Quote:
Originally Posted by JJDOBBER79
Hi Profiler,
All the rest of my gear is 1.25" and I think the 17ethos is 2" so 13 would probably be as low power as I would want to go
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Are you sure? The 13mm Ethos will give you a true field of only 0.85 degrees.
Using 1500mm focal length and field stop diameter in mm from Tele Vue here are some more examples. TFOV=FSDx57.3/FL
FL=Focal Length of Telescope
AFOV=Apparent Field of View
FSD=Field Stop Diameter
TFOV=True Field of View
EPD=Exit Pupil Diameter
32mm Plossl, AFOV=50deg, FSD=27mm, TFOV=1.03deg, EPD=6.4mm
24mm Panoptic, AFOV=68deg, FSD=27mm, TFOV=1.03deg, EPD=4.8mm
31mm T5 Nagler, AFOV=82deg, FSD=42mm, TFOV=1.60deg, EPD=6.2mm
26mm T5 Nagler, AFOV=82deg, FSD=35mm, TFOV=1.34deg, EPD=5.2mm
20mm T5 Nagler, AFOV=82deg, FSD=27.4mm, TFOV=1.05deg, EPD=4mm
16mm T5 Nagler, AFOV=82deg, FSD=22.1mm, TFOV=0.84deg, EPD=3.2mm
21mm Ethos, AFOV=100deg, FSD=36.2mm, TFOV=1.38deg, EPD=4.2mm
13mm Ethos, AFOV=100deg, FSD=22.3mm, TFOV=0.85deg, EPD=2.6mm
I suggest you create a spread sheet using data from the following table
http://www.televue.com/engine/TV3b_p...4#.U0csPmcRC9I
Note the same FSD gives the same TFOV. Note also if you have a different brand eyepiece and do not have FSD then you can use TFOV=AFOV/Magnification. The result is very close and any difference is due to distortion effects within the eyepiece. See
http://www.televue.com/engine/TV3b_p...3#.U0czJWcRC9I
See also
http://www.televue.com/engine/TV3b_p...9#.U0cyzWcRC9I
There are many ways to select a series of eyepieces and magnification spread is only one. However when there are different AFOV you could end up having the same TFOV. TFOV spread is another method and yet another is exit pupil spread giving say 6mm, 4mm, 2mm, 0.5mm (maybe 3mm & 0.7m) exit pupils.
I personally like observing with the same AFOV since TFOV and magnification spreads then correlate. (Of course this is not critical).
The actual magnification, TFOV & exit pupil depend a lot on what you a looking at. See also this article
http://www.televue.com/engine/TV3b_p...4#.U0czT2cRC9I Yes it is promoting Ethos but there is a lot of useful info there. Also refer Para beginning "The 1991 article ..."
On the issue of exit pupil, if you are young you can easily accommodate 7mm or more and provided the sky is dark enough you will get a spectacular view. A 41mm Panoptic with your telescope will give you the maximum TFOV of 1.76 degrees at the expense of a large exit pupil.
41mm Panoptic, AFOV=68deg, FSD=46mm, TFOV=1.76deg, EPD=8.2mm
35mm Panoptic, AFOV=68deg, FSD=38.7mm, TFOV=1.48deg, EPD=7mm
The 35mm Panoptic is a very good alternative to the 41mm being lighter and giving slightly less exit pupil.
Note if your purpose is to have as a large TFOV for easier star hopping then a large exit pupil does not matter. The amount of contrast you have will depend a lot on you location and sky conditions but under a dark sky a 41mm Panoptic will still give a very good view. If your eye cannot accommodate 8.2mm exit pupil (and only the young can barely) it just means you're not using all of your mirror diameter but if you're only finding stuff it really doesn't matter.
The 13mm Ethos is a nice choice and if you like it you may end up complementing it one day with a 21mm Ethos.