Baader 4nm F3 Narrow Band OIII Filter - CMOS Optimised
Baader 4nm F3 Narrow Band OIII Filter - CMOS OptimisedBaader 4nm F3 Narrow Band OIII Filter - CMOS OptimisedBaader 4nm F3 Narrow Band OIII Filter - CMOS OptimisedBaader 4nm F3 Narrow Band OIII Filter - CMOS Optimised

Baader 4nm F3 Narrow Band OIII Filter - CMOS Optimised

£211.00✓ 2 year warranty


Out of stock due 15-20 working days

About this product

Model:  baader_2961425
Part Number:  2961425

Introducing the latest CMOS Optimised High Speed F3 Narrow Band OIII Filter from Baader Planetarium designed for a working focal ratio of F2.3 to F3.4

The new generation of Baader CMOS optimised filters feature

  • Increased contrast
  • Ever more narrow passbands
  • Reflex-Blocker™ coatings, for largest ever freedom from halos, even under most adverse conditions concerning aux-optics
  • FWHM on each filter category carefully designed to allow for 1:1:1 exposures, matched for typical CMOS quantum efficiency and s/n ratio
  • Identical filter thickness to existing standards, with utmost care for parfocality
  • Blackened edges all around, with filter-lead-side-indicator in the form of a black frontside outer rim, to additionally eliminate any reflection due to light falling onto the edge of a filter
  • Each filter coated individually, with sealed coating edge (NOT cut out of a larger plate with coatings left exposed)
  • Life-Coat™: evermore hard coatings to enable a non-ageing coating for life – even in a most adverse environment
  • 3.5 / 4 nm FWHM (to harmonise the exposure time from H-alpha with O-III / S-II), recommended for optical systems from f/2.3 to f/3.4
  • Optimised for modern CMOS cameras, but likewise excellent for CCD camera technologies
  • For highest contrast even at strongest light pollution, minimises star size, emphasises finest nebula detail – indispensable under Bortle 9/8/7/(6) skies


Available in all the common filter sizes the new 3.5nm Ultra Narrow Band Ha Filter features Baaders Life-Coat: Limited-Lifetime-Warranty

A Baader Planetarium exclusive. Filters incorporating Baader Planetarium’s Life-Coat™ technology are so durable that Baader Planetarium warrants the coatings for the life of the filter. Baader Planetarium has developed Life-Coat™ using materials and processes that result in coatings which will not degrade or fail for a lifetime. Life-Coat™ coatings resist moisture, temperature extremes, and are hardened to withstand normal cleanings.

Baader Planetarium guarantees their Life-Coat™ coatings will not peel, flake or physically degrade and will withstand repeated cleaning with fine optical cleaning equipment. The Limited Lifetime Warranty applies to the original purchaser and does not cover failure from mis-use, physical damage through improper handling, or improper cleaning.

To make a warranty claim, simply contact your retailer where you purchased the filter with proof of purchase (commercial invoice) and a detailed failure description. The dealer will then advise of the next steps.

Preshift and further Information Concerning Baader CMOS Optimised Narrow Band Filters

During the last couple of months after introducing their new CMOS-optimised filter families, Baader have received lots of positive feedback from customers.

However, there was an increasing number of reports and discussions concerning low signal, halo strength and so on, which increasingly showed  that the complexity of this topic needs further detailed explanation.
In particular when going to the physical limits, as is the case with Baaders Ultra-Narrowband (UNB) filters, there is no standard solution for every user with different telescopes, sky and weather conditions

This situation was intensified because the f/2 Ultra-Highspeed filter category especially needs a more selective categorisation, in order to select filters with the correct PRESHIFT, to comply with the f/ratio and the amount of obstruction of the respective telescope optics.

Until some weeks ago most customers received filters which worked very well for them, while some others received the same filters –that worked poorly on their scope. It took Baader some time to prepare the necessary insight and knowledge base on how to select filters matched for the many different f/ratios, as well as the different levels of secondary obstruction specifically in the case of Catadioptrics and Newtonians.

Pre Shift - What Is It?

When working with f/2 (to a lesser degree also with f/3 and longer focal lengths), the incoming angles of light will cross any filter with an inclination from 0° to a maximum of 14° with f/2 telescope optics. This requires the bandpass of High speed-dedicated Narrowband filters to be red-shifted by 1 nm to 2,5 nm, depending on the optical parameters of the telescope. Most filter suppliers simply do not preshift their filters peak wavelength at all and customers always receive a filter that is produced "straight on band". This will work well for most telescopes as long as the filter FWHM stays wide and the f/ratio stays well above f/3.4 and only if the telescope optics have zero central-obstruction. But downwards of < f/4 and in conjunction with increasing obstruction of faster optical telescopes, nothing is "on band" anymore, since the transmitted wavefront experiences an ever increasing "blue-shift" – as mentioned, depending on the f/ratio and the amount of obstruction. Without compensating this inevitable blue-shift by a suitable red-preshift, a 95% peak-transmission filter may mutate into a 30% transmission filter. For this reason Baader Planetarium always differentiated between regular narrowbands without preshift and highspeed filters featuring a preshift, in order to provide maximum filter transmission for the wide range of telescope f/ratios.

So far filter manufacturers have not offered any information on the necessity of preshifting narrowband filters at all (very few do offer preshifted filters "on special demand" – at high cost). From a manufacturers perspective this might be somewhat understandable as it requires stocking a multitude of different filter sizes and different filter families for different telescope optics, each filter category redshifted differently by a closely defined degree. And in order to make things even more difficult, even temperature shift needs to be considered to a minor degree. For example, under actual imaging conditions on a winter night at - 20°C, the same filter will have a slightly different shifted transmission window than when used in the lab at +20°C. This will remain insignificant for long focal length optics but becomes increasingly relevant with faster optics. 

The Problem Identified

Usually for O III (and S II) a signal strength similar to H-alpha is expected/hoped for, but this is limited by physics. Equalising the signal strength of narrowband O III / S II data to those gained at H-alpha always requires longer or more exposures than in H-alpha, owing to much less energy supplied by O III and S II nebula emission lines. Hard stretching of insufficient depth of data will certainly bring out halos. It is the main reason why there is seldom any halo complaint for H-alpha-filters, compared to O III and S II filters. This fact led Baader to not listen closely enough when feedback occurred concerning insufficient signal – mostly concerning the Highspeed-Version of Ultra-Narrowband filters used at fast optics.

Baader were convinced they were already providing a good service to the astro community by offering preshifted filters at all, in the form of their f/2 highspeed filter category, while keeping prices for such small and dedicated production runs well within affordable limits. For many years Baader offered f/2 high speed-filters that work very well, while having slightly wider passbands than the current 6.5 nm f/2 Highspeed filters. Both their  current 6.5 nm CMOS-optimised high speed filters as well as the former CCD-high speed filters have been produced with a preshift to serve a telescope f-range from f/3.4 down to f/1.8. This works well for filter FWHM-passbands as narrow as 6.5 nm but it was not decently explained until now about how to select between regular narrowband versus high speed-narrowband filters – which has caused difficulties for users to differentiate between the two, with some users ending up with a filter providing less signal than expected. 

But what works well with only two filter categories and a FWHM of 6.5 nm (Narrowband and f/2 Highspeed)  does not work as well with FWHM 3.5 nm and 4 nm filters. At such narrower filter passbands the two available filter families of Ultra-Narrowband and f/2 Ultra-Highspeed did not make it possible to match filters perfectly to some users telescope(s) in the f/range of f/3.4 down to f/2.3. So in the event your telescope optics work between F2.3 and F3.4 the chances are that you were supplied with the wrong 3.5/4 nm filter preshift and rightfully report about low filter transmission. For all other f/ratios the decision between Ultra-Narrowband or f/2 Ultra-Highspeed does work well and here we got only very limited feedback on signal loss.

It took Baader some time to find that the cause of reports concerning low signal and halos originates almost entirely from customers who have purchased an f/2 declared Ultra-Highspeed filter – but are using it in the focal range of f/2.3 to f/3.4. Or customers having bought a 3.5/4 nm Ultra-Narrowband filter but using it with optics below f/3.4.

This situation had caused Baader to suspend filter deliveries in recent weeks and use the time to redo their filter categorisation on the 3.5/4 nm Ultra-Narrowband / Ultra-Highspeed Filters into three distinctive categories

  • Changed working range:
    3.5/4nm f/2 Ultra-Highspeed, with preshift exclusively for telescope optics faster than f/2.3  (formerly declared from f/3.4 to f/1.8)
  • NEW:
    3.5/4nm f/3 Ultra-Highspeed, with preshift exclusively for telescope optics ranging from f/2.3 to f/3.4
  • Unchanged:
    3.5/4nm Ultra-Narrowband, without preshift – for telescope optics slower than f/3.4


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