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  1. In an earlier post I examined the noise in single dark frames over a range of exposure times. My conclusion was that the Nikon D7500 was a lower noise camera than the D5300. This was also backed up by an examination of master bias frames that again strongly favoured the D7500. However ... The first image I attempted to produce with the D7500 exhibited very strong streaks in the noise. I had been in the habit of not using dark subtraction with the D5300 as it has very low thermal pattern noise. Accordingly, I again only used bias and flat frame calibration in the workflow that produced the above images. Whilst the streaks are due to patterns in the noise being spread across the image due to errors in the application of dithering during tracking, it did indicate that the D7500 did have significant thermal pattern noise. I found this surprising because the noise in individual frames ( when looked at in isolation ) seems to be completely random. I thought that perhaps my memory had failed me and maybe the D5300 has the same level of pattern noise but my memory was being tricked. That is, all my recent images with the D5300 were taken at lowish air temperatures ( ~5 deg or so ) whereas the image above was captured on warm nights ( low 20s ) and so maybe the D5300 would be just as bad at higher temperatures. To test this I produced bias corrected master darks for both the D7500 and D5300 from images all taken at around 20 deg or just over. The images below have all been stretched using the same screenTransferFunction applied to the Pixinsight histogram tool. The results are striking ... D5300 master dark ( 47 subs, bias corrected ) - red channel: D7500 master dark ( 281 subs, bias corrected ) - red channel: D5300 master dark 100% centre crop - red channel: D7500 master dark 100% centre crop - red channel: And the histograms of the full size images ( red channels ) ... D5300 master dark ( red channel ) histogram: D7500 master dark ( red channel ) histogram: The Pixinsight statistics tool calculates the following: D5300: mean 2.3, standard deviation 9.3 D7500: mean 7.5, standard deviation 20.8 ------------- Analysis: The images and histograms clearly show that the D7500 has higher pattern noise than the D5300. In particular, from the histograms, 0.1% ( 6,286 ) of D5300 pixels are more than 44 ADU whereas, for the D7500 this figure is 27 times as great at 2.7% ( 141,305 pixels ). Furthermore, the master dark for the D5300 was only produced using 47 images -v- 281 for the D7500 so I would expect that this difference would be even higher with more D5300 frames. On the other hand, whilst not shown in the histograms above, my D5300 does have more 'very hot' pixels than the D7500 ( 579 pixels greater than 400 ADU -v- 10 pixels greater than 400 ADU ). However, these hot pixels are very easily removed via dithering during tracking and sigma clipping when integrating. The very large number of warm pixels however are very difficult to remove as dithering just places different warm pixels on top of each other. I went back and examined the 'random' noise seen in the individual D7500 dark frames ... and yes they do look random when seen individually, however, when flicking between a number of frames it is clear that the 'random' pattern is repeated in each frame! Conclusion: My D7500 has very significant thermal pattern noise, albeit randomly distributed in a fixed pattern. ............ Next steps (?) - I could use in dark subtraction during calibration to reduce the impact of pattern noise - however, as my camera is not cooled and the night's temperatures are constantly changing, any master dark will not closely align to the actual thermal pattern noise and as such dark subtraction may help but will not solve the problem - Using in-camera dark subtraction ( Nikon's long exposure noise reduction feature ) would almost completely remove the pattern noise from each frame. However, due to the extra random noise being introduced by subtracting another noisy dark frame from each light frame, as well as the reduction in total light frames by 2, the resultant images will suffer from higher levels of random noise. So whilst this would be an improvement with respect to the pattern noise, it is not a complete solution. - Third option, sell the D7500 and go back to using the D5300 ...
  2. Below is a comparison between single dark frames taken with the Nikon D7500 and D5300 with exposure durations varying from 1 sec to 240 sec ( my usual main light frame exposure ) all at ISO400. Firstly a graph of the standard deviation of the noise in the dark frames versus exposure time: The standard deviation of the noise is a fairly constant 2 ADU less for the D7500 compared to the D5300 ( pretty much the difference in the read noise between the two ) However, the difference is not just in absolute terms but also in the quality of the noise ... Below are the dark frames - ranging from 240 sec exposures at the top to 1 sec at the bottom: D7500 D5300 The D5300 dark frames clearly show the pattern in the read noise ( banding down the bottom ) and also have far more chrominance noise compared to the D7500. At 240 seconds ( the main exposure I have been using ) the difference is starkly different; the D7500 produces images with much lower noise that is significantly more even and random and hence more likely to be reduced during integration.
  3. It was love at first sight when I first laid eyes on the gorgeous noise curves of my dear Nikon D5300; young and beautiful with a great body and a large sensor, I was smitten. Now, well, what can I say... my roving eye has spied a new beauty; younger with heaps of energy that should go all night and , whilst I do prefer larger sensors, I can’t stop thinking about the noise curves hidden beneath that lovely new body... I struggled with my guilt for ages, I really did, but the time has come - I’m dumping the D5300 for the new love of my life; the Nikon D7500.
  4. I have completed a comparison of the level and pattern of read noise in bias frames between the Nikon D7500 and the D5300, please see the blog page ...
  5. The Nikon D5300 has a well-earned reputation as one of the lowest noise DSLR cameras used for Astrophotography. Now that I have a new Nikon D7500, I was keen to see how it compared to the D5300 in terms of the level of read noise and the extent to which that read noise is non-random ( and thus needs to be removed using a Master Bias frame to prevent it summing up during image integration). So here goes ... ......... A single bias frame Nikon D5300: ISO400, 1/4000th second: This may look pretty bad but really the extreme stretch is bringing out the very small variations across the frame: Standard Deviation: 3.53 ADU ( note: in this context, 3.53 ADU means 3.53 "units" on a real number scale ranging from 0 to 16,383 ( ie. a real conversion of a 14 bit digital scale )) The master bias frame looks like this: Nikon D5300: ISO400, 1000 x 1/4000th sec frames Standard deviation: 0.48 ADU The bands at the bottom are each 1 ADU brighter than the one above. Now for the D7500 ... A single bias frame from the Nikon D7500: ISO400, 1/4000th second: It is immediately clear that the single bias frame is cleaner. The statistics confirm this: Standard Deviation: 1.37 ADU And the master bias ... Nikon D7500: ISO400, 1000 x 1/4000th sec frame Standard deviation: 0.07 ADU The band at the bottom is 1 ADU brighter than the background. The improvement is very obvious Std. Deviation single bias frame: 1.37 versus 3.53 Std. Deviation master bias ( 1000 frames ): 0.07 versus 0.48 In graphical form ... Conclusion: The read noise in a single frame from the D7500 is around 40% of that in one from the D5300. This should give me greater flexibility to reduce exposure times and still ensure that the read noise is only an insignificant component of the overall noise. I will need to consider further the impact of the very low level of pattern noise in the Master Bias; it is so low that I will think about whether or not I still need to calibrate my lights with a Master Bias ( particularly for long exposures when the noise is dominated by light pollution and thermal noise ).
  6. The Rosette Nebula and Cluster ( NGC 2237 and 2244 ) in the constellation Monoceros edit: updated 30th Dec with improved colour balance and slightly increased brightness ... ...... original: ( please click / tap on image to see larger and sharper ) Still a work-in-progress really... with only 10 x 4min exposures for the main 'lights' before the clouds came over. I will try to add some more data when the moon has gone I am still experimenting with how to get the best out of the D7500. With the very warm nights ( low to mid 20s all night ) the 'warm pixels' are very noticeable so I reverted to my old practice of in-camera dark subtraction. This worked quite well and produced a nice smooth noise floor in the integrated images - albeit at the expense of more exposures. ................. Identification: The Rosette Nebula ( NGC 2237 ) is a large, circular emission nebula in the constellation Monoceros. It surrounds a cluster of hot, young stars known as the Rosette Cluster ( NGC 2244 ). ( SkySafari ) NGC 2237, 2244 Caldwell 49, 50 North is up. .................. Capture Details: Telescope: Orion Optics CT12 Newtonian ( mirror 300mm, fl 1200mm, f4 ). Corrector: ASA 2" Coma Corrector Quattro 1.175x. Effective Focal Length / Aperture : 1400mm f4.7 Mount: Skywatcher EQ8 Guiding: TSOAG9 Off-Axis-Guider, Starlight Xpress Lodestar X2, PHD2 Camera: Nikon D7500 (unmodified) (sensor 23.5 x 15.7mm, 5568x3712 @ 4.196um pixels) Location: Blue Mountains, Australia Moderate light pollution ( pale green zone on darksitefinder.com map ) Capture ( 23 Dec 2017 ) 9 sets of sub-images with exposure duration for each set doubling ( 1s to 240s ) all at ISO400. 10 x 240s + 5 each @ 1s to 120s imaged ~ +/- 1.5hrs either side of meridian maximum altitude ~ 51.3 deg above north horizon Processing ( Pixinsight ) Calibration: master bias, master flat and in-camera dark subtraction Integration in 9 sets HDR combination Image Plate Solution =================================== Resolution ........ 0.633 arcsec/px ( full size image ) Rotation .......... 0.181 deg Focal ............. 1367.90 mm Pixel size ........ 4.20 um Field of view ..... 58' 59.4" x 39' 15.0" Image center ...... RA: 06 31 55.638 Dec: +04 56 30.84 ===================================
  7. The Great Barred Spiral Galaxy ( NGC 1365 ) in the constellation Fornax edit: new version with new long exposure data ( 52 x 240sec ) and better dark subtraction / dithering to remove streaks in the noise and amp glow. This also allowed for a greater stretch revealing more faint data in the galaxy and small faint fuzzies in the image .. The Great Barred Spiral Galaxy ( NGC 1365 ) in Fornax ( please click / tap to see larger ) and below I have added a 100% crop of new version: ........ original image: NGC 1365 ( please click / tap on image to see larger ) ............... The Great Barred Spiral Galaxy ( NGC 1365 ) in the Constellation Fornax Below the equator, not seen from much of the Northern hemisphere, NGC 1365 passes very nearly directly overhead an observer situated near Cape Town, as Sir John Herschel was in November of 1837, or near Sydney, as I was, almost exactly 180 years later, when I photographed this “remarkable nebula” that is numbered 2552 in his book of observations from the Cape. Not called a “nebula” now, of course, this striking object is one of the nearest and most studied examples of a barred spiral ( SB ) galaxy that also has an active galactic nuclei resulting in its designation as a Seyfert galaxy. At around 60 M light years from Earth, NGC 1365 is still seen to occupy a relatively large area ( 12 by 6 arc minutes ) due to its great size; at some 200,000 light years or so across, NGC 1365 is nearly twice as wide as the Milky Way and considerably wider than both the Sculptor and Andromeda galaxies. This High Dynamic Range ( HDR ) image is built up from multiple exposures ranging from 4 to 120 seconds with the aim of capturing the faint detail in the spiral arms of the galaxy whilst also retaining colour in the brightest star ( the orange-red 7th magnitude giant, HD 22425 ). Also, scattered throughout the image, and somewhat more difficult to see, are numerous and far more distant galaxies with apparent magnitudes of 16 to 18 or greater. Mike O'Day ................. Identification: The Great Barred Spiral Galaxy New General Catalogue - NGC 1365 General Catalogue - GC 731 John Herschel ( Cape of Good Hope ) # 2552 - Nov 28, 29 1837 Principal Galaxy Catlogue - PCG 13179 ESO 358-17 IRAS 03317-3618 RA (2000.0) 3h 33m 37.2 s DEC (2000.0) -36 deg 8' 36.5" 10th magnitude Seyfert-type galaxy in the Fornaux cluster of galaxies 200 Kly diameter 60 Mly distance .................. Capture Details: Telescope: Orion Optics CT12 Newtonian ( mirror 300mm, fl 1200mm, f4 ). Corrector: ASA 2" Coma Corrector Quattro 1.175x. Effective Focal Length / Aperture : 1400mm f4.7 Mount: Skywatcher EQ Guiding: TSOAG9 Off-Axis-Guider, Starlight Xpress Lodestar X2, PHD2 Camera: Nikon D7500 (unmodified) (sensor 23.5 x 15.7mm, 5568x3712 @ 4.196um pixels) Location: Blue Mountains, Australia Moderate light pollution ( pale green zone on darksitefinder.com map ) Capture ( 22 Nov 2017 ) 6 sets of sub-images with exposure duration for each set doubling ( 4s to 120s ) all at ISO400. 70 x 120s + 5 each @ 4s to 60s total around 2.5hrs Processing ( Pixinsight ) Calibration: master bias, master flat and no darks Integration in 6 sets HDR combination Image - Plate Solution ========================================== Resolution ........ 1.328 arcsec/px Rotation .......... -0.008 deg ( North is up ) Field of view ..... 58' 8.6" x 38' 47.5" Image center ...... RA: 03 33 41.182 Dec: -36 07 46.71 ==========================================
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