Confused ? !

[Kevdan]

Hi All,

How long is a reflecting scope with a focal length of 750mm?

Mine says 150mm dia and 750mm focal length but when measured it's only 600mm long from front to back of OTA, has my scope been sawn off or are they all shorter than the stated focal length?

Oh and today I saw my first images of Our sun, using the baader film and the continuum filter, absolutely brilliant views untill mr cloud went passed.

Kev.

[RikM]

Hi Kev are you sure you measured it right? My 150P is 70cm long including the dust cap.

[Kevdan]

Yes, well its probably a little longer but not 70cm

but even if yours is 70cm long where's the other 5cm for the focal length?

Kev

[E621Keith]

I am not sure about the exact length of the 150p, but did you take into account the distance from your secondary to the end of your focuser?

[RikM]

The focal length is the distance from the primary mirror up the tube, bounce off the secondary and out the focuser to the focal plain somewhere near the bottom of the eyepiece. Is has little to do with the length of the tube. Mine normally has a 30cm dew shield stuck on the front.

I'll see if I can find a diagram but someone else will no doubt post one first.

[RikM]

Try this.

[Kevdan]

Once more I feel educated, I've looked at pics like this loads of times but never "looked" if you know what I mean, I always thought the focal length was between the two mirrors.

Rest easy Brian Cox it'll be a long time before I take you job LOL.

Kev.