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weega

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Everything posted by weega

  1. a laser diode without collimating lens is as safe as a red led. Without lens it do not produce a collimated beam, it simply sends light all around in a light-cone wider than the light-cone of a led and almost with the same total power. e.g.: http://uzzors2k.4hv.org/index.php?page=violetlaser ciao dan
  2. The BEST artificial star you can use? Take a red laser pointer (few £), remove the collimation lens and you will have a safe point-like light source. Red laser diodes have an emitting area few micrometers wide, surely less tha any commercial artificial star that uses optical fibers or pinholes. The copyright of this idea is of an italian amateur astronomer (not me :-D) on http://forum.astrofili.org/viewtopic.php?f=15&t=53433 ciao dan
  3. can you tell me the size (and thickness) of the "L-section" of the trusses? I'm planning to rebuild the trusses of my 200mm f/4.5... thanks and clear skies! ciao dan
  4. Hi! IMHO filters are useful only on nebulas, since most of them produce light with a "line-spectrum" (lines of O-III, H-beta). With line-filters or narrow-band-filters you can improve the contrast several times cutting everything except theese emission lines. For example with my baader O-III filter (8.5nm bandwidth) I've seen the veil nebula with the 35cm of a friend of mine, from suburban sky (I live near Milan, one of the worst skies of Europe)- UHC filters do a good job, they are less selective but still they improve contrast very well. Broad band filters (LPR, UHC-S, UHC-E) are useful only if the street-lamps neay you have a line spectrum as low-pressure sodium lamp (Sodium-vapor lamp - Wikipedia, the free encyclopedia). The real problem of nebular filter is that each of them works well with a particular exit pupil: eg: O-III filters need a lot of light, so you cannot use them on small telescope at high power, UHC filters need less light etc.etc... With your 8" I suggest you a good UHC filter (not baader UHC-S nor astronomik UHC-E). Very good are Thousand Oaks LP-2, Meade narrowband or Orion Ultrablock. There are cheap UHC filters on the market, like Omegon UHC, but I don't know them (sincerely I would like to test them since they are very cheap). Here you can find a very useful article with perfomance of different kind of filters on different deep sky object: Filter Performance Comparisons - Article ciao dan
  5. mmm, 38mm SWA is an Erfle: only 5 elements in 3 groups (doublet - singlet - doublet). Moreover this barlow works very well with my vixen LVW 13mm that has 8 elements in 5 groups! I don't think the problem is in the number of lenses Surely SWA uses "exotic" glasses to reach 70°, since the classic erfle has an AFOV of only ~60°....mumble mumble... I want a nagleeeeerrrr ciao dan
  6. Hi! I've just bought a 2" 2x ED barlow lens (brand "Telescope Service", built by GSO), I've already tried it on my dobson 15.5", f/4.3. Mechanically it's very well built. Optically it works very well with all my eyepieces (SP26mm, LVW13mm, PL 6.7mm). When I say "very well" I mean I can't distinguish it from Celestron 2x ULTIMA barlow (I have it since 8 years). It works very well both at 2x or at 1.5x, obtained using the barlow's doublets screwed on the 2"=>1.25" reducer. But on my SWA 38mm 70°AFOV (Kunming United Optics) at 1.5x the outer field is horrible, worse than without barlow... It can't be coma, since without barlow the outer field of the eyepiece is better... It seems astigmatism. Anybody has tried it a similar combination? thanks ciao dan
  7. Don't worry Dave: I had a look inside on amazon: Amazon.com: Sky Atlas 2000.0 Companion (9780521008822): Robert A. Strong, Roger Can I ask you why don't you use it very much? They seem very interesting, the only "problem" is that they are HUGE. I already own the three Burnham's handbooks and I was looking for something more compact. I'm starting to think that a good thing would be to copy only the initial pages of each chapter of the Burnham's handbook (the ones with the list of DSO) and bring only them on the field...:-) Thanks and ciao! dan
  8. Hi! I observe with a 15.5" dobson using Tirion's SkyAtlas and the free alternative (Toshimi Taki's 8.5mag atlas). I'm looking for a field deepsky companion where I can quickly check if an object on the atlas is worth to be pointed :-D Which one would you buy and why: Sky Atlas 2000.0 Companion, 2nd Edition or Observing Handbook and Catalogue of Deep-Sky Objects or others? thanks ciao dan
  9. in my 390mm f=1550mm (f/4.3): 13mm LVW vixen does 60% of the work (in Italy I observe principally deep sky object) 26mm meade 4000 plossl => 25% of the work 38mm SWA United Optics => 10% of the work 6.7mm meade 3000 plossl => 5% of the work When I'll get the money I'll buy a premium eyepiece with same quality but longer eye relief and larger AFOV With the 6.7mm (~248x) It's hard to find calm nights.... Moreover on bright objects (moon, planet) it is easier to appreciate details even if the seeing is not very good. While on faint objects, if the seeing is not good, it's hard even to recognize dim stars from planetary nebulae or small galaxies. ciao dan
  10. a good alignement is the first step to obtain great pictures: without a good alignement you can guide as well as you want, but There will be field rotation, look here: orion's sword [1] :: spada_ori.jpg :: Fotopic.Net on the right it's ok, on the left the stars are blurred. Once you have aligned very well your mount, the next step is the guide. With the PEC the mount records all the corrections you do during one rotation of the worm gear. In the next rotation, the mount knows theese corrections and does them automatically. So all you have to do are the corrections caused by the irregularities of the worm-wheel. The simplest way to obtain a perfect polar alignement (afther the use of a polar scope) is the Bigourdan's method: <<You aim a star on the celestial equator at the meridian, in a reticulated ocular. And you follow it with the motor running. If it is necessary to elevate the nose of telescope to keep the star on the recticle line, that means that you must move the north of the platform toward east., and vice versa. When the star follows accurately, you aim at another star on the equator, but relatively low in the west. If you must elevate the nose of scope to maintain the star on the line, you must elevate the north end of the platform, and vice versa. After two or three passages progressively refined, you are sure that the platform axis is aimed to celestial pole.>> from Celestial Alignment I suggest you do do the alignment with the highest-power eyepiece you have. ciao dan
  11. weega

    varie

  12. o__0 it's astonishing! I've reported the link in an italian astroforum. They seem twins. I believe in the reviews that say the two eyepieces work in different way. So I can only suppose that the choise of the right glass is a very important parameter, like the secret ingredient of CokeCola :-) ciao dan
  13. thanks for the info, I'll try with some cans (with different hole's size) I've already tried to take picture with a pinhole camera, but only daytime photos: http://weega.fotopic.net/c777777.html To build the camera I've taken a camera found in a box of laundry soap (in Italy it was quite common some years ago) and I've excanged the lens with a pinhole. I made some pinholes, I measuared their size with a scanner (at high dpi) and then I've chosen the good one that was in accord with Rayleigh criterion ( Pinhole camera - Wikipedia, the free encyclopedia ) I'm curious because with the Rayleigh criterion on negative film the exposure time in daylight is around 1 minute (it depends from the focal lenght and form film's ISO), probably photographic paper has a very low sensivity respect to negative film. Thanks again for the info ciao dan
  14. good to know! I didn't want to say that meade tele extender is not good I'm sorry if it seemed so (I need to improve my English! :-P) ciao dan
  15. On my PCs is the default image viewer since a lot of year. It's my favorite tool to -visualize images (almost every common type) -resize single images or whole directories -prepare fast preview from raw files (I use a canon 300d...) This program is very very fast to open and very light: I use it also on an old 600MHz with win98.... Moreover it can do some basic actions on jpeg files without re-compressing, as 90° rotation, flip or crop. I highly reccomend it! ciao dan
  16. they are spectacular! what is the focal ratio of your pinhole-can? Can you tell us something more about the kind of paper (eg, which contrast index) and the kind of development? I have in the garage some sheet of Tetenal paper (contrast index 4, hard) and some Agfa Neutol to develop.... :-) thanks ciao! dan
  17. I would say <<tailS>>: ion tail is very long, but also dust tail is clearly visible, wonderful! I haven't seen yet this comet :-( ciao dan
  18. Burnham Celestial Handbooks (Vol. 1, 2, 3) => the whole sky in 2100 pages, it's old ('70) so the astrophysical information is very often uncomplete or wrong, but it's very rich of info about deep sky observation, also with quotation from litterature (from ancient greek to today) and from famous astronomers of the past. Simply AWESOME! Available in paperback or hardcover. ciao dan
  19. I wanna rebuild the mechanics of my 390mm dobson to make it more transportable in my chevrolet matiz: now to put the dobson in the car I need to bend down both rear seats... ciao dan
  20. You have a f/10 telescope: with a slow ratio telescope it's hard to vignette even with ultra wide angle eyepiece. Look here: Seeking barlow "clear aperture requirement(?)" formula - Astronomy Forums | Telescope Forums & Reviews | Astronomy Community (please erase it if it's forbidden linking to other forums) <<You could trace rays. Put the largest eyepiece you want to use in your hypothetical barlow and see how large the negative lens would have to be avoid cutting off the edge rays. The answer would be roughly the size of the field stop of the eyepiece plus the distance from the negative lens to the eyepiece divided by the product of the telescope's focal ratio times the magnification factor of the barlow. D = d + s/(f * m) approximately. D = diameter of negative lens d = diameter of eyepiece field stop s = distance from negative lens to field stop f = focal ratio of telescope m = magnification factor of barlow>> ciao dan
  21. link please very very good to know, thanks! ciao dan
  22. it depends form what kind of eyepiece are you looking for. eg: my high-power eyepiece is a 6.7mm plossl (meade 3000 made in japan): it's really sharp, but 1) it is very unconfortable (very short eye relief) 2) it has a small AFOV To reach the same quality in wider and more confortable eyepieces yuo have to spend much more. TMB planetary eyepieces and clones are a very good compromise: alsmost the same quality of excellent plossl, but a 60° AFOV and much confortable eye relief (I've tried Telescope Service Planetary HR) and they don't cost like a pentax :-) ciao dan
  23. good to know! now I'm going to surf the web searching for some reviews thanks ciao dan
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