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Hello everyone. After a few years photographing small things, I decided to capture larger fields. I purchased a Samyang 135 f/2 lens and a Sigma DG HSM 50 mm f/1.4, which I have been able to use with a modified Canon EOS 60D. The problem is that now I try to use these lenses with astrocameras and narrowband filters, and here the trouble begins. The backfocus of Canon lenses is 44 mm, which is not generous. The ASI 1600 MM has only 6 mm metal-sensor distance, and with an adapter that has a built-in manual filter holder, I have been able to adjust to this distance. But in my first shots with the 50mm lens, the stars are a disaster even near the center, much worse than with the Canon. Before continuing to battle, I wanted to know if anyone has experience on the subject. I consider some possibilities: |
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It's doable and in my opinion very rewarding. I took this image of the Horsehead and Barnard's Loop region last month with a Sigma Art lens through a mono camera and filter. Read the description to see what I did to get around the star aberrations caused when shooting wide open. As you'll note, I had to use the Astromechanics adapter to get this result because my lens also does not have manual iris control. The Astromechanics adapters are a godsend for lens users. They don't just allow iris control but also accurate and repeatable autofocus. Thankfully, they are back in business now. It took me a few weeks to get my adapter to ship from Russia to the US. W |
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Although I don't use it anymore, I used a Canon 200 mm custom setup for quite some time. I would suggest using the lens wide open and then buying some metal screw on lens caps (Ebay) and then boring a hole in the cap of the proper size on center for whatever F ratio you want. That gets rid of the diffraction spikes from the iris. Only N/A if it is wide open (which normally they are not due to aberrations). If you have access to a 3D printer you could also probably 3D print the aperture stop as well. Example Image |
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| Hello, Jose. The Samyang 135mm F2 is a great pairing with a dedicated astro camera. Here is an NB image captured with the Samyang 135mm and ZWO ASI183 2023-11-24_08-45-15.jpg and here is what the setup looks like RLx2IfW2kbyS_1824x0_esdlMP5Y.jpg. It is all connected via a 3D-printed mount from Thinkable Creations Hyperpod 135 |
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Camera lenses works very well. I'm a Nikon user, so certain Canon specifics I can't really comment too much on but here goes: 1) Doesn't Canon have the possibility to pre-set the aperture on a camera body before moving it to a dedicated camera? I don't know for sure but the thing that might be the biggest issue could be the actual focusing, if it's relying on a in camera focus motor like the Nikon G lenses. 2) Backfocus is extremely sensitive with camera lenses, the faster the optics are the more sensitive the issue becomes. The optimal starting point would be the lens flange focal distance + 1/3 of your filter thickness. For Canon that would be 44mm initially, but if you used filters with 3mm thickness you would need an additional 1mm to get it right. This can be time consuming since each 0.1mm makes a huge difference in my experience. Especially if you are using F/2, even more so at F/1.4 if you intend to use it wide open. Once you've got this down there's the possibility of tilt, although with a small-ish sensor that should be less of a problem. Lens adapters that aren't firmly holding the lens (read wiggle room) can possibly be an issue as there's a bit of weight hanging of the back of it. A side note to this would be filters as faster optics require filters made for fast f-ratios, otherwise you won't actually gain anything at all by using it wide open. 3) The Samyang has a good reputation and works extremely well. Nikon F mount has a bit more room, but I use my lenses with a Nikon adapter from ZWO and a EFW for my filters. My Samyang is held by the Astrodymium kit. I'm sure the same thing is possible with Canon as well. |
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1) Doesn't Canon have the possibility to pre-set the aperture on a camera body before moving it to a dedicated camera? I don't know for sure but the thing that might be the biggest issue could be the actual focusing, if it's relying on a in camera focus motor like the Nikon G lenses. Yes, you set the aperture while on the DSLR and then remove the lens while the camera is still switched on and it will stay as set. BUT, If you are anything but wide open you will get many spikes on bright stars from the iris which, with the FOV of lenses, can be distracting. That is why an aperture fabricated to a higher f ratio is best - slower but no spikes and better corner stars. |
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I too have been wondering about this. I have an adapter from the Canon EF mount to my ZWO camera. The lenses I was planning to try are Canon EF 400mm f/5.6L USM and Canon EF 200mm f/2.8. On my scopes, I use a Pegasus motor focuser and use Hocus Focus in NINA, but presumably for camera lenses you have to focus manually with a Bhatinov? Thanks |
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| @Stuart: manually or with the ASCOM Canon Lens Controller from https://astromechanics.org |
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Bill McLaughlin: That's what I thought, but I don't know Canon that well so I wasn't sure. To be fair, the star spikes from the Samyang are quite small and unnoticable even at F/4. That's why I'm personally doing F/2 for background and up to F/4 for stars. Some get pinpoint stars even at F/2 or F/2.2 but that's probably going to take patience with both backfocus adjustments and getting a sample with very good QC. Another option could also be to simply get a step down ring, I have no experience with those for this particular use though. |
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Bill McLaughlin:1) Doesn't Canon have the possibility to pre-set the aperture on a camera body before moving it to a dedicated camera? I don't know for sure but the thing that might be the biggest issue could be the actual focusing, if it's relying on a in camera focus motor like the Nikon G lenses. Just as a note, not all lenses can do this. On my Sigma Art 105, attempting any kind of aperture mask just blackens the edges of the frame. Thankfully the diffraction spikes are not obtrusive on my lens so it’s no issue. |
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Stephan May: Fantastic! I didn't now such a device existed. But it figures! Thank you |
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| Set the aperture of the lens on a Canon body, set to a 30' exposure, remove lens during the exposure- it stays there. |
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Thanks for the answers. They have been very useful to me. In summary: 1) The 44 mm backfocus must be very precise, it is difficult to achieve and in no case is it compensated by good focus. 2) For lenses without manual aperture adjustment, we can adjust it by removing the lens from the camera while it is connected (version 1) or while taking a long shot (version 2). 3) Limiting light entry with an additional diaphragm in front of the lens (cap with hole) can work, but it can also cause significant vignetting. Fixes diffraction in stars for those who find it bothersome. 4) Controlling the lens from a computer with a device like the one from Astromechanics works well and is worth it for controlling the diaphragm, but also for focusing, being able to autofocus. In my configuration I have: lens + ZWO adapter with filter holder (26.4mm) + T2 female-female thread adapter (9mm) + ASI1600MM camera (6.5mm to sensor). Total 42 mm. With the appropriate rings you should achieve good backfocus, and with the lens mounted on the camera, reduce the aperture to f/2.8 or f/4. For the Astromechanics device I see a problem. It measures 17 mm, which is little, but it is too much. With the ZWO filter wheel there is no problem, as it measures 20 mm and has two T2 female threads. The chamber has a male thread. But manual filter boxes (16mm the narrowest) have a male thread on one side and a female thread on the other. To screw them to the camera you need an adapter, which measures 9 mm. They are already 17+17+9-6.5=48.5 mm. It seems that with the Astromechanics device I will be forced to use the filter wheel and buy 1.25” narrowband filters. Unless someone knows a filter holder box with M42 female-female threads. The truth is that the Astromechanics controller seems very useful and is not overpriced. |
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José Manuel López Arlandis: You can use the 17mm M42 Astromechanics adapter with the Starizona filter holder/drawer designed for RASA, which is M42 F-F. This puts you around 43mm total, so you'll need to add some spacers. You can also ask Sergey (owner of AM) if he can design an 18mm adapter so you don't have to mess with shims too much. Another option is an ASG Photon Cage with the ASG filter drawer. The entire thickness of this whole assembly is only 19mm and can be adjusted by a few mm using the photon cage adjustment bolts. This may be a better option overall as you'll be able to precisely control tilt and backfocus without any shims at all. I've thought about this option myself for my 2600MM and might ask Sergey for a custom threaded adapter for it. |
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Or you can DIY an adapter - I made mine from an inexpensive modified Pentax K C-mount attached to a T2 adaptor with locking ring to allow me to fine tune backspace. My most recent image with manual Pentax 70-210 zoom lens 150mm at f5.6 and Altair 183 OSC camera.  |
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| I gather the Astromechanics ASCOM adapter is made in Russia and so cannot be purchased any longer outside Russia due to sanctions |
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Stuart Taylor: |
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Stuart Taylor: No. AM will ship to the US now. I just bought an adapter last month - took about 3 weeks to arrive. Contact Sergey and he will guide you accordingly. |
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Stuart Taylor: Sorry, just to be clear I wasn't thinking Sergey would be reluctant to ship. My concern was whether it would be legal for a buyer to buy goods from Russia at the present time without risking sanction-busting. |
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Stuart Taylor:Stuart Taylor: Yep, I understood you 🙂 As I understand, AM was out of commission for a while because Paypal and other services left Russia and there was no way to send money. Alternative methods are available. The products will clear customs (at least for US buyers) just fine. Plenty of people overseas are still buying goods from Russia (e.g., Vostok watches). |
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Stuart Taylor:Stuart Taylor: Good enough for me! Thank you |
