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Equipment Discussions >> Eyepieces

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bdawg6381
sage
*****

Reged: 12/04/08

Loc: Blacksburg, VA
Master Eyepiece Focus Distance Chart
      #4360360 - 02/03/11 11:20 AM Attachment (476 downloads)

While trying to determine a good length to cut my poles for my truss dob I'm building I wanted to make sure that I had enough in-travel distance for future upgrades. What if I get a paracorr? What if I get an EP that requires more in-travel? What if I upgrade my focuser???

There was a good post by Tamiji Homma last spring I was referred to (thanks catboat) that listed a lot of eyepieces and their respective focus distances that were measured with the "zero" point being somewhere in the middle of the measured ones (Pentax XW 20mm). Luckily one of my eyepieces I owned was on his list so I had a reference point for all of my eyepieces to compare to.

I turned "Tammy's" list into an excel file and added more information to it. The first 6 EP's listed are my own, after that the rest are the ones that were measured by Tammy in order of increasing required focus length. I also added other EP specifications I could find through some internet searches. At the bottom of the list is the avg. paracorr in-focus distance you need to subtract as well as the shoulder distances of several focusers in case you want to upgrade later. Some blocks are blank because either I didn't know what he was referring to or I couldn't find the information. Regardless I think that this list will be helpful to people trying to determine the placement of their focusers so that they can accomodate all future EP purchases.

FWIW the MAX drawtube travel needed (not counting different focuser base heights) including the use of a paracorr is 1.94".

Maybe we can turn this into a working list and get a true "Master" chart going as an eventual "sticky" post.

Enjoy!


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bdawg6381
sage
*****

Reged: 12/04/08

Loc: Blacksburg, VA
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4360379 - 02/03/11 11:29 AM

Original post by Tammy here.

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dyslexic nam
Carpal Tunnel
*****

Reged: 01/28/08

Loc: PEI, Canada
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4360488 - 02/03/11 12:17 PM

Useful info. I am trying to rig a filter wheel into my newt's optical train, so knowing which ep's require extra out-travel is very useful. Kudos to you and Tammy.

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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: Master Eyepiece Focus Distance Chart new [Re: dyslexic nam]
      #4361144 - 02/03/11 04:57 PM

Nice find thanks for sharing.... dont forget the Paracorr in of 0.25"

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bdawg6381
sage
*****

Reged: 12/04/08

Loc: Blacksburg, VA
Re: Master Eyepiece Focus Distance Chart new [Re: gatorengineer]
      #4361308 - 02/03/11 06:15 PM

I've found conflicting info on the paracorr... on my list I have the distance as 0.5" (12.7mm). Is this too much? I know the tunable top makes it a range instead of exact... just wondering what the "middle" setting is.

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Plan9
professor emeritus


Reged: 01/28/08

Loc: Northern VA
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4361670 - 02/03/11 08:57 PM

*BLEEP*,

Thank you for compiling this! I've been about to try to work out these distances for eyepieces I own for a scope I'm building. At least a few are on your list.

I'll be happy to send along some additional measurements for other eyepieces; any good links/pointers on how to do this accurately? I'm not sure I could achieve .01mm precision, or put another way, I'm sure I could not achieve .01mm precision!

I'll also try to give you a measurement for my HC-1 focuser, although there's really a question of where the bottom is, since it comes with a variety of spacers. (I take it you were measuring from the underside of the mounting plate?)

Cheers!

Bill


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bdawg6381
sage
*****

Reged: 12/04/08

Loc: Blacksburg, VA
Re: Master Eyepiece Focus Distance Chart new [Re: Plan9]
      #4361780 - 02/03/11 09:53 PM

The .01mm precision comes from Tammy's numbers, not mine. The closest I could estimate was about 0.5mm. I had to normalize my numbers with Tammy's hence the "0.01" accuracy showing up in my numbers (the first 6 on the list). I just did it visually using the scale etched on my drawtube that has 1mm increments on it. Maybe if you had one of those fancy WO digital readouts you could get some repeatable accurate readings. I'm not trying to be the compiler of the list but feel free to add numbers to it and repost an updated one on this thread.

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Tamiji Homma
Post Laureate
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Reged: 02/24/07

Loc: California, USA
Re: Master Eyepiece Focus Distance Chart new [Re: Plan9]
      #4361797 - 02/03/11 10:01 PM

Hi Bill,

I used caliper with digital reading to measure distance between end of OTA and diagonal. Regarding accuracy of the measurement, it has more errors in longer focal length eyepiece because it is bit harder to get critical focus right every time.

I used a target (Satellite TV dish antenna, focusing to get the best reading of letters on the dish) about 1Km away. Obviously, with long local length eyepiece, letter was too small to read so that I may get the focus bit off.

The data was compiled with multiple measurements different times. You would get different reading depending on condition that the measurement was done. The condition includes temperature of scope, air turbulence, eye, etc. So I removed data bit too off each extreme to minimize errors.

Hi *BLEEP*,

Thank you for making effort. I'll have a few more data coming later

Tammy


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bdawg6381
sage
*****

Reged: 12/04/08

Loc: Blacksburg, VA
Re: Master Eyepiece Focus Distance Chart new [Re: Tamiji Homma]
      #4378631 - 02/11/11 11:12 AM

Didn't realize my name was a "bad word" here... maybe some spillover from the EP forum discussions of a certain eyepiece... funny

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Mike Hosea
Postmaster
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Reged: 09/24/03

Loc: "Metrowest" Boston
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4378711 - 02/11/11 11:48 AM

Hopefully a temporary thing. Let's give it a try today: Brandon, Brandon, Brandon

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djeber2
Pooh-Bah
*****

Reged: 07/02/04

Loc: Cloudy Midwest
Re: Master Eyepiece Focus Distance Chart new [Re: Mike Hosea]
      #4378747 - 02/11/11 12:07 PM

Great info, thanks for posting!!

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bdawg6381
sage
*****

Reged: 12/04/08

Loc: Blacksburg, VA
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4378893 - 02/11/11 01:08 PM Attachment (260 downloads)

New list attached. Tammy provided me some more numbers (and repeated measurements for some already there). For the repeats I averaged his results. I switched the display on focus distance to within 1/2 a mm (hard to be repeatable under that amount anyway). If you really want to know Tammy's actual number to the "0.01mm" then just click on that cell.

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Jitou
super member


Reged: 07/29/08

Loc: France
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4380799 - 02/12/11 11:09 AM

Thanks for the update. So 2 inch travel is all what is needed including with the use of a Paracorr. What about using a Paracorr type 2 ? According to TV this new Paracorr needs at most 3/4" in travel more compared to using the EP alone but I have no mean to verify that for now.

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bdawg6381
sage
*****

Reged: 12/04/08

Loc: Blacksburg, VA
Re: Master Eyepiece Focus Distance Chart new [Re: Jitou]
      #4406096 - 02/23/11 01:42 PM

From several posts I've read and doing some reasearch the paracorr values should be 12.7mm to 19mm, NOT the 6.8mm to 12.7mm that I have listed. Still need to confirm this with an actual measurement though... any takers? Just need the difference in focuser positions (delta), don't have to worry about normalizing your numbers to the chart.

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gatorengineer
Carpal Tunnel
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Reged: 02/28/05

Loc: Hellertown, PA
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4469806 - 03/23/11 10:04 PM

Any thoughts on paracorr ii values? This thread really deserves a stickey

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bdawg6381
sage
*****

Reged: 12/04/08

Loc: Blacksburg, VA
Re: Master Eyepiece Focus Distance Chart new [Re: gatorengineer]
      #4470803 - 03/24/11 11:39 AM

It would be really easy to measure with anyone that has a marked drawtube. Just insert an EP and then insert the EP with a paracorr II and post the focus distance difference in mm... Any takers?

(I thought this would get a sticky status too... )


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csa/montana
Den Mama
*****

Reged: 05/14/05

Loc: montana
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4470870 - 03/24/11 12:10 PM

Quote:

(I thought this would get a sticky status too... )






How about in the sticky "Best of Eyepieces Forum"?

http://www.cloudynights.com/ubbthreads/showthreaded.php/Number/2305143/


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bdawg6381
sage
*****

Reged: 12/04/08

Loc: Blacksburg, VA
Re: Master Eyepiece Focus Distance Chart new [Re: csa/montana]
      #4470926 - 03/24/11 12:35 PM

Sweet! Thanks Carol!

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csa/montana
Den Mama
*****

Reged: 05/14/05

Loc: montana
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4470939 - 03/24/11 12:39 PM

You're most welcome! It certainly deserves to remain a sticky.

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catboat
professor emeritus
*****

Reged: 12/01/09

Loc: Maine
Re: Master Eyepiece Focus Distance Chart new [Re: bdawg6381]
      #4471445 - 03/24/11 05:02 PM

Hi Brandon,

It's great that you took this and ran with it.

When I posted last April, Tammy replied with his mind-boggling list of relative focus travel. And BillB9430 provided very useful info on when you need still more in-travel (for barlows or for near-sighted viewers who are viewing without glasses). I'd hoped that others would add to Tammy's list, but it didn't happen -- until now!

Your additions and excel file are invaluable in the age of short drawtubes and low-profile focusers. Anyone building a scope, replacing a focuser, or buying a new ep should consult your updated list. I hope others will add to it -- there are a lot more ep's out there.
(And thanks for a great Brandon thread!)

Edward


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