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Grumpy Martian

Telescope focal ratio comparisons.

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I have a 115mm Vixen refractor which has a focal ration of f7.8. Nothing wrong with this. It gives good views of the moon, planets and double stars. But I do enjoy general wide field star gazing aswell.

I was thinking of getting a wide field telescope to complement the refractor. Perhaps a six inch f 5 Newtonian? My thoughts are that it would give a much wider field of view, plus brighter views. Or am I deceived into thinking that there would be that much difference to warrant the investment? 

What do you think 🤔

Edited by Grumpy Martian
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Hi Martin, if you wait a couple of weeks you can have the WO Megrez 72FD to play with.

Dave

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I had a 6” f5 newt, it was a very nice scope but I missed sitting down behind the scope like you do with a frac. 

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What eyepieces are you using Martin? It might be worth investing in a couple of 2" wide fields if you don't already have any? The ES 68s can be found secondhand reasonably and work very well in my experience.  I also have a 2" Vixen NLVW 65deg which is nice too. Of course, you may already have some wide field Televue or similar in which case ignore the above comments! 

You might get a wider field with the F5 reflecdtor, but I would expectd the Vixen to give far more pleasing images to be honest..

Dave

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U can keep the scope and add a focal reducer it will turn it into an f5.8 which is only 1 focal ratio more then your talking about.

2nd or get a 2 inch diagonal and a low power  ep like a 50 to 55mm 2 inch

Or both and that's probably just even wider than the 6 inch f5

Joejaguar 

Edited by joe aguiar
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If you use a 40mm 68 degree 2 inch eyepiece with the 115mm F/7.8 refractor you will get a 3 degree true field of view.

With a 6 inch F/5 the same eyepiece would create an  rather large exit pupil which would not be as effective IMHO so you would need to reduce the focal length of the eyepiece to something like 30mm. With a 30mm 82 degree eyepiece the 6 inch F/5 would show you a true field of 3.3 degrees so not a lot more than the 115mm F/7.8 can muster.

Depends whether you feel that the relatively small additional field and some additional light grasp is worth owning another scope for :icon_scratch:

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47 minutes ago, F15Rules said:

What eyepieces are you using Martin? It might be worth investing in a couple of 2" wide fields if you don't already have any? The ES 68s can be found secondhand reasonably and work very well in my experience.  I also have a 2" Vixen NLVW 65deg which is nice too. Of course, you may already have some wide field Televue or similar in which case ignore the above comments! 

You might get a wider field with the F5 reflecdtor, but I would expectd the Vixen to give far more pleasing images to be honest..

Dave

Good point. I have the 28mm 68 degree ES and Williams Optics 40mm.

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1 hour ago, John said:

If you use a 40mm 68 degree 2 inch eyepiece with the 115mm F/7.8 refractor you will get a 3 degree true field of view.

With a 6 inch F/5 the same eyepiece would create an  rather large exit pupil which would not be as effective IMHO so you would need to reduce the focal length of the eyepiece to something like 30mm. With a 30mm 82 degree eyepiece the 6 inch F/5 would show you a true field of 3.3 degrees so not a lot more than the 115mm F/7.8 can muster.

Depends whether you feel that the relatively small additional field and some additional light grasp is worth owning another scope for :icon_scratch:

Thanks for doing the maths John. It does rather put it into perspective. 

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Focal Ratio has nothing to do with field of view (fov). Fov is entirely controlled by the focal length of the telescope for any given eyepiece.

Your 115mm f/7.8 Vixen has a focal length of 897mm (115 x 7.8) while a 6" f/5 is 750mm. The ratio 897:750 gives you the difference in field of view and will remain at that ratio between the two telescopes for any given eyepiece. Using a simple ratio you will get between 16 and 19% ( depending on which way you work it ) extra linear field of view with the 6"f/5 using your existing eyepieces. Personally, I would go for wider field eyepieces rather than the Newt as they will give bigger and more pleasing images. More field stuffed into narrow field of view eyepieces makes everything smaller.

Nigel

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I would rather go for a 6" F5 achromat as a rich field scope rather than a 6" F5 reflector. Having used several of them they've never failed to deliver stunningly sharp star fields, impressive light grasp, and are very comfortable to use. They make brilliant comet seekers! ☺

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