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Re: (IAAC) Using High Magnification on Deep-sky Objects

I'd like to present some evidence favoring the use of  high magnifications when
observing small deep-sky objects (very small open clusters, globular clusters,
planetary nebulae, and galaxies).  Of course, one must take into consideration
the aperture of the telescope being used and the quality of  the seeing.  Small
instruments quickly "run out" of light as magnification is increased and poor
seeing makes high power counterproductive.

1. Sky & Telescope's Backyard Astronomy Article "Secrets of Deep-sky Observing"
by Alan MacRobert (reprinted as a pamphlet)

"Using high powers.  Another point made by (Roger) Clark involves the use of
high magnification on faint objects.  The conventional wisdom is that low power
works best for deep-sky viewing. ...The tremendous boom in low-f/ratio Dobsonian
telescopes has been fueled in part by the belief that low power shows deep-sky
objects best.*

Clark demonstrates that this assumption is usually false. ...

The essential point is that the retina, unlike photographic film, has very poor
resolution in dim light.  That is why you can't read a newspaper at night - even
though you can see it andy your eye lens theoretically resolves the letters just
as sharply as in daylight.

As Clark reports, the eye can resolve details finer than 1arc minute in bright
light but can't make out features smaller than about 20 or 30 arc minutes across
when the illumination is about as dim as the dark-sky background in a telescope.
... So details in a very faint object can be seen only if they are magnified to
such a large apparent size - which can require using an extremely high power!

The explanation lies in how nature has adapted the visual system to cope with
low-light conditions. ... In dim light, the retina compares signals from
adjacent areas.  A faint source covering only a small area - such as a small
galaxy in the eyepiece - may be completely invisible at the conscious level.
But it is being recorded in the retina, as evidenced by the fact that a larger
galaxy with the same low surface brightness is visible easily.  In effect, when
receptors see a doubtful trace of light they ask other receptors nearby if
they're seeing it too.  If the answer is yes, the signal is passed on up the
optic nerve.  If it's no, the signal is disregarded.

When the image is magnified, its surface brightness does indeed grow weaker.
But the total number of photons of light entering the eye remains the same. ...
It doesn't really matter that these photons are spread over a wider area; the
retinal image-processing system will cope with them.  At least within certain
limits.  A trade-off is needed to reach the optimum power for low-light
perception: enough angular size but not too drastic a reduction in surface
brightness. ...

What does all this mean for deep-sky observers?  Simply that it's wise to try a
wide range of powers on any object.**  You may be surprised by how much more
you'll see with one than another."

2. Mr. MacRobert goes on to reprise the above on pages 55-57 of his fine book
_Star-Hopping for Backyard Observers_.

3. On page 47 of _The Guide to Amateur Astronomy_ by Jack Newton and Philip
Teece the authors state, "Faint galaxies require a different approach.  Although
they may be most easily found by sweeping with a very low-power eyepiece, they
often appear brighter when studied at high magnification.  A power of about 2x
per centimetre of aperture is useful for sweeping, but 5x or 6x per centimetre
will probably show a better contrast between the galaxy's pale surface and the
surrounding sky."

4. John Sanford says in his _Observing the Constellations_(page 8), "A higher
power eyepiece will make the sky darker and frequently helps see fainter objects
better."

5. On page 90 of _The Backyard Astronomer's Guide_ Terrence Dickinson, when
discussing Telescope Myth #2.  Images Appear Brighter in Fast Telescopes,
comments, "... Therefore, contrary to popular belief, long-focus telescopes can
be used for deep-sky viewing, especially when equipped with a low-power eyepiece
(such as one with a 40mm-to-55mm focal length).  The drawback is that it is
difficult to reach extremely low powers and wide-fields with f/10 to f/16
telescopes which hampers views of deep-sky objects that extend over a large
area.***  This is the real reason that deep-sky fans are advised to stick with
fast-focal-ratio instruments.****"

6. And finally Phil Harington asserts on page 41 of his new book _The Deep Sky:
An Introduction_,***** "Base your choice of magnification on what you are trying
to see.  Widely scattered star clusters and large nebulae are best viewed with
low-power, wide-field eyepieces, while smaller deep-sky objects (e.g., planetary
nebulae and most galaxies) require higher power."

* that and the fact that the tubes can then be made conveniently short
** no argument there
*** exactly, a 40mm ocular in the ASH 17" f/15 clasical Cassegrain yields a "low
power" of 162x!
**** and perhaps the fact that it is much harder to locate (by star-hopping)
those DSO's that will fit into the field of view
***** http://ourworld.compuserve.com/hompages/pharrington/Sw12.htm

So what should we conclude from the above?  Simply to use the appropriate
magnification when doing deep-sky observing, low power for large, extended
objects and high power (if your telescope is large enough to compensate for the
light loss at higher magnifications) for small, dim ones.  Don't be afraid to
experiment!

Dave Mitsky
ASH, DVAA
References: