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focal reducer calculation

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Ive got a 0.5x reducer, but i cnt for the life of me work out what effective value ep it gives me!

i have a barlow which halves the ep size, eg 10mm with 2x barlow ~ 5mm ep.

however, what happens with the 0.5x reducer!!? surely it cant be 10 / 0.5 = 20mm ep?

so if i have 20mm widefield ep and use the 0.5x on it, i will get an equivalent 40mm ep view?

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Yes in effect you will get the same magnification as a 40mm eyepiece. however the field of view may be not be quite what you expect due to restrictions in the optics and the size of the eyepiece.

You cannot make a 64mm eyepiece with a 32mm and a 0.5x reducer. You might get the expected magnification but no better field of view than the 32mm assuming a 1.25 inch eyepiece.

Barlows and focal reducers change the effective focal length of the telescope and not the eyepiece - thats why they also work with prime focus photography with no eyepiece.

Hope this helps


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thanks doc :D

so an actual 40mm ep will have a better fov, but a 20mm with a .5 reducer will have the same mag, but limited to the 20mm fov? or will it be a smaller fov than the 20mm has?

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Unfortunately it's not as simple as that.

Basically as you increase the focal length of the eyepiece you start to see the inside of the scope - whether it's the focus tube or baffles inside the body of the scope.

This is where a 2 inch set up will give wider field of view with the appropriate eyepieces - assuming the scope body can handle it.

My 102mm MAK gains no advantage with a 2 inch set up because the internals of the scope obstruct the view.

With shorter length eyepieces you do get an increased field if view in proportion to the apparent increase in the eyepiece focal length or reduction in the scope focal length - so your 10mm with 0.5x reducer will give the same view as a 20mm and no reducer.

It's at the limits that the internals of the scope start to take effect.

Still confused... There are thread on the use of focal reducers that probably explain this better than I can.

In the end with a 1.25 inch set up a 32mm eyepiece will give the widest possible views.

Clear skies


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