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Posts posted by weega

  1. 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



  2. 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



  3. [cut]

    I have heard, though I dont know how true it is that these Barlows generally tend to not work well with complex lensed EPs. As the SWA 38mm is a complex bit of glass that may be causing your problems.

    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



  4. 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?




  5. I have the Sky Atlas 2000.0 companion but don't use it very much. It may be the sort of thing you are after. If you give me the ngc number of an object you may be interested in I could scan in and post a sample page.

    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?

    A very highly rated pair of books is The Night Sky Observer's Guide Volumes 1 and 2, by Kemple and Sanner.


    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!


  6. 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


    Observing Handbook and Catalogue of Deep-Sky Objects






  7. 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.



  8. 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.



  9. 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:


    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



  10. I also stuck it in a little Konus 4.5" F/4.4, and had no problems, same for a little F/5 refractor. In the latter it did show serious chromatic aberration, but that was purely due to the cheapo achromatic objective (it was really only intended as a BIG finder scope).

    good to know!

    I didn't want to say that meade tele extender is not good :rolleyes:

    I'm sorry if it seemed so (I need to improve my English! :-P)



  11. IrfanView is a very useful tool and you can 'batch' process, which can save a lot of time

    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!



  12. 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.... :-)




  13. 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.



  14. I can vouch for both the 2x and 3x TeleXtenders of Meade (1.25"). Really good piece of kit. Neither shows any vignetting on my 14mm UWA, though the 3x UWA and 14mm combo is only useful in EXTREMELY good conditions on my scope (F/10).

    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>>



  15. [cut]

    Someone on "Cloudynights" posted X-ray photos of an Ethos 13 and an ES 14 100 - the internal lens layout was remarkably similar !.


    link please :rolleyes:

    [cut]I'm very happy with the the ES but I think at <f5 maybe "try before you buy" might be good advice as different people have different thoughts/ tolerances to edge of field imperfections.


    very very good to know, thanks!



  16. 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 :-)



  17. I should get Castell O-III filter this week. It should be broaded than typical O-III - thats how it looks on the spectral charts.

    Let us know how it works! :-)

    I'm very curious to know how do theese "cheap" filters work

    I'd like to buy the Omegon UHC filter: his transmission curve is very similar to Orion Ultrablock, but at a third of the price.



  18. I have a 30mm Vixen NPL and it is "DA BOMB" (image is sharp to the edge with my F5 scope,contrast is great etc). The AFOV is only 50 degrees but feels like its more. I am not a person who worries about sharpness to the edge of FOV, but the NPL delivers this anyway and it is pleasing on the eye. The LVW's look and sound AMAZING. They cost more then i am willing to pay but i cant rule them out at some stage in the near future.

    I've got the LVW 13mm, I paid for it about 50pounds because it was fallen and the barrel's end was a little crooked (you cannot mount filters), but the optics is OK (I checked it against another LVW).

    It's a fantastic eyepiece, even on my dobson that is f/4.3, better than nagler 13mm type 6: surely nagler has a bigger afov, but not as corrected and clear as LVW, moreover observing with an eye relief of 20mm is very very confortable. The only limits of LVW are the weight and the size, both very large. I've tried all focal lenght from 8mm to 42mm (but not 5mm and 3.5mm): all amazing except the 42mm, I dont know if with it the problem was the eyepiece or my scope (exit pupil of 10mm and a lot of coma).



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