus ring on the base of the tube turned so hard that I preferred to focus by moving the diagonal or eyepiece in and out of the tube by loosening the set screws. Set-screw focusing is a kludge at best, but the last thing I wanted to do was to lose my hard-won telescope aim by twisting on the tube. This is an annoying feature. Coronado should loosen the focusing ring or consider finding another way of focusing the instrument (I suggest a lockable rack-and-pinion or Crayford). I am guessing the helical focus unit has been adapted from an existing finder-telescope back end. It may serve well to focus a low-power finder with one eyepiece (and that only rarely) but it is inadequate to focus a high-power instrument having various accessories, all requiring a much different focus.
Second, the instrument is too short. It's handy being small, but this thing is really out of hand. If only the telescope were lengthened to f/8 or f/9 (560 to 630 mm, still shorter than 25 inches), you would not be tempted to purchase exotica like 3-mm eyepieces and the maximum eyepiece focal length would have drifted up toward the common 25-mm eyepiece. The extra elbow room would perhaps have freed up the back of the tube so that they could have achieved longer in-travel and reached 35-mm prime focus. Also, the lack of a 1/4-20 hole would have been more excusable. This instrument is so short you have to be careful when selecting a mounting that dual rings are not too far apart to hold the telescope!
I can't help the nagging feeling that Coronado selected the focal length of the NearStar to fit in the carrying case. Perhaps it is related to the desire to make it fit the size limits of carry-on luggage, but I can't imagine people wanting to carry them by air very often, and you still have the problem of transporting the mount. Maybe Coronado also feels that they couldn't get by with increasing the in-travel without increasing the size of the blocking filter, but I have already parted with $3K for the instrument. I would cheerfully pay a little more for a larger blocking filter on a somewhat longer telescope.
Third, part of the reason this instrument was purchased was because it was stated to be compatible with C-mount video adapters. I half expected a male C-mount would be included that would enable you to attach to the female thread on a surveillance camera. Perhaps, I thought, it would thread directly into the tube assembly or diagonal, giving a firm, non-wobbly fit. The Coronado website touted the C/CS compatibility. But when I received it, there was no connector. What I realized afterward was that the advertisement meant that the telescope had sufficient in-travel to reach a C-mount focus (which is no big deal; nearly all refractors do). You are on your own as to actually figuring out the couplers. One way of proceeding is to get a male C-mount to female T-mount adapter, and then a male T-mount to 1 1/4-inch tube. The C-to-T thread adapter is item NT53-483 from Edmund Industrial Optics, and the T-mount to tube adapter is T125 from Scopetronix. (I think this is the same item. I got mine from Edmund years ago, but I was unable to find it in the new Edmund catalog.)
And that brings us to the last gripe. There is very little documentation associated with this instrument. You get a laminated card with basically the specs and a lawyerly "what-voids-the-warranty" statement. I can understand that there is no requirement to teach you solar astronomy, but they could have warned you what to look for if the scope had been damaged in transit. Coronado could also have made suggestions about the photographic adapters I mentioned above, as well as a note about the foreshortened range of useful eyepieces. How about a beginning photographic exposure for a given ISO film at a given effective f/ratio? What about suggesting methods of supporting big eyepieces, binocular adapters, and cameras on that little 45-degree diagonal?
Surely they have had to deal with these and other questions before and should be passing on this information in a general and archival manner. (I know they answer questions on the web, but that stuff quickly gets lost on the pile.) If a printed manual is too expensive, the answer is simple: just put the manual in an Acrobat file on the website. If they are too busy to write one, my advice is to just wander over to the University of Arizona and find a starving graduate student in one of the science or engineering departments. He or she would be very happy to write a manual for a few hundred dollars and some peeks through this fabulous etalon.
Despite the griping that appears in the previous section, I must say that I like the NearStar. It feels eerie to see the prominences nearly as well as I did in the 1979 total eclipse -- only here the eclipse is on tap every clear day. I am convinced the NearStar will be one of my most used telescopes.
- high-contrast views of the Sun disk as well as prominence features
- idiot proof -- can't detune or render dangerous
- works instantly without electric power
- advertised bandwidth is accurate
- good etalon alignment yields centered image.
- lacks integral 1/4-20 photo-tripod mounting hole
- hard to focus without disturbing image
- doesn't quite reach prime focus in 35-mm SLR cameras
- no manual.
Harold R. Suiter has been an amateur astronomer for over 30 years. He was a prepublication technical reader/reviewer on Rutten and van Venrooij's Telescope Optics and The Dobsonian Telescope by Kriege and Berry. He was one of the technical editors on the reworked Amateur Telescope Making series, and is the author of Star Testing Astronomical Telescopes, published by Willmann-Bell.