Observing Odds & Ends
By Paul Markov, May 30, 2001
On clear and transparent nights I still set up in my backyard in Scarborough, if I am unable to head out to darker skies. From this location I can see stars as faint as magnitude 5.1 with the unaided eye on the best nights. I was curious to find out approximately the magnitude of the faintest galaxy I could see from the city, so I set out to observe a series of galaxies, mainly in Virgo. The results shown below were achieved with a 10-inch Schmidt-Cassegrain telescope with a magnification of 78x.
|Object||Mag.||Surface Brightness||Size (arc min)||Observations at 78x|
|M49||8.4||13.2||8 x 7||Fairly easy to see, slight elongation, uniform brightness|
|M58||9.7||13.0||6 x 5||Somewhat difficult to see, easy to overlook.|
|M60||8.8||12.9||3 x 2||Easy to see, almost stellar|
|M61||9.6||13.4||7 x 6||Could NOT see|
|M84||9.1||13.0||5 x 4||Easy to see.|
|M86||8.9||13.2||12 x 9||Difficult to see, very diffuse|
|M87||8.6||13.0||7 x 7||A little difficult to see, small|
|M89||9.8||13.2||3 x 3||Quite faint, stellar nucleus|
|M90||9.5||13.4||11 x 5||Very faint, cannot tell shape.|
|M104||8.0||11.6||9 x 4||Very easy to see, nice elongated shape|
|NGC3941||10.3||12.5||4 x 2||Easy to see, bright nucleus|
|NGC404||10.3||12.8||4 x 4||Fairly easy to see, small bright nucleus|
|NGC6207||11.6||12.9||3 x1||Very difficult to see|
Based on the observations above, if you also observed with a 10-inch telescope and your unaided eye magnitude limit was about 5, then you can expect to just detect galaxies as faint as magnitude 10 or brighter, whose surface brightness is about 13 or brighter. If you use an 8-inch scope, you will still be fairly close to these upper limits.
Much to my surprise, I was able to see some rather faint galaxies beyond the Messier list from my backyard. The most amazing observation was that of NGC 6207, a galaxy just a degree north-east of M13. I have seen this little galaxy on five different occasions during the past year from my urban location. Of course, averted vision was a must and the galaxy could only be glimpsed during moments of steady seeing, however it was definitely there and I proved it to myself by drawing and comparing what I saw to a detailed star chart for a perfect match.
The critical piece of information for this type of exercise is the surface brightness of the galaxy. You can see in the chart that the last three NGC objects have a faint magnitude, but their surface brightness is higher than the Messier object listed above this is why I was able to still see them from the city. So, if you are stuck in the city, try looking for some galaxies that have a surface brightness of about 12.5 and chances are you will be rewarded.
I always observe with my right eye. Just recently I tried observing with my left eye and I noticed I could not see deep sky objects as well through the eyepiece. Compared to my right eye, the left eye was seeing low contrast, washed out images. It was like there was more light pollution present in the field of view. I am not sure why the remarkable difference between my two eyes, however some have said I could train my left eye to see as well as my right eye. Personally, I tend to think it is more of a physiological difference. Next time you are out observing, try the other eye while at the eyepiece and find out for yourself which eye is better at seeing faint objects there might be a difference!
I typically do not obtain satisfying views with magnifications above 100 times. This has been the case with all three Schmidt-Cassegrain telescopes I have owned, so I always use 50x to 80x when observing deep sky objects. At these lower powers I can always achieve a sharp focus where the stars are crisp pinpoints of light, whereas above 100x, star images become bloated and shimmer with spikes making the views less pleasing. In the past I often thought my optics were not of the highest quality, however, I have heard from other observers that not being able to use higher magnifications is mostly caused by poor seeing (i.e. the atmosphere is not steady enough), and that it is rare to have nights when the atmosphere is steady enough to use higher powers and still obtain sharply focused stars.
I have always had mixed feelings about GOTO telescopes; these are computerized telescopes that slew to anywhere in the sky with the press of a few keys. My predicament is that someone who regularly uses the GOTO feature on such telescopes will never learn the sky because he is relying on the computer too much. On the other hand, these can be extremely useful in many situations - for example, computerized telescopes can become instructors by pointing the way and allowing new users to familiarize themselves with the appearance of objects, fields of view differences, magnitude differences, and differences in deep sky object types. It is up to the user to diligently turn off the GOTO feature every so often and try star hopping. This past spring I upgraded to a Meade 10-inch LX200 and, after 19 years of star hopping, I must say I really enjoyed the GOTO feature of this telescope! I put it to good use for spotting all the objects discussed at the beginning of this article after all, I have completed the Messier list 4 times over, and I certainly did not want to spend much time hunting down those Messiers so I could write my short report. For the record, I plan to continue using the star hopping method for finding any deep sky object I have not seen before, this will ensure 10 years from now I will still know my way around the sky!
I have been using an equatorially mounted telescope since 1984 and have gotten very accustomed to the way telescope movements (left / right and up / down) follow the lines of right ascension and declination in the sky. This matched motion is extremely useful for star hopping because each movement matches exactly the grid lines in a star atlas. As noted above, this past spring I upgraded my telescope and for a few weeks I was without a wedge (for equatorially mounting an LX200). Reluctantly I had to set up my scope in alt-azimuth mode and found star hopping to be much more difficult. Hats off to those of you who routinely star hop with alt-azimuth mounted telescopes your skills are superior to those of the equatorially mounted star hoppers!
Copyright (C) 2001 by Paul Markov
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