This Unihedron Sky Quality Meter (Ser. No. 5739 / Mod. 2.17) has been used over a six-year period at various venues in South Africa. I noted that the first four to six readings in every series of measurements we took drifted lower before stabilizing. My revised operating procedure discards the first four reading in every series and this has virtually eliminated the initial drift of earlier measuring sessions. My results showed and still show inexplicable outliers which have a negative impact on the error profile of the results produced. By mounting the unit on a tripod and making sure it is level and exactly vertical instead of operating it while hand-held, I have reduced the number of outliers but not eliminated them.
The results of the first experiment were obtained at Bergwater Lodge (-33°39’47.06″, 20°00’44.13″, 501m) near Montagu. The objective was to see if the readings taken with the Sky Quality Meter at the zenith (0°) and at 30° and 60° down from 0°, toward each cardinal direction varied and, if they varied, by how much. In experiment 1 the meter was hand-held.
Results for Experiment 1.
Discussion of Experiment 1.
The results in Figures 1 to 6 show that the values given by the Sky Quality Meter do differ at the different cardinal points and also at 30° and 60 ° down from the zenith (0°). At both 30° and 60°, the readings were darker toward the west and east than at 0° and less dark toward the north and south.
In order to illustrate the differences more clearly, the Sky Quality Readings were calculated as a percentage of the value obtained for the zenith (0°) and these results are depicted in Figure 7 (above). Here it is clear that W30°, E30°, W60° & N60° readings were consistently darker than the readings at 0° (average -0.4222) while those for N30°, S30°, N60° & S60° were lighter by a larger margin (average 0.8175).
One set of results is not enough to draw any general conclusions from and I suspect the results will vary from site to site. In all probability, they will also vary from season to season for the same site as the position of objects in the sky changes seasonally. I will select a suitable site and repeat the test at three monthly intervals over the course of one year.
The values in the second experiment were obtained at the SAAO Hostel near Sutherland (-32°22’35.29″, 20°48’18.56″, 1703m). I wanted to verify that my Unihedron Sky Quality Meter’s readings were consistent at 0° when the instrument was rotated 90° between consecutive readings around the vertical axis. In experiment 2 the meter was tripod-mounted and the tripod was set to level and vertical using a spirit level.
Results for Experiment 2.
Discussion of Experiment 2.
From these results, it appears that the Unihedron Sky Quality Meter I use does, in fact, give different readings in the two 90° orientations around the vertical axis that were tested. Even if one takes into consideration the three outlier values the S/N values are consistently darker than the W/E values. It seems possible to produce quite different sets of values, and therefore draw different conclusions depending on how the instrument is orientated around the vertical axis.
I will repeat the test using all four possible 90° orientations around the vertical axis. On that occasion, it will hopefully have the time to take two or three sets of readings on the same evening and also to repeat all of them on two or more consecutive evenings.
Exploring the Astronomy Potential at Kopbeenskloof
This report is a bit late. I thought I had written it and obviously I hadn’t so now I have to write it and look as sheepish as possible about the fact that it’s so late.
In May Lynnette and I went of to investigate a place we’d had our eye ever since we’d visited Leeuwenboschfontein a few kilometres further down the road in 2012. Other than the ominous sounding name it has the added attraction of being situated at over 1200 m above sea level and that, makes it one of the highest farms in the Western Cape Province. For the astronomers who also hike or walk during daytime, there are lots of trails and hikes and the farm is also ecologically interesting as it straddles the transition from typical Renosterveld to the Cape Mountain Fynbos.
The farm is run by Wouter and Elsabé Stemmet and regularly hosts groups of up to 200 from various youth organizations such as the Voortrekkers and Land Care & Land Services. These groups make use of a number of very basic wooden chalets containing only bunk beds. For the chalets there are adequate ablution facilities and a large, well equipped kitchen as well as an outside braai area. Wouter and Elsabé also cater for large functions and large groups, if asked to do so.
A separate camping area for tents and caravans that has its own ablution facilities is also available.
There is a rustic cottage for four that has no electricity and one heats the shower water by making a fire, situated about 200 m from the main house and the chalets.
The guest house, that could sleep nine in three en-suite bedrooms with double beds and three single beds in open areas, is situated about two km away from the main house and the chalets. The house has a kitchen with a stove, microwave and fridge and also has a large inside fireplace suitable for having a braai, should the weather turn nasty. All bedding and towels are supplied and further details can be found on their website if you go here. One last thing is that mobile phone reception is limited to one spot in the garden of the main house and I do not know if one can get a signal for all three service providers there.
To get there one takes the R318 from the N1 and, after 26 km you turn left onto the Nougaspoort gravel road. After six km the turnoff to Kopbeenskloof is on your right and then you just follow the road to the main farm house, the chalets and the rustic cottage, or go past the house, keeping to the left and after two gates, you will be at the guest house.
The reason we went there was to try and evaluate Kopbeenskloof for future astronomy outings. It might work for a Star Party but the chalets are very basic and the area around the chalets does not really lend itself to placing telescopes and I can also see the astrophotography people experiencing problems with power. I think the chalets and the camping are too far apart to allow the groups to interact easily. I would, however, not rule the site out for Star Parties, as one could possibly overcome some of the perceived difficulties by negotiating with Wouter and Elsabé.
The rustic cottage could work for a small group with Dobbies, or even equipment that will run off batteries, which I am sure the owners would be more than happy to allow one to re-charge the next day.
The guest house is a suitable site for doing astronomy and astrophotography but obviously only for small groups. There are some limitations as the house has tall pine trees on both the western and southern sides. The trees are about 25 m from the house but the area other side the trees is open so one can put a telescope there. The areas north and east of the house are open but the house then creates a high horizon to either the south or the west. The site is dark but unfortunately when we were there the weather was not good and we had partially cloudy weather on the first two nights and complete overcast on the third night. One aspect of the site that needs mentioning is the very noticeable sky glow from Worcester if there is even the slightest hint of cloud along the western horizon.
Here are my Sky Quality Meter readings taken at the guest house on Kopbeenskloof.
Donors Alan & Rose Cassells – the two lights for the interior of the marquee.
Speakers (in order of appearance) Martin Lyons – “Live Video Astronomy Demo”
Kos Coroniaos – “Amateur Astronomy in South Africa”
Martin Lyons – “Basic Spectroscopic Astronomy”
Lia Labuschagne – “Brown Dwarfs”
Alan Cassels – “Comfortable observing – Moving the Beast”
Auke Slotegraaf – “Photometry with a DSLR Camera”
Kechil Kirkham – “The SKA – The story so far”
Kerry Patterson – “Planetary Models of Hu Aquarii”
Brett du Preez – “Collimating a Newtonian Reflector Telescope”
Newcomers Dwayne Engelbrecht, Lia Labuschagne, Charmaine Pretorius, Anneke Steyn, Bernie, Michelle & Keagan Swanepoel, Gerhard Vermeulen and Wendy Vermeulen
Day visitors Anneliese & Tertius Carstens, Willie Lombard & Elise, Inus & Elsophie van Staden
The “old hands” Alan & Rose Cassells, Charl and Yolandi Cater, Kos Coronaios, Brett du Preez, Wim Filmalter, Iain Finlay, Kechil Kirkham, Martin Lyons, Kerry Paterson, John Richards, Leslie Rose, Auke Slotegraaf, Willem van Zyl, and of course Lynnette and myself.
Lynnette, Snorre the cat and I got off to a very early start in Brackenfell and arrived at Night Sky by about 09:30 on Thursday. Alan and Rose were already settled in and shortly after 10:00 Anneliese Carstens of Bonnievale Verhuurings and her two assistants were on site to pitch the marquee tent. The whole process took just over 70 minute. As soon as that was done, we started putting up the banners and posters and doing all the other essentials to get the inside organized. Alan and Rose pitched in to help with this and without their very able assistance, the job would have taken a great deal longer. Thanks guys, as usual your convivial company made an otherwise boring task seem quite fun. Lynnette got the Admin table sorted out, as well as the chairs in the main tent and the coffee bar at the back of the tent.
Thursday evening didn’t offer much by way of stargazing as it was one of those evenings where the clouds opened up and, just as one thought it was telescope time, they closed in again. Somewhere between 02:00 and 03:00 I got up to have a peek and it was totally clear, but I was too lazy to take the telescopes outside.
During the course of Friday most of the others trickled in, but by sunset there were still people on the list who had not turned up so we decided to move the Pub Quiz forward to the Saturday.
The weather looked indecisive about being cloudy or not, so we socialized and had an uncharacteristically late braai while Martin Lyons started setting up for his live video astronomy demonstration. Unfortunately the electronic gremlins were out in force and even with the able assistance of Kos Coraniaos the system obstinately refused to perform to expectations. As the clouds had cleared up we all dispersed to the telescopes and spent the rest of the evening observing or introducing the newcomers to the wonders of the night sky.
On Saturday Kos kicked off the programme followed by Lia in Martin’s vacant slot. The planned braai turned into a bit of a fiasco, because it started raining quite heavily just as the fires got going nicely. Kos and Wim’s attempts to rescue one of the fires by putting the braai drum on Brett and Leslie’s stoep, had unexpected consequences. Despite the fact that they closed the doors and windows the smoke from the fire still got into the house and tainted everything with a distinct “wood smoke” odour.
Even though Martin’s talk had been cancelled as a result of his equipment failure, leaving an open slot for an extended lunch, the braai was so late that Alan had to wait for people to finish eating before he could start his talk. After Alan’s talk, Auke presented Alan with his long-awaited observing certificate and a special t-shirt commemorating his acquisition of a 12-inch telescope. After that ceremony we took the group photo and set ourselves up for the Pub Quiz after supper.
The Pub Quiz was contended between six teams. After the first three rounds, three teams were eliminated leaving three to contest the next four rounds before Lia and John’s team made it through to the finals. It was a tight race between these two teams until John’s team took the lead. By that time the weather had cleared completely leaving everyone itching to get to the telescopes, so this year’s Quiz, like the Spring SSP in 2013, was not taken to the individual level. Probably a good thing because John has already won the Quiz twice before on an individual basis!
For the evening and late-night coffee we had a very nice cake baked by Gesina to commemorate the seventh Southern Star Party. The cake was iced in blue and decorated with silver stars. The cake also served to celebrate John Richard’s 70th birthday and we wish him well on his next 100 years. A very nice gesture indeed, thanks Lynnette.
Sunday’s day-programme went off without a hitch. Auke had to leave just after his talk to get Kos back to Somerset West and then he unfortunately encountered a problem and was unable to return as planned. By evening the sky was clouded over, much to everyone’s disappointment, so we also cancelled the scheduled “Ethno-What’s up Tonight”. Anneliese and Tertius had come over specially for the stars, so it was a great disappointment for them. However, just as we were sitting down having coffee with them, Leslie knocked on the door and announced that the clouds were clearing so we could set up a telescope and take them on a tour. Before midnight Lynnette and I gave up the unequal battle with the dew and went to bed.
On Monday morning everyone was packing up and saying goodbyes. In-between all the farewells, we took down the posters and banners in the tent, folded up tables, stacked chairs and loaded the trailer. By that evening it was only Lynnette, Snorre, myself and Iain and Willem left.
On Tuesday morning with the clouds building up, Anneliese and her crew arrived to take down the marquee. Iain and Willem were also packing up and when Anneliese and her crew had finished one of her crew remarked, “Look at those two still struggling with that little tent and we’ve already finished with the big one.” Iain and Willem did eventually finish and left shortly after midday. Lynnette and Snorre and I stayed till Wednesday before we left for home via the Pitkos Farm Stall. Regrettably they had no more roosterkoek by the time we got there so we just had coffee and bought a box of Nuy Muscadel for the next SSP in October 2014 before heading home!
This table gives the sky brightness readings obtained during the Autumn 2014 Southern Star Party.
DarkSky Weekend at Bergwater Lodge, Pietersfontein, Montagu: 26th February to 02nd March 2014
26th of February Lynnette, Snorre the cat and I left home in the Vito on Wednesday the 26th of February hoping to get in at least one extra night’s observing at Bergwater Lodge. On the way there we stopped off at the Pitkos Farm Stall to buy ripe figs and at Pinto’s Butchery in in Paul Kruger Street in Robertson to buy cheese grillers. In Montagu we stopped off at Marina le Roux the vet’s surgery for Snorre’s annual check-up, which he passed with flying colours and then on to Bergwater. It was hot when we left home and even hotter when we arrived at Bergwater Lodge. After unloading and storing everything away we relaxed and waited to see what the weather would do in the evening. The weather turned out to be superb for stargazing, crystal clear and the upper atmosphere nice and stable. At 02:00 the average of 20 sky brightness readings, taken with a Sky Quality meter, was 21,6395 magnitudes per square arcsecond (MPSAS) which translates to a naked eye limiting magnitude (NELM) of 6,4544 V mags. By 04:00 we were falling over our own feet so we packed it in and went to bed.
27th of February On Thursday Jannie and Eddie Nijeboer, who are old hands at Bergwater, arrived during the course of the afternoon and settled in. Thursday evening started out very nicely and then, shortly after 22:00 the clouds moved in. By 01:00 it was completely overcast and we decided it was not going to clear so we all went off to bed.
28th of February Friday dawned bright and clear but day still ended up being very hot. The rest of the group slowly trickled in. Wendy Cooper (Secretary of the South Peninsula Astronomy Club) arrived in good time followed by a first timer at Bergwater, John Richards (a long standing member of the Cape Centre), Ralph and Sheila Baker (novice stargazers and guests of Jannie and Eddy) also arrived with daylight to spare. The rest, Martin Coetzee (Orion Observation Group), Lia Labuschagne, (Chair of ASSA’s (Astronomical Society of Southern Africa) Cape Center), Wendy Vermeulen (Cape Center member) and Brett du Preez (astrophotographer of considerable repute – see the cover of the 2014 SkyGuide), all arrived after dark.
Lia and Wendy, in keeping with the well-established tradition at Bergwater, went sightseeing after dark up to Doringkloof. We did not know about their intention to recce the area so Martin kindly drove me back to the R318 to try and contact them but without any success. We returned to Bergwater hoping they were not in trouble and were quite relieved when they arrived shortly afterwards.
Auke Slotegraaf could not make it due to family complications and was missed by all. Henry Oliver, Jaco Wiese and Raoul Schwenke, were due to arrive on the Saturday as Henry still had teaching commitments in Wellington.
Friday night started out with some instability in the upper air but that improved as the night progressed. By midnight conditions were really good. At around 04:00 on Saturday morning I managed, much to Brett’s amusement, to identify the rising Venus as a plane with its landing lights on or, alternatively, as a possible Alien UFO! At 01:00 the average of 30 sky brightness readings was 21,7023 MPSAS which translates to a NELM of 6,4851 V mags. The serious observers got to bed well after 04:00 so none of them were about much before 11:00 on Saturday.
1st of March Saturday was another scorcher but after rising fairly late many of the group members took the opportunity to cool off in the swimming pool. Brett set up his binoculars with solar filters so that everyone could look at the sunspots during the course of the day. With Brett’s help I also managed to take a more or less reasonable photograph of the Sun later in the afternoon.
By the time it was completely dark Henry, Jaco and Raoul had not turned up and it later materialized that Jaco’s wife Jacoleen, who is pregnant, had been admitted to an ICU for observation. The rest of the night was perfect for viewing. Brett eventually sorted out the problem with his equipment – a too long connection cable. As he did not have a shorter cable there, he did the next best thing and switched to binocular observation, as he had done on the previous evening. At 11:00 the average of 30 sky brightness readings was 21,5970 MPSAS which translates to a NELM of 6,4333 V mags. Lynnette and I only got to bed well after 04:00.
2nd of March At around 05:00 on Sunday morning the power went off, not only at the Lodge, but in the entire valley. Apparently ESKOM was carrying out scheduled maintenance but somehow the Lodge had not been notified. This caused special problems at the Lodge because it put the pumps that supply the Lodge with water out of action which meant that, when the toilets had been used once, the cisterns did not refill. The power only came back on after 17:00 that afternoon.
The power outage and associated water problem meant that nobody could prepare breakfast so most people left earlier than usual to have breakfast in Montagu or Robertson. Lynnette and I relaxed for the rest of the day and made our preparations for the guests we expected for the stargazing event on Sunday evening.