Samyang 135 not enough back focus

[Phillyo]

Hi all, 

I have the Samyang 135mm lens with the Astro Essentials Samyang lens to m48 adapter (https://www.firstlightoptics.com/adapters/astro-essentials-samyang-lens-to-m48-adapter.html), then a slot in filter drawer (not the Canon to eos lens version, the slimmer one). (https://www.firstlightoptics.com/zwo-accessories/zwo-2-filter-drawer-m42-m48.html) 

However, I'm finding that I cannot rack the focus out far enough to achieve focus. It gets close, but not quite far enough. 

Anyone know where I'm going wrong please? 

[woldsman]

You should be able to achieve back focus as I have the same adapter and a filter wheel in the image train. Could you post a picture of your setup? Possibly you have not removed the 11mm extender if the pairing is with a zwo camera. A pair of callipers to measure distance and a set of spacers should sort this out.

[Budgie1]

So, you need 44mm back focus between the adaptor and the camera sensor.

If you replace the 11mm spacer on the front of the camera with the 16.5mm spacer then you should be at the required 44mm.

6.5mm sensor to front face of camera + 16.5mm spacer + 21mm filter draw = 44mm

[malftobe]

I had same problem very recently with getting close but not close enough with the samyang focus. It’s on the last few pages of the imaging with samyang thread. It may help you. 
 

 

My problem was that the short adaptors don’t screw down all the way onto longer threads, so give an increased backspacing to the expected.  

[Elp]

The focus point is very sensitive, you hardly have to turn it to be in focus and completely out.

[Phillyo]

I've taken some pictures and it looks about right to me? Like I say, it gets super close but it just doesn't quite get there. 

Thank you, 

Phil. 

[Budgie1]

Your sensor if further back into the camera than where you're taking the measurement from:

See if this helps:

Edited July 20, 2022 by Budgie1
Added image

[Phillyo]

Ahhh. So I'm approx 2.5mm too close? 

[Budgie1]

4 minutes ago, Phillyo said:

Ahhh. So I'm approx 2.5mm too close? 

Maybe more.

[Phillyo]

The 16.5mm spacer doesn't screw directly onto the camera soooo not sure where to go now lol. I have lots of 'bits' but can't get them to fit together and tighten up. Always something!

Phil

[Phillyo]

Do I need one of these? https://www.365astronomy.com/zwo-m42-m48-adapter-16.5mm-extender-ring

[Budgie1]

I need to look at my ASI294MC & filter draw so this is just an idea but, would the filter draw screw directly onto the camera and put the 16.5mm spacer on the lens side of the filter draw? 

[Phillyo]

4 minutes ago, Budgie1 said:

I need to look at my ASI294MC & filter draw so this is just an idea but, would the filter draw screw directly onto the camera and put the 16.5mm spacer on the lens side of the filter draw? 

Hmmm, just tried and it's still not fitting. The males and the females are all the wrong way around  

[Elp]

I've got the eos filter drawer and this is the measurement prior to the cooled camera (183mm pro):

Needs a bit of fine adjustment but add in the 6.5mm base of camera thread to sensor dimension and around 0.3-1mm added to the 44mm required for a filter it's near enough there give or take a fraction of a mm.

So 44mm + 0.3-1mm for filter, minus 37.6 above, minus 6.5mm = 0.2 to 0.9mm left to add to the spacing, on my last test stars were slightly elongated at the edges.

[Phillyo]

1 minute ago, Elp said:

I've got the eos filter drawer and this is the measurement prior to the cooled camera (183mm pro):

 

Needs a bit of fine adjustment but add in the 6.5mm base of camera thread to sensor dimension and around 0.3-1mm added to the 44mm required for a filter it's near enough there give or take a fraction of a mm.

So 44mm + 0.3-1mm for filter, minus 37.6 above, minus 6.5mm = 0.2 to 0.9mm left to add to the spacing, on my last test stars were slightly elongated at the edges.

Hi Elp, thank you. I use a different filter drawer to that one as I've replaced the Samyang EOS connector for a screw on one for a more rigid attachment. I used to use the one you have above and I could get to focus with it, but now I have the Astro Essentials Samyang Lens to M48 Adapter I can't quite get the back spacing. I think I need a 16.5mm adapter somewhere in there rather than the 11mm one but I'm just trying to get it all connected up and play nicely  

Phil

[Elp]

Can you remeasure. From the back of the lens mounting face (it should sit flush onto the adaptor face, if not from the back of the lens mounting face anyway) to the front face of your camera (not the sensor "nose" face, the face of the body of the camera).

Edited July 20, 2022 by Elp

[Phillyo]

14 minutes ago, Elp said:

Can you remeasure. From the back of the lens mounting face (it should sit flush onto the adaptor face, if not from the back of the lens mounting face anyway) to the front face of your camera (not the sensor "nose" face, the face of the body of the camera).

Like this? 

 

That says 38mm. Bad reflection. 

Edited July 20, 2022 by Phillyo
Added distance.

[Elp]

Thanks.

You're currently at 44.5mm atm, add 0.3mm for a filter and that's 44.8, add 1mm for a filter it's 45.5mm, so you need to reduce the distance by a range of 0.8 to 1.5mm if using a filter. If no filter then you need to reduce by 0.5mm.

I've found that purchasing a set of astro essentials m42 spacer sets imperative for backspacing as I'm always using different setups so need to adjust this all the time, I think your camera ring attached to the zwo is 11mm depth, so a 10mm will get you near enough. For very fine adjustments you can get 0.1-0.2 and 0.5mm spacer rings, I got some free with one of my adaptors, you can simply put them over the nose of the camera and tighten up your rings onto it.

Just thought, if you don't use the 11mm zwo ring, you'll need some sort of thread converter for the m42 spacer as the camera is male, and the filter drawer thread is also male, unless if you can shorten this on the lens side.

Edited July 20, 2022 by Elp

[Elp]

Just out of curiousity, how are you assessing the focus (asiair, software on a screen, bahtinov)?

[Phillyo]

1 minute ago, Elp said:

Just out of curiousity, how are you assessing the focus (asiair, software on a screen, bahtinov)?

I was using the asiair last night but I have a bahtinov that I can use next time it's clear. Like I say, it's super super close but just not quite there. 

I have a bunch of those spacers but I can't get them to connect in. I have the wrong male/female situation going on as stated above. I need to have a good look through all my kit to see if I have any other connectors/adapters to see if I can get it to fit together. But I'm not confused that you're saying I need to reduce the distance a little bit, but Martin is saying I need to extend it a bit lol. I'm obviously missing something obvious but I can't figure it out. I'm supposed to be WFH at the moment so don't have much time to check other than dashing downstairs for measurements n stuff but thank you both for your input so far.

Phil

[Elp]

Asiair focus routine is fine. Once I used it I didn't bother with a bahtinov as the focus routine provides a number when focusing the star size and is not subjective.

[Phillyo]

Also, the Antlia ALP-T filter is supposed to be 2mm thick if that makes any difference while inside the filter holder?

[Phillyo]

10 minutes ago, Phillyo said:

I was using the asiair last night but I have a bahtinov that I can use next time it's clear. Like I say, it's super super close but just not quite there. 

I have a bunch of those spacers but I can't get them to connect in. I have the wrong male/female situation going on as stated above. I need to have a good look through all my kit to see if I have any other connectors/adapters to see if I can get it to fit together. But I'm not confused that you're saying I need to reduce the distance a little bit, but Martin is saying I need to extend it a bit lol. I'm obviously missing something obvious but I can't figure it out. I'm supposed to be WFH at the moment so don't have much time to check other than dashing downstairs for measurements n stuff but thank you both for your input so far.

Phil

I bought the M48 exention tube set. I'm not wondering if I should've got the M42 set instead...

[Elp]

Hard to say, backspacing is a pain if you don't have the right bits at hand, the ZWO filter drawer is m42 on the back end and m48 on the lens side.

[Phillyo]

I think I've got it pretty close, so I'll check it next clear night. 

6.5mm from sensor, 11mm extension, 21mm filter drawer, 5mm extender. = 43.5mm. Add on my 2mm filter, which adds about 0.6mm, that gives 44.1. I have some little delrin spacers I can add for final adjustment. Hopefully that'll do the trick!

Phil