Windows Phone 7 - how to center a popup in a scrollviewer larger than the screen?

I hope someone can help with what should be a very simple problem to solve... I've spent hours on this and it's just driving me crazy!

In my Silverlight WP7 app I have a ScrollViewer control that is 600x600 and offset to the left by 60 pixels (to center it horizontally) and in this I have a number of images overlayed on each other (see the code below).

I also have a Popup control defined which pops up while the images are downloading from the web but I just can't get the PopUp centered - and even worse, the text inside it is NEVER centered even though I've specified it to be... it always justifies left no matter what I try.

    <Grid x:Name="LayoutRoot" Background="Transparent">
        <Grid.RowDefinitions>
            <RowDefinition Height="40"/>
            <RowDefinition Height="600"/>
            <RowDefinition Height="50"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <ScrollViewer Grid.Row="1" Margin="-60,0,0,0" Height="600" Width="600" VerticalScrollBarVisibility="Disabled" HorizontalScrollBarVisibility="Auto">            
            <Grid x:Name="RadarImages" Margin="0,0,0,0">            
                <Image Height="600" Margin="0,0,0,0" HorizontalAlignment="Left" Name="imgBack" Stretch="Fill" VerticalAlignment="Top" Width="600" Opacity="100" />
                <Image Height="600" Margin="0,0,0,0" HorizontalAlignment="Left" Name="imgObs" Stretch="Fill" VerticalAlignment="Top" Width="600" Opacity="100"  />
                <Image Height="600" Margin="0,0,0,0" HorizontalAlignment="Left" Name=开发者_Python百科"imgLoop" Stretch="Fill" VerticalAlignment="Top" Width="600" Opacity="100"  />
                <Popup x:Name="StatusPopup" Margin="-200,-100,0,0" HorizontalAlignment="Center" VerticalAlignment="Center">
                    <TextBox Text="...loading..." Width="200" Height="60" HorizontalAlignment="Center" VerticalAlignment="Center" Background="Black" Foreground="White" BorderBrush="White" BorderThickness="3" />
                </Popup>
            </Grid>
        </ScrollViewer>

   </Grid> 

I've tried adding a Grid inside the popup to explicitly control the layout with no joy: the darned PopUp just won't center on the screen and the text inside won't either.

Can anybody suggest what I need to do to fix this?

TIA...

Mike

---

I've managed to get the Popup content using the following XAML (I added a couple of fixed background colors to show where different elements are):

<Grid x:Name="LayoutRoot" Background="Transparent">
        <Grid.RowDefinitions>
            <RowDefinition Height="40"/>
            <RowDefinition Height="600"/>
            <RowDefinition Height="50"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <ScrollViewer Grid.Row="1" Margin="-60,0,0,0" Height="600" Width="600" VerticalScrollBarVisibility="Disabled" HorizontalScrollBarVisibility="Auto">            
            <Grid x:Name="RadarImages" Background="AliceBlue" Margin="0,0,0,0">            
                <Image Height="600" Margin="0,0,0,0" HorizontalAlignment="Left" Name="imgBack" Stretch="Fill" VerticalAlignment="Top" Width="600" Opacity="1" />
                <Image Height="600" Margin="0,0,0,0" HorizontalAlignment="Left" Name="imgObs" Stretch="Fill" VerticalAlignment="Top" Width="600" Opacity="1"  />
                <Image Height="600" Margin="0,0,0,0" HorizontalAlignment="Left" Name="imgLoop" Stretch="Fill" VerticalAlignment="Top" Width="600" Opacity="1"  />
                <Popup x:Name="StatusPopup" IsOpen="True">
                    <Border Background="Red" Height="768" Margin="60,0" Width="480">
                        <TextBox Text="...loading..." HorizontalAlignment="Center" VerticalAlignment="Center" Background="Black" Foreground="White" BorderBrush="White" BorderThickness="3" />
                    </Border>
                </Popup>
            </Grid>
        </ScrollViewer>

   </Grid>

Bascially, you can't use the Popup itself for position or alignment, you have to work with the root element inside the Popup instead.

One other thing: the valid range of values for Opacity are 0 to 1, not 100.