The back focal length is advised by telescope manufacturers. Since most telescopes have a 55mm back focal length, we are here to provide detailed instructions for all ASI cooled cameras.
Please be noted that this tutorial only applies to 55mm back focal length. And some adapters are not included in your camera box, thus you might need to purchase them separately.
1. 4/3”, 1” and small format cooled cameras:
ASI1600/294/183/533/991/990
1) Without filter drawers or filter wheels:
2) With 1.25”/36mm/31mm EFW:
3) With 2” EFW:
Note: The total back focal length of this solution is 56mm; The two adapters T2-M48 16.5mm and M48-M42 could be replaced by the M48-M48 16.5mm extender.
Note: The total back focal length of this solution is 56mm; The T2-tilter needs to be bought separately.
4) With 2” filter drawer:
Note: The two adapters T2-M48 16.5mm and M48-M42 could be replaced by the M48-M48 16.5mm extender.
5) With OAG-L and 2” filter:
If you have an OAG-L, but do not have a filter wheel to reach 55mm back focal length. Then you can use the M54 adapter instead. It also allows you to add a 2” filter into the image train.
2. APS-C cooled cameras:
ASI2600/071
1) Without filter drawers or filter wheels:
2) With 36mm EFW:
Note: T2-T2 2mm adapter can be found in the package of 36mm EFW;
Note: The M42 sensor tilt plate attached to the camera needs to be taken off. You may need to re-adjust the tilt when re-installing the plate; The original OAG can not be replaced with OAG-L, but if you have the M68 OAG, you can use the M68 OAG instead; This method does not support the telescopes with M42 threads at the end side.
Note: This method only supports the telescopes with M42 or M48 threads at the end side. The M42M-M48F 22.5mm extender needs to be bought separately.
3) With 2” EFW:
Note: The total back focal 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.
Note: The total back focal length of this solution is 56mm.
Note: The M42 sensor tilt plate attached to the camera needs to be taken off, and you may need to re-adjust the tilt when re-installing the plate back; The original OAG can not be replaced to OAG-L, but if you have the M68 OAG, you can also use the M68 OAG instead; This method does not support the telescopes with M42 threads at the end side.
4) With 2” filter drawer:
The two adapters T2-M48 16.5mm and M48-M42 could be replaced by one adapter M48-M48 16.5mm.
5) With OAG-L and 2” filter:
If you have an OAG-L, but do not have any filter wheel to reach 55mm back focal length. Then you can use the M54 adapter instead. It would also allow you to add a 2” filter into the image train.
ASI2600MC Duo
1) Without filter drawers:
1) With 2” filter drawer:
Note: The back focal length of this solution is 56mm.
3. Full frame cooled cameras:
ASI6200/2400
1) Without filter drawers or filter wheels:
Note: The above two solutions will both block the sensor a little bit.
2) With 2” EFW:
Note: The M42 sensor tilt plate attached to the camera needs to be taken off, and you may need to re-adjust the tilt when re-installing the plate; The original OAG can not be replaced to OAG-L, but if you have the M68 OAG, you can also use the M68 OAG instead; This method does not support the telescopes with M42 threads at the end side.
3) With 2” filter drawer:
Note: The back focal length of this solution is 56mm.
Note: This solution only applies to telescopes with M68 interface threads. If the threads of your telescope is M42 or M48, then we recommend you DIY the relative adapters or choose the 2” EFW.
4) With OAG-L and 2” filter:
If you have an OAG-L, but do not have & want to use any filter wheel to reach 55mm back focal length. Then you can use the M54 adapter instead. It would also allow you to add a 2” filter into the image train.
ASI camera connecting with OAG diagram：
When using an heater band additional 0.5mm should be added to the backfocus?
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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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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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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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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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I’m afraid it is.
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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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We have IR/UV cut in our website.
You can have a look.
We recommend you to find a place in the image train to place the filter.
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The same question. How to deal with extra filter in case if there is no EFW? I have color camera. I obviously don’t need wheel.
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You can insert 1.25″ filter into the camera with T2-1.25″ adapter.
For other filters you may need to find a place.
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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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The backfocus of a asi 174mc is 12,5 mm.
How do I reach the 55mm required for my flattener?
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Hi Alessandro,
The back focus of 174MC is 6.5mm, you can add the 11mm T2 ring, then it will be 17.5mm.
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and for the 178 mc?
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178MC back focus 12.5mm
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585 mc
Back focuse?
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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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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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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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You can refer to our 56 mm back focus solution, it is all considered into it.
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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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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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Normally telescope has some tolerence for back focus.
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Normally telescope has some tolerance for back focus.
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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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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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Threads 4mm,
Extend distance 1.5mm,
Total length 33mm（including threads and extend distance）.
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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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After the main camera was fixed, adjust the position of the guide camera by pulling to fine tune the guider camera.
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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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ADC is not a DSO imaging accessory, please tell me why to use that?
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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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Just like the diagram about 1600 mono camera imaging train.
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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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Our T2 extender only have one standard, It’s M42*0.75 thread pitch.
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Thank you Sean. That’s all good now.
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You are welcome!
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What a great guide!!
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Thank you!
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Huge Help!
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Thank you~!
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Printed, laminated, and kept in the case with my cameras. Thanks Sean!
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You are welcome, Edward!
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What a great resource to make sure that you are setup correctly.
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Thank you!
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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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Oh, I forget one thing. Small format camera don’t have thoes adapters. I need to mention this in article.
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Excellent guide!
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Thank you!
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