Tripod Astrophotography: Part 2 - The Andromeda Galaxy - Techniques For Locating Deep Sky Objects

In my previous blog post on astrophotography I describe the set up and procedures that I used to start taking images of the stars.  After some initial success in taking some single long exposure images and stacking some shorter exposure images of the Milky Way, I was ready for my next challenge, that being some deep sky objects.  After some research I decided upon M31 the Andromeda Galaxy.

Now I had literally looked for months for this object, with no success.  My research had indicated that the key was finding the constellation Casiopea.  Casiopea looks like an irregular "W", with one of the points deeper than the other.  If you use the deep "V" as a pointer, and follow that a bit (a fists length when held out in front of you) from the "V", you should be able to find it.  Well I tried this on many occasions with no luck.

After many failures, I tried another strategy.  I decided to take a wide field shot of the sky (at 50 mm) which included the constellation Casiopea in it.  In fact I took many shots (30 or so) and decided to stack the results.  I then post processed the image and cranked up the contrast.  Low and behold in the lower right of my image was a smudge.  I had found the Andromeda galaxy ! !

Now the picture below which is the first image that I found the galaxy in, isn't very good.  There are star streaks, and it is way over exposed.  But I wasn't going for pretty, I was going for identification.

Andromeda Galaxy with respect to the Constellation Casiopea

Closeup of Andromeda Galaxy (left) from previous image, vs a image at 50 mm unknown source on internet

With my new found knowledge of the approximate location in sky I decided to try again a few night later to get a better image.  Here is what I did.

• I took a long exposure image to locate the Andromeda galaxy
• centered the galaxy in the image
• changed focal length of lens and refocused the camera
• took another long exposure, found galaxy and proceeded to centered the galaxy
• Finally changed lenses, took long exposure image to find galaxy
• centered the galaxy
• began taking shorter exposure images for stacking

Here are some more details on some of the steps.

.  Initially I took a long exposure image (20-30 seconds) at 50 mm focal length (after focusing the camera and then putting focus to manual.)  I examined the image and found the smudge that represented Andromeda galaxy.  I did not care that the image might have star streaks.  I was just trying to find the object.

Initial image where the Andromeda galaxy is found (about 1/3 down, 1/3 from the right)

I then took several more long exposures trying to center the galaxy in the frame.  Centering Left/Right was obvious, but Up/Down I initially got backwards.  One thing that I learned is that to lower an object in the field of view, you need to tilt the camera higher on the tripod.

Andromeda is now nearly in the center of the image.  Time to increase the focal length of the lens.

Once the object  (Andromeda galaxy) was centered.  I change the focal length from 50 mm to 135 mm.  At this point I took the camera off the tripod BUT LEFT THE TRIPOD POINTING AT THE SAME SPOT IN THE SKY (My tripod has a snap clamp so that the camera can be loaded/unloaded with the flip of a lever).

I then turned the auto-focus back on, an focused the camera at 135 mm.  I said this in my previous astrophotography blog post but it is worth repeating.  FOCUS IS CRITICAL ! ! !  It is the single most important thing you can do to have a successful photo shoot !

In my case to focus, I used the moon.  The moon is both the best and worst object in astrophotography.   It's the best object for focusing your camera, but the worst object if it is in the part of the sky you want to photograph.  If the moon is not available then you are forced to find a bright light source in the distance (and make sure it is far enough away) to focus on.  You might also be lucky enough to use a bright star as your focus object.  The later may be hit or miss.

Once you have achieved focus (no small deal if the moon is not available) turn the auto-focus off (manual focus), AND KEEP YOUR HAND AWAY FROM THE LENS AND FOCUS RING.   Put the camera back on the tripod.  Hopefully, the above step did not take too long.  Take another long exposure image. Find the object of interest again.  It hopefully has not drifted out of the field of view (if it has then go back to the beginning and try again).  You now need to center the object using the same techniques as before.  I actually do not center the object but put it such that it is about 1/3 from the edge. and will drift toward the center and will eventually move to the other side approx 1/3 from the edge.  I try to keep the object in the middle 1/3 of the images when I am actually taking the "Light" frames, and then readjust the camera.

Andromeda at 135 mm, 20 second exposure.  Note the star "streaks" at this exposure time

Once you have the object in the proper location.  Change your camera settings to use the proper shutter speed using the "RULE OF 600".  For my camera (a Canon 60D) everything is multiplied by 1.6 due to some "cropping" done internally by the camera so in my case it is the "RULE OF 375".

I take my focal length (in this case 135) and divide it into 375.  So 375/135 = 2.7777 seconds.  The is the maximum exposure time I can have before (theoretically) stars begin to streak because of the movement of the earth.  On the Canon there is a shutter setting for 2.5 seconds and another for 2.0 seconds.  I believe to be safe, I chose the former (2.5 seconds).  Another thing to do is make sure your camera is set to record both jpeg and RAW images.  Finally if you have a cheap tripod like I do, you may want to make one more modification.  Something in the "Custom Function" menus.

Go to menu and scroll over to the custom function tab.  Choose C.Fn III:  Autofocus/Drive and press the "set" button.  Then use the left right ring outside the set button to advance to "5" "Mirror lockup"  .  It by default is on 0: Disable.  Press the set button and then the down on the ring surrounding the set button to change it to "1:enabled".  Then press the menu button until you have exited out of the menu.

Mirror Lockup Menu on the Canon 60D

Set Button, thumb wheel and "toggle" buttons on Canon 60D

What this will do in combination with a 2 second delay from your IR remote will be that when the remote button is pressed, the mirror inside the camera with flip up out of the way to expose the sensor to the lens. Then a two second delay will occur (hopefully settling the camera so there is no "shake" and finally after 2 seconds the shutter will open and close.  This particular setting helps for those of use who have marginal tripod equipment.

Here were my settings for this astrophotography session.
ISO 1600:  2.5 seconds, f5.6, 135 mm focal length

Now that everything is all set, use your remote and start taking your light frames.  I like to cover the lens (but not touch it ) to take a blank frame marking the start and end of good images (as we will see in a bit).  So start taking your light frames.  After about 20 to 25 frames, I would suggest seeing where the object is in your field of view.  If it has drifted out of the center 1/3rd of the frame, my recommendation would be to realign the camera and have the object back where it started for frame #1.  To do this I take a black frame (cover lens but don't touch it) then change my exposure to 10 to 20 seconds and take a picture.  Adjust the pointing of the camera, take another picture, etc etc.  Then take a black frame.  Finally start taking "light" frames again.

A Single Light frame At 135 mm, 2.5 second exposure.  Yes there is data there, it's just very faint.

Repeat this process every 20 to 25 images, readjusting the pointing of the camera as necessary.  When you are finished taking you light frames, do not forget to take your "dark frames" and your "bias frames" as explained in my "Tripod Astrophotography Part 1" blog post.

When you are finished then it becomes time to stack the image.  Before you stack, you should manually go through the images and remove any "light" frames where the camera obviously moved and caused the stars to streak.  I also remove any frames where clouds or airplanes are in the image.  With my rudimentary setup, this can be as much as 50% of the original "light" frames that I took (remember the more light frames you have the better in theory you final image will be).  Then stack the images in DeepSkyStacker and post process with an image processing software package of your own choice (I use GIMP but not well, I may try IPhoto at some point).

Here is a the non cropped version of my stacked images

Here is the cropped version

Andromeda Galaxy at 135 mm.  Messier Object 110 (M110) is shown with the arrow

Satisfied with the results I tried again with a 70-300 mm lens, but this time I used a 200 mm focal length and 2.0 second exposure.  It turns out that 2.0 seconds was probably too long an exposure at this focal length as I did get some star trails.

Andromeda Galaxy at 200 mm without tracking

Here is the cropped version of the Andromeda galaxy (show the one that has the other messier objects), and also the comparison with an internet image.

Andromeda Galaxy at 200 mm without tracking.  M110 (top) and M32 (bottom) are indicated by the arrows.

Here were my settings for this session.
ISO 2500:  1.6 seconds, f5.0, 200 mm

Though the third attempt was better than the second attempt I think I can do better.  I need to do the following to improve my final images.

• The proper shutter speed.  I think I would use 1.3 seconds at 200 mm focal length instead of 1.6.
• More images.  I needed to throw 1/2 the images away because of camera jitter.  Then DeepSkyStacker threw more out.  I was left with only 68 images.  Next time I need to take 200 to 250 images so that I may have the chance of  have 125 of those frames "good enough" for stacking.
• Darker skies (I need to try to take some images not from my back yard).  The darker the sky's the better the final image will be
• Make sure the camera is focused.

I will post some update of my next attempt at the Andromeda galaxy.

Next deep sky object up for me....The Great Orion Nebula !