Observation time: Fun with a 10″ Dobsonian Stargazer

Posted: January 17, 2011 by tallbloke in Astronomy

Late last night the sky cleared, so my friends Rob and Johnny and I drove up out of the orange glow syndrome to the top of the Chevin, a hill near where we live. To assist our Stargazing we took Johnny’s new toy with us, a nice 10″ Dobsonian “Stargazer” telescope with 40x and 120x eyepieces.

Johnny and Rob hunting for the Andromeda Nebula

Johnny and Rob hunting for the Andromeda Nebula. The red spot to the right is an aircraft warning signal.

We set it up away from the carpark up on the ridge and got some great observations of, amongst others, the Orion Nebula, Jupiter and its four main moons, and of course, our own Moon. See the photo I managed to get below the break.

Photo taken at 40x Magnification with Fujifilm EXR compact camera

Photo taken at 40x Magnification with Fujifilm EXR compact camera - click for full size image

I got this shot of the Moon by simply offering my handheld compact camera to the eyepiece and taking a snap. Johnny has a neat piece of astronomy freeware on his Nokia N900 smartphone which helped us navigate to the harder to find objects. This was really useful when hunting for faint objects in unfamiliar starfields using the 120x magnification eyepiece. I’ll update with the link to this.

It was a nice cold night with occasional wispy clouds drifting around. To stay warm we took a thermos of hot sweet tea and a hip flask containing a ten year old single malt whiskey. A really enjoyable few hours of observation.

  1. Doug Proctor says:

    I took photos of my son BMX jumping at a construction site at night (trespassing, right?), and PhotoShopped them. Select on the black sky, then crank up the contrast and the saturation. The sky filled with stars!

    This was done with film, mind you, so I don’t know what your CCD sensitivity or resolution was. But it is worth a try.

  2. tallbloke says:

    Hi Doug,
    At 3/4 the Moon pretty much obliterates the starfield for about 15 degrees of arc around it. But a good tip for creative work. The camera I have is pretty good for contrast work. It is 12 megapixel, but in contrasty locations, it uses 6mp for the bright parts and 6mp for the shady areas, at a different setting. Only a 5x zoom, but very little colour aberation, and no barrel distortion on straight lines.

  3. Gerry says:

    I was a professional observer for five years (1960 through 1964) at the U.S. Naval Observatory in Washington D.C. I even observed a transit of Venus with the 6″ transit circle once as it transited the Sun and the local meridian at the same time! The other astronomers were envious of my opportunity (it was my scheduled night to observe). I was able to time both limbs of the Sun and of Venus as they transited the meridian in rapid succession. Kept me busy tracking and retargeting the crosshairs, but the observations had a high weight in the FK4 star catalog, published in 1963. The 6″ had exquisite optics, so I had some good looks at the detailed features of Mars and Jupiter in those years.

    The first quartz crystal clocks were first used while I was working at the USNO. Those crystals were huge! Of course they were replaced by atomic clocks after I left the USNO for a more exciting career at Caltech-JPL in California.

  4. Gerry says:

    (It was my mid-day time to observe as well);)

  5. Gerry says:

    In case you are wondering how observations of the Sun and planets are used for a star catalog, it was a necessary part of determining the 1950.0 equinox and equator reference plane. Now everything is referred to the J2000 equinox and equator.

  6. tallbloke says:

    What sort of magnification were you observing Mars and Jupiter at Gerry? We thought we could see some colour tingeing on Jupiter last night, but it may have been aberration.

    We have another transit of Venus coming up soon – 5th or 6th June 2012 depending where you are.

    How close are the predicted times to actual?

  7. Gerry says:

    tallbloke says:
    January 17, 2011 at 10:41 pm
    What sort of magnification were you observing Mars and Jupiter at Gerry? We thought we could see some colour tingeing on Jupiter last night, but it may have been aberration.

    We have another transit of Venus coming up soon – 5th or 6th June 2012 depending where you are.

    How close are the predicted times to actual?
    I don’t actually remember the magnification of the micrometer eyepiece used on this instrument. It was pretty high, though. Does 100X sound plausible for a six inch refractor? It had a really solid mount on bedrock. The seeing in Washington D.C. was not the best, but there were days when it was surprisingly good. Here’s a scanned historical document about the telescopes at the USNO in the late 19th Century:

    Venus-Sun limb contact times for transits of Venus have a high uncertainty because of the black drop effect and the aureole effect. Even now, observed contact times are only published to the nearest second. Of course we could not observe limb contacts with the transit circle.

    The meridian transit observations of stars were good to a few milliseconds of arc, and the observed minus computed (O-C) errors in our least squares fits were not a whole lot higher.

  8. Gerry says:


    I found the transit circle eyepiece magnification in the document. 67X.

  9. Gerry says:

    Declination accuracy of a single meridian transit was about 0.01 arcsecond. The few milliseconds of arc was the final standard deviation after many observations.

  10. tallbloke says:

    Since my crash I often these kind of calcs wrong but 3 milliseconds of arc at 149 million kilometers (average Sun-Earth distance) is (2*pi*1.49^8)*(360/60/60/60/333) = 4686km (maybe!)

    If so, considering the transits takes around 17 minutes, we should know when ingress is going to occur to within an error of around 3.4 seconds.

    Does that sound right?

  11. Gerry says:

    I calculate the chord length at one AU for 3 milli-arcseconds as
    2AU*sin(.0015/3600 deg) = 2.17581 km.

    However, the above calculation should be made for Venus distance, not Sun distance, and the positions of the planets are not as well determined as star positions. The Sun’s position is less well determined yet. I’m not really sure what it all adds up to though. Not an easy number to get with simple calculations, and the diameters of the Sun’s photosphere and of Venus also have some uncertainty attached to them.

    The simple conversion of angle to time, in terms of Earth rotation, is of course 360 degrees in 24 hours. One second of time is then 24/360, or 0.06667 arcsecond.

    All in all, the theoretical predictions of Venus transit egress and exit times are good for better than a second of time, I’m pretty sure.

  12. Gerry says:

    I should have said predicted “ingress and egress times.” Getting accurate observations of solar transits of Venus is another matter altogether.

  13. tallbloke says:

    Ok, I fouled it up. 🙂

    The software I mentioned in the intro is available from http://www.stellarium.org/
    This is a great program so give it a try. It will even send control data to a ‘goto’ telescope.

  14. Layman Lurker says:

    When I was a kid, my dream was to become an astronomer. Funny thing is, I had never even looked through a telescope until a couple of weeks ago on the Big Island of Hawaii. We went on a small tour bus to the summit of Mauna Kea amongst the big observatories. After sunset we descended to just below the Visitor’s Center (still above the clouds) and the guides pulled out some pretty impressive looking instruments for an evening of star gazing. An incredible night.

  15. tallbloke says:

    Hi LL and welcome to the talkshop.
    Mauna Kea is home to the Keck Observatory isn’t it? SOunds like an exciting trip. Got any photos online?

  16. Layman Lurker says:

    Here is a picture of Subaru (Japan) and Keck I and II atop Mauna Kea. Keck is also known for it’s Visitor’s Gallery (open only till 4 pm) which was closed when we arrived at the summit. I believe there are a total of 12 installations on Mauna Kea all told. This is another image from the Summit just before sunset.

    For anyone who has ever contemplated this trip – you won’t be disappointed.

  17. Zeke the Sneak says:

    Here’s Ian Morrison’s January Night Sky episode for the N. Hemisphere and John Field on the S. Hemisphere:


    Scroll down for the MP3.

  18. RobB says:

    Is that Otley Chevin? I know it well!

    [Reply]Yes! where are you from?