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KaStern_Former_Member

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Everything posted by KaStern_Former_Member

  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
  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 loo
  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
  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 no
  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 li
  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. J
  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 sh
  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
  16. Salut Erathostenes, c'est dommage. Excessive strailight because of many scratches is not normal. The sample you bought might be a used refractor wich has been sold to you as new. You should call the dealer and claim for a refund. Cheers, Karsten
  17. Hello Rik, sorry, this is definitely not true! A Kellner Typ eyepiece performes worse than a ploessl type eyepiece at f/5. Shure, at f/5 he ploessl is bad too, but it is better than the MA. Cheers, Karsten
  18. Hi Astro_noob, I fully agree to you! They supplie eyepieces that are not great at all. But used in a fastish scope they are a pain. In a slowish scope they work better. This is true too if they are used in combination with a barlow. I think they should at least supply a an eyepiece in the range of these ploessl: http://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-sp-plossl-eyepieces.html Cheers, Karsten
  19. Hi Martin, for planetary observation I use my dob with barlows plus orthoscopics or an RKE. The barlows I use are a 2,8x Klee Barlow or a modified 2.2x Klee Barlow. These are coma-compensating. Just as a coma corrector will do too. The benefit is the much wider diffraction limited field of view, wich is nice to have if the scope is undriven. The modified 2.2x Klee might be fine with your 12mm Pentax XF. The view through the 2.8x Klee plus 12mm Ortho is better than with a 4mm alone. And so is the 2.2x Klee with a 9mm ortho. If you prefer wider fields a 5.2mm Pentax XL may be good too. I like mi
  20. Hi Folks, great news! Hope they get along with them soon. Cheers, Karsten
  21. Hi Alan, the right angke Finder is a nice thing to have. But you need one more : A red dot finder, Telrad finder, Rigel Quickfinder. With one of these you point your newtonian-like scope towards to object and then you are naer enough to center it in the right-angle- finderscope and the see it in your eyepiece. This is the way I do it with my Newt + 90° Finder + Rigel Quickfinder. In most cases this works very fast. Cheers, Karsten
  22. Hi Kerry, the smaller the part of your eye you use for observation the lesser the effect of your eye's astigmatism. It may be bad with 5mm exit pupil, but it won't be bad with 1mm eyit pupil. So if you use an eyepiece with 10mm focal legth or shorter the exit pupil at your f/10 scope will be 1mm or less and your eye's astigmatism will no longer effect the perceived image negatively. For your 230mm f/10 scope would recommend an eyepiece with about 10mm or 9mm focal length. This may be a good orthoscopic or a good ww like a Pentax XW or a useed Pentax XL or a TV Delos. Cheers, Karsten
  23. Hi Steve, your 150/1200mm is an f/8 scope and f/8 is not very demanding for eyepieces. The other advantage of f/8 is: Coma does not be bothering, the diffraction limited field of view is relatively big. devide the eyepiece focal length by 8 and you have the resulting exit pupil (ep) when used in your scope. I recommend to take from about 0.66mm to about 6.6mm exit pupil. The corresponding magnifications are from 225x to 22.5x Knowing the field stop diameter you can calculate the true field of view when used in your scope. For example the 5mm eyepiece with 7.1mm field stop gives you 0.339° tru
  24. Hi spudlet, 4" refractor, afffordable, low colour aberation, reasonable quality. I suggest a 100/900mm Synta ED. Or a used 4"f/15 achromat. Has more ca but a used one might be very affordable nowadays. Better beware of cheap fast ED Doublets. It is much more difficult to maintain good quality when f/ratio gets fast. Cheers, Karsten
  25. Hi Russel, you might try a used PEntax XL. To me it is a very good eyepiece with long eyerelief and good contrast.It works even well at f/5. On the other hand you can try a quality barlow. It makes things easier for eyepieces in fast scopes. Cheers, Karsten
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