Going back to 135mm

There is a new Sigma 135mm F1.4 for full frame; it is quite expensive, but judging by the reviews of its sibling the 105mm f1.4, it could stand as a promising lens for astrophotography.

andrea tasselli · Oct 9, 2025, 08:49 AM

There is no comparison, the 135mm wins hands down.

Why is that?

andrea tasselli · Oct 9, 2025, 06:13 PM

One is a f/2 67.5mm clear aperture lens with potentially no significant aberrations on APS-C format (at f/2!) the other one is a tiny refractor of 40mm aperture @f/4.5, that is nearly two stops of aperture more for the Samyang. I mean my guiders are bigger than that!

Thank you for the explanation! My only concern is there are too few objects for that wide, only thing I can think about is Orion with part of Barnard ring, and rho ophiuchi area. I’ve looked into these areas many times with longer FL, do you think the fast but wider one still worth?

While I don’t have any of your scopes as to compare to a Samyang/Ronikon 135, I do own a Samyang 135. Love that lens. I have several images on my page with it. Those images were well received with likes from the AB community. I have used it with all of my cameras and delivers great data. I don’t know if I would want it as my only portable rig, however, I take it with me every time I travel to image. There is always a widefield target to be captured. I also notice interesting things in a wide field, usually because those items are out of the FOV of captures taken with a “traditional” FL for the target…

CS, Tim

I’ll echo what Arnie said (and btw Arnie, I’d love to get a copy of your target list too). I’ve posted a few photos with this rig; the most recent had the Garnet Star Nebula (IC 1396), the Flying Bat and the Giant Squid in one frame. It’s such a treat to be able to image the Rho Oph nebulosity and many other huge objects without having to create a mosaic; and the f/2.0 ratio puts a lot of faint objects within reach (Spaghetti Nebula, anyone?).

If you buy this lens, I highly recommend you also get the ringset custom designed for it by Astrodymium (available from Agena); it’s inexpensive, greatly simplifies the process of attaching it to a mount, and provides an attachment and belt drive for a ZWO autofocuser.

I downloaded a setup for the Samyang 135 from Thingiverse. Had my friend 3D printing it up and have been very happy with this setup. No off axis weight to contend with.

📷 20230910_123222.jpg

Wow! What a difference! I guess I’ll stick to the commercially produced gears, then.

I wonder if the outer surface of the 135mm where the belt rides has a more robust texture than my old 180mm Nikon. It was so frustrating having the belt slip on that lens in the cold weather that before I switched to the nFocus I actually put a dab of caulk on the barrel of the lens and pressed the belt down into it!

Adam Drake · Oct 16, 2025, 02:26 PM

I switched from the Astrodymium system to the 3D-printed solution Tim is using, and prefer it over the former since putting the EAF below the lens makes it much easier to balance.

Interesting observation about the focus ring contracting in the colder months to come. I had not considered that and would be interested in sourcing a shorter belt to use instead.

I agree with putting the EAF below the lens, should be more stable as well. Another thing to consider with temperature/environment, is if you happen to leave your gear outside. For a while during the summers I’d just leave our gear out, uncovered, when temps were mild and I knew it’d be clear with no risk. But the UV damaged some 3D printed parts, including one of the gear rings (can’t remember if it was for the Redcat51 or the Samyang), over time. I really noticed the AF routines failing with the gear ring at the start of one winter when temps would still be in the 50’s-60’s during the day, but drop into the 30’s at night. For several nights I remember banging my head wondering what the heck was going on with the AF curves, they all looked so bad or would just fail. Finally I went out to our observatory to watch the equipment while the AF was running and sure enough, the whole thing was just binding up. Luckily I had a belt and cog lying around so I ripped off the geared rings and threw the belt on, no problems after that.

For the belt, unfortunately I had to order 3-4 different ones to get one that fit just right, they’re not expensive, but it is a little hassle. What I would do now is get a piece of string, get the rig fully assembled, then run the string around the lens and down/around the cog on the EAF to get a somewhat close number for the length needed. This is where I got the belts from before, based on my order history I bought 280mm, 320mm, 300mm, and 308mm. But I cant remember which were for the Samyang and which were for the Redcat.

https://a.co/d/9Wn4wM6

My 3D parts were printed using PETG. Imaging from Missouri with 90 degree summer to -20 winter nights have never had an issue with slippage during focus, etc. This system has focus motor belt tension adjustment if you have shrinkage or expansion for the belt… but never an issue with the 3D cog ring on lens…

Printed with an Ender 3 3D Printer

CS Tim

Eric Benedetti · Oct 16, 2025, 06:11 PM

The surface of the 135mm will depend on which version of the lens you buy, I currently have a Cine version which has a smooth outer surface EXCEPT for a toothed ring at one point. The toothed ring makes it easier to incorporate a belt with teeth, but you still have to print something to make sure the belt doesn’t slide off that toothed ring. The previous versions of the 135mm I had were just the normal version, which has a tacky rubber sleeve on the outside with some indentations across the width, that was good enough to give the belt some bite and not slip during autofocus if you got the belt tight enough. I imagine some strong double sided tape or some caulk would also be more than sufficient in the case of lenses with very smooth surfaces, otherwise I’d design and 3d print a toothed ring to attach to the lens to help.

That was exactly the problem I was having. My older Nikon 180mm had the rubber sleeve with very shallow indentations. To keep the belt from slipping when it got cold I had to snug it so tight that I was causing tilt in the image. Adding a dab of caulk allowed the belt to be a little looser but then backlash was a problem. The Rigel Systems nSTEP fixed both problems. In a rare stroke of serendipity, the gear for the Nikon 180mm lens also fit the Roki 135mm (at least the Nikon version).