Thursday, January 13, 2011

Astrophotography, Part 1; Supplemental Post (A) - Photography with Imre - Episode 32

The world of astrophotography is not only an enormous topic, but a fascinating one too. Episode 32 of my photography series gave a very brief overview, but in this supplemental post I endeavor to provide much more detail. In future episodes relating to astrophotography, I'll be examining specific topics in more detail such as photographing auroras, planets and processing such images.

Shooting the Celestial Wonders Without a Telescope
I have a feeling that if you asked some people to describe astrophotography they would likely include the word "telescope" in their sentence. Yet, a telescope is by no means required to take some amazing photos of the night sky or even in daytime for solar photographers... here comes the warning.

Warning: Seriously said, viewing the Sun, whether with the naked eye, through a camera, telescope or some other means, can be very dangerous and can result in serious eye damage or even permanent blindness. Simply said, take extreme care when photographing the sun and ensure you've done your research and know what you're doing, otherwise don't do it. In almost all cases, special filters should be utilized on the camera lens or telescope (or other device) and ensure that you do not use filters that are attached to eyepieces. I don't believe these have been produced for many years due to safety issues, but they tended to crack as focused sunlight heated them up, which as far as I know led to some people getting eye injuries and even being blinded. Proper filters are generally attached securely to the end of the lens or telescope where the light enters.

For the rest of this post I'll be sticking to astrophotography during the night.

So like I said in the video, you can simply take your camera, attach a nice lens and start shooting. Well ok, it might not be that easy, but here are some things to keep in mind. First of all and as you have likely already discovered, shooting things in almost total darkness will not only require lengthy exposures, but something to keep your camera stable. I have to say, I'd be pretty impressed with anyone who could hold perfectly still while taking a fifteen minute long exposure to get some star trails. Thus, if you're keeping things fairly simple and do not require tracking (which I'll talk more about in the second part of this post), then whip out your tripod. Oh, and you'll also likely be shooting in manual mode, along with manually focusing, but I talk about that further down.

Next up is sensitivity, but what setting you should use may not be that straightforward. Many people would think that if conditions lack light then high sensitivity levels should be employed like 800, 1600 or even 3200 ISO. This of course means that the picture will be noisier than at lower lowers such as 100 or 200. Ok, fine, but how does this translate into what is needed for, let's say, some nice wide field shots? Well here's the trade off you'll need to consider:
  1. Using a low sensitivity setting will obviously reduce the noise that manifests on an image, but this of course also means that the sensor will not be as sensitive to light. So in a given exposure time, the sensor may not pick up some fainter stars/objects as it would at a higher setting.
  2. Using a high sensitivity setting will result in more noise being present, but in this case for a given exposure time the sensor will pick up fainter stars/objects than if at a lower setting.
So at this point you're probably getting the technical idea, but you might still be wondering which setting is better for certain types of shots like doing star trails, of the moon, or wide field shots with no star trails. Well, I think these three common scenarios deserve an explanation.
  • Star trails: In general, I would suggest trying lower sensitivity settings at first, even down to 100 ISO. In the end, the result will be a cleaner image and with a fast lens wide open you should still discover that a very large number of stars show up on the photo; in many cases more than you can see with the naked eye. In addition, if you are taking star trail type photos within a large town or city, light pollution will usually wash out the star trails much faster at high sensitivity levels. If possible, escape the confines of the city and perform such shooting in the peace, quiet and darkness of the countryside.
  • No star trails (e.g. constellations): Now we have another situation, and remember, in this case we only have a tripod; no tracking mount. In photos like this, we want to get nice little bright dots as seen in Figure 12, a photo I took of Orion (details under image). If the camera sensitivity is lower, then longer exposures will be needed to pick up all those lovely details and even the different colors stars give off. But as the earth rotates, the stars glide slowly across the sky as well as the frame, hence star trails. So we need to use a shorter exposure (due to lack of tracking mount) to limit the star trail effect. Therefore, in these types of shots increasing the sensitivity will help, even if it does introduce some noise into the equation; using a fast lens is also helpful to limit light loss.

    Speaking of lenses, there are two other very important factors to consider: focal length and where the lens is pointed to in the sky (thanks to a viewer for asking this excellent question which you can see in the video's comments). If you are shooting toward or at the celestial poles, then the length of exposure you can use will increase as stars don't move as quickly. However, if you're shooting toward or at the celestial equator, where stars are zipping across the sky more rapidly, then exposure times must be decreased because star trails will form faster. By the way, if you're not familiar with the idea of the celestial sphere, check out the Web resources section below. Moving on, this factor interacts with the focal length of the lens you're using. For example, with a fisheye lens (e.g. a diagonal type where one would get 180 degree field-of-view from corner to corner) you would get a very large swath of the sky and stars appear as very tiny dots. Because everything appears smaller, movement is not magnified as much, so you can get away with longer exposures than compared to using a telephoto lens with a greater focal length like 300mm or 600mm. But keep in mind, even pointing a wide angle lens toward the celestial equator will mean you'll have to reduce the exposure time if you don't want to see trails

    The viewer also asked how long the exposure times can be before trails appear. Unfortunately, I don't have any specific information in regard to this, but after a little poking around the Web I did find this post in a forum: http://www.clubsnap.com/forums/showthread.php?t=219390 The information seems valid enough, so this may act as a good starting point.
  • The moon: The moon is a fascinating subject, but in most wide angle lenses it looks like a bright white dot with little or no detail. But with a 150mm lens or larger you can take some very nice shots with detail showing on the lunar surface. Since the moon is fairly bright (although the brightness does change depending on the phase it's in), a moderate sensitivity setting is usually sufficient like around 200-400 ISO. You may have to play with the shutter speed to get just the right look, otherwise the moon may be blown out or underexposed, but don't be surprised if you're getting speeds around 1/250 of a second in some cases. Lastly, keep in mind that like stars, the moon too appears to travel across the sky due to the earth's rotation (in fact, the moon "moves" at a slightly different speed). By sticking to faster shutter speeds, you'll get a sharper image of our natural satellite.
Before moving on I just want to add that the above are just recommendations and with some practice and experimentation you may find that various other settings will provide you with the outcome you want, so as they say, have at it! In addition, for some shorter exposures a few of you may notice that the shot could be a tad sharper. Well that slight amount of blurriness could be caused by the mirror in the camera slapping up, thus creating vibrations that don't dissipate before the shutter opens. To help overcome this, and assuming you have the feature on your camera, use the mirror lock up function. This will slap the mirror up and out of the way when you are ready to take the picture, but the machine will let a few seconds pass (usually user customizable) before opening the shutter. Come to think of it, if you don't have a shutter release cable and must depress the shutter button by hand (how I do it), this can also allow for enough time to pass for the tripod to stop wobbling. For interest's sake, I have my camera set to two seconds.

Next up, I'd like to discuss the aperture of the lens and here are some of my thoughts about it. If my goal is to capture very faint stars/objects, then I generally use the widest aperture on my lens, which of course allows the most light through. On the other hand, although not usually very noticeable for some lenses, vignetting may occur (especially present when the sky isn't completely black) and often lenses are a bit sharper when closed down a little. So, if you aren't happy with how crisp the shot is, then try a smaller aperture. The evident downside to this is that less light will come through, so depending on the scene you're shooting, you may need to up the sensitivity or increase the exposure time if possible. As for the moon, you may find that stopping-down is fine, because as mentioned, the moon can be quite brilliant.

White balance should also be taken into account and I had a question about this on my earlier blog post. Most notably, you may have seen some wide-field photographs where the sky appears an odd orange tone instead of black. This color cast is caused by sodium type street lights and is prevalent in large towns and cities. But there are a couple of ways around this. First off is to simply travel to a location that is far from the light pollution; this generally means around a 30 minute to an hour drive away from the city. Another benefit of doing so, amongst others (like peace and quiet), is that you'll be able to capture and see much fainter stars and celestial objects than in the city. However, if travel is not possible, not all is lost. In my case, I almost always shoot RAW+JPEG and for such space based photos, I tend to exclusively use the RAW file. Although I could set the color temperature to something like tungsten, which will help cool off the image, for me, having the ability to set the temperature to wherever I want is a plus.

If you've already been involved with some astronomy, you've likely discovered how seeing conditions change due to many factors such as upper level winds, air temperature, dust/pollution levels in the atmosphere and moisture content in the air. Because these factors change from day to day, or even hour to hour, on some days the seeing conditions might be great and not as much of an orange cast shows itself on the image, while on other days the image looks less than desirable. So, this is basically why I like to take control. My advice to you here in regard to setting white balance, especially since one single color temperature generally won't work for all cases, is to experiment a little which will be a great way to see the results.

Great, we're getting there. You have your tripod up, camera securely attached, lens on and you've made up your mind about basic camera settings. Although a little tricky to see details in the camera's viewfinder, you manage to pick out a part of the sky to shoot and then press down on the shutter button only to discover that your camera doesn't want to take the shot. Why? Because it couldn't focus (ok, some cameras will take the shot depending on how they're set up and depending on the object you're shooting, but more often than not, focusing under such conditions can be an issue). Here are a couple of tips that should help. If the moon happens to be up in the night sky then use that to focus on. Even though it's much closer to us than even the nearest of stars, earth's satellite is at such a distance that this works well. There is a little downside though. Because of the moon's brightness, you may temporarily lose some of your night vision; just close your eyes for about 20-30 seconds after you're done focusing and your vision should recover.

Another thing you can do is to set your camera to focus on the center dot, aim at a really bright star, press the shutter button half-way, and once the camera is done focusing immediately switch to manual focus and do not touch the focusing ring. Most dSLRs are good enough to "see" that bright dot and lock onto it. Lastly, you may simply have to aim at a bright star like previously mentioned, but use manual focus and a few test shots to see if you got the focus right.

Holy smokes! I was going to write about using telescopes, but seeing how large this post is (and that I've been writing this for many, many hours over two days or so) I'll write a second part geared to that a little later (plus I'll have more resources to add). So happy shooting, be mindful of cold weather shooting if in such a climate and cya L8r!

Web Resources
http://en.wikipedia.org/wiki/Celestial_sphere
http://en.wikipedia.org/wiki/Celestial_equator




Figure 1 - An old photo of my 10" Newtonian telescope on its Dobsonian (an alt-azimuth) mouth. A 28mm eyepiece is attached and you can also see the blue finderscope on top.

Figure 2 - Close-up view of the eyepiece holder and finderscope.

Figure 3 - A 1.25" 10mm Plossl eyepiece. Decent quality for viewing purposes.

Figure 4 - A 2" 28mm Plossl eyepiece. Excellent and bright wide-field viewing with this optic.

Figure 5 - This is my pride and joy, a high-end 1.25" 7mm eyepiece; almost 90 degree field of view, razor sharp and huge eye-relief which works well for photography and those who wear glasses. You can't quite tell from the photo, but this thing is huge; almost hard to wrap your hand around.

Figure 6 - Unlike cheap telescopes, higher quality ones require you to place an eyepiece adapter in them depending on the size of eyepiece you want to use. On the left is a 1.25" adapter and to its right is a 2" one.

Figure 7 - Here's a 1.25" variable Barlow lens. You put this into the telescope first, then the eyepiece goes into this unit, and the image is magnified by the value its set to. I've rarely used this one at 3X as the image becomes quite dark and somewhat soft.

Figure 8 - And here's my other Barlow lens, this one being a 2" model which magnifies the image 1.6X.

Figure 9 - Back almost a decade ago, I got this t-mount adapter so I could place my father's Contax camera (35mm film type) onto the telescope. This one didn't see much action and these days I use the Four Thirds one for my Olympus digital SLRs.

Figure 10 - This is an interesting 1.25" camera adapter (notice the threaded portion where the t-mount would screw onto) as you can either use it as an empty tube (prime focus) or place an eyepiece inside of it (afocal).

Figure 11 - This is also a camera adapter, but a 2" model.

Figure 12 - The constellation of Orion. Taken with the Olympus E-500, lens at 14mm (28mm in 35mm equivalent), f/3.5, 200 ISO, 30 sec. exposure

2 comments:

You may comment, but if it's spam or advertising, I'll be deleting your comment and blocking your arse!