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Meade 3X Barlow vs Celestron X-Cel 2X Barlow Mystery

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#1 JimOfOakCreek

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Posted 08 May 2021 - 04:51 PM

This afternoon is overcast in SE Wisconsin, a good day to play with my 4” APO FL = 714mm, 2X and 3X Barlows and eyepieces during daylight hours.

 

So I compared a (Celestron X-Cel LX 7mm eyepiece & Celestron X-Cel 2X barlow) combination with a (Celestron Ultima 10mm UFF eyepiece & Meade 3X barlow).

 

Doing the math, the X-Cel combo should yield 204X. The Ultima/Meade combo should yield 214X.

 

However, after carefully examining the images, it was clear the X-Cel combo is yielding more magnification than the Ultima/Meade combo. I would guess the X-Cel combo is yielding images that are at least 20% larger.

 

Comparing the Ultima/Meade combo with an ES 52* Series 4.5mm eyepiece, the images of the Ultima/Meade combo were only slightly larger, about 10% larger than the 4.5mm eyepiece. That would make the 10mm Ultima a 4mm eyepiece when used with the Meade 3X barlow.

 

Based on my observations I did a recalculation and came to the conclusion that the Meade 3X barlow is really a  2.5X barlow. Does anyone have more information on that Meade 3X barlow? 

 

BTW: The images were very good with all combinations.

 

E14CFFD7-90BA-4F7C-8F39-12629ABD620E.jpeg

 

 

 

 

 

 

 

 

 

 

 


Edited by JimOfOakCreek, 08 May 2021 - 04:54 PM.


#2 russell23

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Posted 08 May 2021 - 04:59 PM

You’ve discovered an important truth - regular barlow magnification factors are often different from what is labeled.  I’ve found this with many barlows.  I measure the magnification factor using the “ruler on the wall” method.

 

For the 2.5x GSO APO barlow I found a magnification of 2.1x.

 

For the 3x Orion tri-mag barlow I found a magnification of 2.9x

 

For the 3x TV barlow I found a magnification of 3.2x

 

So it is very likely that the magnification factor is different than what it says on the barrel for one or both barlows.  I’ve not measured those barlows.

 

Here is how the method works:

 

1.  Tape a measuring tape/ruler/meter stick on a wall.  Make sure it is level.

 

2.  Set up your scope as far away as possible and still fitting the eyepiece field within the measurement tape.

 

3.  Record the field diameter without the barlow.  

 

4.  Record the field diameter with the barlow.

 

5.  Calculate barlow magnification factor by:  diameter without barlow divided by diameter with barlow. 

 

I set up my refractor on a Vixen SuperPolaris mount set into alt-azimuth mode.  I then can use the slow motion controls to precisely align the ruler across the center of the field. The method is very consistent and accurate. 


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#3 SteveG

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Posted 08 May 2021 - 07:38 PM

To add to what Russ said, I did similar measurements about 10 years ago. He an I both agree with the Orion Trimag being 2.9x.

 

I did measure the Meade, and I came up with 2.7x.

 

YMMV


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#4 PJBilotta

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Posted 08 May 2021 - 07:47 PM

From what I recall, the X-Cel LX "2x" barlow actually clocks in at about 2.2x. This is quite common with these 3-element "apochromatic" shorty barlows. The Celestron Ultima, Orion Deluxe, and GSO all are actually more than 2x.

Edited by PJBilotta, 08 May 2021 - 09:46 PM.

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#5 JimOfOakCreek

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Posted 08 May 2021 - 07:47 PM

To add to what Russ said, I did similar measurements about 10 years ago. He an I both agree with the Orion Trimag being 2.9x.

 

I did measure the Meade, and I came up with 2.7x.

 

YMMV

Your measurement of the Meade 3X is likely more accurate than mine. I was just eyeballing estimating. 


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#6 kel123

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Posted 08 May 2021 - 08:32 PM

The only issue I see with some of these measurements is that the rulers are not very precise.
Also, remember that the magnification factor of a barlow depends on the distance from the eyepiece.
Unless ,it is telecentric. Hence, this might have contributed to the results.
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#7 russell23

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Posted 08 May 2021 - 08:58 PM

The only issue I see with some of these measurements is that the rulers are not very precise.
Also, remember that the magnification factor of a barlow depends on the distance from the eyepiece.
Unless ,it is telecentric. Hence, this might have contributed to the results.

I assure you my ruler measurements are quite robust. For Telecentric barlows I get exactly the specified value.   The problem with regular barlows is that many of them are not designed to place the eyepiece at the right distance for the specified magnification factor.  That is why the ruler method is helpful because it provides a more accurate determination.

 

I did not mention before that TV eyepieces provide a different magnification factor than other brands.  Typically with the barlows it is ~0.1x.  For example, the GSO 2.5x APO barlow was 2.0x with TV eyepieces and 2.1x with non TV eyepieces.  The 3x TV barlow is 3.05-3.08x with TV eyepieces and 3.17-3.22x with non -TV eyepieces. 

 

The Nikon 1.6x EiC barlow is pretty much right on 1.6x.  I get 1.6x with TV eyepieces and 1.62-1.64x with other eyepieces.   

 

Another nice thing about the ruler method is if you are using an extension tube with the barlow you can determine the actual magnification factor.  For example, with a 1” extension tube I found the 3x TV barlow is 3.65x with TV eyepieces and 3.75x with non-TV eyepieces. 


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#8 BillP

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Posted 08 May 2021 - 09:45 PM

The only issue I see with some of these measurements is that the rulers are not very precise.
Also, remember that the magnification factor of a barlow depends on the distance from the eyepiece.
Unless ,it is telecentric. Hence, this might have contributed to the results.

Rulers are as precise as they are marked...so 1/16th to 1/32 to 1/64 inch.  The procedure russel23 points out is extremely accurate also.

 

You do have a point on the Barlow-Eyepiece distance though.  It is actually the distance of the top of the Barlow housing to the Field Stop of the eyepiece that is important.  Not all eyepieces have their Field Stops exactly at the shoulder of the eyepiece so the Field Stop is right at the top housing of the Barlow when the eyepiece is inserted.  As the TV Spec Table shows, many of the TV eyepieces are made with the Field Stop 1/4" below the shoulder of the eyepiece (i.e.,1/4" below the shoulder towards the barrel).  So this means when those are inserted into the Barlow the eyepiece's Field Stop is positioned below or within the Barlow housing by 1/4" thereby reducing its marked magnification factor.  The moral of the story is that with each different eyepiece, given that their Field Stop can be in a different position from some distance above the shoulder of the eyepiece to some distance below the shoulder of the eyepiece, this will mean a given Barlow will operate at slightly different magnification factors with each eyepiece.  And with shorty type Barlows the amount of offset of the eyepiece's Field Stop will cause a greater magnification factor than with the longer Barlow types.  Sure would be a convenient feature if eyepiece makers etched a line on the housing or barrel of their eyepieces to show the position of the eyepiece's field stop as it is sometimes a useful bit of information as is the focal length of the eyepiece!


Edited by BillP, 08 May 2021 - 10:16 PM.

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#9 CeleNoptic

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Posted 08 May 2021 - 10:45 PM

The only issue I see with some of these measurements is that the rulers are not very precise.

 
As a scientist I used to trust rulers and they never failed lol.gif


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