MAS Frequently Asked Questions
Frequently Asked Questions about astronomy (see also MAS Beginners page)
This page answers many commonly asked questions about astronomy. If you have a question on a subject any not covered here, please help support this page by using our on-line enquiry form (requires Java Script to be enabled).
Much of the material here has been taken from topics presented at out monthly meetings. If any member has subject they would like to present for 15-20 miniutes as a 'second half' topic, please contact the Meetings Secretary (to contribute directly to this page, please contact the Webmaster
(+) 0001 How do I find a local Astronomical Society ?
(+) 0002 How is Star brightness measured ?
(+) 0003 What are the Constellations ?
(+) 0004 What is the shape of the Earth ?
(+) 0005 What equipment do I need to start astronomy ?
(+) 0007 What telescope should I buy ?
(+) 0008 How much Magnification can I get ?
(+) 0013 What are Equatorial and Alt Az mounts ?
(+) 0100 How do I use my first telescope ?
(+) 0109 What is a finder ?
(+) 0110 What is an eyepiece ?
(+) 0111 What is a Barlow ?
(+) 0112 What is a focal reducer ?
(+) 0114 What are Nebular filters ?
(+) 0116 How do I use Setting Circles ?
(+) 0120 What is collimation ?
(-) 0125 How can I safely observe the Sun ?
Generally, the only really safe way to observe the Sun with a normal telescope is by using the 'projection' approach = and that should really be restricted to non-mirror telescopes (i.e. refractors) ..
The problem with reflectors is that the mirror coatings do not reflect IR (heat) very well (unlike the typical lens which is very efficient at focusing IR - indeed, glass is so good at passing on IR that digital camera sensors have to be fitted with IR blocking filters !). A good fraction of the IR energy is absorbed and the mirror heats up and expands, which can result in it cracking when it reaches its mountings limit.
The mirror surfaces that are subjected to 'concentrated' IR in the focus path (i.e. the diagonal of a Dobsonian / standard Newtonian - or the secondary mirror on a folded mirror design (like the Schmidt–Cassegrain / Maksutov-Cassegrain) are even more 'at risk' as local intense heating can damage the coating or lead to the surface distorting or cracking.
For sun-projection, to limit the energy going into the optics, larger telescopes should be 'stopped down' (a 3" opening is more than enough to pass sufficient sunlight for a 6" projected image)
Note that, with projection viewing, your eyepieces are also at some risk (as they must pass the full concentrated sunlight). You need to keep them perfectly clean as any grease or dust can be enough to 'catch' the full energy of the sun and overheat the lens (leading to cracking, distortion or even 'etching' of the lens surface !)
White light observing (including the corona)
Direct 'white light' solar observing with a reflector (mirror design) telescope is possible, however it requires a blocking filter (ND 5) be fitted over the front of the telescope. Due to the wide range of telescope objective sizes, solarfilter 'foil' is typically sold in sheets for DIY solarfilter construction at approx £20 for an A4 sheet of standard 'Baader' film (which is availbel in two types, one for visual observers, the other for photographic users - the photographic version passes more light). Glass (and pre-made foil based) filters are also available, however these are typically 5 to 10 times the cost of the foil (and are aimed at the more 'up market' or 'branded' makes of telescope)
Needless to say, any damage to the filter puts your eyes at risk - and using a solarfilter during a public viewing session requires extreme measures be taken to prevent the filter being dislodged !
For refractor (lens design) telescopes, an alternative to the front mounted 'tin foil' filter is a special 'sun diagonal' Solar Wedge or Herschel wedge, which can be used with standard eyepieces. The 'wedge' reflects only about 5% of the visible sunlight upward through a ND filter (which can sometimes be replaced), whilst passing almost 100% of the IR straight through(1).
They are safer than the removable front filters, however the 'blind end' of the diagonal has to absorb the IR and can get quite hot. Whilst Sun diagonals are more expensive than filter foil, they are not much more than the cost of a pre-made filter (at approx £125 for a 1.25") and 1/5th of the cost of the most basic 'real' (H-alpha) sun viewing telescope.
(1) You can't use a Herschel wedge with a reflector (mirror telescope) because the 'IR pass-through' depends on the telescope objective lens focussing IR at a different point to visible light - a mirror focusses all incoming sunlight at the same point so the IR will be also reflected and damage your eyes (or the ND filter)
Some designs (such as the Astro Engineering AC635 combine a Herschel Wedge for direct viewing with a 90 degree mirror for sun-projection (a rather more sensible thing to do with the excess sunlight than to heat up the back of the wedge housing :=) )
What's needed for Solar H-alpha observing ???
'H-alpha' means the frequency of Hydrogen alpha light being emitted from glowing hydrogen gas at the surface of the Sun. To see it, you need to block out all the other light frequencies being emitted by the Sun.
This means both a special filter at the front of the instrument (an "etalon") PLUS a secondary 'blocking filter' mounted just in front of the eyepiece (this is usually positioned just inside the 90 degree diagonal of most 'SolarScopes'). The blocking filter cuts about 99.9% of the light wavelengths leaving a band 0.7 angstroms wide at the H-alpha frequency (some solar scopes allow a limited degree of 'tuning'). 'Double stack' blocking filters takes the 'passband' down to 0.5 angstrom.
Blocking filters are very expensive to make, so normally it is placed as close as possible to the focus point, allowing the size to be kept down to 5mm (known as a 'BF5'). However, when adding an adapter to use a camera the blocking filter usually has to be moved away from the focal plane to make room for the adapter - in which case the filter has to accommodate a larger part of the light 'cone' and 5mm is typically too small. This results in light loss from the 'edges' and there will be noticeable vignetting. The only solution is a larger (i.e. more expensive) blocking filter, BF10 (10mm) or BF15 (15mm) being typical options.
Eds note. SolarScopes are stil specialist items that allow manufacturers to make huge margins, as well as allowing them to offer an 'entry level' version deliberatly designed for visual use only (thus 'differentiating' their even more premium priced products). Hopefully a UK company will come up with a more flexible design (without stupid camera adapter restrictions), however I fear that Patent or other restrictions will mean this is yet another product that will end up with the Chinese.
If your main interest is visual observation with a single eyepiece then a 5mm blocking filter may be sufficient. If you want un-vignetted photos then choose a 10mm or 15mm. If you want to use a 'bino-viewer' then a 15mm is advisable.
The Coronado 'PST' and Lunt 35mm H-alpho (OTA only) start at about £700. The Coronado SolarMax 60 H-alpha range costs twice this, and the SolarView 50/60 Hydrogen-Alpha about double again.
Whilst it is possible to buy a front filter "etalon" + 90 diag. with blocking filter to 'convert' a 'normal' telescope, such kits are typically more than twice the price of a basic 'dedicated' solarscope (such as the PST) !
Advice: The Sun is so bright that you will need to cover most of the objective end of your telescope to avoid setting fire to things at the eyepiece end ! Dobsonian users should also note that as the Sun moves across the sky, unless they are very careful to keep adjusting the Dob. tube direction, the 'light cone' will drift away from the secondary mirror. This means that the sunlight will come to a focus more or less at the top of your tube - as you will discover when your objective cover starts to smoke !
This note last modified: 5th Feb 2015 22:05.
(+) 1030 How can I take photos of the stars ?
(+) 1033 How to take photos of the Aurora ? - (Northern Lights)
(+) 1036 How do I calculate FOV for Eyepiece projection ?
(+) 1038 What is Star trailing ?
(+) 1039 How can I take photos of Meteors ?
(+) 2100 What is Universal Time (UTC) ?
(+) 2114 What are AUs Parsecs and Light Years ?
(+) 2115 What is Bodes Law ?
(+) 3010 When was Neptune discovered ?
(+) 4000 How do I update Stellarium with new Comet data ?
(+) 5000 How To build the MAS (Raspberry Pi) photoframe ?