An Introduction to Astrophotography

What follows is a "reprint" of a three part series on astrophotography. Some readers of Regulus! missed one or more portions, so here it is in its entirety -johnb

There has been much discussion lately of astrophotography both on the web and amongst amateurs and publications. Most of this is due to the arrival of comet Hale-Bopp which has been getting a lot of publicity lately. This, the first article in the series, will therefore discuss some basic ways to get good results when photographing comets and other wide-field objects, like constellations, star clusters, meteor showers and the like.

First we shall talk about the essentials: films and cameras. Astronomical photography usually deals with dim objects in a dark sky. It would follow that you want a film that is very light sensitive, readily available, and fits most cameras. With the advent of new technologies there are some great films out there. I like the Kodak Royal Gold 1000 film, a color film with a fast 1000 ISO rating (The higher the ISO or ASA number, the more sensitive to light the film is. "Fast" films are more light sensitive). Other good films include Kodak Pro 400, Fuji Super G Plus 800. All of these can be used right out of the box with no extra preparation.

The camera you use must have some basic qualifications. First, it must be able to open the shutter and keep it open for long periods of time, usually 30 seconds to several minutes. This is called a "B" or "Bulb" setting. Many modern day "point-and-shoot" cameras do not have this feature that was once a standard for all 35mm single lens reflex cameras. The camera should be mountable on a tripod, and should obviously have a lens that can be focussed out to infinity. Lens choice is up to you. A 50mm lens will easily frame large constellations and show comets. A telephoto or zoom lens can also be used successfully, but realize that the longer the focal length, the less light typically gets to the film, thus requiring longer exposure times. You will also need a tripod or other stable platform to hold the camera while aimed at your subject, and a cable release which allows you to lock the shutter open for the exposure and not shake the camera.

Ok. You have all the basic equipment, you have a clear dark sky, and maybe even a comet to photograph. Here's how:

1. Set up the camera (with film) on the tripod and attach the cable release.
2. Aim the camera at the area of sky to be photographed.
3. Focus to infinity, and make sure the f-stops are as open as possible.
4. Using the cable release, lock open the shutter and count to 30.
5. Keep notes of shot numbers and exposure times.
6. Close the shutter, wind the film, do it all again with progressively longer exposure times (60 sec, 120 sec...etc). Use the whole roll!

This technique works best for large extended objects like constellations, meteor showers, comets and aurora. Usually exposure times under 2 minutes will produce fine results and not show too much star trailing.

Here are some images I have taken over the years. You will see that some are guided exposures. All of these were "piggy-backed" on a larger telescope. These were taken in a light polluted area and required longer exposures to get what some would normally see in half the time at a dark site.

Andromeda with M31: October 31, 1986, 5 minutes guided 50mm 100 ASA f/1.4 Note the edge of the telescope at bottom.

Orion's Belt and M42: January 12, 1986, 130 seconds guided 135mm 100 ASA f/3.5 There is some reciprocity failure in this image.

Comet Hyakutake March 26, 1996, 1 minute unguided 70mm 1000 ASA. Note edge of house.

Hale-Bopp March 15, 1997, 1 minute unguided 135 mm 1000 ASA.

Hale-Bopp March 15, 1997, 1 minute unguided 70 mm 1000 ASA.

More Picures:

Hale-Bopp April 2nd 1997 70 mm 1000iso 30 sec.

Hale-Bopp April 2nd 1997 150 mm 1000iso 30 sec. Enhanced.

Hale-Bopp April 2nd 1997 70 mm 1000iso 30 sec. Enhanced.

By now you have noticed that almost all astrophotos are time exposures. Some can be hours long, others only ten to thirty minutes in length. This is to allow the film the time necessary to capture as much light as possible from these faint astronomical sources. But what happens when the camera's shutter is left open and not guided? Well, the Earth turns and the image you were so patiently recording on film has a bunch of streaks instead of star points. To overcome this astronomers use a variety of mounting equipment that follows the stars by counteracting the Earth's rotation.

Southern Orion 150mm 2 minute exposure unguided 1000 ISO Kodak Royal Gold. Note the star trails caused by the lack of guiding.

In all arrangements, these mounts have one axis that is perfectly in parallel with the north-south running polar axis of the Earth. This way, a single motor can be used to move the camera to follow the stars. These mounts are called equatorial mounts.

The simplest of these systems is a poncet mount which holds a standard 35mm camera and lens and usually incorporates a motor drive that moves the mount in step with the stars. These can be home built, and you can search for plans in older astronomy magazines and on the internet. Search for terms like: "poncet", and "ATM" (amateur telescope making).

Another popular way to get introduced to this type of photography is to mount the camera onto the back of a telescope (piggybacked) and use the telescope's drives and optics to guide with the stars. This is a simple and rewarding method. The tolerances are not very demanding when using a piggybacked system, even when using a telephoto lens, and with a high power eyepiece in the telescope, guiding can be very precise. This can be made even easier with the purchase of an illuminated reticle eyepiece: an eyepiece with cross hairs and a faint red LED lighting them. These come in a variety of styles and can be expensive. I prefer the simpler double wire sort, because I can get a star centered in the central square easily without losing it behind a wire. Try to get one with about 150 to 200x magnification, which would be about a 12.5mm focal length with the now common 8" SCT scopes. Look for adjustable reticle brightness, and, if you also wear glasses, look for good eye relief.

A typical view through an illuminated reticle eyepiece while guiding.

So, you ask, how do you keep that star centered in your field of view? Many telescope mounts come with slow motion controls which allow for fine adjustments in right ascension and declination. These you can use alone for guided exposures of up to five minutes. Any exposures longer than that will necessitate a clock drive just to preserve your sanity! Often the slow motion controls can be used with the clock drive to make minor adjustments required to offset periodic drive errors (caused by the actual gearing being slightly out of round) and fluctuations in power (which is more apparent in older A/C synchronous motor drives). You will have to read the instructions that came with your telescope and mount. Some modern scopes come already equiped with dual axis drive systems and hand held units for making small adjustments. Just remember that extra batteries are a smart thing to carry with you, and the colder it gets, the faster batteries die.

Southern Orion 150mm 5 minute exposure 100 ISO Kodak. This shot was guided by piggybacking the camera and lens on a larger telescope. Guiding was done with an illuminated reticle eyepiece on the telescope. Note how sharp the star points are.

For those interested in some of the mathematics, one can easily calculate the width and height of the photographed area via the following formula:

Field Covered = 57.3/EFL x FrameSize

Where:
Field Covered will be the field in degrees covered by the photograph.
EFL is the lens' effective focal length in mm.
FrameSize is the physical width or height of the film emulsion in mm.

For example:

Standard "35mm" film has 24x36mm dimensions. With a standard 50mm focal length lens the equation would read:

Field Covered = 57.3/50 x 24 = 27.5 degrees height
Field Covered = 57.3/50 x 36 = 41.3 degrees width

What to take pictures of? Since a 50mm camera takes a good wide field image, try projects like collecting all of the constellations. There are 88 of them, and travel will be necessary to get images of constellations on the Earth's other hemisphere. With a moderate telephoto or zoom lens, one could easily capture all of the Messier deep sky objects to form an impressive collection. Other projects include the photography of comets, meteor showers, & planetary conjunctions. Some astronomers use this technique to scan for supernovae. Whatever your plans, this is a fantastic way to get accustomed to guided astrophotography in preparation for part three in our series: Prime Focus Astrophotography, which every amateur wants to do from day one: taking pictures through the telescope itself.

This is perhaps the most tedious and rewarding astronomy project amateurs attempt. It will demand patience, time, and perseverance to accomplish good results. Excellent results demand excellent equipment and trial and error. Over the last twenty years, I have gone through numerous rolls of film and taken maybe a dozen truly good astrophotos. I know that I have a long way to go, but it is the journey that is the fun!

Equipment:

A steady uninterruptible power supply is another essential. If you are using AC power, then be ready for brown outs and possible power outages. The ability to end an exposure in an instant is necessary. For those using DC power supplies, be sure that you have a newly charged battery.

You will definitely need a guiding eyepiece for time exposure photography. Invest in a good 12.5mm illuminated reticle eyepiece. For those with glasses, some manufacturers are making 25mm eyepieces that can be used with a barlow lens to both increase magnification and keep the long eye relief needed for glasses wearers.

To mount the camera to the telescope there are many accessories available. You will want an adapter that connects directly to your telescope (either by 1.25" or 2.00" O.D. barrel, or by a direct screw-on mount for SCT's like Celestron & Meade). The other end of the adapter should accept standard T-ring threads that allow you to attach the camera using any store bought T-ring. Some adapters have multiple uses and allow for insertion of an eyepiece for projection photography. This is useful for lunar and planetary photography where high powers are desired. For long time exposure photography where guiding is necessary, you should obtain an off-axis guiding assembly. This allows you to pick off a bit of light from the primary focus for guiding. This is useful in place of a separate guidescope because it overcomes at least two known problems:

• Primary mirror shift in SCT's.
• Mount flexure between the main scope and the guidescope.

• Have a bulb or "B" setting for long time exposures.
• Allow for the use of a cable release to avoid camera shake.
• Does not use a battery to open and close the shutter.
• Has removable lenses and allows for the use of T-Ring adapters.

	Prime Focus Adapter for shooting quick prime focus images of bright objects that require little magnification such as the moon.
	Eyepiece projection for shooting quick images that require higher magnification. Good for the moon, sun, planets, and some planetary nebula.
	Off-Axis Guider for shooting long guided exposures at prime focus. This allows the photographer to guide on a star within the actual image field of view.

Polar Alignment:

A popular and accurate method of polar alignment is the star drift method. Follow this procedure, and you will have a well aligned telescope shortly after night begins:

1. Align the telescope to read 90 degrees of declination and aim the polar axis at Polaris. This can easily be done by centering the star in a high power eyepiece. Just remember to move the whole mount (tripod and all if necessary), leaving the declination at 90 degrees.
2. Now rotate the telescope to point at a star that is both on the meridian (the imaginary line running from north to south) and close to the celestial equator. Center that star in a high power eyepiece. An illuminated reticle eyepiece helps. Rotate the eyepiece in its holder until one reticle line is aligned with RA, and the other is aligned with declination.
3. If the telescope is not polar aligned you will see drift in declination. Ignore any drift in right ascension. If the star drifts south, then the polar axis is too far east. If the star drifts north, then the polar axis is too far west. Make adjustments in the telescopes azimuth until all drift is eliminated.
4. Rotate the telescope now to aim at a star near the eastern horizon (15-20 degrees elevation is fine) that is also near the celestial equator.
5. Still ignoring all drift in RA, look for drifts in declination. If the star drifts south, then the polar axis is aligned too low. If the star drifts north, then it is aligned too high. Make small adjustments in the polar axis altitude until this drifting is imperceptible after 10-30 minutes.

You now have a well aligned telescope. Why do you have to guide it during the exposures? Depending on the type of scope you have, there are any number of reasons:

• If the scope is driven with AC current, any small fluctuations in power will cause slight variations in tracking speed, usually in a sinusoidal fashion. These are somewhat periodic but still unpredictable.
• No matter what power source you are using, there are slight imperfections in the actual drive hardware. Gears can be, and often are, just slightly out-of-round. This will cause periodic errors that can be predicted. In this computer age, some mounts have small circuits that can be programmed to counteract periodic error. This will make your job of guiding much easier. See your telescope mount manual for details.
• The mount of a telescope may seem rigid and unyielding, but it isn't. As the telescope rotates in RA, the weight shifts. This shift in mass will cause a bending in some mounts that is called mount flexure. Obviously the more sturdy the mount the less of this you will have. Also, the shorter the exposure, the less likely you are to encounter this. Either way, you will have to guide to avoid this.
• For those of you with the ever popular Schmidt-Cassegrain optics, like Celestron & Meade, there is a unique problem that will force you to guide. These telescopes actually move the primary mirror to make images come to focus. Shifting RA positions cause the weight of the mirror to shift in its cell. This can cause image shift and it can cause images to defocus. The image shift can be guided out, but not much can be done about changing focus during exposure. If your optical assembly does this, then you should either have it repaired, returned, or scrapped. There is some information in Usenet archives about this problem. Some amateurs have removed the primary and added extra grease to its slide guide, but I do not recommend this for most people. Optical alignments are easy to wreck in this type of telescope.

Focus:

Exposure:

A typical long exposure routine would be the following:

1. Attach camera and off-axis guider to the scope. Also attach the guiding eyepiece making sure everything is tight.
2. Check power supplies to the scope's drive, the camera and the guiding eyepiece.
3. Focus the camera on a bright star, then focus the guiding eyepiece by raising or lowering it in the off-axis guider housing.
4. Center the object to be photographed in the camera viewfinder and make sure the scope is guiding.
5. Find a suitable guidestar in the guiding eyepiece. This can take a few minutes.
6. Recheck everything!
7. Manually lock up the camera mirror (if possible), and set the camera's exposure time to "B".
8. Open the shutter and lock it with the cable release.
9. Begin guiding.
10. Close shutter when the time is right.

A typical short exposure routine would be the following:

1. Attach camera and to the scope with either the prime focus or projection assembly.
2. Check power supplies to the scope's drive, the camera and the guiding eyepiece.
3. Focus the camera on a bright star.
4. Center the object to be photographed in the camera viewfinder and make sure the scope is guiding.
5. Recheck everything!
6. Manually lock up the camera mirror (if possible) and set the camera's exposure time to suit the needs of the film and the object being photographed.
7. Using the cable release, trip the shutter.

Conclusions:

Examples:

M42: Orion Nebula. Taken 1/1/87 with an 8"SCT at prime focus, f/10, on 400 ISO Kodak Technical Pan film. 10 minute guided exposure with an off-axis guider. Select image for full view.

First Quarter Moon taken at prime focus, f/10 8"SCT with 1000 ISO Kodak Royal Gold film. 1/125th second exposure on 4/14/97. Select image for full view.

Lunar Detail around crater Philolaus taken with an 8"SCT on 9/15/86 with 400 ISO Kodak film using a 6mm eyepiece for projection. Select image for full view.

Last Modified: 3/19/03 8:55p
This page:© Copyright 2005 by John A. Blackwell