Focus with Barlow

[Crossway]

I have recently purchased a 2X Barlow to extend my Astromaster  EP kit.  Using it intitially with the 20mm I found that the focussing barrel would not rack in far enough to  achieve  focus . Almost but not quite !  I could have machined off about  5mm from the base of the EP but  thought this a bit extreme.  Then I discovered that the  black metal trim  at the top of the focus barrel  was threaded and  could be  removed  and  would thus allow the barrel  to be racked in  just that little extra  and sufficient to get perfect focus with the Barlow and 20mm combo.  So it works just fine now but I would be interested tp hear if anyone else  has come across this problem.

Pete

[Cornelius Varley]

See 

it seems to be a common problem.

[Louis D]

I'm presuming you're using a refractor with a diagonal?  If so, then yes, certain long focus barlows can fail to come to focus.  If the optics nosepiece unscrews from the barrel and is threaded with 1.25" filter threads, you can just screw it onto the front of your diagonal for powers usually higher than 2X (around 3X, give or take).  "Shorty" type barlows became popular in the last 25 years to fix this problem with refractors because they don't require as much focuser in-travel as longer designs.  The downside is that they usually show more aberrations for a given amount of money spent than long focus barlows.

If you're using a Newtonian, the barlow can be inserted all the way into the focuser and the focuser usually has to be racked outward to reach focus.  Really handy with eyepieces that won't reach focus otherwise.

[Crossway]

Thank you for your reply and sorry that  I am a bit late in responding.  It is  an Astromaster newtonian  that  was causing the problem and  pleased that I have been  able to sort it out  by removing the focus barrel trim but  note that other people have had  similar issues where a barlow is concerned. 

I recall that when I  used the scope for the very first time it would not focus  at all with the standard EPs supplied, never mind a barlow and I discovered that there was  a serious collimation issue with the secondary mirror which was  way out of  touch  with the focus draw tube and this obviously affected the scopes focal length. I corrected this and  all was well.   But  I know not the cause of  the  problem with my Barlow...... It is a Bresser 2X  and  in appearance is a lot shorter than other Barlows I have seen, however as already stated  it works perfectly .......I just have to make sure that I don't  lose my barrel trim!

[Cornelius Varley]

33 minutes ago, Crossway said:

Thank you for your reply and sorry that  I am a bit late in responding.  It is  an Astromaster newtonian  that  was causing the problem and  pleased that I have been  able to sort it out  by removing the focus barrel trim but  note that other people have had  similar issues where a barlow is concerned. 

I recall that when I  used the scope for the very first time it would not focus  at all with the standard EPs supplied, never mind a barlow and I discovered that there was  a serious collimation issue with the secondary mirror which was  way out of  touch  with the focus draw tube and this obviously affected the scopes focal length. I corrected this and  all was well.   But  I know not the cause of  the  problem with my Barlow...... It is a Bresser 2X  and  in appearance is a lot shorter than other Barlows I have seen, however as already stated  it works perfectly .......I just have to make sure that I don't  lose my barrel trim!

If the telescope has been recollimated due to an earlier issue then maybe the problem is that the primary mirror is now not in the correct location. The distance between the secondary and primary mirrors can affect the focussing distance of an eyepiece. If the primary is further away for the secondary, the focuser has to be wound in further, if the primary is closer then the focuser is wound out more to achieve focus.

[Crossway]

Yes, understood Peter but I am sure that the primary is not out of kilter, in fact I am happy that the scope is  perfectly collimated when using the standaed EPs without the Barlow.  Using them  plus the Barlow with  focus  barrel slightly " dismembered" is  quite tolerable !.

Pete.

[Louis D]

I've got either a Celestron or Orion shorty type barlow like that.  I think it came to focus in my Newt when I was still using it.  It's not a very good barlow compared to my 1.25" TV 2X, Orion Deluxe 2" 2X, 1.25" Meade APO 140 2x, or GSO ED 2" 2X barlows, so I quit using it years ago.  I loan it out locally to folks who have no other barlows to experiment with.  They always end up returning it after getting something better.  If I think about it, I'll dig it out and see where it comes to focus relative to a bare eyepiece.

That Japanese made Orion Deluxe 2" 2X barlow in particular requires about an inch or more of focuser out travel to reach focus, so I typically insert it until the image comes to focus again and tighten the compresson ring around its midsection to avoid massive refocusing.  It's probably 6 inches long.  I've never tried it in a refractor, but I'll bet it wouldn't come to focus if inserted in the diagonal.  Unfortunately, the optics section doesn't unscrew from the tube to be screwed onto the front of a diagonal.  It's just one massive piece from eyepiece holder to field lens.  Excellent correction, however; better than the 1.25" TV 2X.

My point is, in my experience, barlows make reaching focus easier, not harder, in a Newt, so I can't fathom what's going on with yours.  Maybe try a long focus barlow sometime?