Same focus for Moon & stars??

[Peje]

Noticed something strange last night,

While I was waiting for my cluster target to clear the roof of my house (after getting good focus with Bahtinov mask) I decided to take a few (250) shots of the moon. I fixed on the target and started to readjust my focus since it should be different for the moon vs. the stars...at least that's what I thought was the case, turns out it's not.

Is this normal? I'm certainly not complaining but just thought I'd check to see if anyone else has experienced this.

Pete

[pipnina]

10 minutes ago, Peje said:

Noticed something strange last night,

While I was waiting for my cluster target to clear the roof of my house (after getting good focus with Bahtinov mask) I decided to take a few (250) shots of the moon. I fixed on the target and started to readjust my focus since it should be different for the moon vs. the stars...at least that's what I thought was the case, turns out it's not.

Is this normal? I'm certainly not complaining but just thought I'd check to see if anyone else has experienced this.

Pete

All objects in the night sky are far enough away to not require refocussing after changing target. Unless your telescope aperture is utterly gargantuan they'll all have the same focus point.

[Peje]

Oh really, not sure where I heard the info to the contrary. That's great, I normally put off grabbing some moon shots as I can't be bother refocusing.

Out of curiosity, when using the 5x zoom in live view this focus point should still say the same shouldn't it as none of the optics are changing? BackyardNikon is due for an update to finally allow us to use this feature. It'd be handy for grabbing some closeups of the moon.

[Ruud]

It's a consequence of the lens formula:

1/object distance + 1/image distance = 1/focal length 

Astronomical objects are at a very large distance, so 1/object distance is practically zero. As a result the image distance equals the telescope's focal length for all astronomical objects.

[SilverAstro]

Interesting to contemplate a(n imperfect ) gravitational lens, where the image distance can vary by days, months or even years depending upon which bit one is looking through ?

 

[hjw]

Hi Pete,

you can test focus during daytime with an object which is a couple of km or a mile away. For the telescope this is already close enough to infinity. Now 300,000 or 300,000,000,000 km doesn't really make a difference

Clear skies

HJ

[Peje]

Strangely I get this from my normal daytime photography, not sure why I thought a telescope would be different. So this also holds true for planets?

[hjw]

10 hours ago, Peje said:

Strangely I get this from my normal daytime photography, not sure why I thought a telescope would be different. So this also holds true for planets?

The closest Venus gets to us is 38,000,000km, so yes way more than 2 km. Planets are also at infinite as far as your scope is concerned.

[Peter Drew]

Try telling this to a visual observer!. Many seem to agree that there seems to be a slight difference in focus visually between stars and extended objects like the Moon and planets. Although this is obviously something of an optical illusion due to the distance the fact remains that the difference is the eye reacting differently to point source and extended objects.

[Stu]

3 minutes ago, Peter Drew said:

Try telling this to a visual observer!. Many seem to agree that there seems to be a slight difference in focus visually between stars and extended objects like the Moon and planets. Although this is obviously something of an optical illusion due to the distance the fact remains that the difference is the eye reacting differently to point source and extended objects.

I'm glad you said that Peter because that is my experience too. I know that they are both effectively at infinity, but I normally do re-tweak between planets and stars.

I guess altitude in the sky and atmospheric refraction may also have a slight affect?

[hjw]

1 hour ago, Stu said:

I'm glad you said that Peter because that is my experience too. I know that they are both effectively at infinity, but I normally do re-tweak between planets and stars.

I guess altitude in the sky and atmospheric refraction may also have a slight affect?

I wonder whether that has something to do with the size of the pupil? On a bright object like the moon the pupils will be small with a large depth of focus and compensate for slight imperfections in the scope's focus while with stars the pupil will be wide open and has only a small depth of focus. For a physical sensor there will be no difference.

[Peter Drew]

Good point hjw.