Filter Calculator
Estimate on-axis narrowband transmission through a converging beam, compare two Antlia filter designs, and check basic 36 mm sizing geometry.
Optical Parameters
Choose a preset or enter your own optical parameters. Accessories are paired with the telescope they are listed beside.
Preset specifications: Apertura CarbonStar + PRCC · APM 1.5× Barlow · EdgeHD 9.25 · EdgeHD 0.7× reducer · Samyang 135 mm f/2.
Bandpass Characteristics
Antlia 36 mm Hα: Standard vs Highspeed
3 nm Pro Hα — standard
36 mm unmounted · 2 mm thick · OD5 blocking · CWL 656.3 nm · FWHM 3 nm · Hα transmission >88% in Antlia’s specification.
Modeled Hα retention of its own peak: —
Spec-scaled illustration: —
Sensitivity sweep: —
3 nm Pro Highspeed Hα
36 mm unmounted · 2 mm thick · OD5 blocking · CWL 657.5 nm · FWHM 3 nm · listed peak 90%; +1.2 nm red pre-shift.
Modeled Hα retention of its own peak: —
Spec-scaled illustration: —
Sensitivity sweep: —
Select an optical setup or enter aperture and focal length to compare modeled Hα line retention.
The headline comparison normalizes each filter to its own peak, avoiding a misleading direct comparison of the standard filter’s >88% specification with the Highspeed’s listed 90% peak. Baseline assumes a shared neff=1.8 and 1 nm flat-top; the sensitivity sweep tries neff=1.6, 1.8, 2.2 and flat-tops 0.5, 1, 2 nm. These are scenarios, not measured limits. Antlia recommends Highspeed at f/2.6–f/3.6 and standard for slower optics, but its standard listing also says use to f/3 with slight loss. Both are 36 mm outside diameter; neither product provides a full numerical angle-dependent curve here.
SHO filter and Samyang f-stop comparison
Calculated on-axis with the same annular-pupil model as above. The percentages are modeled line transmission relative to each filter’s own peak, not measured absolute throughput. For SII, the comparison assumes a 50:50 mix of the 671.6 and 673.1 nm lines.
CarbonStar 150 mm: native f/4 and 0.95× reducer f/3.8
| Setup | Line | Standard transmission | Highspeed transmission | Better in model |
|---|
Samyang 135 mm: all 14 f-stops × both filter versions × three lines
| Line | f-stop | Standard transmission | Highspeed transmission | Better in model |
|---|
View all 84 Samyang combinations
| Line | f-stop | Standard transmission | Highspeed transmission | Better in model |
|---|
All 84 Samyang combinations are included, even outside Antlia’s stated use ranges. The chosen f-stop above still maximizes a modeled line-photon proxy proportional to transmission ÷ f-number²; that calculation is kept out of the comparison columns. Antlia lists Highspeed for f/2.6–f/3.6 and recommends standard beyond f/3.6; its standard product pages also say use to f/3 with slight loss.
Inputs: Antlia’s published 3 nm CWLs/FWHM and stated transmission levels for standard Hα, standard OIII, standard SII, and Highspeed SHO. The coating index, actual angle-dependent curves, lens exit pupil, SII line ratio, and exact peak transmission of each filter are not published as comparable numeric data here. The ranking is therefore a conditional model result, not a verified purchasing verdict or guarantee of image quality. Camera QE, target spectrum, sky background, seeing, and full-field vignetting can change the practical choice.
36 mm filter clearance check
Enter sensor diagonal and filter distance to estimate the minimum clear opening for an image-space cone.
ZWO sensor diagonals: ASI533MM Pro and ASI2600 manual. The standard and Highspeed Antlia product pages give 36 mm nominal diameter but no numeric clear-aperture specification; this field is intentionally not preset.
Approximation: required clear diameter ≈ sensor diagonal + distance ÷ f-ratio, assuming a centered filter and perpendicular chief rays. Off-axis chief-ray displacement can require a larger opening. A 36 mm unmounted filter may not fit a given wheel or lens adapter; Samyang’s front filter thread is 77 mm, not 36 mm.
Bandpass Curve
How the calculation works
The model approximates the image-space cone as an annulus. For each pupil radius r, ray angle θ ≈ atan(r / focalLength). A thin-film interference passband blue-shifts approximately as λ′ = λ₀ √(1 − sin²θ / n_eff²). The line transmission is weighted by annular area 2πr dr and integrated with Simpson’s rule. Central obstruction removes the inner annulus. For a complex camera lens or reducer, the true exit pupil can make this angle distribution differ from the simple f-ratio model.
Without a measured curve, the passband is an assumed flat-top with Gaussian shoulders: this construction gives the entered FWHM, but is not Antlia’s measured curve. A measured normal-incidence curve is evaluated at the wavelength corresponding to each ray’s shifted passband. The result is one on-axis line-throughput estimate; it does not include full-field throughput, quantum efficiency, or filter-holder vignetting. Manufacturer f-ratio guidance is context, not a substitute for measured transmission through your setup.