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Eyepiece Backfocus Newbie Question

outreach observing eyepieces equipment beginner
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#1 Peabody1998

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Posted 20 June 2018 - 08:19 AM

Sorry for the basic question but....

 

How do I determine the backfocus used with an eyepiece?

Is it the same as the eyepiece focal length?

 

TIA

Rich



#2 Garyth64

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Posted 20 June 2018 - 08:56 AM

With all the different eyepieces out there, there isn't anyway to give an answer to the first question.  You can just stick in the eyepiece into the focuser, focus on a star, and take a measurement.

 

And any back focus is not the same as the eyepiece f.l.

 

If I may ask, why do you need to know the backfocus of an eyepiece?



#3 John Turley

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Posted 20 June 2018 - 09:03 AM

I also get a bit confused by this, I thought it related to the distance of travel on the telescope's focusing mount, but with my ES Refractor the spec stated 130mm backfocus, but there was only 45mm of travel on the focusing mount, I think they also added on the two 40 mm extension tubes supplied with the instrument, which I thought was misleading.

 

John



#4 Tamiji Homma

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Posted 20 June 2018 - 09:07 AM

Sorry for the basic question but....

 

How do I determine the backfocus used with an eyepiece?

Is it the same as the eyepiece focal length?

 

TIA

Rich

 

Hi Rich,

 

Check out this thread.

 

https://www.cloudyni...t/#entry3990093

 

Tammy



#5 Garyth64

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Posted 20 June 2018 - 09:20 AM

That is a very nice chart, but only for those eyepieces listed.  So it is just a slice of all the eyepieces that are out there.



#6 Starman1

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Posted 20 June 2018 - 01:49 PM

Only TeleVue, Nikon, Pentax, APM/Lunt, Baader, and Explore Scientific have ever stated in printed exactly where the field stops are in their eyepieces. 

Other companies have not, though this would be useful information.

Some companies don't even bother to tell you details like apparent field.

As long as we deal with such detail-free marketing, we are relegated to figuring out the focus positions and coma corrector positions for eyepieces by an empirical (often trial and error) methods. 


Edited by Starman1, 20 June 2018 - 01:56 PM.

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#7 Starman1

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Posted 20 June 2018 - 01:53 PM

Here is a link to TeleVue's information:

http://televue.com/e...=214&plain=TRUE

I attach the spec sheets for the ES eyepieces.

One sheet has some newer models, but the other has an explanation of the numbers:

Attached Files


Edited by Starman1, 20 June 2018 - 01:56 PM.


#8 Peabody1998

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Posted 21 June 2018 - 11:08 AM

Sorry I was not clear in my original post. Thanks for all the responses but I still don't understand. :(

 

In answer to the why....

I'm buying a scope with the following specs and want to know what spacers & eyepiece I need to get the eyepiece to come to focus.

 

FIXED backfocus of 6.2" behind the focuser"

Focuser travel: 0.5"

Focal Length: 1570 mm

F5

 

How do I calculate the required spacing for a given eyepiece???

What information about the eyepiece is required??

 

Thank you



#9 John Turley

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Posted 21 June 2018 - 11:40 AM

 

 

FIXED backfocus of 6.2" behind the focuser"

Focuser travel: 0.5"

Focal Length: 1570 mm

F5

 

 

Focuser travel of just 0.5in appears ridiculously small, the Baader Focusing Mount on my 14in Newtonian, and the Hex Focuser on my ES refractor both have 45mm (about 1.75in), and I consider the latter too small, but there are limits as to how much you can have on a Newtonian.

 

John



#10 Garyth64

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Posted 21 June 2018 - 02:49 PM

Sorry I was not clear in my original post. Thanks for all the responses but I still don't understand. frown.gif

 

In answer to the why....

I'm buying a scope with the following specs and want to know what spacers & eyepiece I need to get the eyepiece to come to focus.

 

FIXED backfocus of 6.2" behind the focuser"

Focuser travel: 0.5"

Focal Length: 1570 mm

F5

 

How do I calculate the required spacing for a given eyepiece???

What information about the eyepiece is required??

 

Thank you

Trial and error.



#11 Starman1

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Posted 21 June 2018 - 02:59 PM

Sorry I was not clear in my original post. Thanks for all the responses but I still don't understand. frown.gif

 

In answer to the why....

I'm buying a scope with the following specs and want to know what spacers & eyepiece I need to get the eyepiece to come to focus.

 

FIXED backfocus of 6.2" behind the focuser"

Focuser travel: 0.5"

Focal Length: 1570 mm

F5

 

How do I calculate the required spacing for a given eyepiece???

What information about the eyepiece is required??

 

Thank you

That's the problem.

Telescope eyepieces focus over a range of about 2" (2" size) or 1.5" (1.25" size).  There will be many eyepieces that come to focus outside the range of your focuser.

However, if only 1.25" eyepieces will be used, set the focuser in maximum out travel to result in the focal plane of the scope to be level with the top of the focuser.

That will allow you to accommodate many 1.25" eyepieces.  Some will require more out-travel, but that's OK, since you can simply pull them out of the focuser slightly and tighten them down, 

or add barrel extenders to the bottoms of the eyepieces to allow them to be pulled even farther out.  There will be some eyepieces requiring more in-travel that you will not be able to use.

Is there a particular reason you cannot use a focuser with a bit more travel, say, 1"?



#12 SteveG

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Posted 21 June 2018 - 04:28 PM

Sorry I was not clear in my original post. Thanks for all the responses but I still don't understand. frown.gif

 

In answer to the why....

I'm buying a scope with the following specs and want to know what spacers & eyepiece I need to get the eyepiece to come to focus.

 

FIXED backfocus of 6.2" behind the focuser"

Focuser travel: 0.5"

Focal Length: 1570 mm

F5

 

How do I calculate the required spacing for a given eyepiece???

What information about the eyepiece is required??

 

Thank you

What telescope?



#13 Peabody1998

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Posted 22 June 2018 - 01:40 PM

Re: Focuser: Optec Gemini Rotator/Focuser has only 1/2" of travel to minimize eating up too much backfocus in an imaging train.

Re: OTA is a AG Optical Harmer Wynne design 12.5"/F5 which uses a fixed primary and secondary with a fixed backfocus of 6.2".

At 6.2" I have plenty of backfocus available but I only have 1/2" of linear travel with the focuser. 

 

Hopefully a diagram helps explain it better.....

 

Image train for camera use:

<-----------------------------------------6.2"--------------------------------------->

<OTA>---<Focuser>---<spacers>---<OAG>----<filters>---<camera>

 

Image train for visual use:

<----------------------------------6.2"--------------------------------------------->

<OTA>---<Focuser>---<spacers>----------<diagonal>---<eyepiece>

 

 

For my imaging train, the backfocus of each component is simply summed to achieve the required backfocus to achieve focus. 

Spacers are used to get "coarse focus" within 1/2" of focus. The focuser is only used for fine focus adjustments.

My question is, Can the backfocus used by an eyepiece be calculated/measured? How?

 

TY

Rich



#14 Garyth64

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Posted 22 June 2018 - 02:41 PM

 I believe it cannot be calculated.

 

I tried:

 

formula.jpg

-sorry, I couldn't resist.  lol.gif

 

 

Seriously, as I said before, you're just going to have to adjust things until you get want you want.  I don't  know how it can be calculated.

For each instance that you do for a set-up for your camera or eyepiece, and you find that you can get things to focus, write down the dimensions, for the next time.  Changing an eyepiece, if they are not parfocal, will change your numbers.  To many variables to plug into an unknown equation.

 

 

 



#15 Starman1

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Posted 22 June 2018 - 04:40 PM

Re: Focuser: Optec Gemini Rotator/Focuser has only 1/2" of travel to minimize eating up too much backfocus in an imaging train.

Re: OTA is a AG Optical Harmer Wynne design 12.5"/F5 which uses a fixed primary and secondary with a fixed backfocus of 6.2".

At 6.2" I have plenty of backfocus available but I only have 1/2" of linear travel with the focuser. 

 

Hopefully a diagram helps explain it better.....

 

Image train for camera use:

<-----------------------------------------6.2"--------------------------------------->

<OTA>---<Focuser>---<spacers>---<OAG>----<filters>---<camera>

 

Image train for visual use:

<----------------------------------6.2"--------------------------------------------->

<OTA>---<Focuser>---<spacers>----------<diagonal>---<eyepiece>

 

 

For my imaging train, the backfocus of each component is simply summed to achieve the required backfocus to achieve focus. 

Spacers are used to get "coarse focus" within 1/2" of focus. The focuser is only used for fine focus adjustments.

My question is, Can the backfocus used by an eyepiece be calculated/measured? How?

 

TY

Rich

Yes, it can, but you have to now where the focal plane of the scope is, and you have to now where the focal plane of the eyepiece is.

For some brands of eyepieces, we can get lists of where the focal planes are.

And you can determine the focal plane of your scope by putting translucent tape over the star diagonal and aiming at the moon and focusing the moon on the translucent tape.

Hopefully, you'll be able to achieve focus that way.  You might have to add spacers to the focal train or even, perhaps, an extension tube.

 

But if you can find out where the focuser is when the telescope is in focus at the end of the star diagonal, then the figures on the focal plane position of the eyepiece will let you know.

If, for example, the focal plane of the eyepiece is 3mm below the shoulder of the eyepiece, it will come to focus 3mm farther out than the focal plane position for the scope alone.

If the focal plane of the eyepiece is 3mm above the shoulder, then 3mm of additional in travel from the focal plane of the scope will be necessary.

 

 

Alas, the focal planes of just 1.25" eyepieces vary from near the bottom of the 1.25" barrel (perhaps 1-1/8" below the shoulder) to as much as 1/2" above the shoulder.

To focus all of them would require over 1.5" of focuser travel.

And 2" eyepieces have a larger range.

That's why refractors for visual use have lots of travel in their focusers.

You'll have to accomplish that with barrel extenders and extension tubes.



#16 Eddgie

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Posted 22 June 2018 - 05:21 PM

As others have mentioned, you will need to find the displacement of the field stop from the shoulder of the eyepeice (the part of the housing that hits the top of the eyepiece holder when you insert it all the way)  Generally the field stop is near this point but not always on it.  Often it is a bit closer to the telescope end of the barrel than behind it.

 

You also have the problem of measuring the exact light path lenght of all of your components.  For example a 2" diagonal will have a light path between about 100mm and 110mm.

 

If you use a prism, this will have a shorter effective light path than a mirror, so you will have to actually measure it directly using a star at infinity (the light path of the prism varies with focal ratio). 

 

If you have your eyepiece, you might be able to actually look in the end and see the field stop and if so, you can probably use a Q tip stick to mark how far in it is, or unscrew the barrel (on many eyepieces) and measure it directly. 

 

Also this.  It is a cheat but a darned good one. If you know the light path of your diagonal, get a spacer that puts you a little on the short side.   Now, buy a 2" extension tube.  These just have M48 male threads on one end and M48 female on the other.  Screw it into the nose of the diagonal.  

 

Set your focuser to mid position.

Now, slip focus the nose of the diagonal in the adapter (which I assume has some kind of clamp for the eyepiece) until you reach rough focus.  Now you can use the built in focuser for fine focus.  If you change to an eyepiece that won't reach focus, once gain, slip focus the diagonal to get it to the new rough focus point.

 

You can get these extension tubes from Stellarvue and others and they come in different lengths.

 

Also, if you have a 2" Barlow tube and sufficent space, you could use this as your extension tube.  Just unscrew the Barlow element and lock the nose of the diagnal into the tube.  Now you can insert this and slip focus.

 

The extension just gives you enough draw tube to make sure the diagonal is well supported.  You only need an inch or so in the holder, maybe more if this is uncomfortable to you.

 

Anyway this makes it so you don't have to be exact with your extension lengths.  Just undershoot by an inch or two of your light path when you get your spacers and slip focus the extended focuser nose as necessary. 



#17 Eddgie

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Posted 22 June 2018 - 05:24 PM

And we could probably get you an exact answer, but it would only be for one eyepeice at a time, and what happens if you get an eyepiece that does not reach focus?  Now you have to redo your spacers.

 

You might be one of those people though that likes to have exactly the right part and I can appreciate that because I am like that.  But I know when to not be like that and if it were me, this would be one of those times.. LOL. 


Edited by Eddgie, 22 June 2018 - 05:25 PM.



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