The 16" F5 I am building will be ready for the pole cutting procedure in the near future. Anybody got any suggestions as to the best eyepiece to use as a center of travel standard? I have one of the JMI units that doesn't move all that much so I need to get this one as close to perfect as I can. In the past I always used my standard starting eyepiece a 2" 32mm Meade UW. I don't want to end up with short poles when my eyepiece collection changes.
What is standard?
Started by David W, 12/26/2003 09:59PM
Posted 12/26/2003 09:59PM
Opening Post
Posted 12/29/2003 06:23AM
#2
Hi David,
If you plan to position the focal plane relative to your focuser so the drawtube travel will encompass all of TeleVue's eyepieces, then almost every non-TeleVue will come to focus within your focuser's range also. The advantage of using TeleVue as the standard is that all of TeleVue's eyepieces have published "flange level" specifications. Flange level is a relative measurement of the distance from focal plane to the eyepiece's flange, or where it seats against the drawtube.
Of all TV's eyepieces, the 31mm Nagler requires the most in-travel (more than almost every other eyepiece out there, including the notoriously in-travel needy Speers-Walers, IIRC). The 31mm Nagler has a flange level spec of -0.38". Others have much higher flange numbers, up somewhere around +0.75" or so. This means that an eyepiece with a flange level of +0.75" will come to focus 1.13" farther out than one with a -0.38" flange level. So, your drawtube need only travel something like 1.25" to adequately cover the range.
Not owning a 31mm at present, I now use my 32mm TV plossl which has a published flange level of +0.25". +0.25" is 0.63" farther out than the 31mm Nagler, and 0.53" farther in than the 12mm Nagler with a flange level spec of +0.78". So, I rack the focuser out around 3/4" with the 32mm plossl in place (the 31mm will require 0.63" additional in-travel than the 32mm plossl, so 3/4" ensures I have the in-travel I need + a little extra) and adjust the mirror until stars focus. Works every time. Also, I use actual stars to prevent errors.
You can find the flange level specs at this site. They are in column "F". Just find your TV eyepiece on the chart and compare it's flange to the 31mm Nagler to find how far out you should rack your focuser before moving your mirror to focus on a star.
http://www.televue.com/engine/page.asp?ID=144
If you plan to position the focal plane relative to your focuser so the drawtube travel will encompass all of TeleVue's eyepieces, then almost every non-TeleVue will come to focus within your focuser's range also. The advantage of using TeleVue as the standard is that all of TeleVue's eyepieces have published "flange level" specifications. Flange level is a relative measurement of the distance from focal plane to the eyepiece's flange, or where it seats against the drawtube.
Of all TV's eyepieces, the 31mm Nagler requires the most in-travel (more than almost every other eyepiece out there, including the notoriously in-travel needy Speers-Walers, IIRC). The 31mm Nagler has a flange level spec of -0.38". Others have much higher flange numbers, up somewhere around +0.75" or so. This means that an eyepiece with a flange level of +0.75" will come to focus 1.13" farther out than one with a -0.38" flange level. So, your drawtube need only travel something like 1.25" to adequately cover the range.
Not owning a 31mm at present, I now use my 32mm TV plossl which has a published flange level of +0.25". +0.25" is 0.63" farther out than the 31mm Nagler, and 0.53" farther in than the 12mm Nagler with a flange level spec of +0.78". So, I rack the focuser out around 3/4" with the 32mm plossl in place (the 31mm will require 0.63" additional in-travel than the 32mm plossl, so 3/4" ensures I have the in-travel I need + a little extra) and adjust the mirror until stars focus. Works every time. Also, I use actual stars to prevent errors.
You can find the flange level specs at this site. They are in column "F". Just find your TV eyepiece on the chart and compare it's flange to the 31mm Nagler to find how far out you should rack your focuser before moving your mirror to focus on a star.
http://www.televue.com/engine/page.asp?ID=144
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