Monday, December 7, 2015

Observing Report: Revisiting the Herschel Sprint

The night of April 16, 2015, I made my second attempt at a drift method Herschel Sprint. I made a few changes from my March 2015 attempt.

I used a 20-inch f/5.25 telescope, so I should have every chance to see all the objects on the list. I used a zoom eyepiece which when set at 18mm yielded 148× and 0.3° true field of view (TFOV). I used HIP48413 (RA 9h 52m 59s, Dec 0° 00′ 13.6″) as my calibration star, as before. The time of its meridian transit, according to Stellarium, was 9:20:30 p.m. PDT. But, rather than using a clock and a list to follow each object’s right ascension, I pressed the “record” button on my digital recorder the moment the calibration star was at meridian and centered in the eyepiece. I hoped this would allow me to simply record my observations without removing my eye from the eyepiece; so long as I said “centered” when the object being described was centered in the eyepiece, the recording would give me a fairly accurate elapsed time which I could later compare to the object list and charts to confirm what I saw. Using my inclinometer to find the upper and lower limits of the sweep’s declination band, I used C-clamps to attach wood blocks to the altitude bearing on my telescope to restrict altitude movement within the band. I was thus freed from worrying about navigation and could concentrate on observing and describing, as Herschel could. All I had to do was gently, so as not to disturb the telescope’s collimation, bob the telescope up and down in an even tempo to cover all of the sky within the band.

I once again observed from the Fremont Peak Observatory Association’s site at Fremont Peak State Park (elevation 2700’), California. The temperature dropped into the 40s after dark and it was windy. Seeing was 5/7 and transparency 4/5; SQML was 20.60.

It’s very difficult for me to untangle my observations and report my results. I spent a good part of this weekend transcribing my recording and noting the elapsed time of each of my comments. When I review these times versus what the elapsed time of the Sprint objects would be from my calibration star, I have very few matches. NGC 3274 is a certain match: according to the Sprint list it would appear at 00:39:18 elapsed time. My comment at 00:39:14 on the recording is “close to center, there's a triangle of stars, between two at the base there’s cloudiness, a galaxy, E-W elongation” which closely matches what I find on the NGC/IC Project photograph and description. But few of the others have an obvious corresponding match.

One reason was the unevenness of my altitude movement tempo. Before starting I timed my calibration star’s transit across the eyepiece from east to west; it took 74 seconds. To cover the band completely I tried to move the telescope up in the band in 20 seconds, then down in 20 seconds, and so on. This should cover the band in altitude in thirds of eyepiece FOV. But the sky moves quickly through the eyepiece at that pace, leaving very little time to perceive and describe what one sees. Very often I could not keep that pace, or I might linger on an object just to make an observation of it, spoiling my timing and making me miss small swaths of sky, which incrementally added up to a lot of missed sky. Often I would see a galaxy just at the western edge of the FOV, having nearly missed it, and have just a few seconds to describe it.

The swiftness of an object’s passage through the FOV is evidenced in the poor quality of my descriptions, which for the most part are brief and just describe the overall impression, and are not helpful to confirm the object from a photograph or another’s description later. As a result, most of my observations are unidentified.

Since my first attempt in March, I read Discoverers of the Universe by Michael A. Hoskin. In it I learned that William’s brother Alexander had fitted the telescope with a mechanism which would ring a bell when the telescope was at the top and bottom limits of a sweep’s declination band, to give William’s assistant, pulling on the ropes, cues when to raise or lower the telescope. I wondered whether William had the assistant use a metronome to help maintain an even tempo — William was a musician, after all. However, metronomes did not come into use until the early 19th century. In any case, a musician’s timing helps. In order to keep a smooth, even movement, I needed to grasp my UTA in both arms, in a hug, and move the telescope with my body, adjusting my head to keep my eye positioned — this became physically tiring as the night progressed.

From the book I also learned Herschel could move his telescope a small amount in azimuth and could track an object for about 15 minutes. This gave him more time to perceive and describe interesting objects. Also, Caroline did not rely on just a sidereal clock but primarily on Flamsteed’s catalogue (volume 3 of the Historia Coelestis Britannica). Caroline would list out the stars which would be in the path of that night’s sweep, and forewarn William of their coming; William would then describe the nebulae’s distance and position from these reference stars. I lacked this second set of eyes to help with navigation.

As the night progressed I realized that my elapsed time would not be enough to help locate an object later; since many of the objects appeared on the edges of the FOV, my elapsed time would be many seconds off. So I tried to remark whether I was near the top or bottom of the band, and to remark on any double star, red star (which might be a carbon star), or pattern of stars which might give me more reference points when reviewing the charts later. This has been of some help so far, but my progress is slow.

In all I recorded 175 observations. The majority of these were not bright, easily seen galaxies, but faint objects which I decided to describe in case later I could confirm them as galaxies. There were a large number of NGC, IC, and MGC objects in the band of the sweep, so it’s possible I observed objects not on the Herschel Sprint list. I wanted to cover my bases by describing everything I had the slightest suspicion of being a galaxy. I separate the objects I observed into three categories:

High confidence: 56 objects were palpable hits, clearly visible as galaxies. Whether bright or faint, I was able to verbalize a complete description of the nucleus, core, halo, size, and position angle. I am in the process of identifying these on my charts.

Medium confidence: 62 objects I suspect are galaxies but I am not sure. My descriptions are typically “low surface brightness,” “dim patch,” “some glow with AV,” “small, faint.” My uncertainty comes from my lack of experience observing anything dimmer than a typical 11-12th apparent magnitude galaxy. These might be the usual fare for those used to observing faint objects; I am unfamiliar how they would appear in the eyepiece.

Low confidence: 57 objects are very uncertain. My descriptions are along the lines of “dim hazy star” or “mistiness.” Too uncertain to have more than a slight suspicion they are galaxies. I’d need to find them on a chart to prove it to myself — and to gain more experience observing very faint objects to recognize them correctly.

It’s frustrating not to have more certainty in what I saw, and I will spend the next few weeks settling a final tally. On the one hand, I had the desire to bear witness and “prove” I saw the objects by identifying them. But on the other hand, through the night I permitted myself to have that feeling of discovery. I can hear the excitement in my voice during playback when an obvious galaxy comes into view, such as at elapsed time 1:09:30: “real pretty one, right near the top of the band, glowing core, stellar nucleus with AV, elongated NE-SW, pretty one. Large. Spanning 1/3 FOV, fat, 3:2 a nice fat one,” or at 2:45:39: “big, beautiful long one, bright core, you can see a dark lane, it's so long, oh my god and it's nearly out of FOV, I just barely got it. That's a nice one. 30 seconds too late.”

And that sums up the experience of visual astronomy for me. I feel the melancholy thrill of beauty fleetingly held; the intellectual astonishment trying to understand our true relation to the universe; the challenge of describing it to others so they may share. That Herschel was able to do these things so successfully speaks to his greatness. As I remarked to another observer, when doing the Herschel Sprint, you know you are in the presence of the Master.

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