Auto Focus in the dark

[leon]

Hi Guys I am trying my hand at some wide field imaging using a Sigma 105mm F/1.4 Lens on a Canon 6D11.

This morning I was nearly impossible to get an accurate focus on a bright star as the AF focus dose not work in the pitch black sky.

So I am going to put a small reflector or light some distance down the drive way and AF on that and then turn the lens to manual.

So how far do i need to go to get an accurate infinity focus please.

Is there a calculation for this somewhere.

many Thanks in advance.

Leon

[RB (Andrew)]

Focus on a distant object (a horizon or a distant tree) during the day and then put it in manual focus.
Don't change focus after that.
You can then use it later at night pre-focused.

Or you can just move the focus of the lens manually to infinity (or very close to it).

RB

[AstroViking (Steve)]

Don't forget to put a piece of gaffer's tape over the join between the focus ring and the lens body to stop any accidental movement.

V.

[Leo.G (Leo)]

Maybe no great help but I always find my terrestrial infinity focus is just the tiniest bit off for the night sky.
What I do is take a series of shots (Samyong 14mm f2.8 manual focus lens with very inaccurate infinity markings) and run inside to the computer and check the larger images on the computer screen because I see nothing under a 27" monitor. Images which look incredibly sharp on my camera review screen I've later found far from sharp after taking 150 or so images and going inside to process them. To me that's a wasted night and a lot of frustration.
I've spent numerous nights out fine tuning the focal point on this one lens for wide filed MW images and when finally spot on, I marked the lens with a line of red nail polish so I can find it easy now whenever I bother going out.

[leon]

Thank You for your suggestions, I have bought a garden solar light from Bunnings and stuck it on a post some 100meters away, so when it shines in the night and although just a small light i can focus on that . Maybe, haven't tried yet see what the morning brings

Leon

[Crater101 (Warren)]

Similar to RB, when I'm setting up I look for the last light of dusk on the horizon, and use the autofocus, aimed at the furthest point of the horizon that I can find. It's usually a fine point (depending on how late I start to setup) as to if I have enough light for the autofocus to work. (I'm using the same lens as Leo, and I sometimes use a Canon 35mm f1.4L). Instead of Gaffer Tape though, I use BluTack in a similar way to stop anything getting bumped or moved.

[bojan]

How about the Moon? Darn thing is always there when clouds go away, and is bright and far enough :-)

[RB (Andrew)]

Originally Posted by leon

Thank You for your suggestions, I have bought a garden solar light from Bunnings….

Leon

Don’t tell me it was $23.35 !!!

[ronson]

A Bahtinov mask generally works well for manual focus on bright stars.

[leon]

Oh you are so funny Andrew, no it was $10.00 and I only bought one.

Leon

[leon]

Hey Ronson, that didn't cross my mind actually, thank you might have to look into that.

Leon

[Cyberman (Rob)]

Leon, I use Backyard EOS programme on my laptop. It has a focus screen/programme where you can magnify the star as much as you like. Then I focus manually til the star is the smallest I can get. It also has FWHM and HFD methods of focussing. Costs about $50 to $60 I think, one off buy. I think it also has a free trial period. I also use BYEOS imaging programme for taking the photos.
Cheers, Rob

[leon]

Many thanks Rob, will have a look at that.

Leon

[RB (Andrew)]

Leon, are you planning on doing long exposures on a tracking mount or is this the static tripod shot you were asking about the other day?
If it’s just a tripod shot then just keep it simple.
Pre focus your lens during daylight, put some BluTack on the focus ring so it doesn’t move like Warren - Crater101 mentioned and have fun.
On a tripod, you’re only going to be doing 4.5 sec exposures anyway.

Unless you go tracked long exposures, I wouldn’t go to the trouble of Bahtinov masks and laptops…

RB

[leon]

Thanks Andrew, mate it is just tripod stuff and i am testing to see which lens suits the shots better.
Tonight, I mean right now I have come inside from an early morning session at about 3.30am.

First lens 105 mm F1.4 @1250 ISO nice results at about 3-4 seconds before trailing.

Then the 50mm and 40mm at F1.4, and 1250 ISO could get a few seconds more but they ended up pretty noisy.

So far the 105mm is superior but the fog and mist moved in and that was that.

Now I need to take a heap of subs and try and stack them all together but to be honest i have forgotten how to do that after so many years out of imaging.

Any suggestion would be helpful.

Have a great day

Leon

[RB (Andrew)]

Originally Posted by leon

Now I need to take a heap of subs and try and stack them all together but to be honest i have forgotten how to do that after so many years out of imaging.

Any suggestion would be helpful.

Have a great day

Leon

Oh gosh, now you're pushing the friendship.
I can't remember the last time I did any astro work.
I used to use RegiStar and DSS (Deep Sky Stacker).
You're better off starting a new thread asking the experts on current techniques.

[leon]

Probably a good idea, I know there is one member, plus probably others that only use DSS, but cant remember who that was, but it will come to me and I will ask the question.

Thanks Mate.

Leon

[ronson]

Hi Leon,
If you are looking at a simple process to stack photos to reduce noise without going the extra mile of darks, flats etc., then I would recommend Sequator https://sites.google.com/view/sequator/

[leon]

Hey thanks Ronson, will certainly have a look at that.

Leon

[leon]

Well Andrew i did it your way and it was a excellent idea.

Did a focus on the Moon at dusk and set the Camera to all required settings, left it in the shed to cool down over night, went out at 4am turned it one and was imaging withing a few minuets.

Perfect and no stuffing around to try and find focus in the dark.

300 plus images later (although short) they all looked great, now comes the fun part, but i will do that later

Thanks mate, that is the way to go from now on.

Leon