Question about focal length

[Zupio]

How do you measure focal length on a reflector telescope? Is it simplly the length of the tube or is there more too it? I recently bought the skywatcher 130pds which is quoted to have a shorter focal length than 130P but the stats are exactly the same, even on the side it says 650mm f5, but the length of the tube is 600mm, would this mean it's f4.6?

[JamesF]

It's not that simple

The focal length is the distance from the primary to the focal plane, which will probably be somewhere outside the drawtube for a 'frac (to allow for the diagonal), and perhaps close to the end of the drawtube for a newt.

So, in your case the focal length is the distance between the primary and secondary, plus the distance from the secondary to the edge of the tube (the radius of the tube, say) plus enough to get you to the outside of the focuser.

James

[Cornelius Varley]

The focal length of the telescope is the distance between the centre of the lens or mirror and the point where the image comes into focus. All the versions of the SW130/650 , including the 130PDS, have a focal length of 650mm. The reason why the PDS has a shorter tube is to allow the point of focus to be farther away from the secondary mirror, making the PDS version suitable for imaging with a DSLR, many reflectors have a focal point too close to the secondary to allow sufficient inwards focus to achieve this.  

[JamesF]

Just looked up the details for the 130P-DS and it says:

"The new Sky-Watcher Explorer-130P DS has the same superb parabolic primary mirror as the Explorer-130P ... a shorter tube length with the secondary mirror positioned closer to the primary mirror for more convenient prime-focus photography."

So what they've done is moved the primary and secondary closer together which effectively moves the focal plane further out of the drawtube.  To be in focus with a camera the focal plane must coincide with the camera sensor which is quite awkward to achieve with the limited range of movement in a newt focuser intended for use with eyepieces, as eyepieces fit inside the drawtube, whereas a camera fits on the end and in the case of a Canon DSLR for instance the sensor is a further 45mm(-ish) back from the front flange of the camera.

It may also mean that the secondary mirror is larger on the P-DS because the light cone is larger closer to the primary.

James

[Zupio]

Thanks for the quick replies

[Louis D]

30 minutes ago, JamesF said:

It may also mean that the secondary mirror is larger on the P-DS because the light cone is larger closer to the primary.

Which can result in reduced contrast due to the larger central obstruction.  If the secondary is the same, the shorter tube version will have more edge of field vignetting.  I'm guessing the former because it is intended for photography, and images show vignetting much more obviously than the human eye can detect in visual usage.