First... what type of telescope do you have (or if you don't know, just give me the brand & model.)
In general categories, there are refractors (which rely on lenses to focus), Newtonian reflectors (which rely on mirrors to focus), and "compound" telescopes (catadioptric telescopes which use a combination of lens and mirrors to focus.)
Ordinarily you buy something called a T-Ring for Canon EOS mount, and a nosepiece that slips into the eyepiece receiver on your scope.
If you have a Newtonian type reflector then you may have a problem... I'll explain.
Suppose you have a telescope that has a 500mm focal length (I'm making this up to illustrate a point). The scope is optimized to deliver a focused image 500mm away from the primary lens or primary mirror. If the telescoope eyepiece is positioned in just the right spot, you get a focused image.
Refractors and compound scopes all have the eyepiece at the "back" of the scope. Newtonian reflectors have the eyepiece at the "front" (but on the side) of the scope. Since it would be very uncomfortable to sit at the back of a telescope which is pointing "up" and try to bend your body down but twist your head to look "up" into the eyepiece, this would be a recipe for back and neck pain and frequent visits to a chiropractor. SO... these scopes always come with a 90º diagonal mirror or prism so that you can look "down" into the telescope for much more comfortable viewing. (you're wondering what this has to do with astrophotography)
It turns out the telescope makers know this... so they design the optical path of the telescope to account for the extra space occupied by that 90º diagonal. It adds about 2" to the focal length of the telescope (we could round it off and call it 50mm).
The sensor on your Canon 650D (aka Rebel T4i) is at the back of the camera body to allow room for the reflex mirror to swing clear when you take a shot. The distance from the lens mounting flange on the front of the body to the image sensor at the back of the body is 44mm on a Canon EOS camera. That's nearly 2" and by the time you add in the thickness of that T-ring that I mentioned earlier, you're pretty much at 2".
If you have a refractor or compound scope, the solution is easy... remove the 90º diagonal prism (which subtracts 2" from the overall focal length of the scope) and then attach the camera (which adds 2" back in to the focal length of the scope) and the scope will produce a focused image on your camera.
But if you have a Newtonian scope... you may have a problem. Newtonian scopes don't have 90º diagonal prisms since the eyepiece is at the front (on the side of the optical tube) and it's already in a comfortable viewing position. This means that when you attach the camera, you've added in 2" to the focal length of the scope and there's nothing you can take out of the focal path to compensate for that extra 2". When you try to focus the telescope, you'll find that you run the focuser all the way "in" and it hits the hard stop and runs out of focus travel. You'll notice the image in the camera was just starting to get close to focus... but ran out of focus travel before the image could become sharp.
There are SOME models of Newtonian reflectors that are designed for this (sometimes sold as "astrographs" (a telescope built primarily for imaging)). Some Newtonian reflector owners "shim" up the primary mirror (this can cause a slight loss of light since the secondary mirror is now too close to the primary mirror -- so this solution isn't really ideal.)
You can get a 2x barlow. The barlow will double the effective focal length of the scope (at the expense of also doubling the focal ratio of the scope), but as a side effect it will also shift the focus plane farther back and you'll discover that when using a barlow, the camera WILL be able to come into focus.
You only have to worry aobut this IF you own a "Newtonian" class reflector telescope (but these are very popular so there's a strong possibility you own such a scope.)
The basic parts you usually need to attach a camera to a scope are:
1) A T-Ring (sometimes called a T-Mount) for Canon EOS mount cameras
This is a ring that has the Canon bayonet type mount on the camera-facing side, but the front facing side has the industry standard "T-thread".
2) A nose-piece or T-mount compatible with your scope.
Some scopes actually have threads on the focuser where you'd normally slip in the eyepieces. Those threads are industry-standard T-threads. That means the T-ring would screw right onto those threads, and then you attach the camera to the T-ring and the telescope effectively becomes your manual lens.
But many telescopes do not have these threads. So you have some choices...
Some telescopes have a rear-cell (very common on Schmidt Cassegrain Telescopes (SCT) or Maksutov Cassigrain Telescopes (MAKs)) and they make an adapter that threads on the SCT rear-cell (which is not the proper T-thread size) and the backside provides the industry standard t-thread size.
But my favorite way is to use a nose-piece. The nosepiece is simply a short 1.25" or 2" diameter tube (exactly the same diameter that an eyepiece would use) but it has no optics... just t-threads at the back of it You screw the tube to the T-ring and attach it to the front of your camera. Now you slip the camera into the eyepiece recevier on your telescope and snug it down and you're ready to shoot.
Canon's forum policies don't allow us to link specific third party products (because it looks like we're trying to promote them) but do a web search for something like "T-ring and nosepiece for Canon EOS" and you'll probably find LOTS of hits... it doesn't matter what brand you buy. Celestron sells these, Orion sells these, Baader-Planetarium sells these... the list goes on and all. Remember... it's a "standard" so as long as the thing complies with the standard then it will work.
Now that you've got the camera attached to the scope... there's one or two more problems... tracking and possibly field rotation.
The moon is very bright. It's easy to capture a photograph of the moon using a telescope. The general guideline is called the "Loony 11" rule (Loony being a play on Lunar). The rule says that if you set the f-stop of your camera lens to f/11, then you can set the shutter speed to the inverse of the camera's ISO setting. So at ISO 100 you would take a 1/100th sec shot. At ISO 200 you would take a 1/200th sec shot.
But you can't change the focal ratio of a telescope. You have to look up the specs of your scope model (If you tell me what you own, I can tell you what your focal ratio is) and if you use a 2x barlow then dont' forget to double the focal ratio. If it turns out that you don't have an f/11 scope (or something reasonably close such as an f/10 scope) then you'll need to compensate.
Regardless... you'll probably be shooting a fairly fast shutter speed. That means the movement of the moon wont be enough to blur your shot.
Planets are a bit trickier because they are much smaller and farther away. One quick shot usually wont be very good. So most people shoot about 30 seconds to perhaps 1 minutes' worth of video and then use stacking software. You can get free stacking software. Look at Registax or AutoStakkert (both are free) and there are numerous YouTube videos that will offer a tutorial in how to use the software.
Deep space objects, however, are a much more difficult challenge. These objects require long exposure times (typically many minutes). If the telescope is not tracking accurately then the object will appear to "smear" in your image and you'll be unhappy with the result. Even if you have a mount that tracks the object, if it's an alt/az oriented mount, the object will appear to twist or rotate (field rotation). There's a special device called a field de-rotator but these are complicated. The easier solution is to use an equatorially mounted telescope (or put the alt/az telescope on an equatorial wedge.)
Even then, the scope's tracking rate usually isn't precise enough... we have to use great care to achieve a "precise polar alignment" of the instrument, and we have to use a second device called an auto-guider to keep the scope on-target (the auto-guider is a second camera and often a second scope on a side-by-side or piggy-back mount. The second camera tracks a nearby star and images that star every couple of seconds ... checking to see if the star appears to drift in the frame. If it does, it sends a corrective command to the telescope mount to keep things on-target. This happens continuously while the main scope and imaging camera capture your night-sky image.
You do want to make sure that nobody touches the telescope while it's capturing images or your'll get blurred shots. You'll want to trigger the camera either by using the self-timer, or a remote shutter release (wired or wireless), or connect the camera to a computer via the USB cable, etc. to trigger the camera shutter.
Tim thank you for that very comprehensive answer.. sorry i did not give more detailed information in my original post..
the scope is a Celestron SE6 I have an attachment for the camera..
the story so far... I attached the camera to the telescope and also to a monitor .. when i switched on the info. appeared on the monitor so monitor and camera are talking. previously i had centered on a bright moon but nothing appeared on screen. i was hoping i was missing something to do with the camera but it appears not, all i can think now is the mirrors in the telescope are slightly off so I will have to do the collimation bit. I had also tried locking the camera mirror up but no difference..
If you can visually see through your scope with a normal eyepiece then the scope is fine. SCTs don't require much collimation. A poorly collimated SCT would cause stars to have a problem called "coma" which causes stars to slightly smear in one direction (you get a bit of a teardrop shape instead of a round point). When we test for collimation, we point the scope at a star and then use a very short focal length eyepiece to over-magnify the star. We then very slightly de-focus the star so that we can see diffraction rings (aka Fresnel rings). If the telescope is perfectly collimated then the rings will appear as a donut-shape with a black center. If the width of the donut is equal all the way round, then you're done. If it's lopsided (donut is thinner on one side then on the other) then your scope could stand to be collimated. Your manual explains the collimation process. You should also look for collimation instructions for SCTs (because collimation for a Newtonian is different and most collimation instructions and videos tend to be for Newtonians which need more frequent adjustment.)
What are you using to attach the camera to the scope?
For you scope, you should be using the Celestron T-Adapter (Celestron part #93633-A) and the Celestron T-Ring for Canon EOS (Celestron part #93419)
(1) remove the 90º star diagonal from the back of the scope.
(2) the star diagonal was previously inserted into something called a "visual back". Your manual has a photo of this in Figure 3-2 (page 7). That needs to be removed from the scope. It unscrews from the back of the scope (aka the rear-cell).
(3) thread the Celestron T-Adapter onto the rear-cell
(4) thread the Celestron T-Ring onto the T-Adapter (I usually just leave those two parts permanently together since the only reason the T-Ring comes off the T-Adapter is to allow you insert T-Rings for other camera models. But if all you own is a Canon EOS, then there's no reason to take them apart.)
(5) Attach the camera to the T-Ring (just like attaching a lens.)
IMPORANT: With the camera attached to the back of the scope, there will NOT be enough clearance for the scope to point straight up to the zenith. Doing this would cause the camera to collide with the base of the Nextstar mount. Just be mindful of this when you slew to objects you'd like to image.
Point the scope at the moon. You will be able to see the moon through the viewfinder, although this requires you leaning over and twisting your neck to look "up" into the camera (not very comfortable). So the more comfortable thing to do is switch on "live view" and use that to focus the moon.
Keep in mind that auto-focus is a feature of the LENS and since you are attached to a telescope and not an EOS lens, your only way to focus is to turn the telescope's focus knob.
If your scope is not perfectly pointed at the moon and it's also out of focus, then the backgroudn stars can be so blurred that you see nothing at all on the camera (not because the camera and scope aren't working, but because everything is too far out of focus.) If you are pointed at the moon and don't see anything then you may want to nudge the scope around to make sure that the moon isn't just barely out of frame. All the fiddling to remove the visual back and attach the T-adapter and camera may have bumped the scope off target.
Your Celestron Nexstar 6SE is an f/10 scope. This means you will need to put your camera's mode dial on "M" and adjust ISO and shutter speed for your fixed f/10 focal ratio. To take an image of the moon, use ISO 100 and 1/125th sec and you should get a very good exposure (nothing should be so bright that it's over-exposed nor so dark that you can't see it.)
With this setup, the scope's angular field of view will be 51 arc-minutes wide by 34 arc-minutes high. The moon itself is usually about 30 arc-minutes wide from edge-to-edge. But the moon's orbit is not perfectly round. It's an ellipse and when the moon is farthest from Earth (apogee) it's about 29 1/2 arc-minutes, but when it is closest (perigee) it's about 33 1/2 arc-minutes. Your scope & camera combination will just *barely* be large enough to fit the moon (it will be a very tight fit at a perigee full moon (aka 'super moon')).
You can get a device called a focal reducer (Celestron Reducer-Correct #94175). To use this, you thread the Reducer directly onto the rear-cell of the scope, then thread the T-adapter onto the reducer-corrector. This lowers the focal ratio of your scope down to f/6.3 (from f/10) and also allows you to image a wider area of the sky.
well the problem is solved it was the t-ring that caused the problem . When the t-ring and adapter are together i have a small cap which fits inside, to protect the scope when the camera is not attached, when working in the dark it was easily forgotten to remove it, I feel a right prat but thats what age does to you.
Thank you so much for the time and effort you have given me and i can only hope a lot of people got some hints and guidance from your posts, so happy stargazing and photography to all.
Glad to hear it's all working out for you now.
I usually set up for imaging during the daytime just so I can see what I'm doing to get everything hooked up.
My most embarassing incident ("most" because it turns out I've had more then one) was at a public star-party (public invited to view through the scopes). My scope was tracking an object and I had a 2" eyepiece in the scope. I wanted to switch to a higher power eyepiece but that eyepiece was a hydrbrid 1.25" / 2" (TeleVue makes special eyepieces that slip into 2" eyepiece seat even though the eyepiece really only needs a 1.25" seat.) I put the eyepiece in, could sort of see light coming through, but could not focus anything... I was running the focus from one end to the other for quite a while. I couldn't figure out what was wrong. I took the eyepiece out and put in a different eyepiece which worked fine... but then I'm holding the non-working eyepiece in my hand, I look down and realize that the dust-cap is STILL on the eyepeice (the dust-cap was a milky plastic so some light could shine through.) It's amazing what we don't notice when we're working in the dark. Of course I'm not doing this in the privacy of my own observing site... I've got 20 people standing in line waiting to look through the scope as I can't figure out how to focus the scope.
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