Focus

[en51nm]

Brief question regarding focusing. I assume that when I focus on a star/nebula/planet/etc I am effectively focusing on infinity? If this is the case, when is it I need to refocus when changing to look at another object? I can't believe (for example) when focusing on Jupiter and then switching to M42 the reason I have to refocus is because one is closer than the other!? Or perhaps i shouldn't actually have to refocus and I'm just doing something else wrong!?

Any and all explanations welcome!

[hemihaggis]

so do you have to refocus when swapping between objects ??

[themos]

No, you don't. But you do have to refocus when changing eyepieces. Unless they happen to be "parfocal" (to each other).

[paf90]

That's a good question! As I refocus between objects as well, but is it because I tend to cycle through EP's (still getting used to what's best to use) and also I guess it's down to the atmospheric fluctuations especially when looking at planets? As a newbie I think I fall into the trap of trying to push the magnification limit and yet keep the image as clear as possible.

[JamesF]

Well, there are big infinities and even bigger infinities

You shouldn't necessarily need to refocus when moving between objects though, all other things being equal. For imaging planets some people will focus on a star using a Bahtinov mask and then slew to their chosen planet without refocussing.

James

[Lancashire Astroguy]

Any object which is far enough away from your telescope to produce almost-parallel rays hitting the objective lens or mirror can essentially be regarded as being at infinite distance. In practise this would mean anything more than a few hundred metres away from the scope! Essentially then if a telescope is focussed on Jupiter it is also focussed on M42.

When you focus a telescope on "infinity" what you are actually doing is moving the eyepiece in and out until its focal point is coincident with the focal point of the objective lens. This is called putting the telescope into "normal adjustment".

You might wonder why astronomical telescope manufacturers don't just set up their telescopes in normal adjustment and do away with the focus wheel altogether! Well there are 2 good reasons. Firstly, when you change eyepieces you have to re-focus to bring the new focal point of the eyepiece back into coincidence with the objective focal point. Secondly, even with a single eyepiece you will often find yourself having to refocus during a nights observing. This is because varying temperature and even the position (angle) of the telescope tube will change the position of the optical components slightly. A tiny amount of refocussing is required to keep the scope in normal adjustment.

Regards, James

[Tim]

It can be tempting to mess with the focus setting when viewing planets, as you always seem to be chasing the best focus. However this is a feature of the atmosphere, not the telescope.

So as James says, it works pretty well to find a bright star, get really good focus, and then view your planets. Just accept that for most of the time the seeing will prevent you getting crystal clear images, but every now and then the detail just pops out at you

Most telescopes though will alter focus a little over the course of a night, especially fast ratio scopes whose depth of focus is shallower. These fluctuations are due to changing ambient temperatures causing elements of the telescope to expand or contract, thus altering the point of perfect focus. This is one reason carbon fibre tubes are popular, as they are thermally stable and alter very little if at all. For astro-imaging, that is a very useful feature.

Cheers

Tim

[UnderTheRain]

My Bahtinov mask arrived in the post yesterday just need a clear night now to give it a go

[en51nm]

Could the object's brightness effect the telescopes focal point?

In theory - as has been said - you can focus on one star, then slew 30 degrees round to another star and this should also be in focus because you are focusing on infinity. However, this doesn't seem to be the case...

As Lancashire Astroguy said, it could be down to (very) small changes in the position of the mirrors as your scope slews to a different angle. Or could it be something like the objects brightness, or is it more likely to be atmospheric?

Perhaps it is a combination of all these things, plus your eye playing tricks on you. Next cloud free night I'll play around with this ideal a bit more with my Bahtinov mask!

[Lancashire Astroguy]

Could the object's brightness effect the telescopes focal point?

Not at all. Light-rays within your scope behave in the same way no matter how many photons-per-second are travelling along them. That said, it is generally easier to focus on a bright star than a very dim one.

There are though two ancient curses of telescopes that you should be aware of. The first is chromatic aberration, which affects refractors. Basically, different wavelengths (i.e. colours) of light reach focus at slightly different points along the optical axis of the tube. Stars produce a spectrum of colours, so you always end up with a technicolour halo around your best attempt to focus a star. Chromatic aberration is largely nullified in modern refractors by the use of compound objective lenses and special coatings, but it is still there to some extent.

The other problem is spherical aberration, affecting reflectors. A primary mirror should be perfectly parabolic in shape to bring all the light rays hitting each part of it from a given direction to focus on exactly the same spot. Any deviation from a parabola (and some primary mirrors on older scopes are actually spherical in shape, not parabolic!) will cause objects near the centre of the field of view to have a different focus from those at the edge of the field of view (and vice-versa). Again, most modern scopes have parabolic mirrors so this isn't something to worry about, although a badly collimated reflector will produce a similar effect!

Regards, James