Sunday, August 10, 2014
Getting Nebulized
Howsomeever, before we put the dadgummed cart before the
horse, we ought to decide if you even want a DSLR. Or, if you already have one, whether you want to use it for astronomy...
What’s a DSLR good for astronomy-wise? One can take nice
wide-field images of the Moon. Cameras with video can make reasonable close-ups
of Luna—and the planets—via frame stacking with Registax. The ground truth, however, is that for the planets a 300
dollar Solar System cam with a small imaging ship, many tiny pixels, and a high frame
rate—like the ZWOs—will walk all over the
most expensive Canon or Nikon. What a DSLR is mostly good for is deep sky prime focus astrophotography,
just like its ancestor, the (film) SLR.
Who is a DSLR not
for? Folks who are determined to do science with a capital “S” with their
telescopes and cameras. A traditional monochrome CCD camera is more effective
for that. One is much easier to calibrate. If’n you don’t know what that means,
don’t worry about it. A DSLR is likely just the thing for you.
Another astrophotography faction the DSLR is not for is
those folks going after the dimmest of the dim. You know, the characters whose
16-hour exposures show up in my buddy Sean’s “Gallery” feature in Sky and Telescope. DSLRs use color CMOS
imaging chips, which will never be as sensitive as monochrome CCD chips (nor
deliver as much resolution).
Once you decide you want to proceed with prime focus deep
sky imaging with a DSLR, that is just the start of the questions. The next one,
of course, is which DSLR? Take a
stroll through the website of Unk’s favorite camera store, B&H Photo, and you will find 322 different DSLRs—or at least
different configurations of DSLRs. Canons…Nikons…Pentaxes…Olympuses… and on and
on and on. What’s an astrophotography crazy boy or girl to do?
Do you want the safe choice? I’ll give it to you: C-a-n-o-n. There are several reasons for
that. First off, while other manufacturers have narrowed the gap considerably,
the Canon cameras are still the most noise-free for long exposure imaging, if
only by a hair. Secondly, if, like Unk, you want to run your camera with
software on a laptop—operate your DSLR as if it were a “real” CCD camera, that
is—there’s more software support for Canon than any other brand. Finally, Canon
is the only maker, as far as I know, who acknowledges their cameras are used
for astro-imaging and—get this—actually
offers a camera designed for astrophotography, the Canon 60Da.
Yep, Canon is the safe bet, but not the only good bet these days. Plenty, and I do mean plenty, of good shots are being taken
with other brands, especially Nikon. If you are already invested in lenses from
another system, if you have plenty of Nikkor (or whatever) lenses left over
from the film days, you’d be silly not to get a Nikon DSLR body when
transitioning to digital.
Most folks serious about photography went digital years ago,
however. What if you went digital with Nikon or Olympus or Pentax? Should you
consider buying a Canon just for astronomical use? Probably not. Try what you have;
you may be happy with it. While, as above, there’s more software for Canon than
any other brand, that’s changing, if slowly. The folks who do the much-loved Backyard EOS
program, for example, are working on a version for Nikons, BackyardNikon.
 |
| Sony's mirrorless A7... |
Mirrorless cameras have the advantages of lighter weight and
no vibration-causing mirror slap when the shutter is fired—there’s no mirror
that needs to get out of the way. Frankly, these cameras may be the wave of the
future for astro-imaging. Most of us aren't using our cameras’ optical
viewfinders for astrophotography, anyway. For terrestrial work? I ain’t
convinced I’d want to give up a DSLR’s non electronic viewfinder for that. Not
yet, anyhow.
Oh, in case you’re wondering, no, your point and shoot camera won’t work well for deep sky astrophotography.
Its lens is not removable, so you will be restricted to afocal shooting (point
lens into eyepiece). That yields small, vignetted fields and is just too
limiting. With a perfectly serviceable Canon T3 available brand new for an
amazing 299 freaking bucks (with a kit lens!), there’s just no reason to fool
around with point and shoot cams anymore.
What’s the next thing astrophotography greenhorns axe me,
sometimes even before they've purchased a camera? “Unk, does my DSLR need to be
MODIFIED?” With the exception of the Canon 60Da, all DSLRs come with strong IR
filters over their imaging chips. Imaging chips are very sensitive in the
red/IR part of the spectrum. Too sensitive for terrestrial picture taking.
Without the built-in filter, your Aunt Lulu and all the guests at her birthday
bash will look as pink as cherubs. The built in filters fix that, making it
easy for the camera to deliver the proper colors.
Unfortunately, for the astro-imager, the dim diffuse nebulae
many folks want to photograph radiate strongly in the red and the built in
filters make it hard to get good exposures of ‘em. So they have their cameras modified. Which means
they have the built in filters removed, or removed and replaced with a more
nebula-friendly filter.
 |
| 30-second stack of M57... |
Star clusters, galaxies, and planetary nebulae are helped
much less by filter removal (if they are helped at all) than emission nebulae.
Keep in mind, too, that an unmodified DSLR will still be able to image the
Horsehead, for example. You may have to expose longer and process more
carefully but you can still get the shot.
Even if you want to photograph nothing but emission nebulae,
the answer may not be getting your
camera modified, but buying Canon’s 60Da. It is not quite as red sensitive as
some modified cameras, but it is much easier to use on non-astro subjects. See
Alan Dyer’s excellent review in the September 2012 issue of Sky and Telescope. That article contains
a rather eye-opening series of comparison shots of the North America Nebula done
with the Canon 60Da, a standard Canon 7D, and a modified Canon 5D MKIII. You
may be shocked at how little difference there really is, especially given what
you’ve prob’ly read about the subject on the pea-picking Internet.
Alrighty then. Let’s say you’ve got a camera. That, of
course, is only the start of what you will need to get going in whole hog fashion.
If you choose to guide your telescope, to make small corrections to its aim so
stars will be nice and round in long exposures, you’ll need a guide camera, a
guide scope, cables, and more. You can read all about that stuff here, but I suggest you NOT guide. Not at first.
Eliminating autoguiding from the astrophotography equation
in the beginning will go a long way toward preserving your interest in imaging—and
your hairline. Many mounts, even the plebeian Celestron CG5 and Meade LXD75 can
track more than adequately enough to deliver good stars in 30-second exposures.
And current DSLRs, most of which will allow you to use ISOs of 3200 and 6400,
can bring home a lot in brief exposures. Stack 20 or 30 thirty-seconders, and
you will be amazed at how good your resulting picture looks.
 |
| 30-second stack (cropped) of M13... |
In order to expose for longer than 30-seconds, you need a
remote shutter release, but you can do 30-second exposures by just pushing the
shutter button. You probably won’t want
to do that, though. Pushing the button will shake scope and camera, ruining
your shot before you begin.
The solution is a remote shutter release/intervalometer,
which won’t just allow you to release the shutter; it will also allow you to
program in a series of exposures. Set it up to take 20 shots, mash the button, and
walk around the field annoying your friends while the camera does the work.
After you FOCUS.
That’s where the intervalometer becomes less appealing. Yes, most current DSLRs
feature Live View. You can focus on an image on the camera’s little electronic screen
rather than having to squint through the optical viewfinder. That’s doable, but
you still need good eyes, much better than my poor old peepers. Even if you’ve
got 20-20, you may not find it simple to achieve sharp focus on the small
display. That, my friends, is one of the things that make Nebulosity better.
Nebulosity 3 by Stark Labs, available for
Windows and Macintosh, is a program with two personalities. First, it’s a
program that controls your camera. You can set exposures, exposure sequences,
and other things from the computer. If you’ve a modern DSLR, all you have to do
is connect the camera to your PC with a USB cable, just like when you are
downloading pictures. Once your Canon is hooked to the laptop (the Canons are
the only DSLRs the program supports) you get to use the big screen of the
computer for focusing. This is a very cool way to work, as terrestrial
photographers have also discovered (they call using a PC with the DSLR
“tethering”).
You can focus in Nebulosity
by either using your camera’s Live View mode or by having the camera take
continuous short exposures. Why would you not want to use Live View? Some Canons don’t have Live View, and even if your camera does, you may find that
mode not sensitive enough to allow focusing on dimmer stars. In my experience, it’s more than good enough
for rough focusing in Nebulosity’s
“frame and focus” mode, however.
Even more amazing is the program’s fine focus mode. Once you’ve got good rough focus on a bright star,
stop the exposures with the “Abort” button, engage fine focus mode with a click,
and select a dimmer, unsaturated star in the field. The program will
automatically zoom in on the star, and you can focus precisely by observing the
size of the star, a graph of its intensity, a max intensity figure, and a “half
flux radius” number. I used to use a Bahtinov mask for focusing, but find I
don’t need it anymore given the program’s wonderful fine focus mode.
OK, you’re focused and
framed and ready to shoot a purty pitcher. How do you do it? How do you get going
with unguided prime focus imaging? Here’s what I do, y’all…
 |
| Neb's Control Center... |
Do you need an equatorial mount? No. At 30-seconds, the
“field rotation” that will cause stars to trail with a driven alt-az mount no
matter how well it tracks is not much of a factor. Since an alt-azimuth mount has to
track precisely in two axes rather than one, however, don’t expect results as
good as with an equatorial.
Scope aligned, I remove the diagonal and eyepiece, and
replace ‘em with the camera. This is actually the second time the camera has been on the scope. Before alignment,
always mount the camera and balance the scope. With smaller, cheaper mounts
balance is purty critical (slightly east heavy is what you want), and balancing
with the camera onboard is mandatory.
I then connect the Canon to the laptop, turn on the camera,
and start Nebulosity 3. Once it’s
running, I select my camera type, set an ISO value (if the sky is reasonably
dark, I like to shoot at 3200) and begin framing and rough focus. The scope
should still be pointed at my last alignment star, and that is what I use. When
the bright star is as small as I can get it, I will begin seeing dimmer field
stars. As above, I click on one and begin the fine focus process. If the star
is too dim to show up well in Live View mode during fine focusing, I’ll go to the
multiple shutter firing mode and increase exposure till a good focus star is
easy to see.
After that? Ain't a whole lot to it, y’all. I set the
exposure (duration) to 30-seconds and usually try a “Preview.” Mash that button
and Neb will take one frame. Don’t freak out too much when the shot appears.
You will be zoomed in purty tight and the stars may look blobby. Reduce the
zoom factor (upper right porting of the screen) to get a better idea of what
you’ve got. Also, don’t be too concerned if the sky background is a funky shade
of brown because you are shooting in light polluted environs. You’ll be able to
fix that easily when you process your final images with Neb. If you’re
satisfied with the Preview frame, select the number of exposures (20 or 30
usually), and hit the Capture Series button.
When the exposure sequence is complete, the program will
play a little fanfare and you can repeat as needed on the rest of the evening’s
targets. Keep going till you don’t want to go no more, disconnect the camera
(in Nebulosity), turn it off, throw
the Big Switch on your scope, and head home for some Rebel Yell or whichever libation suits your fancy.
Next morning, it’s time to process your pix. That's the other side of Neb. It's pretty powerful when it comes to image processing. You can do that
simply, like Unk, or get purty complicated—Nebulosity
places plenty of tools at your disposal. Mr. Craig Stark, the program’s author,
has an excellent tutorial posted, but I can
give you the basics rat cheer.
 |
| Frame and Focus Mode... |
When the subframes, the individual 30-seond exposures of my
target, are loaded into Nebulosity, I
stack ‘em. The stacking feature is another of Neb’s strengths. It has never
failed me, and is easier to use and has yielded better results than Deep Sky Stacker, for example. To stack,
click a non-saturated star and Nebulosity will show the next sub with that star
circled. You can click it again and proceed to the following frame if
everything is Jake, or you can just tell Neb to stack ‘em all automatically.
When it’s done, you’ll have a single image composed of all
those subframes, which you can save as a common file type (.bmp, .jpg, etc.).
You will probably want to do some processing first, however. Since I almost
always have to shoot from light pollution, the first tool I go to is Adjust
Color Background, which easily dispenses with that murky brown sky. From there,
it’s up to you. In addition to noise reduction functions, Neb includes a
“digital development” routine that almost
automates processing. Yes, I’ll probably run my finished pictures through Photoshop (or Adobe Light Room), but Nebulosity
3 is, frankly, feature-laden enough that I could really get away without
doing that.
Nebulosity is a
powerful and stable program. That’s good, but what does your old celestial
snapshooter of an Uncle really like
about it, muchachos? It is simple to use. So simple that even after laying off
the DSLRing for months due to weather, I still remember how to work the thing.
Oh, and the price is crazy
reasonable: $80.00. Typo? Nix, that ain’t no
typo, Skeezix: “eighty bucks…eighty simoleons…eighty dineros.” Can’t beat that with a stick,
now can you?
Next Time: The Next
Observing Project…
Comments:
<< Home
Rod - Now that now Nikon has removed the low pass filter, and that they has a better DR, and the fact that cameras like the D7100 have a built in intervalometer making it quite handy. I find with a good polar alignment my old LXD75 and C8/FR give very good stars. Even from my house I can use 1600/3200 with out getting over exposed.
The 13 x 20 native file size gives plenty of room for cropping, so I continue to use the FR and still get decent object size.
Have not been to a star party for several years due to Parent issues, but I did some astro photography a few weeks ago with my D7000 & D7100. As I wrote you I was thinking of get a new mount, but for what I'm doing the LXD75 does work well (I can see Polaris from my yard, so alignment is pretty easy). With my limited activity, think I'll wait until the 75 dies.
This week's post is a great one to get people to use what they have and not worry about the next best thing.
The 13 x 20 native file size gives plenty of room for cropping, so I continue to use the FR and still get decent object size.
Have not been to a star party for several years due to Parent issues, but I did some astro photography a few weeks ago with my D7000 & D7100. As I wrote you I was thinking of get a new mount, but for what I'm doing the LXD75 does work well (I can see Polaris from my yard, so alignment is pretty easy). With my limited activity, think I'll wait until the 75 dies.
This week's post is a great one to get people to use what they have and not worry about the next best thing.
Forgot to mention for those of us who are "computer free" when using our scopes, a Bahtinov Focus Masks and the current crop of cameras LCD screen make focusing very easy.
one small point, older nikon lenses work on canon slr's with after market adaptors quite nicely in manual mode. Howard
Post a Comment
# posted by 
: 1:54 PM
: 1:54 PM << Home
