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randomic

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Posts posted by randomic

  1. 39 minutes ago, lukebl said:

    But, like I said, if the meteors are travelling at 30,000mph and the earth is rotating at 1000 mph, it wouldn’t make much difference whether the earth was rotating into the storm or away from it. i.e. the speed of the meteor entering the atmosphere would be c. 29,000mph in the evening and 31,000mph in the morning. Not much difference. 

    You misunderstand me. I mean not the Earth's rotation but it's motion in orbit around the Sun. Which is more like 70,000 mph!

    Meteors are not so much being fired at the Earth from a particular direction and are more debris in the path of the Earth's orbit with which it collides.

    Remember this is also not about the speed at which the meteors hit the atmosphere but the frequency.

    During midday to midnight, the same number of meteors are hitting Earth, it's just that most of them are hitting the other side of the planet from you.

    • Like 2
  2. Could it be a doppler effect? When the surface of the Earth is moving towards the source of the meteors the observed frequency will be higher, and then lower when moving away.

    I can't imagine it would account for more than a 5% frequency difference though.

     

    Edit: Hang on, just occurred to me that the rotation is probably completely irrelevant:

    Isn't it just that from midnight-midday you're on the side of the earth which is going "forward" into the meteors? And from midday-midnight you're going "backwards"? The bugs on the windshield analogy seems spot on.

  3. 12 minutes ago, Damien420 said:

    am I stupid for downsizing?  

    Definitely not. At the end of the day, if the scope is so big that you dread taking it out on a clear night then it doesn't matter how big its aperture is!

    12 minutes ago, Damien420 said:

    Second I am not that clued up on where everything is in the night sky other than the basic objects

    I can recommend the book "Turn Left at Orion"; learning your way around the night sky is extremely rewarding.

    That said, absolutely nothing wrong with wanting a GoTo mount. What sort of astronomy do you do/want to do?

    • Like 4
  4. 56 minutes ago, GalaticBoba said:

    I would like to eventually get into Astrophotography

    For astrophotography the mount is arguably the most important part. It might well be the most expensive part. Make sure you get an Equatorial mount, alt-az is fine for visual and planetary but difficult to use for long exposures. A common beginner mount is the SkyWatcher HEQ5 Pro. If I was looking for an entry level mount today I would strongly consider the iOptron CEM26 as it's very lightweight for its payload capacity.

    https://www.firstlightoptics.com/equatorial-astronomy-mounts/skywatcher-heq5-pro-synscan.html
    https://www.firstlightoptics.com/ioptron-mounts/ioptron-cem26-center-balanced-equatorial-goto-mount.html (most recommend the 1.75" tripod for better stability)

    There is a learning curve to an EQ mount but it's not too bad, especially with things like iPolar.

     

    56 minutes ago, GalaticBoba said:

    I do want an all purpose telescope, deep space and planetary

    I'm sure this is something every astronomer would love! Sadly, such a telescope doesn't exist. The closest thing is probably a Schmidt–Cassegrain telescope (SCT) but they tend to have very local focal lengths which can be tricky to manage with entry-level equipment and expertise. Wide field targets (like Andromeda) are impossible to capture without putting a lot of effort into a mosaic.

    If you were mainly looking at visual, most would recommend a Dobsonian (best aperture for your money) - example https://www.firstlightoptics.com/beginner-telescopes/skywatcher-skyliner-200p-dobsonian.html
    If you were mainly looking at DSO imaging, most would recommend a lightweight refractor (SkyWatcher 80ED is popular) - https://www.firstlightoptics.com/pro-series/skywatcher-evostar-80ed-ds-pro-ota.html
    If you were mainly looking at planets, visual or imaging, most would recommend a Newtonian - example https://www.firstlightoptics.com/reflectors/skywatcher-explorer-200p-ds-ota.html

    There are also Maksutov–Cassegrains and Classical-Cassegrains which, from what I understand, are similar to SCTs (I'm sure someone with more knowledge than me can go over the pros/cons).

     

    The most important thing above all else is get equipment that you will realistically use. There's nothing worse than seeing a clear sky and dreading the equipment setup. A scope with a quick setup will generally see far more use.

    (maybe you're a masochist like me and consider the debugging part of the hobby)

     

    56 minutes ago, GalaticBoba said:

    I want to hook the mount up to a laptop, wireless is preferable

    Some manufacturers do offer mounts with built-in wifi but it is not necessary for remote control. Almost every mount can be computer driven via a standard called ASCOM. You can plug your mount into astroberry, Stellarmate, ASAIR (all basically raspberry pis with astronomy software) and control them from a phone/tablet/laptop on the same network.

    Feel free to ask any questions!

    • Like 2
  5. 1 hour ago, scitmon said:
    1. Back focus... what is it?  I'm guessing its the required distance needed from the back of the telescope to achieve focus with the camera?  ZWO say most telescopes require 55mm, but is that a rough guide or does it have to be exact?  Will my FF/FR on my 80ED eat into that back focus?

    The term is somewhat overloaded.

    For a refractor and some reflectors, backfocus is how far behind your telescope the focal plane is. The more equipment you have behind your focuser, the closer in you have to move it to put your sensor at the focal plane. This is what people mean when they talk about "using up" backfocus or having enough room in the backfocus for some equipment.
    If you're using a flattener/reducer then it will have its own backfocus distance which will be different (not necessarily smaller or bigger).
    The important thing is just whether or not you're able to achieve focus.

    Some SCTs focus in a slightly different way, rather than moving the sensor to the focal plane they move the primary mirror to move the focal plane on to the sensor. This means you're able to reach focus at a wide range of backfocus distances.
    If you introduce a flattener/reducer (or if there's an internal one) then it will have an "optimal backfocus" listed. You need to put the sensor as close as possible to the distance to get the maximum performance out of the scope. This can mean a lot of playing around with extenders.

    tl;dr For most scopes the focal length is fixed so you just need to make sure you can reach focus. For some SCTs you need to be really careful about where you put the sensor to get the best performance.

    • Thanks 1
  6. The perhaps more astound non-uniformity is the lack of anti-matter we observe in the universe. All things being symmetric (which things tend to be in the standard model), there should be equal amounts of anti-matter and matter in the universe. The apparent dominance of matter in the universe (among other things) opens the question of what the mechanism for this symmetry breaking is.

    Another important thing to note is that the CMB isn't as far back as things go, it manifested when the universe became transparent (before then matter was ionised and so light couldn't get very far before being absorbed).

  7. Here's a calculator which has a bunch of explanation about optimal image scale: https://astronomy.tools/calculators/ccd_suitability
    It depends on focal length rather than focal ratio.

    For long exposures you aim for 1-2" per pixel (as a rule of thumb, it depends on how still the atmosphere is). If your scale is too large it takes longer to collect the light, if it's too small then you're missing out on resolution.

    For planetary work you're doing tens or hundreds of exposures per second and picking ones which come out best (where the atmosphere is stillest), a process called lucky imaging. Here you're operating much closer to the optical limit so you aim for somewhere between 0.1-0.5" per pixel.

    Planets are bright so it's hard to run in to but as long as your exposure time isn't reducing your framerate you're golden!

  8. I don't think you'll need a barlow at all. If I'm running the numbers correctly you're at ~1800mm focal length. With the 462 that puts you at an image scale of 0.33" per pixel which, if anything, is on the large side for planetary. I don't think you'd want the image any larger (at least, not for the drawback of putting more glass in the light path).

     

    Might want to consider an IR-pass filter too.

    The 462's bayer filter is almost transparent in IR and IR is less affected by seeing. Therefore, you can get some really high res mono shots of the planets which you can combine with colour data (using the IR-cut filter).

     

    I've never used an ADC so can't really give my experience.

     

    ASI AIR Pro could probably stream to an indoor PC, it depends what your wireless signal is like at the scope. Bear in mind that (in hardware terms) it's basically a Pi 4 so a laptop might be capable of getting better framerates from the camera.

     

    Lastly, the 462's chip is very small, see how you get on but I find it very difficult to get the target on the chip.
    To help, you could consider a flip-mirror so you could line up the object with an eyepiece (optionally one with a reticule).

  9. 59 minutes ago, DaveL59 said:

    well glass is considered to be a fluid, so I guess over a long enough period of time it could if left undisturbed eventually flow such that the lens changes shape. That'd apply whichever orientation you store it in but in our lifetimes and our grandchildren's even it'd not be noticeable. Grease in the focuser on the other hand might flow if temperatures get high enough but other than that I'd expect they'd be just fine.

    The whole glass being a very slow flowing liquid thing is a myth. You don't have to worry about your lenses.

    https://www.cmog.org/article/does-glass-flow

    • Haha 1
  10. 29 minutes ago, Mick_1960 said:

    I would like to be able to see close planets with some detail and galaxies.  Am I asking too much?

    The EdgeHD 800 is amazing for planets. On a good night I can make out details on Mars and can see the disk and colour of Uranus, it's awesome. I use the Baader Zoom 8-24mm eyepiece. For imaging planets I have a ZWO ASI 462MC but I've not been able to take it for a spin yet due to weather. You technically can do it with a DSLR but what you really want is high framerate video capture.

    For galaxies, the EdgeHD 800 is again pretty great for visual. I use the 40mm eyepiece which comes with the OTA and can make out the core of Andromeda even from very light polluted skies. Your experiences here will depend mostly on light pollution. The darker the skies the more you'll see.

    When it comes to photographing large objects like galaxies the EdgeHD 800 will struggle. You say you have a full frame camera which is good but even with that your field of view is narrow enough that you'd have to mosaic to get a complete picture of, say, Andromeda. If you're photographing smaller galaxies or nebulae then it's absolutely fine. Just be aware that the mount is the biggest factor here, even small imperfections in tracking can easily ruin long exposures when you're at 2000mm focal length.

    • Like 1
  11. 1 minute ago, Mick_1960 said:

    I had thought about possibly getting a Celestron Advanced VX 8” Edge HD but  I'm concerned about making mistakes.

    I almost bought that combo but the general consensus is you can get better mounts for the same money. So, I bought the EdgeHD 8" OTA and HEQ5 Pro separately. CEM25p is a good alternative.

    An 8" SCT OTA is on the limit of what these mounts can perform well with for long exposures so you might want to stretch for a Celestron CGX or CEM40 mount. Alternatively you might get an autoguiding setup to help the mount perform at its best.

    I've really enjoyed my EdgeHD 800 so far but it might be just as good getting a C8 XLT, unless you know for sure that you want the internal flattener (which comes with its own set of pitfalls).

    • Like 1
  12. If I had to spend that much I'd probably go with iOptron CEM40 and a Celestron EdgeHD 925 but it depends a LOT on what kind of targets you want to shoot.

    BUT

    If you're just starting out I don't think you need to spend that much. A SkyWatcher HEQ5 Pro or iOptron CEM25p paired with an 80mm apo like SkyWatcher Evostar 80ED will net you some amazing shots of wide field targets.

    • Like 3
  13. I made a diagram which might help visualise. These points are not interacting so in the right-hand case they would drift apart indefinitely. Obviously, the oceans are gravitationally bound to the Earth so they don't just leave 😂

    The red ellipses are not supposed to be an accurate representation of the tidal bulge shape, rather just a visual aid.

    tqtxQuK.png

    P.S. The end of the caption on the right hand diagram should say "the outer two move away from it."

  14. 1 hour ago, JamesF said:

    I have been wondering if the demosaic process might be something suitable for pushing off onto a GPU.

    Hypothetically, couldn't everything happen on the gpu? Since as long as the frames end up in the right order it doesn't matter in what order they're processed (assuming you don't run out of vram).

  15. On 21/10/2020 at 09:42, JamesF said:

    Thanks for that.  I often wonder myself if I'm interpreting the output of lsusb correctly :D

    James

    Out of boredom I built oacapture from source this afternoon and you'll be pleased to know that the ASI462MC is now recognised correctly. It seems to work fine although only manages to record at 6 fps.

    On a semi-related note, is a 67 frame SER supposed to be ~500MB?

  16. 13 minutes ago, JamesF said:

    First I tried to decide, if the Moon and Earth were stationary relative to each other, would there be a tidal bulge

    Yep, the bulge is caused by the non-uniform gravitational field across the Earth. We see the bulge in the oceans because the Earth is (relatively) rigid and water is not. Check my posts in this thread for more information.

  17. 8 minutes ago, vlaiv said:

    However, if we look at three points - one in the Center of the earth and one on each side - those distances are much larger in comparison to distance to the Moon than are with respect to distance to the Sun. Not sure if I put that correctly, here is another attempt - radius of earth in comparison to distance to Moon is much larger than in comparison to distance to the Sun.

    It shouldn't be too hard to run the numbers. We'll keep things Newtonian for simplicity. So a = GM/r^2

    G is Gravitational constant 6.67e-11
    Radius of Earth, re, is 6,371km
    Mass of Sun is 2e30 kg
    Distance to Sun from Earth centre, rs, is 149,785,000km (going off the above diagram)
    Mass of Moon is 7.3e22 kg
    Distance to Moon from Earth centre rm, is 384,835km

    a at Earth centre due to Sun = 6.67e-11 x 2e30 / (1.5e11 + 0)^2 = 0.0059289 m/s^2
    a at Earth close edge due to Sun = 6.67e-11 x 2e30 / (1.5e11 - 6.371e6)^2 = 0.0059294 m/s^2
    The difference between these two is the tidal force due to the Sun = 0.0000005 m/s^2

    a at Earth centre due to Moon = 6.67e-11 x 7.3e22 / (3.8e8 + 0)^2 = 0.000033719 m/s^2
    a at Earth close edge due to Moon = 6.67e-11 x 7.3e22 / (3.8e8 - 6.371e6)^2 = 0.000034879 m/s^2
    The difference between these two is the tidal force due to the Moon = 0.00000116 m/s^2

    So the Sun imparts a tidal force around 43% of the Moon's tidal force. It's a bit different from the 46% in the diagram above because I did a lot of rounding.

    To me, it's astonishing that the Sun imparts such a significant tidal force given how far away it is. It really helps appreciate just how ridiculously massive the Sun is compared to everything else in the solar system.

    • Like 4
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