See the following posts for why this post has a mistake in logic...Don
Ethos Paracorr Settings:
TeleVue's recommended settings are incorrect, according to both my eye and the two excellent calculators on this thread.
Here's what I found:
Used as 2" eyepieces in the Paracorr:
13mm Ethos--Setting 1, with the eyepiece pulled out of the Paracorr 1.5mm
8mm Ethos--Setting 1, with the eyepiece pulled out of the Paracorr 12.5mm (not practical unless a 2" skirt is added to the eyepiece)
Used as 1.25" eyepieces in the Paracorr:
13mm Ethos--Setting 4
8mm Ethos--Setting 1 with the eyepiece pulled 2.5mm further out.
It seems TeleVue's recommended settings may have been calculated using pre-production design specs. TeleVue says that the 1.25" settings for the Ethos are 5(13) and 4(8) respectively, and that isn't even close on the 8mm.
I used the empirical method of actually looking at the edge-of-field star images, and the settings I derived agreed with the 2 calculators on this thread.
The focal plane of the 8mm is 0.7" (17.78mm) below the 2" shoulder, or 0.43" (10.9mm) lower in the barrel than the 13.
If the 13 uses setting 1 in the Paracorr in 2" mode, the 8mm is going to need to be at least 10.9mm farther out than setting 1. If used with a Paracorr, there's no way around a lot of infocusing on the eyepiece/Paracorr combination. I caution any potential buyers to look at their focuser position for the 13 before you buy an 8mm--you will need around a half inch of additional in-focus.
My recommendations:
Use the 13 Ethos as a 2" and use setting 1.
Use the 8 Ethos as a 1.25" and use setting 1 and pull the eyepiece out about 2mm. This will require a LOT of infocusing.
-------------------- Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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