Andrew Welsh's DSLR Astrophotography Website
Tutorials and Links explaining how I do it all
To make an astro-photograph using my telescope, I perform the following sequence of steps, starting roughly around sunset, ready to photograph in about 90-120 minutes.
Following these steps, even if you don't understand them, will ensure you have quality raw data that you can use months/years later as your image processing skills, software or equipment improves. This tutorial is based roughly on Jim Solomon's Astrohpotography Cookbook, a superior resource, and the best on the web... if you know some basics.
This tutorial is written based on my location in the northern hemisphere. It can be used for southern hemisphere observers-- just reverse all the directions (i.e. north for south, etc.). This is for "prime focus" photography, and is not the easiest astrophotography to start with-- but we crave a challenge! An easier start is "piggy back" photography, using a regular camera lens and mounting the camera to the top of the scope. Piggy back is far less sensitive to tracking errors and focusing, which are the two largest challenges to taking good photographs, film or digital. This tutorial also covers the challenges unique to digital imaging.
Table of contents
A. Equipment Required
B. Setup Equipment
C. Flat Frames
D. Polar Alignment
E. Periodic Error Correction (PEC)
F. Align the Telescope's GOTO
G. Photographing- Focusing the telescope
H. Photographing- Taking the picture
I. Dark Frames
J. Image Processing
Equipment required to do this procedure (top)
- Digital SLR camera (these instructions can also work for a webcam, CCD or Meade LPI camera)
- Tripod and Equatorial wedge OR some sort of equatorial mount
- Illuminated reticle eyepiece
- A source of electricity
- Remote shutter release cord
- A computer to download your photos onto
- IRIS processing software (100% free!)
- (OPTIONAL) Off-axis guider or guide scope- allows for much longer exposures
Set up equipment (at/after sunset) (top)
- Cart equipment out to observing site. Can be my back yard, can be a field or wherever.
- Set up the tripod and level it. Have the tripod aim roughly north. Usually one of the legs will point due south or north, depending on the model. For my LX200 classic, one leg points due south. The little bubble level embedded in the tripod works sufficiently. Leveling makes the polar alignment step faster later on. When at home, I will leave the tripod legs adjusted to be level and set it up in the exact same spot the next session. Equitorial wedge is attached. Using the wedge allows the scope to only have to rotate east to west (left to right when facing south), eliminating any up/down movement. This allows for long exposure photographs and prevents the image from rotating in the picture (called "field rotation").
- Mount scope on tripod. Tighten screws.
- Plug in declination (north/south or up/down motion) cord.
- Plug in keypad controller.
- Plug in AC adapter to scope base and other end to outlet or vehicle (if in the field without an outlet). I might get a battery pack someday. DO NOT TURN ON TELESCOPE YET.
- Plug in AC adapter for camera, if plug is available.
- Unpack eyepieces, diagonal mirror, finder scope, eyepiece adapter (to attach eyepiece and/or diagonal to scope), and off-axis guider (or camera adapter if not using the guider) from box. Remove all dust covers, caps and bags.
- Unpack camera and blow dust off of sensor (using "sensor clean" option in the custom functions menu of the Rebel XT). I use a can of air, which is risky. So long as I blow a puff of air away from the camera first, to clear out any liquids, it's OK to squirt the sensor. The safest bet is to use a bulb blower (available at camera shops) which uses just air. Digital camera sensors get dusty no matter how careful you are.
- Remove body dust cover from camera, and attach off-axis guider (OAG)/camera adapter to camera (using t-adapter already threaded onto OAG)
- Thread OAG onto rear of the telescope. Loosen the declination (up/down) and right ascention (left/right) locks so I can aim telescope freely by hand.
- Insert AC adapter into camera, if I'm using it. Turn camera on and check settings. I use manual mode ("M" on the dial on top), mirror lock-up OFF, ISO set to same setting I'm going to photograph with (currently using ISO400), noise reduction OFF, image format RAW (JPEGs are sub-optimal for astrophotography due to image data loss, even at maximum or best setting).I'll also adjust the LCD brighness to the minimum if I've used the camera in the daytime, so I don't blind myself when it's dark.
Flat frames (still twilight, can't see stars yet) (top)
- Now we take some pictures, called a flat frame. The flat frame is an image of evenly illuminated nothingness. The goal is to take a picture of any dust or hairs on the camera chip or in the telescope so they can be digitally subtracted out later on. Flat frames also compensate for any vignetting (edges of the photo being darker than the center) caused by the telescope. Aim the telescope to a clear patch of sky, roughly 20-40 degrees above the horizon, in a southeast or soutwest direction. I'm avoiding getting a gradient of sky brightness (brighter towards the sunset) as best I can- trying to get as even an illumination as possible.
- Hold the camera button down halfway to activate the automatic light metering.
- Adjust the shutter speed with the little wheel, so the light meter shows a properly exposed shot (the line is in the middle on the LCD display on the back). Usually 1/4 second is the right speed by the time I've finished all the above steps.
- Take one photo. Review the photo immediately, and using the camera's "info" button I look at the histogram. The peak should be roughly in the middle or slightly below the middle. Adjust the shutter speed/exposure time as necessary.
- Take a total of 9 or 15 flat frames, moving the telescope left/right and up/down in random directions to average out the sky brightness. If a star happens to get photographed, it will move around between images, and be averaged and processed out later on. The photos are taken rapidly one after another... the sky is getting darker every second, you know!
- Lock down the RA and declination clamps so the scope doesn't flop around once done. I like to put it back in the "startup" position, which is pointing at the meridian ("straight", relative to the base- it's marked on my scope-- due south) and zero (0) degrees declination (there's a wheel on the side which roughly tells you the declination on the LX200). The telescope should be parallel with the wedge, not the ground.
- Remove the camera from the OAG/adapter and put the body cover back on the camera.
- Now take the same number of "dark" frames as you did "flat" frames. 15 flats, take 15 darks. 9 flats, take 9 darks. Leave the camera at the same shutter speed and settings as before. Using a digital SLR, it is important to cover up the viewfinder, as light does leak through this onto the sensor. We want a truly dark picture. All it usually takes for me is to put my thumb over the viewfinder. Most DSLR's come with a viewfinder cover which slides on over or in place of the viewfinder rubber bumper.
- Some people suggest saving these pictures to a PC right at this moment, in case of data loss or whatever. Although a safe bet to do so, I've never had any problem with losing this information. Besides, I can always take flat frames at sunrise or during the day next day, so long as I keep the camera attached to the scope. This step is optional. Sometimes if you're out in the field, you don't have access to a computer or laptop.
Polar aligning the telescope (still twilight, a few stars barely visible) (top)
- Drift alignment of the telescope is crucial for accurate alignment of the telescope's rotation (clock drive) to the Earth's rotation. This is referred to as polar alignment. Drift alignment is a technique used to progressively get the telescope's rotational axis closer to the Earth's.
- Drift alignment is an involved process itself, and is best described (and practiced!) with the drift alignment simulator. I recommend practicing drift alignment on a cloudy night or during the day, so when you get to the scope you don't burn precious observing/photographing time aligning your scope. I've wasted several hours trying to figure this out myself. Once you have this procedure down pat, you should be drift aligned before the sky has reached astro-dark / the end of twilight. Here is another drift alignment article offering an explanation with examples. The Meade LX200 instruction booklet also has directions on how to drift align in the appendix.
- Remove the OAG/camera adapter, thread on the eyepiece adapter, and attach the diagonal mirror.
- Turn the telescope on. Do NOT do any alignment procedure yet. We just want the clock drive for now.
- We're going to focus the telescope for the reticle eypiece now. This is a good time to adjust the finder scope alignment if it happens to be off. Slew the scope to a bright star, the moon or a planet.
- If the finder scope alignment is good, skip this step. If the finder scope alignment is off:
- Insert a wide angle/low power/ high mm eyepiece to help find your target. I use a 26mm eyepiece for this purpose.
- Rough-focus the object in your wide angle eyepiece. Don't even bother with exact focus, just get it close- it will be readjusted real soon anyway.
- Center the star/ planet/ moon in the eyepiece so it will still be there when switching to the higher power eyepiece.
- Replace the wide angle eyepiece with the reticle eyepiece as soon as the object is centered.
- Insert the illuminated reticle eyepiece. If you have a corded model, plug in the cord to the telescope base now. It may still be light enough out that turing on the reticle is not necessary.
- Once the telescope is slewed to a bright star, the moon or a planet, focus the reticle eyepiece.
- Slew the telescope to the first drift alignment position- at the meridian, +20 degrees declination- and start drift aligning per the procedures linked above. I find 5 minutes of no drift is sufficient for manually guided exposures of 10 minutes and completely unguided images of 2-3 minutes.
TIP: now is a good time to take a few "dark frames" with the camera sitting on the sidelines while you drift align. Generally, the camera will be warmer than when you're taking pictures, but not by much. Besides, the software can scale the intensity of the dark frame. Skip to the dark frame section on how to do take darks.
- Once drift alignment is done, we're almost ready to photograph! You should have a nice set of dark frames by this time to start with.
Periodic Error Correction (PEC) (top)
This step can be done one time or averaged over a several nights, but once done it is not absolutely necessary to repeat it unless you have demanding needs (small object requiring tight tolerances for example).
- With the illuminated reticle eyepiece still inserted, conduct the periodic error correction routine per the telescope mount's instruction manual. For the Meade LX200, this means slew the scope to the meridian (due south) and zero (0) degrees declination, then initiate the PEC learn/update routine. The illuminated reticle eyepiece is required to do this step as well, unless you're using an autoguiding system of some sort (outside the scope of this tutorial).
- The PEC routine will take about 8-12 minutes to finish. Once done, it is time to reconfigure the scope for photographic work.
Align the Telescope's GOTO- last step before photographing (top)
- Return the scope to it's home position. For the Meade LX200 classic with an equatorial wedge, that's pointing towards the meridian, and zero (0) degrees declination.
- Turn the scope off, then back on.
- (This is for the LX200 Classic) Once the scope is ready, punch in a star number in the keypad for a star that is visible at this time of night. I usually use one that is in the same quadrant of sky as my photographic target of interest.
- Center the first star and synchronize the computer by pressing and holding down the "Enter" key on the keypad.
- Punch in a second star in the database. I generally use an alignment star on the other side of the photographic target of interest, to "bracket" or "frame" the alignment. This increases the accuracy of the alignment in the vicinity of the photographic target.
- Center the second star and synchronize by pressing and holding the "Enter" key on the keypad. The telescope is now polar aligned and ready for GOTO useage. Sometimes I sneak in a little visual observing at this point, since I have the eyepiece and diagonal still in place.
Photographing- focusing the telescope (top)
- Although there are a litany of focusing methods, I'm presenting here a technique I consider "good enough" for beginning to intermediate astrophotography. Start out by removing the eyepiece adapter and diagonal, and threading the OAG / camera adapter onto the telescope.
- Attach the camera to the OAG/ adapter.
- If you're using an OAG, insert the illuminated reticle eyepiece into the OAG but you can leave it off for the moment.
- Slew the telescope to a bright star nearby the photographic target(s) of interest. I often use one of the alignment stars. If you're lucky, what you want to photograph will have a bright star already in the field of view. Generally I use stars around magnitude 2, as any brighter it's too hard to tell if I have focus or not.
- Set the camera to manual mode ("M" on the dial, should still be set to this if you didn't change it).
- (Specific to Canon Rebel XT DSLR) In the custom functions menu, turn Mirror Lock to ON; Noise Reduction is still OFF.
- Change the shutter speed to 10 or 15 seconds. Plug in the remote shutter release cable.
- Looking through the viewfinder of the camera, focus the bright star as best as you can. Using an SCT-type telescope, I like to end my focus in a counter-clockwise direction. This motion pushes up against the mirror against gravity, instead of pulling it back with gravity. Microscopic / minute finger adjustments are key. Having a large focus knob helps. I use the Peterson EZ-Focus kit for my telescope. I like to think I can see diffraction lines or spikes coming out of the star once I'm in focus. (By the way, be sure to have your viewfinder's dipoter focus adjustment done during daytime, otherwise this step will be useless for you)
- Change the ISO setting on the camera to 1600 (or the maximum). Take a quick photo- use a shorter exposure time for a really bright star. Press the shutter release button once using the remote (to lock the mirror up), wait 10 seconds, then press it again to fire the shutter. The mirror lock causes a lot of vibration in the telescope, especially at prime focus.
- When the exposure is done, review it on the LCD screen on your camera. This is where having the LCD brightness turned down helps. Zoom in to the maximum setting and scroll around the photograph until you find a dim star in the background nearby the really bright one. Examine the picture, and determine if the focus looks a bit soft or not. The star will be round or blob shaped.
- Adjust focus as necessary, finishing the focusing in the counter-clockwise direction. If you focus past the focus point, back up and start again. Take pictures each time you think you have focus and zoom in and compare the same dim star. Once you think you have focus, we're ready to move to the next phase. I usually delete my focusing photographs as soon as I review them, so I don't confuse them with everything else.
Photographing- taking the picture (top)
- Slew the telescope using the GOTO function to your target. If you used a bright star nearby, the short slewing movement shouldn't disturb the focus on the telescope. Sometimes you can even faintly see the object through the camera's viewfinder (although it takes some neck craning if it's overhead!)
- Set the exposure time to 30 seconds, and take a photograph. Press the shutter release cord button so the mirror locks up, wait 10-15 seconds to let vibration die down, then press the shutter button again. Once done, review the composition of the object in your picture. Sometimes you can barely see it, sometimes not at all. The ISO should still be set to the maximum at this point, so you'll get the most out of the exposure time. Try to get the object as dead center as possible.
- With the object centered, synchronize your telescope hand controller on the object. If you changed hand paddle menus, re-enter the object (say, M13) but don't press GOTO, just hold the enter key until it synchronizes. This step is very important.
- Now comes the tricky part, specific to using the off axis guider. (Skip this step if you are doing this unguided). Looking through the reticle eyepiece (OFF), if there are no guide stars, loosen the threading and rotate the guiding eyepiece around until you see some guide stars. Sometimes you can nudge the scope a little to get them closer to the center.
- Re-tighten the OAG onto the telescope.
- Now we have to rotate the reticle eyepiece so the lines correspond to N/S and E/W movements. Turn on the reticle eyepiece.
- Using the reticle adjustment screws and/or the hand controller, put a star on one of the lines, and move the scope back and forth (E/W or N/S), adjusting the eyepiece by rotating it in the OAG, until the star stays on the line all the way across the field of view of the eyepiece. If the N/S is adjusted, E/W is as well (so you only need to align one direction). Tighten the lockdown screw for the OAG eyepiece slot.
- Given you've moved the composition of the shot to align the illuminated reticle eyepiece, re-enter the object on the hand controller, then press GOTO. The scope will return to it's synchronized position, which is with the object centered on the camera frame.
- Take another 30 second exposure at the maximum ISO using mirror lock to double-check the focus and composition. Readjust focus if necessary. Nudge the scope to get the guide stars within reach of the eyepiece.
- After each movement of the scope to put guide stars in the OAG, take another 30 second picture to verify the object of interest is still in the shot. If the framing is not satisfactory, find different guide stars and repeat the above 3 steps. (rotating the OAG changes the orientation of N/S and E/W in the eyepiece, thus the eyepiece needs to be re-rotated)
- Now that we are framed, have guide stars in the OAG and are focused, we're ready. Adjust the reticle crosshairs to line up on the guide star(s).
- Set the camera to the ISO speed you want to photograph at (if different than 1600/maximum). Also, set the camera to "bulb" if you want to do more than 30 second exposures. (I forget this step often, but am reminded 30 seconds into guiding that I forgot this step, when the shutter closes)
- Put your eye to the reticle eyepiece. Set the hand controller speed to "guide" or the slowest setting.
- Practice really quick which button presses move the guide star in what direction. Re-center the guide star on the crosshairs.
- Press the shutter release button. Wait 10-15 seconds for vibrations to die down.
- Given I have an inexpensive shutter release button with no lockdown feature, I use a clothespin to clamp down on the button, holding the shutter open. If you have a shutter release with a lock, depress the shutter and lock it open. Keep track of the time. The Meade LX200 hand controller has a running clock (press the "Mode" button a couple of times) which lets me time my exposures.
- Take as many photographs for as long as planned, being careful not to bump one's head on the eyepiece, telescope or finderscope while guiding. NOTE: for best results, manually guide continuously, looking up to rest your eye for only a half-second or second. A good polar (drift) alignment and PEC training will make this part easier. The star will move around on you, though. If you go the wrong way, try to correct as soon as possible. Even being off for a split second can show star trails on the photo. The exposure may be ruined.
- Another technique to overcome pattern noise of the camera is to dither. This means moving the telescope slightly in RA and declination between each exposure to randomize the pattern noise when stacking the exposures later on during image processing.
Dark Frames (top)
- Remove the camera from the telescope. Cover up the viewfinder, or set the camera on the ground with the viewfinder facing the ground. Careful not to step on it!
- Turn the mirror lockup feature OFF. It's just an annoyance when taking dark frames. Double check that the in-camera noise reduction is OFF-- we're trying to record the in-camera noise!
- Double check that the ISO setting is the same as what you plan the actual photos to be at (or leave it the same if you're doing the dark frames after the exposures).
- Take several dark frames at the same exposure times as your regular photos. If you did multiple exposure lengths (say, 10 images at 30 seconds, two at 4 minutes, and 2 at 10 minutes, even on different objects), take multiple dark frames for each exposure time. Use a minimum of 3. Have an odd number of exposures, since median-combining during image processing likes an odd number of choices. Ideally, more exposures are better, in order to average out random camera noise and get a true representation of the dark frame. The temperature when taking the dark frames should be within +/- 5 degrees of the temperature of the actual pictures. This is for the most accurate results possible. It is possible to sequence a few dark frames in between photos, but this runs the risk of knocking the focus out of whack with the camera being removed/replaced each time.
Image processing. We're done... well, not really. (top)
- Load the pictures from your camera into your computer. Make a separate directory/folder called "working" and copy all of the raw frames into this new directory/folder.
- Get yourself a copy of IRIS and install it if you haven't done it already.
- Read Jim Solomon's Astrohpotography Cookbook, specifically the image processing in IRIS sections. You now have all of the necessary photographs to do the processing per his instructions.
© 2006 Welsh House Productions