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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:
If you are using an M42 filter drawer, the connection setup is as follows:
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:
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:
If you are using an M42 filter drawer, the connection setup is as
follows:
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:
If you are using the 7×50mm EFW, the connection setup is as follows:
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:
OAG Connection Diagram：
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
Most of the images seem to be missing!
Charley Wang
Thanks for the feedback, it has been fixed.
Bob
Can you update please? Include 585 Pro and CAA addition.
diane.tien@zwoptical.com
Hello Bob,
We are planning to update this blog article. In the meantime, you can refer to the product detail pages for information on the 55mm back focol length:
ASI585 Pro: https://www.zwoastro.com/product/asi585mc-mm-pro/
Camera Angle Adjuster (CAA): https://www.zwoastro.com/product/camera-angle-adjuster/
Evgeniy
Any update? I’ve boughten CAA and don’t understand how to use it with 48mm comma corrector, 36mm EFW and ASI294MM PRO
sasa.wang@zwoastro.com
The coma correction lens is connected to the front, the camera is connected to the filter wheel with screws, and the filter wheel is connected to the CAA.
Kurt
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 ?
diane.tien@zwoptical.com
Kurt
Could you please translate to english ?thx , kurt
diane.tien@zwoptical.com
Cedric
Hello, would that work with a efw mini instead of 2″ efw ?
diane.tien@zwoptical.com
The components cannot be directly interchanged. The specific connection method depends on the equipment they are paired with.
Cédric
What about with 7×36 II ?
diane.tien@zwoptical.com
They both have M42 threads and the same thickness. You can remove the EFW mini and replace it with the 7×36.
Wes
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?
diane.tien@zwoptical.com
Cedric
Hello. Would this work with a mini EFW instead of 2″ filter wheel?
Christophe
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?
diane.tien@zwoptical.com
Hi Christophe, please refer to the following image.
Michael
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.
sasa.wang@zwoastro.com
The three screws of the target ring can be removed. There is no thread after removing the target ring.
papayo@hotmail.it
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.
sasa.wang@zwoastro.com
You can refer to this solution.