Observing the sun

The Sun

 

Astronomy is not simply the study of the night sky. There is a lot that can be seen in the daytime as well. The Sun is the power house of our solar system. This dynamic star changes every day and emits light in all wavelengths of the electro-magnetic spectrum. Because of this the Sun can be a great object to study from a hobbyist or scientific perspective. In this section we are going to cover the Sun and how to observe it safely. 

 

Understanding our Star

 

The Sun is a massive nuclear furnace that runs via a process called nuclear fusion (the process of fusing smaller atoms into large, more complex atoms). With this process the Sun consumes nearly 550 million tons of Hydrogen per second. Nuclear fusion in the core of the Sun produces enough energy to run everything on Earth for eternity!  The Sun is what is known as a main sequence star, not too big, not too small.  The Sun contains 99.8% of the mass of the solar system, nearly 70% being Hydrogen gas.  The Sun measures 864,948 miles in diameter at it’s equator.  Over 1.3 million Earths could fit within the volume of the Sun’s sphere.  

 

Being the correct distance away from the Sun is important, too close and we would be too hot.  Too far and our planet would be too cold.  Earth sits in a zone astronomers like to call the "Goldilocks Zone" which means its temperature is just right to support life, as we know it.  The Earth orbits at an average distance of 93 million miles from the Sun, this distance is also called an astronomical unit (distance from the Earth to the Sun).

 

 

QUESTIONS ABOUT THE SUN?

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Image of the Sun showing its varying layers. Image Credit: NASA

Charlie Bates Solar Astronomy Project

 

The Charlie Bates Solar Astronomy Project (CBSAP is a 501c3 non-profit organization) is the largest astronomy outreach program in the world. Based out of Atlanta, Georgia, the CBSAP was founded and is directed by Stephen W. Ramsden, a master of the Sun and all things solar. 

This program provides an advanced science based program to schools all over the world, reaching over 350,000 kids annualy with hands on solar observing and imaging, CBSAP affiliates can be found in all corners of the globe bringing like minded people together to share the wonders of our Sun.

 

Focus Astronomy is proud to be the official partner for the CBSAP here in Los Angeles. Our solar programs are based around the same concepts that are done in all other branches of CBSAP.

To learn more about CBSAP or how to get involved in this program please visit the official website at www.solarastronomy.org.

White Light Filters

 

The most basic form of filter is known as a white light filter. These filters allow us see the "surface" (photosphere) of the Sun and displays sunpots (cooler regions of the Sun). These filters reduce all colors of visible light equally and display a yellowish/white solar disk.

 

For those looking for a simple way to view a planetary transit (like the May 9th, 2016 Mercury Transit) or solar eclipses (like the August 21, 2017 total eclipse) solar glasses can be a fun, affordable and easy way to observe the Sun safely without the need of expensive optics. 

 

The easiest and most cost effective white light filter is black polymer or RG solar film.  This film comes in sheets that can be cut to size to fit the optic you are using.  These films are to be placed over the entire front of the optic being used.  Solar film can be mounted by making a mounting cell out of cardboard or other materials or even rubber banded over the front of your optic of choice.  

 

Pre made full aperture white light filters are also avaliable (as seen in the image to the left).  These filters are available in RG Film of glass.  RG film provides a more true color view of the Sun but can be easily damaged. Glass filters provide an orangish view of the Sun but are more durable.  These filters are mounted within a sturdy aluminum cell that can be sized to fit camera lenses, binoculars or telescopes. Filters such as these come in a variety of sizes to fit any aperture optic. 

 

NOTE TO PHOTOGRAPHERS:

Some larger telephoto lenses have filter drawers within them. This makes it easier when using filters instead of having to purchase a large front filter. These drawers DO NOT work for solar work. The light from the Sun will be partially focused before it gets to the filter drawer and that will damage the filter and lens.  To ensure safety please use a full aperture filter over the entire front element of your lens of choice.  

 

 

A more advanced White Light filter is known as a Hershel wedge.  A Hershel wedge should only be used with a refracting telescope. A Hershel wedge is a specialized prism that is uncoated. No filter is used on the front of the telescope. All the light is focused onto an uncoated prism. Nearly 99.9% of the focused light is passed through the prism into a vented heat sync which dissipates the heat. The remaining light is reflected up into a neutral density filter and then through a polarizer and safely into the eyepiece or camera. 

 

The image through a Hershel wedge provides the most accurate view of the color of the Sun’s photosphere (as seen in the image on the left) as no wavelengths are lost as is the case with many front mounted filters.  A Hershel wedge is a highly specialized instrument so please make sure you are well versed before adding one to your line up.  These types of filters are available from Lunt Solar Systems and Baader.  

Layers  of the Sun

 

As with most objects in space, the Sun is composed of many different layers. The Sun’s has seven major layers. The inner layers are the core, the radiative zone and the convection zone. These inner layers are not visible to use as they are below the point where the Sun’s gasses are transparent to visible light.  The outer layers of the Sun such as the photosphere, chromosphere, transition region and corona are visible using special equipment.  In amateur astronomy the photosphere, chromosphere and corona are the only visible layers. 

 

Photosphere

The photosphere is the deepest visible layer we can see in the visible light spectrum. This layer of the Sun is visible in White Light filters (we will cover solar equipment below) which display sunspots, faculae and granulation at times. The Photosphere is about 300 miles thick with a temperature of nearly 11,000 degree F at the bottom and 6700 degrees F at the top. 

 

Chromosphere

The chromosphere is the layer directly above the photosphere. Surprisingly this layer of the Sun, while further out, is nearly 14,000 degree F towards the upper edge. The chromosphere is most often observed by amateurs in H-alpha and Calcium-K/H wavelengths (we will get into these filters in the equipment section below).  The chromosphere displays sunspots (cooler regions of the Sun), filaments, prominences, active regions, spicules, solar flares and many unclassified magnetic phenomena.

 

Corona

The corona is the outermost layer of our Sun with a temperature around 900,000 degrees F!  The Corona is the Sun's outer atmosphere and can be difficult to see. The only time we can view this layer of the Sun from within the Earth’s atmosphere is during a total solar eclipse. Specialized scientific instruments called coronagraphs can also be used to observe this layer but are rarely available to the public.

Observing the Sun

 

Observation of the Sun can be an amazing activity especially when viewing eclipses or planetary transits.  With the correct equipment it’s easy to understand why so many people become interested in this aspect of astronomy.  Before we dig into observing the Sun and the equipment needed, there is one important thing we need to cover, safety.

 

PLEASE READ:

Observing the Sun can be very dangerous! Due to the amount of light and energy, very specialized equipment is needed. DO NOT look at with Sun with your naked eye or with optical aid without the use of certified filters. Welders glass does not count as a safe filter, it does not block out harmful UV rays that are emitted by the Sun. Ensure all safety precautions have been met before attempting to view the Sun.

 

Now that we have covered the safety aspect lets look into the equipment that makes observation of the Sun possible.  All the instruments listed in the section below have been proven to be safe. Focus does not recommend anything that would put one in harms way.   

Narrow Band Filters

 

Aside from the popular white light filters above another method of observing the Sun is by using narrowband solar filters.  Narrowband filters are, as the name suggests, capable of isolating very small wavelengths of the visible spectrum.  Isolating these wavelengths allows us to study different layers of the Sun in greater detail.  Narrowband filters bring a different dynamic to solar observing and can provide impressive and dynamic views as well as details about the Sun's chemistry.  Narrowband filters are designed to very tight tolerances thus making them much more expensive than full spectrum white light filters.  Below we will cover the varying types of narrow band solar filters and how to use them.  

 

 

Hydrogen-alpha (H-alpha) solar filter is a specifically designed filter that allows us to view the glowing Hydrogen gas found in the Sun's chromosphere (top layer). With an H-alpha filter we are able to view 

amazing solar events such as: 

 

  • Solar Prominences 

  • Solar Flares

  • Active Regions

  • Filaments

  • Surface Detail

  • Sun Spots

 

H-alpha filters are generally rated by their band pass (how narrow a band of light they allow to pass through). This band pass is usually measured in a unit called an Angstrom (one ten-billionth of a meter or 0.1 nanometer).  The narrower the band pass, the more detail that can be see on the Sun's disk.  The wider the band pass the better we can observe edge details like prominences or coronal loops.

 

In today's market, H-alpha filters generally range from 1 angstrom down to an amazing 0.2 angstrom. The narrower the band pass the more expensive the device due to the tolerances needed to obtain such a narrow filter. 

 

There are two major types of H-alpha filters: solid etalons and air spaced etalons. An etalon is the heart of an H-alpha system, these filters allow light the bounce between to highly polished glass plates creting an interference pattern which allows the specific wavelength to pass through along with several other peaks on each side of centerline. The 2nd required stage of filtration in an etalon based system is called a blocking filter and it eliminates the other peaks on each side of the desired wavelength. In order for an etalon to work correctly light must enter the filter at right angles to the glass surfaces so additional collimation is required before and sometimes after most filters.  These systems cannot be used without the blocking filter. Blocking filters are sold based on the size of the glass opening. Different sizes are of the same quality but have larger or smaller fields of view. 

 

Solid Etalons

Solid etalons produced by companies like Daystar and Solar Spectrum offer a wide range of band passes.  These filters are generally mounted to the focuser of a telescope and have to be used on long focal length (f/30) instruments in order to obtain a parallel beam of light.  To achieve this, larger instruments must be stepped down in aperture or a barlow lens is needed to increase the focal ratio.  These systems also require an energy rejection filter in front of the objective to prevent focused infrared radiation from striking the filter surface directly.

In order to tune a solid etalon to centerline, the temperature must be carefully controlled.  These filters require built in heaters and external power sources to stay on band.  Operating at such high focal ratios normally makes it impossible for a solid etalon system to display a full disk view of the Sun.  These solid etalons are excellent choices for extreme close-ups and provide very even illumination.  

Daystar recently introduced the “Quark”, a solid etalon, all in one “solar eyepiece” H-Alpha and Calcium H filter that can be used with smaller refracting telescopes to provide a full disk or close-up view.
  

Air Spaced Etalons

 

Air spaced etalons work just like they sound; two solid glass surfaces are separated by an air gap.  Instead of using heat to tune these filters tilting, physical pressure or air pressure it used to tune them onto the correct band.  Air spaced etalons are more difficult to produce due to small spacers that are needed to produce the small air gap.  Air spaced etalons can be found from companies like Coronado, Lunt and SolarScope.  

Unlike solid etalons an air spaced etalon can be mounted to the front of a telescope using machined adapter plates.  Air spaced etalons work very well without any external heat and at the native focal ratios of most refractors.  Most air spaced etalon systems are around 0.7 angstrom band pass. This is good for observing both prominences and some surface detail but not nearly as good as narrower filters.  

Combining two or more etalons in the same system is called double or triple “stacking. Each additional etalon will reduce the bandpass of the filter at the expense of a 40% image brightness reduction per additional etalon. A Lunt Solar Systems single etalon filter normally has around a .7 Angstrom bandpass while a double stacked Lunt scope will normally be around .55 Angstroms. Triple stacking further reduces the bandpass but in almost all cases, the image is too dim to visually observe at a comfortable level for most people and the reflections created by all of the additional glass is usually problematic.

Dedicated Solar Telescopes

 

Now that we have covered H-alpha filters, lets discuss dedicated solar

telescopes.  

Most H-alpha filters systems we have mentioned need to be used in tandem with another telescope.  However another option is a telescope that has the filters built into it.  A dedicated solar telescope can be an elegant answer to those looking to observe the Sun.  Most solar telescopes today are based around an H-alpha filter.  

A dedicated solar telescopes is an entire H-alpha filtration system contained within the telescope. A package such as this can make a great addition for educational organizations who need a simple set up with little to no removable parts.  Dedicated solar telescopes are available from nearly all major solar manufacturers such as Coronado, Daystar, Lunt and SolarScope.  

Sometimes a solar scope can provide a larger aperture for less money when compared to a filter set.  This is mainly due to the size of the etalon used.  Most solar scopes can use a smaller etalon inside the optical tube rather than a full aperture etalon.  Solar telescopes can range in sizes from the smaller and affordable (under $1000) Coronado PST and Lunt LS50THa to the high level research class such as the Lunt LS152Tha ($6000) and LS230THa ($25,000). 

Calcium-K & H Filters

 

Another type of narrow band filter is Calcium-K (CaK) and H (CaH) line filters.  Calcium filters such as this display a lower level of the chromosphere below what is visible in an H-alpha filter.  Calcium filters display the Sun in a deep violet color in the near ultra-violet portion of the spectrum.  Calcium filters are excellent for observing:

 

  • Sun Spots

  • Plages

  • Active regions

 

 

Being on the very edge of the normal visible light sensitivity of most people (390nm-700nmn), it is difficult for most people to see CaK or CaH.   Those with younger eyes are normally able to see the dark violet disk much easier than older eyes. This is due to our eyes yellowing as we age.  Because of this, CaK filters are generally recommended for imaging purposes.  CaK filters are avaliable through Daystar (which requires power) and Lunt Solar Systems (which does not require power).  

Coronado has manufactured dedicated CaK solar telescopes in the past but are no longer marketing them. Lunt CaK modules are available starting at around $600. 

 

Another line of Calcium is the Calcium-H line (CaH).  CaH filters see 397nm light from the solar spectrum. This wavelength is slightly more visible to the human eye and is equally good for imaging purposes compared to CaK.  Daystar is currently making a CaH “Quark” which can be used, with external power, on most small refractors.

 

Over the years Focus has used CaK filters in our solar programs.   As of February 2016 Focus has replaced our CaK filter with the Daystar CaH Quark which has expanded the ability for people to view the Calcium line of the Sun. For those teaching about the Sun we highly recommended looking into the Daystar CaH Quark to expand your outreach efforts across the spectrum.  

 

 

 

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