The Clark "dome" is actually of the inverted bucket design. It was designed and built by Godfrey Sykes of local Ponderosa pine. The telescope inside enjoyed first light on July 23, 1896.
In 1999, I was invited to Lowell Observatory to have a look at Mars through their 24" Clark refractor, and I enjoyed it so much, I decided to do it again this year. These expeditions are joint ventures by myself and my observing pal, Dave Healy. This year we were joined by several guests, including Jay LeBlanc, who handled our photography, and Brent Archinal, who ran some ephemerides for us. From time to time a few other people might be found in the dome as well.
The 2001 opposition of Mars came in late June, which is a bit dicey as far as the weather in Arizona goes - it is getting a bit close to monsoons. In southern Arizona, at my home, the monsoon officially began on June 12, and the monsoon-like cloud patterns began to make their appearance early enough to significantly degrade the skies for the Grand Canyon Star Party a couple of weeks prior to our trip to Flagstaff. Although there was risk from the monsoon clouds, Dave and I wanted a big Martian disk to look at on our trip, so we scheduled our observing time for a few days after Mars' closest approach.
For our first night, at least, that gamble paid off. We arrived at Mars Hill early in the evening to find a huge public outreach event in progress. Our observing time began at 10:30, and we knew to expect the public there earlier in the evening, but I had not understood the magnitude of these events. Lowell's parking lot was stuffed, and some people were walking up Mars Hill after parking in a scenic overlook on the access road. The line for Percy's telescope must have been a couple hundred people long, and the Lowell visitor center staff had several other telescopes set up for use as well as opening the 24" for public viewing. At least half of the visitors were minors, and I was impressed at the show that Lowell puts on for Flagstaff residents and passers-through every clear night of the week but Sunday. The public faded away and by the time 10:30 came around, the Hill was almost empty.
The dome housing the 24" is the work of Godfrey Sykes and his brother Stanley, two handymen who made a good deal of equipment at Lowell Observatory. The dome - which is actually an inverted-bucket design - is made of pine, with fine wood floors of the sort that you would see in an old Victorian home. Built into the floor is a series of circular steps to accommodate the swing-through of the telescope, and this provides a comfortable observing height for most locations in the sky, without the use of a ladder, adjustable flooring, or other contraptions. (Unfortunately, Mars' low declination meant that we did have to use a ladder to reach the eyepiece.)
Stowed somewhere on the observatory grounds is the old "Clark Stretcher." According to legend, in the event of fire, this emergency tool was to be used by inverting the telescope, removing the lens cell retaining screws, allowing the cell to drop onto the stretcher, and then, of course, you simply carry the whole thing outside - saving your skin and Alvan G. Clark's glass all at once. Fortunately, no-one has had to test their loyalty to the instrument by actually using the device.
On our first night, the seeing was about an arcsecond, not atypical for Mars Hill. Mare Sirenum was clearly visible along the south side of the planet. Solis Lacus was in evidence as a comma-shaped spot, along with the very low contrast spots Phoenicis Lacus, Daedalia, and Tithonius Lacus in that general area. Tharsis in general was not glaringly obvious, but the various light albedo spots could be seen. Olympus Mons was visible as a light colored spot surrounded by a darker albedo area. To the north, Mare Boreum was clear but much lower contrast than Sirenum to the south. Some of the westernmost spots in the Propontis Complex were visible as well.
Jeff Medkeff, looking through the 24" f/16 Clark refractor at Lowell Observatory. The International Planetary Patrol clock can be seen as a blue cylinder on top of the telescope at the eyepiece end. Control rods for RA and Dec slow motions, as well as for the iris diaphragm, extend up the tube along the side facing the camera. The wire hanging from the telescope's eyepiece end controls dome rotation. Not shown in this photo are additional telescopes, including a 12" Clark, attached to the 24" on the other side.
On our first night, thanks to Brent Archinal's ephemeris plots, we were able to see Deimos near its elongation without difficulty. Phobos would wait until the second night, when it came to maximum elongation just at the end of our observing session. We saw Phobos starting about 20 minutes before its greatest elongation. Since we were using rather high power, simply shoving Mars out of the field of view managed to quell the glare enough to see the moon.
The second night was similar to the first. The public event before our session was smaller than the one the night before, but was still substantial. I'm told by people at Lowell that the public programs on Friday and Saturday nights attract quite large crowds, sometimes of 1000 people. By 10:30, Dave Healy and I started our session with Jay LeBlanc, Brent Archinal, and Jim Misti. At various times people from Lowell would drop by to say hello, and Brian Skiff spent about an hour with us, though he was nonplussed at the prospect of seeing Phobos.
The view of Mars on Thursday night was about the same as on the first. The seeing early in the night was pretty good, but it got worse as the night progressed. All of the same surface features were visible, in precisely the same aspect, except that there seemed to be fewer (or not as bright) limb clouds.
The Telescope:
The Lowell telescope is the last large Clark objective that Alvan Graham Clark made, later Clark objectives being figured by Carl Lundin of the Clark firm. Since the telescope is so "fast" relative to its aperture size, chromatic aberration is significant and there is a substantial purple halo around any bright object. The telescope is equipped with an iris diaphragm made of at least 26 curved blades, actuated with a knob at the eyepiece end, which can stop the telescope to any aperture between full and 6 inches. This contraption is wonderfully engineered, and was made by Stanley Sykes. At 6", the telescope is nearly color free, being in that configuration a 6" f/64. For most of our observing, we used the Clark either full-aperture, or at 18". Strapped to the back end of the Clark are at least three other Clark telescopes, including a 12", a smaller refractor, and a finderscope. Although I had several looks through the 12" two years ago, I didn't bother to have a peek this year.
Amateur astronomers with substandard equipment may be glad to hear that problems plague larger telescopes as well - the RA slow motion wasn't quite healthy during our 2001 trip (it worked rather better in 1999). Also, loosening the clutch on the focus drawtube would, we were told, offer the opportunity for drawtube, diagonal, and eyepiece to plunge to the floor unless we had a grip on the focusing pinion knob to hold it steady. We didn't test this theory to see if the night assistants were right.
For a diagonal, we took the Observatory's equipment out and replaced it with a Tele Vue Everbright. Most of the observing was done with a 25mm Abbe orthoscopic (390x) and an 18.5mm (528x) from the University Optics line. A red #25 filter and blue #80A were both used, as well as unfiltered views, at various times. The red filter did a bang-up job of showing extremely low-contrast features on the disk, enough of them that at times there was considerable difficulty in interpreting what we were seeing and correlating the features to the names of albedo areas. In the end, we saw a good deal more than the IAU has seen fit to offer names for. The blue was handy for picking out some of the clouds, but in general, I think that such a strong blue filter was pressing our luck with this telescope, as good focus was quite difficult to achieve.
Observing with such a long focal length telescope presents opportunities that amateur astronomers do not often enjoy - such as the opportunity to set records for empty magnification. Unable to quell our competitive spirit and in celebration of our disdain of department-store telescopes advertising 600x, we set about to push the 24" Clark way beyond its limits.
The author making adjustments to the terminal hardware. Several barlows and a pair of stacked eyepieces were added to the optical train to render a completely unreasonable magnification, but only for long enough for everyone to have a quick look - then it was back to business.
Our strategy was to stack barlows and eyepieces, and gain the largest magnification we had ever heard of. We began by adding a Tele Vue Big Barlow to the telescope, prior to the star diagonal - so that the diagonal operated as though it were an extension tube. A University Optics Klee barlow (2.8x) was then added after the diagonal. After adding a short extension, Brent Archinal's 2x Meade barlow was added to the train. For an eyepiece, a 5mm University orthoscopic was screwed into a 4mm University orthoscopic with a small extension tube between them, the effective focal length of the combination in that configuration being less than a millimeter. According to my final calculations, we achieved a magnification of:
534,520 x
We believe this to be an all-time record for highest magnification ever used on an astronomical telescope. Brent Archinal supplies an accurate description and then a whimsical analysis of what could be seen through this contraption:
Surprisingly, light from Mars was making it through this system. The appearance of the field of view was one of looking down a long shiny pipe, with a pinhole at the other end. I.e. there was a small concentrated point of light, with a radial pattern of faint 'nebulosity' around it. Not quite like a faint galaxy, but that may have been the closest thing to it in appearance. No detail seen on Mars however, not even the nameplate on any of the landers!
Note that assuming your current estimate of 534,520x, a maximum resolution of the unaided eye of 2', and ignoring such things as seeing and optical theory, the resolution of this system was about 1/4 of a milliarcsecond (coincidentally, about the current astrometric accuracy of VLBI measurements). Given Mars' approximate distance at that time (6.78x10^7 km), the resolution at Mars is therefore about 74 meters - so not quite enough to see the landers. We'll have to add a few more Barlows next time!
No change to the appearance of the field could be made by adjusting the focuser, so it is impossible to prove that we were out of focus. So much for the whimsical side of life!
In summary, our traditional post-opposition trip to Lowell for Mars observing went off a good deal better in 2001 than in 1999 (during 1999, one night was cloudy, and the other night had sub-arcminute seeing). I've rarely seen more 'stuff' on Mars than with the 24" the first night of our run this year. Bagging both moons so easily was another bonus. The Lowell Clark is no slouch when it comes to the planets, and we're looking forward to a repeat performance during 2003.
The author, at Denny's after the first night's run. We're making plans and strategies for the following night, including making arrangements for borrowing some Barlow lenses for the magnification record.
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