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Viewing 15 posts - 1 through 15 (of 101 total)
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  • in reply to: Milky Way #17981

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    There you go. Astrophotography is simple isn’t it!

    in reply to: Fog on eyepieces #16138

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Wiping EP’s is not a problem Tej. If the coating was so delicate it came off that easily it would indicate very poor quality. Modern coatings are pretty good. Not military grade perhaps but more than good enough for nomral use and cleaning. There is no harm in giving an EP a wipe but obviously you use a soft cloth like a microfibre and not one that has been used to clean your shoes beforehand. A bit of common sense is needed.

    All lens cleaner really is normally is isopropyl alcohol, sometimes watered down. This is a solvent which evaporates very quickly. Again common sense prevails. You don’t apply a gallon and leave to soak overnight!

    Best cleaning kits on the market is one of the Zeiss kits.

    in reply to: Fog on eyepieces #16134

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    If you are regularly changing eyepieces then the easiest solution is to keep the caps on the eyepieces. If they fog in use then clean them (you should only ever need to do the ‘eye’ end. Wiping an eyepiece down is a much more straightforward task to cleaning the lens, or in your case, the corrector plate. That should not be done. Leaving that to dry naturally is the best solution.

    Putting them in you pocket works just as well although there is always the chance you can put your fingers on the optics when reaching for them.

    Dew heaters for eyepieces are only effective if sticking to one eyepiece. I never recommend this as a solution.

    You also need to be aware of your breath. It is quite easy to breathe on an eyepiece when lowering your head which naturally causes it to fog.

    in reply to: M57 #16120

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Well done for getting out there. The Ring Nebula I presume?

    in reply to: Refractors vs reflectors #16052

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Your recent ‘bargain’ purchase highlights a fundamental issue that is all too common among astronomy retailers. This being not offering a customer any advice and not making sure they know how to use what is supplied. I try to make sure all my customers get advice, even if it means we supply something that is less than they were going to spend.

    It also highlights that such is the depth of poor service offered by many of the astronomy retailers out there that this poor guy did not feel he could seek advice from the retailer he got the scope from. Had he done that, or had the retailer actually cared the customer was happy with his purchase, he would now be using his new telescope. Instead this guy is now almost certainly lost to our hobby.

    in reply to: M13 Hercules Cluster from ahem…my Living room window #16029

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Well done Tej. I am convinced there is still loads more to come from it. The Sky is not that black so I reckon the cluster could be twice as large. Needs a Star Mask.

    With a Canon, put it on ISO800 and leave it there. 1600 is very noisy. Better to stretch gently in processing than let the camera increase gain.

    in reply to: M13 Hercules Cluster from ahem…my Living room window #15984

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Hey Tej, you have very neatly demonstrated the main thing about Astrophotography. That is to just do it! I bet it was a bit more comfortable than sitting in a damp cold field!

    A good result considering you broke all the rules! Looking at the image I think this could stand a lot more stretching.

    So will you be now offering remote hosting?

    Re your fears / questions. I don’t know what Canon quote for shutter life but most Nikon SLR / DSLR’s are rated for 150,000 actions so there’s plenty of life left in your camera yet.

    Dark frames are used to subtract noise. The more you take the better. You can throw away the individual frames and just keep a master. However what you have done is created a picture with a signal to noise ratio equivalent to a bit less than 50 frames overall. This is because if you took say 500 frames at ISO 1600 and stacked them you have a low noise result. If you then subtract a dark frame that is from 50 frames at ISO 1600, you have added back in the noise those 50 frames have. So although you have a lower noise result with 500 lights plus 50 darks, than 50 lights plus 50 darks, more darks is better. You should also have masters for each ISO. Don’t combine them. The noise at ISO 6400 is really bad. I don’t know how much worse it is relatively than at 1600 but its a lot that’s for sure. Signal to Noise Ratio follows a square root rule so if you take 9 images then the S/N increases by 3 (square root of 9). This being the case, you need to take 9x as many frames (square of 3) to overcome the effect of adding one noisy frame. So 9 light frames cancels out the negative of adding only 1 dark at the same ISO. You can see that 50×9=450 so 500 frames only just deals with the effect of adding only 50 darks. You might find that by dropping all your ISO 6400 (and maybe also 3200) images that the noise level is so much lower it can be stretched more in processing without the same noise penalty. You certainly have enough data to experiment with things like this.

    • This reply was modified 2 years, 7 months ago by  Astrograph.
    in reply to: National Geographic's top places for dark skies #15473

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Cristina

    The equalize function is just to show what is there. There will be lots of nasty’s. Don’t worry. Can you dropbox the original RAW file for the ISO 800 one so I can play?

    in reply to: National Geographic's top places for dark skies #15472

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Cristina,

    The 500 rule is a usable rule for imaging without visible star trailing so you should be able to get more than 10s with a wide angle. Understand that star trailing is still there but the reduced star size and wider view mean you can’t see it easily.

    Stacking is a method used to increase signal to noise ratio. Every image stacked has only one constant, the stars. Sigma Kappa type stacking will reject data that is not duplicated, so as noise is random, it gets rejected while the stars are kept. Up to 30 frames this means the S/N becomes much greater. Therefore you can then stretch to amplify the image without amplifying the noise as much. You can’t fake it by taking one picture and duplicating it because the picture is identical so nothing is rejected and the S/N stays the same.

    Taking more shots on a fixed tripod is OK if you don’t have a foreground subject. If you do then you have to fake it and take a foreground image to place over the sky (that you have lowered).

    Your camera has low noise due to on board handling of the image but it is not magic. Go above ISO1250 and it will become visible. The Sony sensors Nikon work with are getting close to ISO invariant. This means that if you took a picture at ISO 100 and one at ISO1600, then increased the exposure in processing on the ISO100 to be the equivalent of ISO1600 it would look the same. The advantage being that software is a bit more subtle doing this than the camera. In reality, its not really the ISO we need for astrophotography, its exposure, we need to capture more photons.

    Why the fascination with getting a super wide angle lens? You will always get more distortion and more problems. Leaving lens aberrations aside a super wide means you will;

    – Have a greater chance of a light pollution gradient

    – If you put a light pollution filter on the front of the lens it will not work as well at the edges because the lens is ‘seeing’ light through the filter at an angle which is not filtered as effectively as light passing through perpendicular to the filters surface.

    – Have to pay more relatively due to the complexity of the lens.

    The wide angle I currently use is a Nikkor 24mm F2.8 AIS. Good, reliable, pro grade manual lens that new was over £500. It has a lens profile built into your Nikon and also in Camera Raw. Cost me £99 off ebay. No brainer. Why spend more until you feel the lens is your limit?

    You really do need to get this ‘I can’t take pictures at F5’ nonsense out of your head. If you limit yourself like that you will end up only taking pictures full of artifacts. As I said, most imagers are using scopes with f ratios ranging from F5-F10. When I do solar system imaging it is at F32+! Yes they are tracked but its photons you must capture which require extra time.

    Tracking mounts are not expensive. You can make one for £50-£100. You can buy a reasonable one for not much more and a really good one for less than that Sigma. Depends what you want to do.

    You will have to come along to the deep sky talk in March.

    in reply to: National Geographic's top places for dark skies #15451

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Hi Christina

    OK well if it was just 10s then the other artifacts I saw were not tracking.

    10s is too little exposure and ISO 8000 too much gain (all digital camera do is increase amplifier gain from a default. This just adds noise). Nikon cameras are better than Canons in this regard but I don’t go above 1250 with a DSLR. The 500 rule is something which works well if you are a bit conservative with timings. So 500/focal length = exposure time with a full frame sensor. Therefore 500/20 = 25seconds, which is why I said go for 20 secs.

    Returns for the Samyang are no surprise. No different to cheap, badged Chinese telescopes. The odd one can be brilliant but on the whole, far from it. Not that £400 is cheap for a lens but for something exotic (24mm F1.4 is) then if you pay Peanuts, you get Monkeys. BTW, if you want a wide angle then the one lens you should save up for is not really the ones I mentioned, its the Nikon 14-24mm F2.8. This is used by a lot of Astro Photographers with Canon, Nikon and CCD alike. A grand used but worth every penny as its a sought after pro lens that is like a bag full of primes. I’m saving up for one.

    If people are returning lenses for coma then you have to ask how are they using them? The only way they can determine coma is with something like a star test and that’s not really fair because if used in daylight on a terrestrial subject they would never see that. Distortion, Chromatic Aberration, Longitudinal Aberration and plain old Sharpness (but in the eye of the beholder) yes. Coma, no.

    F2 is not really any different from wide open. Only at F5.6 does a lens really deliver. Most lenses sweet spots from F5.6-F8. Always have been because this is the aperture which uses the best part of the lens. A caveat needs to be mentioned here because I should really say approx 2 stops from fully open. Your F4 kit lens in theory would be best at F8. Having said this the faster the native F Ratio the smaller the radius of the curve in the lens and the more distortion because that is harder to make so the edge of the lens suffers. If you want to see how lenses perform then I suggest looking at either DXO or Photozone (just google with the lens) as they offer tests.

    F4-F5.6 aperture range most people imaging with telescopes would kill for. The key to seeing anything is not to end up with a picture you can see. It is to end up with anything at all! The Rosette image I took at E-Eye was 4 images (about 20 less than ideal) of 5 minutes each and this was the actual result after stacking that. ISO would have been equivalent to about 200.

    RGB Stack by Rupert Smith[/url], on Flickr

    As you can see there is not a lot there. In Photoshop, if you go to Image/Equalize then this will show you image with every level maxed out so you can see whats in there. This is waht that ‘nothing’ image reveals.

    RGB Stack Equalize by Rupert Smith[/url], on Flickr

    I use this as a guide and also a challenge. I am by no means a talented processor but I got the nothing to this so far. Getting there but really more data for longer is the way to go.

    Rosette 210117 v3 by Rupert Smith[/url], on Flickr

    Your image as shot is fine for enhancement. If you shot with less ISO, longer exposure and at F5.6-8 you would be able to bring more detail out because there will be less noise or aberrations in it. Slowly, slowly, catchy monkey as they say.

    in reply to: National Geographic's top places for dark skies #15448

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Christina,

    There are a number of things to consider here. No 1 is that using ANY camera lens from ANY maker wide open will ALWAYS result in some coma and possible other aberrations if you are critical enough. I make this point so many times to people. A camera lens has so many jobs to do compared to a high end telescope lens that it just cannot be effective on point sources of light. Using one wide open will always lead to issues. They must be stopped down to at least F4, preferable F5.6 before aberrations like this will effectively disappear. You will find it much reduced from others and you may be able to use a lens with a lower F ratio but never fully open. Testing of the 20mm Art shows a massive increase in performance from centre to edge at F4.

    No 2 is a wide angle lens will also suffer more from barrel and pincushion distortion, which when wide open will be accentuated. No 3 is that the bottom corners of an image can often seem more distorted due to dispersion in the atmosphere. This will be more noticeable with longer exposures but is a factor.

    No 4 is that this is a 20mm lens on a full frame camera (I assumed you used you D750?). This is very wideangle, much more so than a 24mm. While I would expect better in the central part of the image I am not surprised by the edges, which if shot at F1.4 are not that bad.

    Out of interest what was the exposure as I can also see some tracking error so I assume quite long?

    Many errors like this can be fixed in Camera Raw if the lens is in Adobe’s DB.

    Re comparison of a Samyang (or whatever other name it appears under) to a Sigma Art, then there is not really one to be made. For all the hype Samyang get, they are still a cheap, manual only focus (nothing wrong with that) except with Samyang the AF focus confirmation can apparently be a bit variable (meaning focus is wrong) so you have to check focus in daylight by eye. The odd lens has punched well above its weight but generally it is the 35mm that is the secret bargain. Sigma are a premium lens maker that have developed and developed to the point where they are easily the best independent lens maker outside of Zeiss (although the new 85mm Art just beat an Otus in the lab at 1/3 the cost). They have for some time been superior to much of Canons output if not quite with Nikon but the Art models are seeing to that. They were not always this good but have progressed.

    Now despite all the BS surrounding the ‘Art’ lenses they are still subject to variance like anyone else’s lenses. I have a customer who was using a triple Canon 300mm F4L lenses on an Astro imaging rig. Generally this is considered a good lens but I got a call asking if I could collimate them. I saw images from the 3 and 2 were decidedly poor. Nothing really can be done so 2 were sold and he bought a proper scope.

    The Art you have borrowed is a first rate lens. You might have borrowed a lemon, but there is also as much chance of that, if not more with the new 20mm Samyang. Personally if you are in the market for a £600 wide angle then the Art is a good choice and I would ask to borrow another if this one is suspect. I certainly would not just buy a Samyang. I would want to test it first. Although more expensive I would also keep my eyes open for a used Zeiss 21mm Distagon. This is F2.8 but will exhibit a level of consistency that is difficult to beat. A daylight image taken with a Zeiss lens is ‘just right’. I had a set a few years back and sold them all when times were tough. I really wish I hadn’t. A used 21mm will cost £7-800.

    Failing that I would also consider a used Nikkor 20mm AI or G lens. They can be a bargain. Remember that for all the hype surrounding digital sensors, sharpness, blah, blah. B&W film has much more dynamic range than digital and the sharpest types would be the equivalent of a 500MP (that is 500 not 50) full frame sensor. A lot of old lenses had to be built to work with this level of quality. Ironically manufacturers are now having to get back up to this quality level and it costs.

    So sorry for the long post, but I would try the lens again at F4 and personally would limit the exposure to about 20s (500 rule suggests 25). I would also aim at the zenith for a pure test.

    in reply to: Images from E-Eye Trip #15002

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Re process of the Rosette. Need more than 20mins of data to do more

     

    Rosette 210117 v2 by Rupert Smith[/url], on Flickr

    in reply to: Refractors vs reflectors #14998

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    MMmm. Clearly the person responsible for that load of tosh would not know which end to look in anyway. If they did they probably would see Space Vampires through the reflector. Of course, upon looking at the same thing with the refractor they would then realise they were stars.

    in reply to: Images from E-Eye Trip #14935

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    First processing attempt of Rosette Nebula taken at E-Eye. Due to the late start I did not have the chance to gather as many frames as I would have liked. The scope guiding did not work first time, so rather than waste time I went with running my Fornax Lightrack travel mount unguided. Never having tried that at 350mm FL was a bit of a gamble. In the end it turned out OK. There were a few frames that were not perfect for various reasons. By the time my first run ended it was -5C and my bed and the thought of being warm was a stronger draw than taking more pictures.

    So this is only 4 x 5 minute frames taken with a one shot colour camera. Quite pleased with the result. Need to play with the processing a bit more to make the gas cloud ‘pop’ but not quite sure how to do that yet.

    Rosette 210117 v1 by Rupert Smith[/url], on Flickr

    in reply to: Images from E-Eye Trip #14926

    Astrograph
    Participant
    Topics: 27
    Replies: 102

    Kayron was my first hosting customer for E-Eye. He actually visited E-Eye on Saturday and we were supposed to meet up but we were in Jerez.

Viewing 15 posts - 1 through 15 (of 101 total)