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Question - Eyepieces, zoom, barlows, etc.


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Evening everyone. Quick question about eyepieces in simple terminology:

I understand that the lower the number of an eyepiece, the more zoom it provides, and different makers provide different FOV. My question is that, for example, if I use a 2x Barlow with an 8mm ep, would I have a 4mm ep? (12mm+2x = 6mm; 40mm+2x = 20mm, etc.) 

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We usually refer to zoom as magnification but your understanding is correct - a 2x barlow lens effectively turns an 8mm eyepiece into a 4mm, a 10mm into a 5mm etc, etc.

 

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To an extent that is true. If an eyepiece gives a magnification of say 50x in your scope a 2x Barlow with the same eyepiece will give a magnification of 100x. The Barlow does not half the focal length of the eyepiece however. It doubles the focal length of the scope.

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As said, basically you are correct. The only other thing to add into the mix is that the quality of the Barlow has an impact. Use a bad Barlow to get higher magnification and you will get a bad result i.e. Poor quality image. Get a decent one (and that doesn't have to be stupidly expensive) and it can be quite effective.

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Also worth noting that a barlow will often extend the eye relief of an eyepiece. This tends effects longer focal length eyepieces more, so if you intend to use a 40mm eyepiece for example you may be better off considering a powermate or similar depending on what scope your using it in of course.

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Most astronomers when starting out automatically assume that they absolutely need a barlow lens. This is not the case though. A collection of 3 well chosen eyepieces (low,medium and high...........so about 8,15 and 30mm)..............will be/is enough.

What will work best with your scope, (i dont know what scope you have or what will work best). I'll leave that to others. I have a wide variety of scopes of different types and size and i personally find that middle of the road (£30-50) Plossl EP's work well in all of them.

I also have an 8-24mm Hyperion zoom EP, but that is a different kettle of fish. Works well, but certainly not everyone needs one.  It's very convieient for me to own one.

Edited by LukeSkywatcher
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1 hour ago, LukeSkywatcher said:

Most astronomers when starting out automatically assume that they absolutely need a barlow lens. This is not the case though. A collection of 3 well chosen eyepieces (low,medium and high...........so about 8,15 and 30mm)..............will be/is enough.

What will work best with your scope, (i dont know what scope you have or what will work best). I'll leave that to others. I have a wide variety of scopes of different types and size and i personally find that middle of the road (£30-50) Plossl EP's work well in all of them.

I also have an 8-24mm Hyperion zoom EP, but that is a different kettle of fish. Works well, but certainly not everyone needs one.  

I have an 8se. I stayed away from buying an ep set due to input from SGL folks. I have the 25mm that came with the scope and also got a celestron 8-24 zoom. I didn't get a barlow because I read, like you said, to stick to a few ep's that are low, med, high. The zoom is good for now until I get some funds built up, but I've been looking at a 30 or 40mm because I get too much magnification from the 2 that I have (lunar/some dso observing made this obvious to me). I was just curious about the barlows. I actually haven't felt the need/want to get one.  

I saw a great difference in lunar observing with the Plossl and the zoom that I have. I'm the kind of person that is skeptical when it comes to things like "a certain type of something is better than another". Put another way, I saw that the Plossl lens gave sharper detail. 

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2 hours ago, John said:

We usually refer to zoom as magnification but your understanding is correct - a 2x barlow lens effectively turns an 8mm eyepiece into a 4mm, a 10mm into a 5mm etc, etc.

 

Thanks! Obviously I'm new to this haha. I'm trying to relate to terms that I know well (photography terms). I'm seeing a difference in photography/astronomy terminology to some degree. On that note, one thing that has been bothering me is that in photography, f/(insert number) is the aperture that a lens allows, but when it comes to telescopes I'm seeing this reffered to as fastness/slowness....input?

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3 minutes ago, crcooney said:

Thanks! Obviously I'm new to this haha. I'm trying to relate to terms that I know well (photography terms). I'm seeing a difference in photography/astronomy terminology to some degree. On that note, one thing that has been bothering me is that in photography, f/(insert number) is the aperture that a lens allows, but when it comes to telescopes I'm seeing this reffered to as fastness/slowness....input?

I'm not an imager so I'll leave others to comment on that.

On the barlow though, if you have a Celestron 8SE then I suspect that a 2x barlow is not going to get much use for visual observing. The 8mm focal length point of the zoom is giving you 250x which is about as much magnification as can be used most of the time.

If you are staying with the 1.25" format eyepieces then a 32mm plossl would be a good low power eyepiece to compliment the others you have.

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5 minutes ago, John said:

I'm not an imager so I'll leave others to comment on that.

On the barlow though, if you have a Celestron 8SE then I suspect that a 2x barlow is not going to get much use for visual observing. The 8mm focal length point of the zoom is giving you 250x which is about as much magnification as can be used most of the time.

If you are staying with the 1.25" format eyepieces then a 32mm plossl would be a good low power eyepiece to compliment the others you have.

Couldnt agree more. Tried and tested by myself (i am by no means an expert), but on the 8SE.........works well for me.

Edited by LukeSkywatcher
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12 minutes ago, John said:

I'm not an imager so I'll leave others to comment on that.

On the barlow though, if you have a Celestron 8SE then I suspect that a 2x barlow is not going to get much use for visual observing. The 8mm focal length point of the zoom is giving you 250x which is about as much magnification as can be used most of the time.

If you are staying with the 1.25" format eyepieces then a 32mm plossl would be a good low power eyepiece to compliment the others you have.

I was just curious about the magnification with barlows. No plans to get one.

I considered a 9mm and a 30 or 40mm when I was buying my scope and the zoom offered an inexpensive alternative in the higher magnification range. I noticed that I actually stay at about 12mm when observing due to the image suffering if I dial in any more. 

Thanks for your suggestion! 

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A Schmidt-Cassegrain requires collimation just as a Newtonian; just as any telescope that utilises mirrors to form an image.

Whilst it's true that a Schmidt does not require collimation as often as a Newtonian, the need is there nonetheless.

If your telescope could indeed use a bit of adjustment, then if collimated spot-on you may then be able to realise a power that even an 8mm would provide(254x), at least on occasion, especially when observing the Moon.

Edited by Alan64
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17 hours ago, laudropb said:

To an extent that is true. If an eyepiece gives a magnification of say 50x in your scope a 2x Barlow with the same eyepiece will give a magnification of 100x. The Barlow does not half the focal length of the eyepiece however. It doubles the focal length of the scope.

Thanks for that info, I wasn't aware that the Barlow doubled the focal length of the scope.  Bearing that in mind, I assume it also halves (for a 2x Barlow) the TFOV of the EP?

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