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alan4908 last won the day on April 4

alan4908 had the most liked content!

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About alan4908

  • Rank
    Proto Star

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  • Yahoo

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  • Gender
  • Interests
    Astrophotography ! ....at the moment I'm concentrating on deep space imaging
  • Location
    East Sussex
  1. alan4908

    Bubble in the TEC140.

    This is very impressive, I particularly like the bubble NB detail and the RGB star colours. Alan
  2. alan4908

    Star shapes

    Yes, for some reason I was. Anyway, at least you get the idea. Hopefully, you'll soon sort out your issues. Alan
  3. alan4908

    Star shapes

    Hi Mike If you put your processed images through CCDInspector then it indicates that you have a significant degree of tilt and collimation error on the M27 image. The Iris has a smaller tilt and better collimation. If you look down the right side of the M27 image you can also see that the RGB channels are incorrectly aligned. The average aspect ratio of the stars is coming out as 24 and 27 for the Iris and M27, respectively - which isn't good. To find the root cause(s), I'd suggest that: 1. Take some very short subs with the telescope pointing vertically upwards - this will minimise any tilt that may be coming from focuser droop and minimize any detrimental effects of guiding/tracking errors. 2. Repeat the above step but this time with the telescope as horizontal as you can get it whilst still being able to take a picture of the night sky. This will maximise the influence of any focuser droop. You then need to examine the resultant subframes. If you like to do things quantitatively then I'd suggest you download a free trial of CCDInspector and put the unprocessed but calibrated subframes though it in its averaging mode. This will enable you to measure the tilt, collimation error and aspect ratio. The easiest way to determine your imaging resolution in arc seconds per pixel for either your main system or guide scope is to put a subframe through a plate solving program, alternatively you can work it out by the formula (206.265/Focal Length)*pixel size (here the pixel size is in microns and the focal length in mm). Your measured guiding RMS of 0.6 arc seconds also seems high - however, before trying to lower this, I'd suggest you concentrate on lowering any camera tilt and collimation issues. Alan
  4. alan4908

    Deep Sky III

    Images taken with a Trius 814 and a Esprit 150
  5. alan4908


    Many thanks Barry - to be honest, I was a little surprised that these revealed themselves so well with the relatively short lum integration time. Alan
  6. alan4908


    Thanks for the comment John. The image was processed using three software packages: CCDstack: calibration, alignment, stacking, data error rejection, DDP stretch (lum) and deconvolution. PS: contrast enhancement, sharpening, colour enhancement, linear stretches, mask generation. Pixinsight: gradient reduction (DBE), photometric colour calibration, sharpening (MLT), background noise reduction (TGV). Alan
  7. alan4908


    NGC4631 is a starburst galaxy that is viewed edge on from Earth and interacts tidally with the dwarf galaxy NGC4627, forming a "light bridge" between the two objects. It has also very vibrant star forming regions along its length. Due to its shape it is commonly known as the Whale and Companion. The image below is an LRGB image and represents about 7 hours integration and was taken with my Esprit 150. Due to poor weather and my multi object acquisition strategy, the image capture ended slightly prematurely since it has now disappeared below my local horizon. Whilst I'd have liked to gather more data, particular on the Lum channel, I was reasonably pleased at the level of detail revealed. Alan LIGHTS: L:10, R:6, G:13, B:12 x 600s, DARKS:30, BIAS:100, FLATS:40 all at -20C.
  8. alan4908

    Optical resolution in DS imaging.

    Vlaiv This is exactly what I was after - well done ! Alan
  9. alan4908

    SH2-140 (reprocessed)

    An LRGB image with an Ha blend into the red and lum channels and representing 16.5 hours integration time which was taken in 2017. In this reprocess I was trying to get a little more star colour by stretching the brighter stars much less than the main image. My original attempt can be seen in my gallery Deep Sky III. Alan
  10. alan4908

    Optical resolution in DS imaging.

    Thanks Andrew for your suggestions. I do like my current camera (SX Trius 814) and given that my site is in the UK (aka my back garden) I'm increasingly of the opinion that the benefit I'd derive from a new camera with smaller pixels and higher QE would not justify the cost. Due to the poor UK weather, I do implement a form of lucky imaging but not in the conventional sense. Since I image automated and unguided, I've configured my system to allow imaging when it is a little cloudy, this is because there's a chance that in the direction my scope is pointing, the sky is actually clear for the duration of the sub frame (normally 600s for LRGB). If a cloud happens to wander into the field of view, the image resolution will be degraded either significantly or hardly at all depending on the duration of the obstruction. Obviously, this approach does generate more throw away subs, due to clouds, but it also generates additional usable subframes that I wouldn't have obtained if I'd configured my system to image only when the skies are crystal clear (a somewhat rare event). Alan
  11. alan4908

    Optical resolution in DS imaging.

    Andrew Thanks for the information, it is not really what I'm looking for but it's interesting that your conclusion for optimal resolution is 3 to 4 pixels per FWHM seems to agree with the Stan Moore analysis (http://www.stanmooreastro.com/pixel_size.htm) which states that FWHM of 3.5 pixels or more is optimum. My Luminence stack for NGC5907 gives an FWHM of 2.43 arc seconds (as measured by CCDInspector) so since I'm at 0.71 arc seconds/pixel that would be an FWHM of 3.42 pixels. However, the best 600s subframe has an FWHM of 1.49 arc seconds or 2.1 pixels. Which suggests that some improvement is possible if I go for a camera with smaller pixel size/higher QE, however, I'd only expect to see this on nights of very good/excellent seeing from my site. Given that I'd be increasing my oversampling rate, stronger deconvolution in areas of high SNR should enable me to extract this higher resolution from the averaged stack. So, does this provide justification for purchasing a new camera with smaller pixels and higher QE ? - I have to say that I'm still not really convinced. Alan
  12. alan4908

    Optical resolution in DS imaging.

    Andrew Having experimented with my own data, the sum of the squares equation does seem to only give reasonably accurate results when used with highly over sampled data. So, I tend to agree with your summary. However, since I'd really like to have a practical analytical guide for the total imaging resolution of a system, surely there must be a way to approximate this ? Alan
  13. alan4908

    Optical resolution in DS imaging.

    Ah yes, well spotted, a typo on the formula which I have now edited. Have another look. It should have been: Alan
  14. alan4908

    Optical resolution in DS imaging.

    Thanks for all the explanations, I think I'm slowing comprehending this (hopefully). What is confusing me is the following. Above, you give a demonstration on what the formula would predict if you took an image, measured the FWHM, binned the image 2x2 and then remeasured the FWHM. As you say, you can test the formula by comparing the measured binned FWHM with the predicted FWHM. After doing this you seem to conclude that the formula is inaccurate. I was attempting to repeat your analysis and I came to a different conclusion. To explain: if you take a well sampled image that you've taken with your scope the formula is saying that the measured FWHM should be: 2.355*SQRT(Image Scale/2.355)^2 + X1). Here X1 represents the unknown contribution from the seeing, guiding error etc. Remembering my math, I can re-express this as: FWHM = SQRT(Image Scale^2 + X2) where X2 is unknown. So, if I take the image and bin it 2x2, X2 will not change but the image scale will be reduced by 2. So: FWHM (binned) = SQRT(4*Image Scale^2 + X2). So, now we attempt to eliminate X2 by squaring both equations: FWHM^2 = Image Scale^2 + X2 and FWHM (binnned)^2 = 4* Image Scale^2 + X2 If you then subtract the two equations to eliminate the unknown X2 you get: FWHM (binnned)^2 - FWHM^2 = 3*Image Scale or rearranging FWHM (binned) = SQRT (3*Image Scale^2 + FWHM^2). In your test you have an image scale of approximately 0.5" and a measured FWHM value before binning of 4.11. So if you put these into the equation you get a predicted FWHM after binning of: FWHM (binnned) = SQRT(3 x 0.5^2 + 4.11^2) = 4.2. This compares to your measured value of 4.26. This appears to suggest a reasonably accurate prediction by the equation, at least when using oversampled data. So what is the error in my analysis ? Alan
  15. alan4908

    Optical resolution in DS imaging.

    Well, I'm still attempting to comprehend all this..... In the meantime, I've found a very interesting post by Jon Rista on the subject of large telescopes/small telescopes and the limits of resolution. He also addresses how you work out the limiting factors for resolution and the calculation for the total system blur for a system. Interestingly, his approach mirrors that of Chris Woodhouse that I mentioned above. Have a look here (you will need to scroll down to his post): https://www.cloudynights.com/topic/559188-understanding-sampling-resolution/ Alan

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