Obtaining focus

[shropshire lad]

This may be a daft question, but it may not be so I will ask it... 

If I get a nice sharp image of the moon (focus spot on) will that focus setting be good for every other object in the sky?. e.g planets, stars and deep space.

Thanks Paul

 

[wimvb]

In theory, yes. But focus can shift during a night, due to temperature change or simply due to slipping. That's why you need to check focus every now and then.

Edited August 4, 2023 by wimvb

[Mandy D]

Thierry Legault answers this question in his book, Astrophotography, on page 74. He calculates that the difference in focal plane position in a telescope of 1000 mm focal length, for the Moon and a star hundreds of light years away is about 1 billionth of a millimetre (10^-12 m). Even the artifical satellites orbiting the Earth can be considered to be at infinity.

Edited August 4, 2023 by Mandy D
typo

[AstroNebulee]

As wim says yes generally this is good enough. But best to use a batinov mask on a really bright star to get the central line intersecting the two lines correctly. 

Lee 

Edited August 4, 2023 by AstroNebulee
Addition

[shropshire lad]

Thank you for the replies, I will focus on the moon (edge probably) lock focus to that and then fit my  batinov mask, take a shot and zoom into my image and see if the line is central.

[AstroNebulee]

11 minutes ago, shropshire lad said:

Thank you for the replies, I will focus on the moon (edge probably) lock focus to that and then fit my  batinov mask, take a shot and zoom into my image and see if the line is central.

What scope and camera are you using by the way? 

If you can find a bright star would be best and take an image of that to check focus. 

Lee 

[ONIKKINEN]

If you are imaging with the help of a computer of some kind (so anything but a DSLR screen) then i would suggest focusing without a bahtinov mask, or the Moon. Both cases can be less than ideal, the Moon has the issue where you have to slew your scope to the target afterwards, which can cause slippage with modest focusers. With the bahtinov mask its easy to misinterpret just how central the line is.

Instead i would recommend focusing by simply looking at reported HFR/FWHM values. The workflow is very simple, just loop exposures and adjust focus to either direction. Once the values no longer get smaller you are at ideal focus. Repeat once you see HFR increase during the night.

Of course relies on a computer to read the values.

[shropshire lad]

Canon 80D and 600mm lens ...  and not using computer

 

Edited August 4, 2023 by shropshire lad

[AstroNebulee]

5 minutes ago, shropshire lad said:

Canon 80D and 600mm lens ...  and not using computer

 

If you can use live view on your camera and up the exposure on a bright star using your batinov mask to achieve focus. 

This is what I used to do. Now I have an astro camera I just use my batinov mask to gain focus and zoom in the check and always worked for me. 

Good luck

 

Lee 

Edited August 4, 2023 by AstroNebulee

[shropshire lad]

@ONIKKINEN .. I'm using AZGTi in eq mode ... so do you sugest I do the 2 or 3 star alignment for polar without worrying too much on the focus, and then slew to a bright star closest to my intended target and do the focus only then.

 

[ONIKKINEN]

9 minutes ago, shropshire lad said:

@ONIKKINEN .. I'm using AZGTi in eq mode ... so do you sugest I do the 2 or 3 star alignment for polar without worrying too much on the focus, and then slew to a bright star closest to my intended target and do the focus only then.

 

I see you are using a DSLR, so if you are worried about the shutter count increasing then do just that with live view and a bright star.

If not then ideally you would focus on the target itself with measured HFR readouts.

[ONIKKINEN]

I see you added the no computer part, so disregard the HFR thing then.

So probably the best way would be a bright star near the intended target then. Using a bahtinov mask or not, up to you. I tried with and without when imaging with a 550D and i preferred not to use the mask in the end and instead look at the star itself on high zoom.

[shropshire lad]

@ONIKKINEN ..  What progs or software will read and show the HFR, as I may be able to use a computer via wi/fi to remote my camera.

 

 

[ONIKKINEN]

13 minutes ago, shropshire lad said:

@ONIKKINEN ..  What progs or software will read and show the HFR, as I may be able to use a computer via wi/fi to remote my camera.

 

 

NINA is an allround great control software. Free and open source, works with just about anything. I think the camera may need to be connected via USB though, not sure at all about using via wi-fi.

If you manage to get things going with a computer then reaching the absolute best focus possible becomes so much easier so i do encourage at least attempting that.

* NINA link : https://nighttime-imaging.eu/

Edited August 4, 2023 by ONIKKINEN
*

[shropshire lad]

Ok thanks again, I will have a play and see what I can achieve, just need the skies to clear of these pesky clouds.

Paul

 

[shropshire lad]

@ONIKKINEN .. I have downloaded NINA but it won't recognise Canon cameras, only nikon in the drop down list.

UPDATE ... OK eos utility was stopping it from seeing my Canon, removed Eos and my canon now in the list.

 

 

Edited August 4, 2023 by shropshire lad

[Chewie65]

On 04/08/2023 at 18:02, shropshire lad said:

Canon 80D and 600mm lens ...  and not using computer

 

Hi, if you have the canon app, then I believe that you can use a phone/tablet as a remote. With this you can somehow zoom in on the screen. I dont have canon, but I did give some general tuition to someone who has them and he discovered that yo7 can , and it helped him. 

[shropshire lad]

Thanks Chewie65 .. actually that is something that I was thinking of trying, just need some dark skies to test it out .... need to see if I fit my batinov mask will the spikes show when zoomed in.

 

[ollypenrice]

There's theory and there's focus. I prefer focus. 

Olly

[bosun21]

3 hours ago, shropshire lad said:

Thanks Chewie65 .. actually that is something that I was thinking of trying, just need some dark skies to test it out .... need to see if I fit my batinov mask will the spikes show when zoomed in.

 

If the spikes are difficult to see just incase the exposure.