Samyang 135mm F2 and QHY183M

[SimonWhitfield]

Hi everyone, 

I have been using a QHY183M and QHYFCW3-M-US for a while and I am now trying to connect it to a Samyang 135mm F2. I say "trying to connect it".. it is actually connected and seems to be spaced out correctly - the Samyang is a Canon EF version, so should need 44mm of back focus. 

I have bolted the QHY183M (10mm) directly to the filter wheel (17mm) using a 020063 (1mm) QHY183 to M54 adapter, and used 6mm of spacers before attaching to the QHY M54 Canon adapter (10mm) for a grand total of 44mm.  The filter wheel contains 7 Baader 36mm unmounted filters, the usual LRGBHOS suspects, and the whole set up is held in place by an Astrokraken 3-ring setup on a dovetail.

In theory, as far as I can figure, this should all work fine. 

I have a windmill that I normally use to test focus, Chesterton, 2.5km away that I can see from my kitchen, and the set up above focuses on the windmill at approximately 9m on the Samyangs focus scale... nowhere near infinity. If I move the lens onto my Canon 6DMk2, i have to rotate the lens to infinity to get focus on the windmill...

I have yet to use the Samyang135/QHY183M at night... everything seems to be fine but that focusing discrepancy seems weird. Everyone else I have seen who has one, where focusing discussions are had, talk about the focus being at or beyond infinity. A little I could understand, but to go from infinity being infocus on an object 2.5km away on one camera to 9m being infocus on the QHy183M seems bizarre!

What am I missing? Why does the focus seem to change so much for the same back focus on 2 different cameras?

 

Regards, Simon

[R26 oldtimer]

I think that this should be fine, looks like you have room to focus past infinity. The opposite would be real trouble.

[Alien 13]

It does sound as if your spacing is a little short, did you allow extra for the filters?

It depends how the focusing is done on this lens, some lenses move the whole assembly and others certain elements only, if its the former then it wont matter at all where the focus position is but might do with the latter.

Alan

Edited April 9, 2020 by Alien 13

[SimonWhitfield]

12 minutes ago, Alien 13 said:

It does sound as if your spacing is a little short, did you allow extra for the filters?

It depends how the focusing is done on this lens, some lenses move the whole assembly and others certain elements only, if its the former then it wont matter at all where the focus position is but might do with the latter.

Alan

I did add an extra 1mm, which I think was over-doing it as the Baader filters are 2mm thick, but I also tried with an extra 0.5mm.  The only difference either seemed to make was to move the focus point slightly closer to infinity, but I still have 60 degrees of rotation before I get close to the infinity mark. 

Simon

Edited April 9, 2020 by SimonWhitfield
Name

[Alien 13]

2 minutes ago, SimonWhitfield said:

I did add an extra 1mm, which I think was over-doing it as the Baader filters are 2mm thick, but I also tried with an extra 0.5mm.  The only difference either seemed to make was to move the focus point slightly closer to infinity, but I still have 60 degrees of rotation before I get close to the infinity mark. 

Simon

I admit that its very odd but still think the infinity mark on the lens is telling the truth as shown when connected to the Canon camera.

Have you tried measuring the overall spacing with a vernier?

Alan

[SimonWhitfield]

11 minutes ago, Alien 13 said:

I admit that its very odd but still think the infinity mark on the lens is telling the truth as shown when connected to the Canon camera.

Have you tried measuring the overall spacing with a vernier?

Alan

I have used a digital vernier and it all seems right, even measured the individual bits to make sure they were accurate. I even used the calculator on my phone to add the individual millimetre measurements together, just in case I was having a brain-fart 😆 

I agree about the Canon... that should be the reference point. This is just weird...  

[Alien 13]

5 minutes ago, SimonWhitfield said:

I have used a digital vernier and it all seems right, even measured the individual bits to make sure they were accurate. I even used the calculator on my phone to add the individual millimetre measurements together, just in case I was having a brain-fart 😆 

I agree about the Canon... that should be the reference point. This is just weird...  

It certainly is strange, from memory every bit of glass in the optical train adds on average 1mm to the required backfocus distance so 1mm for the filter and 1mm for the sensor window. It could be interesting to see how much extra spacing is required to get the focus mark in the correct place..

Alan

[fwm891]

Don't know the lens but is there a macro setting?

[SimonWhitfield]

31 minutes ago, Alien 13 said:

It certainly is strange, from memory every bit of glass in the optical train adds on average 1mm to the required backfocus distance so 1mm for the filter and 1mm for the sensor window. It could be interesting to see how much extra spacing is required to get the focus mark in the correct place..

Alan

Hmm, hadn't considered the sensor window. I've got another 10.5mm of mixed spacers I could throw into the mix, and it looks like it would take that much to get the infinity mark anywhere near the focus mark but that would just be silly, surely? From 44mm to 55mm?

Tonight looks like it might just be clear enough to run off a few test images, just to see how the stars look. I guess that's the ultimate test, regardless of anything else...

 

Simon

[SimonWhitfield]

30 minutes ago, fwm891 said:

Don't know the lens but is there a macro setting?

Nope, just a simple focus ring, simple f/stop ring...

 

Simon

[Adreneline]

1 hour ago, SimonWhitfield said:

the Samyang is a Canon EF version, so should need 44mm of back focus. 

My experience with the Canon fit Samyang plus Baader filters is that the spacing between the end plate of the lens and the sensor is 46.75mm and that is assuming that the sensor is at 6.5mm back focus as specified by ZWO (although I read somewhere that they quote a manufacturing tolerance on the sensor position of (I believe) +/- 0.25mm). The focus position is then dead centre in the 'L' mark - anywhere in the 'L' mark is good. There are numerous sources on the web advising how to use/interpret the 'L' mark. 

Adrian

[Alien 13]

1 minute ago, SimonWhitfield said:

Hmm, hadn't considered the sensor window. I've got another 10.5mm of mixed spacers I could throw into the mix, and it looks like it would take that much to get the infinity mark anywhere near the focus mark but that would just be silly, surely? From 44mm to 55mm?

Tonight looks like it might just be clear enough to run off a few test images, just to see how the stars look. I guess that's the ultimate test, regardless of anything else...

 

Simon

I agree Simon, a star test will show if the spacing is critical or not..

Alan

[SimonWhitfield]

9 minutes ago, Adreneline said:

My experience with the Canon fit Samyang plus Baader filters is that the spacing between the end plate of the lens and the sensor is 46.75mm and that is assuming that the sensor is at 6.5mm back focus as specified by ZWO (although I read somewhere that they quote a manufacturing tolerance on the sensor position of (I believe) +/- 0.25mm). The focus position is then dead centre in the 'L' mark - anywhere in the 'L' mark is good. There are numerous sources on the web advising how to use/interpret the 'L' mark. 

Adrian

Now, the white focus line on mine is the other side of the 10m mark when attached to the QHY183M and filter wheel and focused on the windmill 2.5km away... but bang on where it is supposed to be when I connect it to the Canon.

 

Simon

 

[Adreneline]

9 minutes ago, SimonWhitfield said:

Now, the white focus line on mine is the other side of the 10m mark when attached to the QHY183M and filter wheel and focused on the windmill 2.5km away... but bang on where it is supposed to be when I connect it to the Canon.

Hi Simon,

My experience is it is all down to trial and error. If your focus mark is the other side of the 10 then I would suggest the spaceing is not correct and the star shapes may suffer. There is no doubt that 44mm is a good starting point but my experience is it is never 44mm. If I replace my Samyang 135mm with my Samyang 85mm the spacing is wrong again! If I use my Canon 200mm that is different again and I have found the correct spacing by trial and error - mainly error!  A 0.15mm shim can make all the difference. I am pretty sure that these minor differences between lenses do not have a huge effect on day-to-day photography although you do see reviews by 'professional photographers' saying some lenses are not as "crisp" or "sharp" as others.

I have spent hours outside on clear nights assembling and disassembling in order to find the correct spacing for my Baader filters. When I recently changed to Astronomik I had to start all over again.

Good luck - it's worth it in the end.

Adrian

[Alien 13]

39 minutes ago, SimonWhitfield said:

Now, the white focus line on mine is the other side of the 10m mark when attached to the QHY183M and filter wheel and focused on the windmill 2.5km away... but bang on where it is supposed to be when I connect it to the Canon.

 

Simon

 

The sensor on the Canon 6D will have been shimmed to minute tolerances so that is your benchmark, as Adrian has said you need to add whatever spacers are necessary to match that with the QHY camera.

Alan

[SimonWhitfield]

14 hours ago, Adreneline said:

I have spent hours outside on clear nights assembling and disassembling in order to find the correct spacing for my Baader filters. When I recently changed to Astronomik I had to start all over again.

Good luck - it's worth it in the end.

Adrian

Hi Adrian,

I think that is what it is going to have to be... fixed under the stars... but this has confirmed my suspicions that something isn't quite right. If I was a few degrees off, fine, but this much?  At least I know my next clear night is going to be spent experimenting rather than imaging.

Thanks, everyone, for your assistance.

 

Simon

[SimonWhitfield]

Update:

15 x 4min Ha on Sadr @ f/4.  The stars not quite right in the corners, with top left being the worst, and they get progressively worse the closer I get to f/2.0.

Currently backspaced at 47.5mm and focusing pretty close to the middle of the 'L' mark on the lens. Also found loosening off the lens and camera rings helped with the image, as tightened up they seemed to introduce some distortion into the image, with the FWHM going wild only in the top left/bottom right corners... squeezed lens?

Anyway, progress has been made, just some fine tuning to go but already, I am liking this lens... even at f4, 30mins of exposures around Polaris started showing the molecular dust clouds. Repeatable accurate focusing might be an issue, but I have already noted Adrians solution to that.

Thanks once again.. I'm sure your help has shortened the time spent resolving this issue already 🙂

Simon