2" Barlow- 2 piece question.

[Swoop1]

I have inherited a 2" Barlow which is in 2 sections- one part has a short slot in the barrel, the other inner section has a thread for a little thumb screw (missing).

What is the purpose of this being in two parts? Is it to fine tune focus/ back focus?

Thanks,

Matt

[Don Pensack]

Are you referring to the 2" to 1.25" adapter, where the thumbscrew fits in a slot in the 2" body of the Barlow, like in this picture of a similar one:

 

[Swoop1]

That does look very similar. I will check tonight.

[Swoop1]

@Don Pensack- yes, that is exactly the same design as my Barlow.

[Louis D]

To be clear, when using 2" eyepieces, just insert them into the big opening without the adapter.  When using 1.25" eyepieces, insert the adapter into the big opening, and tighten the big opening's screw to hold the adapter firmly. Next, insert the 1.25" eyepiece into the adapter's smaller opening and tighten its screw to firmly hold the eyepiece.  I hope that makes sense.

[Swoop1]

So the slot is just to allow full insertion of the adapter whilst maintaining the correct length of cell for focus?

[John]

26 minutes ago, Swoop1 said:

So the slot is just to allow full insertion of the adapter whilst maintaining the correct length of cell for focus?

Yes. A number of 2 inch diagonals, 2 inch barlows and Moonlite focusers use a similar approach. It enables a low profile design 1.25 inch adapter to be used.

 

[Louis D]

In the case of GSO 2" to 1.25" adapters, it maintains parfocality between 2" and 1.25" eyepieces if both have their focal plane at the same distance relative to their shoulders.  Notice below how the 1.25" adapter's opening is recessed exactly as much as the thickness of its lip so the shoulder of the 1.25" eyepiece will be at the same level as a 2" eyepiece's shoulder in the absence of the adapter:

It actually works quite well to avoid lots of refocusing if your eyepiece collection is parfocal between 2" and 1.25" eyepieces.

[Don Pensack]

1 hour ago, Louis D said:

In the case of GSO 2" to 1.25" adapters, it maintains parfocality between 2" and 1.25" eyepieces if both have their focal plane at the same distance relative to their shoulders.  Notice below how the 1.25" adapter's opening is recessed exactly as much as the thickness of its lip so the shoulder of the 1.25" eyepiece will be at the same level as a 2" eyepiece's shoulder in the absence of the adapter:

It actually works quite well to avoid lots of refocusing if your eyepiece collection is parfocal between 2" and 1.25" eyepieces.

IF your 1.25" eyepieces don't rest on the outer lip of the adapter, like Hyperion, Morpheus, LVW, and many other 1.25" eyepieces.

[Louis D]

3 hours ago, Don Pensack said:

IF your 1.25" eyepieces don't rest on the outer lip of the adapter, like Hyperion, Morpheus, LVW, and many other 1.25" eyepieces.

True, but enough don't to still make it a useful feature.