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Baader FFC--Barlow or Telecentric?

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#1 Kent10

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Posted 25 February 2021 - 11:53 AM

I think the Baader FFC (Flatfield Corrector) is a barlow.  Baader calls it a barlow on their page

 

https://www.baader-p...fc)--3x-8x.html

 

And I think it is a barlow because it increases magnification when increasing the eyepiece distance.  The FFC can be used from 3X-8X.

 

However, it has 4 lenses instead of the typical 2 that barlows have.  Are the 2 additional lenses only used to create a flat field?  Why 4 lenses instead of 2?

 

Also, I see that some telecentrics can increase power with distance as indicated on TV's page with the 5X Powermate.  Or am I misunderstanding the diagram?   https://www.televue....?id=53&Tab=_app

 

Why does the 5X Powermate increase magnification while the others don't or do only marginally?

 

I couldn't find a ray trace diagram of the Baader FFC.

 

Thanks.

 



#2 Thomas_M44

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Posted 25 February 2021 - 08:01 PM

I think the Baader FFC (Flatfield Corrector) is a barlow.  Baader calls it a barlow on their page

 

https://www.baader-p...fc)--3x-8x.html

 

And I think it is a barlow because it increases magnification when increasing the eyepiece distance.  The FFC can be used from 3X-8X.

 

However, it has 4 lenses instead of the typical 2 that barlows have.  Are the 2 additional lenses only used to create a flat field?  Why 4 lenses instead of 2?

 

Also, I see that some telecentrics can increase power with distance as indicated on TV's page with the 5X Powermate.  Or am I misunderstanding the diagram?   https://www.televue....?id=53&Tab=_app

 

Why does the 5X Powermate increase magnification while the others don't or do only marginally?

 

I couldn't find a ray trace diagram of the Baader FFC.

 

Thanks.

I can only comment upon the TeleVue 5X Powermate.

 

I conjecture the 5X TV PM is actually somewhat of a "hybrid" in it's performance, somewhere between a classic 2-element Barlow and a 'true' telecentric amplifier which would give a truly centric light column.

 

The increase in magnification the TV 5X PM delivers as PM to eyepiece spacing increase indicates to me a conic, non-centric light column.

 

Having said that, I find the 5X PM to be a *very* clean and transparent amplifier.  It produces no visible vignetting even in my longer-FL TeleVue 32mm 26mm or 20mm Plossls, which are notorious for vignetting when amplified with classical Barlows.

 

I use the magnification effect of the TV 5X Powermate to good advantage. Inserting a simple 1-inch extension tube between the PM and the eyepiece increases the PM magnification to about 5.75X. If instead, a 1.5-inch extension tube is used, the magnification is 6.2X

 

In fact, my absolute favorite planetary viewing configuration these days is to use the 5X PM, with or without an extension tube, in combination with my Plossl, Abbe Ortho, Kellner and RKE eyepieces from between 32mm and 18mm. This way I achieve  super-sharp high-magnifications with the comfortable eye-relief the longer FL eyepieces afford cool.gif

 

FYI: I also have the 2.5X and 4X Powermates plus the Televue 2X and 3X Barlows. Additionally, the Vernonscope Dakin 2-inch 2.4X Barlow; and the Nikon EiC16 1.6X Barlow. 

 

I tend to use the 5X Powermate more often than the other Barlows/amplifiers due to my above mentioned strategy.

 

The 5X Powermate is a very good unit --if and when it suits one's needs.



#3 MartinPond

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Posted 25 February 2021 - 08:21 PM

-----------------

I think the Baader FFC (Flatfield Corrector) is a barlow.

  Baader calls it a barlow on their page...

-----------------

 

---->That makes me want to take them at their word..

 

--------------------

And I think it is a barlow because it increases magnification when increasing the eyepiece distance. 

The FFC can be used from 3X-8X.

--------------------------

 

-----------------> A telecentric cannot do that.   It is a Galilean optic

                        of fixed power. 

 

 

------------------------------------

However, it has 4 lenses instead of the typical 2 that barlows have. 

Are the 2 additional lenses only used to create a flat field? 

Why 4 lenses instead of 2?

-----------------------------------

 

------>Barlows can have 1,2,3, or 4, or more lenses.  There is no specific number.

          It is probably to create a flat field, but more correction

            of any kind is possible.  

         The common tele-converter you attach to a camera behind a regular lens

             is almost always 4 elements......but it is almost always a Barlow.

        Interestingly, the teleconverter you attach to the end of a camera lens

           is always telecentric.  


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#4 Kent10

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Posted 26 February 2021 - 11:34 AM

Thanks Thomas and Martin.  I don't own the 5X Powermate.  Thomas, I have been following your successful use of the Powermates with RKE eyepieces.  I have quite a variety of barlows and a few Powermates and I do use them from time to time. 

 

The reason I would like to know about the FFC is because I have been using it in front of my ADC to correct aberrations created from the prisms in the ADC.  If the FFC is not actually a barlow, then it wouldn't be correcting these aberrations.  I did a star test with and without the FFC and I didn't see a difference when the seeing was quite good.  But the aberrations might be too subtle for me to notice in the star test.  Still it is fun for me to try.  I am using an F8 refractor and should use a barlow to correct for aberrations when using the ADC but so far I haven't been able to see a difference with the FFC.  I may try some that I know are barlows to see if I can discern a difference.

 

Thanks again.



#5 Thomas_M44

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Posted 26 February 2021 - 01:59 PM

Thanks Thomas and Martin.  I don't own the 5X Powermate.  Thomas, I have been following your successful use of the Powermates with RKE eyepieces.  I have quite a variety of barlows and a few Powermates and I do use them from time to time. 

 

The reason I would like to know about the FFC is because I have been using it in front of my ADC to correct aberrations created from the prisms in the ADC.  If the FFC is not actually a barlow, then it wouldn't be correcting these aberrations.  I did a star test with and without the FFC and I didn't see a difference when the seeing was quite good.  But the aberrations might be too subtle for me to notice in the star test.  Still it is fun for me to try.  I am using an F8 refractor and should use a barlow to correct for aberrations when using the ADC but so far I haven't been able to see a difference with the FFC.  I may try some that I know are barlows to see if I can discern a difference.

 

Thanks again.

Perhaps the aberration you are seeking to correct is indeed significantly below the threshold of visibility.

 

And hey, if so --then than would be good news smile.gif



#6 Kent10

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Posted 26 February 2021 - 02:25 PM

Perhaps the aberration you are seeking to correct is indeed significantly below the threshold of visibility.

 

And hey, if so --then than would be good news smile.gif

You could be right on that.  Others who have tried the barlow with this ADC have not found a difference on planets.  I have tried before, too, and haven't seen a difference but I still enjoy trying and I believe there must be a difference even if very small.  According to some tests by the maker of the ADC, without a barlow and at focal length 7.5 you would get .9 strehl at best, and with the barlow extending the focal length to F11.5, you would get .94 strehl.  I am starting at F8 with my refractor so the difference might be smaller.  Difficult to see but I like to try to squeeze every bit of performance I can from my scopes.  It is more effort to use the barlow, however.  That wouldn't stop me if the improvement is there.  Also, I have fewer options with eyepieces using the FFC which increases the power so much.  I can't use my typical planetary eyepieces.  On the other hand, I get more eye relief.  So I continue to try both with and without FFC for different options and experiences. 



#7 rkelley8493

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Posted 26 February 2021 - 03:08 PM

It can be used in both ways as a Standard Barlow or Telecentric Lens.. The best of both worlds ubetcha.gif

 

Standard Barlow config

ffc-standard barlow.jpg

 

Telecentric config

telecentric.jpg


Edited by rkelley8493, 26 February 2021 - 11:48 PM.

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#8 MartinPond

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Posted 26 February 2021 - 11:01 PM

Wild!



#9 Kent10

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Posted 27 February 2021 - 04:41 PM

Thanks rkelley for the pictures.  I am trying to understand the difference between the 2.  Is it simply one is after the diagonal and the other before?  I don't see the FFC in the 2nd pic.  Thanks.



#10 rkelley8493

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Posted 27 February 2021 - 10:23 PM

Thanks rkelley for the pictures.  I am trying to understand the difference between the 2.  Is it simply one is after the diagonal and the other before?  I don't see the FFC in the 2nd pic.  Thanks.

Oh, sorry! Here's the before picture.. It required a ton of back space to reach focus when placed ahead of the diagonal. The FFC is attached within the extension tubes on the previous photo in Telecentric config.

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Edited by rkelley8493, 28 February 2021 - 01:38 PM.

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#11 Kent10

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Posted 28 February 2021 - 11:14 AM

There it is!  Thanks rkelley.


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