Having covered my high power needs with shorter efl EPs, I'm wondering what "rig" would work best with the C-14 to get the widest possible views on extended deep-sky objects without vignetting and light loss? I've read that the lowest usable power in a C-14 is about 56X. This would require something like a 70mm EP. I suppose there may be a few narrow-field Plossls in this range, but I really don't care for narrow AFOVs when observing wide field objects. What about using a 35 Panoptic and a focal reducer? Would the field illumination suffer as a result? To make a long story short, what would be the maximum TFOV that I could achieve and what eyepieces or combination of eyepiece and reducer, would you recommend to get there? Thanks! - Chuck
Lowest Power and FOV in a C-14
Started by TheDesertSage, 03/05/2006 11:12PM
Posted 03/05/2006 11:12PM
Opening Post
Posted 03/06/2006 01:30AM
#1
It is my understanding that you cannot use a 2" wide field eyepiece in a focal reducer without serious vignetting. The widest true field with the focal reducer and a 1.25" eyepiece would be about 0.75 degrees in your scope.
A slightly larger true field can be had using a 2" wide field eyepiece in a 2" diagonal. Something like the 41mm Panoptic or the UO 40MK70 would get you about 0.80 degrees, which would be about your maximum true field.
Hugh Bartlett
A slightly larger true field can be had using a 2" wide field eyepiece in a 2" diagonal. Something like the 41mm Panoptic or the UO 40MK70 would get you about 0.80 degrees, which would be about your maximum true field.
Hugh Bartlett
"Praise the Lord for the expanding grandeur of creation, worlds known and unknown, galaxies beyond galaxies, filling us with awe and challenging our imaginations." 2007 Reform Siddur
Posted 03/06/2006 06:47AM
| Edited 03/06/2006 06:59AM
#2
Chuck Burton said:
Having covered my high power needs with shorter efl EPs, I'm wondering what "rig" would work best with the C-14 to get the widest possible views on extended deep-sky objects without vignetting and light loss? I've read that the lowest usable power in a C-14 is about 56X. This would require something like a 70mm EP. I suppose there may be a few narrow-field Plossls in this range, but I really don't care for narrow AFOVs when observing wide field objects. What about using a 35 Panoptic and a focal reducer? Would the field illumination suffer as a result? To make a long story short, what would be the maximum TFOV that I could achieve and what eyepieces or combination of eyepiece and reducer, would you recommend to get there? Thanks! - Chuck
Hi Chuck,
Just some general thoughts. First, the 41mm Pan is an excellent eyepiece and provides the widest FOV through a 2" you can obtain (See Al Naglers notes on this on the TV website). I also like the UO 40mm MK80 which is an excellent EP especially for the price.
If you want to go with a longer FL and retain a reasonable FOV I can suggest the 60mm Masuyama and the 65mm Clavé. (I would avoid the 75mm Clavé as it provides a distinct port hole view.) Neither of these are going to provide the wide angle view it sounds like you're looking for though. Without checking, I would guess these eyepieces are relatively narrow, about 52° to 55°. I also like the Vixen K70 but it is a Kellner and has an even narrower FOV.
I also have a Russell 85mm Super-Plossl which is a really fun eyepiece but I doubt it's what you're looking for. See the review/article written about this eyepiece here on AM some months ago. ( http://www.astromart.com/articles/article.asp?article_id=235 )
Bottom line is the Field Stop eventually limits your FOV. There's only so much you can squeeze through without developing aberration. With a 2" eyepiece that limit is about 40mm for the Panoptic/Konig design and ranges up to about 55mm to 65mm for a plossl and similar designs. At least that's my experience. You can find longer focal lengths but they will not provide any wider AFOV.
Hope that helps.
Clear Skies, Bill
Posted 03/06/2006 07:56AM
| Edited 03/06/2006 07:58AM
#3
Chuck,
Lumicon makes a focal reducer "Giant Easy Guider" that gives about f5 to f6.5 with an 80mm lens, so it should allow you to fully illuminate a 2" ep.
At f5, a 31 Nagler gives 57.4X @ 1.43 degree.
However there could be obstruction issues showing up at these low powers, central blackout effects, distortion because the reducer lense is not fully corrected etc.
I have used one of these with mixed results visually. I may try it with the 31mm Nagler if we ever get another clear sky here in North CA ?!
As the exit aperture of the C14 is about 3", I suppose a special EP could be built with that size field lense, but I have not yet seen anything like this. There may be some military ep's like this though.
Lumicon makes a focal reducer "Giant Easy Guider" that gives about f5 to f6.5 with an 80mm lens, so it should allow you to fully illuminate a 2" ep.
At f5, a 31 Nagler gives 57.4X @ 1.43 degree.
However there could be obstruction issues showing up at these low powers, central blackout effects, distortion because the reducer lense is not fully corrected etc.
I have used one of these with mixed results visually. I may try it with the 31mm Nagler if we ever get another clear sky here in North CA ?!
As the exit aperture of the C14 is about 3", I suppose a special EP could be built with that size field lense, but I have not yet seen anything like this. There may be some military ep's like this though.
Attached Image:
David Simons
