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Optolong Clear Focusing Filter
Optolong Clear Focusing Filter
Optolong Clear Focusing Filter
Optolong Clear Focusing Filter
Optolong Clear Focusing Filter
Optolong Clear Focusing Filter
Optolong Clear Focusing Filter

Optolong Clear Focusing Filter

by Optolong
SKU OPT-12401
CA$34.00 - CA$42.00
CA$34.00
CA$34.00 - CA$42.00
CA$34.00
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  • Achieve Critical Focus with Dim Stars
  • Transmission >98.5% Across the Visible Spectrum
  • λ/4 Transmitted Wavefront for Image Integrity
  • Parfocal with Other 1.85mm Optolong Filters
  • Functions as a High-Quality Sensor Dust Shield
1 in stock
Qualifies for FREE Shipping in Canada on orders over $500.
  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • Optolong Clear Focusing Filter

    The Optolong Clear Focusing Filter is a dedicated utility filter designed to solve one of astrophotography's most persistent challenges: achieving perfect focus with dim stars. By passing over 98.5% of visible light, this filter ensures your focusing star is as bright as possible, a critical advantage when narrowband or light pollution filters would otherwise render it nearly invisible. Constructed from B270 glass with a transmitted wavefront of λ/4 and a thickness of 1.85mm, it maintains the optical path's integrity, allowing you to focus with confidence before swapping to your imaging filter.

    Critical Safety Warning: Not for Solar Use

    This filter is NOT designed for solar observation. Looking at the Sun with this filter, or any filter not explicitly certified for safe solar viewing, will cause immediate and irreversible eye damage, including permanent blindness. Do not point your telescope at the Sun with this filter attached.

    Achieve Critical Focus with >98.5% Transmission

    Narrowband and aggressive light pollution filters drastically dim incoming starlight, making it difficult for autofocus routines to lock on or for you to judge manual focus accurately. The Optolong Clear Filter solves this by passing more than 98.5% of light across the entire 420-680nm visible spectrum. You simply use this filter to achieve a sharp, bright focus on a star, then swap it for your narrowband or LRGB filter to begin imaging, knowing your focus is already set.

    Parfocal and Protective with λ/4 Wavefront Flatness

    A focusing aid is useless if it degrades your image or shifts the focal point. The Optolong Clear Filter is made from B270 substrate polished to a λ/4 transmitted wavefront with 30s parallelism, ensuring it won't introduce aberrations. Its 1.85mm thickness makes it parfocal with other Optolong filters of the same thickness, minimizing the need to refocus after a filter change. It also serves as an excellent dust shield, protecting your camera sensor or reducer optics from dust during long imaging sessions.

  • Why do I need the Optolong Clear Focusing Filter? Can't I just focus without a filter?

    While you can focus without a filter, adding a narrowband or LRGB filter into the optical path will slightly shift the focal point. The Optolong Clear Focusing Filter has a thickness of 1.85mm, designed to be parfocal with other Optolong filters. By focusing with this filter first, you ensure the focal point is correctly set for when you swap to your imaging filter, saving time and guaranteeing sharp results.

    Is the Optolong Clear Filter the same as a Luminance (L) filter?

    No. A Luminance filter is an imaging filter with specific UV and IR blocking characteristics. The Optolong Clear Filter is a utility filter designed purely for focusing. It passes a wider band of light (>98.5% from 420-680nm) to maximize star brightness for focusing software and visual confirmation. It is not intended for capturing final image data.

    How does the Optolong Clear Filter help when imaging the Horsehead Nebula (B33) with a narrowband filter?

    Imaging Barnard 33 requires a strong Hydrogen-alpha filter, which blocks almost all starlight except for a single wavelength. This makes focusing stars extremely dim and difficult. With the Optolong Clear Filter, you can select a nearby bright star like Alnitak, achieve perfect focus using the full visible spectrum, lock your focuser, and then switch to your Ha filter to capture the faint nebulosity without struggling to see the dim, red-only star.

    Will the Optolong Clear Filter improve focusing with my modified DSLR and f/5 Newtonian?

    Yes. A modified DSLR is more sensitive to a broader spectrum of light, and this filter's >98.5% transmission takes full advantage of that. In an f/5 Newtonian, stars will be bright and sharp through the Clear Filter, allowing your camera's live view or focusing software to achieve a much more accurate focus than with a light pollution or narrowband filter installed.

    Can I leave the Optolong Clear Filter in the optical train as a dust shield?

    Absolutely. With its high-quality B270 substrate and λ/4 wavefront, it will not degrade your image quality. It acts as an effective barrier, preventing dust from settling on your camera sensor or reducer optics during a long imaging night.

    Can I use the Optolong Clear Filter for solar viewing?

    NO. ABSOLUTELY NOT. This filter provides no protection from the Sun's harmful radiation. Using it for solar observation will cause instant and permanent blindness. Only use filters specifically designed and certified for safe solar observation.

  • Substrate B270
    Thickness 1.85mm
    Average Transmission >98.5% (420-680nm)
    Surface Quality 60/40
    Transmitted Wavefront RMS λ/4
    Parallelism 30s
    Available Sizes 1.25" Mounted, 2" Mounted

    • Optolong Clear Focusing Filter (Selected Size)

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  • Optolong 3-Year Warranty