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Please Help with NEW ROKINON 135mm f/2.0

Astrophotography Beginner DSLR Equipment
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#1 CaptainRMWExplorer

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Posted 08 May 2021 - 11:23 AM

Hi, I’m new to cloudy nights. My name is Kat but growing into CaptainRWMExplorer.

 

Anyways, I recently got my Rokinon 135mm f/2.0 and I have no clue how to get my star to be points! And I’m having a hard time getting the Exposure and ISO right for Nebulea and Galaxies. Specifically Owl Nebula, Bode’s Galaxy, and the Orion Nebula.

 

Equipment:

• Sky Watcher Star Adventurer 2ii

• Canon T3i unmodified (I know. “Get A Full Frame!!”)

• Rokinon 135mm F/2.0

• Introvelometer

I started with the canon rebel Ti and Tamron 18-200mm 3.5-5.6. So this is a BIG upgrade. For me.

 

ROKINON:
I have polar alignment down pack, but the stars still look like unfinished circles. I set it to all the way infinity but it still looked weird. When I did put it a little before infinity the stars sort of looked better.

 

Where should I set the focus to get point stars? Are there any methods to get my stars to be points and not weird rings or bloated stars? What is the best F stop?

 

 

CANON T3i:

I have used SO many different exposure, ISO, and camera settings and all I am able to see is stars. Or it’s over exposed and I see the brightness of my light polluted sky.

 

What settings should I use with this set up? 
 

Is the Owl Nebula, Bode’s Galaxy, and Orion Nebula bright enough for my Rokinon Lens to see? 
 

I know it can be done but I’m struggling. YouTube videos don’t explain everything. Can anyone please help me?

 

Pictures are in my gallery titled: Struggling Astro Images


Edited by CaptainRMWExplorer, 08 May 2021 - 11:42 AM.


#2 Houthans

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Posted 08 May 2021 - 11:53 AM

Hi Kat,

 

Welcome aboard. As to focus, most lenses focus a bit past infinity, so turning the lens all the way to the infinity stop may not be the best thing to do. Most of my lenses need to be just shy of the infinity stop. I focus using a star in maginified live view, but you can also use a Bahtinov mask for accurate focus.  As regards the f-stop: I've seen excellent images with this lens at f/2 and some who needed to stop it down one or half a stop. This brings me to another potential explanation for your bloated stars: Rokinon is not known for its excellent quality control and quite a few people have had to return their lenses for a copy that did not have a decentred element. Your lens could just be a dud.

 

For ISO: I mostly use 800 or 1600 ISO and expose for 30 - 60 seconds, but I'm in a Bortle 3 area and mostly limited by my mount. The brightness of objects will not be the limiting factor for this lens. Basically, you want big objects with this focal length. The Orion Nebula will be nice, but Bode's will be very small (as will be most other galaxies). You can definitely do some nice nebula with this lens.

 

Finally, just try and struggle. Struggling means that you're learning and this is a great place where people can and will help you learn astrophotography.

 

Good luck,

Hans


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#3 bobzeq25

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Posted 08 May 2021 - 12:47 PM

Focus.

 

Get to your target, or at least close.

 

Look at the LCD with LiveView.  Forget about the bright stars.  Focus to reveal as many dim ones as you can.

 

Another alternative is a Bahtinov mask, but I find the above method works about as well.

 

Exposure.

 

ISO with an early Canon should be either 800 or 1600.  Nothing else.  If you want to know why read this end to end.

 

https://dslr-astroph...trophotography/

 

This hobby requires a _lot_ of research, my bookshelf is extensive.

 

Subexposure time.  You should see some light pollution, but the screen shouldn't be anywhere near white.  Stacking subs will help you reduce the brightness of the background.

 

Below is an example, your color may be different, and you will not see anywhere near that many stars.  It's just to show you what the background should look like.  Dimmer is good, too, but you shouldn't go for black.

 

Better way to set subexposure time.  Get the histogram on the LCD screen.  Set the obvious peak about 1/3 over from the left.  The histogram is shown on the example below.

 

Not to worry about, _at all_.  Full frame.  A bit wider field of view, but the stars at the edges are likely to be unpleasant anyway.

 

_Really_ big deal, this and the next paragraph.  For dim things, your intuition will say longer subexposures are better.  Wrong.  What's better is longer total imaging time, more subs.  What counts is how many total photons you collect.  How you slice the total into subs is far less important. 

 

And the calibration frames.  Bias, flats, darks are not optional.  They reduce noise that obscures dim stuff.

 

Patience.  There's a lot to learn.  It's never the case that there is nothing more to learn.  Part of the charm of the hobby for many of us.

 

Helpful, but not as fundamental as the above.  Astro Pixel Processor is an excellent program for calibrating/stacking/processing.  Yeah, it's not free.  It's really "worth it".  Has an excellent gradient reduction tool for reducing the effects of light pollution.  Set up in a way that actually teaches you about processing.

 

Stop the lens down to 2.8.

 

Back to the important stuff.  Shoot more subs.  <smile>  At least an hour of total imaging time.

 

a sub with histogram.jpg


Edited by bobzeq25, 08 May 2021 - 01:17 PM.

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#4 Andynator

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Posted 08 May 2021 - 03:38 PM

Dr. Benway here on Cloudy Nights will make a Bahtinov mask for you.

https://jwbozeman.com/

Highly recommended.

What works for me is setting focus with the mask and F-stop wide open, then when finished turn the stop in one click and leave everything else alone.

#5 DJL

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Posted 08 May 2021 - 04:48 PM

You may find it helpful to get a low cost clear / skylight filter for the lens and click the Bahtinov mask into that. In my case it took a little work with a file. Then you can put the filter into the lens and screw it in a quarter turn or so to stop it falling out. The trick is to remove it after you are done, and not go on to shoot your subs with it still in place. The other trick is to remove it without nudging your polar alignment.

 

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