For Prime Focus photography, just check that your Dob will allow sufficient “in-focus” with a DSLR attached. I understand that the focuser on Newtonian telescopes sometimes bottom out. That is, even when racked all the way in, the image in the DSLR has not yet come to focus.
Some users have solved this with using low profile focusers, low profile adapters, etc. whereas others have raised the Primary Mirror up the tube so the light cone extends further out of the focuser.
For eyepiece projection (the DSLR pointing into the eyepiece) there are special adapters that fit on the focuser and you can screw the DSLR baseplate onto the adapter to hold it steady, although a DSLR is quite heavy for this technique and may have a vignetted field of view unless you can get the camera lens very close to the eyepiece.
I suspect that attaching a DSLR plus lens to the eyepiece of a Dobsonian (eyepiece projection photography) would dramatically affect the balance of the tube. Additionally, the image of the brighter objects (such as our Moon and the Planets) would be so highly magnified that it might also be quite challenging keep the object in the field of view.
Cheers
Dennis
Last edited by Dennis; 29-05-2014 at 06:28 PM.
Reason: Added last paragraph...
As Dennis has said, in-focus could be an issue, as might the % illuminated field. Dob Newtonians are usually built for visual observing. A Newtonian built for photography is known as an astrograph.
If you are able to reach focus, then yes you will need a T-mount adapter, you will also need an EOS to M42 adaptor.
This will get you started however you will see Coma (elongated stars at the corners of the photo) at some stage, if the photography bug bites, you may want to purchase a coma corrector which essentially replaces the t-mount, you will still need the EOS adapter.
With Skywatcher Newts [including Dobs] you do not require a T-adaptor.
You unscrew the barrel on the focuser that 1.25" eyepieces slide into, and the T ring screws directly onto the exposed male thread for prime focus work. The weight of a DSLR is not a real problem, as a simple counterweight can be devised. My 30mm 2" eyepiece is heavier than my Canon 1100D, and the load is even heavier when I add the 1.5x barlow.
raymo
With Skywatcher Newts [including Dobs] you do not require a T-adaptor.
You unscrew the barrel on the focuser that 1.25" eyepieces slide into, and the T ring screws directly onto the exposed male thread for prime focus work. The weight of a DSLR is not a real problem, as a simple counterweight can be devised. My 30mm 2" eyepiece is heavier than my Canon 1100D, and the load is even heavier when I add the 1.5x barlow.
raymo
Hi Raymo,
The advice I've been given for the Skywatcher I'm looking to purchase is that it does require an adaptor. This advice was from Andrews Communications.
you do not need a T- adaptor with a SW Newt. If you look at a T- adaptor
you will see that one end slides into the focuser, and the other has a male
thread on it. That male thread is the same as the one on the SW focuser that is revealed when the barrel for 1.25" eyepieces is unscrewed. That thread is almost flush with the open end of the focuser. When the T-ring
[not T-adaptor] is screwed onto the male thread the camera is hard up against the focuser, which is where it has to be in order to come to prime focus. If you introduce a T-adaptor between the T-ring and the focuser,
the camera cannot come anywhere near focus. When a Canon DSLR is
at prime focus, the focuser is only racked out around 10mm. The SW focuser/adaptor setup is a bit confusing, and many new owners have
thought that their scopes cannot come to prime focus, until told how
to do it by various IIS members.
raymo
That's o.k. It is of course possible that the latest available tubes have a
different focuser arrangement, but I doubt it very much, they've been
the same for many years. I am taking some pics to show how it works,
and will post them very shortly. I suggest you have a look at them before
contacting Andrews again, so you have a better idea of what you are talking about.
raymo
I am taking some pics to show how it works,
and will post them very shortly. I suggest you have a look at them before
contacting Andrews again, so you have a better idea of what you are talking about.
raymo
T-Ring...T-Adapter?
Looking forward to your photos.
O.K. Loosen off the 2 knurled knobs at positions 2 o'clock and 6 o'clock in
pic 1. Remove the eyepiece holder/adaptor assembly so you are left with the open end of the focuser as in pic 2. Separate the 2 parts of the assembly you removed from the focuser.[ pic 3, ] so you end up with
pic 4. Attach the disc with the male thread to your T-ring as shown in
pic 5. Attach the T-ring/ adaptor assembly to your camera. Attach the camera to the focuser as shown in pic 6. Rotate the camera to desired position, and nip up the 2 knurled knobs that you earlier loosened off.
Sounds a bit of a pain, but after you have done it once or twice you can do it in about 30 seconds, or even less.
Pic 6 shows the focuser in approx. the position that it is in when at prime focus. If you want to know what a T-adaptor looks like you will find a pic of one on Andrews website.
raymo
Last edited by raymo; 30-05-2014 at 01:44 PM.
Reason: more info
Yes you need a t-ring should mount like the pic below , mine does to a skywatcher dob
But I need the adapter to mount it to my refractor
Edit , t-ring should mount straight to the focuser minus the ep holders
Some clarification on astro-photography methods with DSLR and telescope:
1. Piggyback: Camera with lens rides on top of telescope using suitable bracket with 1/4-20 screw for camera. Great for wide field photography.
2. Prime focus: Camera without lens is attached to the telescope. The telescope is the lens whether it is refractor or reflector. First a T-Ring that matches your camera body must be used. The T ring converts your camera body bayonet mount to T2 female thread. Then a T2-adapter that has a T2 male thread screws into the T-ring. The other end is either a 2" nose-piece that slips into the focuser. (1.25" is also available but will vignette with cameras with larger sensors). e.g. if you telescope is 200mm dia./f5 then you have a 1000mm f5 telephoto 'lens'.
3. Eyepiece projection: The camera with lens removed and T-ring added screws onto a special eyepiece projection adapter. This adapter holds an eyepiece inside and the focal length of eyepiece and distance to film plane determines the effective focal length of the system. This method was popular in the film days for photographing planets and close-ups of the Moon. It is not used much anymore due to video astronomy offering far better results for planetary and Luna. Some eyepiece projection adapters were variable with a sliding upper body. Note only small narrow body 1.25" eyepieces with a short upper section could be used with this method.
4. Afocal: The camera with lens is placed via a suitable bracket (or hand held if steady enough for bright objects) in front of the eyepiece. The eyepiece used is typically 32 to 40mm or so with good eye relief to avoid vignetting. The zoom feature of the camera can be used to enlarge the object. Again this method is best for the Moon and planets and using small compact digital cameras.
Note I've not seen a Skywatcher Newtonian focuser but from what others have said it appears that it has a T2 male threaded adapter. It is also possible to get a 2"-1.25" adapter with T2 male thread. However I particularly do not like these as you could easily damage your field lens on say a 2" Nagler if you happened to forget to remove the 2"-1.25" T2 threaded adapter from the focuser.
Note often the problem with prime focus on a standard focuser is that you cannot rack in enough and hence the need for a low profile focuser, (or first try and release the primary mirror springs all the way and then tighten only enough to get collimation). The Skywatcher system overcomes this with a different design. I have a Tectron focuser with two upper bodies; one visual standard rack & pinion and then another photographic for prime focus photography. You simply undo two nylon screws, remove the visual and add the photographic body to get the camera much lower. This focuser is not made anymore that I know of.
Its hard to see but there's a very fine thread on the t-ring
So the top one you listed should be fine
The one with the 1.25 nosepiece might not reach focus due to the extra length.
