A quick FOV question.

[lonebod]

If you have 2 telescopes with a 6" aperture, one with a focal length of 750mm and one with a focal length of 1500mm. These obviously have different FOV.

If use 2x Barlow on the 750mm this gives the equivalent of 1500mm, so will the 750mm with Barlow have the same FOV as the 6" native 1500mm?

Also, if I had a 0.5 focal reducer and put it on the 6" 1500mm son it had the effective focal length, would it have the same FOV as the 6" bat be 750mm?

Thanks,

[Astro Imp]

In my view both your assertions are correct.

[lonebod]

Thanks. Might influence my decisions re a Mac + focal reducer, or a mac and a newt.

[Uhwaz]

http://www.12dstring.me.uk/fov.htm

great FOV calculator 

[nicoscy]

Rough FOV formula:

FOV = Apparent Field of View of Eyepiece / Magnification

Therefore FOV = Apparent Fielld of View of Eyepiece x focal length of Eyepiece x Barlow / Reducer factor / Focal Length of Telescope (Obviously when no Barlow or Reducer is used, the factor is 1)

[ollypenrice]

Beware of other factors which mean that the initial assertion is not always correct. It is always 'optically correct' of course, but many catadioptric designs have a field of view limit imposed by their baffle tube which creates a physical field stop smaller than that that allowed by the optics themselves. The well known example arises with the popular American SCTs for which an F6.3 reducer is available. If you have a widefield 2 inch EP, using this reducer will reduce magnification but not increase the FOV because you are 'looking down the straw' of this restrictive baffle tube. I don't know the limitations of the Mak in question but do some homework to be sure that the full FOV will be available. With a 2 inch EP and 0.5 reducer I'd be surpised, but that's a guess. As ever, no free lunch....

Olly

[Owmuchonomy]

Aha yes, the much promoted F6.3 reducer, the lump in the throat of my 9.25". As Olly says, do the sums.