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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
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.
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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: 205
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.
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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
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
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
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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
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
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
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
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
super member
Reged: 12/04/06
Posts: 125
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?
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Brandon
AstroSystems TeleKit 18" f/4.6; Orion XT8 7.4" f/6.4
TeleVue 26Nt5, 13E, 3-6NZ; Antares 2" 1.6x
Astro-Physics 15x70; Nikon Action EX 12x50
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
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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Fireball
sage
Reged: 03/24/06
Posts: 270
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Can somebody help me with the Meade 5000 UWA ?
I have 6.7 - 18 - 31mm.
What about Hyperion, e.g 13mm ? Or QX26mm ?
TIA
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20x90 Bino
12" Lightbridge
Meade UWAs, TV Ethoi, Pentax XW, Hyperion, AT Titan.
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
Loc: Los Angeles
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It might be helpful to review how to pick a proper setting for the Paracorr:
1) If you have a TeleVue eyepiece, set the Paracorr for that eyepiece (from the TV website) and focus. Remove the eyepiece and insert another. Focus using the tunable top on the Paracorr, not the normal focuser knobs. Note the setting, write it down, because that's the setting for that eyepiece.
2) If you don't have a TeleVue eyepiece, start in setting 1 (tunable top all the way turned out), and put the eyepiece in. Focus. Evaluate star images at edge of field. Still flared in a radial direction slightly? Move Paracorr to setting 2. Refocus. Star images at the edge better or worse? Worse? Setting 1 is your choice. Better? Move Paracorr to setting 3 and focus again. Evaluate stars at the edge. Worse? Go back to setting 2. Better? Continue to setting 4. Etc.
You will find one setting that is the best for that eyepiece. Write it down. Return Paracorr to that setting.
Remove that eyepiece, insert other eyepieces one at a time, and focus using the Paracorr tunable top instead of the focuser. Note the setting for each eyepiece where it is in focus--that's the Paracorr setting for that eyepiece. In this way, you can do a whole collection of eyepieces with experimentation only on the very first eyepiece.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Robert Provin
professor emeritus
Reged: 08/14/06
Posts: 565
Loc: Northridge, California
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Quote:
It might be helpful to review how to pick a proper setting for the Paracorr:
1) If you have a TeleVue eyepiece, set the Paracorr for that eyepiece (from the TV website) and focus. Remove the eyepiece and insert another. Focus using the tunable top on the Paracorr, not the normal focuser knobs. Note the setting, write it down, because that's the setting for that eyepiece.
2) If you don't have a TeleVue eyepiece, start in setting 1 (tunable top all the way turned out), and put the eyepiece in. Focus. Evaluate star images at edge of field. Still flared in a radial direction slightly? Move Paracorr to setting 2. Refocus. Star images at the edge better or worse? Worse? Setting 1 is your choice. Better? Move Paracorr to setting 3 and focus again. Evaluate stars at the edge. Worse? Go back to setting 2. Better? Continue to setting 4. Etc.
You will find one setting that is the best for that eyepiece. Write it down. Return Paracorr to that setting.
Remove that eyepiece, insert other eyepieces one at a time, and focus using the Paracorr tunable top instead of the focuser. Note the setting for each eyepiece where it is in focus--that's the Paracorr setting for that eyepiece. In this way, you can do a whole collection of eyepieces with experimentation only on the very first eyepiece.
Don, your posts here have been very helpful, thank you! 
BTW, it was nice meeting you up at Mt. Pinos last weekend.
Robert
--------------------
Robert W. Provin
Dept. of Geography
California State University
Northridge, CA 91330-8249
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peter k
member
Reged: 02/03/07
Posts: 99
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Quote:
It might be helpful to review how to pick a proper setting for the Paracorr:
1) If you have a TeleVue eyepiece, set the Paracorr for that eyepiece (from the TV website) and focus. Remove the eyepiece and insert another. Focus using the tunable top on the Paracorr, not the normal focuser knobs. Note the setting, write it down, because that's the setting for that eyepiece.
2) If you don't have a TeleVue eyepiece, start in setting 1 (tunable top all the way turned out), and put the eyepiece in. Focus. Evaluate star images at edge of field. Still flared in a radial direction slightly? Move Paracorr to setting 2. Refocus. Star images at the edge better or worse? Worse? Setting 1 is your choice. Better? Move Paracorr to setting 3 and focus again. Evaluate stars at the edge. Worse? Go back to setting 2. Better? Continue to setting 4. Etc.
You will find one setting that is the best for that eyepiece. Write it down. Return Paracorr to that setting.
Remove that eyepiece, insert other eyepieces one at a time, and focus using the Paracorr tunable top instead of the focuser. Note the setting for each eyepiece where it is in focus--that's the Paracorr setting for that eyepiece. In this way, you can do a whole collection of eyepieces with experimentation only on the very first eyepiece.
This is a very useful post--thanks, Don. However, there seems to be something I'm missing. The above quote suggests (to me) that for a given scope there is a single, fixed point of best focus for the Paracorr, and once this is established, focus is obtained for different eyepieces by use of the tunable top, leaving the focuser untouched, regardless of eyepiece. This does not seem consistent with earlier posts, including Don's just a couple up, which states that additional in-focus is needed for the Ethoses. Can someone (Don?) clarify?
Thanks much.
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
Loc: Los Angeles
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Sure.
There is one distance from the paracorr lens to the eyepiece's focal plane that results in the best coma correction.
But how do you find it?
Answer: correct the coma completely for one eyepiece. You have found that eyepiece's lens-to-focal plane distance.
Insert another eyepiece and focus using the Paracorr top. Note that while doing so, you are effectively moving the eyepiece in and out to adjust the eyepiece position relative to the focal plane of the telescope plus Paracorr. This is really no different than you would do adjusting the focus of the eyepiece in the focuser without the Paracorr, only instead of moving the focuser, you're moving the Paracorr's top.
What it implies, and it would be true, that there is only one position for the focal plane of the telescope, whether the Paracorr is used or not. The Paracorr moves the focal plane position slightly, but the idea is the same as having no Paracorr--you are moving the focal plane of the eyepiece to the focal plane of the scope in order to focus.
The Paracorr does not focus, per se, so you are not focusing the Paracorr, merely putting it into the system to modify the focal plane slightly.
Now, if your eyepiece's focal plane is low in the eyepiece, you will have to move the eyepiece back relative to the Paracorr's lens in order to maintain the correct distance to the lens for best correction. But that means the focal plane of the system is now higher than before. In order to bring the focal plane of the paracorr plus eyepiece to be coincident with the focal plane of the telescope, you will have to move the Paracorr plus eyepiece in to make the focal planes coincident.
So, eyepieces with low focal planes require more out travel with the Paracorr and more inward movement of the focuser.
In a sense, that's no different than an extension tube. Add an extension tube to the focuser, and you will have to move the focuser in to get to focus: eyepiece goes out, focuser goes in.
Hope that makes it clearer.
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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