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Camera / lens settings using Rokinon 135mm

Rokinon 135mm f/2.0 ED UMC ZWO ASI2600MM Pro Acquisition Deep Sky
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Chase Davidson avatar
Chase Davidson avatar
Hi everyone, I was super excited when my new Rokinon came and the clouds cleared up. I bought it specifically to take the Rho Oph complex this summer. After the first night, I went back and looked at my 60 seconds subs and saw that everything is very bright. I'm used to capturing on a RedCat 51 or 91 at 120 second subs so I think I just need to get used to a faster lens. I went back last night and tried it at 30 second subs for my lum filter and it was still really bright. My question is, what is everyone setting their lens / camera settings to? I use a zwo asi 2600mm pro with Antila filter set. I the lens set to f2.8 and it is close to no moon light. I am in a bortle 7-8 zone thought so maybe it's jsut too bright for a fast scope?

Using the ASIAIR, should I lower the gain, increase the f ratio, or just keep lowering the exposure times? I'll post some pictures when I get home.
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Dan Kearl avatar
Dan Kearl avatar
You need to post an image for anyone to answer the question.
I use a Rokinon 2.0 that I use at 2.8.
I have used it with a 2600, 294 and 533.
Depending on the subject I shoot from 60 sec to 5 min at 2.8.
I am in bortle 8 crappy skies,
I shoot everything on all my cameras at default gain settings.
You can look at my portfolio, I think the last couple I have posted were with the Rokinon 2.0 lens.
I shoot with Radian or Antila OSC filters, I would not use a Lum filter ever in my skies on any telescope.
That is your problem, shoot RGB or SHO.
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Dan Watt avatar
Dan Watt avatar
Chase Davidson:
Hi everyone, I was super excited when my new Rokinon came and the clouds cleared up. I bought it specifically to take the Rho Oph complex this summer. After the first night, I went back and looked at my 60 seconds subs and saw that everything is very bright. I'm used to capturing on a RedCat 51 or 91 at 120 second subs so I think I just need to get used to a faster lens. I went back last night and tried it at 30 second subs for my lum filter and it was still really bright. My question is, what is everyone setting their lens / camera settings to? I use a zwo asi 2600mm pro with Antila filter set. I the lens set to f2.8 and it is close to no moon light. I am in a bortle 7-8 zone thought so maybe it's jsut too bright for a fast scope?

Using the ASIAIR, should I lower the gain, increase the f ratio, or just keep lowering the exposure times? I'll post some pictures when I get home.

You went from F5 to F2.0 which is a difference of 2-2/3 stops. Every 1 f-stop is twice the brightness. So around 20 seconds at f2 on the Roki would be close to 2 minutes with the Redcat. There are many other variables at play here which is why I wish manufacturers would also use T-stops but that's another conversation. How do the stars look at f2? Do they look better at F2.8? F4? There is a lot of variability with Rokinon's manufacturing (hence the price point) so there isn't always a one size fits all approach with these lenses. If you do like to shoot at f2 and don't want to deal with hundreds of subs to process then decreasing the gain might be a viable strategy.
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Jon Woodhead avatar
Jon Woodhead avatar
If you stop down to f4 then your exposures would be more comparable to your Redcat? Certainly worth a test to help determine what is going on - I normally use my Samyang at f4:

samyang 135

I have somewhat darker skies (B5) but never go beyond 60s for RGB or 120s for narrowband. All at unity gain.

I agree with Dan - even with B5 skies I find that luminance filters give bloated stars and so stick to simple RGB...
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Chase Davidson avatar
Chase Davidson avatar
Thanks gentlemen! I’ve made it home and gathered a few subs I took at f2.8 (these are auto stretched from ASIAIR)

lum 60 seconds, 100 gain:


red 60 sec, 100 gain:


lum 30sec, gain 100:


It could be the auto stretched from ASIAIR that’s making it so bright. The more I look at it, the 30s lum has good star size, I’m just afraid it’s losing the details of the nebula. The red 60s is more like what I’m used to seeing in a sub. Thoughts?
danieldh206 avatar
danieldh206 avatar
Rho Ophiuchi is bright, so use zero gain to get the maximum well depth. This calculator will help visualize what happens with a fast scope in Bortle 7, and it is not pretty.  https://tools.sharpcap.co.uk/

I know it can be challenging to get out to a dark site, but with a fast lens, you only need a couple of hours at a dark site to get a great Rho Ophiuchi image.  Light pollution is not kind to fast lenses with a really wide field of view.  

What latitude are you imaging from? If Rho is under 25 degrees from the horizon, the light pollution is going to cause bad gradients at 135mm.
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andrea tasselli avatar
andrea tasselli avatar
It's Rho Ophiuchi.