MP #204: Getting started in astrophotography
What photographer has never dreamed of imagining the starry sky?Astrophotography is a discipline that requires a lot of personal investment, but which is incredibly rich.
Personally, I thought I knew a lot of things in optics and in photo in general, but when I seriously put myself in Astrophoto, I discovered that I did not know much!Oh, Socrates, that you were right!
Immerse yourself in the splendors of the night and the deep sky will make you discover that the universe is not black, that it conceals thousands of wonders and will make you end dozens of misconceptions.
© Guillaume Seigneuret
It is also a noble practice that "increases your senses"!Indeed, while in the diurnal photo, the device captures part of what your eye perceives, in Astrophoto, it is able to put in image what your eye will never be able to perceive.Astrophoto is also a way to recharge your batteries while enjoying the calm of the night!
After an introductory article to allow you to photograph the Milky Way and the Stars, we will see in this Practical Wednesday how to prepare well to do astrophotography, including photo equipment (device, lens, frame and tripod), the essential concepts to know for the shooting and the compulsory passage by the box of the processing of the photo in order to obtain a finalized astro photo.
Material to start well in astrophotography
You can get striking results with the equipment you already have in your photo bags!Forget the received ideas that try to say that you need a telescope to take superb photos of the objects from the deep sky.
What can we hope to get with classic daytime equipment?
Most of the time we tend to think that galaxies or nebulous are very small and that it takes a very long focal length to see them.It is not so.The problem is that these objects are very weakly bright and that the human eye is not sensitive enough to see them.We think then that if we don't see them it is because they are small.
The best example is Andromeda's galaxy.It has an apparent size in the sky of 7 times the diameter of the moon!It is only 60x less bright!It will then suffice for a 50mm lens to see it.
This example is not isolated, in the boreal sky (our sky of the northern hemisphere), there are a good thirty objects all more impressive than the other who can perfectly have the portrait unless100mm.
Trifide and nebula nebula of the lagoon, multiple heaps and Milky Way - EOS 5D MK III @70mm f/4, 60 minutes of pose.- © Guillaume Seigneuret
NOTOTOTébuleuse du coeur et de l’âme EOS 50D défiltré @200mm f/4, 60 minutes de pose- © Guillaume Seigneuret
NOTOTOTorth America and Pelican EOS 50D defiltrated @70mm f/4, 60 minutes of installation.- © Guillaume Seigneuret
Camera choice for astrophotography
You can do astrophotography with any diurnal device as soon as it has the ability to make a manual focus.If he can make exhibitions of more than 30 seconds, it's even better, but to start we rarely make longer poses, because they are more difficult for a lot of reasons that we will mention in the rest of the article.
What is very reassuring is that in Astrophoto, high -end devices often do not get better than entry/mid -range SLRs.Or if this is the case, they are very little advantaged.
The annoying point is the IR filter (infrared) of these boxes.Indeed, by day it is important to have an IR filter in front of the sensor to prevent it from saturation because of the immense quantity of these photons.The problem is that they are not very precise and eat a small part of the visible red very important in astro: the H alpha, a dark red massively emitted by magnificent nebulae.
This filter absorbs between 70 and 90% of HA depending on the model and it will therefore be necessary to pose between 3 and 5x longer to bring out the nebulae.It is nevertheless possible to modify most SLRs either yourself if you are a handyman, or by a professional who will replace the filter with a more precise who still blocks the IRs but does not block the dark reds visible.The cost of such an operation is between 250 and 500 € depending on the model of your camera and if you want to have a replacement filter affixed.For my part, I transformed my old eos 50D by removing the filter and he found a second youth.
Les appareils à gros photosites
Many have started to do astrophoto with Canon EOS 350D because there are only 8 MPs on the surface of this APS-C sensor.It is therefore quite sensitive.If you already have it or have a very tight budget, it is always a good candidate to start.Of course, the sensors have improved over the generations and have become more and more sensitive.
Les bons appareils petit budget :
Les très bons appareils numériques pour l’astro :
NOTOTOT.B pour les full frame : à moins que vous ne souhaitiez faire de la photo diurne en plus ou que vous possédiez déjà ces APNOTOTOT, n’achetez pas ces appareils pour l’astrophotographie uniquement.They are very expensive compared to dedicated astrophotography cameras and are much worse.Examples if you want to go further: zwo as 1600 or qhy 163m.
Choice of objectives
What objective to use for astrophotography?The rule is simple: it takes the most open objective as possible, which will significantly reduce the total exposure duration.In general, to take a nice astro photo in a large field, you need at least 30 minutes of exposure with a goal open to f/2.8.For the same result at f/4 will take 1 hour, 2h to f/5.6, 4h to f/8.
So unless you are very patient and love the long nighttime evenings, plan a minimum f/4 lens.
Astrophoto highlights all the faults of your goals.If you believe that your goal is very stung because by day it produces superb images, you could be disappointed with your first astro photo.Indeed, the stars formative of very bright punctual points bring out the various aberrations (coma, astigmatism, chromatic aberrations, vignetting).Depending on the final quality you are waiting for, it will therefore probably be necessary to close your goal from one or two diaphragms to draw the best quality.
A compromise will therefore be made according to the exposure time you have in front of you.If you know that you only have 30 minutes in front of you and you have a goal with a maximum opening at F/2.8, too bad for aberrations, the photo will be prettier with a few aberrations than with an irrecoverable noise which makes the photo completely unusable, especially that it will often be possible to reframe to remove the edges which are often the most problematic areas.
Start with great angle lenses (between 17 and 35mm).They forgive tracking errors and encompass a staggering amount of medium -sized objects that are already noticed very well!
Choice of a frame
The frame with follow -up will be the compulsory choice if you want to get high -quality photos and you will superimpose your images.There are a plethora of mounts, I will therefore go here to the essentials by offering the best quality/price ratios to start.
The question of the type of frame does not arise in photos.You need an equatorial frame, either an equatorial board (rare, often amateur construction), or a so -called German frame.
The equatorial mount has an axis of rotation parallel to the axis of terrestrial rotation.Unlike a standard ball joint, the equatorial ball joint will follow the star during its journey, allowing to obtain clear images, which compensate for the rotation of the celestial vault linked to the rotation of the earth.
If you want to be nomadic, excellent small German frames exist at very honest prices, and will allow you to obtain very convincing results up to 300mm focal lengths.Their other advantage is their extreme portability which will allow you to take them on a trip with you without worrying too much about the overweight of your luggage.
German frame Skywatcher Star Adventureut and Vixen Polarie
Vixen Polariemport: 3kgpoids: 740gprix: € 375
Skywatcher Star Adventurer (German frame) Takes: 5kgpoids: 1.2 kgprix: € 349
In addition to the focal limitation, you will have a price weight limitation, the weight you can put on it.The Vixen Polarie is limited to 3kg and the star adventure at 5kg.In this weight you have to count everything, the little ball joint that you put on top, the remote control, any heating resistances to fight against humidity.The weight rises very quickly.
It will take you a few nights of acclimatization to master them, because to obtain good results with it, you will have to know how to make a precise polar alignment.The longer the focal length, the greater the density of pixels, the longer the duration of exposure, the more precise the alignment must be.To start, make your weapons with focal lengths of less than 50mm, the inaccuracies will be erased by the resolution of the image.
Attention !For these frames you will need to have a good quality photo tripod with a high loading capacity (8 to 10 kg) and if possible absorbing vibrations.Carbon or wooden tripods are excellent for that, see the next part.
NOTOTOT.B: In astronomy we express the resolution (or sampling) in second of arc by pixel.
The mounts presented above follow the sky with an approximate error of + or - 20 ″ arc over a period of ten minutes.
An EOS 450D mounted with a 50mm lens will have a resolution of about 22 ″ Arc/Pixel so you can exhibit as much time as you wish without risking appearing elongated stars and blurred objects.
An EOS 450D mounted with a 135mm lens will have a resolution of approximately 7 ″ Arc/Pixel so you can make poses of about 3 minutes.
An EOS 450D mounted with a 300mm lens will have a resolution of approximately 3.5 ″ arc/pixel and there you will be limited to one minute poses or less, with probably some waste here and there.
Choice of a tripod for travel frame
Here are three tripods that I recommend to support your Astro equipment:
If you want to invest in larger caliber frames directly, which have better quality follow -up and which allow more significant weight, here are some essential models that have their own tripods:
Astrophotography returns very glass dear as soon as you want to get excellent results (but you are used to it since the same goes for daytime photography).Skywatcher frames are Chinese design and manufacturing frames that compete in their quality/price ratio.It takes about 100 € per kilo against 350 € per kilo for Italian / American / Austrian / Japanese frames, etc.. NOTOTOTéanmoins, ces montures de qualité d’usinage sont correctes sans plus et il faudra avoir recours à du bricolage pour en obtenir les meilleures performances.
The quality is unequal from one frame to another and you can hear saying on the part of a person that his mount has been impeccable since unpacking like another person say that his mount is good for anything and that hewill have to dismantle it to adjust it, to change a gear or other so that it works properly.
So I voluntarily included in the list an Italian frame (the Avalon) which is of impeccable manufacturing quality and which offers perfect results without having to ask questions.Obviously, you will have to pay almost 2000 € more to have this serenity.
NOTOTOT.B: If you want to acquire a second -hand frame, you will be more serene if you opt for a Japanese takahashi type frame, Italian type Avalon, American frame type 10micron or gemini, Austrian frame type asa.If you want to take a second -hand Chinese frame (Skywatcher, Ioptron), ask for a control bulletin, recent autoguidation curves or better a trial of the frame.Indeed, these frames are quite badly aging.
NOTOTOTotions essentielles pour bien réussir ses clichés astro
Your digital camera allows you to capture many things that are out of reach of your eye. NOTOTOTéanmoins, pour obtenir des résultats sans défauts, il va falloir un peu faire connaissance avec votre capteur.
If the notions of reading noise (reading noise) and thermal noise (dark noise) are unknown to you, forget everything you think about the ISO settings. NOTOTOTous allons découvrir dans cette section comment tirer le meilleur de votre appareil photo la nuit.
Understand the behavior of the sensor
A sensor is like a bucket of water which fills up a rainy day, except that instead of collecting water it collects light.Like the bucket, it has a capacity and a size.
The bigger the photosites of your sensor, the more sensitive it will be.In other words, according to the surface (µ 4/3, APS-C, APS-H, Full Frame), the more megapixel, the less sensitive it is.This is the equivalent of the diameter of the bucket.
The greater the dynamics, the more its ability to collect light is important.This is the equivalent of the volume of the bucket.(If we want to continue, the optical objective will be considered a funnel above the bucket, the more it is open and the more the funnel will be large diameter).
That's all.The ISO setting does not allow more light to be collected, it is just a parameter that will change the way you empty your bucket once you think is satisfied with the amount of water collected.
During emptying, you go:
At the end you can see a return.Namely a quantity really collected on quantity received. En photo on appelle cela le rapport signal/bruit (SNOTOTOTR – Signal to NOTOTOToise Ratio).If by day of tall rain (day photo), you collect a large amount of water and few impurities, it is easy to have a good yield.
At night, it's like trying to fill the bucket with the morning dew.It's long, it's difficult and the amount of impurity in the face of the amount of water collected will be much more important.The goal is therefore to get the best signal/noise ratio.
Always take your RAW photos, the JPEG or any other compression format deletes a large amount of useful signal that does not necessarily appear on the device of the device.
Reading noise (or Read Noise) is characterized by sensor reading errors when the shutter closes.It is equivalent to the ability to pour the contents of the bucket precisely in bottles without putting next to them.
Each sensor has its own behavior, and moves very little during the life of the device.
This error depends on the ISO setting.As digital reflex manufacturers design their devices for day photos, they don't care about reading noise errors, since it is very negligible in a scene in full light.Thus at low iso the reading noise will be very important.He will quickly drop by going up the ISO then stagnate.
To find out more about your sensor, go to the Photons to Photos website (in English).
So we would be tempted to use very high iso.But now, by increasing the ISO, we decrease the dynamics and sensitivity to the colors of the sensor.
This is the most important parameter, it will be necessary as much that can be positioned at the ISO setting generating the least reading noise but by maintaining good dynamics and good color sensitivity.
A bias type photo whose curves have been stretched (highlighting the sensor reading noise)
Your sensor is an electronic component powered by electricity.Like (almost) any electric component, it heats up and disrupts itself.This gives rise to thermal noise (or noise of dark).
The parameters playing on thermal noise are the duration of exposure and the ISO.The higher the ISO, the higher thermal noise.The longer the duration of exposure, the higher the thermal noise.
Unlike the reading noise which remains the same photo after photo, the thermal noise is randomly moving on the photo and is dependent on the outside temperature.So, if we take enough photos and overlap them, the thermal noise will disappear.This is the key concept!Take lots of photos to make the thermal noise disappear and garner a maximum of real signal!
Brute (1 photo of 60 ″)
Integration of 3 hours of poses (180 × 60 ″)
You will tell me: but then how many photos should I take?
And there I answer you as usual ... it depends.
It will depend on:
At least it will take 30 minutes of installation.There is no maximum even if after 8 to 3 p.m. your device will no longer be able to make much more information.
For example :
It's good, I have 100 photos!And now what am I doing these 100 photos!?
Photo processing in astrophotography
Photo processing is such a large subject that it would take several articles to cover the subject.So I will try to guide you in a concise way.
In astrophotography, as explained above, we favor multiple shooting for a very long pose.So you have to stack the photos to get a final image.You can use different software, so here is the most used.
To process photos, some software has been developed with special processing for astrophotography.
Bias, Dark and Flat to improve the final rendering in astrophotography
Bias, Darks and Flats are never absolutely necessary but can significantly increase the final rendering of your photo.
In order to eliminate part of the Read Noise, it is possible to take a series of photos with only the reading noise.We call it a master bias or a master offset.
Pour l’obtenir, il faut réaliser une suite d’une centaine de clichés, dans le noir (avec le capuchon de l’objectif ou de l’APNOTOTOT) avec les paramètres ISO utilisés lors de la prise de vue et avec une obturation à la vitesse maximale de l’appareil.
Then you have to stack them a bit as we stack the photos of your shooting.
Il n’est pas nécessaire de les refaire à chaque fois, le bruit de lecture étant constant tout au long de la vie de l’APNOTOTOT, et n’étant dépendant que du réglage ISO.
In order to reduce thermal noise (or to remove some imperfections specific to the sensor), it is possible to make a Master Dark.
To obtain it, you must take a series of around thirty shots (in the dark, ditto as for the bias) with the same ISO parameters, the same duration as during the shooting, and especially the same temperature.
The realization of the Darks is very restrictive but can save an under-exposed shot.
For purists, and in order to eliminate the vignetting effects, dust on the lens and/or the sensor, it is possible to make a Master Flat.
To obtain it, a few shots must be made with a homogeneous lamp in front of the goal used during shooting, with the same opening parameters.
Personally I never do them, because it requires significant logistics, and if you are careful with its equipment (so you eliminate the dust aspect), the vignetting is easily correct in post-treatment.
For the stack of dark, bias, flat, you can use Deep Sky Stacker or Pixinsight.
In one sentence: "Be persistent!"If in diurnal photo you can take" luck "photos, in astrophoto, any successful photo is the fruit of a hard work.But what a reward!
As Corneille said, "to defeat without danger, we triumph without glory.»»