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Recently I entered a discussion with someone that has a relatively popular YouTube channel which says among other things that the earth is flat. I said that earth isn't flat because you can see the ISS by yourself through the telescope. I was reminded that I'd only seen it on the internet, but not actually seen it directly myself. I was told that if I buy a telescope and film by myself they will also buy one, do the experiment, and post the footage on their YouTube channel.

So, what's the cheapest telescope that I can buy and record with a cellphone? I've seen people using a Nikon P9000 but what matters is its lens, which is already expensive, and the camera is expensive too. Maybe it's possible to buy a cheap photography lens that has both good quality and high zoom and can be used with a cellphone?

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    $\begingroup$ Hi ! The forums on cloudynights.com are a good place for this type of info. And other stargazer forums, of course ... good luck ! $\endgroup$
    – user31179
    Commented Dec 24, 2019 at 19:27
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    $\begingroup$ Errr ... Jupiter's size is 30 - 50 arcseconds, ISS around 50 ... yes, tracking and focussing is the problem for a newcomer. There are only seconds. $\endgroup$
    – user31179
    Commented Dec 24, 2019 at 19:56
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    $\begingroup$ If I may suggest, Jupiter and its moons are a better target than the ISS. A decent scope and good weather would show the terminator lines when it is around "half Jupiter". $\endgroup$
    – user31179
    Commented Dec 24, 2019 at 20:00
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    $\begingroup$ @ebv Since Jupiter is ~5x as far away from the Sun as we are, its phase is never much smaller than 99%. $\endgroup$
    – Mike G
    Commented Dec 25, 2019 at 0:16
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    $\begingroup$ I made some edits to your question to better fit the Stack Exchange format. I predict that if the flat-Earther does buy the telescope and do the experiment and post footage, they will use it to prove that the Earth is flat and the ISS is a hoax, and you'll be forever tied to it. I recommend that you walk away from flat-Earthers, rather than enable them, but it's your call. $\endgroup$
    – uhoh
    Commented Dec 26, 2019 at 2:08

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I think you'd want a focal length of 700-1000 mm, and the cheapest way to get that would be a basic Newtonian reflector of 100 mm aperture. Examples: Meade Polaris 114mm, Celestron PowerSeeker 114EQ, Orion SkyQuest XT4.5. There are brackets available to mount a smartphone on any telescope.

Before trying to track ISS, practice acquiring easy targets such as the Moon, the Pleiades, etc. Then use Heavens Above to predict ISS passes visible from your location. After you collect your evidence and your hyper-skeptical acquaintance rejects it, you can still use the scope to enjoy the planets and a few dozen star clusters, nebulae, and galaxies.

On the other hand, maybe you don't need a telescope. ISS is visible to the naked eye, is silent, and doesn't appear to slow down as much near the horizon as aircraft do. Seeing it disappear as it enters Earth's shadow could be persuasive.

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No such thing as a Nikon P9000 .... there are the P900 and P1000 model cameras, which come with a non-detachable lens, which can be zoomed to extreme focal lengths, included. These have indeed a good reputation for a viable solution for moon or even basic planetary shots, however these - being NON-interchangeable lens cameras - would be an exceptionally BAD fit for attaching them to a telescope or dedicated telephoto lens.

People have succeeded getting recognizable pictures of the ISS with this camera - eg https://www.youtube.com/watch?v=pDIPZFqfGGo, though there might be significant planning and skill involved.

Photography-oriented telephoto lenses for DSLR/DSLM cameras of that focal length are a) rarely cheaper than a P900 if you want them to have even the resolution the P900 provides, b) be significantly harder to use - affordable ones are going to be bulky and heavy and have no image stabilization or autofocus, these would take a lot of experience in handling tripods and manual focus in the dark. There are eg cheap 650-1300 zooms available for $200ish, resolution is questionable though and all the handling difficulties I mentioned apply in full - and these are only at all useful with an interchangeable lens camera, NOT a cellphone.

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The biggest difficulty for taking a photo/video of the ISS through a telescope is the tracking. Not all motorized mounts are able to do such a follow. And if you follow the ISS by hand with the telescope, you will get a pretty blurry image.

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