Wednesday, June 10, 2009
NGC 6910 The Rocking Horse Cluster
NGC 6910, a small and compact open cluster in Cygnus. When observing this open cluster you see bright yellow stars, that are in fact B-stars. So they should appear white. What causes the yellowish appearance is explained in this article, where I got some great help from Professor James Kaler, author of some of the best books on stars, and Dr. Franz Gruber, who sent me a few magnificent deepsky images of the Cygnus area to illustrate the high degree of nebulosity in the Cygnus area. To read the full story, and have a look at the wonderful images of Dr. Franz Gruber, follow this link to


Image by Dr. Franz Gruber

Posted by Math on 06/10 at 12:30 AM | (0) Comments | filed in: Deepsky observing | Print
Tuesday, June 02, 2009
Star Observer is online!

Today I launched my new website, Star Observer, a website dedicated to the observation of stars and open clusters. I created this website to have a more structured way to present my observing reports from my favorite objects: stars and open clusters. At the core of Star Observer you will find a new and growing collection of visual observing reports. There is a separate page for every object observed. On an object's page you will find my personal observing report, a sketch (or image) and some notes and background information about the object. The observing reports can be found in the top bar menu under "Single and multiple stars" and "Open clusters". In the section "observing stars" you will find some information about the equipment and resources I use for planning, executing and evaluating my observations.

In the future new observing reports will be added on a regular bases. I am also planning to publish a few articles about stars and clusters, and the method I developed for planning, executing and evaluating / publishing my observations.

You can navigate through using the different menus or the sitemap. If you have any questions or suggestions, please feel free to contact me, using the contact form.
Updates on will be announced in this blog. Follow this link to go to the homepage of Star Observer.

Clear skies

Math Heijen
Posted by Math on 06/02 at 02:34 AM | (0) Comments | filed in: General | Print
Wednesday, February 04, 2009
Working on a new website.......

It's been a long time since you heard from me, but during the last few months I've been working on a new website dedicated to the observing of stars and star clusters. It will take a little while before it will go online, but I will let you know! In the meantime I'm working on the design and content for this new site, and every now and again I look outside, and wow, in the last week of January Venus and the Moon were lined up in the evening sky. Hope you like the images. Please click to enlarge.



Posted by Math on 02/04 at 02:05 PM | (0) Comments | filed in: Solar sytem scraps | Print
Sunday, October 19, 2008
The Moon........


Last week I shot a few images with my Canon Powershot handheld at the 85mm Zeiss Diascope. Here's a nice sample!

Click to enlarge

Posted by Math on 10/19 at 09:32 AM | (0) Comments | filed in: Lunar scraps | Print
Wednesday, August 13, 2008
First light for the DMK 21AF04 AS
At the moment I'm testing my new camera, the DMK 21AF04 AS. I first tried the camera on the Coronado PST, and I was surprised by the result. The first image is a typical frame I selected from the 1-minute movie I shot from a prominence. The second image shows the stacked (600 frames) and processed version.



I will inform you about the equipment set-up and the stacking/processing software in the near future. Right now I'm busy reading the software manuals and experimenting with the few movies I recorded. In the end this monochrome camera will be used to shoot detailed images of the Moon, the Sun in combination with the Baader Herschel Prism/Continuum filter and the Sun in H-Alpha with the Coronado PST. I will keep you updated!

Posted by Math on 08/13 at 01:39 PM | (0) Comments | filed in: Solar scraps | Print
Sunday, August 03, 2008
Update on the spectral riddle in the "Rocking Horse"

I would like to thank Jim Kaler (Prof. Emeritus of Astronomy, University of Illinois), for helping me with my “spectral riddle” in NGC 6910, the Rocking Horse cluster. Here’s why the B1 supergiant looks yellow visually instead of bluish-white what you would expect from a B-type star.

As you know it is in the middle of Cygnus, near Gamma Cygni, not far off the galactic plane (you can see my wide angle picture at  As such it is subject to a great deal of interstellar extinction and reddening from interstellar dust. The dust selectively absorbs and scatters blue light (roughly 1/wavelength), so when you look at a star thru dust it will appear redder than it really is (basically the same reason the sun is reddened at sunset).  The intrinsic B-V color of a B1.5 Ia supergiant is -0.2. The observed color is 0.83, about the color of a K0 dwarf or a G5 giant, so the star appears yellowish to the eye.

The whole cluster is highly reddened in fact by about the same amount. The “color excess” (E) is the observed minus true color, which is about 1.0 for this cluster. The absorption at V is usually taken as 3.2E, so Av (abs at V) is 3 magnitudes. If there were not dust, a 7th magnitude star would appear 4th magnitude, and the cluster would be visible to the naked eye. Good observing on your part to notice that. You can see the effect in many other distant clusters near the galactic plane.

Jim Kaler

Thanks again!

Posted by Math on 08/03 at 09:28 AM | (0) Comments | filed in: Deepsky log | Print
Sunday, July 20, 2008
A spectral riddle in the Rocking Horse
In the night of July 13th/14th 2008 I had a short observing session with the 300mm Dob. I took a quick peek at a few old friends (M27, M29, the Blinking Planetary and 16 Cygni) trying out my new 35mm Panoptic. While sweeping through Cygnus, I noticed a bright, small clump of stars North-Northeast of Gamma Cygni. It was very easy to spot with the 35mm Panoptic. I increased the power to 230x with the 7mm Pentax XW, and I was looking at a wonderful little asterism of stars that, as a group looked like a little dog or horse. I made a rough sketch of this object, and when I later checked the sketch with my planetarium program (Voyager), it proved to be NGC 6910, an open cluster from the Herschel 400 list. According to some sources on the Internet, NGC 6910 is also called the Rocking Horse cluster. I had never heard about it or observed it before, so my first impression that it looked like a dog or horse, was not that strange. Others thought of it as a little horse as well.

Where can you find NGC 6910? The map below should give you a rough idea. Just center your telescope on Gamma Cyngi and move just about half a degree north-northeast. There you will find this nice clump of stars.


(Image from Voyager 4.5,

When you start looking for NGC 6910, bear in mind that it is a small cluster, only 7' in diameter, but then again, at lowest magnification it jumped out at me in my 12-inch scope. With the 22mm Nagler (fov 68') I already saw the complete outline of the little horse, and with the 7mm at 230x I could see a few dimmer, magnitude 12 stars. NGC 6910 is a Y-shaped cluster oriented northwest southeast. I counted between 20 and 25 stars but its always difficult to tell which do belong to the cluster and which are not included. The two brightest stars looked definitely yellow and are from the 7th magnitude.
There is however something that riddles me about these two yellow suns. According to all planetary programs and Internet databases, the Northernmost of the two stars is SAO 49556, a spectral type K1III, which explains the yellowish color. The southernmost of the two is SAO 49563 (or V2118 Cygni), a variable star of spectral type B1.5Ia. Normally I would think that a B1 star shines Bluish or at least mainly white, and not yellow. I checked other observing reports and I found that Sue French's (Celestial Sampler, page 132) reports:

At 87x, two yellowish stars of 7th magnitude and a pearly, split chain of eight 10th magnitude stars unite in a Y-shaped pattern about 5' long.

Sue sees two yellow stars as well, so I'm very curious why this B1 star seems to appear yellow instead of white. Does it have something to do with the Variable character of the star? Maybe one of you out there can help on this one.

Anyway, visually it is a very interesting group, so I made a sketch. The sketch below was made with the 300mm dob, a 7mm eyepiece (fov 18') giving a magnification of 230x. South is up and west is to the left. The next time when you're in the area observing, check out on NGC 6910 and let me know which colors you could see.


Posted by Math on 07/20 at 12:35 PM | (0) Comments | filed in: Deepsky log | Print
Sunday, July 13, 2008
Eye-relief for my 12-inch telescope


About one and a half years ago I bought a 12-inch telescope for deepsky observing, an f/5.3 Newtonian telescope from Orion optics UK. The tube is mounted on a dobsonian rocker-box and equipped with the Argo Navis Digital Telescope Computer. During the past 18 months I have put together my set of eyepieces for this telescope. I had four major criteria for selecting the eyepieces:

1. High quality
2. Large true fields of view
3. Weight between 400 and 800 grams
4. Lots of eye-relief

All selection criteria were important, but the most important was number four, eye-relief. In this article I will tell you why these criteria were more or less important to me, which eyepieces I finally chose and why I chose them.

Posted by Math on 07/13 at 12:54 PM | (0) Comments | filed in: Equipment | Print
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