Barlow Out Of Focus

[7oddy]

Hi all,

I have recently got a Sky Watcher 250 px and have been using a 2.5 x GSO barlow lense.( I have used a SW 130 before with this barlow and had no problems ) What I have noticed is that there isn't enough range on the focuser on the scope to get the barlow lense in focus. What I've found I need to do is unscrew the body of the barlow from the barlow lense eye piece which brings everything into focus. I was just wondering if anyone else has had this type of issue before ? I have used a 3 x barlow in this scope ( not a very clear barlow ) and there have been no focusing issues. Any advice would be appreciated.

[acey]

I take it that you mean you partially unscrew the lens holder, so that the overall distance from barlow lens to eyepiece is increased. If that works for you then the problem is solved - maybe put some tape round it to hold it in the required position.

The magnification of a barlow lens depends on its distance from the eyepiece. By increasing this distance you have slightly increased the magnification - it's not x2.5 now, but a bit more. You previously managed fine with a x3, and if both barlows had the same focal length then this would make sense.

[7oddy]

Hi Acey,

Yeah thats it, I partially unscrew the lens holder to get the Barlow into foucus. It works but the danger is that if I unsrew the lense holder too far it detaches from the eye piece end, and like I did the other night forgot I'd unscrewed the lense holder. I was changing eye piece and undid the thumb screws holding the eye piece of the barlow in the focuser and it dropped into the scope......could have been costly. Think I'll have to purchase a new 2 x or 3 x barlow lense.

[acey]

Some tape round it would hold it in place. As an alternative, don't insert the eyepiece fully, but instead have it only partially in the tube. This will have the same effect of lengthening the distance between barlow lens and eyepiece. Experiment by sliding the eyepiece in the tube, adjusting the focuser with each new position, to find a point where you can bring the image to focus.

[Stephen]

You need a short extension tube by the sounds of it, hanging kit out the back of the barlow or focuser isn't the best idea in the world as you've found out!

Astroboot normally have a few extension tubes pretty cheap

[7oddy]

Thanks, thats some good advice. I'll have a play around with those ideas tonight.

[7oddy]

I was wondering about extension tubes......Thanks Stephen, I have a look on Astroboot

[Peter Drew]

If the Barlow lens/eyepiece assembly is already in as far as the focuser will allow, incresing the distance between the Barlow and the eyepiece will not help as to refocus will require even more in focus.

[Darren1983]

I have the same problem with my sky watcher 200p,if i could just move the focuser in a tad more it would focus perfect,but can't manage too,no matter how i set it up.

Apparently the supplied lenses are not that great,although some people have used the barlow and got it to work fine,and im sure my scope is set up right as i get crystal clear views without the barlow but would like to get in abit more so i can make stuff out.

[Zachariah]

Surely the supplied barlow lens should work? Whats the point in it? My SW 150 barlow works fine with all my lenses including bst and celestron, sounds as if something not quite right somewhere.

[acey]

Just another little point to consider: if the primary has been raised quite high in the tube then this might affect focusing - in the past I found some eyepieces wouldn't come to focus in my 300p, I unscrewed all the primary bolts to get the mirror as far down as possible, then recollimated and found I could reach focus again. An additional issue with the flextube (which I have) is if the upper section is not completely extended: this will obviously affect focuser travel.

[7oddy]

Thats a good point, I'll check to see if the Primary Mirror has been raised too high. I'll unscew the Primary Bolts and recollimate and see if this makes a difference.