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I am able to find the moon with the finder scope during the day. However, when I use my 20mm lense, it seems like I am unable to focus the moon at all. Is there a fix to this?

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  • $\begingroup$ What happens when you use the finderscope to sight the moon at night? $\endgroup$ Commented May 11, 2022 at 12:12

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There should be absolutely no problem at all seeing the bright, sun-illuminated part of the Moon during the day at 1000 mm / 20 mm = 50x through your telescope. The part that's not illuminated by the Sun will still be illuminated by "earthshine" - light reflected by the sunlit part of the Earth like for example where you are looking from - and since we can see parts of that with our eyes and 7x to 10x binoculars as brighter than the sky itself, it should also be easily see-able at low power in a telescope.

The problem might simply be that you've started too far out of focus and/or that your finder scope wasn't aligned.

If you are very careful to never point your telescope towards the Sun, find some object far away on the horizon, at least several hundred meters or a kilometer away and point your finder scope at some feature. Then focus your telescope on that, and confirm that your finder and main scope are aligned to each other.

Then go back and try again. It really should just work.

Note that if you can only focus on a closer object, you can easily refocus to infinity using math.

Your focal length is 1000 mm (= 127 mm x f/7.87) or 1 meter. If you first focus at an object at say 50 meters, your eyepiece will now be 0.02 meters (2 cm) further out than it should be for infinity and you can just move it by that amount back in to your scope and trust you'll be close to in-focus for infinity (assuming you measured that 50 meters correctly).

If your focal length is $f$ and the distance from your scope to the object is $L$ and $d$ is the distance you need to move your eyepiece back in to achieve an infinite conjugate focus, then starting from

$$\frac{1}{f} = \frac{1}{L} + \frac{1}{f+d} $$

we can get to

$$d = Lf \left( \frac{1}{L-f} - \frac{1}{L} \right).$$

Alternatively just focus your telescope at night with this eyepiece then find some way to note and record the eyepiece position so that you can reproduce it during the day, or simply just leave it there.

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