Thursday, 26 August 2010

Glance at some old books

Earlier this week I got the privilege to visit Helsinki Observatory library and notably get my hands on the only copies available in Finland of two legendary books: Catalogue of nebulae and clusters of stars by Sir John Frederick William Herschel (1864) and Observations of Nebulae and Clusters of Stars Made With the Six-foot and Three-foot Reflectors at Birr Castle From the Year 1848 up to the Year 1878 by William Parsons (3rd Earl of Rosse) (1878). What a great experience!

Here are some low quality samples:

Friday, 20 August 2010

19.8.2010 - Closing down Cygnus

Telescopes: 8" Orion DSE
Obs. place: Koivukylä, Vantaa, Finland
Date: 19./20.8.2010
NE Lim.mag: ~5.7 (zenith)
SQM-L reading: 19.06 (zenith)
Background sky: 5
Seeing: 7
Transparency: 5
Weather: +8.0°C, humidity 80 - 90%, 1011 HPa, faint gusts of wind, clear.

The night of the 13th was one of the most uncomfortable I care to remember. With the temperature of +21.5°C (71°F), the session was far from enjoyable. Tonight the temperature was far more to my liking: +8.0°C (46°F) and now I could concentrate on observing and not sweating. Compared to the observing conditions on the 16th, the night was far from it, but now wasn't the time to be picky. With the 20 point increase in humidity, the NELM dropped 0.3 magnitude, transparency was slightly worse but seeing on the other hand was better.

First in line was NGC 6846. The cluster is mainly know for its distance as it is one of the most distant open clusters known (4450 pc = ~14 500 ly). As can be suspected, the given magnitude for the cluster is usually listed as fainter than 14th magnitude so I knew I really had to work to get this one. Using Uranometria and a printed map from MegaStar I could easily find the correct position. With the sun only -14 below the horizon and the less than ideal conditions, the task proved to be quite difficult. @ 200x I could - with optimal averted vision - see "a single, faint ~13.5 magnitude star at the correct position and a very faint haze in the background". As the view was less than admiring and too difficult for a proper sketch I decided only to a rough "check up" sketch of the cluster's possible position and leave it to that. The brightest star and a red giant in the center (magnitude 13.0) of the cluster, as well as the other two brightest stars, might be a member of the group. Looking at the cluster's CMD the stars in main sequence are magnitude 16 and fainter. If not too interested in open clusters, there is something else at the field as well. A faint, stellar planetary Kohoutek 4-41. I decided to give it a shot but clearly failed the task. Using OIII filter @ 200x I could not see anything apart from the 13th magnitude GSC 2673-1251. I suspected that the magnitude for this might be at least fainter than 15 as it was invisible. Looking at it now, not only is the planetary listed as a magnitude 15.9 (p) object but also it is in the wrong position in SkyMap Pro 10. Arrows shows the position of the planetary nebula Kohoutek 4-41.

Field around NGC 6846 with 8" Orion DSE @ 300x

Next in line was 4 Cygni nebula (Waldee 1) which was pointed to me by Californian amateur astronomer and good friend Stephen Waldee. You better believe it when I say Stephen discovered this visually using his 10" telescope but I had serious doubts how well my 8 inch telescope and suburban light pollution hell could match up to his observing site in the Santa Cruz mountains. To my amazement, I was in for a treat instantly when I turned the telescope towards 4 Cygni. Keep in mind that apart from knowing that I was looking for a nebula I had no advance knowledge of this object. Using 38x (32mm Ortho), comparing the view to both 8 Cygni and Theta (21) Lyrae, 4 Cygni had a subtle but definite "milkiness" to it so I'm fairly certain I got it and I'll surely be trying it again under darker skies. The notes are as follows: "Surrounding a 5th magnitude star 4 Cygni. Milkiness apparent even @ 38x being clearly nebulosity and not a reflection or glow of stars. Nebulosity appears as a soft, elliptical glow on the N and W side of the star. UHC filter has no effect, sketched @60x. Wonderful object". Simple comparison of the sketch compared to the POSS2/Blue image can be found here.

4 Cygni nebulosity (Waldee 1) with 8" Orion DSE @ 60x

At Waldee's urging, California amateur astrophotographer Al Howard obtained this digital image of 4 Cygni in July 2010, showing evidence of the nebula, using a Tele Vue NP101 telescope and five 10-minute exposures. Stephen says that the nebula seemed even more obvious *by eye*, in repeated observations, than from the picture in normal positive mode. (It even shows up, barely, on various Palomar Observatory Sky Survey plates, though affected by severe burn-in artifacts.) Here is Al Howard's picture, rendered in negative mode and with some contrast enhancement:

Steve's extensive notes of the nebulosity, discovery information and all details you ever need to know about the topic can be seen here.

While moving away from 4 Cygni towards an open cluster Skiff J1942+38.6, I stumbled upon 11 Cygni. Surrounding it was quite a lovely asterism that reminded me a bit of Messier 11 in Scutum. Another asterism that caught my eye was GSC 2698-688 group which appeared as a tent-shaped clustering near open cluster Ruprecht 174. With the few asterisms out the way, I had one "must" object in my list. Earlier during the day I noticed that I had not seen cluster NGC 6874 which was something to change immediately. The group proved to be better than I'd estimated with a slightly triangular shape just like described by William Herschel.

Of the roughly 40 objects observed during the night, the first object that turned troublesome was NGC 7011. I had little trouble locating the correct field but a cluster or an asterism was nowhere to be found. Modern sources seem to have little doubt with the identity as many of them list this possible open cluster as a V-shaped group of about a dozen stars as the object (X in the sketch). With my telescope this group was far from obvious and I didn't even make a note of it. Herschel's description of "Cl" doesn't help much either. From my perspective the V-shaped group is far too uninteresting and unobvious for Herschel. There was a little more obvious grouping of stars north of 4th magnitude 59 Cygni (arrowed) but it is unlikely this is what John Herschel noted.

Field around NGC 7011 with 8" Orion DSE @ 60x

Last and least were three Basel open clusters (12-14) of which Basel 13 ended up being the most difficult. The cluster is located at 21 12 18 +46 34 00. MegaStar lists this cluster further to the east as does and Kahanpää. With a bit of stretch, one can see a faint cluster at 21 13 11 +46 33 23 which is exactly what I, with luck, managed to sketch. This means that WEBDA coordinates for this clusters are incorrect unless they're right and we, of course are wrong. The field is troublesome with other fairly obvious groupings of stars visible so I encourage observers to have a look at the field themselves and see what they can come up with. I do warn you though, looking for sparse, often uninteresting and lost objects isn't something you see in the Raiders of the Lost Ark.

Basel 13 with 8" Orion DSE @ 100x

After this session I only had half a dozen open clusters and asterisms left to sketch from Cygnus. The total number of open clusters in the constellation is roughly 160 and with asterisms we can probably get closer to 300, so the objects won't end quite that easily.


Optical photometry and basic parameters of 10 unstudied open clusters. Annapurni Subramaniam et al. 2010.