2 Inch Barlows or 1.25 inch Barlows>

[Guest]

Having the Sky-Watcher Skyliner 200P Dobsonian, gives me the choice of either a 2" or 1.25" Barlow.  What's the pros and cons, please?

[John]

Are you intending to barlow 2 inch eyepieces ?

 

[Guest]

9 minutes ago, John said:

Are you intending to barlow 2 inch eyepieces ?

 

No, I was just wondering what the difference was (apart from cost, etc)?

[johninderby]

Size and weight. Some of the barlows or telextenders can be pretty big and heavy. 

Here’s a 2” telextender with a Morpheus eyepiece which is a fair sized eyepiece.

[John]

Things can get big and heavy with 2 inch barlows (Powermate in this case):

 

[Don Pensack]

2 hours ago, merlin100 said:

Having the Sky-Watcher Skyliner 200P Dobsonian, gives me the choice of either a 2" or 1.25" Barlow.  What's the pros and cons, please?

Most people use a 2" eyepiece at low power to get a wide field.  And people usually have 1.25" medium power eyepieces, so why double a 2" eyepiece to yield a duplicate medium power?

Ergo, most barlows, and rightly so, are 1.25".  So if choosing, choose a 1.25" barlow to yield high powers from a medium power eyepiece.

[Don Pensack]

2 hours ago, johninderby said:

Size and weight. Some of the barlows or telextenders can be pretty big and heavy. 

Here’s a 2” telextender with a Morpheus eyepiece which is a fair sized eyepiece.

Though probably not the case with the ES TeleXtender, a normal 2X 2" Barlow would not yield 2" with the Morpheus used as a 2" eyepiece.

The focal plane of the Morpheus is closer to the top of the 1.25" barrel, so using them as 2" eyepieces places the focal plane closer to the barlow lens, reducing the magnification.

[johninderby]

No longer have the 2” Telextender. Just too big and heavy and not a lot of use so sold it on.

Edited April 20, 2020 by johninderby

[Guest]

59 minutes ago, Don Pensack said:

Most people use a 2" eyepiece at low power to get a wide field.  And people usually have 1.25" medium power eyepieces, so why double a 2" eyepiece to yield a duplicate medium power?

Ergo, most barlows, and rightly so, are 1.25".  So if choosing, choose a 1.25" barlow to yield high powers from a medium power eyepiece.

Yep, that's what I thought, but wasn't 100% sure.

[Ruud]

The Morpheus eyepieces above have a 1.25" barrel but can also be used directly in a 2" focuser. There's little point in doing this however as  they sink in unusually deep. Using a 2" to 1.25" adapter  works better.

+1 for the notion that a Barlow is most useful in combination with mid and short focal length eyepieces, which tend to be 1.25".

I have four 1.25" Barlows. The newer 2x and 3x get the most use, but the older 1.8x and 2.5x are also quite nice and just too difficult to part with. I do not own any eyepieces with focal lengths under 4.5 mm, but with the Barlows and my 4.5, 6.5, 8 and 12mm eyepieces I have a wide range of high to very high magnifications to choose from using my f/5 refractor.

[johninderby]

BTW the Morpheus eyepiece  was never used in the 2” telextender. Just used to show size of the telextender.. 

Edited April 20, 2020 by johninderby

[Louis D]

I sometimes put my TV PBI in my 2" GSO ED 2X Barlow for the night in my Dob to double my focal length for all my eyepieces.  Do not try to use a 2", non-telecentric Barlow with near maximum field 2" eyepieces.  They will have edge cutoff, not vignetting.  It's like there's a new field stop inboard of the physical field stop.  The discontinued TV PBI (Panoptic Barlow Interface) works well with either the TV Big Barlow or the 2" GSO ED Barlow (nearly the same focal lengths) to convert them into telecentric magnifiers like the TV Powermates.

[Guest]

To be honest, I've gone off Barlows altogether, just preferring a range of eyepiece focal lengths.