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About Fraunhoffer

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    Star Forming

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    near London
  1. one is 10/6 and the other is 20/6. exit_pupil = ep.focal_length / (t.focal_ratio) = ep.focal_lenght / (t.focal_length / t.aperture) = (ep.focal_length / t.focal_length) * t.aperture = t.aperture / (t.focal.length/ep.focal_lengh) = t.aperture / gain both give the same answer. sorry for the shorthand.
  2. The SCT and focal reducer is superfluous to what I was trying to work out. I was trying to understand why the larger aperture scope would give the better view. Since they both have the same overall F ratio in this example and the fov using the 2 eyepieces is the same. Im thinking that (assuming the eye pupil remains fully open and is larger than the exit pupil) the exit pupil is effectively controlling the aperture of the eye. Since size of the rods/cones are fixed in size (and treating them like a camera sensor) - so the larger exit pupil in this case is allowing the eye to have a higher f/ratio of its own and hence brighter and more differentiated light levels between the background and the subject view. (as well as more detail due to the higher aperture resolution of the scope - and ignoring any sampling at the eye).
  3. lets imagine I wasn't to see a nice DSO about 15' size and I think it should look good nicely framed with a 1 deg field of view in the EP.. Which would give the better (or higher probability of seeing anything at all ) view from a semi urban light polluted home site (e.g Bortle 6)? a) an 100mm f/6 refractor (fl 600mm) and a 10mm EP (60 deg afov, gain 60x = fov pf 1 deg) (and exit pupil of 100mm / 60 = 1.6mm) or b) a 200mm SCT with focal reducer to give f/6 (fl 1200mm) and a 20mm EP (60 deg afov, gain 60x = fov of 1 deg) (and exit pupil of 200 / 60 = 3.3mm) My gut feeling is that the SCT should give a better view just based upon its 2xaperture - but Im not sure I understand fully the maths why. Is the larger exit pupil going to result in a better / brighter / more successful view? Or will the view be 'roughly' the same ? Or have I got it all wrong..... Thanks.
  4. From my very limited experience of 2 visits to a darker place it made an incredible difference. E.g with binoculars, at home I have to carefully work out how to hop from place to place to see star clusters and look carefully. The 2 visits I have made - no problem - straight there, obvious where they were and they stood out clearly. At the second one, the milky was very clear - something I ve not seen for many years, so many stars, it was actually a bit confusing to pick out some of the constellations.
  5. New diagonal prism. It better be good or I'm in deep trouble. Goodbye plastic unbranded mirror with repaired screw hole. Now, what happened to those DSO's Later: Goodness _ I can see a difference just eyeballing through it out the window as some thin wispy clouds, and next doors bricks, brighter and more contrast. The twist focus bit feel lovely and smooth.
  6. Yes thanks for the link. Its hopefully clear later on tonight so will try out a few variations.
  7. That would be assuming that I stacked all the sub-stacks. I was imagining that lets say I can either stack 120x30s for 60 mins and have a bad 5-10 minutes somewhere in there and hope the greater number of good subs will outweigh the bad subs. or, I can do some 10 minute sub-stacks (say I do 20 x30s in each, so Ive got 6), realise ive got a bad set after and so reject it, leaving me 5x(20x30s) to final stack. or something like that - maybe 60-90 minutes total. I don't spend days doing the same subject, whatever comes out in 60-90 minutes is it (even if I get that long) and to do the best I can with it.
  8. Hello Olly Ill have a look at the sigma routine. I don't think I will often get the chance to restack. I have just about 2 1/2 - 3 hours from when the street lights turn off to the sun rising. In that time I have to locate the object visually (no goto mount) and then set up the camera and take a picture. The picture is really there for my reference and to share if its any good - occasionally I have to use the greater sensitivity of the camera to actually 'see' the object. I see one thing from the other discussion is to keep the number of subs the same in each stack before trying to re-stack those (assuming the snr is the same in each). I guess my scenario might be stacks of 20x30s (10m) over an hour and one of these might be bad due to a cloud/mist/helicopter etc. So at least the 'bad' sub-stack can be removed and that way it doesn't degrade the overall snr of the final stack.
  9. Great - thank you for the link. Ive consumed that discussion with interest. I see its a question asked by many before me. I wonder if its a milestone on the path to better imaging met by many.
  10. I like live stacking as I can see the image develop and it saved my hard drive being cluttered up with thousands of images. The downside, I have found is that if a cheeky wisp of cloud comes over, it often gets included (in spite of trying to clamp down the FWHM settings). Some of my images then have a like a grey smear over them. Then after and hour of live stacking I find it looks just like a greasy smear over the picture- which is a real pain to try and post process out. Always seems to happen when I pop indoors to make a drink etc. I noticed that there was a setting on SharpCap. to save the stack- save and reset. Which I have not used yet. So Im thinking maybe I should do shorter live stacks eg. 30s for 5-10 minutes and then stack the results in A!S or DSS, so I can weed out the poorest sets, and I least I get something. Gives me a chance to do any slight re-adjustment too. Is a stack of stacks the same as doing a longer stack? My rusty school maths says it should be - but thought I'd see if anyone else does this. (I guess the setting wouldn't be there otherwise) Thanks
  11. I came across the Topaz Denoise AI and Sharpen AI links from another forum. Although pricey, the demos look impressive and I wondered if anyone was using them for their astro photos. Tx
  12. looks great. I must go and find these next time Im out.
  13. I was reading 'Choosing and Using a Schmidt-Cassegrain Telescope' by Rod Mollise and his recommendation to use felt between the tripod and mount head to reduce vibrations. As I was planning to do some tinkering with the scope and accessories today I made a felt disk and also put some felt under the RA motor. One benefit I noticed was that's it's now easier to use the Azimuth tripod adjustment bolts (especially if the leg/tray spreader is done up). Whilst I was at it, I turned the RA gear wheels around to reduce variations due to eccentricity of the RA worm shaft caused by those little grub screw fixings. Also I have some thin plastic washers (plastic milk bottle) under the slow-mo flange so that it is not wearing metal to metal.
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