I want to image wide angle areas of the sky, and for that I am using a Sigma lens with an astro camera, but I can’t obtain the focus. I have tried different backfocus distances, see below, but I’m unable to get anything close to focus. Does anyone have experience in connecting photography lenses with astro cameras? What am I missing?
My lens is a Sigma 24-70mm F2.8 DG OS HSM | A. I’ve been quite successful using this for wide-angle imaging with my Nikon DSLRs (D780 & D810A).
Per ZWO, the ASI2600MM Duo backfocus is 17.5mm.
The filter wheel is directly connected to an ASI2600MM Duo. The filter wheel is 20mm thick and provides an M54 interface.
A Nikon F-mount adapter connects the lens to M54 threads; it is 12.5mm thick.
Here’s what I tried.
Using the instructions provided in the documentation for the ASI2600MM Duo:
Filter Wheel = 20mm
Nikon F-mount adapter = 12.5mm
ASI2600MM Duo backfocus is 17.5mmm
Total back focus = 50mm ⇒ focus could not be obtained
Notes
This appeared to be the closest to focus, but was still badly out.
This is the minimum backfocus I can obtain with the required components.
As that failed, I contacted ZWO, who recommended this:
Filter Wheel = 20mm
Nikon F-mount adapter = 12.5mm
Spacer between Filter Wheel and lens adapter = 5mm
ASI2600MM Duo backfocus is 17.5mmm
Total back focus = 55mm ⇒ focus could not be obtained
I also tried other spacer thicknesses, none of which worked. Focus appeared worse with larger spacers.
Spacer between Filter Wheel and lens adapter = 4mm
Total back focus = 54mm ⇒ focus could not be obtained
Spacer between Filter Wheel and lens adapter = 9mm
Total back focus = 59mm ⇒ focus could not be obtained
Spacer between Filter Wheel and lens adapter = 16.5mm
Total back focus = 66.5mm ⇒ focus could not be obtained
Spacer between Filter Wheel and lens adapter = 21mm
Total back focus = 71mm ⇒ focus could not be obtained
Spacer between Filter Wheel and lens adapter = 37.5mm
Total back focus = 87.5mm ⇒ focus could not be obtained
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As others have mentioned the backfocus required is 46.5 mm (minus 1/3 the filter thickness). Depending on the FW make there might or might not exists a F-mount adapter. If you're desperate about making it work there are options to have one made to specs.
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According to Nikon the backfocus for the Nikon F-mount is 46.5mm.
The Nikon F-mount adapter from ZWO is exactly the required 29mm -> https://www.zwoastro.com/product/zwo-new-nikon-t2-adapter-suitable-for-all-asi-cameras/
Edit: I guess you won't have any luck with a mono camera, as there is no F-mount adapter of only 9 mm thickness to allow for the filterwheel. Thank you, you were very helpful. That is depressing. I guess I will have to go back to my Nikon D810A. I think ZWO needs to update their instructions for connecting a photo lens.
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Tony Gondola:
This chart might help...
https://en.wikipedia.org/wiki/Lens_mount Thank you, the link is helpful.
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Frankly, it is rather easy to make one.
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The T2 filter drawer goes directly onto the lens and the 2600 screws into the T2 drawer.
You might need to shim the camera to get flat field.
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I have solved my problem. The ZWO instructions appear to be too brief. I found more detailed instructions on the Agena Astro site. They had me remove the sensor tilt adapter, and that solved the problem by removing 5mm from the backfocus. I can now focus. Thank you all for your help.
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Here is an implementation of the suggestions above, that works under the stars and on the bench. ASI 2600 MC Duo Standard Spacers and Sigma Art 85mm for Nikon lens... ![IMG_6492[1].JPG](https://cdn.astrobin.com/ckeditor-thumbs/14617/2025/6c742343-20fd-40fa-b71c-b4667689877c.jpg) |
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The backfocus I needed to be able to focus was more then 17mm. I attached a ZWO filter wheel + 16.5mm + 21mm connector pieces. Then I'm at the edge of having sharp images although I push it near the other end. My autofocusser doesn't work though as I attached a connector piece to much and it doesn't have enough space to go through a cycle. Manual focus works fine this way. (using Askar FRA500)
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Dave Erickson:
Here is an implementation of the suggestions above, that works under the stars and on the bench. ASI 2600 MC Duo Standard Spacers and Sigma Art 85mm for Nikon lens...
What is the bracket your are using to support the lens?
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According to Nikon the backfocus for the Nikon F-mount is 46.5mm.
The Nikon F-mount adapter from ZWO is exactly the required 29mm -> https://www.zwoastro.com/product/zwo-new-nikon-t2-adapter-suitable-for-all-asi-cameras/
Edit: I guess you won't have any luck with a mono camera, as there is no F-mount adapter of only 9 mm thickness to allow for the filterwheel. I am using ZWO Nikon Lens Adapter for ZWO 2" Electronic Filter Wheel # EFW2-Nikon https://agenaastro.com/zwo-nikon-lens-adapter-for-2-electronic-filter-wheel-efw2-nikon.htmlBut I am not sure how much of the dimensions actually account for a "spacer". I measure 12.5mm, but wish I could verify that number.
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Its a ZWO holder ring "ZWO-RINGD90". one for the camera one for the lens. I added a bracket for the flip flat to the one on the camera. For the Nikon Interface to the ASI2600mc-Duo I used the " ZWO NEW Nikon N54 adapter" There are 2 parts: the first is a ring that adapte to the Nikon Camera. It measures 10.5mm thickness from the threaded land to the camera interface, not included are the threads. the second is an M54 x0.75 threaded adapter ring 18.15mm thick. there is likely small production variations in these thicknesses.. The two components are shown below...  |
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![IMG_6492[1].JPG](https://cdn.astrobin.com/ckeditor-files/14617/2025/380bde1e-c190-4f6c-b01f-d00c74eb340b.JPG)
