Televue 2x Barlow and c925

[Hemi]

..... I think I'm doing something wrong again!

Just got this and can't see a thing. I've searched on how I attach it, but all I get is either a massive central obstruction (40 celestron plossl) (sometimes off centre oddly enough) or nothing at all.....ie black!(12.5 Orion edge on).

I've mounted it before and after the diagonal....makes no change to the result down the EP.

What am I doing wrong folks?

[Hemi]

....... I should add that when attaching after the diagonal, it feels as if the Barlow is not going all the way in. Only to about 4cm.lots of shiny silver remains out of the diagonal.

[julianh72 (Julian)]

Normal use is to insert the Barlow in the diagonal, and the eyepiece in the Barlow.

I would guess that it's just way off focus - especially if it can't seat fully in the diagonal.

Try using it on a terrestrial target in daylight hours to find out how much focuser adjustment you need (and which way to turn the focuser) when swapping the Barlow in and out. It's a LOT easier to experiment with new items in your optical train (Barlows, cameras, etc) in daylight (or pick a bright terrestrial target such as a distant streetlight if working at night), when you can get enough information from even a badly out-of-focus image to work out which way you need to adjust the focuser, which can be extremely difficult when you are looking at dim star fields.

[Merlin66 (Ken)]

Hemi,
The Powermate can be used before the diagonal. It is not sensitive to backfocus distance like the traditional Barlow.
Which 2" adaptor are you using?
Sounds like a focusing issue.

[casstony]

A barlow makes the light cone less steep and pushes the focal point out past the eyepiece. To bring the focal point back to the eyepiece the main telescope mirror needs to be moved toward the rear cell by turning the focuser clockwise. It will take quite a few turns of the focuser to achieve focus; if you're going in the right direction the doughnut will be getting smaller.

A reducer has the opposite effect and the focuser has to be turned anticlockwise a lot, pushing the main mirror forward in the tube.

[Hemi]

Thanks everyone,

I'll try tomorrow night, tonight is cloudy! The Barlow sits out by about 2-3 cm which is normal I think. John at bintel checked it out on their floor scope, so it isn't that im not setting it up correctly. Just need to get it in focus. I thought it might have been that, but wasn't sure if I could just keep turning the focuser. So stopped, not wanting to see the primary mirror drop on the floor!

Will try tomorrow when the clouds have rolled away!