Samyang 135/f2 Focus

[groberts]

I'm planning to fit my Samyang 135/f2 to a ZWO ASI1600MM-Cool camera via a ZWO x8 EFW with ZWO filters using screw in M42/T2 adapter between the lens and EFW. 

I'm fairly sure that someone will have this popular set-up and would appreciate guidance on focussing - will I need any spacers etc to achieve focus, if so what thickness?

Graham 

Edited April 15, 2022 by groberts

[newbie alert]

My understanding is the backfocus of a canon version is different to the Sony.. so better clarify what one you have otherwise its not going to work

[groberts]

2 hours ago, newbie alert said:

My understanding is the backfocus of a canon version is different to the Sony.. so better clarify what one you have otherwise its not going to work

Not sure I understand your question but if you mean the adapter it's an NSA M42/T2 which replaces the Canon fitting fitting on the lens and screws directly into the EFW providing a more stable connection betwen the lens & EFW:  https://ensoptical.co.uk/index.php?route=product/product&product_id=13220

Otherwise everything else is standard ZWO camera and EFW.  Hope that helps?

Graham 

 

Edited April 15, 2022 by groberts

[Tomatobro]

If its the Canon version of the Samyang 135 lens then you are good to go with the adapter. The Sony version, while being a lot cheaper to buy used requires a special adapter to get the correct back focus

[groberts]

Good news, thanks.

Whilst I'm on the subject, can anyone recommend a set of step-down rings that will fit this Samyang 135/f2  lens?

Edited April 15, 2022 by groberts

[newbie alert]

6 hours ago, groberts said:

Not sure I understand your question but if you mean the adapter it's an NSA M42/T2 which replaces the Canon fitting fitting on the lens and screws directly into the EFW providing a more stable connection betwen the lens & EFW:  https://ensoptical.co.uk/index.php?route=product/product&product_id=13220

Otherwise everything else is standard ZWO camera and EFW.  Hope that helps?

Graham 

 

If you bought that product then you have a canon fit samyang? As it says in the blurb.. it also says about the 44mm spacing, Sony has a much shorter backfocus so if you required to use it with a filterwheel then you may not have enough room..

[groberts]

OK thanks, I completely understand the principal but need to clarify some details please:

• Is the 44mm back focus taken from the flange or the rear optics of the lens? 
• Either way, it seems to mean a spacer of about +/- 17.5mm:  44mm minus (camera sensor to EFW 6.5mm + EFW 20mm).  If this is correct can somebody point me to a suitable source of spacer i.e. M42/T2 male + female, to screw onto the lens adapter at the front and the EFW behind.  I'm thinking slightly shorter so that I can then use thin spacers to finesse the distance if required, if that makes sense?

Would still like to hear from anyone who has the ZWO 1600 camera + ZWO EFW + Samyang 135 set-up and what their experience and solution to this issue was?

Graham         

[aleixandrus]

Hi @groberts! Have you found a valid solution for your setup? I'm exactly in the same situation so any information would be welcome.

My equipment is: ASI183 (6.5mm) + EFWmini (20mm) + Samyang 135mm f2 with Canon mount replaced by M48 adapter

[ollypenrice]

A cheap alternative to step-down rings...

You also have an infinite choice of apertures.

Olly

[geeklee]

5 hours ago, aleixandrus said:

My equipment is: ASI183 (6.5mm) + EFWmini (20mm) + Samyang 135mm f2 with Canon mount replaced by M48 adapter

I have exactly this on my Samyang @aleixandrus ( @groberts )

As stated, it's 44mm from the M48 adapter.

ASI183 (6.5mm) + EFWmini (20mm) + 16.5mm ZWO adapter (from ASI183MM box) = 43mm

1mm spacer and something for your filters (0.3 x filter thickness).  This will give you 44.Xmm where X is covering for your filter thickness.

With my Astronomik filters (1mm thick) this is shown below - Baader 1mm aluminium spacer (gold), 0.3mm spacer, 16.5mm ZWO adpater.  Then it's just the mini EFW and camera.  This brings focus in under "L" on the focus ring for me.  

As mentioned elsewhere, back focus is critical on the Samyang and 0.1mm can throw the focus point out considerably.

There is a huge thread on SGL discussing the Samyang 135.  It contains images and tips on using it in a variety of ways:

 

[aleixandrus]

3 hours ago, ollypenrice said:

A cheap alternative to step-down rings...

Olly

Yeah, I already have considered doing that... but once I have all setup working 

 

1 hour ago, geeklee said:

I have exactly this on my Samyang @aleixandrus ( @groberts )

As stated, it's 44mm from the M48 adapter.

ASI183 (6.5mm) + EFWmini (20mm) + 16.5mm ZWO adapter (from ASI183MM box) = 43mm

1mm spacer and something for your filters (0.3 x filter thickness).  This will give you 44.Xmm where X is covering for your filter thickness.

With my Astronomik filters (1mm thick) this is shown below - Baader 1mm aluminium spacer (gold), 0.3mm spacer, 16.5mm ZWO adpater.  Then it's just the mini EFW and camera.  This brings focus in under "L" on the focus ring for me.  

As mentioned elsewhere, back focus is critical on the Samyang and 0.1mm can throw the focus point out considerably.

There is a huge thread on SGL discussing the Samyang 135.  It contains images and tips on using it in a variety of ways:

 

Oh, thanks a lot! Huge help. My math said that but it is good to know it is right! Let me check all your info carefully, I may have some issues with that extra 1mm I need to check.

By the way, I'm aware of the mega-thread about the Sammy but it is so large it takes me a while to read and extract the most important info. But I'm on the way

[geeklee]

11 hours ago, aleixandrus said:

I may have some issues with that extra 1mm I need to check.

The Baader solution is expensive on the face of it, but they seem high quality and very rigid considering the thickness.  I got a pack of M48 ones which includes three thickness options.

11 hours ago, aleixandrus said:

But I'm on the way

Yep, it's a beast!  In hindsight perhaps some tagging on posts would have been helpful so people can search for just "spacing" stuff etc!  Good luck, I joined it a bit earlier so have managed to read a lot as it was added.  Was a great read to get up to speed though.

[900SL]

I've asked this previously but not sure I had a reply.

If using step down rings:

1: Does this increase the depth of field for focusing (ie similar to stopping down the aperture)?

2: Do step down rings create any issues with flats (ie light reflecting internally and back from the rear of the step down ring)?

 

Thanks!  

[ollypenrice]

51 minutes ago, 900SL said:

I've asked this previously but not sure I had a reply.

If using step down rings:

1: Does this increase the depth of field for focusing (ie similar to stopping down the aperture)?

2: Do step down rings create any issues with flats (ie light reflecting internally and back from the rear of the step down ring)?

 

Thanks!  

Step down rings do stop down the aperture and so increase the depth of field. The stopped down light cone has a shallower angle than the wide open one so the focal plane can be moved forwards and backwards by a larger amount while retaining acceptable focus.

Since the rings are designed primarily for daytime use, which will include aiming at bright blue skies, I imagine they must be effectively blackened against internal reflection.

Olly

[groberts]

23 hours ago, aleixandrus said:

Hi @groberts! Have you found a valid solution for your setup? I'm exactly in the same situation so any information would be welcome.

My equipment is: ASI183 (6.5mm) + EFWmini (20mm) + Samyang 135mm f2 with Canon mount replaced by M48 adapter

I purchased this variable extension locking ring, which with a 13.85 - 18.00mm range is right in the sweet spot, thus providing plenty of scope for fine adjustment. 

https://www.harrisontelescopes.co.uk/acatalog/altait-t2-varex.html#SID=1664

I have set it at 17.5mm from the adapter flange to the EFW, thereby plus the EFW thickness (20m) and 6.5mm back-focus results equals 44mm, which is the general guidance for this lens.  Using it in daylight & some distant trees, it looks like it's close to focus but will probably need some further tweeking when I'm next out at night.

Graham   

[aleixandrus]

Thanks for your input, very valuable. I think my comments are in line with yours but I don't want to 'steal' this thread with more questions so, if you find my questions too intrusive, please, let me know and I'll open a new one (or if any admin considers split this conversation, go ahead).

Yesterday night I set all the equipment outside to do some (very poor) tests. Many things went wrong, as expected, but that was the goal, find out what's wrong. This includes power and cable management, drew protection, better wifi coverage and test software better (I installed NINA on the field 🙈🤣).

Regarding the main point of this thread, I set the following backspace: ASI183 (6.5mm) + EFWmini (20mm) + 16.5mm ZWO adapter + (0.1+0.2+0.2+0.5 spacers) = 44mm, which is most of you pointed out. I tested this without filters and with the ZWO 1.25" L filter, which I think is 1.9mm thick, so I should need ~44.6mm total. I keep the 44mm distance as I didn't have more spacers... I focused using the EAF 12V, manually, until the stars on the laptop screen seemed 'right'. I used ASI Studio just for simplicity.

I've attached a preview of two single shoots: 120s at f2 and f4. No calibration frames nor stacking. What I found is a a strong vignette and worse star shapes @f2 compared to @f4. Brighter stars seems bloated but there was a lot of dew and I didn't use a lens warmer, so not sure if was due to filters quality the automatic STF in PixInsight, focus issues or anything else. Star trailing seems ok to me spite of being 120s with no guiding. Using no filter but keeping the same focus, stars were *completely* out of focus, but I can achieve focus after refocusing. I think 44mm backspace works for me but... What's your impressions? What additional test should I perform? Should I meassure star diameter for better confirmation? What to improve?

Thanks a lot

star-test.zip

[900SL]

Edit wrong thread 😂

 

 

 

Edited May 28, 2022 by 900SL

[groberts]

On 26/05/2022 at 10:33, aleixandrus said:

Thanks for your input, very valuable. I think my comments are in line with yours but I don't want to 'steal' this thread with more questions so, if you find my questions too intrusive, please, let me know and I'll open a new one (or if any admin considers split this conversation, go ahead).

Yesterday night I set all the equipment outside to do some (very poor) tests. Many things went wrong, as expected, but that was the goal, find out what's wrong. This includes power and cable management, drew protection, better wifi coverage and test software better (I installed NINA on the field 🙈🤣).

Regarding the main point of this thread, I set the following backspace: ASI183 (6.5mm) + EFWmini (20mm) + 16.5mm ZWO adapter + (0.1+0.2+0.2+0.5 spacers) = 44mm, which is most of you pointed out. I tested this without filters and with the ZWO 1.25" L filter, which I think is 1.9mm thick, so I should need ~44.6mm total. I keep the 44mm distance as I didn't have more spacers... I focused using the EAF 12V, manually, until the stars on the laptop screen seemed 'right'. I used ASI Studio just for simplicity.

I've attached a preview of two single shoots: 120s at f2 and f4. No calibration frames nor stacking. What I found is a a strong vignette and worse star shapes @f2 compared to @f4. Brighter stars seems bloated but there was a lot of dew and I didn't use a lens warmer, so not sure if was due to filters quality the automatic STF in PixInsight, focus issues or anything else. Star trailing seems ok to me spite of being 120s with no guiding. Using no filter but keeping the same focus, stars were *completely* out of focus, but I can achieve focus after refocusing. I think 44mm backspace works for me but... What's your impressions? What additional test should I perform? Should I meassure star diameter for better confirmation? What to improve?

Thanks a lot

star-test.zip 29.44 MB · 1 download

You don't mention what mount and camera you're using but I presume you're tracking, though + guiding too would probably help to ensure best possible image capture in order to come to any fosussing conclusions. 

I understand that f2 is generally regarded as undesirable for this lens with astro, so your observation between f4 & f2 are perhaps what might be expected; I recall something about this in the mega thread you refer to.  Ordinarily with terrestrial images the depth-of-field increases with decreasing aperture, though as this is being shot at infinity I'm not sure that's relevant?  Personally, I'd track + guide + use the smaller aperture setting f2.8 / f4.0 and see what you get. 

You also don't say how you actually focus?  Whilst many use autofocus, when using this lens with a DSLR I've been successfully using a micro-focus ring from AstroKraken, which allows very fine adjustments + the ability to lock the focus ring setting once achieved.  If you are just focussing by hand + without locking this could also be your problem?

Graham