Selecting a Baader Planetarium Star Diagonal

By: Brian Ventrudo and Manish Panjwani
December 11, 2017
Choosing Baader Diagonals
Figure 1 – A Baader 2" star diagonal with nosepiece and Clicklock eyepiece holder.

1. Overview

For many years, Baader Planetarium offered just a small handful of premium quality star diagonals. Over the past few years, however, they have introduced the innovative T-2 system of diagonals and accessories along with a half-dozen otherdiagonals to the product mix. There are now some 13different Baader diagonals in production, which offers amateur astronomers a wide range of choice and configurations.

Because all the light from your telescope objective is reflected by your star diagonal, the choice of a star diagonal is at least as important as the choice of eyepiece. Baader makes excellent diagonals, but with so many choices, it might be hard to figure out which diagonal is right for your situation. Should you choose adiagonal with a right-angle prism or Amici prism? Or 45-degree or 90-degree viewing angle? How about Maxbright or BBHS coatings? Or T-2, 1.25", or 2" diagonal? Baader or Zeiss prism? If you’re confused, you’re not alone. We were too. That’s why we sat down to sort out all the options and developed thisguide that we hope will help you navigate the Baader diagonal landscape and pick the diagonal for your telescope and observing interests.This guide will examine both the various physical configuration of Baader diagonals and the options for reflective mechanisms by which the diagonal does its job of directing light from the objective lens into your eyepiece.

2. Baader Diagonal Part Numbers

First, to follow the many the options for a Baader diagonal, it’s helpful to understand the part-numbering system for these products.

There are two part numbers associated with each Baader product sold in the United States.Baader itself uses a single 7-digit part numberto identify each unique product it sells. For example, the 2" Baader Clicklock Mirror star diagonal has the global part number 2956100. However, Baader’s U.S. distributor uses a differentalphanumeric part numbering system to denote Baader products. For example, the same diagonal in the U.S. has the part number CLDIAG-2.While this may seem confusing at first, compared to the long string of numbers used by Baader in Europe, the U.S. designations are easier to use and remember, and they are more descriptive of the products themselves.

Agena AstroProducts uses both part number systems on its Baader diagonal product pages. In this guide, we’ll only use the U.S. part numbers to describe Baader diagonals. In the reference table at the end of the guide, we will use both numbering systems.

Table 1 shows a summary of currently available diagonals from Baader Planetarium. All will be discussed in this article.

U.S. Part NumberGlobal Part NumberConfigurationComments
PRISM-22 2456117 2" diagonal, Clicklock eyepiece holder, Zeiss-spec right-angle prism 2" diagonal with ultra-premium prism and short optical path
PRISM-1 2456005K T-2 body with right-angle prism, 1.25" nosepiece, and 1.25" Clicklock eyepiece holder T-2 prism body with and short optical path, nosepiece, and EP holder
T2-01C 2456005 T-2 body only with right-angle prism T-2 prism diagonal body with and short optical path
T2-01B 2456095 T-2 body only with Zeiss-spec right-angle prism T-2 ultra-premium prism diagonal body with and short optical path
MAX-1 2456100 T-2 body only with Maxbright T-2 diagonal body with Maxbright dielectric coating
MAX-1S 2456103 T-2 body only with BBHS Silver-coated reflector for accurate color reproduction
MAX-2S 2456115 2" diagonal with BBHS 2" silver-coated reflector for accurate color reproduction
CLDIAG-2 2956100 2" diagonal with dielectric coating (not Maxbright) 2" diagonal for general astronomical viewing
AMICI-45 2956150 1.25" 45-degree diagonal with Amici prism Correct-image terrestrial viewing with most telescopes
AMICI-45-2 2956151 2" 45-degree diagonal with Amici prism Correct-image viewing with SCT telescopes
AMICI-DX1 2456130 T-2 body only 90-degree prism Correct-image viewing with most telescopes
AMICI-DX2 2456120 2" 90-degree diagonal with Zeiss-spec Amici prism, Clicklock Premium diagonal for correct-image viewing
AMICI-2 2956152 2" 90-degree diagonal with Amici prism Correct-image viewing with most telescopes

3. Baader Diagonals – Physical Configuration Options

Baader 1.25" and 2" Diagonals

Baader offers diagonals in three basic size formats: 1.25", 2" and their proprietary T-2 configuration. The 1.25" and 2" diagonals are meant for use with 1.25" and 2" eyepieces, respectively, in 1.25" or 2" focusers. Other than an eyepiece, no additional parts or accessories are required. If you wish to use 1.25" eyepieces in a 2" Baader diagonals, you will have to buy a separate 1.25"-to-2" adapter because oneis usually not included with the diagonal.

Baader 1.25" and 2" diagonals come with reflective surfaces made with right-angle prisms, Amici prisms, or glass mirrors made with dielectric coatings (see Section 4).

Choosing Baader Diagonals
Figure 2 – A 1.25" Baader diagonal (PRISM-1) and 2" Baader diagonal (MAX-2S)

Baader 1.25" and 2" diagonals can be used with most any telescope by sliding the diagonal into the telescope focuser. The 2" Baader diagonals can also be connected to Schmidt-Cassegrain telescopes (SCT) directly. When the nosepiece of a Baader’s 2" is removed, it exposes a female thread that allows a direct connection of the diagonal body to the back of an SCT with a 2" male thread. However, a separate L-ring is recommended to help you control the orientation of the diagonal. This article shows you the details of how to attach a 2" Baader diagonal to an SCT.

Baader T-2 Astro System Diagonals
Baader also has a proprietary T-2 system for their diagonals and accessories. A T-2 diagonal comes only with the diagonal body which housesa reflective element such as a right-angle prism, Amici prism, or dielectric mirror. A T-2 diagonal housing does not include a nosepiece to slide into a focuser or an eyepiece holder. These elements are chosen separately to match your viewing requirements. A single T-2 diagonal housing can even be manually reconfigured for several applications.

The name of the BaaderT-2 system arises from the threads used on the diagonal bodies and accessories. A T-2 (sometimes called T2) thread is the same as a standard “T thread", that is, a M42x.75 thread. At Agena, for clarity, our product names and descriptions usually list both T and T-2 designations. You simply need to keep in mind that T and T-2 essentially mean the same thing.

A Baader T-2 diagonal housing has standard female T/T-2 threads at the input face (on the telescope side) and male T threads at the output face (on the eyepiece side). This lets you add Baader T-2 adapters and accessories to configure the diagonal to your needs.One of the key features of all Baader T-2 diagonals, whether they use prisms or mirrors, is the large free aperture and the flexibility to directly adapt to almost any telescope system without the restriction of a 1.25" barrel size.

Choosing Baader Diagonals
Figure 3 – A T-2 diagonal body (left) and an exploded view of a T-2 body, spacer rings, nosepiece, and Clicklock eyepiece holder

The most typical configuration of a T-2 diagonal involves adding a nosepiece and eyepiece holder.You can add a 1.25" nosepiece and 1.25" eyepiece holder. Or, for example, you can add a 2" nosepiece to a T-2 diagonal housing to fit into a 2" focuser and add a 1.25" eyepiece holder. This prevents vignetting when using long focal length 1.25" eyepieces or when using the diagonal with a binoviewer, for example. That’s because the large clear aperture of the T-2 diagonal housing far exceeds the field stop of any 1.25" eyepiece.

Another important benefit of T-2 diagonals, is that they allow users to configure shorter optical path lengths. This can be critical in imaging applications, especially when used with additional imaging accessories like reducers or filter wheels, or for use with binoviewers where you want to minimize the light path due to focus-travel limitations. They also are ideal for use with fork-mounted Schmidt-Cassegrain telescopes where a conventional 2" diagonal may not otherwise have the required clearance with the scope pointed at or near the zenith. Connecting a T-2 diagonal directly to an SCT requires a separate SCT adapter. These adapters come in two lengths, and orientation control is built in to the adapter.

Baader T-2 diagonals do not, however, have enough clear aperture to work with all 2" eyepieces. While we do not recommend using 2" eyepieces with T-2 housings, it is possible to configure a T-2 diagonal to accommodate 2" eyepieces. This post at shows how to do it.

The input and output threads of a T-2 diagonal also integrate with the full range of Baader Astro T-2 System components and other T-threaded adapters. That includes a Baader's quick change ring to mount the Mark V binoviewer directly to the diagonal to achieve the shortest possible back focus distance between telescope and binoviewer. With the right accessories, a T-2 diagonal body also integrates well into digital imaging applications.

4. Baader Diagonals – Reflective Element Options

Despite the many specifications and physical configurations of star diagonals, these devices have a simple job. They redirect light from the objective of a telescope (usually a refractor or compound telescope) into an eyepiece at an angle that is more convenient for visual observation. The redirection of light is accomplished by one of three types of reflective elements:

  • A glass prism that reflects light according to the principle of total internal reflection
  • A mirror made of a glass surface coated with several layers of hard dielectric coating
  • A mirror made of a glass surface coated with a thin layer of silver or aluminum

Baader makes diagonals base on each of these three configurations, and each configuration has its own pros and cons which depend in part on the quality of coatings and materials.

4.1. Right-Angle Prism Diagonals

In prism-based star diagonal, light passes through a glass prism and reflects off the hypotenuse of the prism at a right angle. Each face of the glass prism is anti-reflection coated to maximum light transmission, but the reflection occurs at the interface between glass and air. No reflective coating is required. These diagonals result in images that are erect by left-right reversed.

Choosing Baader Diagonals
Figure 4 – The optical path through a right-angle prism

Prism-based diagonals have many advantages, including:

  • Virtually no light scatter at the reflective surface, which leads to an image with very high-contrast
  • Wavelength-independent reflectivity for good color rendition of planets and stars
  • No change in performance over time since the reflective surface does not degrade
  • No need for collimation, assuming the prism is ground and mounted at the correct angles upon manufacture

Another big advantage of prism-based diagonals is their inherently short path length. Shorter prism-based diagonals are a big advantage when used with telescopes where space is a premium such as fork-mounted Schmidt-Cassegrain scopes. They are also useful when used with binoviewers.

Prism-based diagonals have some disadvantages. While they work very well with telescopes with a focal ratio of f/7 or longer, they do cause some chromatic aberration in telescopes with faster focal ratios, especially at high magnification.

Also, if dirt or dew accumulates on the reflective surface of the prism, the performance of the diagonal can be degraded. To counteract this effect, prism diagonals can be constructed with a reflective coating of silver or dielectric material on the hypotenuse to ensure a high degree of reflection even when dirt or dew is present.

Baader’s prism-based diagonals are renowned for their precise manufacturing, mounting, and coatings, and they have been independently reviewed as excellent performers, especially in applications such as planetary observation where minimal scatter is a big advantage. They are constructed with a premium-qualityprism or with anultra-premium ‘Zeiss-spec’ prism made under license for Baader by the same production facility that made such prism for Zeiss and to the same exacting Zeiss specifications.

Baader right-angle prism-based diagonals currently come in the following models (listed by part number):

  • PRISM-2Z is a full 2" diagonal with Zeiss-spec prism
  • PRISM-1 is a T-2 prism diagonal body that comes with a removable 1.25" nosepiece and 1.25" eyepiece holder with helical focuser
  • T2-01C is a T-2 prism diagonal body only. It’s the same diagonal body used in the BP-PRISM-1
  • T2-01B is a T-2 Zeiss-spec prism diagonal body only

4.2. Amici Prism Diagonals

Baader also makes a wide range of diagonals with Amici prisms, a prism long favored as the best choice for producing correctly-oriented images in refractors and Schmidt-Cassegrain telescopes. If you need an image that’s right-side up and left-to-right correct, a diagonal with an Amici prism is the right choice. These diagonals are ideal for terrestrial viewing or for observers who wish to get a correct image view of astronomical objects to match, for example, lunar maps or finder charts.

Choosing Baader Diagonals
Figure 5 – The optical path through an Amici prism

Amici prisms are hard to make, however, and they require a much higher level of quality control and manufacturing precision than mirror or prism diagonals. The faces of an Amici prism must be polished smooth and flat at the proper angles to produce symmetrical images without astigmatism. The sharp roof edge of the prism must also be made vanishingly fine to prevent a single prominent diffraction spike in images. Not all Amici prism are created equal, and Baader’s Amici-prism diagonals are among the best on the market.

Also, while Amici prismdiagonals rely on total internal reflection within the prism to direct light to the eyepiece, this can result in degraded performance if dew or dust gets on the surface of the diagonal hypotenuse. For their highest quality Amici prism diagonals, Baader employs a BBHS (Broad-Band Hard Silver) coating on the hypotenuse to enhance performance.

Baader Amici prism-based diagonals currently come in the following models (listed by part number):

  • AMICI-45 is an affordable 45-degree diagonal for terrestrial viewing
  • AMICI-DX1 is a T-2 90-degree diagonal housing
  • AMICI-45-2 is a 45-degree diagonal that threads onto the back of Schmidt-Cassegrain telescopes
  • AMICI-2 is a 2" 90-degree diagonal with 1.25" eyepiece adapter; it works with most telescopes with 2" focusers
  • AMICI-DX-2 is a 2" 90-degree diagonal with Zeiss-spec prism, BBHS reflective coatings, and Clicklock adapter; this premium Baader diagonal works with most telescopes with 2" focusers

4.3. Dielectric and BBHS Diagonals

Baader also produces diagonals with mirrors formed from ceramic Sitall glass substrates with two types of reflective coatings, either Maxbright dielectric or BBHS (silver). Both types of diagonal are excellent for use with nearly any telescope on essentially any type of celestial object. They do not suffer from chromatic aberration that may be apparent in prism-based diagonals with telescopes of f/7 or faster. And they are engineered to minimize scattered light from their reflective surfaces.Like right-angle-prism diagonal, these diagonals result in images that are erect by left-right reversed.

Baader’s Maxbright diagonals are made with multiple layers of hard dielectric material deposited on a glass substrate that result in reflectivity as high as 99%. Dielectric coatings are very hard, so these diagonals are more robust to the environment and are much easier to clean without damaging the reflective surface.

Baader’s BBHS (Broad Band Hard-Coated Silver) diagonals have a thin layer of silver deposited on a ceramic glass substrate. Silver coatings give very high and uniform reflectivity across the visual spectrum, which is ideal for achieving vivid and accurate colors and saturation when observing the planets or colorful stars. However, silver by itself is a soft material and easily scratched and degraded by the environment. Baaders BBHS diagonals have an additional hard protective layer deposited the silver to reduce or eliminate this degradation and to reduce scatter at the surface. The reflectivity of Baader BBHS diagonals is not specified.

This review of the MAX-2S BBHS diagonal by Bill Paolini at discusses the effectiveness of this diagonal for rendering vivid and accurate colors when observing planets and stars.

Baader’s Maxbright and BBHS diagonals include:

  • MAX-1 is a T-2 housing with Maxbright dieletric on ceramic glass
  • MAX-1S is a T-2 housing with BBHS coating on ceramic glass
  • MAX-2S is a 2" diagonal with BBHS coating on ceramic glass Baader also currently offers the CLDIAG-2, an affordable2" dielectric-coated diagonal that is not branded as a Maxbright.

5. Additional Reading

This guide has reviewed and explained the many options available for star diagonals from Baader Planetarium. The information presented here will help you choose the right diagonal for your telescope and observing interests. The links below offer more information on this product line: