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# Prefocus on mountain X miles away?

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### #1 orionic

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Posted 22 February 2020 - 04:14 AM

What is the minimum distance X (miles), such that if I focus on a mountain at that distance (in daytime), I can then lock my focus and expect perfect focus for celestial objects?

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### #2 Astrojensen

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Posted 22 February 2020 - 04:48 AM

Depends on the depth of focus for your system. I calculate, that if you want a precision to around 0.01mm at a focal length of 1,000mm, the mountain needs to be 100km away. A distance of 10km will give a difference of 0.1mm.

A longer focal length will need a greater distance. A focal length of 2,000mm will give a focus difference of 0.4mm at a distance of 10km.

I used the formula: I = (O x f) / (O - f), where I is the focus distance, O is the object distance and f is the focal length with the object at infinity.

That was the only formula I could find, but it seems to me, based on my experience of how much I have to defocus my telescopes, when observing the landscape or airplanes, that it's not entirely correct and tend to underestimate the amount of defocus.

Clear skies!
Thomas, Denmark

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### #3 TOMDEY

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Posted 22 February 2020 - 04:59 AM

Cool! OK, I just derived it here. Casually, you would want it to exceed Rangemin = D2/(2 x lambda), where D is the aperture of your telescope and lambda is the wavelength of the light. That would be a quarter-wave, in the wavefront, of pure defocus. e.g. for your 8-inch telescope, that range would be twenty-four miles for ~green light. The quadratic relationship means that the wavefront would be twice as good, if the mountain is thirty-four miles away. That's really all there is to it!    Tom

Edited by TOMDEY, 22 February 2020 - 04:59 AM.

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### #4 GaryShaw

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Posted 22 February 2020 - 10:35 AM

Interesting stuff...... and fun, as long as we acknowledge that calculations or simplistic vacuum-based math fails to account for a great many practical considerations such as optical train type and quality, scope and air temperature fluctuations, atmospheric conditions, etc. If an observer is interested in optimizing focus, the act of focusing is best conducted at the time and place of observation and using proven focusing techniques. When using new optics I’ve also found it helpful to use these ‘prefocus’ techniques to get gear ‘close’ to focus in daytime and then optimize adjustments as I commence observing.

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### #5 orionic

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Posted 22 February 2020 - 05:37 PM

Wow, thanks everyone, and it's interesting to see such different ways of going about the calculation, but with answers in the same ball park.  Tomdey, your physics are completely beyond me, and I am glad it was a stimulating exercise!

I guess one does need to define an acceptable range of focus.  In practical terms, on my SCT there is a range of "good enough" focus, within which the variation is completely indistinguishable to my eye, and I guess that parameter is hard to define.

My mountains are about 8 miles away, so I suspect it is pushing the boundaries of acceptable precision.  But I will test it empirically and see if it leaves me outside the "good enough" range for astronomical use.

### #6 OleCuss

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Posted 23 February 2020 - 06:21 AM

Interesting stuff...... and fun, as long as we acknowledge that calculations or simplistic vacuum-based math fails to account for a great many practical considerations such as optical train type and quality, scope and air temperature fluctuations, atmospheric conditions, etc. If an observer is interested in optimizing focus, the act of focusing is best conducted at the time and place of observation and using proven focusing techniques. When using new optics I’ve also found it helpful to use these ‘prefocus’ techniques to get gear ‘close’ to focus in daytime and then optimize adjustments as I commence observing.

^^^This is where it's at.^^^

There are all kinds of things which go into getting your focus right.  And the atmosphere is a huge part of that.  It doesn't help that things tend to get a bit fuzzy looking at distant terrestrial objects so dialing in the focus quite precisely doesn't work so well.  Remember that most of us use a Bahtinov mask to really nail down our focus on the stars and there is simply no way you can do that on almost any distant terrestrial target.

And then you have to remember that your focus at the zenith is different than is your focus closer to the horizon.

And your focus changes with temperature.  So even if you ignore everything else, your daytime focus isn't going to work because your OTA's temperature will almost certainly be different during the daytime than it is at the time you are imaging at night.

So you get a rough focus during the daytime and count on having to refine it when you start imaging/observing at night - and especially if you are imaging you have to count on periodically re-focusing throughout the night as the temperature changes and as your targets change.

Remember, this can be sufficiently repetitive and boring that a whole lot of people get focusers which can be controlled by a computer and then throughout the night the focuser, computer,and software conspire to handle the focusing issue.

Edited by OleCuss, 23 February 2020 - 06:23 AM.

### #7 Clouzot

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Posted 23 February 2020 - 07:25 AM

Apart from the ubiquitous Bahtinov mask, what do you guys use to check focus? I found it quite hard to get the perfect V-shape curve out of Sharpcap’s FWHM or the multi FWHM. And putting on the Bahtinov mask in the middle of a session means going outside when you’re working remotely

### #8 GaryShaw

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Posted 23 February 2020 - 11:14 AM

“And putting on the Bahtinov mask in the middle of a session means going outside when you’re working remotely“

Right you are but that’s the way it is if you do want truly optimized focus. Frankly, for EAA, most nights I use the mask to start and don’t update focus again. It’s really about observing and learning about objects and not about sharpest focus and picture-taking. My sessions don’t often exceed 3-4 hours so focus doesn’t drift all that much.
Cheers
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### #9 Clouzot

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Posted 23 February 2020 - 04:46 PM

Indeed. I was just wondering whether I had overlooked something evident or not. I usually focus once (after the OTA seems stable) and never look back, unless donuts start appearing (which I take as a sign that I should pack up things and go to bed)

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