DeltaOptical dark blue filter #38A 1.25″
If you want to get the most out of your planetary observing sessions, a well-chosen color filter is one of the simplest and most effective tools available to you. The DeltaOptical dark blue filter #38A in 1.25″ size is a classic astronomical filter that generations of observers have come to appreciate for its ability to significantly enhance the contrast of surface details on the planets of our solar system.
How the #38A filter works
Filter #38A transmits primarily the blue portion of the spectrum while suppressing red and green components of light. The result is increased contrast between different regions of a planetary surface or atmosphere - particularly where bright and dark structures of varying chemical composition meet.
- Jupiter - the filter helps distinguish cloud belts and intensifies the visibility of the Great Red Spot against surrounding structures.
- Mars - at opposition, it makes it easier to follow polar caps and dark albedo features.
- Saturn - the filter helps reveal finer structures within the atmospheric belts.
Construction and compatibility
The filter is manufactured as a colored glass optical element set in a precision-machined metal cell with a standard 1.25″ thread, which fits the vast majority of eyepieces and diagonals of this size. Installation is immediate - the filter simply screws into the bottom of the eyepiece. The glass element is anti-reflection coated on both sides to minimize unwanted reflections and light loss.
Who is the #38A filter suited for?
Filter #38A is particularly appreciated by observers who focus on planets and want to extract as much detail as possible from their telescope with an objective diameter of approximately 100 mm or more. The filter can also be used with smaller instruments, but the visual benefits grow with aperture - a darker filter like the #38A reduces the overall brightness of the image, which is advantageous with larger telescopes where there is plenty of light. A planetary filter for a 1.25″ telescope is an excellent choice as the first color filter in any active observer's collection.
Combining with other filters
The filter can be used to great advantage in combination with other color filters depending on the object being observed. The DeltaOptical color filter series covers the full range from light yellow to dark blue and red tones, making it easy to assemble a complete set tailored precisely to your observing priorities.
About DeltaOptical filters
DeltaOptical filters have built a solid reputation among astronomers over the years thanks to thorough quality control of the optical glass, precision mechanical finishing of the cell, and reliable anti-reflection coatings. The brand offers a wide range of products for both visual and photographic astronomy - from eyepieces and filters to telescopes and mounts - and all products are designed with the practical needs of real observers in mind. Filter #38A is proof of this: a simple, reliable and immediately usable tool that will make a visible difference at every planetary observing session.
Key specifications
| Parameter |
Value |
| Filter type |
Color glass - dark blue |
| Filter designation |
#38A |
| Thread size |
1.25″ |
| Anti-reflection coating |
Both sides |
| Cell material |
Precision-machined metal |
| Recommended aperture |
From approx. 100 mm |
| Primary use |
Planetary observation - Jupiter, Mars, Saturn |
