Monday, July 22

Set your filtered sights on the Sun – Astronomy Now

Sunspots were widely spread on the Sun on 1 August 2023. Image: Sheri Lynn Karl.

The Sun is our very own special star, which we rely on totally for our continued existence. The long summer days, hopefully blessed with the year’s best weather conditions, when the Sun rides high in the sky, are ideal for becoming better acquainted with our life-giver. When thinking of astronomy, the Sun isn’t the first thing that springs to mind but you would be missing out on glimpsing an extremely dynamic world. How is it possible to see anything when the Sun is so intensively hot and bright and what will I be able to see anyway? 

A suitably filtered small telescope can reveal a wealth of detail at various wavelengths, including spectacularly in hydrogen-alpha (at 656.3nm), but observing sunspots crossing the solar surface, or photosphere, in white light is perhaps the easiest and cheapest method to see and image the Sun in action.

Seeing sunspots

Sunspots are temporary regions of intense magnetic disturbance where the energy from below is blocked by the magnetic force. This causes the area to cool down slightly compared to the surrounding surface creating the sunspots. If you look carefully, you will see there are two parts to a sunspot, a nearly black area called the ‘umbra’ (there may be several of these if the sunspot is extensive) with a much lighter region called the “penumbra”.  Sunspots can be found on their own but quite often they come in groups which are then grouped into sunspot types depending on their extent and structure.

Sunspots occur over an average 11-year Sunspot Cycle and vary in number, size and structure of sunspots during that time.  At Sunspot Minimum there are usually few if any sunspots for many days or occasionally for several weeks at a time. Gradually the number of sunspots begins to rise and their sizes increase. Some sunspots are occasionally big enough to be glimpsed with the eyes alone by looking through an eclipse viewer. After about three or four years we usually reach Sunspot Maximum when the Sun seems to never be without some sunspots. This situation persists for about two or three years and then the intensity of sunspots starts to fall away over the remaining years until we are once again back at Sunspot Minimum. 

There are a good number of dedicated amateur observers report observations of the total number of sunspots and groups each month. By having observations of the Sun on as many days as possible each month means a graph can be created showing the rise and fall of sunspot activity. We can see from recent sunspot that the Sunspot Maxima over the past have not all been the same – Cycle 24, which started in 2008, was of lower intensity than the two before, though Cycle 25, which, according to recent forecasts, has already hit maximum or will do anytime now .

It is also really useful to find out the position of the sunspots on the solar disk which can be done using a solar latitude and longitude system, as we can plot these against time to produce a rather beautiful graph called a “Butterfly Diagram”, so-called because the plotted positions looks like the wings of a butterfly.

Projecting the Sun  

So how can I see these sunspots? One of the easiest methods is to project the solar image using a small telescope. To do this first insert a fairly cheap eyepiece into the scope and cap the finder if you have one (avoid using your best eyepieces as they can be damaged by the Sun’s heat). 

Do not look through any part of the telescope but aim the telescope sunwards by looking at the shadow of the telescope tube on the ground. When the shadow is at its smallest, a fuzzy bright spot should appear within the shadow. Hold a piece of white paper or card about 20 centimetres behind the eyepiece to catch the bright spot. Now gently focus the telescope until the projected image is sharp. To make the image bigger, move the paper or card away from the eyepiece. You should be able to see at least one of two sunspots on the bright face of the Sun. 

A more permanent solution is to use an adjustable screen or box attached to the telescope to catch the image. If your telescope is made of plastic, or uses a mirror, don’t try projection as the Sun’s heat can damage them.

An eyepiece projection set-up. Any small telescope can be kitted out with its own solar projection screen. This method of observing the Sun, which reveals sunspots and limb darkening, is particularly useful for group observations such as public outreach sessions. Image: Steve Ringwood.

Full aperture filters; glass and solar film

The alternative to projection is the solar filter that fits securely over the front of the telescope. These are often called ‘full-aperture solar filters’. They block all the energy from the Sun except for a tiny proportion of the light. Such filters must meet stringent safety tests to ensure they are safe to use provided we follow the maker’s instructions. 

There is a choice of either a film-like coated solar filters or glass-coated filters. Whichever you choose, they often come fitted in a frame that needs to match the aperture of your telescope, so you need to pick the right one for your scope. Film-like coated filters while cheaper give excellent views of the Sun. Because they are easier to damage you must check them thoroughly before use. The glass-coated filters cost more, give great views of sunspots and are a bit more robust but you still need to check them before every use. 

A Celestron C5 telescope fitted with a Thousand Oaks full-aperture glass solar filter. Image: Steve Ringwood.
EMC S100 is a polymer solar filter, which employs Baader Solar Film. Image: Steve Ringwood.
A group of sunspots on 23 April 2022. Image: Rob Parsons.

Herschel Wedge

The best view of sunspots can be achieved with a Herschel Wedge (a device named after Sir John Herschel, who invented it). These can only be used at the eyepiece end of a refractor (or lens) telescope. It uses an un-coated prism to reflect just under five percent of the Sun’s energy towards the eyepiece, the remainder including infra-red and ultra-violet, are refracted away from the observer. The Sun is still unsafe to look at so a neutral density filter and polarising filter are also required to make it safe to view. 

The Herschel Wedge costs more than a full-aperture filter and cannot be used on telescopes over 150mm aperture as they can overheat. The manufacturer’s instructions should also be followed carefully.

Baader’s modern version of the Herschel Wedge, featuring a ceramic cooler. Image: Steve Ringwood.
Lunt’s version of the Herschel Wedge, with energy vent.

 

Large sunspot AR 3366 on the solar limb on 8 July 2023. Image: Sheri Lynn Karl.

Safety first always

Never make your own solar filter from items found around the home.  This is because while it is easy to find something either dense or reflective enough to dim the Sun, neither will block infra-red or ultra-violet causing permanent damage to the eyes or even blindness. Never use solar filters that screw in to the telescope eyepiece as they are now known to be unsafe. 

Usually in astronomy we like telescope aperture. Light from most night-time stellar objects is quite low so the bigger the aperture the better as it collects more light helping us to see more. For solar astronomy, a small refractor telescope of between 60-100mm aperture is best preferably on an equatorial mounting to allow you to track the Sun. Try to pick an eyepiece that lets you to see all of the solar disk at once. An eyepiece magnification of about 60x will achieve that and if the air is steady enough use a slightly higher magnification for close-up views.   

The Sun, our nearest star, is a highly dynamic object forever-changing. Both amateur and professional astronomers have found that it is full of surprises and only gives up its secrets slowly.  It’s well worth a look.

source: astronomynow.com