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Master Eyepiece Focus Distance Chart

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#1 bdawg6381

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Posted 03 February 2011 - 11:20 AM

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! :grin:

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#2 bdawg6381

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Posted 03 February 2011 - 11:29 AM

Original post by Tammy here.

#3 dyslexic nam

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Posted 03 February 2011 - 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.

#4 gatorengineer

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Posted 03 February 2011 - 04:57 PM

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

#5 bdawg6381

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Posted 03 February 2011 - 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.

#6 Plan9

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Posted 03 February 2011 - 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

#7 bdawg6381

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Posted 03 February 2011 - 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. :)

#8 Tamiji Homma

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Posted 03 February 2011 - 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

#9 bdawg6381

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Posted 11 February 2011 - 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

#10 Mike Hosea

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Posted 11 February 2011 - 11:48 AM

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

#11 djeber2

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Posted 11 February 2011 - 12:07 PM

Great info, thanks for posting!!

#12 bdawg6381

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Posted 11 February 2011 - 01:08 PM

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.

Attached Files



#13 Jitou

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Posted 12 February 2011 - 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.

#14 bdawg6381

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Posted 23 February 2011 - 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.

#15 gatorengineer

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Posted 23 March 2011 - 09:04 PM

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

#16 bdawg6381

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Posted 24 March 2011 - 10:39 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... :shrug:)

#17 csa/montana

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Posted 24 March 2011 - 11:10 AM

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



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

http://www.cloudynig...Number/2305143/

#18 bdawg6381

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Posted 24 March 2011 - 11:35 AM

Sweet! Thanks Carol!

#19 csa/montana

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Posted 24 March 2011 - 11:39 AM

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

#20 catboat

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Posted 24 March 2011 - 04:02 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

#21 gatorengineer

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Posted 24 March 2011 - 07:59 PM

Thanks for the Stickey

#22 sixela

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Posted 15 December 2011 - 06:50 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.


The "extra" focuser in-travel when inserting a Paracorr is not a constant. It needs less focuser in-travel for eyepieces that need a lot of focuser in-travel in the first place (because you will set the tunable top lower for them).

In fact, once you have a Paracorr, the amount of focuser travel you need for all the eyepieces that have a correct tunable top setting is zero. The tunable top is a parfocalising device to ensure the Paracorr always ends up at the same spot.

In my experience, to use both Paracorrs correctly, you need to place the focal plane roughly 22-25mm over the fully racked in focuser. That leaves you roughly 2-5 millimetres or so on both sides of focus.

The Paracorr 2 needs a bit more distance from the fully racked in focuser to the focal plane of the scope (my guesstimate is 4-5mm, at least that's what it felt like when I had to move the mirror forward a bit when going from a type 1 to a type 2). That's to be expected, since it does have a correct tunable top setting for some eyepieces for which the type 1 doesn't have a correct tunable top setting (e.g. a 21E or 31T5).


#23 gatorengineer

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Posted 10 September 2014 - 07:40 PM

lets bump this paracorr 2 settings anyone?



#24 Starman1

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Posted 11 September 2014 - 01:34 PM

Determining the correct settings for a whole slew of eyepieces only needs you to determine the correct setting for one.

All the rest of your eyepieces will be in the right place if you focus with the tunable top of the Paracorr.

So all you need is to optimize one eyepiece.

 

Take your eyepiece that requires the most In-focusing of all your set to achieve focus.

Put it in the Paracorr, with the tunable top all the way UP (or out).

Focus the scope.  Look at the edge.  You will almost certainly see coma there because it is highly unlikely that will be the optimum

setting for that eyepiece.  Run back and forth through focus so you can ascertain what is astigmatism and what is coma.

Coma makes the star look a little like a comet, not a seagull or an oval.

 

Turn the Paracorr setting in one letter and refocus the scope.  Look at the stars at the edge of the field.  Better?  Good.

Still displaying a bit of coma and not perfectly round like in the center of the field?

Move in one more setting and refocus the scope.

Repeat as necessary.

 

When you arrive at the correct setting, coma will be gone.  If the eyepiece has field curvature, the star images at the edge may be out of focus, but round.

In that case, a touch of the focuser and the stars at the edge will be small points.

If you still see classic astigmatism (star image elongates one way on one side of focus and 90 degrees from that on the other side of focus,

but in focus appears to be a small dot without radial smearing, then you've arrived at the correct setting of the Paracorr for that eyepiece.

Don't worry if it's all the way in to the in-most setting as long as you get there.

 

Write down the correct setting so you can return the Paracorr's top to that setting before the next use of that eyepiece.

 

For any other eyepiece, simply insert in the Paracorr and focus by turning the tunable top.  Look at the setting and write it down.  That's the setting for that eyepiece.

In a few minutes, you can do a whole collection of eyepieces.  if the eyepiece requires an adapter, use the adapter.  A few eyepieces, like the 17.3mm and 14mm Delos,

require a special adapter that allows the shoulder of the eyepiece to get closer to the lens than the top of the Paracorr's 2" body.  Such adapters are available from TeleVue dealers and others.

 

When you're done, all your eyepieces will be parfocal.  You will "pre-set" the top for each eyepiece before or after you insert it, and you may need to "touch up" the focusing

a bit to get it perfect, but your focuser will only move in and out a millimeter or so from that point on.








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