Telescope Reviews: Re: binos and filters?

Calan,
I've taped a pair of UHC filters in front of my 50mm binos, and faint nebulae really jump out!

In fact, if the glass substrate used for the filters is reasonably good, in front of the objectives is the best place for narrow band filters, as opposed to the behind the eyepiece. This is because for even a wide-angle bino, the maximum off-axis angle for objects at the edge of the field will be no more than about 4 degrees (for, e.g., an 8 degree fov). This is quite close to perpendicular, and therefore the filters will work perfectly well across the entire field.

Compare this with behind-the-eyepiece filters, where the light rays exiting the eyepiece, which for even a small apparent fov is at least 25 degrees off-axis. In this instance, not insignificant "de-tuning" occurs for off-axis imagery, and hence proper performance is obtained in perhaps no more than the central half of the field.

The reason why I mentioned the qualifier regarding filter substrate quality is because a friend's Lumicon UHC filter was quite bad. In front of a 10X bino the image was astoundingly astigmatic, and even when used in the normal position in front of an eyepiece (where the tolerances are rather more relaxed) the image was just not quite right.

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I've also tried a mix of UHC and O-III filters. I can only say that I really don't like it! The brightness difference in the sky between the two is too uncomfortably large, and it feels like I'm going blind in the O-III eye! And no, there are no gains in nebula visibility, either... it's best to use the same-type filters for both eyes, methinks.

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2" telescope eyepieces require a 48mm diameter thread.

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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV

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