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Piero

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Piero last won the day on June 6 2018

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About Piero

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    White Dwarf

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    Cambridge
  1. Star names - their lore and meaning by R.H. Allen is a classic one. It is mentioned quite a lot by Burnham in his Celestial Handbook, which is another classic. Both are highly recommended.
  2. Sidgwick's Amateur astronomer's book is excellent in my opinion. Very inspirational and rich of insights. A keeper.
  3. Thanks. Burnham cites that book quite often. That's how I became aware of it.
  4. By 'tipped' do you mean off-axis towards the primary mirror? 1. A primary mirror axial misalignment will cause coma on axis (coma due to misalignment). 2. A focuser axial misalignment will cause the stars to focus at different points across the focal plane. 3. A secondary mirror (severe) misalignment will cause unequal field illumination. It's rather obvious that only the first one degrades on-axis, therefore 2 and 3 reveal nothing on a star testing as this is conducted on-axis. Of the 3, the last one is the less critical for visual astronomy. Without coma corrector, the first one is the most critical. With coma corrector, also the second becomes rather critical. All of them become more critical in faster newtonians. There is nothing to fear in this process. If wrong, it can be fixed and rather easily.
  5. If collimation of a, let's say, f5 Newtonian telescope without coma corrector is checked with a star test, the star must perfectly be on axis, otherwise the coma-dependent misalignment is also visible. This cannot be really distinguished by optical misalignment. So, again, to me at least star testing is not the right way to check collimation as it is too sensitive, unless the seeing is very good, the high power eyepiece is good, coma corrector is used, and the mount tracks automatically. Even so, this test doesn't tell you anything about secondary and focuser alignments. Coming back to the topic, my HG laser and Catseye telecat give the same reading consistently.
  6. Not sure what some members above meant by star testing in this thread. Do they mean "checking" or "collimating" with a star test? In my opinion, I wouldn't suggest the latter, particularly with manual driven mounts. Regarding the former, the procedure can be rather complex as it is easy to get errors due to other factors which don't have anything to do with misalignment. Of course, if one knows how to star test, the feedback given by this tool is incredibly useful in order to understand what does not work properly in a telescope. Regarding the importance of collimation, well Suiter's book on star testing offers some quantitative data on this subject. Of course, a slightly miscollimated telescope still works. Said this, if one pays for premium optics and is happy to have an "okay-ish" collimation, the same person should reflect that some okay-ish optics with excellent collimation could probably give the same results in terms of view quality.
  7. Withdrawn. Feel free to archive this advert.
  8. I tend to stand while observing as that's my preferred way. This with both my dobson and refractors. The refractor mounts have an extensible column which I love as it avoids contortionism or "yoga postures".
  9. Last Thursday night the seeing was great with very good transparency. Observing Aristarchus crater and plateau from 160x to 630x was amazing. In particular, I was impressed by the kind of "river" approaching the crater to the left and how the light played fantastic shades inside. From Wikipedia: Aristarchus plateau (NASA).
  10. Found these books in the second hand market.
  11. I observed Copernicus past night with my dobson up to 533x without any sign of image degradation. It was wonderful!
  12. Last night I had a chance to try the telescope following the last work on the mirror cell. The sky conditions were clear, windless, and rather stable temperature between 8.20pm and 9.20pm. After that the temperature started dropping. The telescope was left outside with the fan on since 6.30pm. The light shroud was also fixed in order to reduce internal and nearby turbulence. Both telescope focuser and primary mirror axial alignments were adjusted with my 2" Glatter laser 650nm. Several stars were used for star testing: Betelgeuse, Procion, Rigel, Pollux, and Aldebaran. This was done in order to have a range of colours and different altitudes. I could not spot any trace of astigmatism. Spherical aberration was highly corrected too. This was at about 300x. The rest of the time was spent observing the moon which was outstanding. Copernicus at 533x (zoom plus VIP barlow) was really impressive, showing fine and minute features (e.g. tiny craters inside and ground detail) that I didn't even imagine were possible to see. Very happy about this result. From sky and telescope lunar map (at the eyepiece I could see far more detail that this):
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