Author Topic: A problem focusing with my Barlow  (Read 1697 times)

alan g

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A problem focusing with my Barlow
« on: June 02, 2013, 01:52:04 AM »
Hi Guys , I've posted this on SGL but thought I might get some help here as well.
I was out tonight looking at Saturn using my Heritage 130p with a BST 8mm and the views were great, but when I dropped in my new Revelation Astro 2.5x Barlow all I saw was a dull image of my primary mirror and a black image of my secondary mirror and its supporting arm.... why is this ? (answers in plain English please as i'm new to this and my brain might melt!) The scope has a diameter of 130mm and is 650mm long.
I did manage to overcome the problem by reducing the length of my scope by about 0.5 of an inch and then everything focused beautifully, a neat thing you can do with this scope.
Last night I used my new Starwatcher ST80 (diameter 80mm and length 400mm) with the same 8mm lens and Barlow and had views that were far better than I had imagined I would get and without any focusing issues.... again why should this be ?
Any help here would be most welcome guys, thanks.
alan g

stuhawk123

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Re: A problem focusing with my Barlow
« Reply #1 on: June 05, 2013, 10:30:22 PM »
tool to work out the size. http://www.12dstring.me.uk/fov.htm
10" Dobsonian Sky Watcher

hemihaggis

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Re: A problem focusing with my Barlow
« Reply #2 on: June 09, 2013, 11:54:31 AM »
its all to do with the amount of magnification , and the "seeing"  using a x 2,5 barlow and an 8mm might just be too much magnification, also the atmospheric conditions vary hugly from night to night ..