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brianb

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Posts posted by brianb


  1. Wind here is worse than it has been all through the last winter. Lots of displaced wheely bins & lots of work for TV aerial repairers but I can't see anything more serious at the moment (80 mph+ winds do occur more or less annually in these parts). Windows are covered in salt spray with a few minutes every time it stops raining .... the most "interesting" bit was around breakfast time, when the cold front went through (& before the wind really started), there was 5 mins of very very heavy rain - approx 10 mm of it - never known rain so heavy, not even during thunderstorms.


  2. Gliese 581D looks like a fantastic discovery, and hopefully we'll find many more habitable planets.

    Ummm. We don't know that the planet is habitable, we've just modelled an atmosphere containing rather a lot of CO2 which would give it a habitable temperature.

    Now an atmosphere containing that much CO2 would be in chemical equilibrium (like the CO2 atmospheres of Venus and Mars); an unmistakable sign of life would be that the atmosphere was out of chemical equilibrium, like Earth's, which contains millions of times more than the equilibrium level of oxygen and only has low concentrations of CO2 because over several billion of years, life has removed carbon, sequestering it in fossil fuel reserves (which we are now doing our best to squander, but that's a different story).

    Anyhoo, the point is:

    1. If Gliese 581D does have enough CO2 to be in the goldilocks zone, it likely doesn't have life;

    2. If Gliese 581D has life, there won't be sufficient CO2 in the atmosphere to keep the planet warm.

    The conclusion seems to be obvious.

    I also have serious reservations about red dwarf stars being suitable for the evolution of life, whatever planets may exist in "goldilocks" orbits; the point being that red dwarf stars tend to have flares of similar magnitude to those on our own Sun; and being 20 times closer in order to receive sufficient average energy flux makes these flares more than a little hazardous.

    Gravity is of course irrelevant for life forms until they leave the oceans.


  3. Orthoscopics are a 3-element design with limited eye relief, but exceptional sharpness all the way to the edge of the field. They were developed for use with longer refractors and excel in planetary and lunar work.

    The Plossl design (developed by Georg Plossl in the 1860's) is a 4-element design that offers better eye relief and a much wider field of view, usually 55-60 degrees, compared to 40-45 degrees for an ortho

    That's just plain wrong. Both orthos and plossls are 4 element designs with very similar eye relief (in fact the standard Abbe ortho eye relief is 85% of the focal length, whereas the standard plossl is 80%).

    Both ortho and plossl designs work well at f/6 and acceptably at f/5 but neither works well in scopes faster than that.

    A standard 4 element plossl will not have an acceptable field of view exceeding 50 degrees. There are modified designs ("super plossl") which have extra elements - 5 or 6 elements in 3 groups - and do have a larger field of view.

    The standard Plossl has more tendency to "ghost images" caused by internal reflections than the ortho design. Super Plossls can be even worse; the extra surfaces reduce transmission and tend to reduce image contrast (the effect is far worse in this respect than the small central obstruction in reflectors).

    Additionally, for long term use, note that the eye facing lens in an ortho (the one which tends to get wiped more than anything else in the optical train) is made of scratch-resistant hard crown glass, whereas the eye facing lens in the plossl design is soft, easily scratched flint glass.

    Monocentrics are to be preferred for planetary work in driven scopes, when their "drinking straw" field of view is not an issue. Plossls of good quality are fine for general use but a good quality ortho like the Baader Genuine Ortho is superior for planetary, lunar and solar work, the higher contrast trumping the slightly wider field of view.


  4. You can reduce the light coming into the scope by partially covering the aperture. Some scopes have a small offset cap on the ota lid for that purpose.
    At the expense of resolution. Filtering is a much better way to reduce light intensity. Why buy (and lug around) a 8" scope and use only 2" of it?

  5. For most purposes you don't need an accurate polar alignment. Pointing north with the aid of a good prismatic compass (allowing for magnetic variation), levelling the tripod head and setting the latitude according to the scale is good enough for most purposes. In the back yard, you can drill holes in a concrete patio so that the tripod always goes back in the same position & get a really accurate alignment by drift aligning at night. Keep the head attached to the tripod & the alignment will be good enough for almost all purposes (remembering that the sun has its own drift rate anyway due to the orbital motion of the earth).

    Then all you need to do is skip alignment, go into setup, select tracking, solar rate & the motor will start tracking ...


  6. i will soon be purchasing a new scope, it will either be a celestron 9.25" or 11" sct, any one recomend a focuser for it, or will it make much difference,

    For what purpose?

    For low power work (DSOs) the stock focuser is just about usable. It's too coarse for high power work, even visually, and mirror shift can be an issue for imaging.

    The Feathertouch replacement focuser knob (with 10:1 microfocuser) is light, convenient & solves all the issues except mirror shift. Bolt on Crayford focusers eliminate that as well but they are heavy and can have slippage issues - why does no-one use rack & pinion instead of relying on metal-on-metal friction? Nevertheless probably the best solution for planetary imaging, if motorized.

    also would a better star diagnal be a big improvement to the standard celestron diagnal,

    The supplied diagonal actually works fairly well, within its limitations ... but IMHO a 8"+ SCT needs a 2" visual back & diagonal. As for the diagonal itself, optically there's not a lot of difference, but which is best for you depends on which eyepieces you have - the WO diagonals have a tendency to bite TV eyepieces, and vice versa, because of a difference in the dimensions and location of the "safety" cutout in the barrel (which actually makes it more likely that you will drop the eyepiece. Why oh why can't we have smooth sided barrels?)


  7. Well it's not necessary to protect the eyes from damage but the moon certainly can be uncomfortably bright when viewed in a dark sky. A deep red (Wratten #29) filter makes it much more comfortable when the sky is dark, increases the contrast against a twilit sky (even when the sun is above the horizon), preserves most of your dark adaptation and the seeing is steadier at long wavelengths ... win, win, win ... can't imagine why anyone would want to be without. The moon is essentially colourless so there is little to be lost by using a deeply coloured filter; some people prefer green but red works better for me.


  8. I did some trials a couple of years ago, where the Baader Solar filter was used with all the different filters I had in my collection.

    I tried the usual coloured filters you get, RBG, BVRI, narrowband ie OIII, Ha and some LPR filters as well as the Continuum filter.

    IMHO the best results ie improved contrast and definition was found with the Continuum filter. A close second was a Light Green. None of the others did much good.

    Yes, the image of the Sun will appear green.

    This does depend to a certain extent on (a) the magnification used, (;) your personal sensitivity to green light. I find that (a) the Baader Solar Continuum filter is very similar in effect to the ordinary (Wratten #58) green, just a bit better; (:o when used in conjuntion with the normal Baader solar film, both are a little too dense for comfort when used at high magnification. Try a deep yellow (Wratten #15) or even orange (Wratten #21). If your eyes (like mine) are relatively more sensitive to red and less to green than "average", you may find one of these works better.

    I find Wratten #29 (deep red) is the best filter for "white light" imaging - the longer wavelength helps steady the seeing.


  9. The good weather having gone away 8-( I've had a chance to catch up on some AVIs which got pushed aside for lack of time.

    A rather nice full disc mosaic:

    Sun-110426-1106-1113-Ha-FLTX1-small10.jpg

    2011 April 26, 1106 - 1113 UT. William Optics FLT 110, prime focus, Solarscope SF-100 Ha filter set, Imaging Source DMK41 - 10 frame mosaic; 50% resize: full size version.

    And a close up of the activity in the south east on the previous day (dominated by AR 11195):

    Sun-110425-1340-1348-Ha-discSE-FLTX2-small06.jpg

    2011 April 25, 1340 - 1348 UT. William Optics FLT 110, 2x Powermate, Solarscope SF-100 Ha filter set, Imaging Source DMK41 - 6 frame mosaic; 33% resize: full size version.


  10. The instructions do not mention this clear film, neither is it obvious

    The last sheet I bought (still have it, unopened) has a "dayglo yellow" sticker on the outer plastic envelope, the text of which reads

    "ATTENTION: For cutting and trimming AstroSolar Film, always keep the film enclosed between the two protective sheets of clear plastic and silk paper, to avoid scratches and fingerprints. Only use one layer of AstroSolar Film for observing the sun in front of your telescope. Dispose the clear plastic film, because only AstroSolar Film has optical quality for high magnification work."

    Clear enough IMHO. Of course your supplier may be obtaining through channels which do not add the sticker.


  11. Nice effort!

    What I do with disk / prom composites is to scale up the disk image by 1.5% ... this helps cover the "gap" that tends to develop at the limb, without actually obscuring any of the detail in the chromospheric image.


  12. The more distance to the moving object the slower it appears to move?

    That's it ... look out of the window of a jet cruising at 500 mph at 35,000 feet and the world outside seems pretty static. OTOH if you look out of the window of a car going 50 mph with objects only a few feet away you appear to be moving pretty fast.

    Now the Pillars of Creation are 6,500 light years away ... if something nearby to them was moving at right angles to our line of vision at the speed of light, it would move about 2 arc minutes in a year ... or the same speed as objects on the ground would appear if observed from 35,000 feet by an aircraft flying at 5 feet per year rather than 750 feet per second. And the speed of the turbulence in the Pillars of Creation is, of course, a few orders of magnitude less than the speed of light.


  13. I'm assuming that there are no known problems with using a full apature Mylar solar filter with a Newtonian reflector at all?

    - If the Mylar where to devolp pin-***** holes in it, is it beyond saving or is there some sort of repair that can be carried out? I seem to recall mention of going over them with a marker pen, but I'm not sure if this refers to something else!

    FYI Baader solar film is not mylar ...

    You still need to take care that the filter is in good condition before each and every use; to remove the finder unless it has its own solar film filter; to ensure that solar film filters are fixed in place so that the cannot be accidentally disloged or blown off by wind gusts. (And that applies to all scopes, not just Newtonian reflectors!)

    A few pinholes are of no consequence, contrast loss will be an issue before there is a danger to eyesight. But splits or cracks of any size are not permissible; check in particular for gaps developing between the film and the mount round the edge.

    Pinholes can be "spotted out" with black marker pen, drops of paint etc. for the purposes of maintaining contrast. If the total area is sufficient to present a hazard (we're talking a couple of square millimetres) then it's better to replace the film. But Baader solar film is pretty resistant to pinholes (it's coated on both sides, so a tiny hole in the coating on one side is of no consequence) ... it does however go brittle over a period of some years, as does any plastic exposed to strong ultraviolet light from the sun. Chemical fumes from cleaning products will also affect the long-term stability & resilience of the plastic base material.

    In practice (& with reasonable care) the limiting factor of the lifetime of Baader solar film is that it is impossible to clean; dust & grime buildup will likely result in needing to replace it before aging of the substrate or deterioration of the reflective films.


  14. Given that the solaric irradiance at top of atmosphere is 1350 Wm^2,

    that the heat capacity of float glass is 500J/Kg Kelvin,

    that the density of float glass is 2.5 g/cm^3 and that the reflectivity of aluminum coating on the secondary is 96%.

    The real issue here is that the beam will not always be falling only on the secondary mirror - some of the energy will get to the supports which have a lower thermal capacity and an absorbtion at least an order of magnitude higher.

    Do not use a solar wedge or attempt eyepiece projection with any scope except a pure refractor (one without flattening optics near the bottom end of the tube). Do not use a barlow upstream of the wedge, the heat will uncement the elements. Do not use an eyepiece with cemented elements for projection for the same reason.

    A reflector with an unsilvered primary mirror is equivalent to having a solar wedge (but of course cannot be used sensibly for anything other than observing the sun).

    If you're at all unsure, play safe and use a solar film filter fitted over the objective.

    This is quite apart from the fact that the use of a diagonal (of any type) with a Newtonian reflector will result in a very uncomfortable viewing position, in the rather unlikely event that there is enough focus adjustment to get a sharp image at all.


  15. No matter how steady or unsteady, the light gathering ability for a wave form for the size of aperture should a be constant. I do not think it diminishes because you have a case of the jelly wobbles.

    True, but if the instrument is supported firmly you can see fainter objects because your eye/brain gets more of a chance to "integrate" the image ... I'd say the difference might be as much as 1.5 to 2 magnitudes between a hand-held 10x50 binocular and the same instrument mounted firmly.

    Steadying the instrument by supporting it on the top of a fence etc. also works & gives nearly the same improvement as a tripod. Where a tripod really scores is that you can "park" the instrument whilst consulting charts etc. without having to find the target object again.


  16. I have had very similar issues with external Hard Drives, and never really work out why or what was causing it.

    Fact of the matter is, USB connectors are not very good, especially after they've been plugged & unplugged a few dozen times. The more power you try to draw through them, the worse they are. Changing the cable for a new one is a relatively cheap way of "repairing" an unreliable setup, you should be able to do this a few times before the fixed connectors in the PC & device finally give up the ghost.


  17. up here in Edinburgh we only have a few weeks left then we dont get ANY ASTRO DARKNESS for a few MONTHS!!!!!

    At my location (55N) true astronomical darkness (sun 18 deg below horizon) is just about gone, till the first week in August (a notoriously cloudy month).

    However many types of observation can be done with twilight ... I find the sky "observable" for the purpose of variable star observations once the sun is down 10 degrees, and planets & moon can be good with much brighter twilight than that, even with the sun above the horizon.

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