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Showing content with the highest reputation since 29/09/20 in Blog Entries

  1. 3 points
    After much advice here and thanks to a friend of a friend I took delivery of this today. 1.3 m high, many kilograms heavy! It's been treated with zinc phosphate primer and will get a couple of coats of Hammerite or equivelant before planting. This weekend I'm running Cat6 cable out to the plinth and starting to build the equipment box which will be fitted to the bottom of the mount. Michael
  2. 1 point
    Telrad fitted. Baader zoom has to be used in place of the 1.25" focuser, straight into the 2" focuser. Had a go at collimating first and managed to mess it up. All I could see after pointing at Vega first was a blob of light. Collimated again and was better, but realised it's too far from the secondary mirror. Removed the 1.25" focuser and fitted straight into the 2" one and that's perfect. Need to grease the focuser. Found Double-Double but was unable to split the 2 stars into 4 at 162x. Jupiter at 162x fitted 4 moons in the fov and shows nice banding. Saturn at max zoom a bit fuzzy but still can see the rings very nicely. Forgot to look for Cassini division but probably need Barlow for that. Was still to bright for Andromeda and was getting late so looked at Mizar but haven't seen the second star, I think. Need to check sky maps. (Turns out my zoom was too big and missed Alcor)
  3. 1 point
    It's a question that comes up regularly, but what is the difference between a Barlow and a telecentric amplifier (TA), otherwise known as a Powermate, ES Focal Extender. Meade Telextender, Bresser SA Barlow, etc? A telecentric amplifier does give a 2x magnification, just like a Barlow but that's where the similarity ends. A Barlow is a negative doublet (Smyth lens) that causes the exiting light rays to diverge and hence deliver the image amplification. If you move the EP further from the Barlow the magnification increases, whilst taking the Barlow nose-piece off and screwing it onto the EP will [generally] give 1.6x magnification, assuming we're talking a 2x Barlow. In the FE/Powermate/TE/SA Barlow (the latter isn't a Barlow, which is a confusion) the negative doublet is followed by a positive doublet that turns the exiting rays back to parallel - ie, telecentric. Because the rays are parallel, the distance between the EP and the amplifier elements is [broadly] irrelevant as the image amplification was done internally, between the TA lens elements. In practice, this still means that the effective focal ratio of the scope is doubled - It's a common misconception that the EP focal length is halved - but unlike a Barlow, the eye relief of the EP in use is unaffected. In other words, you insert an ES FE in the scope and the EP behaves exactly as it did before and the scope has effectively doubled in focal length. The down side is that double the number of lens elements costs more, but whereas a Barlow (which has other uses because of what it does) tends to feel like a second-best-to-an-additional EP, the ES FE simply feels like you have an extra EP. In visual terms, it's a less intrusive and more transparent solution and a more transparent device. So the Barlow is second best? Well no, not all of the time. For the reason why, you only really have to look at Televue Naglers and the clones thereof. They weren't the first (contrary to popular forum lore, but they're certainly the most successful) to use the idea, but what Unc Al realized was that whilst it was easy(ish) to create a wide field EP, the difficulty was in creating them at shorter focal lengths with an eye relief usable by humans AND with a well corrected field of view, especially in fast scopes like large Newts. Essentially, what he did was create longer focal length wide field EPs and then fit them with a Smyth (Barlow) element in the nose. Thus, you got an EP that acted as a shorter effective focal length, but had greater eye relief than it would have without the Smyth element. Very cool. In fact, this is the source of the reason why Naglers (and there derivatives) are renowned as well corrected in fast scopes. The Smyth element does increase eye relief, but as per a Barlow, it effectively increases the focal length and therefore focal ratio of the scope. As we know, a slower scope is less prone to aberrations, but in this case, it's the EP that is effectively delivering it. Your Nagler is better corrected, because it effects a better correct scope. So, this is also what your Barlow can do. A 20mm EP in a Barlow (and TA) will give a better corrected view than a 10mm EP, all other things being equal. This is handy, especially if you like your Orthos and Plossls which tend to have ever shorter eye relief with decreasing focal length. A Barlow can be partnered with a longer FL EP to give an effective shorter FL EP, without the need to glue your eyeball to the EP it emulates. Whereas a TA uses up it's focal length in the focal path, a Barlow does the opposite and pushes the focal point outward - It adds optical path length. How is this handy? Well if you have a binoviewer that uses up 110mm+ of focal path, the scope (refractors in particular) may not have enough space available to rack the focuser inward to compensate. A Barlow, or at lest the doublet element from the nose of it, screwed into the Binoviewer is enough to push the focal point outward and get you that focus point back. That's just one example. The important point is that whilst a TA is, as long as it has room to work, a generally superior device, there are times when a Barlow has qualities all of it's own. A good example of both will be a one off purchase and both will deserve space in your EP case. Buy right first time and you may find they remain a constant, whilst your prized EPs come and go..... Russell
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