Barlow Lens

[Crossway]

I am somewhat confused by the specifications of  barlow lenses which are advertised  via Amazon.  My initial interest was in the 2X  which I  reckoned would  adequately extend the  capability of my small  Celestron reflector  ( with standard 20mm and 10mm EPs ).   and understand that the Barlow 2X will double the  scopes focal length and double the magnification of the EP.   There are also other Barlows advertised  up to 5X.....does this mean that the focal length is increased by the  indicated factor : and following this ,  the magnification by the same amouint .!!!

The Celestron 10mm EP is OK  ( moon and planets) but  has taken a bit of getting used to vis a vis the  small aperture which necessitates a bit of a "squint".  Am I correct in my understanding that  a Barlow ( say 2X) will  improve eye relief  and reduce the squinting.

Pete

[baggywrinkle]

Yes that is correct.

A 5x would make your OTA 5 time longer in focal length.

As a rule of thumb the theoretical maximum magnification of a telescope would be 2x the objective size, so if 130mm the 260 would be the max. That would be on a good night with clear skies and seeing.

For a small scope I would not go more than 2x or 2.5x for a Barlow.

Generally the stock ep's provided with a scope are never of the best quality and they are usually the first items to get replaced.

[Cornelius Varley]

Yes, a 5x barlow would increase the effective focal length of the telescope by a factor of 5. This increase in magnification would probably be unusable so it is better to stay with a x2 barlow.
The eye relief is not affected by the use of a barlow lens.

[Putaendo Patrick]

27 minutes ago, Crossway said:

The Celestron 10mm EP is OK  ( moon and planets) but  has taken a bit of getting used to vis a vis the  small aperture which necessitates a bit of a "squint".  Am I correct in my understanding that  a Barlow ( say 2X) will  improve eye relief  and reduce the squinting.

No - your 10mm will have all the same problems. The difference is that if you got a 5mm EP, you might have to squint more, but if you use a x2 Barlow on your 10mm EP, you effectively get a 5mm while maintaining the original eye relief of the 10mm.

[Alan64]

This is my 4mm orthoscopic, and uncomfortable if not downright painful...

 

Instead, I combine a 12mm eyepiece with a 2.8x barlow, for a simulated 4.3mm...

...and much more comfortable to use.

 

5x barlows are for photographic use; for imaging planets, I think.  A 3x barlow would be the highest for visual use, with eyepieces.  I have three better-quality barlows: a 2x, 2.8x and 3x, with the Antares 2x on the left being quite the performer, as my 102mm apochromat would testify, if it could speak...

This is the current incarnation of that Antares 2x barlow... http://www.rothervalleyoptics.co.uk/antares-x2-twist-lock-barlow-lens-125.html

[John]

Barlow lenses do extend the eye relief of the eyepiece so can make an eyepiece with tight eye relief easier to use.

With regards to the multiplying effect that you go for (ie: 2x, 3x etc etc) bear in mind that the maximum useful / practical magnification limits of your scope and the viewing conditions will still apply - there is no "free lunch" with barlows !

 

[Mak the Night]

Barlow lenses do indeed extend the eye relief, even if the Barlow element is threaded directly into the draw tube itself or placed into a diagonal nosepiece. I'm fairly sure that some amplifiers, such as TeleVue Powermates, don't extend eye relief though.

[Crossway]

Many thanks to fellow members for all  for your replies.....a lot of very useful and  practical  information  which  is of great help to me.

Thanks again,

Pete. 

[Charic]

Crossway........your assumptions are true, and others have added a little more info, hope this helps.

Your 10mm & 20mm Celestron eyepieces will not be too dissimilar to my 10 and 25mm Sky-Watcher eyepieces. I chose to upgrade my two Sky-watcher eyepieces in favour of better eye-relief and a much better field of view.

The Barlowing  effect  of  my upgraded eyepieces still applies, but the eyepieces are just more comfortable in use. 

Maybe consider an eyepiece upgrade to give yourself a wider field of view, this may help with the squinting through a tiny eye-lens, similar to those depicted above?
 

[Mak the Night]

6 hours ago, Alan64 said:

This is my 4mm orthoscopic, and uncomfortable if not downright painful...

 

Instead, I combine a 12mm eyepiece with a 2.8x barlow, for a simulated 4.3mm...

...and much more comfortable to use.

 

5x barlows are for photographic use; for imaging planets, I think.  A 3x barlow would be the highest for visual use, with eyepieces.  I have three better-quality barlows: a 2x, 2.8x and 3x, with the Antares 2x on the left being quite the performer, as my 102mm apochromat would testify, if it could speak...

This is the current incarnation of that Antares 2x barlow... http://www.rothervalleyoptics.co.uk/antares-x2-twist-lock-barlow-lens-125.html

Do you know whether the Antares 2x current incarnation has a smooth draw tube or features a safety undercut?

[Dave In Vermont]

51 minutes ago, Mak the Night said:

Do you know whether the Antares 2x current incarnation has a smooth draw tube or features a safety undercut?

If it's any help, I've bought many Orthoscopic EP's - and seen many others. None have had a 'safety' on their barrels. Thank goodness - those things drive me nuts.

Have fun,

Dave

[Mak the Night]

5 minutes ago, Dave In Vermont said:

If it's any help, I've bought many Orthoscopic EP's - and seen many others. None have had a 'safety' on their barrels. Thank goodness - those things drive me nuts.

Have fun,

Dave

Yeah, for a long time I couldn't understand why the undercuts were a problem. My diagonals and Barlows had no brass retaining rings. Then I upgraded my Barlows (to TeleVue 2x, 3x) and started to discover that my (predominantly) Celestron EP's were having extraction problems from them. As I acquired better EP's (often TeleVue) the problem was not so apparent. The brass compression ring can snag the older 'squared' undercuts. 

Above are three 15mm EP's: A TV Plossl, a Celestron Kellner and a Celestron Omni Plossl. The Omni's undercut is 'squared off' and not tapered like the TeleVue. The Kellner is a 'smoothie'. The smooth draw tube of the Kellner allows it to be smoothly inserted or extracted into any Barlow or diagonal, whether it has a brass compression ring or not. The TeleVue's tapered lower undercut lip allows it to be extracted fairly easily from a compression ring, although not as well as a smoothie. The Omni (and others) would often get caught up on extraction causing much profanity from me! lol 

I am so glad that most orthoscopics manufacturers haven't decided to add undercuts.

[John]

With the Omni's I recall you can simply screw the 1.25" barrel on the other way around so the compression ring is nowhere near the safety undercut. That also works on Revelation / GSO plossls apart from the 32mm where I think the field stop is fitted to the inside wall of the 1.25 barrel.

 

[Mak the Night]

2 minutes ago, John said:

With the Omni's I recall you can simply screw the 1.25" barrel on the other way around so the compression ring is nowhere near the safety undercut. That also works on Revelation / GSO plossls apart from the 32mm where I think the field stop is fitted to the inside wall of the 1.25 barrel.

 

That's interesting. I always wonder if all the lenses will drop out if I unscrew anything though lol.

[Alan64]

2 hours ago, Mak the Night said:

Do you know whether the Antares 2x current incarnation has a smooth draw tube or features a safety undercut?

There's no undercut on the Antares barlows, that I can see...

A call to the vendor would verify that, and prior to purchasing.

[Mak the Night]

28 minutes ago, Alan64 said:

There's no undercut on the Antares barlows, that I can see...

A call to the vendor would verify that, and prior to purchasing.

OK thanks. They look good.

[John]

1 hour ago, Mak the Night said:

That's interesting. I always wonder if all the lenses will drop out if I unscrew anything though lol.

With GSO and Omni plossls and most orthos the lenses are generally held in place by a lower lens retaining ring which doubles as the field stop. It's not connected to the 1.25" barrels. The 1.25" barrel just screws onto a thread cut into the bottom edge of the black anodised part of the eyepiece body. In these models this thread is the same size as the 31.7mm filter thread so the barrel will go on either way around. This is not the case on all plossls though.

Some of the Tele Vue plossls (the 32mm ?) do use the barrel to hold the lens elements in place though.

 

 

 

[Mak the Night]

7 hours ago, John said:

With GSO and Omni plossls and most orthos the lenses are generally held in place by a lower lens retaining ring which doubles as the field stop. It's not connected to the 1.25" barrels. The 1.25" barrel just screws onto a thread cut into the bottom edge of the black anodised part of the eyepiece body. In these models this thread is the same size as the 31.7mm filter thread so the barrel will go on either way around. This is not the case on all plossls though.

Some of the Tele Vue plossls (the 32mm ?) do use the barrel to hold the lens elements in place though.

 

 

 

I know some of the TeleVue Plossls and other EP's of theirs are held together by the draw tube. I often wonder about the threading on Celestron eyepieces or filters. It doesn't seem the same as virtually everything else. My Baader filters thread into all of my TV, WO, Hutech EP's and even the Sky-Watcher giveaways. I have some Celestron Wratten filters (from the Eyeopener Kit) that only properly thread into Celestron EP's.