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Starman1
Postmaster
Reged: 06/24/03
Posts: 10636
Loc: Los Angeles
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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
Edited by Starman1 (08/07/08 01:52 AM)
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Tigerider
Vendor (Televue Rep)
Reged: 02/21/06
Posts: 187
Loc: Torrance, CA.
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Quote:
Ethos Paracorr Settings:
TeleVue's recommended settings are incorrect, according to both my eye and the two excellent calculators on this thread.
<snip>
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.
Wow thats quite a statement...
Let's see : The TV website recommends the #1 setting for the 13 Ethos also,
so how are we incorrect there ?
As for the 8 mm: Your's & our settings are 12 mm apart, which 1.25 adapter are you using on the 8 mm Ethos ?
We recommend the "lower" adapter that comes with the Paracorr.
--------------------
John Rhodes (TV Rep.)
TeleVue: NP 127is,101is,101
102,85,76,60is,60 all EP's+
Celestron Ultima 11
Celestron C-5
Meade Research Grade 856
Celestron 20 x 80
Canon 20 Da
Canon "is" Binoculars 10x30 & 15x50
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Starman1
Postmaster
Reged: 06/24/03
Posts: 10636
Loc: Los Angeles
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John,
Yes, the 13Ethos works great at setting 1 (2").
I'm using the standard 1.25" adapter that comes with the Paracorr.
Here's the math:
The focal plane is 0.70" below the shoulder, per TV's chart.
The 1.25" adapter is 0.379" tall (calipers measurement).
That combination results in a focal plane to optimum Paracorr lens distance of 2.76mm too low for the 8mm Ethos when the Paracorr is set to setting 1, that is the 8mm eyepiece needs to go up another 2.76mm to achieve the optimum 55mm distance from focal plane to lens.
Have I missed something?
I could be wrong, but the calculator shows setting 4 would be 12.28mm too low, and that setting 1 is 2.76mm too low.
I'm forgetting something--see below
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
Edited by Starman1 (08/07/08 01:54 AM)
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Starman1
Postmaster
Reged: 06/24/03
Posts: 10636
Loc: Los Angeles
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John, et al.,
My wife just pointed out how much of a bonehead I was. It took her to show me the error in my logic. 
Now I eat crow....
The focal plane of the 8mm Ethos is 0.7" below the 2" shoulder.
When I put the eyepiece in the adapter, it automatically raises the eyepiece by the length of the 2" skirt (1.04"). Hence, the focal plane of the eyepiece is now 1.04-0.7 ABOVE the adapter (or 0.34"). So long as I also take into account the thickness of the adapter, the calculator should work fine.
According to the TeleVue website, that means the focal plane is above the reference surface (the bottom of the 2" skirt the eyepiece is resting on).
That means I should be able to plug the 0.34" figure into the calculator and get the proper setting for the Paracorr.
OK. I get setting 3.
It gets setting 4 for the Type 6 Naglers (agreeing with TeleVue), and setting 5 for the 22T4 (again agreeing with TeleVue), and setting 3 for the 17 T4 (same as TeleVue).
But I get setting 3 for the 8 Ethos (no agreement there--TV says 4) and setting 3 for the 16T5 (where TV says 4), but the difference is now 1 setting (0.125" or 3mm).
That says the 8mm Ethos is best used as a 1.25" eyepiece in the Paracorr.
So, John, is my math now better? Or have I overlooked something else?
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
Edited by Starman1 (08/07/08 01:56 AM)
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Starman1
Postmaster
Reged: 06/24/03
Posts: 10636
Loc: Los Angeles
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Plugging the focal plane positions from TeleVue's website into Walt's calculator (and compensating for the skirts on the 12mm T4 and Ethos eyepieces), I get the following optimum(55mm spacing) settings for the Nagler eyepieces(1.25 eyepieces use adapter):
31T5--6 (there is no 6, so use setting 5)
26T5--3.3
22T4--5.2
20T5--1.7
17T4--3.2
16T5--3.7
13T6--4.0
12T4--3.9 (with 1.25" adapter)
11T6--4.0
9T6--4.0
7T6--4.0
5T6--4.0
3.5T6--4.0
2.5T6--4.0
13 Ethos--.8 (as 2")
8 Ethos--4.2 (as 1.25")
Rounded off to the nearest whole number, the settings seem to agree with TeleVue's chart.
It should be noted that TeleVue says +/- 4mm is an acceptable margin for error on the setting. Since the settings on the Paracorr are 3.2mm apart, that means there is quite a bit of leeway in the setting of the Paracorr (i.e. that if optimum is setting 4, good correction can be anywhere from setting 3 to setting 5).
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
Edited by Starman1 (08/07/08 02:27 PM)
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Zoomit
member
Reged: 12/04/06
Posts: 87
Loc: Tehachapi, CA
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Quote:
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)
Don, if you would...can you confirm this is still accurate?
--------------------
Brandon
18" f/4.6 AstroSystems TeleKit
7.4" f/6.4 Orion XT8
15x70 Astro-Physics, 12x50 Nikon Action EX
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Starman1
Postmaster
Reged: 06/24/03
Posts: 10636
Loc: Los Angeles
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WEll, not quite.
I'm sorry if it wasn't clear.
Close, but not exact.
I was slightly mistaken in how to measure the eyepieces with a skirt.
The Ethos 13, if used as a 2", requires setting 1 (all the way out). That is close to the optimum setting. If used in the 1.25" mode, setting 5.
The Ethos 8mm, if used as a 1.25" eyepiece with the standard Paracorr adapter, requires setting 4. It is not usable in a 2" mode unless a barrel extension is added to the eyepiece, in which case the eyepiece would be pulled out 12.4mm from the Paracorr, with the PC in setting 1. In other words, this eyepiece requires the use of the 1.25" adapter in setting 4. I wouldn't bother to try to use it as a 2" eyepiece in the Paracorr. Just remember that this requires some in-focus movement of the focuser.
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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