Discovery Astrophotography with ZWO ASTRO

ASI Camera 55mm Back Focus Solution-2025 Updated Version

                                (2025 Updated Version)

Unlike planetary imaging, deep-sky astrophotography demands extremely high field
flatness across the entire image frame. To achieve this, telescope manufacturers typically use two approaches:

1. Built-in Field Flattener Design
Some telescopes integrate the field flattener optics directly inside the optical tube. With
these systems, once the camera reaches focus, the image field is naturally flattened—no
additional adjustment is required. Users of such telescopes don’t need to worry about the back focus issue discussed in this article.

2. External Field Flattener Design
Other telescopes use a detachable external field flattener. The advantage of this approach is flexibility—the flattener can be removed when doing visual observation or planetary imaging, where a flattener isn’t needed and may even introduce unwanted optical effects.

Typically, external field flatteners, reducer–flatteners, and MPCCs are designed with a specified optimal imaging distance, known as the back focus length.
At this precise distance, the optical system delivers the best field correction—most clearly seen as perfectly round, sharp stars all the way to the corners of the image.

Achieving this optimal back focus usually requires a combination of adapters and accessories. That ‘s exactly what we ‘re going to explain here.

The good news is that most manufacturers have reached a consensus on a 55mm back focus standard, mainly to maintain compatibility with DSLR cameras. When a DSLR is connected via its standard adapter ring, the resulting back focus distance is exactly 55mm.

Based on this shared standard, we’ve designed a full set of matching adapters. Next, let’s explore how to achieve the perfect 55mm back focus through different configuration combinations.

Part 01

4/3″, 1″, and Small-Format Cooled Cameras

Currently, ZWO offers several cooled cameras in the 4/3″, 1″, and small-format categories,
including: ASI585MC Pro, ASI533MC/MM Pro, ASI294MC/MM Pro, and ASI183MC/MM Pro.

Cooled Color Cameras

Cooled Monochrome Cameras

For cooled monochrome cameras, we generally recommend pairing them with an EFW or a filter drawer for optimal performance.

If you’re using a 1.25″ / 36mm / 31mm EFW, the connection setup is as follows:

If you’re using a 2″ EFW, the connection setup is as follows:

Note: The back focus: 56mm.The combination T2–M48 (16.5mm) + M48–M42 can be replaced with an M48–M48 (16.5mm) adapter.
Note: The back focus: 56mm; the T2 tilter is sold separately.

If you are using an M42 filter drawer, the connection setup is as follows:

Note: T2-M48 16.5mm + M48-M42 can be replaced with M48-M48 16.5mm.

If you are using the OAG-L and require a 15mm extender, the connection setup is as follows:

Part 02

APS-C Frame Cooled Cameras

ZWO currently offers several APS-C frame cooled cameras, including ASI2600MC/MM Pro,
ASI071MC Pro, and ASI2600MC/MM Duo.

Cooled Color Cameras
APS-C Frame Cooled Cameras Connection Solution

APS-C Frame Cooled Cameras + OAG

Cooled Monochrome Cameras
Note: For APS-C frame cooled cameras, ZWO recommend using 2″ or 36mm filter wheels or filter drawers. Smaller filters may cause vignetting or frame obstruction.
If you are using a 2″ filter wheel, the connection setup is as follows:

Note: The back focus: 56mm.The combination T2–M48 (16.5mm) + M48–M42 can be replaced with an M48–M48 (16.5mm) adapter.
Note: The back focus: 56mm;
Note: This setup requires removing the M42 sensor tilt adapter that comes with the camera. If you have an M68 OAG, it can also be used as a replacement for the OAG -L.

If you are using the OAG-L and require a 15mm extender, the connection setup is as follows:

If you are using the 36mm EFW, the connection setup is as follows:

Note: The T2-T2 2mm adapter can be found in the 36mm EFW package. This configuration is suitable for telescopes with a focal ratio greater than F5.4. 
Note: This setup requires removing the M42 sensor tilt adapter that comes with the camera. If you have an M68 OAG, it can also be used as a replacement for the OAG -L. This configuration is suitable for telescopes with a focal ratio greater than F4.8.
Note: This setup requires removing the sensor tilt adapter that comes with the camera. It is only compatible with telescopes featuring an M48 or M42 thread on the telescope side. The M42M–M48F 22.5mm extender needs to be purchased separately.

If you are using an M42 filter drawer, the connection setup is as
follows:

Note: T2-M48 16.5mm + M48-M42 can be replaced with M48-M48 16.5mm.

Part 03

ASI2600MC Duo

Note: For the ASI2600MC/MM Duo cooled camera, which integrates guiding and imaging
in one, the connection setup is as follows:

ASI2600MM Duo

If you are using an M54 filter drawer, the connection setup is as
follows:

Part 04

Wireless Smart Cameras

Currently available wireless smart cameras include ASI585MC Air and ASI2600MM/MC
Air.

ASI585MC Air：

ASI585MM Air：

ASI2600MC Air：

ASI2600MM Air：

Part 05

Full Frame Cooled Cameras

ZWO currently offers the following full-frame cooled cameras: ASI6200MC/MM Pro and
ASI2400MC Pro.

Cooled Color Cameras

Cooled Monochrome Cameras

Note: For full frame cooled cameras, only 2″ EFW or filter drawers are recommended.
If you are using a 2″ EFW, the connection setup is as follows:

Note: This setup requires removing the M42 sensor tilt adapter that comes with the camera. If you have an M68 OAG, it can also be used as a replacement for the OAG -L.

If you are using the 7×50mm EFW, the connection setup is as follows:

Note: This setup requires removing the sensor tilt adapter that comes with the camera. The 5mm rear sensor tilt adapter needs to be purchased separately.

If you are using the OAG-L and require a 15mm extender, the
connection setup is as follows:

If you are using the M54 filter drawer, the connection setup is as
follows:

Note: The back focus: 56mm
Note: The M68 OAG has been discontinued.

OAG Connection Diagram：

Note: The 1.25″ extender is optional.

Part 06

Medium Format Cooled Camera

ZWO currently offers medium-format cameras, including the ASI461MM Pro.
The connection setup is as follows:

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23 Comments

  • flappah@icloud.com
    Posted December 27, 2024 at 4:39 am

    Most of the images seem to be missing!

    0Likes Reply
  • Bob
    Posted March 19, 2025 at 7:57 pm

    Can you update please? Include 585 Pro and CAA addition.

    0Likes Reply
  • Kurt
    Posted March 31, 2025 at 7:38 pm

    I purchased the OAG-L + 2″ EFW and the ASI 533 Pro and woud like to connect it to my M42 Celestron T-adapter.
    I don’t see a connection guide on this page . Can you please help me ?

    0Likes Reply
  • Kurt
    Posted April 6, 2025 at 4:37 am

    Could you please translate to english ?thx , kurt

    0Likes Reply
    • Post Author
      diane.tien@zwoptical.com
      Posted April 24, 2025 at 1:28 pm

      connection guide

      0Likes Reply
      • Cedric
        Posted April 29, 2025 at 4:09 am

        Hello, would that work with a efw mini instead of 2″ efw ?

        0Likes Reply
        • Post Author
          diane.tien@zwoptical.com
          Posted April 29, 2025 at 3:21 pm

          The components cannot be directly interchanged. The specific connection method depends on the equipment they are paired with.

          0Likes Reply
          • Cédric
            Posted April 30, 2025 at 12:36 am

            What about with 7×36 II ?

            0Likes
          • Post Author
            diane.tien@zwoptical.com
            Posted April 30, 2025 at 12:26 pm

            They both have M42 threads and the same thickness. You can remove the EFW mini and replace it with the 7×36.

            0Likes
  • Wes
    Posted April 26, 2025 at 3:24 am

    My recent purchase of the ASI533MM Pro bundled with the 1.25” 8-position EFW, did not come with an 11mm ring. Is there a 55mm solution for the 533MM Pro with 1.25” EFW without using the 11mm ring?

    0Likes Reply
  • Christophe
    Posted May 15, 2025 at 9:41 pm

    Hello,
    I have purchased the following equipments:
    – ASI 2600MC Duo
    – EFW 7×50,8mm
    – FF80 APO
    To respect the backfocus, I need to do the mount “APS-C with 2″ EFW”.
    The problem is that the camera is delivered with 2 M45M-M54F adapters (16.5mm and 21mm) but not with the T2-M48 16.5mm and M48-M42 adapters as shown on your diagrams.
    So I can’t get all the parts together. Have I missed something?

    0Likes Reply
  • Michael
    Posted September 27, 2025 at 10:50 pm

    What do we do for AIR cameras such as the 585 and 2600. It appears we cant remove the 11mm on the tilt plate and still also have a thread on the camera.

    0Likes Reply
    • sasa.wang@zwoastro.com
      Posted September 30, 2025 at 5:45 pm

      The three screws of the target ring can be removed. There is no thread after removing the target ring.ZWOZWO

      0Likes Reply
  • papayo@hotmail.it
    Posted October 23, 2025 at 8:30 pm

    Hello, i just bought a Asi 2600 mc air with m54 filter drawer, m54m-m48f-2, m48-m48 16.5mm. This is the scheme you recommended and the only one to achieve a backfocus close to 55mm. This way you get to 56mm. When I want to shoot with only the UV-IR filter included with the 2600, what should I do? I don’t use any filters, so 56mm is too much.

    0Likes Reply

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