Saturday, January 29, 2011

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

In this supplemental post I'll be discussing photography using telescopes, in contrast to part "A" of this two part text that is focused on photographing the night sky without one.

A Little about Telescopes and Eyepieces
Before getting into the photography side of this topic, let's first take a quick look at the three families of telescopes that I mentioned in the episode. I've only written fairly quick summaries below with the intention of providing you with some overall information. If you are interested in finding out more (and believe me there is a lot to learn about scopes), check out the Web resources section.

Refractors: Refractors are basically your classic telescope style; a tube with an objective at the front and eyepiece at the back. The objective is usually comprised of two or three optical elements, the former design generally referred to as achromatic and the latter as apochromatic. Both designs reduce the effects of spherical and chromatic aberrations, but apochromatic ones are more effective. As such, these versions tend to cost more, but for those with cash to spare and the longing for very good optical quality then they're tough to beat (within reason of course). But it's not like the achromatics are that bad either and are still a great choice for those on lower budgets.

There are some nice advantages to refractors, one being that they are very easy to use and most don't require collimation (alignment of the optics) out of the box as that has been done at the factory. Since there is nothing in the way of light getting into the tube as there is with reflectors and catadioptrics, no diffraction spikes/effects occur so you get lovely tiny round dots for stars (no "X" shapes). On the downside they tend to be quite pricey for their size. Last I checked, a 6" achromatic type is around $2,000 with high-end apochromatic ones (like Takahashi telescopes with fluorite lenses) coming close to or just over the five figure mark. In contrast, a 10" Newtonian reflector on a dobsonian mount is about $1,000. Fast telescopes (e.g. f/7) of this type also tend to exhibit more chromatic aberration than slower ones (e.g. f/16).

By the way, some spotting scopes have the ability to be attached onto cameras; they are basically refractors. I have a couple of these, one of which is this model: Although it doesn't provide outstanding image quality, for its price, it's a ton of fun to play it! I'm digressing...

Reflectors: Unlike refractors, reflectors have no lenses. Instead, mirrors are used to focus incoming light. This has several advantages over using glass (or fluorite) lenses, in that chromatic aberration is no longer an issue as mirrors can focus all frequencies (colors) of light into a single spot. However, for our purposes there are two common types of mirror shapes used in these types of scopes to focus light, one being spherical and the other parabolic. Spherical mirrors suffer from, you guessed it, spherical aberration, which means that parallel rays of light do not get focused into a nice little spot (hence a softer versus sharper image). This is the advantage that parabolic mirrors have, because in their case parallel rays of light are focused into a more precise spot, thus yielding better image quality.

So why are telescopes made with spherical mirrors then if they are inferior to parabolic ones? Well, primarily cost. Spherical mirrors are easier to grind/manufacture and for smaller telescopes, say 5" or less, the effect of spherical aberration isn't as noticeable as it is on larger and faster scopes. I can personally say this is true as I own a 4" Bushnell with a spherical mirror and a 10" scope with a parabolic one, both Newtonian designs. Indeed, the smaller 4" scope actually has very good quality in comparison to what the 10" delivers, aside from the obvious such as more light gathering ability and resolution advantage of the larger one.

Reflectors, such as the common Newtonian (see figure 1; figure 2 shows a closeup of the focuser with an eyepiece in it), are overall quite inexpensive and can produce stunning results, but do have a few disadvantages. Faster scopes of this type, around f/5, have visibly more coma, which makes stars towards the edges of the image look like little comets. This may not be much of an issue if one is just viewing with the naked eye, but photographers might not take well to this artifact. However, this effect can be corrected with various types of eyepieces, an example of this being the 7mm Speers-Waler you see in figure 5. Another matter concerns collimation, the lining up and adjustment of the mirrors/optical elements. Having done this a few times with my scopes, I can comment that it's not very difficult to do, but can be a little time consuming to get it just right. The better the alignment of the mirrors, the better/sharper your image will be, so it's a necessary aspect to such telescopes; there's more information on the art of collimation in the Web resources section below. The last item I'll touch upon is the secondary (aka diagonal) mirror; this mirror held by its four supports (sometimes one on smaller scopes) is affectionately known as the "spider". This smaller mirror, in comparison to the primary, is what redirects light to the eyepiece on the side of the telescope body and blocks a little bit of it from getting into the tube. But this amount of blockage is usually quite minimal, generally a loss of light in the single digit percents. Some photographers may not like the diffraction spikes caused by the spiders "legs". You've surely seen this on some astronomical photographs where stars have "X" like spikes extending out from them. As with most things, this is a personal like or dislike. I don't mind the look of them myself.

Catadioptrics: And now we come to the last type of telescope I'll discuss here. These are very similar to reflectors, but with a couple of slight twists. In regard to similarity, both reflectors and catadioptrics have a primary and secondary mirror, but in catadioptrics the secondary mirror does not reflect light out to the side. Instead, light is funneled back down the tube and through a hole in the primary mirror. As such, the eyepiece holder is at the bottom of the scope. In addition, there is a lens at the front of the scope which plays a role in getting light to focus more accurately off the primary mirror; this is termed "corrector plate" in Schmidt-Cassegrain scopes and "meniscus lens" in Maksutov types. These are very popular scopes as they aren't very expensive, although more than Newtonian reflectors, and they are usually sealed units with the mirrors/optics aligned at the factory, hence requiring minimal maintenance. Overall there are fairly few downsides to catadioptrics. Cost can be seen as one as they are a tad more than reflectors, and the secondary mirror blocks a little light from getting in, but not a major issue.

Again, I'd like to emphasize that there is a great deal more information available about telescopes and if you'd like to find out more, feel free to check out the Web resources section below. Some of the links to go Wikipedia, but after perusing through the articles they seemed to be accurate.

Now for a little bit about eyepieces; figures 3, 4 and 5 show a few of mine. As with telescopes, there are a surprising number of different types out there from simple (and outdated) one lens designs to complex (and usually expensive) multi-element types like Plossls, Erfles and Naglers. In a nutshell, each type has its pros and cons, generally hovering around factors like image quality, field of view and eye relief. Some are also better for looking at dim objects like star clusters or nebulae, others for planetary viewing. There are three common sizes in regard to the eyepiece barrel, the part that slips into the telescope's focuser. Your cheap department store variety of scopes usually have a focuser that can only accommodate 0.965" (24.5mm) eyepieces, whereas many higher quality telescopes you can purchase from dedicated telescope shops accept both 1.25" (31.7mm) and 2" (50.8mm) eyepieces (usually an adapter is needed or is supplied with the scope; see figure 6). My recommendation is that even if you don't have a large budget, at the very least try to get a telescope with a 1.25" focuser, otherwise you may find it difficult down the road to locate good quality eyepieces in the 0.965" size. In addition, those really cheap scopes are exactly that, cheap; you do get what you pay for.

Barlow lenses can be used with eyepieces as they can increase the magnification you can achieve. Common factors include 1.6X (figure 8), 2X, 3X and some are variable (see figure 7). If you're on a budget, Barlow lenses can be a nice way to increase your magnification without requiring additional eyepieces, even if it means some loss of light.

Playing with Numbers
Most of us with telescopes love the fact that we can get massive magnification with them, but it has to be understood that there is a limit. Unfortunately, many department store variety of scopes advertise that their small units can provide ridiculously high values, such as five or six hundred. As far as legal issues go, this is true; put together the appropriate eyepiece with a Barlow lens and there you have it. But in regard to seeing anything useful other than a horribly blurry, faint blob of some sort in your eyepiece, not going to happen. So to roughly find out what a telescope is reasonably capable of in terms of magnification (still yielding a fairly sharp, good quality image), I like to use the following which from experience has proven quite valid: multiply the size of your scope in inches by fifty, then divide by two. So if I use my 10" scope as an example, then (10 X 50) / 2 = 250. For you metric folks out there, you can take the size in millimeters and multiply by two, then divide by two and you'll get nearly the same answer. The "seeing" quality will also affect this, but I've dedicated a section to that topic below.

Ok, so now we have an idea of what the telescope is capable of, but of course you likely won't always be using your peeper tube at this level of magnification. Many night sky areas are just gorgeous with a wide field eyepiece that provides very little magnification, like under 50. Therefore, to find out what level of magnification you're getting with a particular eyepiece, simply divide the focal length of the scope with the focal length of the eyepiece. My 10" Newt has a focal length of 1,200mm and one of my favorite eyepieces is 7mm, so 1,200 / 7 = 171. A 28mm eyepiece on the other hand will give me 42.9 times magnification.

Camera Adapters
To begin with, there are actually quite a few different ways that a camera can be attached onto a telescope. When using an SLR (digital or film), you can often find a t-mount adapter for it (see figure 9). These adapters have a mount on one side that attaches to the camera and a threaded side that screws directly onto some eyepieces, an adapter ring on an eyepiece, or onto a camera adapter (see figures 10 and 11). Some camera adapters, like that shown in figure 10, allows a 1.25" eyepiece to be slipped into it (of course not all of this size will fit). If you have a point and shoot model, one of these types of adapters can be used (sorry, I was lazy to take a photo of mine). The camera is screwed onto the small base and that "O" shaped section is tightened around an eyepiece. Then the height and position of the adapter is adjusted so that the lens of the camera is as close as possible to the eyepiece. On point and shoot cameras with larger lenses, don't be surprised if the image shows vignetting. Those cams with tiny fixed (no zoom that is) lenses generally work well, especially with eyepieces that provide large eye relief; many of the images in the episode were taken with such a setup and turned out quite well.

Do-it-yourselfers have also created their own mounts. In fact my old Fuji was in a holder I made out of nothing more than paper, chopsticks as a frame and lots of tape; worked great on the 7mm Speers-Waler. I've also seen old film canisters modified to accept webcams, and you can purchase CCDs made specifically for astrophotography, but I'll let you poke and prod around the Web to find out more about that.

Cool it! "Seeing" what I'm saying?
At this point I could probably get right into photography with a telescope, but you might be a bit disappointed with the results, especially if you aren't familiar with thermally stabilizing your telescope or bringing it to equilibrium with the environment's temperature before using it. "What!?" you say. All it really means is to let your telescope's temperature match the temperature it is outside (usually cool down). For example, there's a scope indoors at room temperature, about 21C (70F), but you want to take it outside where the temperature is only 10C (50F). If you looked through the scope the moment you took it outside, and for a while afterwards, I can best describe the appearance of the image as if it was underwater. This is caused by a couple of factors, one being that the mirrors and/or lenses (depending on the scope you're using) are contracting and deforming as the material looses heat energy. This movement is incredibly small, microscopic, but more than enough to ruin the quality of the image. In addition, as the scope looses heat it causes weak air currents to form, which also degrade the image quality. Reflectors, with their wide open and unsealed tubes, are most prone to this.

The greater the temperature difference, the longer the time it will take for the telescope to reach this equilibrium. I remember taking my 10" Newt outside when it was a chilly -20C (-4F) and it took over two hours for the image to stabilize. Of course in other not so extreme cases this time span decreases to around under 30 minutes.

Now I've already touched upon this a little in part "A", but "seeing" conditions play a big part in not only how well you can see objects through your telescope, but also how photographs will turn out (or not). Upper level winds/turbulence high up in the atmosphere, level of humidity in the air, amount of dust/pollution and light pollution all play a part in adding gunk between you and the stars, planets and other celestial wonders. It's a combination of these things that make stars twinkle, and indeed, less twinkle is usually a good indicator of better seeing. On some days you'll find that you can easily see the dark division in the rings of Saturn, while on other days it's just a bright oval shape... yet seemingly to our eyes the dark night sky may look no different on either occasion.

Anyway, seeing is nice to know about because if you know what to look for it may save you some time and effort. Imagine if you're about to spend around three hours worth of time taking multiple exposures of some faint bodies in the sky, only to discover that waiting a day or two would have resulted in much clearer photographs. One thing I've certainly become accustomed to when working with my telescope is being patient; whether hunting for that dim nebula, focusing carefully or just plain waiting for the camera to finish exposing.

Photography Using a Telescope
In my video I discussed two methods of photography that can be performed with telescopes, but actually there are three (I made little boo boo) and I discuss them in a little more detail below. Since I have experience using digital SLRs in this case, that's what I focus on --just a fair warning that if you're looking for CDD, webcam or film astrophotography material you'll have to do some Googling on your own.

Prime-focus: As mentioned in the episode, you simply attach the camera body directly onto the telescope's focuser without using an eyepiece or any camera lens. Indeed, this basically turns your scope into a huge lens. Although you don't get a lot of magnification, you don't lose a lot of light either as no lenses get in the way. On my 10" scope (1,200mm focal length) I can basically image the full moon, or about half a degree of the sky. This is also a wonderful method to take some wide-field shots.

Eyepiece Projection: To err is human... :P In the video, I said that attaching your camera body to an eyepiece (no camera lens used) is the afocal method. In fact, I messed up on the terminology. When you attach the camera body to a telescope without a camera lens but are using an eyepiece, you are employing the eyepiece projection (aka positive projection) method. The image the eyepiece delivers is projected directly onto the camera's focal plane (the sensor or film). Thankfully though, the photographs of the planets reflected afocal photography, as the Fujifilm FinePix 40i camera I used has a fixed lens and I was taking pictures through an eyepiece.

Afocal: The afocal method is similar to eyepiece projection, but differs in that a "regular" camera lens is attached or you're using a point and shoot model, which of course has the lens built in. For example, an eyepiece is in the focuser and a point and shoot camera or a dSLR with a lens attached is used to capture photographs.

With both the eyepiece projection and afocal methods, you may experience vignetting depending on the eyepiece you use. But unlike prime-focus, you can achieve much higher magnification levels.

To track or not to track?
With a telescope, not only is the image magnified, but so is the movement of the celestial bodies as they pass across the sky. For example, if I'm looking at Jupiter with my 7mm eyepiece, which gives 171X magnification, every 5-10 seconds I have to nudge my Dobsonian mount a little to keep the gas giant in view.

In regard to photography, this means that the use of long exposures will be next to unusable; even shutter speeds around 1/2 second might end up producing a soft image. So if you do not have a tracking mount at your disposal to counteract the earth's rotation, then you need to look for ways to reduce the exposure time. This means doing such things as using the brightest lens you have wide open (if utilizing the afocal method) and bumping up the ISO as high as you dare. Your selection of eyepiece will also play a role in this too, as the higher the magnification gets the darker the image will become. Then you'll likely need to use slower shutter speeds to get reasonably well exposed photos, but the result may be undesirable. Lastly, you'll also very likely be limited to the brightest of objects in the sky; the moon (easiest target, even at high magnification it's still very bright), Mercury, Venus, Mars, Jupiter, Saturn, Uranus, maybe Neptune if you have a large scope, bright stars, some open clusters, and maybe a couple of nebulae (again with a large fast scope).

On the other hand, if you have a tracking mount then long exposures become easier to perform (assuming of course that the mount has been properly polar aligned or set up). Since the shutter can be held open for several minutes, you can start to see stunning details develop on photos that would otherwise remain invisible with previously mentioned methods. For example, dust bands in galaxies and even color from otherwise dim dark gray objects come forth. And although I won't get into this, you can also get various filters, like H-alpha, to bring out details that wouldn't normally show up. When thinking of tracking mounts, most people envision an equatorial or alt-azimuth fork style that is driven by a motor. But there are a few manual tracking mounts out there such as the barn door tracker. Motorized tracking mounts aren't inexpensive (especially better quality ones that can support larger scopes), so for those who are happy using a long lens on their SLR to take some celestial shots, these are easy to make and, best of all, quite inexpensive.

Stay Sharp
Before ending this post I want to briefly discuss focusing. I talked a little about this in part "A" in regard to using the camera with a regular lens, but using a telescope can be a bit trickier. Speaking of tricks, there are a few that can be used to make things easier, but I'll start with some of the simple things first. Like astrophotography with a regular lens, aim for the moon. It's so far away that once you focus on it, stars, planets, etc., should all be in focus as well. Be warned though, your night vision might be temporarily compromised due to the brightness of earth's companion. If you're shooting using the prime-focus or eyepiece projection methods then of course the telescope's focuser will need to be adjusted, and if using the afocal setup, both the focuser and camera lens might need to be varied to get a sharp image. Planets and bright stars can also be aimed for in the moon's absence as they can generally be seen well enough.

By chance if you've already tried focusing on planets or stars, you might have noticed that it's still quite cumbersome to perform. The camera's LCD screen or viewfinder just doesn't give you as much luxury as using your own eyes directly with an eyepiece. But that's where some clever people came up with masks that you can temporarily attach to the front of your scope to aid with focusing. The cool part is that with a little effort these masks can be made with supplies as simple as a knife, cardboard and duct tape. Below are some links to these masks so you can read more about them:

Bahtinov mask
Carey mask
Hartmann mask / Scheiner disk

Keeping to the topic of nice sharp images, when shooting bright objects, strongly consider using your camera's mirror lock-up feature and if possible, use a remote to trigger the exposure. For example, if you just use your finger to press down on the shutter button, you'll likely notice that the telescope vibrates slightly for a few seconds; poorer quality mounts tend to be more prone to this, but even sturdier ones can exhibit some flex. Since the shutter speed for bright objects is quite fast, the resulting vibration form the button press may ruin the image. But by delaying the actual exposure for a few seconds this issue can easily be overcome. Now if you're using a point and shoot camera, or there is no mirror lock-up feature, not all is lost. In situations like this you can hold a matte black card in front of the lens, press the shutter button, wait a few seconds after the exposure is triggered, and then quickly move the card away from the lens; poor man's mirror lock-up. Remember to add a couple of extra seconds to the exposure time to compensate for this action.

Although I think I've broken a personal post size record, I've barely scratched the surface of astrophotography. In case you have a question or two, feel free to ask away and if I know the answer I'll be glad to help or point you in the right direction, likely in the form of a blog post. I definitely need a break from all this writing and I have yet to touch my paper airplane-a-day calendar, so I think I'll try my hand at that. L8r!

Web Resources

Telescope related: - Lots of articles here

Astrophotography related:

Astronomy magazines:
Amateur Astronomy
Astronomy Now
Sky and Telescope
The Astronomer

Telescope manufacturers:


General astronomy sites:
Astronomy Pic of the Day
Canadian Space Agency
European Space Agency
NASA Jet Propulsion Laboratory
Royal Astronomical Society of Canada
Science @ NASA
Institute for Space Imaging Science

Astronomy related software:
Sky View Cafe

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.

No comments:

Post a Comment

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