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KaStern_Former_Member

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About KaStern_Former_Member

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  1. Hello Gerry, here are two links with some information on Schmidt-Cassegrain-Telescopes (SCT) and Cassegrain-Telescopes: http://en.wikipedia.org/wiki/Schmidt%E2%80%93Cassegrain_telescope http://en.wikipedia.org/wiki/Cassegrain_reflector Both share some properties, but differ inothers. The SCT does have a spherical primary mirror and a spherical secondary mirror, plus a Schmidt plate with one flat surface and one aspherical surface The Cassegrain Telescope does have a paraboloidal primary mirror and a hyperboloidal secondary mirror. Bot aspheres are more dificult to make than a spherical surface, especially the strongly aspheric convex secondary. The SCT has strong off axis coma, the classical Cassegrain has coma as strong as a parabolidal Newtonian with the same f/number. Nowadays you will not find a classical Cassegrain, allthough some mirror makers will make the mirrors on request. Takahashi did make the CN 212 reflector, but it has been dicontinued. Cheers, Karsten
  2. Hi Gerry, do you wnat to know about Cassegrain telescopes or Schmidt-Cassegrain Telescopes? They are very different allthout they may look relatively similar. Cheers, Karsten
  3. Hi folks, Hi folks, no false colour? For an achro this is impossible. Better get your eyes checked. When it comes to the 4"f/10 TAL achromat some people seem to loose the ground contact... Or whatever it is. That thing got lots of chromatism at 120x on the moon. You even do not need 120x. 50x is enough to see it. If have seen it with my own eyes. Cheers Karsten
  4. Hello Michael, " What people often forget about observing is that comfort is as important as optical quality " good point. That is why I own a mixed set of Pentax XL / XW eyepieces too. I bought them used one after another. They combine good optical quality with long eye relief and good comfort. Cheers, Karsten
  5. Hello ajohnson, much depends on if you want to image or if you want to observe. If you want to make images you definitely should chose the RC over a 6" SCT. The RC does not have off-axis coma, the SCT does have a lot of it. Off-axis image quality is much better with the RC. If you want to observe this does not matter that much. The eye is only capable of high resolution in a very small area of the retina, the fovea centralis: http://en.wikipedia.org/wiki/Fovea_centralis So you only see sharp objects when you look directly to them. Therefore you will only notice off-axis unsharpness if you look directly to the object. In theory the SCT should have a small on-axis advantage over the RC because of its slightly smaller central obstruction. But the obstruction is bigger than tthe secondary alone, the baffletubes cost additional obstruction. And a telescope has to be collimated very well. This is true for the SCT and the RC. In practice the collimation can cause bigger differences than the obstruction. Both SCT and RC are lightweight and compact. But the SCT may have a slight advantage. If you can manage this you should try to do a direct side by side comparison between these two scopes. Cheers, Karsten
  6. Hello John, the 5mm Pentax XO has a very tight eye relief too. It is only about 3.6 mm: http://www.tele-optic-tecnica.de/pentax-xo.html Ragards, Karsten
  7. Hello John, " I also reckon Thomas Back was quite capable of designing an excellent eyepiece. " me too. But what if the eyepiece manufacturer choose to build the eyepieces not according to the specs of Mr Back? Here is a link to where M. Back writes upon the TMB/Burgess Eyepieces: http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/BO-TMB-review.html " The design has a 2-element air-spaced field group followed by a 2-1 positive assembly. It is thus a 1-1-2-1 design (I have disassembled it to determine this). " I have "tested" some of the TMB/Burgess, first and later ones, and some with other brandnames on them. They all weren`t flawless. Some exhibited strong flaring, some minor light scatter issues. I ended by using further my orthos + a barlow. If this barlow is a 2.8x Klee Barlow this barlow is a 3-element in 3 groups barlow. Together with the 4 lenses of the ortho it sums up to 7 lenses in 5 groups, wich translates to 10 air-to-glass-surfaces. Despite this light scatter is very low and contrast is high. Good manufacture quality seems to be more important than counting the number of lenses. I have seen the lowest light scatter ever using a 5mm Pentax XO. It was even slightly better than a 5mm TMB monocentric. The XO is a 1-2-2 design, the monocentric has 3 lenses in 1 group. Cheers, Karsten
  8. Hello, this scope has a quite big central obstruction. Therefore the contrast transfer suffers, wich is most obvious for low contrast detail. The scope is o.k. for lunar observation, where most detail is high-contrast detail, but for planetary observation where many detail is low-contrast detail this scope is somewhat limited. This scope was meant to serve for astrophotography. In this part it is good. Due to the RC-Cassegrain Design it is free from off-axis coma. And there is no colour aberration at all. But there will be a bit astigmatism off-axis and there is field curvature too. Please note that the RC must be well collimated! Cheers, Karsten
  9. Hello Steve, " At present myself and Astro baby are trying to do a three way shoot out between the ES 30mm, Nagler 31mm and Pentax 30mm. " very interesting! I have one proposal to make: Could you please test them with a very slow scope too? For example with an f/20 or f/25 or even an f/30 or so Schiefspiegler? I asume the differences in light scatter would be seen better at higher magnification. Cheers, Karsten
  10. Hi folks, good orthoscopics are not sharper, but contrastier than the TMB planetaries. I fould less scatter, no ghosting and no of the annoying flares I could see in some TMB Planetaries or their clones with other "brandnames". Eye relief of an Ortho is about 4/5 of the focal length. I routinely use Orthos with a barlow for high magnifications. Cheers, Karsten
  11. Hello Donaldo, exit pupil diameter = entrance pupil doiameter devided by magnification. Your entrance pupil sice is 80mm, with your 10mm Pentax XW you get 600mm : 10mm = 60x So the resulting exit pupil diameter is 80mm : 60x = 1,333mm You can make an experiment: Take your scope and aim it twoards the distant horizon, put in your eyepiece and focus it. The step back a meter and look at the eyepiece. You will see a brightly illuminated disc of 1,333mm diameter. That is the area where the light exits the eyepiece. The light is parallel light, like from a very distant object. This light is captured by your eye and then focussed onto the retina. There are light sensitive cells located. The way they are packed can differ slightly from individual to individual. " As I've got a BST 8mm ED and an Orion 'Shorty' 2x Barlow, that combination will give me x150 " Try it. You can aim the scope towards polaris, wich does not move. Observe it carefully. Do you see it slightly disc-like, surrounded by a faint ring? Or do you see it as a point-like object? Try different magnifications. The higher the magnification you need to see a star as atiny disc the worse are your eyes. Try different magnification when observing the moon. The moon will take relatively high mags because it has some strong contrast at the border between light and dark. Jupiter all in all has low contrast features. If you push magnification too high the contrast drops too low. For me obseving Jupiter is best with magnification of about 0.8mm to 0.7mm exit pupil. This is all individual. The phsics behind it is true for all people. But the physiology can differ a bit from individual to individual. So you should try out different eyepieces and magnification and see what is best for you. Cheers, Karsten
  12. Hello Donaldo, I do not think that it is a good Idea to push the magnification that high. Please remember that you will be able to see the airy disc http://en.wikipedia.org/wiki/Airy_disk as a tiny disc when magnification is about 1mm exit pupil or slightly higher. 0.7mm exit pupil is a good number where the visible unsharpness caused by diffraction and image scale is well balanced for observers with normal eyesight. 0.7mm exit pupil translates to 114x Therefore I would rather recommend to get a 2x barlow. It would give you about 120x wich is a very reasonable magnification for a 80mm scope. John did mention some very good points. A barlow will alter the way the rays go through the eyepiece. The consequences will be an enlarged eye relief and for off-axis ray bundles vignetting in the eyepiece can result. Extended eye relief is helpful when you use short focal length simple typ eyepieces like Ploessl, Ortho, Kellner, monocentrics and so on. But it is not when you use eyepieces with long eye relief. The exit pupil is moved further outward and you will have difficulties to position your eye since the mechanics of the eyepiece will not support the eye anymore. The Pentax XW like the XL or the Vixen LVW sipports 20mm eye relief. The effect depends on the barlow magnification factor and the barlow "focal length". The shorter the barlow and the stroger the added magnification the further is the exit pupil moved out. Both the above problems are avoided by the powermates. You may try a longish conventional 2x Barlow too. The Televue is one example. There was a baffled 2x Barlow with 2 elements made in Japan (discontinued as far as I know) distributed by Orion in the US wich may be useful too. But if I were you I would rather try to get a used 5.2mm Pentax XL or 5mm Vixen LVW than a new Barlow or Powermate. Cheers, Karsten
  13. Hello John, no. The only thing that helps is to contact the vendor. But depending on wich dealer sold the the TAL to him it will have no effect at all. Cheers, Karsten
  14. Hello Rob, because of the colour aberration free views I even prefer the TAL 110mm Newt to the TAL achromat. If I add a bit more aperture like with a 150mm f/5 TAL newt or 150mm f/8 Synta Newt I get both colour aberration free views and higher resolution. To be fair I have to say that all these Newts need to be collimated welll and a user who cannot collimate or does not want to collimate should better buy a refractor (and hope it will come perfectly collimated). Of the achromatic refractors I prefer the classical long ones, like the japanese 80mm f/15 or a 4" f/15 over the somewhat short 4"f/10. The modern 4" f/9 ED refractors are not very expensive, but they cost about twice the price of the TAL achromat. Cheers, Karsten
  15. Hello Folks, I was not so impressed by the views a 4" f/10 TAL gave. The aperture is limited and the views are further compromised by visible colour aberration. I prefer views free from visible colour aberration and with more detail. Cheers, Karsten
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