Refractor - Barlow before or after the diagonal.

[Daf]

Hi all,

Dob 5hinking about getting a 4" refractor (for g&g/travel). I have a 2" Barlow but the refractor I was looking at has a 2" focuser, but I'm more inclined to go with a 1.25" diagonal so that I can use it with a small mak I plan on picking up at some point.

Is it possible/are there any issues with putting the Barlow in the focuser, before stepping down to the diagonal?

Thanks,

Daf.

[Mr Spock]

Yes, you'll need an extension tube to reach focus.. I found on mine I needed 35mm.

[wookie1965]

Diagonal Barlow eyepiece. 

[Stu]

Putting the barlow in front of the diagonal will increase the factor it magnifies by. How much depends on the type of barlow but it will have quite an impact.

[Louis D]

Vintage long Barlows simply won't come to focus in many refractors because they don't have enough focuser in-travel if used after the diagonal.  This isn't an issue with Newtonians.  When used before the diagonal, they will produce more magnification than what they're marked for because the optical path length is longer.  Focus should be achievable in this mode, though.

Shorty Barlows will work well in either, but cheap ones tend to degrade the image somewhat.  Shorty Barlows also tend to increase magnification faster with longer path length because they have a shorter focal length (there's an inverse relationship between them).

If you have a GSO/Revelation 2" ED 2x Barlow, the nosepiece is M48 threaded and can be screwed onto the nosepiece of a 2" diagonal like a filter.  In that mode, the optical path length will be somewhat longer than when used after the diagonal with the original tube and eyepiece holder, but not a lot longer.  As such, you might get 3x at most, so it might be a good option for slightly more magnification.  I'll have to try it sometime with mine to see what the resultant magnification and focuser travel requirements are.

[Daf]

Thanks all. It is the 2" Stella Lyra (I guess GSO?) ED 2x Barlow, I already have.

I know at some point I'm going to end up getting the two sizes for diagonals and Barlow's... at least knowing it can be done I can just put off buying a 2" diagonal and 1.25" Barlow and filters for another time, to spread the cost.

That is interesting to know the Barlow position will change how it behaves! 🤔

[Mr Spock]

44 minutes ago, Daf said:

It is the 2" Stella Lyra (I guess GSO?) ED 2x Barlow, I already have.

Much like the one I have. I don't know about a 1.25" diagonal, but when placed in front of a 2" diagonal it gives x3.68. The nosepiece of the Barlow screwed into a 2" diagonal gives about x2.3.

[Louis D]

3 hours ago, Daf said:

Thanks all. It is the 2" Stella Lyra (I guess GSO?) ED 2x Barlow, I already have.

Marked Taiwan with the recessed 1.25" adapter, it's GSO.