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Recently I have been doing some wide-field (all-sky) imaging with the Sigma 40mm/ZWO2400MC combo. This is in support of the ABC survey, to collect data on large-scale gradients in the southern sky. While I am not too worried about star-shape, it is rather disappointing to note that I don't appear to be achieve to achieve perfect focus with this set up. Using the autofocus routine in the ASIAir and a printer belt to correct the EAF to (and rotate) the lens barrel, at wide-open (f1.4) I achieve best focus (around 3 pix) at 2m on the lens focus window. However, the images don't look great, with halos at the centre of the sensor and coma like wings at the outside. I suspect this might be a back-focus distance problem, but I have tried with both the ZWO Canon adapters with/without the filter slide. The results are indentical - best focus at 2m and still ugly stars. FWHMEccentricity in PI finds that both sets of images are spherically symmetric for FWHM and Eccentricity with things going off fast towards the corners. [The wings/halos don't have much light in them, so they don't look as bad in the numbers from the fitting as they do by eye. In addition, to create all-sky mosaic images, I am binning up 2x and using BXT which lessens the impact. Nevertheless this is bugging me. Quite apart from the cosmetics I must be losing SNR. I also tried using this lens wide open with my Canon 6D, and everything work OK - focus at infinity and round stars. OK, that's the details. My questions are as follows: Has anyone else experienced this with the Sigma lens [or the ZWO adapters] Do I need to shim - if so by how much? Is this calculable without a raytracing program? Is there a procedure for doing this? Many tbanks Brian |
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Hi Brian, I'm not sure this is exactly what I experienced, but I had some issues with the Sigma 105 art at F1.4 (and actually all the way until about F5) that sound familiar. My issue looked like backfocus combined with astigmatism. ZWO's adapters don't leave any ability to deal with tilt/backfocus to adjust though. I got shims to test the tilt, but my images appeared to require closer backfocus (not further). My guess was that the adapters aren't as exact as needed. For my Rokinon 135, I bought the replacement flange from FLO and it reduced the issues I saw immensely. Outside of the Rokinon135 though, I haven't found relief. If you do, I'd love to hear it |
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Hi Brian I don't know exactly which EOS adapter you have (there are many, but afaik the ZWO ones are for T2 and as such unsuitable for FF). Like Nick I had similar problems with my Samyang 135, and replace the EOS flange with and M48 one and then worked my way through the backfocus calculations as for a telescope. But for the Sigma this is not an option. My approach would be the shortest possible EOS-M54 adapter, followed by the new ASKAR backfocus adjuster, followed by the camera - then you can use the ASKAR to adjust the BF on sky using NINA/Hocusfocus. it has to add up to 44mm, 17.5 are used by the 2400, leaves you with 26.5mm. The ASKAR needs at least 16mm, the shortest EOS adapter I could find is 10mm, so it will be tight. and the 10mm EOS adapter is only to M48, so you would need a zero backfocus M48-M54 adapter. They exist, but are not common, basically a thin ring with an M48 thread inside and M54 outside. Alternatively you could put a Neumann CTU in between to find the best back focus (it is much easier to adjust than to unscrew, adding shims, screw again ...) - the come as thin as 11.3mm - and then when the proper backfocus is found with hocusfocus replace it with a proper distance ring (this is how I optimized my Samyang 135). Matthias |
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What great answers from @Nick Grundy and @GalacticRAVE . Thank you so much Matthias and Nick. It is good to know that it is not just me. Matthias, your answer was particularly helpful. Yes, I am using the T2 adapter, which is not great for a FF sensor. My mosaic fields have a fair degree of overlap, but this vignetting does hurt the SNR in the overlap areas - particularly where you need it. So it is certainly time to ditch these adapters. I hadn't heard of the ASKAR backfocus adjuster, but it sounds really great. However, as you say with this set-up it is rather a tight fit, and is quite an expensive solution . Thank you so much Matthias and Nick. It is good to know that it is not just me. Matthias, your answer was particularly helpful. Yes, I am using the T2 adapter, which is not great for a FF sensor. My mosaic fields have a fair degree of overlap, but this vignetting does hurt the SNR in the overlap areas - particularly where you need it. So it is certainly time to ditch these adapters. I hadn't heard of the ASKAR backfocus adjuster, but it sounds really great. However, as you say with this set-up it is rather a tight fit, and is quite an expensive solution [once you include the new thin EOS adapter and conversion ring (which doesn't seen to be readily available anyway). But your post set me thinking. I do have an 11.5mm EOS-M54 connector [designed for the ZWO 2in EFW and I have just purchased 6mm, 7.5mm and 15mm M54 extensions with 0.15mm, 0.3mm, 0.5mm and 1mm spacers. With the 17.5mm plate-sensor distance in the camera, this should allow me to produce back focal distances of between 42.5 and 45.5mm at increments of around 0.15mm. This is a rather brute force method, but it is a bit less expensive and should keep me occupied for a bright night or two! Will keep you posted on the results. Many thanks again Brian |
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ASKAR backfocus adjuster i had one of these, because it seemed like a great idea. Be careful not to put too much torque on it though. Mine is in 5-6 pieces now. |
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Hi Brian, You can get a ZWO M54 filter drawer and use the flange from that on the Canon filter drawer. You may need to slightly adjust the spacing because the two are of slightly different thickness. |
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| I think it'd be great is they'd sell M54 of M48 replacement flanges for most of the lenses out there. I ended up designing my own for the Zeiss Milvus 135 I want to use more, but getting it made from a cnc shop is almost $200. |
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Technically you can get the M48 adapter made for the Samyang 135 (Canon version), drill new holes on it and match it to a Nikon (as an example). The issue is that you then have to use lots of spacers to get the incremental backfocus correct. Some lenses also don't focus to infinity, check this carefully with a DSLR on distant objects. I have several Samyangs now and two out of three actually manages to focus to infinity or beyond. I modded my 135mm although it didn't need it, my 85mm F/1.4 seems very good but a 35mm F/1.4 I got can't even focus on a mountain in the distance, let alone stars. I don't know about the QC on Sigma, but it could be something to check. Certain lenses are just can't handle it and will fringe, produce coma and other abberations no matter what backfocus adjustments you make. For imaging at F/1.4 it requires extraordinary precise collimation of the lens elements as well as backfocu/tilt adjustments. Make sure there's actually any benefit from shooting at those apertures as opposed to F/2 or F/2.8 in the first place. Faster is better but only till a certain point where you actually start loosing signal. |
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Nick Grundy: It would wouldn't it. But I guess we are a small, specialized hobby - and we are probably lucky to get what already have! Brian |
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Jan Erik Vallestad: Hi Jan Erik, Good points. But in this case I did check with my DSLR. The Sigma lens was able to focus on infinity (at the infinity marker on the barrel) with sharop stars across the field at f1.4. Pretty sure it is the ZWO adapters that are the weak points in the system CS Brian |
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I have been dealing with the same problems, coupling the Sigma 40 mm EF to ASI6200MC. Initially, I purchased a ZWO EOS adapter, but this has M42, so it was vignetting like crazy and the stars were ugly in the corners. There is an M54 drawer, with holes/threads at the back side, so you can switch it and directly connect to your camera at M54, but the vignetting was not solved and the stars were even uglier than before. Here is some discussion about that: https://bbs.zwoastro.com/d/16529-eos-flange-to-m54-interface First I found out, that the petal lens hood must be aligned with the sensor. In the case of DSLR, it's always in the correct position, but not if you screw the drawer with the camera. This solved the vignetting. Then I realized that the M54 is 0.5 mm shorter than the M42 back ring. The infinity was not at the mark on the barrel. I purchased the M54 rings and experimented with back focal distance https://www.baader-planetarium.com/en/m54-fine-adjustment-rings-(03--05--1-mm)-aluminium.html I tried many different combinations - and concluded that without the filter I need a 1 mm spacer, with NBZ 1.5 mm.  The stars in the corners are anyway not perfect, but BluXterminator can fix it. |
