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Posts posted by digital_davem

  1. 14 hours ago, Alan64 said:

    When you unlock the clamps, the slow-motion assemblies are indeed bypassed, and for freedom of movement.

    Incidentally, when tightening the clamps, don't overtighten them, if such as possible, as the moun int is made of aluminum, and can result in stripping out the threads.

    Given the amount of slop, i wont be locking them at all. Still don't understand why there so much movement. I don't think k the gears in this thing would be much use in a clock or watch....?

  2. 1 hour ago, Knighty2112 said:

    Any system with gears in them will have play in them, this mount is no different as the gears that control the slow motion have this play also. A simple camera ball head has no gears, and clampling is done directly onto the ball head. With no gears in its system there is no play, so can be secured with no play. 

    Makes sense. So the movement is in worm drive rather than the main bearings?

  3. Hi

    I'm not sure what to do with the counterweight on an alt az (it's kind of obvious on an EQ) so I screwed the bar into the top mount and put the weight in the middle. It does help - now I can nudge with the clamps unlocked reasonable smoothly and it doesn't drift under the scope's weight. It's very usable like this.

    I still don't really understand why such a huge mount isn't rock steady, though. There should be zero movement with the clamps locked. Given that a simple cheap camera ball head is completely steady within its weight limit it doesn't say a lot for SW's quality control. The clamps are effectively useless as they don't clamp!




  4. 11 hours ago, Knighty2112 said:

    When tracking object at high magnifications there is no effect of this play at all. It only becomes an issue if you knock the scopes on the mount. All you then need to do is move the scope back in the opposite direction of this play and you will have the object in view again quite easly. In practice when observing I don't find this play a problem at all. I believe all mounts like this even other manufacturers will have a slight play in them still. 

    Hi Knighty

    I wish I could be so sanguine as you, but my experience has been very disappointing.

    Example:  observing Jupiter.  I line up the planet in my finderscope and lock down the mount. Jupiter in smack in the middle of the eyepiece view. I then adjust the focuser - the resulting movement is enough take the planet entirely out of the field of view of the eyepiece.

    Or when "nudging" with the clamps unlocked I position the planet right in the centre of the view, then let go of the eyepiece and it jerks to some other position. It can end up anywhere in the field of view, usually right at the edge necessitating further adjustments.  Observing then can only get done in brief interludes between adjustments.

    I do agree that the slack doesn't seem to affect use of the slow motion controls so much, once the scope has moved and settled at the lowest point of the slop.  However, the lack of tightness with the clamps unlocked makes it difficult to track by nudging. And when you clamp the locks tight, there really shouldn't be any movement at all from a 3Kg scope on a mount rated for 2x 13kg. My manfrotto camera tripod heads are perfectly rigid when clamped and they are one tenth the weight and dimensions.  It's just simple engineering.

    I remember seeing posts on here that say the skytee can have the play adjusted out by tightening internal bolts and I have seen a step by step guide for the left-right axis but not the up down.

    Can anyone help out with this?




  5. Hi

    I got my Skytee just over a month ago and have had a small problem with movement of its axes. Basically, there is about 1mm of play in both the main axes with everything clamped down as tight as can be. This shows up without a scope mounted: just gripping the "T" and twisting back and forth with my hands reveals a sudden movement.

    I've sent videos to the supplier and obtained permission to attempt to tighten it up without risk to the warranty.

    However, how to do it? 

    I've seen talk of grub screws and lock nuts and the like but it would be handy if anyone has a step by step guide, particularly on how to adjust the up/down axis which is the one that is most annoying. Using the right tools is a concern as well, that warranty amnesty might go out the window if I'm belting it with a club hammer :-)

    This link goes to a video showing the problem:  video

    Any help would be appreciated.





  6. Had a brief outing in the garden last night before it clouded over. Seems to be quite a lot of movement in both axes of the Skytee.  Even with everything clamped up tight, if I grab the eye piece and wiggle it, the objective end wobbles up and down about an inch! Not smooth movement but simple slop.

    I know the Skytee has a bit of a reputation of being quirky but is this just an adjustment issue or should I be thinking of returning it for replacement?



  7. 40 minutes ago, kilix said:

    I thought about this earlier. We know that we are *stretching away from certain objects in our universe faster than the speed of light. Therefore we are behind an event horizon for those objects, we are outside their future light cone, no information can ever pass between us and them. Therefore we live in a black hole?

    *stretching as in a sense that the universe is expanding, we are not physically moving through space away from each other FTL.


    EDIT> but what does it have to do with this topic? We are offtopic, Dave!

    Just to stay off topic for a bit longer...

    A black hole event horizon marks the point where gravity is so fierce, light would have to exceed the speed of light in order to achieve the escape velocity necessary to get away from the black hole. As light can't go faster, it is the cause of not being able to see what it is going on in that particular region. 

    The expansion of the universe at a rate greater than the speed of light is another cause of not being able to see what it is going on in a particular region.

    In both cases the seeing problem is a limitation produced by the light speed barrier but the regions are not the same thing.  One is a hidden region because escape velocity is higher than light, the other is a hidden region because it is moving away from us faster than light can come towards us.

    The qualities of each region are very different, the only commonality is that we can't probe those regions using light. So, no, not being able to probe those regions is frustrating but not evidence that we live in a black hole.

    • Like 1

  8. 2 minutes ago, Ricochet said:

    In theory the 50x aperture in inches (2x aperture in mm, 0.5mm exit pupil) applies to the splitting of double stars. The optimum magnification for planetary detail is lower at 25-30x aperture in inches (0.85-1mm exit pupil) while unfiltered DSOs (galaxies!) are best viewed with an exit pupil of 2-3mm which is the same range as your daytime pupil and the best corrected part of your eye lens. However, this is all based on the average telescope and the average eye and so it should all be taken as a rough guide with a pinch of salt and the actual optimum/maximum values are going to vary from person to person. 


    That makes sense in that it explains why I prefer the crispness and "look" of views at 100x compared to 150x with scope. I'll try some double stars to see if the higher mag works better.


  9. 23 minutes ago, John said:

    I think Dave's F/16.5 is a refractor.

    Sometimes you can see spikes through bright object if there is something like a powerline or telephone cable crossing near the line of sight. It does not actually need to be right through it, just near to it and you get a diffraction spike from it. I've discovered this the hard way by having a few such lines crossing my garden !

    Also a 45 degree / 90 degree erecting prism type diagonal can cause a spike through bright objects. I think the Celestron 70 comes with such a diagonal as standard, which might not be helping things, astronomically speaking.

    Yes, a refractor.

    According to much published wisdom aperture in inches x 50 is the max magnification you should use. My scope ought to be good at 150x by the reckoning but I prefer it at 100x, just looks better, if smaller.  I suppose these formulae don't really take into account seeing, transparency, light pollution, thickness of the atmosphere. Perhaps if I were pointing the scope at the zenith from the Atacama desert 150x would be a piece of cake. Perhaps not so easy in south London, especially to the north where on a good night the sky is a yellow/orange glow...


  10. 47 minutes ago, Macavity said:

    Not necessarily a BAD telescope --- Just a £70 telescope?
    (Assumed Celestron Travelscope 70). Sadly, "superlative"
    view come at greater cost... Mostly. And even then? lol. :p

    Welcome to the real world of Astronomy... "It isn't going
    to look like they said in the adverts"? But I hope this will
    be an incentive to understand... and encourage a certain
    amount of persistence - And stubbornness we needed! :evil4:

    Jupiter is not the ideal target for this (many) scope(s).
    But Planets are not the only target around...
    The Pleiades are still on view etc. etc. :)

    Don't knock £70 telescopes! My 76mm f/16.4 classic refractor cost £27!  My 130p reflector with eq2 and tripod cost £45. :-)


    • Like 2

  11. When I look at Jupiter through my 76mm f=1250 f/16.5 achro I see a very small circle with a line across the northern hemisphere and a line across the southern hemisphere, no more detail (the moons are pinprick sharp). This is at 100x with a 12.5mm plossl.  If i use my xcel LX 7mm for 178x, I see a larger, blurrier, dimmer Jupiter with the same detail. F16 protects me from chromatic aberration unlike your fast scope but not sure that huge magnification with a small aperture is the way to go. 100x should be OK through a 3" refractor but Jupiter won't be huge at 100x, certainly not a centimetre across. A 4mm eyepiece is tricky to see through. My scope was originally fitted with a 0.965" 4mm ortho and I could barely see anything through it!



  12. 11 hours ago, Swoop1 said:

    Ok. Thanks for the answers.

    I have the standard alloy tripod for my 150P Newtonian on an EQ3/2 mount. Swapping the legs out for wooden ones is a DIY mod I would consider as an improvement.

    I have a 130p on an Eq2.  With the legs at their shortest, I find it fine when seated. It's nothing like the challenging of keeping a f/16 long tube achromat steady. Is the 150p so much wobblier?

  13. On 27/02/2017 at 07:23, kilix said:

    one thing, that helped me realize what is really happening was using a Nikon camera in telescope's prime focus. All that theory that I had read suddenly clicked into understanding.
    Using the camera in prime focus, I realized that the telescope produces some kind of image, which is static basically. All changes to this image happen inside the eyepiece. With that eyepiece removed, you get to see what the telescope actually produces (but you cannot do that with your own eyes).
    Then I realized, that all the magnification is just the eyepiece "looking" at a smaller part of this static image produced by a telescope. The bigger the mag, the smaller part of the original image is viewed. I imagine it somewhat like this:

    The telescope collects light according to the surface area of the lens/mirror. The bigger the mirror/lens, the more photons are falling on it's surface because of bigger surface area. Image from mirror/lens gets projected somewhere to be actually used by humans - this is the focal point, point where the eyepiece rests. Now, the eyepiece decides if it will project the entire image to your eye (low mag EP), or if it will use and project only a small part of the image (high mag).

    My brain understands it something like that. Very similar situation to understand would be this: telescope projects an image on a piece of paper, this image cannot be magnified or changed, it is still the same, only other telescope will provide other image. Using the eyepiece, you view only a small part of the paper and magnify it.

    With bigger aperture scope, you collect more photons, so the produced image is brighter. And you can go to bigger magnifications without loosing too much contrast because of more light collected (pi * r  squared).

    That's why it eventually starts loosing brightness and - accordingly - contrast and detail with big enough magnification.

    What I still don't practically understand is how those widefield EPs work compared to normal EPs.

    Wider field eyepieces.

    This was a puzzler for me to start with because of my understanding of optics from a photographic background. Some said a photographic background is a hindrance but it isn't once you understand the somewhat different terminology used in astronomy and the various mechanisms at work.

    Think of it this way, a telescope tube without an eyepiece is exactly like a camera lens.  Light enters the front and is focused at the focal plane. At the focal plane, the image that is formed is not a point but a circle.  The shorter the focal length of the objective, the wider the field of view it can see and the lower the magnification.  In 35mm camera terms, a 20mm lens is a wide angle lens with a big field of view and everything looks pretty small. A 1000mm lens has a narrow field of view and everything looks big. A telescope is just the same.  A 300mm telescope is considered to best suited to wide field, low magnification work. A 1500mm telescope is best suited for moon and planets where a wide field isn't needed. So, without an eyepiece, the field of view and the magnification seen at the focal plane is determined only by the focal length of the objective. This is the "first stage" of the telescope.

    The "second stage" is the eyepiece. Cameras don't have them!

    Eyepieces are really just kind of like magnifying glasses that look at the optical circle produced by the objective and and additionally magnify parts of it.  Short focal length eyepieces magnify parts of the optical circle more than longer ones but see less of the circle as a result. Longer ones magnify less but can see more of the optical circle so give wider fields of view.

    What has this got to do with the so called super/extra/ultra wide eyepieces like Naglers and the like?

    I'll use an imaginary telescope and made up number to illustrate as I don't have real figures to hand:

    Let's say I have a simple refractor with a 1000mm focal length.  Let's say that without an eyepiece it creates an image circle at the focal plane that is 40mm in diameter.   If you stick a camera at the prime focus, that is what it would record, a circle 40mm in diameter (let's assume we have a big sensor!).

    When you use a "standard" eyepiece, you zoom in and see a magnified piece of that 40mm circle.

    The amount you will see depends on the focal length of the eyepiece. A short FL ep will "zoom" in an magnify the central region and exclude the edges of the circle from view. A longer ep will see more of the circle but everything will be less enlarged.

    However, short or long focal length, the eps will still see only a part of the circle, say the inner 10mm section for a high magnification ep and the inner 20mm for a low magnification ep.  There will still be some parts of the out ring of the circle that is missed by the eyepiece. It is this missing part of the circle that explains where widefield eyepieces do their stuff.

    So what do exotics like Naglers do?  What they do, is at any given focal length they see more of the objective's optical circle than the standard ep.  A 10mm Nagler will magnify the same amount as a standard 10mm (eg an ortho or a plossl) but they can see more of the optical circle. Using these exotics you can have increased  magnification AND a wider field of view because they can make use of some of the objective's optical circle that is wasted by lesser optics.  It's similar to the using the same full frame 35mm lens with a 35mm sensor, a APS-c sensor and a 4/3 sensor. The lens produces the same optical circle with all three camera types but only the full frame sensor can see the full circle.

    There are limitations, of course:

    No matter how wide field the eyepiece, it obviously can only go as far the edge of the circle produced by the objective.  If you use a exotic eyepiece of low magnification, eventually it will see all of the circle and the limit of eyepiece tricks will have been reached.

    Typically, other factors come into play, such as vignetting by the eyepiece barrel or the focusing tube itself.  The reason 2" eyepieces permit wider fields of view in simply because the bigger eyepiece barrel and focusing tube remove a source of vignetting and thus you can get to see more of the circle produced by the objective.

    Sorry the above is a bit of a baby talk explanation but I think it covers the essence of how exotic eyepieces do their stuff. 

    • Like 4

  14. 7 hours ago, Swoop1 said:

    So, did you replace basic aluminium legs?

    is the stability and vibration damping an improvment?


    The aluminium legs are lightweight but weak with a telescoping action secured by a screw clamp plus they attach to the tripod top plate/spider with bolts that go through plastic end pieces.  My DIY legs are not height adjustable but single lengths of wood from top to bottom. The side pieces of each leg are glued and screwed into wooden blocks that make up the centre of each leg. This leads to a heavy and stiff set of legs despite the soft wood used.

    It is difficult to mount a long tube refractor, even if it isn't that heavy. The long tube gives a big lever action that means the slightest vibration causes considerable movement of eyepiece and objective.  I started off using a photo tripod and it was wobbly. The porta ii was an improvement, even more so on the wooden legs.  It was perfectly steady if you were just looking through but focusing still caused movement. The jiggling vibration took maybe 7 secs to calm down after every touch and made focusing tricky.

    I was hoping the Skytee would make it rock steady. My brief foray so far suggests it is a massive improvement but it is not rock steady. Touching the eyepiece with a 100x lens in cause noticeable movement in the field of view but only for about a second which is manageable. There are still a few things I have to play around with:  adding a spreader to the tripod, adjusting the skytee tension, using the counterweight. 


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