2x Barlow for 1.5x magnification-question!

[Rhushikesh-Canisminor]

Hello!

So I wanted to ask this question since a long time. I read somewhere (don't remember where now!) That I can use my 2x Barlow lense to give 1.5x magnification. Here is what am I doing-

The Barlow that I have is skywatcher achromatic 2x Barlow. It is kind of  hollow, with the actual lenses attached at the bottom. It is removable. So I simply remove the lense and attach it to my plossl eyepiece like a filter.

I don't know exactly what's happening here. In my 90mm f/10 refractor, 25mm eyepiece gives 36x magnification. With the setup I mentioned above, image is definitely bigger than what 36x gives, but smaller that what it gives using 25mm + 2x Barlow (72x magnification). I can focus the image easily and image is definitely clear as well. No clue what am I doing here.

I like to use this setup during public observations for moon and double stars.

I also use it for my H-alpha solar stuff with my 10mm plossl. It still gives a full disc image.

Haven't tried it out for galaxies yet.

Here are photos of the setup!

[xtreemchaos]

I do the same with my dslrs t adapter and lunt wedge to take full disc images with my fracs, works a treat. clear skys ,charl.

[Ricochet]

Yes, screwing the Barlow lenses directly to the eyepiece as your are doing should result in roughly 1.5x. With a Barlow the magnification is related to the distance from the focal plane of the eyepiece so reducing that distance reduces the magnification and increasing the distance increases magnification. This also means that while the Barlow is called a "2x" Barlow the actual magnification will vary from eyepiece to eyepiece. 

[LukeSkywatcher]

Its been so long since ive used a barlow that i just cant remember how to do this. I have a 1.5x-2x Revelation barlow and i think all i had to do to make it 1.5x was to remove the lens from the barlow and then just insert an EP into the barlow as normal. In this way the barlow acts like an extension tube?.I dont remember ever having to screw the barlow lens glass onto an EP (like you would do with any filter).

 

[Charic]

I use the Barlow lens directly on my eyepieces for the same reason, 1.5x is comfortable  in use. I have the Skywatcher De-Luxe (the one I use) and a Meade Tele negative Barlow lens.
But just reading this thread has got me thinking, maybe now the  spare Barlow tube could suffice as a Moon filter holder, or not!

[LukeSkywatcher]

10 minutes ago, Charic said:

I use the Barlow lens directly on my eyepieces for the same reason, 1.5x is comfortable  in use. I have the Skywatcher De-Luxe (the one I use) and a Meade Tele negative Barlow lens.
But just reading this thread has got me thinking, maybe now the  spare Barlow tube could suffice as a Moon filter holder, or not!

So you dont have to keep swapping the moon filter when changing EP's. I cant think of a reason why it wont work. I was going to suggest you put the moon filter on the end of the star diagonal in your scope but noticed you use a Dob, so no diagonal there. 

[Ricochet]

1 hour ago, LukeSkywatcher said:

Its been so long since ive used a barlow that i just cant remember how to do this. I have a 1.5x-2x Revelation barlow and i think all i had to do to make it 1.5x was to remove the lens from the barlow and then just insert an EP into the barlow as normal. In this way the barlow acts like an extension tube?.I dont remember ever having to screw the barlow lens glass onto an EP (like you would do with any filter).

 

You have to use the Barlow lenses to get any effect. Using just the tube does not change the focal length of the telescope and will not provide any additional magnification. 

55 minutes ago, Charic said:

I use the Barlow lens directly on my eyepieces for the same reason, 1.5x is comfortable  in use. I have the Skywatcher De-Luxe (the one I use) and a Meade Tele negative Barlow lens.
But just reading this thread has got me thinking, maybe now the  spare Barlow tube could suffice as a Moon filter holder, or not!

It will work if your focuser has enough inward travel to accommodate the extra length that the tube adds to the focuser. 

[Charic]

Cheers Richochet, I'll give it a try sometime.

[LukeSkywatcher]

"You have to use the Barlow lenses to get any effect. Using just the tube does not change the focal length of the telescope and will not provide any additional magnification". 

Thats what i was thinking. As i said, its been so long i just cant remember.

[Rhushikesh-Canisminor]

12 hours ago, LukeSkywatcher said:

I was going to suggest you put the moon filter on the end of the star diagonal in your scope 

I had no clue that we can do something like this!

Will try this next time wwith my nebula filters...(this changes everything!)

Thanks a lot everyone for their inputs!

[Charic]

13 hours ago, Ricochet said:

It will work if your focuser has enough inward travel to accommodate the extra length that the tube adds to the focuser. 

The first eyepiece I used failed to reach focus using the Barlow extension tube. I did not try any other eyepiece.

[JOC]

I didn't realise a Barlow would unscrew like that, but my cheap second Barlow does the same - I see a possibility which is going to work - I've just tried it.  Half the variable polarising filter in the barlow, half on the bottom of the EP makes twisting the polariser to find a sweet spot/adjustment much easier!  :-D

So whilst we are on the subject of Barlows - how come if I hold the Barlow up to an object it appears to reduce the object size rather than increase it?

[Louis D]

Here's another trick to try with removable barlow optical sections.  Thread it onto the front of your 1.25" diagonal for you refractor/CAT users out there.  You'll get about 3x or so.

3 hours ago, JOC said:

So whilst we are on the subject of Barlows - how come if I hold the Barlow up to an object it appears to reduce the object size rather than increase it?

Because it is a negative focal length lens rather than a positive focal length lens like a refractor objective.  By diverging the light rays, it lengthens the effective focal length of the scope.  A focal reducer is just the opposite.  It is another positive lens and further shortens the effective focal length of the scope.

[SilverAstro]

By attaching the barlow lens element to the eyepiece you are reducing its penetration into the light cone from the primary, hence the reduction in x,  compared to when it is mounted in its tube ( if or not the change is to exactly 1.5x depends upon the construction of the eyepiece and hence the exact positioning of the negative element )

Similar, if you construct an extra long barlow tube to carry the lens further in you can get 2.5x or even 3x etc, but the further in ( the greater the x) you begin to get extra distortions.

 

[JOC]

Louis D and SilverAstro - looks like I shall spend the night on the internet finding the diagrams to make sense of what I am certain are actually fine explanations!

[SilverAstro]

27 minutes ago, JOC said:

spend the night on the internet

actually fine explanations!

I'll let Louis speak for himself but you may be assured that mine is,  LOL !

But seriously though, ask and it shall be given,  but later I'm cooking, , , unless Louis wants to draw some pictures

else - we give the hints you do the research , mind you dont get too engrossed, keep a sharp eye out for a parting of the clouds !  I wouldnt want to be responsible for you missing that !!

 

[Louis D]

This diagram isn't perfect, but it helps to convey the diverging rays caused by a barlow lens.  This diagram isn't too bad either showing the apparent increase in focal length.

[JOC]

Ah, so a Barlow prevents the light from converging by effectively lowering the angle that it approaches the eyepiece at so the point of convergence - that which determines the focal length becomes longer.  This must also be why someone told me previously that a Barlow will increase the eye relief on the EP - the convergence point of the light becomes a greater distance from the plane of the EP lens because the angle starts off wider when it hits it due to the effect of the Barlow lens.

Def. one of those occasions when pictures work!

Edited March 13, 2017 by JOC

[Louis D]

12 minutes ago, JOC said:

Ah, so a Barlow prevents the light from converging by effectively lowering the angle that it approaches the eyepiece at so the point of convergence - that which determines the focal length becomes longer.  This must also be why someone told me previously that a Barlow will increase the eye relief on the EP - the convergence point of the light becomes a greater distance from the plane of the EP lens because the angle starts off wider when it hits it due to the effect of the Barlow lens.

Def. one of those occasions when pictures work!

Pretty much from a hand wavy description point of view, that's correct.  That's also why Powermate style focal extenders (telecentric barlows) don't mess with the exit pupil.  They have a second set of optics between the negative lens and the eyepiece to reconverge the light cone to a near parallel orientation so it doesn't mess with the eyepiece's exit pupil.  Increasing the distance between them and the eyepiece has little effect on increasing the power.  They're also handy if the eyepiece's fieldstop approaches the size of the eyepiece barrel.  Ordinary barlows will tend to vignette the outer field of view while telecentric barlows are completely unnoticeable other than increasing power.

[SilverAstro]

2 hours ago, JOC said:

 on the internet finding the diagrams

Well it seems that the internet is a bit short on diagrams of what is going on with Barlows

Here is a Chinese one that addresses the variable spacing / x magnification

http://www.skylook.org/info/info-tw/info_2106.html

good luck with Google translate

It gives two examples of the insert distance (A) 25 or 3mm resulting in magnifications of 2x and 3.1x

The magnification of the Barlow mirror is changed with its position (distance A). Example:

Set the focal length of Barlow f = 50 mm, A = 25 mm.
The imaging distance must be B = f 'A / (f - A) = 50' 25 / (50 - 25) = 50 mm
Magnification = B / A = 50 mm / 25 mm = 2 times

Let f be still 50 mm but A increase to 34 mm.
Imaging distance B = 50 '34 / (50 - 34) = 106 mm
Magnification = B / A = 106 mm / 34 mm = 3.1 times

 

 

[John]

I found this on the difference between a barlow and a telextender / Powermate. At least it explains why a Powermate / telextender is no good for barlowed lased colimation - it does not disperse the laser beam !

 

[Dannomiss]

I am late to the party, but I tried this with my new BST StarGuider lens and doesn't work. Not enough space between the lenses