Imaging with the Samyang 135mm f2

[Adreneline]

20 minutes ago, Uranium235 said:

The right hand side of your images displays out of focus stars. This also corresponds with the master flat you posted earlier.

Whether that is due to droop in your imaging train, I dont know. But an idea would be to test the lens on a DSLR and take some flats with that - if you get the same vignetting pattern, it will be the lens. If not, then you need to look at the way its attached to your astro camera as to what is causing it.

Thanks for the feedback and advice. Re. the flats I recalibrated the filter wheel which has improved the flat situation but it is still not totally symmetrical. I do wonder if the lens is slightly suspect so when the opportunity presents itself I will use it on my 70D and see what results I get.

I mount the lens on the filter wheel using the ZWO EoS adapter. Placing a digital level on the camera and the lens gives the same result so there appears to be no differential droop between the lens and the camera. I have to hope the camera sensor is orthogonal to the lens plane.

This is a fairly ridiculus shot of the Pacman taken after I calibrated the filter wheel - not an ideal choice of lens for this target but useful for testing the tracking and imaging train I guess. There is only minimal edge cropping to the image. This is 12 x 120s of Ha, OIII and SII - all without guiding.

Thanks again for your help and the critical assessment of my images - much appreciated.

Adrian

Edited September 16, 2019 by Adreneline
Clarification

[Alien 13]

15 minutes ago, Adreneline said:

This is a fairly ridiculus shot of the Pacman taken after I calibrated the filter wheel - not an ideal choice of lens for this target but useful for testing the tracking and imaging train I guess. There is only minimal edge cropping to the image. This is 12 x 120s of Ha, OIII and SII - all without guiding.

Thanks again for your help and the critical assessment of my images - much appreciated.

Adrian

Thats a cracking shot, I like lots of space around an image, far too many crop the life out of a good composition.

Alan

[Adreneline]

34 minutes ago, Alien 13 said:

Thats a cracking shot, I like lots of space around an image

Thank you!

I have to say I do like to see images of galaxies with a fair amount of 'space' around them - I feel it represents the reality far more - they are one heck of a long way off! It is interesting to compare different nebula with the same FoV - IC1396 and the Heart & Soul fill the frame - I'm just imaging Sadr and that also fills the frame - diminutive Pacman often appears to be of equal size but is so much smaller.

The ED80 never gets a look in these days - I love my 135/1600 combination.

Thanks again.

Adrian

[Adreneline]

Still looking to see what my CEM25-EC + 135mm + ASI1600MM-Pro can achieve unguided.

This is the Sadr region : 12 x 120s of Ha, OIII and SII - pre-processed in APP and post-processed in PI with a final colour tweak in PS - I've tried not to overprocess the image. Cropped the very edges of the image to ensure DBE wouldn't get confused. Star shapes toward the extremes are still not perfect but I am pretty sure it is the lens and not droop or lack of guiding. Again I would welcome the views of others. A 90% moon didn't exactly help but clear nights are not to be squandered!

I've uploaded this as a png high resolution image.

Thanks for looking.

Adrian

[Uranium235]

The corners on the pacman are much improved. That leads me to think that the focus can be tweaked slightly to improve the overall field.

One thing I used to do is use a ROI box on the offending corner (for quick loop), take an exposure, then tweak the focus very slightly.... "are the stars better or worse?", if not, go the other way... if yes - check the other corners to see if youve knocked anything out of focus - if its still good, youre ready to rock. Sometimes its bit of a compormise. It takes a bit of time to do, but its worth it.

[Adreneline]

2 hours ago, Uranium235 said:

That leads me to think that the focus can be tweaked slightly to improve the overall field.

Hi Rob,

Thanks very much for the comments.

I use FWHM in SharpCap to focus. One really odd thing I find is that I can achieve the best FWHM with the Ha filter - OIII and SII never seem to be quite as good - 2.0 for Ha compared with 2.5 for OIII and SII, however, when I pre-process in APP it gives an estimate of the FWHM range               for each sub and the OIII and SII are always lower than the Ha - it's a complete role reversal!

2 hours ago, Uranium235 said:

use a ROI box on the offending corner

SharpCap requires that you select a ROI and in particular one star. It never occured to me to focus on a star in one corner of the image and then see if the result holds true for the other three corners.

I am pretty sure it is not alignment of the imaging train but I am not convinced as yet it is not the lens. I need to take the same star field using my 70D with the 135mm and see what FWHM value I can achieve with BYEoS.

Thank you very much again for you help and advice.

Adrian

 

Edited September 18, 2019 by Adreneline

[Uranium235]

Measuring fwhm on one star only measures it for that particular area of the corrected field. You need to take the whole field into consideration when focusing. That's why it's a bit of a compromise at times

[wxsatuser]

I normally check several stars and if they are close that will do.

[Adreneline]

On 20/09/2019 at 13:12, Uranium235 said:

Measuring fwhm on one star only measures it for that particular area of the corrected field. You need to take the whole field into consideration when focusing. That's why it's a bit of a compromise at times

 

On 20/09/2019 at 13:19, wxsatuser said:

I normally check several stars and if they are close that will do.

Thanks for all the advice Rob and Mike.

I've followed your advice and am getting pretty much the same result on the resulting image. However, I have recently acquired (purchased!) a Canon 200mm f2.8 and I used that the other night to image IC1396 - so 200mm mounted on exactly the same setup as the 135mm with just a minor adjustment to the tube ring I use to support the lens.

The result was surprisingly good with good and consistent star shapes across the whole image and in all four corners. (I won't post on here because this is 135mm only).

It makes me think maybe my 135mm is not quite 100%, which is a bit disappointing. If I crop the 135mm image down to the same as the 200mm I lose all the offending stars but it kind of defeats the objective of using the 135mm!

Umm? Not sure what to do now. At the next opportunity I will try the 135mm on the Canon 70D and see what I get on a widefield image; I'll post the result.

Thanks again for your help.

Adrian

[Space Oddities]

Interesting article from a Polish astrophotographer, who seems to be also a talented DIY guy! He sales some very interesting 3D printed accessories for the Samyang, with a very reasonable price I must say  

https://astrojolo.com/gears/thirty-months-with-samyang-135-f-2/

@Uranium235, check out his custom made T2 adapter!

[Rush]

This is an HALRGB version Orion taken with Sammy 135mm and SBIG ST-8300M.

Was a windy night , focus shiffting at regular intervals. Star spikes, not thin edged due to

vibration i guess.

CS

Rush

[KyleStoke]

Hi All 

Took this one a week or so ago, first go with a my new 135 and ASI1600, can somebody let me know if they think the corner star shapes are spacing related or something else. I should be well inside the 44mm  back focus 

[Adreneline]

1 hour ago, KyleStoke said:

can somebody let me know if they think the corner star shapes are spacing related or something else

Well not sure I really know the answer to this one but I experience exactly the same problem in the extremities of my images. I've played around extensively with spacing and the results are always similar. My spacing on the current set up achieves focus just before the end-stop on the focussing ring but I still get slightly mis-shaped stars in the corners.

Can't recall now where but I saw someone advising the use of step-down rings on the front of the lens to correct/ameliorate these sorts of issues. I think they had reduced the aperture right down to 50mm which slows the system down to around f5 (still keeping the aperature setting on the lens at f2) but I've not tried that approach as yet, although I do have a ring that would step mine down to 67mm.

As an aside I've also tried using the 135mm/ASI1600 combination with both 1.25" and 2" filters and the result is still the same.

I will be interested to see what others think/advise.

Adrian

[Mark1489]

1 hour ago, KyleStoke said:

Hi All 

Took this one a week or so ago, first go with a my new 135 and ASI1600, can somebody let me know if they think the corner star shapes are spacing related or something else. I should be well inside the 44mm  back focus 

 

I’ve found that the focus is extremely critical, tiny movements have changed the elongated stars in my corners/switched the bad corners. To be honest I crop most of it out but I’m lazy! 

[KyleStoke]

24 minutes ago, Mark1489 said:

I’ve found that the focus is extremely critical, tiny movements have changed the elongated stars in my corners/switched the bad corners. To be honest I crop most of it out but i’m lazy! 

Hi Mark 

I don't think its the focus, its all mechanically controlled and seems to be in focus across the majority of the picture. 

42 minutes ago, Adreneline said:

Well not sure I really know the answer to this one but I experience exactly the same problem in the extremities of my images. I've played around extensively with spacing and the results are always similar. My spacing on the current set up achieves focus just before the end-stop on the focussing ring but I still get slightly mis-shaped stars in the corners.

Can't recall now where but I saw someone advising the use of step-down rings on the front of the lens to correct/ameliorate these sorts of issues. I think they had reduced the aperture right down to 50mm which slows the system down to around f5 (still keeping the aperature setting on the lens at f2) but I've not tried that approach as yet, although I do have a ring that would step mine down to 67mm.

As an aside I've also tried using the 135mm/ASI1600 combination with both 1.25" and 2" filters and the result is still the same.

I will be interested to see what others think/advise.

Adrian

Hi Adrian 

Ill try stepping it down a couple of stops and see if that clears it up, I don't want to go down to far else start to lose one of the main reason for having the lens.  

[Adreneline]

16 minutes ago, KyleStoke said:

Ill try stepping it down a couple of stops and see if that clears it up

Hi. Just to be clear the recommendation was to leave the lens setting at f2 but to use a step-down ring on the front of the lens - like one of these . I have a 77mm - 67mm (which allows me to fit a 67mm filter I have to my 77mm lens for regular photography) but have not used it as yet for AP.

The article implied going further to like 50mm which will slow the lens down to more like f5. If I can remember where I read about it I will post again.

Adrian

P.S. Should have said right at the outset that star shapes aside it is a really nice image. The H&S are one of my favourite targets to image with the Samyang. I am gradually building up a huge amount of multi-session data I need to combine; hopefully it will have been worth it!

 

Edited October 13, 2019 by Adreneline
Clarification

[Adreneline]

31 minutes ago, KyleStoke said:

I don't think its the focus, its all mechanically controlled

Sounds like you've got your focus system in place but it maybe of interest of others to see what I have now done with my continually eveolving setup.

I used Araldite to attach a surplus toothed belt to the lens (great care needed!) to give a more positive engagement with the mounted SW focus motor - rather than relying on friction between the focus thumb grip and the toothed belt. It has pretty well eliminated all the backlash problems I was having. Might be of interest to someone.

Adrian

P.S. The brass lab weights are to balance out the d.c. focus motor in the DEC axis.

 

[Alien 13]

3 hours ago, KyleStoke said:

Hi All 

Took this one a week or so ago, first go with a my new 135 and ASI1600, can somebody let me know if they think the corner star shapes are spacing related or something else. I should be well inside the 44mm  back focus 

 

Not sure if you are using a mono cam and filters but are they rated for use at f/2 as some struggle?...

Alan

[KyleStoke]

3 hours ago, Adreneline said:

Hi. Just to be clear the recommendation was to leave the lens setting at f2 but to use a step-down ring on the front of the lens - like one of these . I have a 77mm - 67mm (which allows me to fit a 67mm filter I have to my 77mm lens for regular photography) but have not used it as yet for AP.

The article implied going further to like 50mm which will slow the lens down to more like f5. If I can remember where I read about it I will post again.

Adrian

P.S. Should have said right at the outset that star shapes aside it is a really nice image. The H&S are one of my favourite targets to image with the Samyang. I am gradually building up a huge amount of multi-session data I need to combine; hopefully it will have been worth it!

 

I will look forward to seeing it. Also one of my faves being flirting with it since I started AP. This is the perfect focal length to frame it nicely. 

I'll try stopping it down if all good ill look into getting a ring as you mention. 

Regarding auto focus I was contemplating an opposing toothed belt but found with it under tension it seemed fine. What is the size of the rings you have holding the asi.? 

 

2 hours ago, Alien 13 said:

Not sure if you are using a mono cam and filters but are they rated for use at f/2 as some struggle?...

Alan

Hi Alan its a badaar 7nm Ha I'm sure I read it was OK at F2 1.25in but as Adrian mentions above he had the issue with both 1.25 and 2 inch but I won't rule it out

[Adreneline]

11 minutes ago, KyleStoke said:

What is the size of the rings you have holding the asi.? 

I bought the ASI1600 mount produced by ZWO specifically for the 1600. I had to space it off the vixen bar with a piece of stock aluminium  bar so that I could position the lens more or less in the centre of the lens support ring (which I bought from TS Optics in Germany).

I found having the toothed belt engaged with the thumb grip worked providing it was under sufficient tension but this Samyang lens is not made like a Zeiss (which is why it is about quarter of the price!) and I have noticed it is possible to distort the barrel if you put too much pressure on it from the support rings. What I really need is a felt lined ring as per the one which support the ASI1600 but so far I've not found one.

If Clear Outside is to be believed I might get the chance to try the step-down ring tonight. CO even agrees with the BBC Weather app - almost unheard of!

Adrian

[KyleStoke]

Yeah potentially clear here too. Might try stopping down and a few test shots. I've got the ts ring also but would prefer something a bit more fitted

[Space Oddities]

I've just purchased the Samyang kit from Astrojolo. Lukas was very helpful and even accepted to 3D print a custom plate so I can mount my ASIAIR on the lens.

In total, I spent 110€ for the rings, the focuser, the mini guider, a dovetail and a custom plate, shipping included. I will give you my impressions when I receive everything!

Also, for anyone interested in the T2 mod for the Samyang lens, Lukas told me he will soon receive some copies from his provider. It is made of CNC aluminium and the price is 33€ (incl. VAT).

[Tarun]

9 hours ago, Space Oddities said:

 

Also, for anyone interested in the T2 mod for the Samyang lens, Lukas told me he will soon receive some copies from his provider. It is made of CNC aluminium and the price is 33€ (incl. VAT).

I placed 2 orders from him last week,told him to swap t2 to m48 so that i can fit 2 inch filters in b./w

[KyleStoke]

16 hours ago, Space Oddities said:

I've just purchased the Samyang kit from Astrojolo. Lukas was very helpful and even accepted to 3D print a custom plate so I can mount my ASIAIR on the lens.

In total, I spent 110€ for the rings, the focuser, the mini guider, a dovetail and a custom plate, shipping included. I will give you my impressions when I receive everything!

Also, for anyone interested in the T2 mod for the Samyang lens, Lukas told me he will soon receive some copies from his provider. It is made of CNC aluminium and the price is 33€ (incl. VAT).

Not a bad price it cost me £60 have m48 version knocked up at a fabricators in UK but it was a massive pain to do

[Space Oddities]

Indeed! I think it's quite cheap for something that allows you to get rid of infamous Canon adapters!