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Barlow Hack: 3X from a 2X

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#1 mikemarotta

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Posted 04 December 2021 - 09:12 AM

... Speaking of Don Pensack... he mentioned this in another topic thread. With a refractor if you put your 2X Barlow ahead of the diagonal, it will be 3X. This gives you more flexibility for your set of oculars ("eyepieces"). For myself, one advantage is that it allows me to use more than one filter at a time. 

 

Venus is very bright. In order to see the disk and its illumination with minimal glare, I have found it convenient to use two filters. That worked for my National Geographic 70-mm refractor, but did not work for my Explore Scientific 102-mm. I could not draw the focusser tube in close enough. On another board, a moderator recommended that I buy a field flattener, a $250 fix to a $250 telescope. However, I found that with the 2X Barlow ahead of the diagonal, the new focal length is so long that I can draw the focusser in close enough with two filters on the ocular and get a nice view of Venus.

 

Also, of course, if you like experimenting, you can see for yourself the differences among this times two and that times three. 

 



#2 vdog

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Posted 04 December 2021 - 09:45 AM

Now you tell me.  I already bought a 3x Barlow.  Oh well, it was a Svbony, so it didn't exactly break the bank.  


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#3 Sarkikos

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Posted 05 December 2021 - 04:31 PM

... Speaking of Don Pensack... he mentioned this in another topic thread. With a refractor if you put your 2X Barlow ahead of the diagonal, it will be 3X. This gives you more flexibility for your set of oculars ("eyepieces"). For myself, one advantage is that it allows me to use more than one filter at a time. 

 

Venus is very bright. In order to see the disk and its illumination with minimal glare, I have found it convenient to use two filters. That worked for my National Geographic 70-mm refractor, but did not work for my Explore Scientific 102-mm. I could not draw the focusser tube in close enough. On another board, a moderator recommended that I buy a field flattener, a $250 fix to a $250 telescope. However, I found that with the 2X Barlow ahead of the diagonal, the new focal length is so long that I can draw the focusser in close enough with two filters on the ocular and get a nice view of Venus.

 

Also, of course, if you like experimenting, you can see for yourself the differences among this times two and that times three. 

Why would he advise you to buy a field flattener?  A field flattener would flatten the field of the 102mm f/6.7 refractor.  Would it give you more back focus?  Is it a reducer also?  In any case, I don't know what that has to do with using a Barlow.  

 

I attach relatively expensive accessories to relatively inexpensive telescopes all the time.  Sometimes the accessory is about the same price as the scope, sometimes it's more, sometimes several times more.  There's no law that says you can't mix and match your accessories to your telescopes.  Do what works.    

 

If you ever decide to get a field flattener, I recommend the TSFLAT2.  I've used it on several of my achromats and APO/ED's to flatten their fields.  None of them objected.  No one called the Astronomy Police.  Split the difference by using it on all your refractors.

 

grin.gif

Mike


Edited by Sarkikos, 05 December 2021 - 04:32 PM.


#4 Jon Isaacs

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Posted 05 December 2021 - 05:47 PM

The easiest way to get a nearly color free view of Venus with an achromat is to use an aperture mask, reduce the aperture by a factor of two. This improves the color correction by a factor of 4.  

 

Many achromats have a small lens cover in the middle of the regular lens cover, just remove the small lens cover. Resolution is reduced but viewing the phase of Venus, this is not an issue.

 

As far as increasing the magnification of a Barlow,  the magnification of a Barlow increases as the spacing is increased.

 

A = D/FL + 1 where A is the amplification factor, D is the distance from the Barlow to the focal plane and FL is the Focal Length of the Barlow.

 

For a 2x Barlow, (A= 2), D/FL = 1 so the focal length is equal to the distance from the Barlow lens to the top of Barlow, the approximate focal plane.

 

To increase the magnification from 2X to 3x, D must be increased from 1 FL to 2 FL. 

 

(A = 2FL/FL +1 = 3)

 

A 1.25 inch diagonal has an aapproximate optical length of 75mm so if the Barlow has a focal length of 75 mm, the placing the Barlow in front of the diagonal with result in a 3x magnification.  (The TeleVue 1.25 inch 2x Barlow has a 75 mm focal length)

 

All this depends on the actual Barlow and the actual spacing so the magnification is approximate unless you know the actual spacing and focal length of the Barlow.

 

This thread discusses all this:

 

https://www.cloudyni...barlow-that-is/

 

Jon


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