SCT/Maks, diagonals & magnification.

[Alfian]

I imagine this may have come up somewhere here before but I've found it interesting all the same. Having just reacquired a 102 Mak and looked at the  possibilities, as you do, I pondered the question as to whether playing around with visual backs/diagonals affected the focal length of the scope. Of the mind that the given focal length of a scope IS the focal length, full stop, and all else absorbed into focusing I was surprised to find that though that holds true for newtonian reflectors and refractors, the focal length of SCTs and Maks by virtue of the design are affected by the visual back/diagonal  light path length. I'll not rabbit on further but this thread over on cloudy nights elucidates the subject well enough. Apologies if people have read about it before but it was new to me.

https://www.cloudynights.com/topic/291340-does-a-diagonal-increase-magnification/

[JamesF]

I've mentioned it a few times in the context of the positioning of a camera sensor making a difference to the effective focal length of the overall system and there have been discussions about the optimal placing of a camera to minimise optical distortion, but it doesn't seem to come up that often.

James

[Davey-T]

There was a post a while ago discussing  it by a member sur la continent I believe whose name escapes me ATM.

Dave

[Paz]

I have read about and noticed this, I use a reticule eyepiece to measure things and it relies on knowing or calibrating the focal length of the scope. With my reflector the focal length is fixed and you change the position of the eyepiece in and out to hit the fixed focal point of the objective. With an off the shelf  maksutov or sct the eyepiece position is usually fixed and you move the focal point of the scope to meet the eyepiece instead. This is possible because the primary and secondary mirrors are both active and you change the effective overall focal length by changing the distance between them.

So I prefer not to do measuring with a mak or sct as you have to be careful to calibrate properly and then be careful not to move the primary, whereas with a refractor (or a reflector with a flat secondary) you know where you are every time.