Here is the procedure I go through for astrophotography of deep sky
objects with a digital SLR. It will be different for different
equipment and situations, but it’s still a basic template of a plan.
Before I go out, I find out basic information for the observing site.
I find latitude and longitude as accurately as possible, and the
magnetic declination (how far true north is from magnetic north) so I
can use a compass to find north. This information can come from
topographic maps, such as at this site. Also, I go to www.time.gov to make
sure my watch is set right, down to the second. You might also want
to consult star maps to see what bright stars are visible at darkness
(for that matter, check sunset time too). Web sites that do this include
US Naval Observatory for sunset and astronomical twilight information, and
Fourmilab for star maps.
You will need to be able to recognize a few bright stars for alignment–at
least two, but have a few backups as well. They should not be too close to
the horizon at the time you align, or they may be lost in the trees, and
should not be too close to the zenith, or the scope can’t reach them. It is
best if they are moderately far apart in the sky.
I have a Celestron C6-R goto doublet refractor scope (6in aperture,
1200mm focal length, f/8) on a CG-5 mount. The first thing I do is
set up the tripod and mount, preferably when it is still day time. I
make sure it points north (I use a compass), and then level it–the
mount has a built-in bubble level. I try to make it as accurately
level as possible, as slight deviations will mess up tracking.
Then, I make finer adjustments to the mount to make sure it is
pointing north–I actually loosen the mount a little so I can move it
clockwise or counterclockwise without unleveling the tripod, and then
tighten it back up (don’t forget that step!). Again, I use a compass,
but this time, I make sure to adjust for “magnetic declination”. The
compass I use is this one. It has a mirror to enhance accurate direction finding.
For example, I was recently at Alpha Ridge Park near Marriottesville,
MD. The coordinates are 39 19 13 N and 76 55 02 W, with magnetic
declination of 11 degrees West. Sine it is West, I rotate the bezel
of the compass 11 degrees counterclockwise, so that the “forward”
marker on the case points to 11 degrees “East” (backward from what you
might think–you want “north” on the bezel to be 11 degrees west of
Then, I site exactly along the latitude-adjustment bolt of the mount,
and hold the compass between my eye and that bolt. I have the case
open about 15 degrees and the bottom of the case (where the compass
is) as level as I can get it. I then rotate the case till I see in
the mirror that the needle is exactly on the north indicator of the
twistable bezel, and at the same time, the marker on the case closest
to me is exactly on the center line (again in the mirror). This
prevents errors due to parallax. Instructions on how to use the
compass mirror are included with the compass.
From this, I see how far off the mount is from where I want it to be,
so I loosen it, move it, and tighten it. Then I repeat the compass
check. I keep doing this (patience is required to get it right!)
until I have it aimed to the north.
Finally, I finish putting the scope together and wait for darkness. I
do a final check–sight with compass–is it exactly north? check the
bubble level–did I knock it out of level doing all this?
When it is dark enough, I align the goto mount. Even with all I’ve
done, there are still some errors in the mount to deal with. With the
CG-5 goto mount, you turn it on and it asks you to align. You hit
“undo” because you want to align to take photos (hit Enter if you
don’t, say you’re using the scope to starhop but want to be able to
nudge it electrically or something). You set the scope to index
position (there are arrows on the mount to show you where this is),
then enter longitude, latitude, and current time, but enter it a few
seconds after what it actually is, then wait for the time to catch up
with what you entered, THEN press ENTER.
Next, it will ask your alignment method–select two-star alignment.
Then, select from the list of stars (using the “rate” or “menu” key to
switch between eastern and western sky stars) one that you recognize.
It will slew to that star, give or take a couple degrees. Use the
arrow buttons on the hand controller to center it (a Telrad makes this
easier). Use the Telrad first, if you have one, then the finder if
you have a separate finder, and then finally the eyepiece. Get it as
exactly centered as you can. You hit Enter, then it will switch to a
slower fine-adjustment rate and you can make sure the centering is
precise, then hit “align”. Now, repeat for a second star.
Next, I select “Polar Align” from the menu. It slews to Polaris, and I adjust the mount itself, NOT the arrow buttons, using the latitude bolt and by loosening, moving, and retightening the mount. I use the Telrad first, then the finder, then the eyepiece. When this is done, I do the two alignment stars again. This results in better tracking.
I usually skip the “add calib stars” at first. As it darkens, you can
do some visual observation through the eyepiece. When you use the
hand paddle to goto an object, if you know what it is supposed to look
like, you can use the arrow buttons to center the object precisely,
then hit undo as many times as needed to get out to the main menu, and
hit “align”. Use up/down (not the arrows, the words “up” and “down”)
to select “add calib star”, and follow the instructions to add this
object (it doesn’t really have to be a star) as a calib star. You can
easily add the four calibration stars this way.
Now, it’s fully dark and you want to photograph something. Choose an
object a bit above the horizon to reduce distortion and skyglow. If
the object is on your hand paddle’s list, just goto it, adjust the
arrow keys to center it exactly, and add it as a calib star (it will
replace something already added). If not, find an object close to it
and do the same. It tracks better if one calib star is close to what you are tracking.
Here is where I take the lens off my Canon Digital Rebel XTi, a
digital SLR. I replace it with a T-adapter made for the Digital
Rebel, attached to a 2in tube threaded for the T-adapter at one end,
and made to fit the 2in focuser. Both are available from Celestron.
I then put the camera body onto the telescope, and rotate it to the
orientation I want.
Now, I focus. I look through the viewfinder and try to focus on the
object. If the object is too faint, I just goto a bright star. In
any case, I get as good a focus as possible by eye, then adjust the
diopter for the camera’s viewfinder (I’ve found this a necessary step
for good focus), and refocus as good as possible by eye. I take my
time on this, as an accurate focus makes a huge difference.
Finally, I center the scope on the object I want to photograph, using
the arrow keys to make fine adjustments so the field of view, as seen
in the camera’s viewfinder, is what I want.
Now, I crank the ISO speed all the way to 1600, make sure automatic
noise correction is on, and take a test photo, 10 seconds for brighter
objects, 30 seconds for dimmer objects. I adjust as needed.
Finally, I set the timer for 30 seconds (10 seconds if the object is
so bright it overexposes, but that’s rare for deep sky) and use a
remote switch to lock the camera into repeated exposures, and let it
take pictures for a while, at least 10 pictures, sometimes more. It
is very important not to touch the scope or camera or do anything to
vibrate it–that will cause streaks in the image.
After all photography is done, I use ImagePlus to stack the photos. I
basically do “automatic file processing” using “sigma clipped average
(s=1.0)” for the stacking algorithm. It is here that I see that even with all I did, there are slight errors in tracking. Then, I use “digital
development” under the “color” menu, and adjust the settings to bring
out a bright image with as much contrast on the features I want to see
as I can get. Depending on the situation, I may use other ImagePlus
features, like color correction (turning the yellow cast to a bluer
one), halo reduction, and maybe others. I then save a copy as .jpg
and exit. Sometimes I finish with Photoshop, to crop, rotate, reduce
skyglow even more, adjust contrast and color, etc.
Here are some
of my results. Note sometimes instead of photographing through
the scope, I put the camera on top of the scope (piggyback) with a
lens to take wider-field photos. Otherwise, the procedure is the