OPFA - Eyepiece Projection adapter (I - VII)

An OPFA eyepiece projection adapter has been the standard accessory for decades when photographing planets with traditional digital cameras. The established applications are described in full detail in the product description below.

However, the OPFA and an eyepiece can be used not only to project images onto a camera sensor. The OPFA eyepiece projection adapter can also be used in combination with the Baader Solar Projection Screen (BSPS) (#2420400 , € 39,- ) . Together, these components form a closed solar projection system behind any refracting telescope, allowing safe visual observation of the Sun in small groups. Using an eyepiece that matches the telescope’s focal length and is suitable for solar projection, the Sun is projected onto the dark screen of the projection box at the optimal size.

For a comfortable viewing position, we recommend to use the telescope with a star diagonal or prism, so you don’t have to kneel directly behind the telescope. Please note that when observing straight through, the focusing range becomes shorter, which may prevent the solar image from being sharply focused on the projection screen. With a star diagonal, the correct distance can usually be achieved easily, allowing several observers to view the Sun simultaneously in white light — comfortably and with all visible surface details. If standard star diagonals are too long, the OPFA can also be directly attached via its T-2 thread to a Baader T-2 mirror or prism to make focusing easier.

 Solar Projection with OPFA and BSPS

Eyepiece projection has long been an affordable method of achieving high magnifications for lunar and planetary photography without the need to purchase an additional Barlow lens or telecentric system. Especially in combination with orthoscopic eyepieces, which are ideal for planetary observation thanks to their high on-axis sharpness, the required magnification can be easily obtained.

With the OPFA (Ocular Projection Focal Adapter), you can also use eyepieces for eyepiece projection that, unlike for example the Hyperion and Morpheus eyepieces, do not feature a camera-side connection thread. The OPFA system can be used with 1¼" eyepieces up to a maximum outer diameter of 38 mm.

The system always consists of the following core components (OPFA-4):

• 2x T-2 Extension Tube – 40 mm (#1508153 , € 27,- ) and
• 1x T-2 Sliding focuser for eyepiece projection (#2458010 , € 41,- ) with a locking ring to align the camera if necessary.

Depending on the model, various telescope-side connections are also included — from standard nosepieces to threaded adapters for an especially secure and twist-proof fit. In addition, you will need a suitable eyepiece and a camera-specific T-ring to attach a DSLR or mirrorless camera. Astronomical cameras with a T-2 thread can be mounted directly onto the OPFA; for planetary cameras, we offer a range of eyepiece clamps with T-2 thread.

The distance — and thus the magnification — is adjusted via T-2 extension tubes and can be fine-tuned using the short, sturdy sliding focuser with a guide slot. Unlike many simpler variable eyepiece projection adapters, there is no risk of tilting, while maintaining full flexibility. By using additional or shorter T-2 extension tubes, virtually any — even extreme — projection scale can be achieved!

All OPFA variants – The vasic components of the OPFA-4 are included in every version

In classic eyepiece projection, the camera body is mounted without a lens behind the eyepiece. The focal image of the telescope is projected onto the camera sensor through the projection eyepiece — similar to a slide projector enlarging a slide onto a screen. The resulting focal length is called the equivalent focal length (f_eq).

The image scale depends on the focal length of the eyepiece (f_EP) and the projection distance (a). The equivalent focal length is calculated as follows — the longer it becomes, the greater the image scale:

f_eq = f_prime × ((a ÷ f_EP) - 1)

From the formula, it is clear that reducing the eyepiece focal length and/or increasing the distance (a) (eyepiece focal point / image plane) increases the equivalent focal length — and thus the magnification. The image becomes larger.

Schematic Representation of the Equivalent Focal Length

In its basic version (OPFA-4), the OPFA consists of the two T-2 extension tubes and the sliding focuser. It features a camera-side external T-2 thread and a telescope-side internal T-2 thread.

In the diagram, the telescope-side connection (i.e. the focuser) is shown at the bottom, and the camera connection at the top. The transparent components represent the nosepieces that are additionally included with the OPFA-1 (1.25") and OPFA-2 (2") versions, as well as the projection eyepiece and the camera-specific T-ring, which are not included with any OPFA.

• Each OPFA has a T-2 internal thread, to which nosepieces or threaded adapters can be securely attached without any tilt. If your focuser has a threaded connection, you should select one of the OPFA versions 3 to 7 accordingly (e.g. OPFA-3 for Celestron and Meade Schmidt-Cassegrain telescopes).
• Next is the sliding focuser with a T-2 thread locking ring. The separately available 1¼" eyepiece is inserted and clamped into this sliding focuser. The function of the locking ring and the sliding focuser is described further below.
• The two 40 mm T-2 extension tubes are then screwed onto the sliding focuser over the eyepiece (maximum outer diameter 38 mm). They serve to establish the required distance (a) from the eyepiece to the camera (the projection distance). By adding more T-2 extension tubes, the distance (a) can be increased as desired.
• The connection between the OPFA and your DSLR or mirrorless camera is made using the camera-specific T-ring (not included).

The sliding focuser allows fine adjustment of the projection distance (a) by gently moving the eyepiece forward or backward within its holder. The screw thread in the inner tube and the guide slot in the outer tube provide additional stability compared to simpler adapters.

By loosening the locking ring, the entire assembly — including the mounted camera — can be smoothly rotated to achieve the desired image framing. With the OPFA-1 and OPFA-2 models, the camera can also be rotated within the eyepiece clamp of the telescope.

For the OPFA-3 to OPFA-7 models, the connection between the focuser and the OPFA is a rigid threaded joint. In these versions, camera rotation is only possible via the locking ring of the sliding focuser.

The components of the basic OPFA-4 version with T-2 connection. Parts not included in the base version are shown in gray. Nosepieces or additional telescope-side threaded adapters are included in the respective OPFA variants (1–3, 5–7). The projection eyepiece and the T-ring for the camera are not included in any set.

Cameras with fixed lenses can also be mounted behind the eyepiece, provided that the lens has a filter thread and is sturdy enough to support the camera’s weight. The lens must be positioned just behind the eyepiece (approximately at the eyepiece’s eye relief distance) without touching it. For this, you will need shorter T-2 extension tubes. If you mainly plan to do afocal photography, it is better to use the ADPS - Afocal Digital Projection System ( I - VII ) (various versions available) instead of the OPFA. It already includes shorter extension tubes. By swapping the T-2 tubes, you can convert the OPFA into an ADPS at any time.

To adapt from the T-2 male thread to the camera’s filter thread, you will need the Adapter T-2i to SP54a – Hyperion® DT-Ring (#2458040 , € 31,- ) and a Hyperion DT ring, which converts the SP54 system thread to your filter thread.

Our brochure Everything About Digiscoping provides a detailed explanation of the different methods by which you can attach a camera to an eyepiece.

Eyepiece projection is an affordable way to get started with planetary photography. As your experience grows, you will likely upgrade to a dedicated planetary camera and Barlow lenses.

Unlike “monolithic” adapters from other manufacturers, the Baader OPFA is not a dead end: when connecting planetary cameras and Barlow lenses, or large deep-sky astro cameras, you will most likely need T-2 extension tubes to fine-tune the distances. The 40 mm tubes can then be repurposed, and the eyepiece clamp with sliding focuser is ideal for precisely adjusting the distance between a planetary camera and a Barlow lens to achieve the optimal spacing and magnification factor.

This way, you can continue to use the components of the OPFA even if you no longer need it for its original purpose!

Saturn Image – © Mario Weigand