What is the effect when you slightly move (withdraw) your eyepiece from the focuser

[Rogering1]

Normally you insert your e.p. fully and focus but what is the effect when you slightly move (withdraw) your eyepiece from the focuser and refocus?

Should this give you....

I suspect it increases focal length, what would this do increase magnification?

Thanks

[michael.h.f.wilkinson]

No. If it was in focus, you get it out of focus. If you do it before focusing, the position of focus changes, that is all. Because you have not changed any of the lenses physically, no changes in focal length (or other optical parameters) occurs.

[LukeSkywatcher]

No. If it was in focus, you get it out of focus. If you do it before focusing, the position of focus changes, that is all. Because you have not changed any of the lenses physically, no changes in focal length (or other optical parameters) occurs.

When i read this i thought it was an eye relief issue. Clearly (no pun intended) it isnt. However if you move the EP in or out of the focuser, doesnt this change the focal lenght, because you are moving the EP closer or away from the secondary focal point of the scope?

I dont see a reason to move an EP in or out from the secondary. If you are doing imaging and cant get perfect focus then you may need an extension tube or something?

[Carbon]

I assume you're talking about SCT's? If you have to refocus using your primary mirror to adjust for the extended eyepiece, then yes, that will somewhat alter the focal length of your scope, most likely not noticeable to the eye.

[JamesF]

When i read this i thought it was an eye relief issue. Clearly (no pun intended) it isnt. However if you move the EP in or out of the focuser, doesnt this change the focal lenght, because you are moving the EP closer or away from the secondary focal point of the scope?

In a 'frac or newt the focal length is fixed. The position of the image plane never moves. Moving the EP in the focuser merely moves the point of focus of the eyepiece such that you need to change the position of the focuser to get the eyepiece back into focus (ie to get it's focal point to coincide with the image plane once more).

If you move the piece in the focuser of an SCT or Mak, the act of refocusing might change the focal length slightly, but that's because of the way the focal length is calculated when you have systems comprising more than one optical element.

James

[LukeSkywatcher]

In a 'frac or newt the focal length is fixed. The position of the image plane never moves. Moving the EP in the focuser merely moves the point of focus of the eyepiece such that you need to change the position of the focuser to get the eyepiece back into focus (ie to get it's focal point to coincide with the image plane once more).

If you move the piece in the focuser of an SCT or Mak, the act of refocusing might change the focal length slightly, but that's because of the way the focal length is calculated when you have systems comprising more than one optical element.

James

Ok. I didnt know that.

Thanks.

[gooseholla]

A 5mm hyperion wouldn't focus in my Newtonian, until I pulled it out a tiny bit and locked it. Then it would focus. So basically, you can, if it is close to focus, focus an eyepiece in some circumstances.

[JamesF]

Yes. That's presumably because the focuser didn't have enough travel outwards for the eyepiece to reach the focal plane when fitted normally. I have the same problem with my 10" dob since I fitted a nicer, but lower profile, focuser.

James

[gooseholla]

Yes. That's presumably because the focuser didn't have enough travel outwards for the eyepiece to reach the focal plane when fitted normally. I have the same problem with my 10" dob since I fitted a nicer, but lower profile, focuser.

James

You can always but an extension bar - although you then run the risk of going too far the other way and still not focusing lol

[JamesF]

That's effectively what I have done, yes.

James