Image scale???

[matt]

Quick question.

I keep reading posts which refer to "image scale". What exactly is that???

Also ... for folks with scopes of shorter focal length like mine ie 8 inch f5 ... now is the time to start turning those towards Mars. What was disappinting a week or so ago ... is now a joy!!!

great views around 3am this morn. Try a 10ml EP with a 2x barlow.

Cheers

[videoguy]

Matt, Image scale simply means how large the object (a planet for example) is projected on to a cameras CCD or film plane. In other words, how large the image appears. Sort of like a slide projector close to the screen produces a smaller picture and as you move it further away the picture gets larger.
The shorter the focal length the smaller the image and vias versa. There is another consideration that affects image scale even if the focal length is fixed and that is the size of the CCD chip in the camera. For example a camera with a 1/2 inch CCD attached to an F/10 scope produced an image slightly smaller than and camera with a 1/3 inch CCD. On the same f/10 scope, a camera with a 1/4 inch CCD produces an image larger than the 1/3 inch CCD camera does.

[matt]

Thanks Steve

Can't wait for that Vixen EP to arrive...

Matt

[iceman (Mike)]

Hi Matt.

As additional reading, check out this thread I posted yesterday.

My 10" scope is 1250mm FL, and at prime focus (no barlow) with a ToUcam, you can see the size of the image it produces. A 10" LX200 SCT has a focal length of 2500mm, twice the length of mine because of the folded light path. It means given the same ToUcam at prime focus, the image will be twice as big as what I capture in my newt.

So generally, SCT's or Mak's with longer focal lengths are more favoured for planetary imaging because of the larger image scale they produce. Newt's can be better for widefield astrophotography though.

The focal length is also something you need to take into account when choosing eyepieces for a given scope. A 13mm eyepiece in a newt like mine or yours is considered medium magnification, good for DSO's. But in an SCT at double the focal length, the 13mm eyepiece is equivalent to a 6.5mm eyepiece in our newt, so it's really a planetary eyepiece.

Hope that helps and not confuses.

[matt]

Thanks Iceman

but tell me ... what exactly does something being at "prime focus" mean???

I keep reading it and I'm sure it's something simple, just haven't found a definition.

Cheers

[iceman (Mike)]

It just means, directly in place of the eyepiece (no barlow or other extension). So the focal length when you capture the image is the focal length of the telescope (in my case, 1250mm).

A barlow can be thought of as a focal extender for a telescope, so a 2x barlow doubles the focal length of your telescope. So instead of a 1250mm FL telescope, you are now working with a 2500mm FL telescope, and as Steve already pointed out, the longer the focal length, the larger the image scale.

[asimov (John)]

I've always wanted to know & not game to ask Mike: Can I image @ prime with my kodak P&S would you think?

[iceman (Mike)]

No John, because your Kodak has a lens you need to use an eyepiece and do afocal as you have been.

If you had an SLR or DSLR and you removed the lens, then you could capture at prime focus like we're doing with webcams (with the webcam lens removed).

[asimov (John)]

Thanks for that.

[matt]

ta ... Iceman

[rumples riot]

Yes, SCT do have their benefits with image scale, and just like Mike said the larger the focal length the larger the image scale at a given magnification or fstop.