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Observing Tips and Tools For Planetary Nebulae

Section Contents

I. BACKGROUND

II. NEBULA FILTERS

           III. DEEP SKY OBSERVING

IV. The CDROM of Planetary Nebulae (Benji Mauclaire)

I. BACKGROUND

The dying stars which have expelled their outer layers of hydrogen and other elements, and which we observe as 'planetary nebulae' exhibit such a range of brightness, size, form and structure that there is no single observing technique suitable for even the most common ones found in our skies.

There are very small, bright planetaries; there are large, very dim planetaries, and of course, there are many small AND dim planetaries.  Fortunately, telescopes of even moderate aperture (70mm to 150mm) are quite capable of detecting numerous types of PNe's, and with the use of certain observing techniques and / or nebula filters, this 'detection', observing, and examining can be very rewarding.  Owners of larger instruments, of course, will be rewarded with the far ranging diversity of PNe's, and can set their sights on detailed observations of the surrounding gas nebula, the structure of the shell(s), and even perhaps, the central star of the nebula.

It is well known that most stars in our observable universe emit an essentially continuous spectra so that neighboring wavelengths in their spectra are almost of equal strength.  For stars that have advanced to the PN stage however, their spectra has been altered considerably due to the nature of their internal nuclear constitution and the greater temperatures generated, on the order of 100,000 degrees K.  No longer is the spectra considered continuous, but instead consists of discrete emission lines which determine how the star's light is emitted.  Most of the known PNe's have been initially detected not by visual means, but by spectroscopy. So even though a number of stars may appear stellar through the eyepiece of a telescope, the examination of their spectrum can disclose whether or not they have advanced to the planetary nebula stage.

In the spectra of PNe's, there are a number of principal  lines that show up at different wavelengths; these lines, and their respective associated ion are shown in the following table:
 

Planetary Nebula Emission Lines
Wavelength  
(Angstrom's) 

Associated  Ion  

Line Names 
and Notes
3727

O II

Forbidden line; often strong
4340

H I

H gamma; = 0.4 of H beta
4861

H I

H beta; = one third of H alpha
4959

O III

Forbidden; = one third of 5007
5007

O III

Forbidden; = usually the strongest
6548

N II

Forbidden; = one third of 6584
6563

H I

H alpha
6584

N II

Forbidden line

(Note: Forbidden line is not one that is actually 'forbidden'; it's just an emission line, or state of atom excitation, that occurs in these stars, but is impossible, at the present time, to duplicate in laboratories located here on Earth.)

It just so happens that the peak response of the human eye occurs at a wavelength of around 5,500 Angstroms (550 nanometers, or nm), which corresponds to the color perception of green and very close to the strong O III emission line found in planetaries and H II regions.  The range of visual color perception of the dark adapted human eye is from approximately 400 nm to 620 nm, which is one very good reason why there are certain planetaries and H II regions in which observers perceive color (mainly green) in these objects. But there has to be a certain minimum surface brightness of the object for this to happen due to the fact that under dim lighting conditions, the "cone receptors" of the eye do not perceive color, and that is left to the "rod receptors".  More on this later.

Shown below is a drawing of the main spectral features of a typical planetary. The spectral range is from 342.6nm to 658.4nm.  The emission lines identified below the spectrum are all so-called forbidden transitions, and the emission lines at 495.9nm and 500.7nm are those emissions that enables stellar planetaries to be detected by prisms, spectroscopes and nebula filters .


Main PN Spectral Emission Lines


Note: The following discussion on nebula filters is being reprinted, with permission, from the author, Jack Kramer, and the Lake County Astronomical Society (Web Master - Michael Purcell).  The information is extracted from a very informative and extensive publication on "The Care and Feeding of an Astronomical Telescope", written by Jack Kramer of the LCAS. Many thanks to these gentlemen on allowing me to reprint this information, as it provides all of the necessary insights into purchasing and using filters of this type.
 

II. Nebula Filters

The term "nebula filter" is a generic description of a variety of filters that block certain discrete frequencies of light to enable the observing of nebulae even where light pollution is a problem. But this technology also produces filters that are intended for observing gaseous comets ("Swan band" filters) and some purportedly to enhance the light emitted by galaxies. Instead of being a piece of glass that's dyed a certain color, these filters use thin film interference technology to discriminate between discrete bandwidths of light, measured in Angstroms. This technology actually spans a wide range. On the low-end are light pollution rejection (LPR) filters that admit light across a broad range and stop only those bandwidths emitted by streetlights; these are referred to as "broadband" filters. The light that the filter admits is in the "passband". A true nebula filter stops more light than it admits, so these are designated as "narrowband" filters. While colored filters sell for about $12.00 apiece, many newcomers to the hobby are astounded to find that the nebula filter that looks like a piece of tinted glass the size of a quarter starts at about $75.00 a pop! In fact, the same concept is applied to hydrogen alpha filters intended to reveal solar flares. Some of these are so sensitive that they must be heated to keep the passband within a fraction of an angstrom of the correct wavelength. It's not uncommon for these filters to cost over $2000.00!

For our purposes here, we'll concentrate on those filters intended for observing nebulosity and identify each by its capabilities.  That's not such an easy task, as you'll see.

The LPR (broadband) filter approaches the problem from the angle of rejecting light emitted by mercury or sodium vapor lights. From this standpoint, it darkens the sky background and may actually make it easier for you to see certain bright galaxies, as well as nebulae. Though it's good for general applications in areas of moderate light pollution, the LPR filter becomes overwhelmed in locations where there's severe light pollution. On the other end, it won't help you see things better from a truly dark site. But within its working range, it can prove very helpful and is what most newcomers to astronomy acquire. Each supplier has its own brand name for this type of filter. Some examples: Lumicon Deep Sky, University Mark V Photo-Visual, Meade Nebula, Parks LPB, Orion Skyglow, and Celestron LPR.

The true nebula filter (narrowband) is able to do its thing because nebulae emit their visible light in very narrow wavelengths. Note that these filters won't work on reflection nebulosity, such as that involved in the Pleiades cluster, because this type is only reflecting starlight, which covers a very broad range of visible light frequencies. One supplier (Lumicon) has a trio of filters for specific purposes:

Ultra High Contrast - provides high contrast on planetary and other emission nebulae.

Oxygen III - has higher contrast than the UHC above, but in a narrower range - primarily that of planetary nebulae which emit much of their radiation in the band of doubly and triply ionized oxygen.

Hydrogen Beta - the most discriminating filter - intended specifically for the California, Horsehead, and other extremely faint nebulae with hydrogen beta emission.

These filters often improve the view even at dark sites, because most such sites aren't really dark - there's natural skyglow (neutral oxygen) and residual glow from far-off towns, especially in the winter when snow adds to the reflection of light. During times of peak solar activity, there may be airglow from atmospheric gases excited by gamma rays from the Sun.
While Lumicon filters are generally regarded as the best in the field, other firms also produce narrowband filters at somewhat lower cost, such as Orion's Ultrablock.

Which filter is best? In all cases, the filters are identified as either broadband or narrowband. Most experienced deep sky observers opt for narrowband filters because they provide the greatest overall improvement. In fact, some nebulae look considerably different when viewed through a very narrow narrowband filter, such as the Oxygen III. The following chart shows the wavelengths of light (in nanometers) that are allowed to pass by some of the filters for which data is available from the manufacturer. Note that light pollution occupies a range primarily in the center of the chart; in all cases, the filters do not pass any light in this range. Actually, light pollution somewhat spills over into other frequencies, so the more discriminating narrowband filters provide a darker sky background. Of course, this is done at the cost of transmitting light only from those particular nebulae that emit radiation in the very narrow ranges of these filters. There is no clear consensus among experienced observers, but many feel that a general purpose narrowband filter such as the Orion Ultrablock or Lumicon UHC is probably the best choice for the first-time purchaser of a nebula filter, especially in view of the amount of light pollution with which most of us have to contend.

One final note: Nebula filters do their job by restricting certain frequencies of light. The object you are observing appears brighter because the background becomes darker, while the light from the nebula is allowed to pass through. In effect, the view is improved by virtue of an improvement in contrast. But in reality, even the nebula is slightly dimmer, because in fact, there is less light passed to your eye. The effect is so minute that it isn't a problem with larger telescopes, but on small telescopes, you may see no improvement whatsoever, or a faint nebula may disappear altogether when using a narrowband filter. The telescope itself must gather enough light to compensate for the loss of light caused by the filter. Personal experience suggests that a narrowband filter works best on telescopes of 6-inches or larger, although I have successfully used an Oxygen III filter when observing bright emission nebulae (such as M42 in Orion) with a 4-inch refractor.  For scopes smaller than 6-inches, a broadband filter will be more useful, and in scopes of 3-inches or less only a broadband filter will somewhat improve the view, and only on the brightest nebulae.



       
 


III. Deep Sky Observing Tips

The following tips and techniques, although certainly not all inclusive, can lead to a more fulfilling and pleasant observing session. I hope they provide some insights, and if you know of any that I have overlooked, please don't hesitate to let me know. Okay? Okay.
 

Top Twelve Planetary Nebulae & Deep Sky Observing Tips

1.  Optics
Regardless if you are using a 100mm or a 75cm telescope, ensure that the optics are reasonably clean and the system is collimated (all the optical elements are aligned to one another). Also, check eyepieces, filters, and other optical attachments (reducers, barlows, etc) for cleanliness and are damage free.  At the observing site, set up your equipment as soon as possible to allow the optics and the system to temperature acclimate before you start observing.

2. Lists
Prepare a short list of objects to observe beforehand.  This list can be made up to include 10 to 15 objects at different levels of difficulty.  Try to familiarize yourself with their approximate location in the sky and within their constellation.

3. Dark Skies
Seek the darkest skies, obviously.  But also take into consideration the benefits of altitude, a dry air environment, windless conditions, and a good local horizon.  Remember, the darker the skies, the better the contrast between faint objects and the background sky.

4. Night Vision Aids
Don't forget to take extra batteries and a bulb for your red-light flashlight. Or take along a backup. Retain your night vision by using a dim red-light.  On the day of your planned observing, try to avoid extended stays out in bright sunlight.  Always take along a very dark or preferably black cloth large enough to drape over your head, shoulders, and eyepiece - this eliminates stray and reflected light while viewing, even under dark skies.

5. Comfort
Don't forget warm clothing for your feet, hands, and head.  Definitely take your favorite astro-food, cold and warm drinks, and water. Do not consume alcoholic beverages. They will definitely affect your night vision, and can make you throw-up on the telescope (Yuckee).  One item you definitely do not want to forget is mosquito or bug repellant.

6. Finding Stuff
Use star-hopping methods to locate your 'targets'.  Set up your preferred star-atlas or finder charts in a convenient location so you can quickly view from chart to eyepiece (or finder) back to chart.  If you are using computer control or setting circles, ensure your alignment is of the necessary precision and the tracking motion is correct.

7. Filters
Keep your filters and their holding cases handy.  Filters that you should have are broadband and narrowband.  For planetary nebulae, you should definitely have the OIII filter and the UHC (Ultra High Contrast).  Both of these are narrowband.  A good broadband filter (such as the Lumicom Deep Sky, Orion SkyGlow & others) will be useful under moderately light polluted skies, but didn't I ask you to go to a dark sky site?  In addition to the filters, always take along a black piece of cloth large enough to drape over your head, shoulders, and the eyepiece to eliminate stray and reflected light from interfering with your viewing and reflecting off of the filter.

8. Visionary
Use averted vision along with direct vision to view extended objects.  Look to one side of the object to bring out detail in other parts of the object.  You are then using a more sensitive region of your retina. It takes a little practice to perfect, but makes a big difference on viewing faint objects.

9. Jiggle
While observing a faint object, try jiggling the telescope a very small amount while viewing through the eyepiece.  This action also 'jiggles' the image and any difference in contrast between the object and the background sky will be (hopefully) more easily perceived by your eye.

10. Blinking
Most planetaries which appear stellar or almost stellar are difficult to pick out from normal field stars. A helpful technique to isolate which object is the planetary is to hold, with your fingers, a nebula filter between the eyepiece and your eye, and by moving the filter in and out of the field, the 'normal' stars will usually disappear (spectral light blocked by the filter), but the planetary will remain in view (emission lines transmitted through the filter).  This is one 'blinking' technique.  In conjunction with this method is the use of a dark, preferably black, cover or 'drape' over your head and shoulders and which also shields the eyepiece.  This eliminates stray light (even under dark skies) and reflections from interfering with your attempts to see both the object and what is being passed through the filter. 

11. Stop & Look
Look for details and structure and faint extensions on all objects. Spend some time using the above techniques, such as averted vision. Examine the field for subtle details. Record the details of what you see, and/or make a sketch of the field, including the brighter stars, or associated objects.

12. Power Struggle
Use different eyepieces. Start off with low or medium power ones, then switch to higher power once the object(s) have been located. On nights with excellent 'seeing', high power eyepieces can bring out extraordinary details.

13. ENJOY!

 

 

IV. The CDROM of Planetary Nebulae

This CD, produced in France by Benji Mauclaire, is packed full of material on planetaries and much more.  Benji has provided the following information on the CD (Thanks to Benji for sending the CD information below in English). For ordering information, please contact Benji at bmauclaire@underlands.org  . Benji notes that this is not a commercial product, but that copies will be made available for planetary nebulae enthusiasts at a reasonable cost that covers the cost of materials and shipping from France. Thanks Benji!

CONTENTS OF CDROM:

Introduction

 

    Both in research and in amateur astronomy, the quest of information involve such a lost of time. As I'm a PN fanatic, I've done a prospection work all around the world wich has been conclued by this CD where astrophysic documents and pictures collection are merged.


Articles

    I've collected here some articles and conferences that I made and which deal with subjects related to PN : from stellar evolution to spectral analysis...
 

 

  Those texts are copyrighted and can't be reproduced or freely distributed without the author permission.


Catalogues

    Here are collected PN catalogues as the SEC (ESO) one, the PK catalogue with visual magnitude or cataclysmic stars catalogue...
 

 


Np_datas

    Ospuscule of useful astrophysics datas.

 


Np_draws_benji

    This directory contains all my sketches about visual obsevation (more than 200) that I made with a 18" dobsonian telescope at the La Sinne Observatory (Vauvenargues  (13) - France).

    Some are merged in a tabular where each sketches are associated with the photographic picture for comparison.


Np_draws_others


    Other amateur astronomers make accurate PN observations :
 




Np_gallery


    Here are collected more than 1 200 pictures of PN that come from several origins : internet, magazine HST surveys, amateur CCD pictures...

    This collection provides a large representation of PN's morphology through several great observatory's telescopes...

    Otherwise, all the 1 143 PN aren't represented even if the DSS gives the pictures of lot of them but not always with the best quality required by morphological analysis..


PreNP_gallery


    A picture collection and text (articles...) about final stage of stellar evolution as Wolf-Rayet stars, PP, OH-IR object useful for PN's progenitor studying.
 

 


Programs

    Some software and tools that deals with stellar evolution, hydrodynamic, etc...
 

 

Tools

    Tools that provides reading postscript files and others.
 

 

Webb_for_np


    Here are the complete web site files or web address that are related to PN ! A lot of stuff is available...
 


                                               En espérant que ce CDRom vous rendra service, faites une bonne visite.

                                                                                                                                         Benjamin MAUCLAIRE
 

This document was translated from LATEX by HEVEA .


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I will continue with this background information and much more on observing PN's in the very near future.
 Last updated May 6, 2001

 
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