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Sony E-mount lens adapter to use with ASI Pro cameras

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#126 Jinux

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Posted 26 February 2024 - 01:45 PM

I have had that issue where the lens hits the hard stop with the EAF, sometimes it does cause problems, sometimes not. With the Samyang 135mm it is possible to adjust the focus ring and extend the beyond infinity focus throw, which eliminates that problem. If you don't want to undertake that task you have two options, decrease the step size and hope the focus curve can still be accomplished accurately over a smaller focus distance OR decrease the back focus distance which will make the camera come to focus before the infinity distance on the lens. Obviously the second one is only possible if there's an even thinner lens adapter, which isn't the case here. You might get lucky and the lens might come to focus a little before the infinity mark and you won't have any issues, otherwise you'll have to decrease step size or adjust the focus collar.

Rokinon 135mm seems to have enough margin over infinity focus. Probably 0.1mm margin I added (0.1mm thinner)  also contributed as well.

On field test, autofocus ran well without hitting hard stop.

All CNC'ed components including ring, clamps and holding plates worked well on the field but critical focus zone was too narrow for this fast optics. 

 

CFZ = 1.6 * lamda * F^2, where lamda is wavelength of green = 500nm

 

With F/2.8 setting on Rokinon, CFZ is mere 6.3um which is lower than my CNC tolerance (I don't know how much precision typical manufacturing has.)

So, I need to adjust some tilt plate using screw. I don't know how Eric got that flat with minor tilt with my 3D printed plate. 

 

Also, light pollution on my backyard is quite high and change over time in frame, so it's very difficult to process.

So, I designed 77mm filter to M48/0.75 2" filter adapter and plan to put this filter in front of the objective lens. This will handle light pollution and step down the F ratio to increase CFZ.

 

Need clear nights to test this out but rain is coming. Should've done this during summer days when temperature is tolerable, abundant clear days, and long list of wide field objects in the sky.

Attached Thumbnails

  • 135_setup.jpg
  • field_test.jpg


#127 Jinux

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Posted 26 February 2024 - 02:39 PM

It would be interesting to add 2" EFW in front of objective lens to have automated NB imaging.

I think it's easy to model and CNC. 



#128 erictheastrojunkie

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Posted 26 February 2024 - 04:34 PM

 I don't know how Eric got that flat with minor tilt with my 3D printed plate. 

 

 

Complete blind luck is all it was. Obtaining critical focus with the lens can be quite difficult, also if your autofocus program selects a focus star that is more towards the edge vs centrally that can also play a big role, imaging with fast optics like this can be VERY difficult when it comes to focus and tilt. One thing I will say is that that Rokinon 135mm holds focus very well over the course of a night, l had looked at some logs saved from multiple imaging sessions and even with big temperature swings the focus point barely shifted 10 steps over the course of multiple nights worth of imaging. If your temps aren't shifting by more than maybe 20 degrees in a night it may not even be worth running the autofocus because it could make focus worse if it happens to be off by a little bit. That's just something you have to test for yourself though. 

 

Gradients with a lens like this can be a beast to remove, especially complex light pollution gradients WITH filters. You also have to be careful with this lens and filters on the front of the lens, I know when I used a filter in front of one of my copies of the lens I got some nasty concentric circles that could not be fixed, maybe filter specific, but I've heard multiple people state the same. 

 

I really like the "retro" look of having wood elements on the setup, something about the modern element of imaging with the mount/lens/camera/computer goes well with the wooden components. 


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#129 Jack’sDad

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Posted 26 February 2024 - 06:27 PM

As someone heavily entrenched in the ZWO & Sony environments who just stumbled upon this thread (and didn’t read it all, yet), color me interested. Looks like I’ve got some reading to do.
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#130 Jinux

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Posted 29 February 2024 - 03:56 PM

Gradients with a lens like this can be a beast to remove, especially complex light pollution gradients WITH filters. You also have to be careful with this lens and filters on the front of the lens, I know when I used a filter in front of one of my copies of the lens I got some nasty concentric circles that could not be fixed, maybe filter specific, but I've heard multiple people state the same. 

I designed 3D printed filter mount for 2" filters and used IDAS NBZ filter for the second light shot. 

 

filter_mount.jpg

 

and I got a decent pictures out of it. Background gradient is okay and not as nasty as no filter one and easily removed by all my processing tools (Siril, Startools and GraXpert) without much differences. 

When I had no filter, only GraXpert was able to deal with that uneven gradients. And of course, background noise is much less.

One of the improvement was to add flocking on lens hood. My club members recommended to do it when I consulted about nasty concentric circle gradient on my first light image.

 

Only 88min available to picture before it sets down over my house roof.  

 

result_rosette_cn.jpg

 

Now I'm convinced and will attach filter wheel in front of the objective lens. This scheme has many advantages over filters before the sensor.

First, 48mm aperture with 135mm focal length lens means 135/48 = F2.81 It's still blazing fast, so I'm not concerned about getting less lights with reduced aperture. Even 36mm filter, opening is 34mm and it would yield F3.97 and many people use this 135mm/F2 at F4 to get better sharpness. 

Second, filter in front of objective lens means, no filter induced halo, no refocus due to filter change, no worry on dusts on filters, no separate flats per filter, no fast optics optimized filter required. Sounds like dream to me.

 

Of course, I would add an UVIR filter on "telescope" side opening of filter wheel with some 3D printed hood to screw in to block sideway lights and dusts.

This is a project until coming two series of storms go away in California. 


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#131 KLWalsh

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Posted 03 March 2024 - 06:26 PM

This might be useful for some:

https://www.amazon.c...,aps,131&sr=8-5

This inexpensive extension tube set includes a section with a bayonet interface for Sony A lenses. The different sections of the tube set screw together using an M57 thread. (That’s what I measured with calipers.) The M57 is an oddball size. Preferably, an M57 to M48 adapter would allow the bayonet section to be mated to an ASI camera.
From my searches online, it appears that Borg telescopes use M57 threads. So perhaps Borg has an adapter that would work.


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