can the barlow position extend a refractor focal length?

[Piero]

I have been thinking about this for a while.. 

So here are the data: 

- refractor f.l. : 360mm

- eyepieces: 9mm (40x) and 5mm (72x)

- barlow vip: 1.5x  (no spacers)

- 1.25" TV everbrite diagonal: estimated path: 40mm 

 

If the barlow is placed between the diagonal and the eyepieces, these become 6mm (=9/1.5, 60x) and 3.3mm (=5/1.5, 109x), respectively. No problem here. 

if the barlow is placed before the diagonal, the VIP should work as if there were a 40mm spacer and should become 2.16x (=(64-15-15+40)/64 + 1). This should change the eyepieces focal lengths to: 4.16mm (86x) and 2.3mm (156x), respectively.

 

However there is an issue.. When the VIP is placed before the diagonal my 9mm shows much more magnification than 86x. It is actually around 110x (very similar to 5mm + VIP 1.5x placed between the diagonal and the eyepiece. This was checked on Jupiter and the Moon many times.

Assuming that the magnifications are exactly the same, this would mean that 5mm / 1.5x = 9mm / y  => y =2.7 , where y is the barlow magnification. So the VIP would operate as if there is enough spacers to change it from 1.5x to 2.7x. If the computation is correct, 2.7x would correspond to a spacer of (2.7x - 1)*64mm - 34mm = 74.8mm. However, this doesn't make sense because the diagonal doesn't have a so long light path inside.

Even assuming an additional cm to the diagonal path, the VIP would become 2.31x which would make the 9mm a 3.89mm (92.5x). Still this is noticeably smaller than something around 110x.

 

 

When the vip is used before the diagonal, the focuser tube is shifted all the way out in order to reach focus. As this is not the normal way how the focal length of the telescope was measured, I wonder whether the extraction of the whole tube to reach focus with the barlow causes an increase in the telescope focal length. Assuming this is the case (and so the VIP barlow only has the additional path, 40mm,  introduced by the diagonal), this would be:

9mm/2.16x = 4.16mm (new eyepiece focal length)

~110x = y / 4.16mm => y = 457.6mm , where y is the new telescope focal length in this configuration, 

This would mean an increase in the telescope focal length of almost 10cm, which coincidentally is about the length of the draw tube.

 

Could it be the case? if not, what am I missing?

[jetstream]

Great question- I have no clue to the answer. I have noticed funny things in my barlow travels as well...

[Peter Drew]

You are correct in your assumptions. As you increase the separation between the Barlow and the eyepiece be it with a spacer, diagonal or more so with a binoviewer the effective magnification characteristic of the Barlow will increase and the focal plane will rapidly increase outwards, the limit is reached when the Barlow is positioned inside the focal plane of the objective by a distance equal to the focal length of the Barlow as at this point the converging rays from the objective will be converted into parallel light. 

[ronin]

Problem here is that we talk of the change in the eyepiece focal length (or magnification) the barlow has no effect on the eyepiece, what it does is alter the effective focal length of the scope.

With the seperation D and assuming hat both operate as a simple lens we have:

1/Ft = (1/Fo) + (1/Fb) - (D/(FoFb))

Ft = Total or Final focal length, Fo = Objective focal length, Fb = Barlow focal length (this is -ve) and D = Seperation.

As Peter says at D = the focal length difference then the barlow negates the objective and you have no image for the eyepiece. Fb is negative.

The other aspect is that the barlow is (if you are lucky) designed for one position and being a basic (probably) doublet it will not be very good for CA or other aberrations. So using one where it is not intended will degrade the final image that it produces and that is then fed into the eyepiece. The presumption that it will work ideally immaterialy of where it is placed is incorrect. It might "work" but it will work worse.

 

[YKSE]

I think your reasoning  looks correct.

Effective focal length with a barlow can be calculated as mentioned here

http://astunit.com/astunit_tutorial.php?topic=barlow

As to the aberration effects of a barlow, I'd like to think it as the total effect of a system, i.e. including the scope, diagonal, EP, etc, as mentioned by experienced visual observers, such as Nils-Olof Carlin:

http://web.telia.com/~u41105032/barlow/Barlext.htm

[Piero]

Thanks for all your replies. 

To add on this, In my TV60, if the Baader VIP is used after the diagonal, the focus point is moved inward, however if I use it before the diagonal, the focus point is moved outward. Very puzzling.. 

If these two focus positions are really different as it seems to me, they both work, and work quite well actually. It doesn't seem to me that any aberration is introduced. The images are just very good.

 

[Piero]

Using the above comments, I think I found the issue.

I measured my telescope throughout. 

 

A=200mm

B=55mm  (coarse focuser tube)

C=50mm (fine focuser wheel)

D=18mm (fine focuser tube)

E=40mm (Baader VIP 1.5x - no spacer)

F=40mm (1.25" TV everbrite)

G=35mm (1.25" TV everbrite nose)

 

From here, it is already clear that I made a mistake in my first post when I calculated the space introduced by the mirror diagonal. It isn't 40mm, but 40mm+35mm=75mm. This means that the Baader VIP 1.5x in front of this diagonal is actually operating at (64-30+75)/64+1=2.7x. 

Anyway, as the Baader VIP requires some inward shift to reach focus in my telescope, I calculated the telescope focal length when the VIP is added and the effect of this when placed before or after the diagonal.

 

Baader VIP 1.5x before the diagonal

Telescope focal length=200mm+55mm+50mm+ (approx) 10mm=315mm   [note: coarse focuser tube fully extracted]

The VIP 1.5x is operating at 2.7x (=64-30+75)/64+1)

New focal length=315mm*2.7x=851mm

New focal ratio=851mm/60mm=F14.2

The 9mm eyepiece gives a magnification of 951mm/9mm=94.5x

 

Baader VIP 1.5x after the diagonal

Telescope focal length=200mm+0mm+50mm+ (approx) 10mm + 40mm + 35mm=335mm  [note: coarse focuser tube fully retracted]

New focal length=335mm*1.5x=502.5mm

New focal ratio=502.5mm/60mm=F8.4

The 5mm eyepiece gives a magnification of 502.5mm/5mm=105.5x

 

These two measures are much closer. There is also an uncertainty regarding the length of the extracted fine focuser tube.

 

 

p.s.

assuming the above calculations are correct, the highest magnification I have used on this telescope on the Moon was using the Vixen HR 2.4mm and Baader VIP 2x before the diagonal ( (64+75)/64 + 1 = 3.17x), telescope focal length=998.55mm, telescope focal ratio=16.64, mag=416x.

[Peter Drew]

An easy way to determine the focal length of a Barlow if unknown is to draw a circle twice the diameter of the lens then shine the Sun through the lens and when the solar image matches the circle the distance from the lens to the circle is the focal length.  

[Piero]

9 minutes ago, Peter Drew said:

An easy way to determine the focal length of a Barlow if unknown is to draw a circle twice the diameter of the lens then shine the Sun through the lens and when the solar image matches the circle the distance from the lens to the circle is the focal length.  

Interesting! But why twice?  Is it for a 2x barlow? 

[Peter Drew]

From what I recall I think it's for any negative lens.  

[Merlin66]

That solar projection "formula" works for all negative lenses.

(See "Astronomical Spectroscopy for Amateurs", p189)