Quote:
Originally Posted by ausastronomer
One of the best tools / tips I have found for observing doubles, particularly those with a bright primary and a dim secondary, is an occulting bar eyepiece. While you can't buy these, they are easily made. Sirius is now a fairly easy split at the moment with a separation of about 11". I was able to split it several times through 1998 to 2000 in an 8" scope with an occulting bar eyepiece, when the separation was ~5". At that time it was an exceedingly difficult split in any amateur telescope and I tried hundreds of times to split it in telescopes up to 20" aperture, without an occulting bar eyepiece and was always unsuccessful.
An occulting bar eyepiece also helps enormously to observe planetary satellites like Phobos and Deimos.
It works by blocking the unwanted light / glare from the primary target which makes it much easier to see the dim secondary target.
You can make one from any simple eyepiece which has the focal plane below the field lens. Orthoscopics are really good as they have the focal plane in the right place and are well suited to this type of observing. If you use the wrong eyepiece the edge of the bar will be fuzzy and out of focus. I have a 6mm UO ortho permanently set up. You just unscrew the barrel and glue in a piece of aluminium foil (cut straight with scissors) or a sliver of timber cut with a Stanley knife just below the field lens. Some people don't use glue through fear of getting it on the lens. I am careful and dab the glue on with a sewing needle. I then put the stop in with tweezers using a loupe. I then take comfort that the stop won't move.
Whilst I only have one eyepiece set up, I sometimes use it in combination with 1.8x and 2.5x barlows.
Cheers
John B
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Thanks for you timely input and tool tip John.
Was thinking of doing a Section on Masks and Bars ..so this is a very welcome addition, which I will include with reference to you.
Another good reason to have an Ortho handy!
With thanks bigjoe.