Pushing Your Telescope, and Yourself,
to the Limit
By Paul Markov, August 2000
As an avid deep sky observer, I often try to push my telescope and myself to the limit. I have also always wondered "what is the faintest possible deep sky object I can see with my telescope"? Of course, the faintest object one can see with a given telescope is ultimately a function of the telescopes aperture, however there are other factors that affect this limit some quite obvious, others more subtle.
Because deep sky objects vary so much in their nature, it is very difficult to decisively state that with "x" inches of aperture an experienced observer under a perfect sky can see deep sky objects as faint as "x" magnitude. I have experienced this uncertainty myself with the two telescopes I have owned in recent times; an 8-inch and a 10-inch SCT. The chart below shows the faintest nine deep sky objects I have ever seen. If you take a few minutes to analyze the chart, you will notice that I have had equal success in observing faint deep sky objects with either telescope, although the 10-inch has allowed me to see fainter surface brightness objects.
|NGC 4186||Coma Bernices||13.8||13.5||8-inch SCT||100x||Long Sault|
|NGC 6247||Draco||13.5||11.4||10-inch SCT||107x||Long Sault|
|NGC 4799||Virgo||13.3||12.9||10-inch SCT||62x||Port Granby|
|NGC 5953||Serpens||13.3||12.8||8-inch SCT||62x||Starfest|
|NGC 5994||Serpens||13.2||Unknown||8-inch SCT||62x||Starfest|
|NGC 4668||Serpens||13.1||13.1||10-inch SCT||62x||Port Granby|
|UGC 2855||Camelopardalis||12.9||15.0||10-inch SCT||107x||Near Parry Sound|
|NGC 2146A||Camelopardalis||12.9||14.1||10-inch SCT||62x||Near Parry Sound|
|NGC 3804||Ursa Major||12.9||14.0||10-inch SCT||62x||Port Granby|
*Magnitude (Mag.) and Surface Brightness
(S.B.) numbers are from the Saguaro Astronomy Club (SAC)
**Long Sault Conservation Area is 20 minutes north of Bowmanville, Port Granby is 20 minutes east of Bowmanville, Parry Sound site is 20 minutes south of Parry sound, Starfest site is 15 minutes north of Mount Forest.
A couple obvious factors that affect ones ability to observe very faint objects are light pollution and sky transparency. Everyone knows the effects of light pollution on deep sky observing so I will not dwell on this. Just remind yourself not to bother hunting for very faint objects unless you know you are under a very dark sky. Good sky transparency is another important requirement. An indication of good transparency is a deep blue sky during the daytime, while at night transparency can be measured by doing a visual limiting magnitude estimate (see Observers Handbook, Sky Transparency). However one must be careful because the visual limiting magnitude estimate could be misleading when applied to deep sky observing - the reason is "seeing" conditions.
"Seeing" is the term used by observers for judging the steadiness of the earths atmosphere. During nights of good seeing the earths atmosphere is free of air turbulence allowing for steady and sharp stellar and planetary images. Some deep sky observers believe that for very faint and low surface brightness deep sky objects, poor seeing will blur the faint light right into the background sky, making the objects invisible. This is analogous to trying to observe small, low contrast planetary features on nights of poor seeing these features will just be blurred into the planets disk. Therefore, just as in planetary observing, keep staring at the field where you suspect the deep sky object might be, and perhaps it might "pop into view" during a moment of good seeing. Personally, I have experienced many nights where the sky appeared to be quite transparent, yet I could not see as faint as I typically do. Perhaps those transparent skies were being severely affected by poor seeing.
It is generally accepted that increasing the magnification will allow you to see fainter objects, therefore if you cannot see that elusive deep sky object you have been hunting for a while, try increasing the magnitude. My personal experience is that increasing the magnitude does not necessarily allow me to see fainter objects than at lower power, rather it just lets me see them better. I do the majority of my deep sky observing at lower powers (50x to 60x on my 10-inch SCT), and occasionally I use a medium magnification of 107x. I never use a higher magnification as the images deteriorate too much and become too dim.
Other factors that will affect the sky, and are not typically considered, are the presence of natural forms of light pollution, such as the Gegenschein, the zodiacal light, and aurora. The first two are quite localized in the heavens and are seasonal, thus they can be avoided with a little planning; look in your Observers Handbook for more details. Aurora may seem as an obvious obstacle and hardly worth mentioning, but this is only true when the aurora is bright and clearly visible. You should be careful when observing deep sky objects in circumpolar constellations because a faint, non-active auroral glow may be present without being very noticeable.
Atmospheric extinction is the term used for defining the absorption of starlight light by the earths atmosphere. At an altitude of 52 degrees and higher, atmospheric extinction has a factor of zero.
The factor increases the closer to the horizon an object is and is the result of starlight having to pass through a thicker portion of the Earths atmosphere. Atmospheric extinction, in terms of magnitude loss as a function of altitude, is given in the Observers Handbook, Sky Transparency section. The meaning of all this is that if youre planning to observe objects at the limit of your telescopes light-gathering ability, you should observe them when they are as close to the Zenith as possible. Notice how the majority of the faintest objects I have seen are in constellations that always attain a high altitude (Coma Bernices, Draco, Camelopardalis, Ursa Major).
Surface brightness is an entire topic in itself. For more details on surface brightness, please see my article on this specific topic in Scope, April 2000 issue. Pushing your telescope to the limit actually turns into a "numbers" game when considering surface brightness. But, if you want to reach your limit strictly in terms of magnitude, then target objects that have the faintest possible magnitude, yet the brightest possible surface brightness, such as NGC 6247 which has a magnitude of 13.5, but a rather high surface brightness of 11.4. On the whole, an object with a brighter magnitude but a fainter surface brightness may be more difficult to see, such as UGC 2855, which has a magnitude of 12.9, but a feeble surface brightness of just 15.
Other factors that may limit the observation of very faint objects through a telescope are the observers experience and visual acuity. If you were to show a magnitude 12.5 galaxy to a novice observer who is just getting used to the fact that M31 does not look as spectacular in his 4-inch as all the photos he has seen, chances are that he will not be able to see such a faint galaxy. Therefore, if you are just starting out in deep sky observing, give yourself some time to gain experience before attempting really faint objects.
In respect to visual acuity there is not much that can be done as this is typically limited by how good your eyesight is in low-light conditions. Perhaps experience will train your eyes to see those faint fuzzies a little better, but an actual physiological limitation cannot be overcome by experience. In fact I have known a few amateur astronomers who gave up visual astronomy for astrophotography because they just could not get satisfactory views of deep sky objects due to their poor eyesight. I have read that taking a few deep breaths just before attempting to view a very faint object may help. The theory behind this practice is that more oxygen is delivered to your body, thus increasing your eyes sensitivity to faint light.
Still on the topic of eyesight, it is generally accepted among serious deep sky observers that exposure to bright sunlight the day before a night of deep sky observing will temporarily "bleach" your eyesight, minimizing your eyes sensitivity to faint light. Thus, it is suggested you wear dark sunglasses or stay out of bright sunshine altogether the day before you plan on pushing your telescope to the limit. A few people have gone as far as suggesting that excessive smoking or drinking the day before a deep sky observing session will decrease your eyes acuity to faint light. Although it sounds like a good thing to avoid, I doubt any studies have been done to verify this theory.
There are another three physical conditions that will affect your ability to view faint objects, and for these I speak from personal experience. The first one is your energy level. Suppose you have had a tough week at work, that you have had little sleep all week, it is 2 a.m. on a Saturday morning and that youre trying to spot a very faint object. I would not be surprised if you fail to spot it! Being rested and having a good amount of energy will help you concentrate on your search and ensure your eyes will deliver every possible photon to your brain. Comfort is another important factor; if you are very cold and on the brink of frostbite or you are bent over backwards because the eyepiece is an awkward position, you will not be able to concentrate long enough to have a good look through the eyepiece. This of course applies to any kind of observing, but will definitely have a major impact if you are trying to push your telescope to the limit. Another factor, which is a combination of the two above, applies if you keep one eye closed while viewing through the eyepiece. Once in a while I experience difficulty in keeping my non-observing eye closed because I am very tired. This then becomes quite uncomfortable and it impacts my ability to concentrate on detecting very faint objects. A solution to this problem is to wear an eye patch over your non-observing eye so that you can still keep it open. You can make your own eye patch or you can buy one for a few dollars at a pharmacy.
One last suggestion is to use a "monks hood" to shield your eyes from stray light. Unless you are lucky to have your own domed observatory, you will experience stray light interference of some kind or another. Throwing a piece of dark fabric over your head to shield your eyes from stray light will increase your chances of seeing very faint objects. This is particularly helpful if your eyepieces have substantial eye relief, as this forces you to leave a gap of several millimeters between your eye and the eyepiece, allowing stray light to interfere.
I am very interested in finding out "how faint you have gone". Please drop me an email at firstname.lastname@example.org with details such as object number, telescope size, magnification, and observing location.
Copyright (C) 2000 by Paul Markov
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