How to reach the 55mm back focus with ASI cameras? 

The back focus is advised by the telescope manufacturers. 
Since most telescopes have 55mm back focus, we are providing the detailed guidelines for all our cooled cameras.
Please be noted that these are just examples for 55mm back focus,  you need to know the back focus of your reducer and flattener first and then arrange the adapters.

Notice: About small format, 174, 178, 385, 290, 224, 185, etc. The pacakge does not include some adapters, please take a look at the diagram of package list in product description.

 

1. 4/3”, 1” and small format cooled cameras:

ASI1600/294/183/533

1) Without filter drawers or filter wheels:
small frame cooled camera -1

small frame cooled camera + OAG

 

2) With 1.25”/36mm/31mm EFW:

small frame cooled camera + 1.25inch 31mm 36mm EFW

small frame cooled camera + 1.25inch 31mm 36mm EFW + OAG

 

3) With 2”EFW:

small frame cooled camera + 2inch EFW

Note: The total back focus length of this solution is 56mm; The two adapters T2-M48 16.5mm and M48-M42 could be replaced by one adapter M48-M48 16.5mm.

small frame cooled camera + 2inch EFW + OAG

Note: The total back focus length of this solution is 56mm; The T2-tilter need to be bought separately.

 

4) With 2” filter drawer:

small frame cooled camera + M42 filter drawer

Note: The two adapters T2-M48 16.5mm and M48-M42 could be replaced by one adapter M48-M48 16.5mm.

small frame cooled camera + M42 filter drawer + OAG

 

2. APS-C format cooled cameras:

ASI2600/071

1) Without filter drawers or filter wheels:

APS-C format cooled camera - 55mm back focus length solution

APS-C format cooled camera + OAG

 

2) With 36mm EFW:

APS-C format cooled camera + 36mm EFW + OAG

 

3) With 2” EFW:

APS-C format cooled camera + 2inch EFW

Note:The total back focus length of this solution is 56mm; The two adapters T2-M48 16.5mm and M48-M42 could be replaced by one adapter M48-M48 16.5mm.

APS-C format cooled camera + 2inch EFW + OAG

Note:The total back focus length of this solution is 56mm.

 

4) With 2” filter drawer:

APS-C format cooled camera + M42 filter drawer

The two adapters T2-M48 16.5mm and M48-M42 could be replaced by one adapter M48-M48 16.5mm.

APS-C format cooled camera + M42 filter drawer + OAG

 

 

3. Full frame format cooled cameras:

ASI6200/2400

1) Without filter drawers or filter wheels:

full frame cooled camera - 55mm back focus length solution

full frame cooled camera + OAG

Note: The above two solutions will both block the sensor a little bit.

 

2) With 2”EFW:

6200-55mm-backfocus-solution-2-2

full frame cooled camera + 2inch EFW + M68 OAG

 

3) With 2”filter drawer:

full frame cooled camera + M54 filter drawer

Note: The back focus length of this solution is 56mm.

full frame cooled camera + M54 filter drawer + M68 OAG

Note:This solution only applies to the telescopes with M68 interface threads. If the threads of your telescope is M42 or M48, then we recommend DIY the relative adapters or choose the 2” EFW.

 ASI camera connecting with OAG diagram:

OAG

106 Responses to The best solution of 55mm back focus length
  1. Avatar

    When using an heater band additional 0.5mm should be added to the backfocus?




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  2. Avatar

    Hello, I have a 1600mono cooled as my main imaging camera and a 120 to use as a guiding camera. I am using an OAG and a filter wheel all ZWO. I need help figuring out the correct spacing configuration so both cameras are in focus. Thank you.




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      This is just an example. You need to know the back focus of your reducer or flattener, and arrange the back focus.




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  3. Avatar

    I just bought the ZwO ASI183 MC pro cooled camera. I intent to use with with a hyperstar lens and a RASA 8 and I am unsure of what adapters I should have in the camera. The camera came with one ring installed but several more in the box. Can you give any guidance?




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    • Avatar

      Hi Luis,
      We suggest you to find the back focus of your telescope first, then you can arrange the suitable adapters.




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  4. Avatar

    Hi!

    I can’t find an easy way to rotate the camera in my ZWO OAG. This is sometimes necessary when looking for a star near the edges of the guide camera image. There are three lock knobs that loosen the camera and allow it to be turned a little ways, but the extensions on the main camera adapter, the round dovetail thing, hit the prism stalk and don’t allow it to rotate very much. Do I need to accept this?

    Thank you,

    James




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  5. Avatar

    Which extenders to use if I want to add an IR/UV filter in the optical path between reducer and ASI294MC Pro? The backfocus should be 55mm. Can you also recommend me the IR/UV filter?




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  6. Avatar

    What about ASI224MC imaging through the OTA – not in an OAG?? Please help




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      If you want to fit the 224MC into the 55mm back focus, you need a 5 mm extender to match the distance of 17.5mm that we designed for cooled cameras.




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  7. Avatar

    The backfocus of a asi 174mc is 12,5 mm.
    How do I reach the 55mm required for my flattener?




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  8. Avatar

    I have an ES 102mm scope with a focal reducer. I am trying to setup up my scope for the following ASI1600pro camera with a ZWO filter wheel in front of it. Is 55 mm back focus a maximum? What if my camera is further back than 55 mm.?




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    • Avatar

      The 55 mm solution is a sample, suitable for the flattrner which needs 55 mm back focus.
      You have to calculate the back focus in accordance to the specification given by your telescope, in your case;ES 102 mm scope and a focal reducer.




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  9. Avatar

    hello, I an considering a 1600mm with your filter wheel to go behind a Skywatcher field flattener and i note the spacing solutions above, but is the 1mm due to the filter width included in these numbers? if not, how is it achievable when all the supplied extenders are of fixed length?
    Thank you for you help
    Andrew




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    • Avatar

      You can refer to our 56 mm back focus solution, it is all considered into it.




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      • Avatar

        To better clarify your response, is the 56mm backfocus distance shown in your diagram the mechanical backfocus distance and the optical backfocus (which is what is important) is the typically desired 55mm, with the 1mm difference being due to the optical path length through the filter wheel being 1mm less than its 20mm mechanical thickness?




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        • Avatar

          yes when you add the filter, it need 1 more mm.




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            So basically you’re saying that it’s not possible to achieve a 55mm back focus with OAG+EFW+ASI1600MM? The diagram shows 56mm just mechanically and any filter, as you said, is going to add another 1mm (more like 0.6mm but…). So the train is pushing 57mm. I know a lot of people supposedly use this setup successfully but it sounds like it wouldn’t come to focus with any standard 55mm back focus reducer/flattener. Am I missing something? Seems like one really needs a 9mm ring not 11mm or, better yet, something adjustable.




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            Ah, I get it now. The filter adds to the back focus *requirement* not the actual effective distance, meaning that the user needs it to be mechanically 56mm (or so) end-to-end to work with a 55mm reducer. Makes sense.




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  10. Avatar

    Hello, the ASI 1600 has 6.5mm if backfocus I believe. Can you let me know how much backfocus is added (at the shoulder of the 1.25 piece) with the T2-1.25′′ T-Mount.
    Thank you




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  11. Avatar

    Hello,
    I’m a little bit confused…I have an ED80 with its flattener who requires 55mm backfocus and I put the ZWO oag and the right spacer between the oag and the main CCD (a Moravian) to reach the 55mm distance. I also want to us a my ASI 120MM as guider camera so I screwed it to the oag with the 5mm spacer. It seems both main and guider camera reach the focus but I tested from home toa far hill. What if I should fine tune the guider camera focus on the actual night sky? No adjustment is allowed now..am I missing something? Thanks in advance.
    Clear skies!




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  12. Avatar

    Would you be so kind to add a diagram that has the following: 1)ASI camera, 2)filter wheel, 3)oag, and 4) adc.




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  13. Avatar

    Hi,

    I use a skywatcher coma corrector model GPU 4 lenses, and it asks for a backfocus of 55, how to have them because I use a 1600 mm pro asi with EFW mini and halrvb zwo filter.
    Thank you




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  14. Avatar

    I’m a little confused with the 21mm T2 Extender, some ZWO Articles specify it as T2 Extender [https://astronomy-imaging-camera.com/…/best-back-focus…], whilst others specify an M42 Extender [Item 6 in the interface drawing: https://astronomy-imaging-camera.com/…/QuickGuide…].
    Wiki defines M42 as 42mm x 1mm thread pitch and T2 as 42mm x 0.74mm thread pitch. Can anyone pleas clear up my confusion as I wish to get focus on my ASI1600C-Cool with ZWO OAG, using the ASI120MC as the guide camera. Many thanks for any clarification.




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  15. Avatar

    Printed, laminated, and kept in the case with my cameras. Thanks Sean!




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  16. Avatar

    What a great resource to make sure that you are setup correctly.




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  17. Avatar

    Dear Mr Wang,
    I purchased the asi224MC cooled in the UK several years ago when it was first released and it certainly was not shipped with the extenders that are illustrated in the diagram. Were they meant to have been?




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  18. Avatar

    Excellent guide!




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