Telescope Can Not Focus With A Barlow :(

[StrainHardening]

Hi All,

I recently bought a Televue Barlow 2X.

Later that night I went to use it and I found that it was impossible to focus the view.

Telescope that i'm using: http://www.ozscopes.com.au/skywatcher-102mm-510mm-az3-refractor-telescope.html
If you dont want to open the link, it's 102mm diamater and 510mm long.

I had the 10mm eye piece that came with the telescope, in the barlow.

I then adjusted the focus setting to it's limit so that the eye piece was furthest away from the main body of the telescope. I then slowly adjusted the focus knobs the other way so that the eyepiece would go closer to the main body of the telescope. I carefully looked through the eyepiece the whole time and found that the moon never came into focus. It seemed that I need to move the eyepiece even closer to the main body of the telescope for it to come into view however it is at it's limit/it is as close to the main body of the telescope that it allows. I would estimate that 1-2 more turns of the knob would get it into focus however there's just no more travel.

I have no problems getting the telescope to focus with just the 10mm eyepiece in with no barlow. No problems at all.
I then decided I would try the 20mm eye piece with the 2x barlow lense so that it would be the same magnification as the 10mm however I had the same problem, I ran out of 'travel' with the focusing.

I made sure the eyepieces are in the barlow properly and really pushed them down to make sure.

So my question is, is there a way to know if a barlow lense is going to work with your telescope?

I really want to get a 2X barlow that will work so i can see a bit more detail on the planets and moon.

Thank you

[Renato1]

Have you put the Barlow into the diagonal (making it 2X) or the diagonal into the Barlow (making it 3X)?

I've had problems with the latter not working, but it's always been okay with the former.

Also, the diagonal in the picture is an erect image one, which has a longer light path than a standard mirror diagonal - which could cause problems.
Regards,
Renato

[julianh72]

Not to mention that a 45-degree erect-image diagonal is great for terrestrial viewing, but gives a slightly inferior image for astronomical work compared to a 90-degree diagonal; AND a 90-degree diagonal will allow MUCH more comfortable viewing positions for anything at moderate to high elevation above the horizon.

[StrainHardening]

I had it set up like this:

1. Telescope
2. 45 degree diagonal
3. 2X Barlow
4. 10mm eye piece
5. my eye looking in to eye piece.

Ok, thanks, i'll have a look into getting a standard mirror diagonal.

[5ash]

Hi Jock,
The Barlow extends the focal length so you would have to rack the focuser out. If you can't get focus try pulling the diagonal, Barlow and even the eyepiece out a little and retighten them to get extra length .if this works then an extension tube will solve the problem/ or even a new focuser.
Philip

[MattT]

I have a 4" f5 Refractor too.
This is what I found with mine.
The R+P it comes with is 120mm long. I replaced that with a GSO Crayford that is 90mm long. The measurment is only the focuser and flange without any adaptors.

With an ES 30 82º eyepiece and 2" mirror diagonal focusing on a tower in the distance the measurment on the focus tube said 55mm, adding a 2x GSO 2" ED Barlow I needed to rack the focuser in toward the objective and focus was there at 27mm out. Did the same with an ES 24mm 68º and got the same amount of in travel needed.

My only other 2x Barlow is an ES telecentric and the focus position stayed the same at 45mm with the 24mm eyepiece, no in/out travel at all.

I presume the f5 achro's were not designed to be used with barlows?

A mirror diagonal will help but I'm not sure it's enough. From memory there is a difference of 9mm between a prism and mirror diagonal. A new focuser will, I got mine from Andrews Commiunications, as will using an 2x ES Focal Extender. Myastro shop sell ES FE's.

Matt