DeltaOptical Red Filter #25 2″
Color filters are an essential part of every serious planetary observer's toolkit, and the DeltaOptical Red Filter #25 is a great example of that. Designed for standard 2″ focusers, it fits the vast majority of today's telescopes without any adapters. If you regularly observe Mars, Jupiter or other Solar System bodies, this filter will reveal significantly more detail than you are used to seeing without filtration.
How it works
The filter works on the principle of selective light transmission - the red filter #25 strongly suppresses the blue and green components of light and reduces the yellow component to some extent as well. The result is a substantial increase in the contrast of structures that are naturally reddish-brown or dark against a bright background. For technically oriented observers: this is a narrowband color filter with dominant transmission in the red part of the spectrum roughly above 580-600 nm, effectively suppressing blue and green detail.
Planetary observation with the red filter #25
Mars
On Mars, filter #25 significantly improves the visibility of polar caps - the white icy regions stand out more sharply against the dark reddish surface, and their boundaries become noticeably better defined. The filter also helps distinguish surface albedo features from atmospheric haze.
Jupiter
On Jupiter the situation is somewhat different: the filter enhances bluish structures in the atmosphere, such as darker belts and storm formations, because the blue component is strongly suppressed by this filter - which paradoxically increases the contrast of these structures against their surroundings.
Daytime planetary observation
An interesting and less common application is daytime planetary observation. When observing Venus or Jupiter in daylight, the filter significantly darkens the bright blue sky, dramatically increasing the contrast of the planet against the background and bringing the overall visual impression closer to nighttime conditions. Observing planets in daylight with a red filter is a technique you will especially appreciate when tracking Venus through its various phases.
Who is this filter for?
The DeltaOptical Red Filter #25 is best suited for planetary observers who already have some experience with a telescope and want to take their observations to the next level. It is recommended for medium and larger aperture telescopes - with a very small objective diameter, the strong light filtration can negatively affect the visibility of fine detail.
| Parameter |
Value |
| Filter size |
2″ (standard focuser) |
| Filter type |
Color - red #25 |
| Transmission range |
Dominant above 580-600 nm (red spectrum) |
| Suppressed colors |
Blue and green (partially yellow) |
| Recommended aperture |
From approx. 100-150 mm and above |
| Primary use |
Planetary observation - Mars, Jupiter, Venus |
| Adapter required |
No - fits standard 2″ focusers directly |
