2" 30mm Superview eyepiece out of focus

[Nexus6]

Hi,

I've got a GSO 8" Dobs and just received my (early) Christmas present, a GSO Superview 30mm (2"). I tried it out yesterday and I have problems to get it into focus. To get it to focus I have to pull out the focuser to the maximum plus pull out the eyepiece out of the focuser by about 3-5 mm. So I though, ok I'll get a 35mm extension tube (not too large), that might do the trick... I was wrong. Now it seems the 35mm is too large (needs to be shorter). Has anyone any idea what I can do? If I pull the eyepiece out about 3-5mm it's ok, but wobbles in the focuser...

Cheers
Tom

[MortonH]

This Lumicon extension is only 25mm (1 inch) and may work.

http://www.astroshop.com.au/products...asp?id=MAS-432

Morton

[Nexus6]

Hi Morton, thanks for your reply... I found out why it doesn't work. The focuser extends to a maximum of 47mm. The eyepiece needs about an additional 3-5mm. The 35mm extension tube brings it (focuser back in) to 50mm. Unfortunately the eyepiece doesn't slide completely in the extension tube (it stops where the thread for the filters starts), so it sticks out even further... to far. If the eyepiece would slide in completely in the extension tube, it probably would work...

Thanks a lot for the link, I'm pretty sure this will solve the problem... . Now I'm sitting on a extension tube I don't need... oh dear.

I really appreciate your help.

Cheers
Tom

[wavelandscott (Scott)]

The other option is to change your mirror position by adjusting the collimation point of your mirror (moving it further up or down the tube as needed with the collimation screws)...particularly if you are only looking for a few mm.

[erick (Eric)]

Originally Posted by wavelandscott

The other option is to change your mirror position by adjusting the collimation point of your mirror (moving it further up or down the tube as needed with the collimation screws)...particularly if you are only looking for a few mm.

That's it! I was trying to work out why I had no trouble with my 30mm in my previous 8" GSO. I suspect your primary mirror is some distance up the tube and the springs aren't particularly compressed. Stick your nose in and have a look from behind and see if that is the case. If so, wind your locking screws well back, then pull the mirror further down the tube with the three collimating screws. The same number of turns on each should keep your collimation close. Turn each a few turns (maybe 2?) at a time. This will pull the focal point deeper in the focusser, the distance the mirror moves towards the rear of the OTA. This may also improve how well your scope holds collimation through the altitude range - the springs are more compressed, less likely for mirror "sag". Perhaps do it during day time when you can test, as you go, on a very distant mountain top or TV tower - make it many km away. WATCH OUT FOR THE SUN - point in the opposite direction and cover your finderscope objective lens when you are not using it.

The only problem now is whether any eyepieces now bottom out in your focusser without reaching focus? Perhaps start by checking the position of the focusser draw tube for each of your eyepieces - on that very distant object. The one that is focussing the closest to the tube tells you how much room you have to move the focal point.

By the way, adding a 2x barlow in the focusser often solves the reaching focus problem, but at a price of 2x magnification!

[Nexus6]

Hi Erick and Wavelandscott,

Thanks a lot for your help... I'm not a big risktaker and I don't want to "break" anything, so colliminating is one thing (and trying to get this right.. puh) and then trying to get this sorted by moving the main mirror back is another... I'll try and see how I go with the 1inch extension tube. Thanks a lot for your tips, great help. I hope you had a very good christmas.

[MortonH]

Another option is to change the focuser to a low-profile model, such as the Bintel Crayford Focuser Low Profile 10:1

Morton

[acropolite (Phil)]

To keep the EP from wobling when it's pulled partly out you could get a parfocal ring, they're only a few mm thick and are inexpensive.

[Jaeger]

Originally Posted by erick

By the way, adding a 2x barlow in the focusser often solves the reaching focus problem, but at a price of 2x magnification!

I struck a similar problem with a Bintel (GSO) 2" SP 40mm eyepiece in my 8" f/6 Dobsonian.

When the SP 40mm was seated normally, I couldn't move the focuser out far enough to get it focus. Lifting it in the focuser tube moved the focus point back within the reach of the focuser, but with the risk that it wasn't "straight". (I suspect the brass compression ring will limit how much it could be out-of-whack compared to a simple locking screw?)

I tried it (and my other eyepieces) with a Bintel (GSO) 2" ED 2x Barlow. The Barlow also needed to be lifted from the seated position to allow the eyepieces to reach focus - I assume this is normal? (The Barlow doesn't have a safety groove like the eyepieces, so there isn't an obvious "home" position.)

I'll try pulling the mirror down as far as possible with the collimation knobs; I don't think any of my eyepieces needed much in-focus, so it shouldn't be a problem.