I have been using Ron Wodaski's CCD Calculator to help me choose the telescope or the barlows when framing.
What it does it gives a pictoral view of what to expect for the final image with your own telescope and camera combination.
Now that the programme is free you can try by clicking
here I found it rather limited in use. It does not cater for wide field images. Not for DSLR and lense combination. But it is free and it is a start.

Nowadays for both laptops and desktops you can purchase ones with the LCD widescreen panels. Hey the LCD widescreen aspect ratio approximates 3:2 ie about 35X24mm frames. What a coincidence

So running a programe like
stellarium another free programme which shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope or DSLR . Better still running on this LCD widescreen can give you an idea of what to expect from your imaging and helps you with your framing. How?
Step
1. Download the stellarium programme by clicking
here . The beta release of version 10 indeed
is a very improved version and very user friendly. Still with bugs but patience soon will be removed. RUN the programme.
2. Use the mouse move the cursor to the extreme left hand and change the location [F6] to Kuala Lumpur. Then set the date/time [F5].
3. We will use M42 as an example. Search [F3] for M42. Rotate the mouse wheel till M42 fill the LCD widescreen. Normally for good composition leave some space all round. Read the FOV= 1.2 (on my laptop)
4. Goto
Lens Focal Length and Field of View converter
5. Convert and get the required focal length of 1377mm. so for a telescope with a focal length of 600mm you will need a 2X barlow to get full frame of M42 using a crop DSLR such as Nikon D200 or Canon 450D.
6. Aim your telescope to the centre of the trapezium and shoot as per universe 24's recommendation.
happy shooting.
cheers