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

    I have the ASI2600MC pro and I want to use it on my 800mm Newtonian with my Baader Mark ii coma corrector. What set of spacers/adapters should I use. My 1st attempt left distortion towards the outer regions of the images, which I suspect was due to incorrect placement of the coma corrector. The Baader is almost the same length as the 21mm spacer adapter provided with the 2600.




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  2. Avatar

    Hi,

    I just received my ZWO 5 position EFW and I followed the connection diagram on ZWO’s site to connect it to my ASI071MC-Cool. The connection diagram for APS-C cameras does not show how the camera connects to the EFW; only how the adapters and spacers attach on the scope side of the wheel. I read instructions for connecting it to a full frame sensor and I removed the tilt plate but I found the holes do not line up. Is there another adapter to make this work?

    Is the 5 position 2” EFW not compatible with my camera?

    – Joe




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  3. Avatar

    I use a Baader 2″ Mark III Multi-Purpose Coma Corrector which has a back focus of 55 mm with asi 1600mm and a filter wheel how can I get back focus of 55 mm??




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    • Avatar

      Sorry we don’t have test how to connect the Mark III Multi-Purpose Coma Corrector.
      You could refer to the first title “4/3″, 1″ and small format cooled cameras” on this website.




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  4. Avatar

    Hi,

    I could really use some help. I purchased a ZWO ASI071 MC Pro and haven’t got a clue if I have my rig setup correctly. One of the main problems with most of the pictures of setups online, discussion forums, and in the manual(s) is that they primarily show and talk to DSLRs while only casually mentioning CCDs. My problem is trying to understand the setup and difference between 55mm and 155mm of back-focus. From the closest part, I can measure to the bottom of the telescope to the bottom of the CCD sensor (17.5mm) is a grand total of just over 153mm.

    My setup consists of an EdgeHD 8″, EdgeHD Compatible T-Adapter, ASI071 MC Pro – Short question is, what parts do I need?

    *I do not have a focal reducer or OAG.
    ** The Celestron T-Adapter came in two attached pieces.

    I’ve looked over all of the ZWO setup diagrams but cannot determine one that fits my case. Specifically, do the M48 threads on the telescope-end include or preclude the T-Adapter?

    Thank you, John




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    • Avatar

      Hi John, please figure out how much does your telescope needs to acquire focus and then arrange the adapters.




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  5. Avatar

    I do not understand “backfocus”. I have a ZWO 1533, and have no problems focusing – I have no extenders. I use both an ED102 refractor ( F7 ), and an 8SE with reducer, (aprox f6.3) – I can focus just fine. I’m very confused as to why all the extension tubes and backfocus documentation.




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  6. Avatar

    I have an ES ED80, with the ES field flattener, which has 45mm back focus, along with the 178MC Cool. I have all the adaptors (21, 16.5, M42/M48)) but I am unclear of which ones to use. Can you help?




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    • Avatar

      Maybe you could remove the T2 ring(11mm) which attached to the camera. And then connect the camera with T2 21mm extender, M48-T2 16.5mm extender, but you should find an adapter to attach the male theard of M48-T2 to your telescope by yourself. We are sorry we don’t have this adapter.




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      • Avatar

        Moson:

        I did the math, and what I currently calculated behind the flattener is:
        17.5 for 178MC Cool
        21 spacer
        6.5 spacer
        total of 45MM.

        ES confirmed today that the flattener to the sensor should be 45, so am I good to go?




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        • Avatar

          Yes, but as I mentioned before, we don’t have a suitable adapter for you to attach the male thread to the telescope. You could do it by yourself.




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          • Avatar

            Now I am a little confused. Currently, the last piece in the chain is the flattener, which clamps into the focuser. Are you talking about this connection? I’d post a picture but it seems no attachments allowed. The chain is; camera @17.5 + 6.5 + 21 = 45, then flattener threads into spacer and goes inside focuser.




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            • Avatar

              The backfocus is depend on your flattener which requires 55mm back focus. Sorry what’s your 6.5mm refer to? We have M48-M48 16.5mm extender, M48-T2 16.5mm extender, and OAG 16.5mm.
              Camera 17.5mm + extender/OAG 16.5mm + T2 extender 21mm = 55mm




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              • Avatar

                So, to clarify, my flattener requires 45MM backfocus, not 55. the 6.5 is a ring I added to get to 45. I was mostly confirming that if the flattener backfocus is different than the 178 backfocus, I should still use the flattener backfocus as my distance for calculations. Make sense? In other words, I’m shooting for 45MM, not 55MM.




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              • Avatar

                Yes, it make sense.




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  7. Avatar

    Hi, I use a Sky Watcher 200 f4 with a coma corrector that need a 54,2 back focus. My camera is the ASI 071 and, with adapters I have 55 mm back focus. How can I achieve 54,2 backfocus? Do you have some recomendation? Regards




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    • Avatar

      There is a tolerance of 1-2mm for the back focus of the telescope, it doesn’t matter if you use the solution of 55mm back focus.
      Or you could try adding a gasket to extend the back focus to 55mm.




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  8. Avatar

    I am trying to find focus of the OAG and a ZWO ASI 1600 colour. I don’t think I have the right spacer for the ZWO and the 2″ OAG because when one is in focus the other one isn’t and there isn’t enough correction on the OAG. Is there a correct distance for both?




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  9. Avatar

    Ciao,
    ho acquistato una camera 533 pro color, ho un problema sulla messa a fuoco senza riduttore di focale, a 600 mm con ED80, cosa posso fare?




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  10. Avatar

    Can you attach the OAG and manual filter wheel and keep the 11mm ring fixed to my ASI 183, I want to stack a HA filter in the wheel and a UV filter in front of the camers

    Thanks

    Nice to see you are back!




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  11. Avatar

    I have the ASI 1600MC (5mm) and a GSO 2″ .5 focal reducer (15mm), T2 extender (22mm), an Optolong L-Enhance (2mm) and ZWO T2 extender (11mm) for a total of 55mm. Can I place the focal reducer closest to the camera, or do I need to put it at the far end?

    My current plan is:
    ===============
    ZWO 1600MC (5mm)–>GSO 2″ .5 FR (15mm)–>T2 extender (22mm)–>Optolong (2mm)–>T2 extender (11mm), or 5+15+22+3+11=55mm of total back focus. Is that an acceptable configuration? Does it matter where I put the FR or IR filter in the image train to achieve the 55mm of back focus?

    Love your products!




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    • Avatar

      Focap reducer should be place close to the scope, then calculate the 55mm from the thread of the thread on focal reducer which face to the camera and to the sensor.
      Thank you for choosing us!




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  12. Avatar

    Io ho una Asi 224- Se voglio applicare la OAG della ZWO Servono delle prolunghe per un back focu?




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  13. Avatar

    Hello,

    i am trying to connect the samyang EF lens 14mmf2.8 to the zwo 294mc pro but i can not reach focus. I am using the ASI T2 EOS adapter and the 11mm T2 extender included with the camera but with no succes. I am missing someting here. Your support will be much appreciated.

    Thank You




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  14. Avatar

    Hi,
    i have a doubt about the right backfocus concerning my camera ASI 1600ProM fitted on my C14 Edge HD with Hyperstar system.
    The right backfocus for my Hyperstar is 67.4mm whereas for the ASI 1600Pro is indicated the best backfocus of 55mm.
    What should i taken into account?
    Thanks




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  15. Avatar

    I am attaching an ASI294 MC cooled camera to an AP105 Traveler f6, 610mm focal length, with a 92/105 AP 92FF field flattener. The recommended camera back focus from the field flattener is 64.1 mm, but the recommended back focus for the ZWO camera is 55 mm. Do I thus want an extender that places the ZWO camera at 64.1 mm, 9.1 mm more than the recommended 55 mm? I am assuming flatness of field (which is optimal at 64.1mm from the field flattener) is more important than the 55 mm ZWO back focus, which I assume simply affects how much of the image from the scope falls on the camera sensor, is that correct?




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  16. Avatar

    Hi
    I have svr102t with focus reducer + light pollution filter.
    The light pollution filter is fixed in front of the extension adapter tube.
    My back focus should be 55mm .
    Do I add 1mm for the light pollution filter?
    Thx




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  17. Avatar

    Hi!
    To get 55mm backfocus with asi 1600mmc: 6,5 + 20 + 11 + 16,5 + 1 mm (filter) = 55. I don’t need T2-T2 adapter, is it correct? Thx !!




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  18. Avatar

    Simple question, but I’m stuck.

    I have a Celestron C8 SCT telescope. I can connect my camera through a 1.25″ nose piece, but I wanted to connect it directly by threading it onto the visual back.

    What do I need to do this? The threads on the “visual back” don’t match anything supplied with my 294MC Pro. Do I need to buy a T-Adapter? What do I need to connect it?

    This must be a very common configuration, yet the threads on the telescope side are not well documented and don’t fit anything I have except the visual back supplied (leading to 1.25″ connection).




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    • Avatar

      ASI294 provide M42 interface, you can use 1.25″ adapter or you can find another adapter, we do not have this telescope and we cannot be sure about this.




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  19. Avatar

    I use a skywatcher coma corrector model GPU 4 lenses. I have m48-m42 adapter that is 5 mm and m42 extender that is 40 mm.
    I have in total 5+40+12,5=57,5. Is it a problem with an asi 224 mc?
    Thanks




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