Orion’s UltraBlock UHC is an excellent light-pollution filter for those on a budget. However, before getting to this review I would like to take the time to explain what a UHC filter actually does with your telescope and what to expect – many beginners have unrealistic expectations or try to use filters on the wrong objects and are left disappointed. However, when used correctly, theOrion UltraBlock UHC is absolutely wonderful to use and makes a world of difference when used on planetary and emission nebulae, no matter your skies.
Like all eyepiece filters, the UltraBlock screws on using the built-in threads on almost all eyepieces. If your telescope has a 2” focuser, most 2” to 1.25” adapters allow 2” filters to be screwed on so you can use the 2” UltraBlock filter with 1.25” eyepieces, and if your adapter doesn’t there are many available at low prices that are threaded for filters.
Why use a UHC Filter & How it Works
Telescope eyepiece filters work by rejecting the light from wavelengths other than the desired ones which are let through. This is accomplished with a specially tinted piece of glass for color filters, and complex multi-coatings for nebula and other emission line filters. For color filters, this is pretty simple – a blue filter only passes blue light, a purple filter passes only red and blue light, and so on. For nebula filters, the process is a little more complex. Most nebula filters only pass narrow wavelengths around the hydrogen-alpha, hydrogen-beta, and oxygen-III lines that nebulae strongly emit – these molecules are typically ionized and lit up by nearby stars, causing them to glow with these very specific colors. So the filter only passes the light at the wavelengths around those specific colors – hydrogen-beta and oxygen-III in the blue-green range and hydrogen-alpha in the deep red.
Obviously, only passing these very narrow wavelengths of light results in a dimmer image. So you’re not going to see anything brighter than the view unfiltered, and also stars and galaxies will be dimmed severely. But for planetary and emission nebulae, the restriction to these narrow wavelengths of light causes the background to turn nearly pitch-black and boosts the overall contrast, allowing you to see more.
A UHC (ultra high contrast) filter like the UltraBlock has a narrower bandpass than a broadband nebula filter (which are almost entirely useless from my experience) and thus the background is much darker and the contrast effect is more pronounced.
In practice, nebula filters do not eliminate light pollution’s effects on nebulae – especially considering the growth of LED lighting which basically voids the effect of these filters – but it is definitely helpful. Nebula filters are still useful even under a dark sky simply due to their contrast-boosting properties, as the natural sky background even under very dark skies is still less-than-black thanks to the glow of faint stars, airglow, Gegenschein, and the zodiacal light.
People seem to like the effect the UltraBlock has on the Moon and planets. I have never tried using the UltraBlock for this task, but after using other nebula filters I honestly cannot come up for any explanation as to why people use them for solar system objects, let alone find an improvement in the image. They really add nothing to the view, and the color shift is distracting, to say the least!
With the UltraBlock UHC, the Orion Nebula is significantly more pronounced than it would be otherwise. With a 6” Newtonian under my Bortle Class 5 suburban skies, the UltraBlock makes the difference between being able to see the Lagoon Nebula or not, as with the Veil Nebula. Using a larger telescope of at least 12” in aperture with the UltraBlock allows you to see the Horsehead Nebula with some difficulty – an H-beta filter is probably far better for this task.
Interestingly, holding two UltraBlocks in front of my eyes allowed me to just barely glimpse the gigantic faint nebula known as Barnard’s Loop in Orion. Normally this is a job for H-beta filters only, but the UltraBlocks do it well.
Planetary nebulae are definitely increased in contrast with the UltraBlock, but not as much as with a dedicated oxygen-III filter. However, spotting small planetary nebulae at low power is still far easier with the UltraBlock than without.
Compared to a pricier filter, the UltraBlock UHC’s principal disadvantage is the overall light transmission. To me, the difference is somewhere around a half to a third of a magnitude. However, the UltraBlock does just as well if not better at bringing out faint detail and provided equally sharp images, all at a much lower cost.