Focal length

[Pryce]

This might be a stupid question but as far as I understand focal length is the distance between the lens/mirror and the focal point(eye piece) 

 

Then,  my question is this; how can the listed focal length of the Explorer 150P and 150P-DS(or any of the P/P-DS siblings for that matter) be the same (750mm) when the primary mirror of the P-DS is closer to the secondary mirror AND the P-DS has a shorter(or lower) focuser? 

Edited August 21, 2020 by Pryce

[Nakedgun]

Perhaps the focal length for this line is an approximation, perhaps they have different primaries, otherwise I don't know.

[johninderby]

It’s all down to the infocus travel needed by a camera. Because of the differences between a camera and an eyepiece the camera usually needs more infocus travel than an eyepiece so that’s why the difference between the two. If you use an eyepiece in one if the pds scopes you will probably need an extension to allow the eyepiece to focus. Remember the focal length isn’t the dustance between the seconday and the primary but the distance between the primary and the eyepiece / camera. In one of the visual (p) scopes you may have trouble using a camera because you can’t get the camera in close enough.

[Pryce]

1 minute ago, johninderby said:

It’s all down to the infocus travel needed by a camera. Because of the differences between a camera and an eyepiece the camera usually needs more infocus travel than an eyepiece so that’s why the difference between the two. If you use an eyepiece in one if the pds scopes you will probably need an extension to allow the eyepiece to focus. Remember the focal length isn’t the dustance between the seconday and the primary but the distance between the primary and the eyepiece / camera. In one of the visual (p) scopes you may have trouble using a camera because you can’t get the camera in close enough.

Indeed, and thats why I'm confused about the focal length, since the primary mirror is higher up the tube and the focuser is shorter.  Making the distance between the primary mirror and the focuser of the camera(on P-DS) shorter than that of the primary mirror to eye piece of the P versions.

[johninderby]

It’s simple. There is no difference in total focal length.. If used visually with the PDS version then you would use an extension in the focuser to bring the focal length to the same point as you would have in the P version. 

It’s all down to the camera needing to be physically closer to achive the same effective focal length. With a DSLR the camera is outside the focuser whereas an eyepiece sits partly inside the focuser.

Edited August 21, 2020 by johninderby

[timwetherell]

The camera needs a real image on the CCD which should sit in space at a distance exactly equal to the focal length of the primary mirror. The eyepiece magnifies this real image and in effect turns it back into a virtual image at infinity. So the optical centre of the eyepiece will need to be beyond the primary mirror focus point by a distance equal to it's own focal length. Which in the case of say a 40mm, is quite a bit. 

[Captain Scarlet]

6 hours ago, Pryce said:

This might be a stupid question but as far as I understand focal length is the distance between the lens/mirror and the focal point(eye piece)

Then,  my question is this; how can the listed focal length of the Explorer 150P and 150P-DS(or any of the P/P-DS siblings for that matter) be the same (750mm) when the primary mirror of the P-DS is closer to the secondary mirror AND the P-DS has a shorter(or lower) focuser? 

The position of the camera in this case depends on the placement of the secondary mirror. Because the secondary is simply a flat mirror, it's just bouncing the primary's light-cone out sideways. As my crude picture here shows, different positions of the secondary result in different positions of the camera and focuser, but the main focal length remains unchanged.

Cheers, Magnus

[Nakedgun]

Not being an imager, this explanation certainly eluded me. 

[Pryce]

So in theory I only need to move the primary a centimeter or two to correct for the adjustment in the focal point? 

 

Or i might not even need to at all since I've bought a M54 to T adapter that I scew right onto the draw tube.  That might solve my focusing problem.