{"product_id":"farpoint-adjustable-tri-bahtinov-mask-2-5-to-4-5-otas","title":"Farpoint Adjustable Tri-Bahtinov Mask — 2.5\" to 4.5\" OTAs","description":"\u003c!--FPSEO|v1|20260521|111111111110000010--\u003e\n\u003cp style=\"margin: 1.2em 0; padding: 0.8em 1.1em; background: #f0f4f8; border-left: 4px solid #2c3e50; border-radius: 4px; font-size: 0.95em; line-height: 1.45;\"\u003e\u003cstrong\u003eNot sure this is the right mask for your telescope?\u003c\/strong\u003e Use our \u003ca href=\"\/pages\/focus-mask-finder\"\u003e\u003cstrong\u003eFocus Mask Finder\u003c\/strong\u003e\u003c\/a\u003e — match your scope to the right Standard, Tri-Bahtinov, or Hyperstar\/RASA mask.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eAdjustable Tri-Bahtinov focus and collimation mask for OTAs from 2.5\" to 4.5\" front aperture.\u003c\/strong\u003e Sets onto the front of your telescope, gives you focus AND a collimation check from a single short exposure, and lifts off cleanly when you're done. The adjustable peg system fits any scope in this aperture range — including OTAs we don't make a custom-fit mask for. Made in our own shop in the USA — American materials, American machinery, by Americans.\u003c\/p\u003e\n\n\u003cp\u003e\u003cem\u003eFits any OTA with a front aperture diameter from 2.5\" to 4.5\" — small refractors, guidescopes, compact astrographs, and any scope in this size range we don't offer as a custom-fit.\u003c\/em\u003e\u003c\/p\u003e\n\n\u003ch2\u003eWhy a Farpoint adjustable Tri-Bahtinov\u003c\/h2\u003e\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eUniversal fit, 2.5\" to 4.5\".\u003c\/strong\u003e The adjustable peg system seats securely on any OTA in this aperture range, including brands and models we don't make a custom-fit mask for.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eMade in the USA.\u003c\/strong\u003e Designed and manufactured in our own shop with American materials, American machinery, and American workmanship.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eIndestructible ABS plastic.\u003c\/strong\u003e Won't crack, warp, or shatter like aluminum or acrylic alternatives. Built to last decades.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eTwo diagnostics, one tool.\u003c\/strong\u003e Focus AND collimation check from a single short exposure — no swapping masks, no second imaging session.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eWhat's in the box\u003c\/h2\u003e\n\u003cul\u003e\n  \u003cli\u003e1× Farpoint Adjustable Tri-Bahtinov Mask\u003c\/li\u003e\n  \u003cli\u003eAdjustable peg system for 2.5\" to 4.5\" OTA range\u003c\/li\u003e\n  \u003cli\u003eInstructions for use\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eHow a Tri-Bahtinov mask works\u003c\/h2\u003e\n\u003cp\u003eA standard Bahtinov produces three diffraction spikes — two forming an X, plus a center spike that moves across the X as you focus. Land the center spike on the X, and you're focused. One pattern, one diagnostic.\u003c\/p\u003e\n\n\u003cp\u003eThe \u003cstrong\u003eTri-Bahtinov\u003c\/strong\u003e rotates that same pattern 120° around the center, three times. The result: three sets of diffraction patterns from one short exposure — three Xs, three center spikes, three independent focus reads at three different orientations.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eFor focus:\u003c\/strong\u003e adjust until all three center spikes land centered on their Xs. When all three are symmetrical, you've hit optimum focus.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eFor collimation:\u003c\/strong\u003e a properly collimated scope reaches focus at the same focuser position across all three orientations. An uncollimated one won't — light from different parts of the aperture takes slightly different path lengths to the image plane, so different orientations reach focus at slightly different positions. If one of the three Bahtinov patterns can't be centered when the others are, that axis is out of collimation, and the offset direction tells you which screw to turn.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eBonus for SCT owners:\u003c\/strong\u003e align the mask's three Bahtinov sections with your three secondary mirror collimation screws. You get a direct visual mapping between what you see on screen and which screw needs adjusting.\u003c\/p\u003e\n\n\u003cp\u003e\u003cem\u003eThe Tri-Bahtinov design was developed by amateur astronomer CY Tan and shared with the astrophotography community on \u003ca href=\"https:\/\/github.com\/cytan299\/tribahtinov\" rel=\"noopener\" target=\"_blank\"\u003eGitHub\u003c\/a\u003e. Farpoint manufactures the masks; CY Tan invented the pattern.\u003c\/em\u003e\u003c\/p\u003e\n\n\u003cp style=\"text-align: center;\"\u003e\n  \u003cimg src=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0567\/6639\/8617\/files\/bmaskanimation.gif?v=1778193011\" alt=\"Animated GIF showing a star coming into focus through a Bahtinov mask\" width=\"256\" height=\"256\" style=\"display: block; margin: 0 auto;\"\u003e\n  \u003cbr\u003e\u003cem\u003eEach of the three Tri-Bahtinov panels produces this pattern — three focus reads at three orientations, simultaneously, from one exposure\u003c\/em\u003e\n\u003c\/p\u003e\n\n\u003ch2\u003eFree software for precise focus and collimation analysis\u003c\/h2\u003e\n\u003cp\u003eIf you want machine-precise readings instead of eyeballing the spike pattern, free open-source software analyzes Tri-Bahtinov diffraction patterns automatically:\u003c\/p\u003e\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eBahtinov-Collimator\u003c\/strong\u003e — a Windows tool that quantifies focus AND three-axis collimation error directly from your image. \u003ca href=\"https:\/\/github.com\/insertnamehere1\/Bahtinov-Collimator\" rel=\"noopener\" target=\"_blank\"\u003eAvailable on GitHub\u003c\/a\u003e.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eAPT (Astro Photography Tool)\u003c\/strong\u003e — Bahtinov mask analysis is built into APT's image-capture workflow.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp style=\"text-align: center;\"\u003e\n  \u003cimg src=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0567\/6639\/8617\/files\/Collimation.jpg?v=1737820356\" alt=\"Screenshot of Bahtinov-Collimator software analyzing a star image\" width=\"551\" height=\"408\" style=\"display: block; margin: 0 auto; max-width: 100%; height: auto;\"\u003e\n  \u003cbr\u003e\u003cem\u003eBahtinov-Collimator analyzing a star image for precise focus and collimation measurement\u003c\/em\u003e\n\u003c\/p\u003e\n\n\u003ch2\u003eJust need focus, not collimation?\u003c\/h2\u003e\n\u003cp\u003eIf your scope is already well-collimated and you only need focus assistance, our \u003cstrong\u003eAdjustable Bahtinov mask\u003c\/strong\u003e in this size range is the simpler (and less expensive) option — same adjustable peg system, same precision focus, just the focus axis without the collimation diagnostic. \u003ca href=\"\/products\/farpoint-adjustable-bahtinov-mask-2-5-to-4-5-otas\"\u003eSee the Adjustable Bahtinov for 2.5\" to 4.5\" OTAs →\u003c\/a\u003e\u003c\/p\u003e\n\n\u003ch2\u003eRelated guides\u003c\/h2\u003e\n\u003cul\u003e\n  \u003cli\u003e\n\u003ca href=\"\/pages\/focus-masks\"\u003eFocus Masks Buyer's Guide\u003c\/a\u003e — material comparison, custom-fit vs adjustable, choosing the right type\u003c\/li\u003e\n  \u003cli\u003e\n\u003ca href=\"\/pages\/telescope-collimation-guide\"\u003eTelescope Collimation Guide\u003c\/a\u003e — full walkthrough on collimating your scope\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp style=\"text-align: center;\"\u003e\n  \u003cimg src=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0567\/6639\/8617\/files\/DiffSpike.jpg?v=1737826264\" alt=\"Diffraction spike pattern at perfect focus through a Bahtinov mask\" style=\"display: block; margin: 0 auto; max-width: 100%; height: auto;\"\u003e\n  \u003cbr\u003e\u003cem\u003eThe Bahtinov diffraction principle — multiplied by three for simultaneous focus and collimation diagnostics\u003c\/em\u003e\n\u003c\/p\u003e","brand":"Farpoint","offers":[{"title":"Default Title","offer_id":40161496400025,"sku":"FP400T","price":24.0,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0567\/6639\/8617\/products\/adjustabletri-bahtinov-1.jpg?v=1647382928","url":"https:\/\/farpointastro.com\/products\/farpoint-adjustable-tri-bahtinov-mask-2-5-to-4-5-otas","provider":"FarpointAstro","version":"1.0","type":"link"}