The Hydrogen alpha Tool Box


For those who have never had the chance to observe the Sun with a hydrogen alpha
filter, let me say, should you get the opportunity, you are in for a real treat! 
And to prepare you for such an experience I am going to give you some tools you
will need in order to understand and fully appreciate what you will be observing
with the filter. First of all, along the electromagnetic spectra lies the
hydrogen alpha line and is numbered 6563 A. or 6563 Angstroms.  (An angstrom is a
unit of length used for expressing the wavelength of light, but the use of
nanometers is now preferred, and the hydrogen alpha line would then be
represented as 656.3 nm).  

This is the first line in the Balmer series and the strongest absorption line in
the solar spectrum and most commonly used by solar researchers.  The hydrogen
alpha line can be divided into "hair thin" slices or "sub-angstroms " or half
band pass (HBP) units and is represented by a zero, followed by a decimal, and a
single digit number (for all practical purposes) and is seen below. Currently,
for amateur and student use, filters range from 0.9 A. to the very expensive 0.5
A., but it is the 0.7 A. filter that will give us a good, overall view of the Sun
and its prominences.  Other filters will tend to bring out various other features
an observer may wish to study.  And it is important to know and remember that
EACH FILTER IS UNIQUE. What is true for one filter may not be for another. Also,
hydrogen alpha filters can be used on refractors and SCT reflecting telescopes. 
However, my technical advisor tells me that hydrogen alpha filters can be used on
a Newtonian telescope, BUT the telescope would have to be drastically modified.

Should such modifications be made, the telescope would be suitable for ONLY solar
observation. There are two types of hydrogen alpha filters available; one type
that uses an "oven" to keep the filter centered on the hydrogen alpha line and
the other type that does not. This type is called a T-scanner as the hydrogen
alpha filter pack is tilted to produce the "on band" image.  For demonstration
purposes, we will be using the DayStar "ATM" 0.7 A. filter equipped with an oven.
 All these specialized filters are made by DayStar Filter Corp. including the
more expensive "University" model which features higher quality "etalons" or
"filter pack components".

Let's go to the telescope now and begin setting up the telescope for our
observing run.  The very first piece of equipment that goes on the telescope is
the ERF mask.  ERF stands for energy rejection filter and has a "window" made of
red optical glass (or slight color variations thereof) that has been figured to
better than a 1/10th of a wavelength.  This filter will serve to protect the eye
from harmful infrared and ultraviolet radiation, but will admit the harmless
sunlight.  The ERF also protects the hydrogen alpha filter "blocker window " from
bleaching and extends its life. The ERF mask comes in various sizes and the
choice of size will depend upon the size of the telescope and what the observer
intends to do with the hydrogen alpha filter.  For example, I use a 6" f/10
refractor for my observations.  But that f/ratio is much too "fast" for my
hydrogen alpha filter which will only center on the hydrogen alpha spectral line
if the telescope is stopped down to about 2" (60 mm) or f/30.

A 2" ERF mask produces a "parallel beam" of light for my particular telescope
size. A 4.25" (100 mm) ERF mask would also create the same focal ratio IF a 2x
negative barlow lens is attached to the hydrogen filter and will give the
observer a wonderful close-up view of the Sun! One last word concerning the ERF
masks. There are two types: on-axis, which is for refractors; off-axis is for the
SCT reflectors.  (Fig. 1, page 4) ERF masks are not recommended for use on the
Newtonians unless the telescope is drastically modified. We will use the 2" ERF
mask for the time being and attach it to the front end of the telescope over the
objective.  After the ERF is in place, we lightly, but securely tape the mask
into place.  (I use pieces of basic black duct tape that has been placed into two
or three positions around the mask.  The tape also comes in a variety of fashion
colors!).  The tape serves to help prevent accidents from happening (e.g. a
sudden gust of wind). I also tape the finderscope closed.  This is especially
important when dealing with the public. In this case, I would remove the
finderscope COMPLETELY.  Besides, the finderscope is not really needed to find
the Sun! Now that the ERF is in place we can begin adding the hydrogen alpha
filter to the diagonal and eyepiece to the telescope.

Later, one may wish to add a camera with the appropriate accessories. As I stated
previously, some hydrogen alpha filters have "ovens" which helps keep the filter
at a constant temperature.  For example, the filter I use runs about 130 degrees
Fahrenheit.  This is important to keep the filter constantly "on band" and
centered precisely on the hydrogen alpha spectral line.  On the other hand, the
T-Scanner is calibrated for the average temperature of the observer's area or
locale, which I thought was interesting . The hydrogen alpha filter is plugged
into regular 120 household current for 10 to 15 minutes to reach the correct
temperature.  Better quality hydrogen alpha filters come with an adjustable
temperature control.  The control is calibrated from 0 to l0 (ten being the
highest and "hottest" temperature).  The knobpot controller resembles the face of
a clock, or perhaps, I should say the face of a wristwatch, since that's about
it's size.

On the filter I use I need to make sure not to exceed the
number 9 setting on the knobpot.  Also, this filter has been found to work the
best on the number 8 setting and so I think it is a good idea to check and make
sure the setting is on number 8 BEFORE and AFTER plugging in the filter.  I will
also glance at the knobpot during the observing run, to make sure the setting is
correct.  Failure to check the setting may overheat the etalon filter pack, 
inside the filter, causing the filter to shift "off-band" into the red wing of
the hydrogen alpha line. 

Some hints:  (1) Don Trombino showed me how to tie the extension cord and the
cord from the hydrogen alpha filter in such a manner to keep the connection from
pulling apart.  (2) Also, it is important to keep the cord (from the hydrogen
alpha filter) hanging straight downwards so as to prevent the system from
becoming polarized. Should the system become polarized the image, of the Sun,
will become blurred and indistinct. (3) While I am making my white light solar
drawings (via projection) I have the filter plugged in and set aside so that it
is ready to go for direct hydrogen alpha viewing when I am finished sketching. 
This helps save some time.

The 10 or 15 minutes are up and we can select the eyepiece we wish to use.  Some
may prefer a 40 mm, 26 mm (or somewhere in that range), but I prefer the 17 mm
Plossl as it gives me a nice image and is easy to work with.  Experiment!
However, something like a 10 mm or 5 mm will cause a loss of light and definition
(so called: "empty magnification").  Add a star diagonal, a rubber eye shield,
maybe throw a dark cloth over the head, and get ready for some thrills!

The Sun is made up of the photosphere, chromosphere, and the corona.  It is the
chromosphere that the hydrogen alpha filter will permit us to see since it is
composed mostly of ionized hydrogen. Focus the telescope AFTER your eye becomes
accustomed to the red color of the solar disk. The first thing you will notice is
the "orange peel" looking "surface" of the Sun.  This feature is called
"mottling."  Mottling consists of spicules (tiny peaks of flame) forming ring
shaped patches.  Now, look for sunspots.  Should you notice lighter areas around
a sunspot or even away from a sunspot this feature is called "plage."  (Plage
rhymes with garage and the credit goes to Don for this one).  Cruise around the
limb and look for prominences. After getting oriented, look for other and more
finer detail or features.  For instance, spicules  can be seen at the limb or the
very edge of the Sun.  Spicules look like the jagged teeth of a hand saw. 
Sometimes these will bunch together and look like gently rolling hills or mounds.
 Then, at tines, I swear I've seen gaps between these "mounds" at the limb!

One might even see a surge, as I did, not long after having the filter!  Then,
there are caps, puffs, and various other types of prominences. And should you see
a gray streak crossing the "surface" of the Sun (in various directions) you will
be seeing a "filament", which is nothing more than a prominence seen from above. 
Flares are another possibility and can be seen in a sunspot as well as away from
it...if one is lucky! Figure 2 shows features which are visible with the Hydrogen
alpha filter.

After you have a pretty fair idea of what is going on, draw your observation, add
remarks, plus, other valuable information, such as date/time, weather conditions,
and seeing/transparency.  If I am working with the number 8 setting I will note
the time (to the second), add a slash, and then note the number setting.  Each
feature I draw will have this information noted close by.  This is very important
when working with the red and blue wings of the filter. On the filter I use, the
red wing is number 9 and the blue wing is number 7.  These two settings show less
of the chromosphere than number 8, which shows the upper chromosphere.

A fair amount of time has passed and we've gotten all the information we can at
this point.  Let's now use the barlow lens and get a good close-up view of the
Sun! In order to do this we need to turn the telescope away from the Sun, remove
the 2" ERF mask, and install the 4.25" mask.  Again, we tape the mask into place.
 At the business end of the telescope we attach both the barlow lens and the
hydrogen alpha filter that have been mated together. Hear that slight rattle
coming from the barlow lens?  Don't worry.  This means that the lens has enough
room to expand because of heat build-up. Normally a 20 mm to 40 mm Kellner
eyepiece is recommended.  But, since I have only Plossl eyepieces, I use the 17
mm, BUT accidently found that the 12.5 mm Celestron Micro-Guide eyepiece gave me
such a super view!  This eyepiece is of the Abbe Orthoscopic design.  I noticed
its super quality while measuring a solar feature one day and use it often when I
want a really good view, even though, the measurement scales get in the way at
times!

Again, sketch your observation, make notes, and include the number setting. When
the observing run is finished, remove the hydrogen alpha filter and place it into
its box.  I carefully wrap the cord so as to keep as much stress off the cord and
connections as possible.  Other attachments should be careful packed away.  Next,
gently remove the ERF and put it into its box or plastic bag.  Both the ERF and
the hydrogen alpha filter should be stored indoors and in a safe place. The
optics in a hydrogen alpha filter should ONLY be cleaned with a blower brush that
is available from a camera shop for about $4.95 (A/O l2/95).  And any cleaning
should be done VERY GENTLY. If the filter is kept in its box the unit should very
rarely need cleaning. Lastly, NEVER use lens cleaning fluid or any liquid to
clean the hydrogen alpha filter! The ERF can be cleaned with a half and half
mixture of distilled water and 70% Isopropyl alcohol.  Don recommends (at this
time) the Kleenex brand of tissue to gently wipe the surface of the ERF.  Also,
once in awhile, check to make sure the screws holding the red optical glass have
not worked themselves loose. With tender loving care the hydrogen alpha filter
will provide the observer with many hours, indeed YEARS, of observing pleasure!


SUMMARY
The face of the Sun is always changing and it is this that makes solar observing
so much fun and so fascinating!  The hydrogen alpha filter adds another dimension
to solar observing and with some of the basic tools in hand the observer will
come to appreciate what he or she is observing. Happy observing! My heartfelt
thanks to Don for his part in making an old dream a reality, for teaching me how
to use the hydrogen alpha filter, and for his help with this magnum opus!  He is
a good teacher notwithstanding!

Nancy L. Thomas
408 Vermont Avenue
Daytona Beach, FL   32118
(904) 252-8049
NLTsolar@aol.com