Visual observation of the Sun

Observing visually our most important star is pretty simple, as long as you apply some basic, while important safety rules and precautions :

Never (NEVER) look a the sun without a proper external Sun filter of density 5 or 4 minimum (Welder glass is not safe enough and old camera black films not at all). This means (see below),  an external filter either made of glass or home made of Mylar and properly fixed to the outside of the camera or telescope.

In no case use the popular "sun Filter" of 24,5mm or 31,75mm diameter built to be screwed inside of the eyepieces !! NEVER..As they are placed at the focal point of the main objective, the heat concentration may break them at any time, burning in seconds the eye placed behind the eyepiece.

Projection vs filtering

The best and safest way to observe the Sun is still to project its image onto a screen placed behind the instrument. The article below from BBC is self explanatory.

The fun with observing the sun is to see every day large or small spots moving across the globe (The Sun surface rotates in 25 to 35 days), and also to measure and calculate the solar activity and observe it grow or decline month after month and year after year. 

This means every day to count and estimate the Wolf number (from the name of the Swiss astronomer who designed the method in 1848 in Zurich). This number enabled to discover the 11 year solar activity cycles. We are currently in the growing phase of cycle 25.

To estimate Wolf number at a given moment, just count the number of individual Sun Spots (s) and sun spot groups (g)

W = k(10g+s) where k is a homogeneity ratio or observer parameter linked to the observation conditions, instrument. You can assume k=1 as a start. Keep records and see the evolution over time.

June 2004 Venus is crossing the Sun

For this event, a Celestron 5 Schmidt Cassegrain was used with a Nikon film Camera F90X. The telescope was driven by a battery powered synchronous motor on one axe and a manual aiming and guiding was necessary.

Used a glass filter from thousand oaks in the USA. I own since 2 full aperture filters, one for visual observations (density 5) and one for photography (density 4).

This Celestron mount technology is obsolete and just a few pictures of the event were clear. 

Outside the C5 telescope itself, which is a wonderful small telescope (I still have it and use it since 1990), no technology from that time is anymore available today (But I also kept the equatorial mount and wedge, renovated it and it is available for a future Telescopes museum).

For solar eclipses or other Sun related phenomenae, a solid long focal Maksutov-Cassegrain or refractor is the best choice. This is also true for regular daily sun observations- We have more than enough light for this kind of star, for once- so a F/D ratio of 10 or more is desirable. 

I describe below how I covered several Sun related phenomena, with a special focus on total eclipses, with description of the equipment and results achieved.

Historically I used to bring along for far away eclipse travels different types of instruments, mainly dedicated to photography, but which can be used also for wild life and other nature portraits in some cases. 

In air travel, weight and size are key elements, as limitations of the airlines policies and the fragile nature of the cargo dictate the constraints. I will describe in the following chapters the evolution of my small eclipse telescope set up.

October 2005 Annular Solar Eclipse Tunisia

In October 2005, I was traveling to Tunisia (Tataouine, deep south in the desert) for an annular eclipse with a regular Celestron 5 telescope (Schmidt-Cassegain with 125mm lens diameter at f10). The advantages of the C5 are obvious (it can be also used on planets and other deep sky objects at night), even if it is not really the best choice for solar photography, so later on I decided to change this equipment for these kinds of activity.

Its focal of 1250mm however, makes it just suitable for use with DX camera bodies (as a rule of thumb, the size of the moon or sun image is 1cm per meter of focal length, so the diameter of the sun at 1250mm fl is roughly 12,5mm, a bit large for a DX sensor (15.8 x 23.6 mm) for practical reasons.

A full frame camera will give a bit more space around the image, as during the shooting period of 2 hours, the telescope will have to constantly and consistently follow the sun so the image will slightly move even with the best mount set up (especially when the alignment is made during the day as it not possible to see any star to confirm the position).

We will come back on the subject of guiding and aligning, but the key element here is the use of a proper Solar filter. It must be a full diameter outside filter in front of the telescope objective (AND NEVER A SCREWED-IN FILTER INSIDE THE EYEPIECE BARREL), preferably in glass and density of 5 for visual use (4 for photography).

The Celestron 5 optical tube was set onto a Meade LX55 equatorial mount for tracking the sun. Just simple battery powered tracking and remote control. It happen that the LX55 can deliver some goto capability, but this mount had a very brief life time ( I am still trying to repair it, but desperatly need a new autostar controler almost impossible to find at a decent price since Meade Telescopes went bankrupt a few years back and belongs now to Orion telescopes)

Although this set up could deliver reasonnable results, the size of the mount and optics made it a little complicated to carry, plus the inherent tendency of Schmidt Cassegrain telescopes to move out of optical alignment after travel and vibrations often forced me to re align the optics on arrival. For those interested by this set up, these are the key elements below. This being said, the C5 and especially the 1990 model above is an extraodinary telescope (It was used onboard Apollo during the Moon missions for a reason). Excellent optic, versatility and portability unmatched for generic astronomical and terrestrial use. A 100% commendable telescope under any of its formats since more than 30 years (classic to computerized models).

July 2009 Tibet total eclipse

For the 2009 Solar eclipse, we carried up the Tibet Mountains during two weeks after landing in ChengDu an other setup :

A Celestron Nexstar 4 telescope on a computerized mount. Not really a light setup (The optical tube alone weights around 2.4Kg or the same as the previous C5 and in total the instrument gets close to 10kg), but more compact, solid and optical alignment secured by the Maksutov-Cassegrain design of the little tube of 102mm diameter for 1325mm of focal length  or f ratio of 13. 

At this Focal length, it becomes difficult to use a DX camera body, unless you add on the back of the telescope a focal reducer (0.63X) to bring down the focal length to 835mm and F ratio of 8.2..This is the perfect image size for the Sun, Moon or eclipse work. Of course, this requires an adaptor as pictured below. Unfortunately, the weather changed brutally at the last moment when the sun was getting covered.

Mars 2016 Total Eclipse Indonesia West Sulawesi

An other set up again was used for the 2016 eclipse in Indonesia : Using a Star Adventurer equatorial mount and a Tamron 150-600mm Teleobjective lens on a very standard tripod. Using also a 180mm very old Nikon lens for larger pics and telephone for landscapes.

Here, the solar filter was home made, using some flexible mylar sheets sandwiched between cardboard frames. It is a pretty useful and safe way to adapt filters on any kind of optical device (inc binoculars). I provide you with links to the equipment elements also.

The sky was clear and the sea beautiful at 6am, and this was a wonderful moment in a very remote and hard to reach destination.

One major attraction there is the incredible Poso lake, deep and clear like crystal.

August 2017 Total Eclipse Nashville USA

Same equipment almost was used in August 2017 in the USA for the total eclipse..But clouds were invited there too

On top of the Tamron telelens, also used a Williams Optics Zenithstar 66mm F 359mm refractor. Using commercial filters this time. This model is discontinued but the SVBony below is equivalent for the purpose of Sun and eclipse pictures (These little refractors are marvelous instruments especially for deep sky imaging, but this is an other subject for an other blog page). Camero bodies were Nikon D750 and D3300 (because the D3300 has a wonderful remote control connector identical to the D750, and this seems to disappear these days of "all wifi" connections)

July 2019 Total Eclipse La Higuera Chile

For this event, we carried around 100kg over the planet, and 2 instruments :

1- The legacy C5, with a Nikon D750 body, but this time connected into a new automatic mount called Sky-Watcher SolarQuest. There is also version of this mount sold by Orion telescopes called Orion Sun Starseeker. 

The beauty of this mount, totally dedicated to solar tracking, is the fully automatic (South and North Hemispheres) pointing and tracking of the sun, using an integrated GPS system and a small camera. The device is powered by 6 AA batteries.

No need to align and follow the sun, just turn on the mount and wait 30 secondes, the telescope is aiming at the sun and keeps following it. Seems this mount is not available anymore for sale at Amazon or at the manufacturer site.

2- The StarAdventurer mount, loyal since 2016, fitted with the Williams Optics 66/359mm refractor and a Nikon D3300 body

This one setup was very useful also for deep sky pictures while visiting the Elqui valley after the eclipse event.

Being at the bottom of the Solar cycle 24, the corona was not very active, and the eclipsed sun not that much "hairy" 

However, next edition in 2024 should show a much bigger corona and more glorious chromosphere..we shall see !!

I discovered lately the optical quality of Meade ETX line of Maksutov-Cassegrain and purchased a used ETX 90, diameter 90mm and fl 1250 (f/D 13.8). Images with this scope are really sharp, more than needed for sun or even moon eclipses.

This is the lightest possible for eclipse trips as it weights only 1.4kg, and fit perfectly on the StarAdventurer mount, using the proper adapter. You can find used Optical Tube assembly (OTA) for around $150 on ebay. 

There is also a 105mm and 125mm models, weighting respectively 2.6kg and 3.3kg so little heavier than the C4.

For Sun and eclipse work 105mm can be useful, but i find the 90mm extremely portable and light, and can adapt also (using the right adapter to SCT above) a SCT meade focal reducer to make it 790mm focal length at F/D 8.75, perfect for a DX camera body.

25 October 2022 partial eclipse UAE

The latest one, with C4 on its Nexstar mount, still active and working after 14 years of good services