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Random tip: Red lights are night vision friendly = white lights are not !

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MAS Frequently Asked Questions


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 ?

(-) 1033 How to take photos of the Aurora ? - (Northern Lights)


Mode = Manual (disable all 'Auto' functions - there is insufficient light for 'Auto-focus', 'Auto-exposure' or 'Image Stabilisation' to work and leaving them enabled will just chew up your battery power as the camera 'hunts' forever) Cameras without a Manual focus mode are going to be 'difficult' (i.e. almost impossible) to use. DSLR's with a separate lens are ideal as the lens focus can be set to Manual and 'locked' (if necessary by using gaffer tape :-) ) Lens = wide angle with a low f/ number (the lower the better, you need the maximum light) ISO = highest you can stand (i.e. with acceptable 'noise'). Modern cameras will typically support a max. of 12800 ISO (especially in the cold at "-X degrees"), however DSLR's (especially Canon's) are usually much more 'conservative' (i.e. offer lower max. ISO). Tripod = a must, because ... Expose time = 1/2s to 15s 1) For very bright coronal aurora or on a full moon night, 0.5 seconds at f2.8, ISO 12800 2) 'On average', try 5 seconds at f2.8, ISO 12800 (and adjust as necessary) 3) For faint aurora on a moonless night, 15 seconds at f2.8, ISO 12800 Aurora intensity can vary dramatically, from barely visible (which will still show up in photos at 15s) to so bright it can illuminate the ground ! NOTE that at 30s even with a wide angle lens you might start to see 'star trailing'. Photo: ../Beginners_and_FAQ/photos/Img_7653-stars-thumb.jpg
In the photo, right, taken by Tim from his back yard 25/25 Oct 2016 after an AuroraWatch UK 'red warning' at 8s F/3.2 ISO 800 (which is about 6s f2.8, ISO 800 or about 0.3s f/2.8, ISO 12800) you can just start to make out some of the brighter stars (click on the image to see a larger size) = sorry, there was no Aurora to be seen :-)


It's a 'good idea' to get some practice shooting the (very) new Moon in the UK before making a (very) expensive trip = the last thing you want to do is spend a week on the Arctic circle at -40 degrees C learning how impossible it is to focus with your specific camera (or how it refuses to take a decent shot ....). You will also discover your cameras limitations (like max. exposure time) and the need for any 'apps' (if you expect to use the camera with a smart phone / tablet). By all means take a red-light torch = whilst it won't stop you ruining others photos at least it won't ruin their (or your) night vision. For the same reason sticking some red cellophane over your LCD (or smart phone screen, if used to control your camera) is also a 'good idea'. Make sure you have more than one extra battery = at "-X degrees" you will be lucky if your battery lasts half the normal time ! You will need a 'mains' battery charger and at least one extra USB power block. Modern camera's will charge via their USB socket but this can take hours and with limited opportunity you may only be able to recharge once a day = there's no point in having half-a-dozen batteries if you can't re-charge more than one at once (see Detail at end) :-). If you use your smart-phone / tablet to control your camera, you will also need to keep that well charged up, so I pack a 6 way UK extension block with a 'foreign adapter' as few Hotel rooms have more that 1 'free' socket. Your Tour Guide will instruct you on the best place to set up your tripod (avoid being placed behind some-one else = when they preview their results on their LCD - or start waving around a torch - it will ruin your own photo) NB. Remember to shoot in RAW + Jpeg at the highest quality possible. Focus will be your main problem. Even with a full Moon, few camera's will manage to auto-focus and with no Moon (the ideal time for Aurora watching) for sure they won't (unless you get very lucky and there are distant town lights you can aim at) So Manual focus it will have to be. For DSLR cameras, you can focus on some distant object ('at infinity') during daylight and then 'lock' the lens (if necessary, using gaffer tape :-) ). For cameras with built-in lens (and no focus ring), if there is a Moon, you can usually (manually) focus on that and then 'lock' the focus electronically (often by holding the shutter button half way down - but read your camera's manual to discover how to do this well in advance). On moonless nights you will have to focus on the lights of a distant town etc. Note, once you have focus, it's a good idea to reduce the brightness of your LCD display (or expect lots of complaints as you review your efforts and ruin other peoples long exposures :-) ) Lens set to wide angle (24mm or less) and 'wide open' (lowest f/ possible). You can correct for any distortion later, it's more important to get maximum light. Remember, a difference in the lens aperture of '1 stop' (one step in the sequence f/1.4, f/2, f/2.8, f/4, f/5.6 ...) halves (or doubles) the exposure time. So (for example) 15s f/2.8 (ISO 12800) would only be 7.5s at f/2.0 (ISO 12800) or about 4s at f/1.4. The same applies to the ISO (800, 1600, 3200, 6400, 12800 ..), so 15s f/2.8 ISO 12800 is only 7.5s at f/2.8 ISO 25600. White balance (if you have the option) is best set to 3700K - 4100K in Moonlight, else 4000K - 5500K (when there is no moon). ISO I suggest you start at the manufacturers 'max' and reduce it if the noise proves unacceptable. By all means experiment with the 'extended' setting = whilst this would normally result in totally unacceptable noise, it may be much better when the camera is nice and cold at "-X degrees C". If you double the ISO, you halve the exposure time. So 5s at f2.8, ISO 12800 becomes 2.5s at f2.8, ISO 25600. However (generally) the lower the ISO the better the noise, even at the expense of a longer exposure time. So going from 5s at f2.8, ISO 12800 to 10s at f2.8, ISO 6400 or even 20s at f2.8, ISO 3200 may well give a better photo - but 20s will show star trails on a 50mm lens, although you should 'get away with it' on a 24mm lens. If your camera has a "Long Exposure Noise Reduction" mode (dark frame subtraction) Enable it (or set it to Auto etc). This works by taking a 'dark frame' immediately after the main exposure. Since the dark frame has to be of the same timing, this means 'saving' the photo takes as long as the main exposure (so don't be surprised when a 5s exposure takes 10s to complete :-) ) Exposure must be set to Manual and 'as short as needed'. You need to keep down the exposure times as the longer the exposure, the more 'noise' (and the more likely some-one will fire off a flash or wave a torch and ruin it). More to the point, Aurora are not 'static' and the longer the exposure the more 'diffuse' or 'smeared out' your Aurora photo will be. In general you will have to 'try it and see'. If your camera supports a 'histogram', use this along with the image review 'zebra' or 'blinking' over-exposure (highlight indicator) to make sure you do not 'clip' the aurora. Aim at a histogram with no shadow clipping (it may be 'left biased', but the histogram should not be touching the left edge of the graph). The idea is to aim at an exposure with clean shadows and less noise (so you don't have to lighten the image in post processing). Use the highlight indicator to make sure that you are not overexposing the aurora itself (or any highlights created by light pollution). If your camera can't show a histogram, on an 'average' night you could try 5s (f/2.8 ISO 12800) and increase the time until it's obviously over-exposed on the LCD and then drop back 1 step (plus take a few shots either side of this setting - what looks OK on the LCD in the dark of the night may not be so good when you get it back to the laptop in your room). Wobble Even with the best tripod poking the shutter button with gloved stone cold fingers is going to wobble the camera. So it's a very good idea to trigger your camera with a remote / cable or use the 'self timer' (delay). DSLR users should check their manual for how to use 'mirror lock up' delay. Noise reduction To reduce the noise, you can drop the ISO, however many modern camera's limit the maximium exposure time to 30s or less, even on the Manual mode (so for faint aurora, where noise is going to be most obvious, 15s f/2.8 ISO 12800 you may only be able to 'drop' 1 ISO step to 30s f/2.8 ISO 6400) Most older DSLR's have a 'B' ('Bulb') mode that allows shutter control via an external cable connected controller. Modern cameras (if they have a B setting at all) typically offer this only via some 'smart phone app'. If you are at the back of the group, by all means dig out your phone and give it a go, but be prepared for complaints (waving a lit up phone around when others are trying to take 2 minute (f/2.8 ISO 1600) or 8 minute (f/2.8 ISO 400) exposures is not going to make you very popular :-) ) Remember, above about 30s on a 50mm lens you will see star trailing, although for some people this can actually 'enhance' the photo :-) Chances are your Tour Guide will be able to advise the 'best' settings just by looking at the Aurora = and if 'all else fails', look at what others are achieving and ask them what settings they are using ! Batteries When I'm on a photo-shoot, I use 4 batteries. One is left in the room charging whilst I'm out with the other 3. On return, the now fully charged battery goes into the camera bag. If I used all 3 batteries, one goes into the charger and a second is charged in the camera, whist the 3rd will be left behind when I next go out, although a modern 'energy saving' Hotel room (with a 'key slot' which will power-off the room whilst you are out) may make this difficult (using the mini-bar socket usually works, as does jamming a couple of business cards in the 'key slot'). If you are going to be away from mains power for more than a day, a Power Bank can help by recharging your camera etc. but they can take 10 hours to recharge (although as few Power-Banks are specified for use below 0 degrees, leaving it in the room later to recharge whilst you are out on the 'shoot' is not a big drawback). Tips from Tim When I took Aurora images in Norway, I didn't take RAW, just jpeg, and then at reduced resolution. This was so that I could take many images in succession (6 per minute) and create a movie. I didn't loose detail in a "medium jpg".  Aurora are diffuse by nature. Ed. RAW lets you 'save' an under-exposed photo, so I recommend you use it when possible, however you may well be limited by storage space. A 16Mpixel RAW+Jpeg will be about 24Mb in size, so at 10s a photo (6 per min) a 32Gb card will fill up (with 1,364 photos) in just under 4 Hrs. On a one week tour you will need up to a dozen cards (or a laptop with lots of spare space). Focusing is a trick, I agree.  I focused on 'infinity' (e.g a distant tree) in daylight on 'auto' and then switched to manual (and got out the insulation tape). Even so, it's a good idea to check with live view (if you can) just in case something has slipped. I can recommend using at least two batteries, preferably new-ish ones of high quality - often the cheaper 'after market' copies don't support the full temp. range and they may just 'die' when they get cold ! When very cold (sub zero), I kept one battery in my pocket with a chemical hand warmer like "HotHands".  They really do provide heat up to 10 hrs, from 0 to 6 hrs the temp is about 40-50C so enough to keep a battery warm.

More information

Organised tours to see the Northern Lights (and photograph the Aurora) can be found on the Internet (just use Google) - indeed we even had a talk by one of the operators (see our 7 Oct 2016 meeting notes) For some more photos, visit this photo.net post
This note last modified: 26th Oct 2016 13:49.


(+) 1035 How to calculate FOV for prime focus ?

(+) 1036 How do I calculate FOV for Eyepiece projection ?

(+) 1037 How do I use a Raspberry Pi camera for astrophotography ?

(+) 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 ?