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Hey there! I'm Dave Hearn and I'm the Director of Kissimmee Park Observatory. In this Star
Hopping "Extra", we're going to continue on our Basic Astrophotography series, and show
you how to capture the Night Sky with your camera, by mounting the camera on the prime
focus of your telescope. More about all this great stuff right after this - stay tuned.
Hey Hello Hi and welcome to Episode 26 of Star Hopping with Kissimmee Park Observatory!
I'm Dave Hearn, and I'm just beside myself to be your host. (duplicate Dave, look at
each other, then back at the camera again surprised). In this series of programs we'll
show you the most beautiful sights in the night sky, and explain exactly how to find
them with your binoculars or telescope.
So tonight we’re going to do yet another episode in our new Topic area of Basic Astrophotography,
and continue to discuss methods to capture the night sky with your camera. This installment
will complete the initial set of basic astrophotography topics we started a few weeks ago.
In the previous two Astrophotography episodes we discussed the three basic methods for taking
an astrophoto, going from the most simple to the most complex. As a reminder, they are:
Time Exposures of the night sky using a fixed tripod, which lets you create star trails
or time lapse movies. We discussed this method in the first installment.
Mounting your camera on a mount or telescope that is tracking the stars, otherwise know
as “Piggybacking”, which lets you take images of large deep sky objects, and we covered
that topic two weeks ago. Mounting your camera on the focus of a telescope,
thereby using the telescope as a long telephoto lens. This allows you to capture smaller objects,
and get closeup images of deep sky objects, and that’s this week’s topic. There’s
a lot of details that we covered in the Piggybacking episode that I’m assuming you know for this
installment, so it may be a good idea to go back and review that episode before taking
in all the info we’ll cover tonight - you can see the episode notes and review the video
on our blog at kpobservatory.org/SH024.
So mounting your camera at the Prime Focus of a telescope means you are essentially using
the telescope’s optics as a long telephoto lens, which gives you much more effective
magnification than using a regular telephoto lens attached to your camera as normal.
The Prime Focus is the first focus of the telescope, essentially where the light waves
first converge to form an image. When you place your camera at that point, the image
focuses on the CCD of the camera, and allows you to capture it digitally.
Normally this is accomplished by mounting the camera to the telescope’s focuser, then
you use the focuser knobs and the camera viewfinder or LCD to focus the image. One special piece
of equipment that is needed to mate the camera to the focuser is the Prime Focus Adapter.
This comes in two parts. First, the T-Ring for your brand of DSLR, which provides on
one side the bayonet mount for your camera, and on the other is a common thread. The second
part is the nosepiece that threads into the T-Ring. The nosepiece has the same diameter
as the barrel of your focuser. That can be an inch and a quarter or two inch diameter.
So once these components are screwed together, and attached to your camera, you can slide
the camera into your scope’s focuser, and lock it down with the thumbscrews. Make sure
they are good and tight so you don’t have any movement.
It’s important that the prime focus of the scope can be accessed by the focuser. The
distance from the front of the DSLR camera to the surface of the CCD is an inch or so,
and to focus the camera with the telescope, the focuser needs to be able to be racked
in far enough to allow the focus to be positioned on the CCD. Otherwise you won’t be able
to focus the image. Most commercial telescopes are set up to allow prime focus astrophotography
to occur. There’s lots more details to work through if your camera doesn’t focus, but
that’s beyond the scope of this basic tutorial. If you have that situation, contact me, and
we’ll figure it out together.
Now if all you would like to do at this point is to take images of the Moon or the planets,
then you’re ready to go and don’t need any other equipment, because the exposure
times for the Moon and Planets are generally very short, and you don’t have to guide.
But if you want to do closeup imaging of deep sky objects with long exposures, there’s
quite a bit more effort and equipment required, similar to what we learned a couple weeks
ago in the Piggybacking tutorial.
When we learned about using your camera for piggyback astrophotos, we used the main telescope
for tracking a guide star with an illuminated reticle eyepiece. But now you have the camera
attached where that guiding eyepiece normally goes, so how will we do our required guiding?
Well there are two methods, and they require completely different setups and equipment,
and there are advantages and disadvantages of each.
The first option is to mount an Off-Axis Guider between your camera and the focuser. The second
method is to use a separate guide scope mounted on top of your telescope.
For the first option, an Off-Axis Guider is a piece of equipment mounted between your
camera and focuser. This device uses a small prism to intercept a tiny bit of the light
passing through the focuser tube on its way to the camera’s CCD, and then sends that
intercepted light 90 degrees out the side to an eyepiece, normally fitted with an illuminated
reticle as we discussed before.
The great advantage to this setup is that you don’t need to buy and mount a guidescope,
so it’s generally a less expensive route. It also eliminates any concern about the guidescope
shifting during an exposure. I’ll talk more about that in the next option.
The disadvantage is that it places your guiding eyepiece very close to the camera, and in
parallel with the telescope tube, so it is difficult to get your eye in there to actually
see a guide star. Comfort is extremely important when doing manual guiding, and the strange
angles that this arrangement causes can be challenging.
It’s also sometimes very difficult to locate a guide star in this configuration. Off-Axis
Guiders do have the ability to move the prism or the guiding eyepiece around to try to find
a guide star, but at the high magnifications that we use with guiding eyepieces, sometimes
it’s downright impossible to get a guide star, unless you’re shooting into a dense
star field.
So the second method is to use a Guide scope, generally mounted on top of your main telescope.
To accomplish this you need to select a guide scope, which must have a long enough focal
length to provide high enough magnifications to guide with. The scope is usually mounted
with rings attached to the main scope of telescope mount. Here is the configuration I use with
the KPO Reflector. In my case I use a Meade ETX-125 compound telescope. This type of scope
uses a folded light path to allow a very long focal length in a small package.
So here are the advantages of using a separate guide scope. First, using a guide scope places
the guiding eyepiece in a position easy to access, for comfortable manual guiding. Secondly,
it’s much easier to find guide stars with a separate guidescope. The guide scope does
not necessarily need to be aligned with the main optics; it can be adjusted or pointed
by adjusting the screws on the rings until you find a suitable guide star.
The disadvantages are that the addition of the guidescope requires some additional counterweights
to balance the new system, so the scope is heavier and it might tax the mount if it’s
too small. So you might need a bigger telescope mount. Additionally, if the guide scope is
not mounted securely, it might flex during an exposure and cause you to make guiding
errors that will show up in the images. And of course you have the added cost of buying
the guidescope and ring mounting system.
So which solution is best? Personally I have always gone with the guide scope option. I
have tried off axis guiders, but the challenges with the eyepiece angle and the difficulty
in finding a guide star have proven too challenging to get repeated good results. I have always
used good strong and reliable ring mounts for the guide scopes, so flexure really has
not been too much of a problem. So that’s my personal recommendation. And it looks awesome
to have that guide scope mounted on the main scope!
So now that we have all the equipment sorted out, let’s talk about actually capturing
images. Going forward from here, I am assuming you have your telescope mount aligned to the
pole and have the clock drive running, as we discussed in the last astrophotography
episode.
Going back to Lunar and Planetary imaging, as I mentioned, this is the easiest effort
because of the short exposures that are required. To shoot the full moon in a reflecting telescope,
you can set your camera on Auto exposure mode, and you will find that the exposure time will
be near 1/1000 of a second. You can play with your camera’s ISO settings to get the exposure
short enough. Planets take a bit more exposure, but you never really have to go more than
a few seconds. Trial and error works best here. With planets though, at prime focus,
the image scale will not be very big, so the disc of the planet will be small in your image.
Its big enough to see the rings of Saturn or the moons and cloud bands of Jupiter. To
get the moons of Jupiter you’ll need a longer exposure, which will totally overexpose the
disc of the planet.
But for Deep Sky imaging, you’ll need to take many subframes as we described last time,
then stack them in software later. The process is exactly the same as we discussed for Piggybacking,
which is: Number 1: Frame up your image in your camera.
For prime focus, you can use a brighter star. Make sure you have a sharp focus. It’s best
to focus on a bright star somewhere else, then move to your target, then tighten your
focus on a fainter star. Here’s a great tip: if your DSLR has the ability to do a
zoom on the LCD to 5X or 10X, do that and focus the star. This will make sure you have
tack sharp star images. Two: Find your guide star in the guidescope
or off-axis guider. As we mentioned, you might have to adjust the guidescope with the adjustment
screws on the mounting rings. Three: Align your illuminated reticle eyepiece
to the motion of the mount, so when you make adjustments the motion follows the crosshairs.
Four: Start your exposure series using your intervalometer or computer. Make adjustments
using your scope’s motor pushbuttons or adjustment knobs, as you see the guide star
drift in the illuminated reticle eyepiece. And Five: Pick your subframe exposure lengths
and do as many exposures as you feel necessary. I usually try for at least an hour of total
integration time, which as we discussed last time, is the sum of the lengths of all the
subframes you take. Remember, if some badness occurs during any
subframe, you can kill it, and get started on a new one. You can just throw away the
bad image during the stacking process.
So with this setup, you can capture closeups of any deep sky object, and this is what you
need to do to capture most galaxies, planetary nebulae, and get the best images of globular
clusters. I have to tell you though: this is the most difficult type of astrophotography,
because since the magnification is higher, it is easier to see any drift - the guiding
must be done with as much precision as you can achieve. If you have drift, you end up
with images like this, which is not what you want! But if your tracking is good, after
processing, you can end up with amazing images, like these from KPO; galaxies, nebulae, and
clusters all await your camera.
So now we’ve talked about all the basic kinds of astrophotography that you can accomplish.
For prime focus imaging, which was tonight’s topic, we first discussed the equipment needed
to mount your camera to the scope. Then we talked about the pros and cons of using off-axis
guiders vs. mounted guidescopes. Next was basic lunar and planetary imaging, then we
reviewed the process for deep sky imaging at the prime focus.
So now you have lots of beautifully guided subframes. In a future episode, I’ll discuss
basic image stacking and processing, which is where these images bloom into the gorgeous
astrophotos that you see from accomplished astrophotographers across the Internet.
I want to do a quick shout-out and thank you to Mark Gallup, who became KPO’s first contributor
on our Patreon site. We really appreciate your support, Mark; it really validates our
work here - thank you for your help.
For our supporters on Patreon, we provide the scripts of every episode, free digital
images of our astrophotos, and other exclusive content. For more information on our Patreon,
check out our episode trailer coming up in a second, and you can also find the link to
the site in the show notes.
So that does it for this week’s show. You can find the show notes on our website at
kpobservatory.org/SH026, where you can comment and leave any questions that you may have.
Thanks for joining us, and I’ll see you next week in our episode of Star Hopping.
I'm Cassie, and I hope you've enjoyed star hopping around the Milky Way. We'll continue
to bring you these video astronomy tutorials every week on Thursday, and in their podcast
format on Fridays. They will be designed to help you find deep sky objects that are up
in the sky at the time we post them on the Internet.
The reason we create these video and podcasts is to help beginning amateur astronomers learn
the sky and get more enjoyment out of their telescopes and astronomy in general. If you
have any requests or suggestions of potential targets in the night sky that you would like
to see us present, just let us know down in the comment section below, or on our website
blog.
Don’t miss our free Field Notes for this episode, basically the script of the show,
with all the images and start charts we use for our star hopping activities. You can get
them for free at kpobservatory.org/FieldNotes.
If this is the first time you’re checking out Star Hopping, and if you found this video
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Also, please follow KPO on Facebook, where we post all of our astrophotos and keep everyone
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All the links to these places including our website kpobservatory.org, can be found below
in the Episode Notes as well.
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Well thanks again for watching, and we'll see you next time on Star Hopping with Kissimmee
Park Observatory.