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I use a scale of 1-5.

Transparency is straight forward. My usual Zentih Limiting Magnitude (ZLM) is 4.5 so I use this as 3 onthe scale. On really excellent nights I have seen down to Mag 5.5 ZLM and this gets a 5.

Seeing is slightly more subjective and before I can make a good judgement the scope has to have cooled down.

As a double star observer I use these as a useful guide. If I take for example the doble-doulbe in Lyra on an average (3)night I can split these as x96 but on a poor night it will take x133. Ona very good night I can eaily split them at x78.

Also if you look at single bright stars they will really twinkle on poor nights through the scope. But make sure it is not the hot scope causing this 1st.



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Talking of seeing, the following (extracted from the latest BAA ECirc) might be of interest:

'If the sky is clear, without obvious haze, on any of the nights between December 21-24 2006, find the constellation of Orion between 8.00 pm and midnight and count how many stars you can see within the rectangle of Orion (bordered by his shoulders and feet but NOT including these four stars!). The more stars you can see, the less the light pollution.'

You can enter your findings on the BAA website or by post.

It is a little late but there is still time to take part (tonight is the last night).

To find out more, visit the Star Count web page at:


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Seeing - if the stars are twinkling it's only worth bothering with low focal lengths for imaging. if you can't get diffraction rings but the star is a bit jumpy thats a bit better. Maybe ok for imaging up to 600mm with my small pixelled camera. If I get nice diffraction rings and a sharp little star I can collimate and go planetary imaging.

Transparency - for me I have a nice haze free sky when I can see 5 stars in the square of pegasus. Now that's moving away I will have to find another bit of sky with mag 5 stars to make a judgement on.

If the sky looks a bit orange I have serious light pollution - time for narrow band filters. Also I measure the sky glow when taking preliminary subs. I aim for the background light intensity to be less than 3000ADUs on a 16 bit image.

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