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#1 PJP

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Posted 28 February 2021 - 06:53 PM

Is it possible to add and extension tube to an eyepiece with the results being a lower magnification factor?

 



#2 RedzoneMN

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Posted 28 February 2021 - 07:04 PM

Never heard of that. On the other hand spacing can sometimes change magnification, for example, you can get greater magnification with spacing your eyepiece from the Televue Powermate 5x.


Edited by RedzoneMN, 28 February 2021 - 07:17 PM.


#3 MitchAlsup

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Posted 28 February 2021 - 07:08 PM

Is it possible to add and extension tube to an eyepiece with the results being a lower magnification factor?

No, magnification = FL_scope/FL_eyepiece

 

The only thing an extension tube can add is the ability to focus farther out than the focuser normally can achieve.



#4 Redbetter

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Posted 28 February 2021 - 07:54 PM

Is it possible to add and extension tube to an eyepiece with the results being a lower magnification factor?

 

Lower?  I am not sure how that could be achieved via extension near the eyepiece alone.  Extensions in specific situations increase magnification.   The eyepiece isn't changing focal length, the focal length of the rays are being altered from nominal. 

 

Higher magnification can be achieved in at least these two situations:

  • If the extension is between an eyepiece and Barlow, then the multiplication factor of the Barlow will increase. 
  • If the scope is a Mak or SCT which focuses by moving the primary mirror, the effective focal length of the scope changes when an extension alone is added.  The factor is usually about 3 to 4x the change in position of the focal plane, but depends on the specifics of the system.

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#5 MartinPond

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Posted 28 February 2021 - 10:09 PM

Is it possible to add and extension tube to an eyepiece with the results being a lower magnification factor?

No, but there is a device that would reduce the apparent focal length of

       the telescope, thus lowering the power.  

Getting something to do this is simple.  Getting something to do this

   without ruining the image in an eyepiece or displacing the focuser too much

   can be quite expensive.    Look up "focal reducer".    



#6 PJP

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Posted 01 March 2021 - 07:56 PM

Thanks to all who responded.

 

The lower power requirement might not be needed. If this approximation is close to correct as applied to camera lenses

Focal Length / 50 = power so a 600mm lens =~6x and a 1800mm =~36x.

 

My brand new Skymax 127 (first ever telescope) came with a 28mm eyepiece (~54x) which will be way to much for capturing a full moon. I've purchased a 40mm which is ~ 37x which should be just about perfect for a full moon based on my assumption about camera lens power.



#7 rexowner

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Posted 01 March 2021 - 07:58 PM

Of course not.  The focus point doesn't change.



#8 Redbetter

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Posted 01 March 2021 - 08:16 PM

Of course not.  The focus point doesn't change.

Actually, with a Mak like the OP has it does change.  An extension will move the focal plane position of the eyepiece out.  That will require the focus to be adjusted to meet this new focal plane position.  In order to to do that the primary mirror is moved, which changes the relative "power" of the secondary, altering the overall focal length of the scope.



#9 Redbetter

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Posted 01 March 2021 - 08:34 PM

The lower power requirement might not be needed. If this approximation is close to correct as applied to camera lenses

Focal Length / 50 = power so a 600mm lens =~6x and a 1800mm =~36x.

 

My brand new Skymax 127 (first ever telescope) came with a 28mm eyepiece (~54x) which will be way to much for capturing a full moon. I've purchased a 40mm which is ~ 37x which should be just about perfect for a full moon based on my assumption about camera lens power.

Not sure what you mean.  Are you doing visual or imaging through the eyepiece?

 

For visual just about any 28mm eyepiece with this scope should have plenty of field for framing the Moon.  The Moon is a little over half a degree across in apparent size.  The included eyepiece is a 2" with a 31mm field stop.  Assuming that the nominal focal length of the scope is accurate, the TFOV = 31*57.3/1540 = 1.15 degrees.

 

Have you actually used the scope and eyepiece yet?  I don't know what 40mm you ordered or what you actually intend to do with it.  The Moon wouldn't be on my list...



#10 rexowner

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Posted 01 March 2021 - 09:04 PM

Actually, with a Mak like the OP has it does change.  An extension will move the focal plane position of the eyepiece out.  That will require the focus to be adjusted to meet this new focal plane position.  In order to to do that the primary mirror is moved, which changes the relative "power" of the secondary, altering the overall focal length of the scope.

You’re right.  I’m more of a refractor person, so missed that.

 

It would be interesting to see this quantified, eg for a Mak eg F15 primary. i’ll bet you a donut it won’t 

be enough to change magnification from 54x to 37x. I highly doubt it, but would love to be shown 

wrong. 


Edited by rexowner, 01 March 2021 - 09:06 PM.


#11 SeattleScott

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Posted 01 March 2021 - 09:31 PM

Typically it is about a 5-15% difference going from 1.25” to 2” diagonal, depending on the scope.

But yeah the OP can fit two full moons in the field of view with the 28mm.

Scott
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#12 Redbetter

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Posted 02 March 2021 - 12:26 AM

It would be interesting to see this quantified, eg for a Mak eg F15 primary. i’ll bet you a donut it won’t 

be enough to change magnification from 54x to 37x. I highly doubt it, but would love to be shown 

wrong. 

His Mak is an f/12, mine is an f/15.  The question here is whether an extension could decrease the focal length of the system by increasing the magnification of the eyepiece.   As far as I know that is not possible, but the focal length can be increased.  I don't know how far as it comes down to how much travel is available for the focuser/moving primary from the nominal focal plane position.  

 

A chart I have seen estimates that that a 4" f/15 would change focal length from 1540 to 2286mm by extending the focal plane 203mm.  That would be a 3.75x ratio.  I don't know if the focuser can provide that much range (it doesn't have to move very far at all, there is a large multiple with respect to changing the distance between the mirrors.)  The factor for f/12 is likely somewhat lower but still substantial.  An f/10 SCT has a ratio of ~3.2 to 3.3. 

 

So if a person used an f/12 Mak with a 28mm eyepiece at 1540 nominal, the mag would be 55x, while with a 40mm it would be 38.5x.  If enough extension was provided to allow the focal plane to move 203mm, and the ratio the system worked out to 3.75x, then with a 40mm eyepiece (as the OP asked about) the system would be at 2286mm for 57x.  I doubt that there is sufficient focuser travel for this since the native set up appears to be for 2" anyway (longer path length), and the actual ratio is  probably about ~3.5 for f/12.   

 

I have drift timed to determine differences in focal length with different set ups for my 127 f/15 Mak.  With a Tak Prism and 13mm T6 Nagler I used the field stop to calculate ~1960mm focal length.  With 2" TV diagonal and highhat adapter the focal length was ~2220 and with an AT diagonal w/SCT threads (regular adapter) it was ~2106mm.


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#13 PJP

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Posted 02 March 2021 - 01:50 PM

Not sure what you mean.  Are you doing visual or imaging through the eyepiece?

 

For visual just about any 28mm eyepiece with this scope should have plenty of field for framing the Moon.  The Moon is a little over half a degree across in apparent size.  The included eyepiece is a 2" with a 31mm field stop.  Assuming that the nominal focal length of the scope is accurate, the TFOV = 31*57.3/1540 = 1.15 degrees.

 

Have you actually used the scope and eyepiece yet?  I don't know what 40mm you ordered or what you actually intend to do with it.  The Moon wouldn't be on my list...

I have tested the scope as best I can with a photo tripod (mount I ordered is back ordered). The 28mm eyepiece provided too much magnification for capturing a full moon using afocal method. I also have tested the Celestron E-Lux 2" 40mm eyepiece and think the lower magnification will work. Since I'm familiar with camera lens focal length and what they display wrt fov I was trying to make and approximation of magnification power and relate that to telescopes.

 

I plan on carefully evaluating afocal imaging prior to spending more money on an ILC camera that will direct focus.    
 



#14 Redbetter

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Posted 02 March 2021 - 04:23 PM

So it is really an imaging question, not about what the eyepiece and scope can do for visual.  Sorry, I can't help with that.


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