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MAS Frequently Asked Questions

MAS FAQ

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 ?

(-) 1030 How can I take photos of the stars ?

Using your camera and telescope to photograph the stars is not a task for the faint hearted. Whilst it's (relatively) easy to simply hold your camera / smart phone / tablet / iPad lens up to a telescope eyepiece and take a photo of the moon, capturing images of the stars is orders of magnitude more difficult.

Indeed, there are many members of MAS who are still struggling to obtain reasonable (or even any) results after years of trial and error (many trials, lots of errors) !

You should also be warned that it can become quite expensive - especially if you become obsessed with IR filter modified DSLR's or 'specialist' RGB filter wheels and dedicated astrophotography cameras

However, if you are determined and persistent, you can join those who 'post' to the MAS Photo Gallery and bath in the glory of the respect this will gain you in the eyes of the rest of the society !

The basics

Generally it's best to 'get started' using your DSLR - which is ideal for 'Deep Sky' objects (star fields - eg Pleiades, nebula - eg Orion, galaxies - eg Andromeda etc.).

For planets, however, you need high magnifications which means small fields of view (FOV). Further, atmospheric distortion will 'smear' surface details - so a 'movie camera' (web cam) will generally be your camera of choice - it has a small sensor (which matches the small FOV) and by taking multiple 'frames' you can 'throw away' the ones too distorted to be of any use and 'stack up' the rest to produce a really clear image.

You may have a 'movie mode' on your DSLR camera, which may even be 'full HD' (1920x1080 = '2 mega pixels') and higher resolution than the typical astro 'web cam' (which can be as low as 0.5 mega pixels). However, HD is always 'compressed' (using a 'jpg' type method known as h.264) which makes it most unsuitable for high contrast stars against a black background, which will be 'smeared out' (clever cameras may even 'remove' the stars as 'defects' !)

Note that the Moon is a bit of a 'special case'. The 'problem' is that while you want a 'movie' (to allow 'smeared' frames to be dropped) the moon is generally much 'too large' to 'fit' onto the small web-cam sensor (although a focal reduction lens can help). So, unless you are prepared to 'mosaic' your web-cam 'movie', you will end up taking multiple short exposures using your DSLR (and when we say 'multiple', we mean hundreds). Some areas of most (if not all) of your photos will be effected by atmospheric distortion - which means they won't 'line up' during simple stacking. Fortunately modern stacking software (such as RegiStax = see MAS Meetings) can be told to 'find' multiple datum points and 'adjust' for distortion

Getting started - using your camera on it's own

Taking photos of the night sky with your camera on it's own will familiarise you with how to adjust the settings to get good results.

Any DSLR camera on a steady tripod that can be set to 'manual' (or 'bulb') mode can take a photo of the night sky. With a maximum (note 1) exposure time of of 10-30 seconds (to minimise 'Star trailing') and an ISO setting of 400 or better can record the brighter stars including planets, Meteors, Iridium flares**, ISS transits***, and Aurora. The longer the exposure, the more light you will gather and thus the 'brighter' the sky in the final image. To avoid the night sky looking like it's been taken in daylight, you will need to use a lower ISO. For really long exposures, an ND Filter will be needed (a '10 stop' filter will allow exposure times of 2 hours).

(note 1) The 'exposure' time setting can usually (only) be found in the 'menu' of most digital cameras when it is in manual (M) mode.

Many cameras are limited to a maximum of 30 seconds. Some have a 'noise compensation' mode - this 'takes' a second exposure of equal time to the first, but with the sensor covered, in order to reduce thermal induced noise. Some modern camera's can be set to take multiple exposures and 'add them together' (stack them) to achieve the same result ** this link is for predictions of Iridium flares visible from Maidenhead. *** this link is for ISS transits visible from Windsor (Maidenhead is not offered as an option on the site) Once you are experienced in taking 'star field' shots, try taking photos of the moon (a zoom lens helps, of course :-) ) Advice: For ISO, choose one back from the maximum the camera offers - this will minimise 'noise'. Use a cable (Bulb) release with the lens set at maximum aperture (eg F2.8) and use manual focus. Digital or film can be used, and with very wide angle lenses, exposures between 30s and 1 min on a tripod can be used in dark sky areas. Here are two examples [ Mercury ] [ Comet Pan Stars ] of photo's taken by MAS members
Using a telescope

The simplest will be a small refractor - indeed many stunning photos of nebula etc. are taken with small refractor 'piggy backing' on the main 'scope (which is used to 'guide' the mount)

There are many ways to mount a camera onto a telescope, however the 'simplest' is to use an adapter which will allow your DSLR to be attached to the telescope in place of it's normal eyepiece. Such adapters have no glass parts and simply allow the camera sensor to be positioned at the primary focus point of the telescope - which can be next to impossible for a Newtonian (the primary mirror focus point may be too far 'in' for the focusing mechanism to reach) - these are known as 'prime focus' adapters.

The more advanced adapter (for a 2" focus tube) has space inside for a 'normal' 1.25" eyepiece - which may 'solve' the focusing problem however it also means that the FOV is reduced (so larger 'targets' may have to be photographed in 'segments' and build up as a 'mosaic') - these are known as 'eyepiece projection' adapters.

Advice: Regrettably, many telescope shops charge 'an arm and a leg' for these simple pieces of metal, so, unless cost is of no concern, you will have to visit eBay and wait whilst it's shipped from China.

Note that, in both cases a Focal Reducer lens can help bring the camera to focus

Adaptors come in two parts - one is the 'bayonet ring' with one side specific to your camera lens mount ring (eg Canon) and the other side a 'T2' screw thread, which then screws into the second part, a 'standard' prime-focus / eyepiece projection tube assembly.

Note that, in the authors experience, by far the most difficult task will be achieving FOCUS. This is helped (to some extent) by cameras that have a 'live view' capability, and especially those that have a live view 'zoom' capability . Note that, whilst you can use the moon to achieve good focus (before going on to take 30s exposures of the stars), the 'best' time for Deep Sky Object photography is when the Moon has set (or not yet risen) since it's brightness will 'swamp' the faint light from the stars.

It is also possible to purchase 'clamps' of various designs that allow cameras without a removable lens (and smart phones etc.) to be held 'next to' (above) the eyepiece and take photos (a process known as 'digiscoping', which is very popular with bird watchers, as you will see from this How-To guide).

The main advantage of these is that, since the camera still has it's lens attached, it can be left to 'auto-focus'. Except, of course, the stars do not provide enough light for camera auto-focus systems to work. Some camera's won't even focus on the Moon ! A DSLR lens will usually have a Manual switch. Cameras with built in lens might have a Manual Focus setting in the menu, many don't.

Dedicated astrophotography cameras

A number of vendors offer what are essentially 'modified web cams', usually with cooling fans (and 'Peltier' devices) to keep the sensor chip as cold as possible (the hotter the sensor, the more 'thermal noise' you find in your photos). These are all offered for the usual astronomical prices. If you want the 'best possible' results, however, you will have to go this route.

Pixels counts are typically very low - SVGA (800x600) being quite a 'high spec' device - however they will deliver 'raw' (typically 10 or 12 bit) uncompressed pixel data direct to you PC via a USB cable. The 'higher end' ones are 'B&W' for use with external filters - a 'filter wheel' allowing you to take 4 shots = 'LRGB' (needed to assemble a colour image) is another add-on component that will cost the usual astronomical price :-)

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MAS Frequently Asked Questions