Afocal / prime focus photography - very naive question

[wulfrunian]

First, as I own a dob I know I'm never going to be taking great images. But I'd like to be able to just have a go at snapping the moon and maybe the bright planets. Despite looking around at different threads, I'm still not clear of one basic thing.... as I understand it, I could attach a camera direct to the focuser with a T-connector, so presumably get no magnification then, or I could steady the camera in some way in front of an eyepiece. Assuming that's right, what are the advantages / disadvantages?

[Astro_Baby]

Afocal is where you use the camera to take a pic through the EP. You can do that in various ways.

Prime focus is where you replace the EP with the camera and the telescope just becomes a kind of giant telephoto lens.

So yes - your right - I cant say what the advantages/disadvantages are only ever having used a happy snappy cam to take pics through the EP (afocally)

[Sierra Golf Mike]

Michael,

I was about to post a similar question so apologies for the slight hijack. If I use my DSLR camera in Prime focus mode, is there any way of magnifying the image? If my understanding is correct the image will have zero magnification so are you as well using a telephoto lens? Sorry if this sounds stupid but as I am a newbie I am just trying to get my head round it all.

Thanks

Steve

[Cornelius Varley]

Michael,

I was about to post a similar question so apologies for the slight hijack. If I use my DSLR camera in Prime focus mode, is there any way of magnifying the image? If my understanding is correct the image will have zero magnification so are you as well using a telephoto lens? Sorry if this sounds stupid but as I am a newbie I am just trying to get my head round it all.

Thanks

Steve

To say that there is zero magnification when using a DSLR attached to a telescope in prime focus is not exactly correct, the magnification comes from the focal length of the telescope. The telescope becomes a big telephoto lens, so, for example, a telescope with a focal length of 1000mm becomes a telephoto lens with a focal length of 1000mm. To increase the magnification of the telescope you need to increase the focal length by the use of a barlow lens. A 2x barlow lens would increase the focal length of the telescope to 2000mm.

Peter

[Sierra Golf Mike]

Peter,

How do you fit a barlow lens when the camera is attached to the focusser?

Steve

[Cornelius Varley]

The barlow fits into the focuser and the camera adapter fits into the barlow.

Peter

[gustavo_sanchez]

Please correctly if I am wrong. I think that for attaching the camera/t-ring assembly to the barlow, you need another piece called the t-adapter which has a screw at one end and a eyepiece-like barrel at the other end that goes into the barlow. That way you can attach the pieces together.

[Rui]

Please correctly if I am wrong. I think that for attaching the camera/t-ring assembly to the barlow, you need another piece called the t-adapter which has a screw at one end and a eyepiece-like barrel at the other end that goes into the barlow. That way you can attach the pieces together.

That's right. But the set up gets much more stable whent the T-ring screws into the focuser

[RobH]

Hello Wulf,

For one-offs of the moon, afocal imaging works fine, even with a compact camera held up to the eyepice with a camera adapter (Telescope house do one).

For both afocal, and prime focus imaging, you could probably get some very good results with a webcam too, and this would be a lot lighter. Dobs generally aren't built to have heavy cameras hanging from the focuser.

With both types of imaging, if you add a barlow you wiill increase magnification, but you will also increase the effect of any slight wobble, and the fact that you're not tracking, and in practice you may find this limits the useable magnification.

Cheers

Rob

[jaskel]

good info guys, I just got a 2" prime focus adapter for my 400D Canon, interested in some nice photos!