What mag is this??

[Starcrazzy]

Hi,
I have a canon 10d which i attach to my 8" newt via a variable projection t mount..The only way i seem to be able to achive prime focus is with a 2x barlow..I have tried eyepiece projection and this seems to acive similar views to the fl eyepiece used..But what sort of mag is the 2x barlow on its own with the camera??
Im stuck

[janoskiss (Steve H)]

Angular size of the image in degree = (size of the sensor or film frame in mm) / (effective focal length in mm) x 57.3

Draw a simple ray diagram if you want to see where this comes from. Note: effective focal length = focal length of the scope x barlow magnification; for your example, with the 2x barlow, that is 2 x scope FL.

If you want a typical eyepiece (say 50 degree field Plossl) power equivalent then divide the apparent field of view of the eyepiece, i.e., 50 degrees, by the above calculated angular size of the image.

[[1ponders] (Paul)]

Hi SC. It's understandable that you are stuck.

Let's start at the beginning. You can't reach focus using your 10D at prime focus. The most likely cause (and while I haven't used a newt to image it is a common problem) is that your mirror is too far down the tube. If you seem to be unable to rack your focuser in enough that is likely the problem. I'm sure the Newt imagers will jump in here, but I can think of two ways to fix it. One drill a few more holes in your scope and move your mirror closer to the secondary or get a lower profile focuser.

The other problem is when attaching your camera with the variable projection mount. I have around 4" of travel at least with my 4" refractor and in some situations that isn't enough out travel. So if you don't have enough out focus then you need to change your attachment type to bring your camera closer to the focuser.

Now using the barlow as Steve has said will give you 2X the focal length of your scope as well as slowing the scope down by a factor of 2. When talking imaging rarely is the term magnification used (except possibly for planetary imaging), especially for deep sky work. Normally the term Field of view is used. DSOers are primarily interested in how wide the object is rather than how big they can make it. But for a rough and ready estimate, if you are using a 35mm film camera then divide the FL of your scope (and barlow if using one) by 50. This will give an approximate magnification. If you are using a DSLR then divide by about 30 to give a rough mag estimate.



BTW using a barlow to image through is called negative projection, using an eyepiece in the teleprojector is called positive projection. Holding a camera up the an eyepiece and taking a picture is called afocal projection and mounting the camera directly to the focuser is called prime focus.

These are the four most common forms of astroimaging with prime focus being number one for DSOs and negative and postitive projection for high magnification planetary imaging.