Moon AND star focus

AnthonyL · Oct 1, 2015

Normally one is either focusing on the moon or on the stars. The eclipse gave an opportunity to see stars behind the moon and that begs the question - is there any noticable difference in the focus when focusing on the moon compared to on the stars? I'm of the impression that even focusing on the moon requires a re-focus to then take a sharp image of one of the bright planets when on max focal length.

Talking about a DSLR here with say lens between 250mm - 600m.

W5JCK · Oct 1, 2015

AnthonyL said:
Normally one is either focusing on the moon or on the stars. The eclipse gave an opportunity to see stars behind the moon and that begs the question - is there any noticable difference in the focus when focusing on the moon compared to on the stars? I'm of the impression that even focusing on the moon requires a re-focus to then take a sharp image of one of the bright planets when on max focal length.

Talking about a DSLR here with say lens between 250mm - 600m.

In my experience I see varying degrees of infinity focus. I don't really notice much difference at WA and UWA, as those lenses tend to have great depth of field. But the longer the FL and the more wide open the aperture, the more variance I see. I don't think I've really noticed much difference between focusing on the Moon and on stars when using a 400mm lens, but perhaps when you get out to a few or many times that FL it can be an issue. Not sure. I did notice that the Moon and stars captured together in both my 400mm and 85mm images were indeed all in focus. However, I have noticed that focusing on a distant airplane or street light does not always mean the stars will also be in focus. I've had many failures at that practice. I'm not a telescope person so I cannot address the much longer FL or telescope usage.

Blucenturion · Oct 1, 2015

Focusing on the moon will not suffice for sharp stars, and to the best of my recollection, neither for Jupiter or Saturn. Seems illogical, but there it is.

Blucenturion · Oct 1, 2015

My answer is based on a 400mm FL. As Jack said, shorter lengths will be more forgiving.

alexisgreat · Oct 1, 2015

AnthonyL said:
Normally one is either focusing on the moon or on the stars. The eclipse gave an opportunity to see stars behind the moon and that begs the question - is there any noticable difference in the focus when focusing on the moon compared to on the stars? I'm of the impression that even focusing on the moon requires a re-focus to then take a sharp image of one of the bright planets when on max focal length.

Talking about a DSLR here with say lens between 250mm - 600m.

I have a related question..... how long of an exposure does it take to bring out background stars during a total lunar eclipse? I took some 1/4 sec exposures at f/4.5 and ISO 3200 at 170mm and cant see any background stars.

Tristimulus · Oct 1, 2015

AnthonyL said:
Normally one is either focusing on the moon or on the stars. The eclipse gave an opportunity to see stars behind the moon and that begs the question - is there any noticable difference in the focus when focusing on the moon compared to on the stars? I'm of the impression that even focusing on the moon requires a re-focus to then take a sharp image of one of the bright planets when on max focal length.

Talking about a DSLR here with say lens between 250mm - 600m.

If the moon is in focus then the stars will be in focus. Problem is that critically focusing (talking about focusing within a few microns here) the moon with a 250-600mm lens is impossible if using the camera without other focus aid than magnified live view. Getting good focus is not that much of a problem but critical focus is something very different.

Using a Bhatinov mask and a bright star might be a better way o focus.

Lots of lenses also have rather sloppy mechanical tolerances making exact focusing impossible. Have some of those lenses in my posession. Great for ordinary photography but not quite up to more serious astrophotography. So the problem mentioned migh be due to sloppy mechanics.

Chromatic abberration is another quirk. When focusing my Zeiss 2/135mm lens best focus is slightly inside the blue focus (red, green and blue light have different focii). This way I get rid of the dreadful red halos but the stars are technically ever so slightly out of focus. Still with this focus setting the stars look far better in the final image than at visibly best focus. And the final images actually look sharper.

Different lenses have different quirks.

And then add the photographer. We all have different standards and different ways to cope with problems. What is a focus problem to one might be nonexistent to another.

W5JCK · Oct 1, 2015

alexisgreat said:
AnthonyL said:

Normally one is either focusing on the moon or on the stars. The eclipse gave an opportunity to see stars behind the moon and that begs the question - is there any noticable difference in the focus when focusing on the moon compared to on the stars? I'm of the impression that even focusing on the moon requires a re-focus to then take a sharp image of one of the bright planets when on max focal length.

Talking about a DSLR here with say lens between 250mm - 600m.

Click to expand...

I have a related question..... how long of an exposure does it take to bring out background stars during a total lunar eclipse? I took some 1/4 sec exposures at f/4.5 and ISO 3200 at 170mm and cant see any background stars.

alexisgreat · Oct 1, 2015

W5JCK said:
alexisgreat said:

AnthonyL said:

Normally one is either focusing on the moon or on the stars. The eclipse gave an opportunity to see stars behind the moon and that begs the question - is there any noticable difference in the focus when focusing on the moon compared to on the stars? I'm of the impression that even focusing on the moon requires a re-focus to then take a sharp image of one of the bright planets when on max focal length.

Talking about a DSLR here with say lens between 250mm - 600m.

Click to expand...

I have a related question..... how long of an exposure does it take to bring out background stars during a total lunar eclipse? I took some 1/4 sec exposures at f/4.5 and ISO 3200 at 170mm and cant see any background stars.

--
https://supermanalexthegreat.shutterfly.com/

Click to expand...

Each lens, camera, exposure settings combo is different. I captured stars with both my a6000 and 400mm lens at f/5.6, 04. sec, ISO 3200, and with my a7 and 85mm f/2.8, 2 sec, ISO 1600. It might come down to few things.

Is the focus really sharp? It is difficult to focus on the eclipsed Moon. If focus is too soft, then faint stars will just disappear.

What is the IQ of the lens? A so-so lens might have more difficulty in pulling in the star light than does a really good lens.

What is the low light capability of the camera/sensor being used? This can make a lot of difference. A full frame camera will have an advantage over cropped cameras, and some cropped cameras do better than others at capturing stars under eclipse conditions.

I do notice that you shot a slower shutter speed with the 170mm lens than I did with my 400mm lens. You probably could have used a 1 sec shutter without issue. The longer shutter would help considerably.

--
Jack Swinden
W5JCK, amateur radio operator
An astrophotography hobbyist and amateur radio instructor and examiner. Sony a7 and Sony a6000. https://www.flickr.com/photos/jackswinden/albums

Thanks Jack I saw both of your collections, the one on flickr and the time lapse on youtube and I thought both were really outstanding. Did you see more stars with the 85mm f/2.8 2 sec ISO 1600 combo than you did with the 400mm f/5.6 0.4 sec (?) ISO 3200 combo?

One thing I was trying to figure out is why right at the midpoint of the eclipse (10:47PM here) and for about the next few minutes after that the moon got super dim even with my given settings, but during the last 15 minutes of totality the moon actually seemed much brighter- even though it was still officially in totality until 11:23 PM. In addition, it seemed the moon was much dimmer in the first half of totality than during the second half, even when the moon had just entered totality at 10:11 PM it was very dim compared to the last 15 minutes of totality.

Should I have used different settings for the first half of totality and the midpoint, versus the second half of totality?

--
https://supermanalexthegreat.shutterfly.com/

W5JCK · Oct 1, 2015

alexisgreat said:
Thanks Jack I saw both of your collections, the one on flickr and the time lapse on youtube and I thought both were really outstanding. Did you see more stars with the 85mm f/2.8 2 sec ISO 1600 combo than you did with the 400mm f/5.6 0.4 sec (?) ISO 3200 combo?

One thing I was trying to figure out is why right at the midpoint of the eclipse (10:47PM here) and for about the next few minutes after that the moon got super dim even with my given settings, but during the last 15 minutes of totality the moon actually seemed much brighter- even though it was still officially in totality until 11:23 PM. In addition, it seemed the moon was much dimmer in the first half of totality than during the second half, even when the moon had just entered totality at 10:11 PM it was very dim compared to the last 15 minutes of totality.

Should I have used different settings for the first half of totality and the midpoint, versus the second half of totality?

--
https://supermanalexthegreat.shutterfly.com/

There are some good animations on this website that illustrate the eclipse.

http://earthsky.org/tonight/total-lunar-eclipse-blood-moon-hunters-moon-september-27-28-2015

Basically the Earth's shadow was much bigger than the Moon. Even though the Moon remained completely covered by the Earth's shadow for over an hour, at the mid point of the full eclipse, the Moon was closer to the center of that shadow. The shadow is darkest in the center and progressively less dark as you move to the outer edges. So as the Moon entered full eclipse, then later exited full eclipse, the shadow was less dark than when it was at the mid point.

--
Jack Swinden
W5JCK, amateur radio operator
An astrophotography hobbyist and amateur radio instructor and examiner. Sony a7 and Sony a6000. https://www.flickr.com/photos/jackswinden/albums

W5JCK · Oct 1, 2015

I did see more stars in the 85mm images, but I suspect that was due to the larger FOV. Most of the stars I saw were too far away from the Moon to make it into the 400mm FOV.

AnthonyL · Oct 1, 2015

Trollmannx said:
AnthonyL said:

Normally one is either focusing on the moon or on the stars. The eclipse gave an opportunity to see stars behind the moon and that begs the question - is there any noticable difference in the focus when focusing on the moon compared to on the stars? I'm of the impression that even focusing on the moon requires a re-focus to then take a sharp image of one of the bright planets when on max focal length.

Talking about a DSLR here with say lens between 250mm - 600m.

Click to expand...

If the moon is in focus then the stars will be in focus. Problem is that critically focusing (talking about focusing within a few microns here) the moon with a 250-600mm lens is impossible if using the camera without other focus aid than magnified live view. Getting good focus is not that much of a problem but critical focus is something very different.

So set the moon in good focus (10x mag in live view), set focus to manual and all other stellar objects will be sharp? And vice versa - get a clear star (I know, not so easy), lock focus and the moon will be sharp? I only have 250mm on a crop but I guess I'm going to have to try it. Ideally I'd like to see whether the lens has moved between one or the other but I suspect I won't be able to see that accurately.

I've had fairly sharp stars, and I've managed to get a couple of Jupiter's moons.

AnthonyL · Oct 1, 2015

W5JCK said:
I did see more stars in the 85mm images, but I suspect that was due to the larger FOV. Most of the stars I saw were too far away from the Moon to make it into the 400mm FOV.

This chap did it with 600mm

http://www.dpreview.com/forums/post/56559812

You'll need to view at 100% but the stars to the top right are quite clear. The brightness of the moon usually masks them from view.

W5JCK · Oct 1, 2015

AnthonyL said:
W5JCK said:

I did see more stars in the 85mm images, but I suspect that was due to the larger FOV. Most of the stars I saw were too far away from the Moon to make it into the 400mm FOV.

Click to expand...

This chap did it with 600mm

http://www.dpreview.com/forums/post/56559812

You'll need to view at 100% but the stars to the top right are quite clear. The brightness of the moon usually masks them from view.

Well, here are two images taken about 20 seconds apart. These are full size 24MP images. One is 400mm and one 85mm. I had plenty of clouds to deal with and it looks like your chap didn't.

a6000 and Canon EF 400mm f/5.6L USM

a7 and Rokinon 85mm f/1.4

--
Jack Swinden
W5JCK, amateur radio operator
An astrophotography hobbyist and amateur radio instructor and examiner. Sony a7 and Sony a6000. https://www.flickr.com/photos/jackswinden/albums

AnthonyL · Oct 1, 2015

Well at 100% I see a couple of things that look like stars to the right of the moon in your zoomed in image.

W5JCK · Oct 1, 2015

AnthonyL said:
Well at 100% I see a couple of things that look like stars to the right of the moon in your zoomed in image.

I'm not sure what your point is. The images are not zoomed, they are both full size 24MP images. There are quite a few stars in both images, though they seem to me to be more easily seen in the 85mm image.

AnthonyL · Oct 1, 2015

W5JCK said:
AnthonyL said:

Well at 100% I see a couple of things that look like stars to the right of the moon in your zoomed in image.

Click to expand...

I'm not sure what your point is. The images are not zoomed, they are both full size 24MP images. There are quite a few stars in both images, though they seem to me to be more easily seen in the 85mm image.

I may have misread your post. The stars you see close to the moon in your 400mm image would not normally show up and are only visible because the moon was dimmed. The ones further out as per the 85mm presumably would show (subject to exposure) whether or not there was an eclipse.

By zoomed I meant at the longer focal length (sorry a bit lazy there), compared to the 85mm.

t_wade · Oct 1, 2015

To all,

The Moon and the stars are both at infinity no matter the focal length. In addition, a point source (i.e., stars) will always be easier to focus on than an extended object such as the Moon. When photographing the Moon, I always use a nearby star to achieve "perfect" focus. If you have a SCT, keep a watch out for mirror flop if you have to move your telescope away from the Moon to find a decent star to focus on. In addition, you don't want to pick a very bright star. Pick a star that is bright enough to comfortably see on the screen at 10X.

In summary, always use a star to achieve near perfect focus. If you are really lucky, fainter stars will appear as you approach critical focus. This way you know you have nailed it.

Wade

Bob in Baltimore · Oct 1, 2015

AnthonyL said:
Normally one is either focusing on the moon or on the stars. The eclipse gave an opportunity to see stars behind the moon and that begs the question - is there any noticable difference in the focus when focusing on the moon compared to on the stars? I'm of the impression that even focusing on the moon requires a re-focus to then take a sharp image of one of the bright planets when on max focal length.

Talking about a DSLR here with say lens between 250mm - 600m.

Simple answer: No, there is no noticeable difference in the focus. The distance of an image from the center of a simple lens is calculated by this formula, which is adequate for our purposes.

1/S1 + 1/S2 = 1/f

S1 is the distance of the object (384,000 km = 384,000,000 m)

S2 is the distance from the center of the lens to the image in meters

f is the focal length of your lens in meters (0.6).

The image of the Moon is 0.600000003 meters from the lens center. A star image, at effectively an infinite difference would be exactly 0.600000000 meters away. The difference is 3 billionths of a meter. (A human hair is 10,000 time larger than this!)

So why does it seem to be a problem?

Trollmannx has explained this ... it is hard to get a good focus on the Moon. The Moon may look pretty good, but it may not be good enough for a star. Stars are so far away that the ought to appear as a point of light. Optics (Airy disk) and atmospheric turbulence (seeing) probably make the star appear to be about 1 arc second, which is in the range of a pixel, depending on your telescope and your camera. Lunar features, even the sharpest ones are 5-10 pixels across. again, depending on your setup.

Bob in Baltimore · Oct 1, 2015

<snip>

W5JCK said:
W5JCK said:

One thing I was trying to figure out is why right at the midpoint of the eclipse (10:47PM here) and for about the next few minutes after that the moon got super dim even with my given settings, but during the last 15 minutes of totality the moon actually seemed much brighter- even though it was still officially in totality until 11:23 PM. In addition, it seemed the moon was much dimmer in the first half of totality than during the second half, even when the moon had just entered totality at 10:11 PM it was very dim compared to the last 15 minutes of totality.

Should I have used different settings for the first half of totality and the midpoint, versus the second half of totality?

Click to expand...

AnthonyL · Oct 1, 2015

Thanks for the detailed responses, techniques and formula. Next time I take a photo of the moon I'll try focus on a star or planet instead and see how I get on.

Moon AND star focus

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