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Whats the widest field of a small refractor?

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#1 Brent Campbell

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Posted 03 December 2012 - 04:23 PM

I just purchased an orion express 80, 550 mm focal length, 80 mm aperture. It has a 2" focuser and according to the formulas it should be capable of a 4.79 degree TFOV.

But what is the practical limit? I've heard that field curvative can affect wide field views. What should I expect out of this scope? I have had constant rain so first light has had to wait. My expectations of this scope is as a wide field grab and go.

My current 24 mm 80 degree can give a TFOV of 4 degrees at 20 X. If I went with a 30 mm 80 degree I could get a 5 degree TFOV (albeit vigetted) but would it be worth it at all? My current eyepiece selection is in my signature.

The reason I am doing this is that I am trying to find out which of my eyepieces go with which scope so I don't waste time changing things out. I think my 40mm is out of the question (too much weight anyway) and my 24 MM, 18mm, and 11mm explore scientifics are my best option on this scope?

The other nagging question is would my zoom eyepiece be a good fit for this? My zoom is 8-24 with a 50 degree FOV at 24 mm. This makes for a nice 2.5 degree TFOV at 24 mm which isn't so bad! Just don't know which way I should try and due to so much bad weather no time to experiment. :help:

#2 Jon Isaacs

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Posted 03 December 2012 - 04:40 PM

Brent:

I believe the Express 80 was had a 480mm focal length rather than 550mm. With an eyepiece that has a 46mm field stop, that would translate into about 5.5 degree TFoV.

The quality of the widefield views in such scopes depends on your tolerance for field curvature and the quality of the eyepiece. The field curvature is there to be seen and with an eyepiece that has aberrations of it's own it will be even messier, particular when looking at brighter Messier objects.

But even with the field curvature and possible astigmatism,, it's definitely worth having an eyepiece that provides nearly the maximum field of view. Those big views can be most memorable. I recently added a 2 inch focuser to an 80mm F/5 achromat. With the 31mm Nagler, the field curvature is quite apparent but even so, the 6.0 degree TFoV is still to kill for when the skies are dark and clear... and at times, I use it with the 35mm ES-70 which provides 6.4 degree TFoV, pretty rough around the edges but with a O-III or Ultrablock...

Jon

#3 Mark9473

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Posted 03 December 2012 - 04:40 PM

I agree completely that a 2.5° FOV is pretty good! I use an 8-24 zoom on my 80mm apo (555mm FL) about 95% of the time - it is just so convenient. I did modify my set-up a bit so that the zoom gives me 32-96x magnification instead of the native 23-69x - sacrificing a bit of FOV at the long end for better reach at the high end. But I could equally be happy with the 23-69x zoom and a single higher power eyepiece.

#4 Jon Isaacs

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Posted 03 December 2012 - 06:07 PM

I agree completely that a 2.5° FOV is pretty good!



One can achieve a 2.5 degree field of view with a scope with a 1000mm focal length. I have an 8 inch F/5 Newtonian that is capable of a 2.5 degree TFoV.

The thing a scope like this can do that other larger scopes cannot is is provide bright, 4+ degree FOV .. from where I sit, it seems sad not to let the scope do what it does best...

Jon

#5 GlennLeDrew

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Posted 03 December 2012 - 11:00 PM

I'm with Jon! Get an eyepiece which will provide the largest field with near to the largest useable exit pupil. Even though the outer field may exhibit significant defocus and astigmatism, you won't notice it so badly as long as you are gazing toward the field center. Let your very low resolution peripheral retina make use of the outer field while your much sharper fovea concentrates on the middle.

#6 Brent Campbell

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Posted 04 December 2012 - 07:07 AM

Thanks everyone! I guess my 40 mm Meade SWA is not out of the question! It s a huge eyepiece however. I probably will more likely use my 24 mm meade ultrawide until I can afford a 32 mm explore 80 degree for this scope.






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