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ollypenrice

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ollypenrice last won the day on July 10

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About ollypenrice

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    http://www.sunstarfrance.com
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    Imaging, Cycling, Thinking, Literature, French culture, Mountains...
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    South east France, Lat 44.19N.
  1. Flushing between subs sometimes comes up in flats problem conversations. You might try building in a longer delay, though perhaps you already had a long one? Olly
  2. ollypenrice

    F Numbers????

    I'd agree with that, though the F ratio of good apochromatic refractors is linked to aperture, so there are superb 4 inch F5 examples but once you're up to six inches it really has to be F7. I gather that the colour correction is the limiting factor. Olly
  3. ollypenrice

    F Numbers????

    It's another Ralf Ottow creation, Peter. A thin primary is separated from a rigid glass blank beneath it by a hermetic ring. When air is evacuated from the space between the two glass elements the curvature of the mirror is increased, so reducing its focal length. Remarkably, Ralf demonstrated this simply by sucking on a tube leading to the air space, though ultimately this will be done by a pump and connected to sensors which will preserve the selected pressure. He paid us only a flying visit so my chance to look through it was in the daytime, but it was very impressive - as are all the instuments he makes. Olly
  4. ollypenrice

    F Numbers????

    Focal ratio is a 'derived variable' and should be treated with caution. Aperture and focal length are primary qualities in an optical system. A few things we can say for sure: - fast (low F ratio) optics are harder to make. It is harder to rid them of field curvature, coma and false colour (in refractors.) If they are good they will be expensive. - fast optics produce a steep light cone which gives a shallow depth of field, making focus critical and requiring a mechanical system free of tilt. - In imaging, exposure times will be reduced as the square of the F ratio if the f ratio is lowered by increasing the aperture. If it is lowered by reducing the focal length then exposures will only be reduced because the light lands on fewer pixels. (This is an example of a derived variable causing confusion!) If money were no object and a brilliant optical engineer could make a perfect design then faster optics would be preferable because they would give you the option of a wider field of view if you wanted it. But, in reality, we tend to go for the compromise which suits our observing prefences and budgets. Last year I had a look through a very rare beast indeed, a reflector with a variable focal length primary mirror. Olly
  5. Colours can be measured and calibrated so that the same camera which can take a convincing colour photo in daylight can take one over longer exposures at night. There is no reason to suppose that it becomes inaccurate in its colour balance when doing so. And there are plenty of stars varying between blue and red which allow the astrophotographer to check that the camera is delivering the right colours. I use a star's published B-V colour index in conjunction with a B-V colour chart to check a selection of stars in the field of an image. The resulting image, I believe, will be very close to what an enormous and colour sensitive eye would see. By the way, Saturn is a deidedly warm colour to my naked eye, tending towards light orange and very different from Jupiter. Olly
  6. ollypenrice

    August 17-18, 2018: NGC7000 to Sadr with Zeiss 85mm

    Remarkably good! Hard to beat Carl Zeiss... Olly
  7. I strongly disagree with this. The last thing I want to do when I prepare an astrophoto for publication is 'to conform to certain accepted conventions.' In general my desire (not always possible to realize) is to produce something new. Sometimes I find I can't do this and settle for trying to produce a familiar view of high technical quality. But with a bit of luck I can bring something new to the table. I might do this by taking an insane amount of data - so the nebulosity around Gamma Cass is far more extended than most images suggest: Or I might colour calibrate my image, check the stars against the published astrophysical data, find that all is in agreement but that my result is very different from the norm, and publish it as I find it. Like it or lump it, my Cone Nebula and Elephant Trunk are far more magenta than most but that's what I found in a sincere effort to record the sky and I won't be changing them until someone convincingly demonstrates that I made a mistake - which they might, of course. High quality astrophotos are highly processed but that doesn't mean that they are highly invented. Much of the processing effort goes into removing errors and calibrating colours. Back to the OP's question: the problem is that astrophotographers are not in the business of trying to replicate the eyepiece view. They are interested in finding what cannot be found at the eyepiece. That's the whole point. Olly
  8. ollypenrice

    william optics flt 132 vs takahashi tsa 120

    I would warn you that Takahashi's quality control has not been of the best in recent years. So I don't entirely agree with Whirlwind. I use a dual Tak FSQ106 rig and cannot fault the optics but they are both old instruments. However, Tak will sort things out. I'd say that Barry has given a good answer to your question. There are plenty of good apo refractors available and you need the right one for your camera and targets and you need a good example of the breed - which means you need a good dealer. Olly
  9. That's great! A new soap bubble. (I find them humiliatingly difficult to spot but I don't tell anyone about that...) Despite the fact that you are pulling out the faintest of stuff you've given the image your signature 'soft touch' giving the unwary the impression that this might be an easy one, but I'll bet my bottom dollar that it isn't... Always nice to see something new, too. Olly
  10. I liked your first processing and it's very rare that I say this (I usually recommend the reverse) but I think you might bring in the black point just a bit. In Photoshop there is an incredibly simple but powerful trick for making Ha nebulosity pop. Just go Image-Adjustments-Selective Colour and it will open by default in the reds. Move the top slider to the left to lower the cyans in red and anything Ha will jump out and shine. Olly
  11. ollypenrice

    M31 Andromeda

    Yup, lots to like. I think that you have discarded some of the faint data by bringing the black point in a bit too far. You end up with the brighter parts of the galaxy jumping out of a very black sky whereas your data probably contains faint extensions making the galaxy larger than it shows here. Might be worth another look. Olly
  12. ollypenrice

    LDN 1251

    Nice one, Yves. Very nice indeed. I'd just pull down those few core-saturated bright stars which would intensify their colour as well, providing a colour contrast with the dust. Super image. Olly
  13. ollypenrice

    M31 Andromeda

    I think you need to post a JPEG here in order for us to see the image. Olly
  14. I'd forgotten the Brontsaurus! Very nice image. Olly
  15. ollypenrice

    Pier & Observatory

    You might be surprised by how small a roll off observatory can be. The most compact design is usually called the 'sentry box' with the entire shed rolling off when observing. This way the shed can be a tight fit around the mount and scope since it doesn't need to house the observer as well. Also they can be made incredibly easily: you start with a small concrete base and pier. You buy the smallest, simplest shed that will fit over your setup. These can be wooden, plastic or metal. You make a plywood floor the size of the shed's footprint and fit it with little wheels running on rails. It needs a slot cutting into the plywood floor to let it run halfway round the pier and then you just build up the shed on the wooden rolling floor. I've made two sheds like this but the simple idea of using a rolling floor to carry a standard bought-in shed didn't occur to me so I spent ages welding up chassis and frames myself. Curses!! lly
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