Samyang 135mm - advice on how to setup

https://www.pierro-astro.com/materiel-astronomique/accessoires-astronomie/roues-%C3%A0-filtres-et-accessoires/support-filtres-%C3%A0-tiroirs-pour-objectif-canon-eos-zwo_detail

You can use something simple like this, or you also Can found an adaptator available with filter wheel 

https://astronomy-imaging-camera.com/product/nikon-lens-adapter-for-2filter-wheel/

You can either print something yourself: https://www.thingiverse.com/thing:4761672
or you can buy something like this from TeleskopService (it comes with everything you need): https://www.teleskop-express.de/shop/product_info.php/info/p14230_Wega-ZWO-EAF-Adapter-mit-Schelle--Schiene-und-Sucherschuh-fuer-Samyang-135-mm-Objektiv.html
The Option from Eteocles looks nice aswell.

Check the Samyang Equipment page, I have uploaded pictures and stl´s for 3D printed parts there.

https://www.astrobin.com/forum/c/equipment-forums/samyang-135mm-f20-ed-umc/zwo-eaf-adapter-ring/

For focussing the Zwo autofocusser works really well, the zwo filter holder for Nikon/Canon allows you to easily use filters and to adapt to your cooled camera.

I think vignetting is quite pronounced even though I only use an APS-C sensor but it can be calculated out well with flats.

Michael

@Jan Erik V , do you have more information on how to directly adapt the canon version to 48mm?

Many thanks! I found a good priced cannon version of the Samyang, hope it is still available… Beeing able to directly adapt to the camera is a big plus!

Michael

James Tickner:
Interested to hear people's thoughts on the best adapter to connect lens and camera.

I have the Nikon version of the Samyang 135 mm and recently bought a RisingCam APS-C format cooled camera. I grabbed a ZWO Nikon lens adapter which *should* do the trick. The optical thickness of the adapter is 29 mm which with the 17.5 mm offset of the camera should exactly match the 46.5 mm back focal distance of the lens. However, analysis of star shapes in ASTAP is showing field curvature which I don't see when I mount the lens to a Nikon DSLR. Shimming the lens out a touch only increases the curvature, indicating that I really need to get the camera closer to the lens which is impossible with the adapter I have.

There also seems to be 'flex' in the adapter, which introduces tilt as you point to different parts of the sky. It's not helped by the fact that the camera is fairly heavy.

Interested to hear if anyone else has seen these issues and managed to overcome them.

I use the ASI533 so I don't get any severe issues regarding tilt, but I have noticed some very slight movement in the adapter. However my adapter is 20mm, then the EFW is another 20mm, + 6.5mm from my camera. My filters are 3mm thick so I have resorted to adding one of the smallest shims.

https://www.astrobin.com/1ds9r7/

This is the image I made from my test session, four frames, the squid was shot with no adjustments and the elephant trunk was done with the shim inserted, the opposite corners were also made with various thickness shims. I used the back focus spacing guide from OPT as a guide:
https://www.google.com/url?sa=i&url=https%3A%2F%2Foptcorp.com%2Fblogs%2Fdeep-sky-imaging%2Fhow-to-set-the-correct-back-focus&psig=AOvVaw3kEpPAtS4CxIYcFTpqdxl5&ust=1691669063424000&source=images&cd=vfe&opi=89978449&ved=0CBEQjRxqFwoTCMCntb_Ez4ADFQAAAAAdAAAAABAE

I haven't done any tests as to how far i can push the lens for good stars but shot mine at F/4, the nebula was shot at F/2. What I can say though, is that the slight movement in the adapter didn't matter on my sensor - the backfocus however did. The squid has a lot of issues compared to the trunk in my image. That said, your sensor is bigger and would suffer more from any inaccuracies.

Does the RisingCam have a fixed 17.5mm backfocus or can you remove parts to get 6,5mm? Getting a holder ring for the camera is one way to go, I have one but it's not compatible with the Astrodymium unless I print parts that matches exactly the height from the dovetail. It has to be very precise.

Michael Ring:
James, you have a very similar config to the one I have, you should use Nina's abberation inspector to properly shim the lens, with this tool it is very easy to get the distance from the sensor properly dialed in. If you want I can look up how much extra shimming I needed, perhaps this info is already good enough to bring you in the right ballpark.

And yes, flex is an issue with the adapter, I wanted to fix the issue by 3d printing another holder for the camera itself, now I will very likely buy a quite cheap Canon Version of the lens on Monday and will use the 48mm adapter that Jan pointed me to...

Michael

The problem is that I need a 'negative' shim thickness! In other words, I have curvature with no shim that gets worse if I shim even a little bit, implying that I need to get the camera closer to the lens. I bought the camera with the AR glass and have added a UV/IR cut filter inside the ZWO adapter, so I wonder if that is the cause of the problem ...

I also came across the Geoptik adapter here Teleskop-Express: Geoptik Adaptor for Nikon lenses to T2 for CCD cameras - w/ 1/4" thread

This seems to have quite good reviews and also has the advantage that the optical distance is only 21 mm, so I could add T2 adapter tubes and shims to get exactly the back focal distance I need. Unfortunately though it only takes a 1.25" filter which seems a bit small for an APS-C sensor. 

Has anyone tried this adapter?

I have both the Canon and Nikon ZWO filter drawer/adapters.  Mine are both pretty solid, though the Nikon one has a tiny bit of slop in the lens connection.  Nowhere near as bad as my QHY lens adapters or the old ZWO lens adapter. 

If you are doing OSC with a filter I think the ZWO filter drawer/adapter is a must-have.  For a FW it gets more complicated, particularly for APS-C.  You'll need 2" filters and those FWs are pretty heavy.  I'm always wary of trying to hold that much weight in a PETG kit.

James Tickner:

I use the ASI533 so I don't get any severe issues regarding tilt, but I have noticed some very slight movement in the adapter. However my adapter is 20mm, then the EFW is another 20mm, + 6.5mm from my camera. My filters are 3mm thick so I have resorted to adding one of the smallest shims.

... 

I haven't done any tests as to how far i can push the lens for good stars but shot mine at F/4, the nebula was shot at F/2. What I can say though, is that the slight movement in the adapter didn't matter on my sensor - the backfocus however did. The squid has a lot of issues compared to the trunk in my image. That said, your sensor is bigger and would suffer more from any inaccuracies.

Does the RisingCam have a fixed 17.5mm backfocus or can you remove parts to get 6,5mm? Getting a holder ring for the camera is one way to go, I have one but it's not compatible with the Astrodymium unless I print parts that matches exactly the height from the dovetail. It has to be very precise.

Which adapter are you using that is 20 mm thick? That could be an option for me with a T2 extension tube. Does the adapter allow a 2" filter to be inserted.

Unfortunately I think there is no backfocus adjustment on the camera itself. There are four mounting bolts on the front surface but they are clearly labelled DO NOT OPEN and I'm not game to break a brand new camera just yet 

The idea of 3D printing a cradle to hold the camera on the correct alignment is a good one, but I was hoping to find an adapter that is naturally stiff enough to make this unnecessary. Maybe this is too optimistic!

Yeah, I wouldn't open it. I didn't think of the tilt plate on those cameras. The adapter I have is for the 1.25" or 31mm filter wheel so there's no filter drawer there unfortunately. https://www.firstlightoptics.com/zwo-accessories/zwo-efw-nikon-lens-adapter.html 

They also make this one: https://www.firstlightoptics.com/zwo-accessories/zwo-nikon-lens-adapter-for-2-filter-wheel.html

I don't know if any of those would make a reliable "MacGyver solution" to your problem somehow, but if you're looking for a factory made option I don't know of any. I would however try it without the filter just to see how it affect things. It seems very strange that the exact focal distance seems to be too long. Have you measured everything to see if it all checks out and ruled out any inaccuracies within the different parts?

Christian Bennich:
I have ordered the Wega option from Teleskop-Express…I don’t have a 3D printer 😉
and also the M48 option as the Samyang lens I have is the Canon version. 
Then I need to figure out what to do with my 36mm ZWO EFW and backfocus etc. 

I can see that it might not be so simple. 

With the Samyang, don’t I still need 55mm backfocus from the lens to the ASI2600?

Backfocus is 44mm for Canon EF.  At f/2 you will also need to take into account any filter thickness.  So, if you have a 2mm thick filter you'll want to add 0.667mm to the backfocus using shims, if possible.

Thanks guys. I love this - it’s a learning experience 🤪🤪

Torben van Hees:
I see you ordered the Wega kit. Be sure to test it quickly within the return window. I‘ve had 2 out of 2 Wega parts fail. This has been some time, so maybe they moved to a different procedure since then.

Thx @Torben van Hees, great info. 
Any particular part that was more prone to failure?