Cannot focus with longer focal length eyepiece

[JezuzStardust]

Hi!

Yesterday I was out with a friend, trying out sky watching for the first time. It was a great experience to say the least!

However, we ran into a few problems. The telescope we used is a Skywatcher 12" Dobson and we managed to get good views on different things like the double star in Ursa Major and the Pleiads. This worked fine when using the two eye pieces which came with the telescope, however, we also tried using a Skywatcher Sky Panorama (23 mm) eye piece, but with that we could not manage to focus! What we saw was an image of the top mirror, including the four bars which hold it in place! No matter how we tried adjusting the focus all stars just looked as the mirror.

Does anyone have an idea what the problem might be? In retrospect I started to think that we did not actually adjust the focus when rotating the controls of the scope, however I do not have access to the scope to test if this is the case. What I mean is that the eyepiece is quite large and heavy and perhaps the focusing mechanism was just sliding when I rotated it since the image did not seem to change very much. (I know the mechanism can slide since we accidentally used it a couple of time when the locking screw was tightened.

Anyway, if someone has any suggestion I would be very thankful.

[paul mc c]

Sounds like you need an extension tube,you probably need more back focus.http://www.telescopehouse.com/cgi-bin/sh000001.pl?REFPAGE=http%3a%2f%2fwww%2etelescopehouse%2ecom%2facatalog%2fStargazing%2ehtml&WD=extension%20tube&PN=Revelation_50mm_Extension_Tube_2__-1%2ehtml%23a1_21FF114#a1_21FF114

[moriniboy]

Before trying an extension tube try upping the tension via the hex adjusters which can be found at the bottom of the focuser, to me this sounds like the focuser not moving out as the ep's weight is overcoming the ability of the focuser to work (hence the view stays the same)

Its better if you don't use the locking screw to do this, save this for when you have cameras etc attached.

[Charic]

........In retrospect I started to think that we did not actually adjust the focus when rotating the controls of the scope,

+1. moriniboy beat me to this,  but that sound like your issue.

[JezuzStardust]

Thanks for the replies! I will try to tension the focuser. If that does not work I will try an extension tube.

I think, in the long run, trying out sky watching will end up costing me a lot of money . Now I want my own telescope.