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Piero

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Everything posted by Piero

  1. I found the amateur astronomer's handbook by Sidgwick so interesting, that I decided to get its complementary book too. £3 delivered and in very good conditions.
  2. In order to improve collimation and remove the block screws, I have been considering the replacement of the default springs at the back of my Sky-Watcher 8" dobson. As the telescope is in Italy, this work will be done at Christmas. Has someone done this before? If so, which springs do you advice? Thanks, Piero
  3. Found a copy of this for just £0.70!
  4. Just thought about sharing these two articles I read this morning. The Clouds of Venus Puzzle Scientists: https://www.skyandtelescope.com/astronomy-news/venus-clouds-puzzle-scientists/ Video: Black Hole Warps Light: https://www.skyandtelescope.com/astronomy-blogs/black-hole-files/video-black-hole-warps-light/
  5. John, I don't think there are things like perfect views, optics, or mechanics, but just things that either work bad or within an acceptable tolerance. I am critical with my equipment because certain things can be improved. Others will just need to be accepted as they are. It is just a matter of recognising them. I see this as a continuous learning process. Your comment pushed me to test that combination again. So, thank you for your feedback. This didn't change my opinion on that eyepiece, but the additional test let me understand the eyepiece and the refractor a bit more, which is a positive thing, I feel.
  6. I found this annotated image from this thread: https://www.cloudynights.com/topic/523392-replacement-focuser-for-sw-ed80/ It contains some good advice. Could it be that your focuser misalignment was due to heavy load?
  7. Nice post, Gerry! It made me smile! Next time then! It will still be there tomorrow.
  8. I apologise with the OP as this post is a bit off-topic. Citing from the link above "Most refractors have little or no coma, contributing to their being well-suited to wide-field viewing and imaging." This doesn't exclude coma in refractors. In addition, coma can be related to the telescope optical design (e.g. Newtonian telescope), but also induced by misalignments (miscollimated axes). If the focuser of a refractor is not collimated, the telescope can show coma, even on axis. My second comment above came from notes I took about 2 years ago when I bought the Takahashi. I just returned from a session outside and checked the 20mm Lunt 100 deg with the Tak-100. I could not detect field curvature. On the other hand I could not exclude that the aberration was a minor astigmatism at the very edge of the eyepiece. Spotting it took some effort, as I personally struggle seeing the field stop in a such a wide AFOV eyepiece. That's also the reason why I seem not to get along with 100 deg AFOV eyepieces, and so this eyepiece doesn't get much use.. Shame as it is a very good performer, light for its class, and has an excellent eyecup. I have seen coma with this eyepiece on my two F5.9 dobsons (8" and 12", no coma corrector) every time. It is tolerable to my eye, but it's obviously visible from about 60 deg AFOV. 60 deg is rather consistent with my other eyepieces. On-axis stars are aberration free. This is now (thankfully!) also with the 12" dob after fixing the issues I had with the mirror supports. In addition, coma is more visible in the 12" than the 8". This should not be a surprise considering that stars are brighter in the larger aperture, and therefore the comatic smear becomes more apparent. It makes me wonder whether misalignment-unrelated coma is less visible 1) under light polluted skies, 2) with dusty optics, or 3) if the telescope is not collimated within tolerances. Points 1 and 2 can affect the darkness of the background sky, therefore reducing the visibility of the comatic tail. Point 3 adds coma on axis, which can make one accustomed to the problem.
  9. Fast optics show coma, whether refractor or reflector. The Televue 100 refractor doesn't because it has lenses in place to correct coma.
  10. In my f7.4 Tak refractor, I can see it at the edge with my Lunt 20mm 100 Deg.. not disturbing, but it's there. In a f5 or f5.3, coma is certainly visible. Of course it's up to the telescope owner to decide whether it is bothersome and / or to do something about it. Consider a dim star on axis. On fast optics without coma corrector, that star can become invisible at the edge due to the fact that it's light is spread on a larger area. This doesn't mean that you don't have coma. Same for seeing colour on a star off axis. Rather than chromatic aberration, that is more likely due to coma. I am not saying that a coma is a must. I'm just saying that coma is there. It affects the off axis star brightness, as light is spread, and therefore it can reduce the chance of seeing dim targets. That's not my opinion, just how a fast newtonian works.
  11. Well, beyond approx 25 Deg AFOV, coma is visible in a F5 telescope. Not that it isn't visible within, it's just that beyond 25 deg the diffraction disk becomes larger than what it should be with the diffraction limited optics.
  12. Keep in mind that some of the Allen screws are for regulating the tension of the focuser - making smoother or firmer. What you will need is to adjust the drawtube axis instead.
  13. Thanks. I need to do some research as I don't have that telescope. From preliminary search, it seems that the focuser can be collimated. https://www.cloudynights.com/topic/140083-sky-watcher-100ed-collimation/ Check jrbarnett's comment. I'll have a proper look at this this weekend.
  14. I think it could lose a tiny bit on axis, but the improvement across the field should be huge at f4. You will have the whole field within diffraction limited threshold. The increase in magnification could also help you spot some other faints..
  15. Curious to know how you will find it, Gerry.
  16. Good to hear that! Anyway, I would also suggest to rack out the drawtube as when a normal eyepiece is in focus. A fully retracted or extended drawtube is not in a good position to check the focuser collimation, as this can be quite different. Also, why should this be done with the telescope pointing to the floor or ceiling? The laser is expected to be quite aligned in those positions and does not stress the focuser off axis. Focuser collimation with a refractor should be done with the tube put horizontally (and drawtube as said above). With your template, you can check that the laser beam hits the centre. If it doesn't and remains within tolerances, it's up to you. Otherwise it should be collimated.
  17. I am not sure you can rely on the mirror on the floor. The returning beam will depend on the angle of that mirror. If the beam goes through the hole of your template, which is supposed to be at the centre, that's a good indication that the focuser is collimated. In any case the tolerance for the focuser axial alignment is 0.03xD, where D is the aperture and here no coma corrector is used. That is 3mm for a 100mm refractor. For checking collimation of the lens, see Suiters' book.
  18. Whilst it is true that a miscollimated telescope can show astigmatism, in this specific case astigmatism dominates over coma when the strehl is below the diffraction limited threshold (about 0.80). At that point the images are quite bad. If your focuser is miscollimated, you should detect coma on axis. That is the predominant aberration. If you are certain you see astigmatism, it could be in the eyepiece or the optics are retained too tightly. Also, make sure the telescope has cooled down properly. For test collimation I use the Glatter square attachment with my Glatter's laser. As others said, the diagonal must be removed.
  19. Mm.. some interesting feedback from this conversation. Thank you all. Food for thought!
  20. In an ideal world, I would love a Vixen HR 5mm, but it seems this cannot be made due to the inherent design of the HR line. So, options are: - 11mm +/- 2.25x barlow - 5mm and use the 24 Pan +/- 2.25x Barlow Yeah.. a Pentax XO 5mm would be great! ... So would be the XO plus the 10mm ZAOII... Tough to find them and on the wallet though..
  21. I currently have a Nikon MC1 zoom 21-9mm which works well although I had to re-do its barrel as it is a spotting scope eyepiece. In terms of optical quality it matched my Vixen SLVs.
  22. Thank you all. From your comments, it reminds me of the Orion super Plossl Sirius 10mm I bought back in the nineties. Good eyepiece, but tight on eye relief.
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