19 replies to this topic

[grace]

Hi all,

 

I have been reading a few threads here about issues with focusing and extenders. I understood that for a given telescope and eyepiece, the focal plane is fixed, that is, it does not depend on the object being focused on. Or at least it should be so for objects at infinity, like stars and planets. If this is true, then once an object is in focus, pointing at another object should not require refocusing. Questions:

1. Did I get it right? Is there a significant discrepancy between theory and practice?
2. How does it work with the moon? I guess it cannot be treated as being "at infinity" in this context, right? I have some short eyepieces (4mm, 6mm) that focus easily on planets but with which I cannot get to focus on the moon. Is there some math I can use to determine the length of the extender I need? (Yes, I can try empirically to move the eyepiece further out, but I got curious about the math...)

[sharkmelley]

The moon, planets and stars will all be at the same infinite focus, so in theory no refocusing is required,  However, in practice refocusing might be required after the scope is slewed to a different part of the sky because of the effect of gravity on parts of the optical train such as the focuser of some telescopes or the main mirror of an SCT.

[grace]

The moon, planets and stars will all be at the same infinite focus, so in theory no refocusing is required,  However, in practice refocusing might be required after the scope is slewed to a different part of the sky because of the effect of gravity on parts of the optical train such as the focuser of some telescopes or the main mirror of an SCT.

Thanks. What you suggest though makes me think about small refocusing adjustments, whilst in my case the same eyepiece works well on planets but cannot focus at all on the moon. Should I look at some other potential root cause?

[Astrojensen]

 in my case the same eyepiece works well on planets but cannot focus at all on the moon. Should I look at some other potential root cause?

How much refocus are we actually talking about? And how does the planets look at best focus? How about a star? Does it have the same focal point as the planets, or the same as the Moon, or a different one? 

 

It's normal to have slightly different focus between planets and the Moon, but this all happens in the eye. Apparently, the eye reacts differently to looking at a small, luminous disk on a black background, vs. looking at a large object that more or less fills the field of vision. I think it wants to perceive them lying at different distances. I always have to refocus slightly. If you then try using a camera, the focus is the same for all. 

 

 

Clear skies!

Thomas, Denmark

[grace]

Thanks Thomas. Planets and stars look sharp at best focus, but I'll have to do some more experimenting with repointing the scope with the same eyepiece without touching the focus - as soon as I get a clear sky...

 

(oh, I'd love to see the sky in Bornholm, here in Copenhagen it's quite a drive for a decent sky!)

[TOMDEY]

You're doing something wrong... not sure what. Stars, planets, and the moon are all at the same focus with possibly only tiny differences because your eye gets dazzled by the moon and your pupil contracts.  What telescope are you using... aperture and focal length?    Tom

[grace]

You're doing something wrong... not sure what. Stars, planets, and the moon are all at the same focus with possibly only tiny differences because your eye gets dazzled by the moon and your pupil contracts.  What telescope are you using... aperture and focal length?    Tom

Hi Tom! It's 8 inch f/6 (Bresser Messier 8 dobson, 203mm aperture, 1218mm focal length). Have this issue on the moon with 4mm and 6mm eyepieces, not with the other ones (15mm, 25mm, 32mm). 

[Gleb1964]

When observing with high magnification and a small exit pupil, the size of the observer's pupil does not affect the observation.

For ideal optics, the focus position should be the same for all types of objects. However, if the optics have some residual aberrations, the best contrast for different spatial frequencies will be at slightly different focal points.

Edited by Gleb1964, 12 February 2025 - 10:35 AM.

[AlamoBob]

The moon, planets and stars will all be at the same infinite focus, so in theory no refocusing is required,  However, in practice refocusing might be required after the scope is slewed to a different part of the sky because of the effect of gravity on parts of the optical train such as the focuser of some telescopes or the main mirror of an SCT.

Wait.  Are you saying that there is a gravimetric anomaly right next to my telescope??? Woo!  I just lost (or gained) 3.06 ounces!

[ChristianG]

Wait.  Are you saying that there is a gravimetric anomaly right next to my telescope??? Woo!  I just lost (or gained) 3.06 ounces!

There is a black hole below my basement. That's why left socks keep disappearing! There must be a reason, like some kind of symmetry in general relativity, as to why only left socks disappear? Like this morning, I put my right sock on, and couldn't find the corresponding left one in the clean sock bin!!! There must be something there.

 

The original poster's assesment was correct. As for the math for checking focus shift, here's a simple formula we learnt in high school for a lens, which is just fine for our purposes:  1/p + 1/q = 1/f. We have f as the focal length of the objective, p the object distance and q the image distance. 

 

If p = infinity (put a REALLY BIG NUMBER if you want), then you will have 1/q = 1/f thus the image distance will be q = f, say 2 m for an 8" f/10 SCT.

 

If you focus on the Moon, then p = 380 000 000 m, in which case 1/q = 1/2 - 1/380000000 = 0.499999997368. The image will now be at q =  2.0000000105, displaced by about 10 nm. For comparison, our good'ol friend Coronavirus is about 100 nm in size. I don't think even the most expensive gold-plated two-speed focuser has that resolution...

 

So as was said earlier, you can take the Moon at infinity for ALL practical purposes. This is analoguous to the reason why climbing a tree at Full Moon is not a viable space exploration strategy... Have fun!

 

--Christian

Edited by ChristianG, 12 February 2025 - 04:42 PM.

[Inkie]

This is a reflector.  Could it be at all possible that the focus does shift, ever so slightly, when the tube is re-oriented?  And that would be because of an inadequacy in the mirror mounting?  And if so, I doubt that it would only be for the Moon.  It should be whenever the OTA is placed in a certain altitude.

[ChristianG]

Yes, that's what post #2 is about in reality...

[WISDOC]

Welcome to C/N grace. As stated above the moon could be treated as infinity for viewing purposes. Just out of curiosity do you use a filter when you view the moon ? For me the moon is so bright as to be blinding to me at small exit pupils. Whithout a moon filter attached to the eyepiece it seems I can't reach focus with higher power eyepieces on the moon. For me it's simply thet Luna is so bright it overwhelms my vision.

  Clear skies and keep looking up. Doc

[Keith Rivich]

Hi Tom! It's 8 inch f/6 (Bresser Messier 8 dobson, 203mm aperture, 1218mm focal length). Have this issue on the moon with 4mm and 6mm eyepieces, not with the other ones (15mm, 25mm, 32mm). 

Methinks you are experiencing bad seeing. Bad seeing on the planets can be hidden a bit by the small angular size of each of the planets. The moon is in your face. Can't hide to much with with that much scale. 

[Gleb1964]

 

.. I understood that for a given telescope and eyepiece, the focal plane is fixed, that is, it does not depend on the object being focused on...

 

 

The focal "plane" is typically a curved surface where the best focus varies with the position of the object within the field of view.

The telescope tube is in continuous thermal drift, which changes the optimal focus position.

Pointing the telescope tube at the sky alters the tube's gravity flexure, affecting the focus.

[grace]

Thanks all for your comments! I think the theoretical side of the question is well-answered now! As for the issue I am experiencing, I will give it a proper check as soon as we have a clear sky here, hopefully in a couple of days!

[WillR]

Welcome to C/N grace. As stated above the moon could be treated as infinity for viewing purposes. Just out of curiosity do you use a filter when you view the moon ? For me the moon is so bright as to be blinding to me at small exit pupils. Whithout a moon filter attached to the eyepiece it seems I can't reach focus with higher power eyepieces on the moon. For me it's simply thet Luna is so bright it overwhelms my vision.

  Clear skies and keep looking up. Doc

+1. Moon should be at same focus point as everything else, or close. However anytime you use a filter, you have to refocus.

 

If he’s not using a filter, the OP’s issue is a mystery to me.

[grace]

Case closed. As it is often the case, the simplest explanation is the right one - I was doing something wrong. Mainly it was the issue of dazzling light and re-adjusting my own eyes. With some patience and an ND filter I had no issue focusing on the moon tonight. Thank you all for your support!   

[triplemon]

You might also want to visit an optometrist.

 

Your eyes could have some defects limited to the very center of your eyelens. In most "normal" daytime use you might not notice this so much, but with a telescope, in particular at very high magnification, all light is limited to a very narrow exit pupil only, often much narrower than your eyes pupils can contract. Its a sortof very selective eye lens test. Issues like that might remain unnoticed until it gets really bad, but may be treatable if detected early.

Edited by triplemon, 14 February 2025 - 05:30 PM.

[WillR]

Yeah, could be a cataract. I have no trouble looking at the moon in my 10” with no filter. A trip to the ophthalmologist might be in order.