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Hi All

I'm thinking of getting a 4"/ F7 refractor  for a wider FOV. There is a great thread here & CN about a new 4" F11 scope. I have a great ,comfortable Meade 82 degree 5.5 eyepiece which would give approx 200x with the F11 scope & I also suffer with floaters at approx 0.5 exit pupil. My question is_

*  What FOV would I have with each scope in degrees with this eyepiece?

The other advantage of this F11 scope is there is a removable 120mm section for Binoviewer use. My other questions are_

* If this section is removed for Binoviewing, could I use a seperate up to 120mm extension tube to reach focus with my Meade eyepiece?

* Would the FOV be the same with the narrower extension tube as the existing wider removable section?

Hopefully that makes sense & thanks in advance for any feedback

Joe

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Stick your 'scope specs into Stellarium to give an idea of the field of view.

Or use Scope Maths if you are looking for something more precise! :D

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Thanks for that Ben. The  Scope Maths link is the ideal answer to that question & just added it "my favourites" for future use

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The things you are discussing regarding fov seem to be a little contradictory to me. Buying a widefield scope ie effectively one with a shorter focal length would you to access wide true fields of view with low power eyepieces. Using the 5.5mm in either scope won't give you a widefield view, it will give you a high or highish power (depending on the scope) with a small true field of view, albeit wider than if you used say a Plossl of similar focal length. Alot of high power planetary and doubles observing is done on axis, so a wide true field of view is nice but not that important. If you are having to nudge the scope then again a wider afov is probably more useful than if you are using a driven mount.

To answer your questions:

A 4" f7 scope should have a 700mm focal length.

An 82 degree afov 5.5mm eyepiece in this scope will give a mag of x127, a true field of view of 0.64 degrees and an exit pupil of 0.78.

The 4" f11 scope has a focal length of 1122mm, so the same eyepiece will give you x204, with a 0.4 degree true fov and an exit pupil of 0.5mm.

You should be able to use a 2" diameter extension tube with the bv section removed. The scope is designed to operate with binoviewers with the focuser in this position so should not clip the light path, although it is worth verifying this for 2" eyepieces too.

What sort of targets are you planning on observing with the scope?

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A pretty accurate idea if true field is apparent field of the eyepiece e.g. 82 divided by magnification so 200x

Therefore 72/200= 0.41 degrees. Exit pupil is simply focal length of eyepiece divided by focal ratio of scope.

If thinking of binoviewers and assuming you want a 4" frac, and assuming it's the new Ascent ED from Altair Astro then I'd go for the f11 without any doubt unless the physical length of the scope is an issue for you.

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Thanks Stu and Moonshane for that feedback & yes that is the scope I'm very close to buying. I only use 1.25" eyepieces and just go for the usual targets such as the moon, planets & stars splitting etc. I love refractors but my issues are floaters, which is why I want to try Binoviewing. Also my present mount is a manual CG4 which may be dodgy, balance wise, with the F11 scope. However, if I'm certain the Ascent can reach focal point with an extension tube with the small section removed, then I believe this setup would work for me. Again thanks

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5 hours ago, Joe G said:

Also my present mount is a manual CG4 which may be dodgy, balance wise, with the F11 scope.

Do you mean the ability to damp a long moment arm's tendency to oscillate everything you touch the focuser?  I know some mounts do fine with a short tube SCT but not so well with a long frac despite both being of equal weight.

 

8 hours ago, Joe G said:

I'm thinking of getting a 4"/ F7 refractor  for a wider FOV. There is a great thread here & CN about a new 4" F11 scope.

Do you want a wider maximum field of view at the expense of a more curved field and more false color in the f/7 tube over the f/11's narrower, flatter, better corrected field of view?  It sounds like you're mostly interested in the f/11 scope.  It would probably work better for the objects you prefer to view.

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Hi Louis

That probably sums up my thoughts on which scope suits best but no more thinking required! My local supplier had the ED102 F11 in stock but their web site stated they only shipped to UK mainland. I then got in touch with RVO & they are sourcing one for me for the same price. I also spoke with Adam, their sales guy & mentioned the issues Astrojenson hightlighted in the CN thread. He will open the box & do a quick QC on the scope prior to dispatch to me. Hopefully it should arrive before the end of the month

Again sincere thanks to everyone who provided their valuable feedback

 

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3 hours ago, Joe G said:

Hi Louis

That probably sums up my thoughts on which scope suits best but no more thinking required! My local supplier had the ED102 F11 in stock but their web site stated they only shipped to UK mainland. I then got in touch with RVO & they are sourcing one for me for the same price. I also spoke with Adam, their sales guy & mentioned the issues Astrojenson hightlighted in the CN thread. He will open the box & do a quick QC on the scope prior to dispatch to me. Hopefully it should arrive before the end of the month

Again sincere thanks to everyone who provided their valuable feedback

 

Good luck.  Make sure to let us know your thoughts on it after a few observing sessions.

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