Trouble using a barlow with my Meade Lightbridge 16"

[adarshajoisa]

I recently acquired a Meade Lightbridge 16" scope (f 4.5), and realized that I'm unable to focus with my barlow attached ( the barlow is a Celestron X-Cel LX 3x barlow). After some experimentation, I found that it's because the focal plane of the primary is too short - it's just at the end of the focuser when it's fully retracted. By removing the 2"-1.25" adapter, I found that I'm able to focus with the barlow if it can go about 6 mm further in. 

Other than cutting my truss tubes to shorten them, or buying a lower profile focuser, is there any cheap, DIY method by which I can achieve focus?

An astronomer friend of mine suggested fully tightening all the primary collimation knobs, so that the primary mirror moves up a little inside the OTA, thus bringing the focus further out of the focuser, but that didn't help ( it probably moved upwards only by a couple of millimeters)

[alanjgreen]

Or if you get a barlow with less magnification ( such as x1.6 or x2) ?

 

[adarshajoisa]

2 minutes ago, alanjgreen said:

get a 2x barlow instead?

 

 Yeah, I'm keeping that option as a last resort. I'm totally broke currently, because of the 16" purchase. I'm hoping for any cheap/DIY ideas that I might've missed.

[adarshajoisa]

3 minutes ago, alanjgreen said:

get a 2x barlow instead?

 

BTW, I don't think I completely understand how barlows work. How would a 2x barlow be able to focus where the 3x fails?

[alanjgreen]

barlow will increase the focal length meaning the eyepiece gets pushed away from the primary

a reducer pushes it the other way

 

[adarshajoisa]

Oh, I thought barlows increased the focal length, hence the extra magnification.

[alanjgreen]

it does, it was a typo (just corrected it)

thats why a barlow can help you get focus when you use a binoviewer (which adds many mm to the light path)

 

 

[alanjgreen]

11 minutes ago, adarshajoisa said:

BTW, I don't think I completely understand how barlows work. How would a 2x barlow be able to focus where the 3x fails?

its in the maths

http://astunit.com/astunit_tutorial.php?topic=barlow

the magnification factor ties into the amount of increase in focal length

 

[adarshajoisa]

3 minutes ago, alanjgreen said:

its in the maths

http://astunit.com/astunit_tutorial.php?topic=barlow

the magnification factor ties into the amount of increase in focal length

 

Wow! That's a great resource! Thanks!